Methods for ligation and uses thereof

FIELD OF THE INVENTION

0001The present invention relates to compositions and methods for ligation. The invention provides novel reagents and methods for ligating an acyl donor compound with a nucleophilic acyl acceptor compound to form an amide bond using a transamidase.

BACKGROUND OF THE INVENTION

0002Protein engineering is becoming a widely used tool in many areas of protein biochemistry. One engineering method is controlled protein ligation, and over the past ten years some progress has been made. For instance, synthetic-based chemistry allows the joining of synthetic peptides together through native chemical ligation and a 166 amino-acid polymer-modified erythropoiesis protein has been synthesized using this method. However, native chemical ligation relies on efficient preparation of synthetic peptide esters, which can be technically difficult to prepare for large polypeptides such as proteins. For example, the reaction sometimes is performed in an organic solvent to produce the requisite protein ester for ligation. Other ligation methods not requiring the production of protein esters generate protein thioesters. An intein-based protein ligation system was used to generate a protein by thiolysis of a corresponding protein-intein thioester fusion. A prerequisite for this intein-mediated ligation method is that the target protein is expressed as a correctly folded fusion with the intein, and that sufficient spacing between the target and intein is needed to allow formation of the intein-thioester. In many instances, the intein-fusion proteins can only be obtained from inclusion bodies when expressed in <i>Escherichia coli</i>, which often cannot be refolded. This difficulty significantly limits the application of intein-based protein ligation methods.

0003Purification of a tag-free recombinant protein often is challenging and often requires multiple chromatography steps. A tag can be linked to a recombinant protein, and after purification, the tag on the fusion may be cleaved from the target protein by treatment with an exogenously added site-specific protease. Additional chromatographic steps then are required to separate the target protein from the uncleaved fusion, the affinity tag, and the protease. For example, an N-terminal 6x His tag from a recombinant protein may be cleaved by an engineered 6x His-tagged aminoprotease, and a subtractive immobilized metal-ion affinity chromatography (IMAC) step can be used to recover the untagged target. Other methods may require two or more chromatography steps and even special treatment of the exogenous protease, such as biotinylation, to facilitate its removal.

0004Methods for the site-specific modification of proteins remain in high demand, and the transpeptidation reaction catalyzed by sortases has emerged as a general method for derivatizing proteins with various types of modifications. Target proteins are engineered to contain the sortase A recognition motif (LPXTG) near their C-termini. When incubated with synthetic peptides containing one or more N-terminal glycine residues and a recombinant sortase, these artificial sortase substrates undergo a transacylation reaction resulting in the exchange of residues C-terminal to the threonine residue with the synthetic oligoglycine peptide.

0005<patcit id="pcit0001" dnum="WO2007108013A"><text>WO 2007/108013</text></patcit> discloses a sortase-catalyzed transpeptidation reaction between a substrate comprising LPXTG peptide motif and biomolecules. <patcit id="pcit0002" dnum="WO2005051976A"><text>WO 2005/051976</text></patcit> dislcoses enzyme-catalyzed methods for linking a molecule to a protein or peptide. <patcit id="pcit0003" dnum="WO03020885A"><text>WO 03/020885</text></patcit> and <patcit id="pcit0004" dnum="WO0062804A"><text>WO 00/62804</text></patcit> disclose sortase-transamidase enzyme from Gram-positive bacteria. <patcit id="pcit0005" dnum="WO2005012541A"><text>WO 2005/012541</text></patcit> discloses a method for creating a cyclic form of peptides and proteins. <nplcit id="ncit0001" npl-type="s"><text>Gruenewald et al., Biochemistry. 2004, 16;43(10):2915-25</text></nplcit> discloses the biochemical characterization of a recombinant nonribosomal peptide cyclase of a streptomycete. <patcit id="pcit0006" dnum="US2002192773A"><text>US 2002/192773</text></patcit> discloses the preparation of macrocyclic molecules from linear, synthetic thioester precursors. <patcit id="pcit0007" dnum="WO9828434A"><text>WO 98/28434</text></patcit> discloses ligation of oligopeptides to form ligation peptide products. <patcit id="pcit0008" dnum="WO9634878A"><text>WO 96/34878</text></patcit> discloses protiens having native peptide backbones produced by a method of native chemical ligation. <nplcit id="ncit0002" npl-type="s"><text>Pritz et al., J. Org. Chem. 2007,70:3909-3912</text></nplcit> discloses an investigation of the reaction conditions of sortase-mediated ligation.

SUMMARY OF THE INVENTION

0006The present invention encompasses the recognition that existing methods of transamidase ligation suffer from competition effects and efficiency issues. The inventors sought to extend the practical applications of sortase-mediated ligation to include ligation/labeling at the polypeptide N-terminus using synthetic peptides containing the LPXTG motif and protein substrates with N-terminal glycine residues. In the course of these efforts it was recognized that each successful transfer of an LPXT unit to a target protein releases a stoichiometric amount of a peptide fragment containing an N-terminal glycine. This by-product competes with the protein nucleophile for the same acyl enzyme intermediate, thereby limiting the efficiency of the ligation reaction. In certain aspects, the invention provides compositions and methods that facilitate N-terminal ligation using a transamidase, e.g., a sortase. According to one aspect, the present invention provides methods of ligation utilizing a modified transamidase recognition sequence. In some embodiments, a modified transamidase sequence comprises an ester group in place of a C-terminal amino acid residue.

0007According to one aspect, the present invention provides a method of ligation comprising the step of contacting an acyl donor compound of formula: <chemistry id="chem0001" num="0001"><img file="EP2391714B2_D0001.tif" /></chemistry> wherein <ul id="ul0001" list-style="none" compact="compact"><li>the transamidase recognition sequence is any peptide motif recognized by a transamidase enzyme;</li><li>X is -O-, or -S-;</li><li>A¹ is acyl, substituted or unsubstituted aliphatic, substituted or unsubstituted heteroaliphatic, substituted or unsubstituted aryl, substituted or unsubstituted heteroaryl, a peptide, a protein, a polynucleotide, a carbohydrate, a tag, a metal atom, a contrast agent, a catalyst, a non-polypeptide polymer, a recognition element, a small molecule, a lipid, or a label;</li><li>R¹ is substituted or unsubstituted aliphatic, substituted or unsubstituted heteroaliphatic, substituted or unsubstituted aryl, or substituted or unsubstituted heteroaryl; and</li></ul> with a nucleophilic compound of formula: <chemistry id="chem0002" num="0002"><img file="EP2391714B2_D0002.tif" /></chemistry> wherein <ul id="ul0002" list-style="none" compact="compact"><li>B¹ is acyl, substituted or unsubstituted aliphatic, substituted or unsubstituted heteroaliphatic, substituted or unsubstituted aryl, substituted or unsubstituted heteroaryl, a peptide, a protein, a polynucleotide, a carbohydrate, a tag, a metal atom, a contrast agent, a catalyst, a non-polypeptide polymer, a recognition element, a small molecule, a lipid, or a label; and</li><li>n is an integer from 0 to 100, inclusive;</li></ul> in the presence of a transamidase enzyme under suitable conditions to form a compound of formula: <chemistry id="chem0003" num="0003"><img file="EP2391714B2_D0003.tif" /></chemistry> wherein the recognition sequence is LPXT, wherein the X of the recognition sequence is any natural or unnatural amino acid, preferably wherein X is the amino acid D, E, A, N, Q, K, or R; or wherein the recognition sequence is selected from LPXT, SPXT, LAXT, LSXT, NPXT, VPXT, IPXT and YPXR wherein the X of the recognition sequence is a natural or unnatural amino acid; or wherein the recognition sequence is selected from: LPKT, LPIT, LPDT, SPKT, LAET, LAAT, LAET, LAST, LAET, LPLT, LSRT, LPET, VPDT, IPQT, YPRR, LPMT, LPLT, LAFT, LPQT, NSKT, NPQT, NAKT, and NPQS; or wherein the recognition sequence is X₁PX₂X₃, where X₁ is leucine, isoleucine, valine or methionine, X₂ is any amino acid, X₃ is threonine, serine or alanine, P is proline, preferably wherein the transamidase is sortase A; or wherein the recognition sequence NPX₁TX₂, where X₁ is glutamine or lysine, X₂ is asparagine or glycine, N is asparagine, P is proline and T is threonine, preferably wherein the transamidase is sortase B.

0008In an embodiment, the present invention provides a ligation method comprising the step of contacting an acyl donor compound of formula: <chemistry id="chem0004" num="0004"><img file="EP2391714B2_D0004.tif" /></chemistry> wherein <ul id="ul0003" list-style="none" compact="compact"><li>A¹, X, and R¹ are defined as described above; and</li><li>R² is a natural or unnatural amino acid side chain;</li></ul> with a nucleophilic compound of formula: <chemistry id="chem0005" num="0005"><img file="EP2391714B2_D0005.tif" /></chemistry> wherein B¹ and n are are defined as described above; in the presence of a transamidase enzyme under suitable conditions to form a compound of formula: <chemistry id="chem0006" num="0006"><img file="EP2391714B2_D0006.tif" /></chemistry> In some embodiments, X is -O-. In some embodiments, R¹ is C_1-6 aliphatic. In some embodiments, R¹ is methyl. In some embodiments, R² is a natural amino acid side chain. In some embodiments, R² is the side chain group of arginine. In some embodiments, R² is the side chain group of glutamic acid. In some embodiments, R² is the side chain group of lysine. In some embodiments, A¹ and B¹ are each selected from the group consisting of an antibody, an antibody chain, an antibody fragment, an antibody epitope, a recombinant protein, a peptide comprising one or more D-amino acids, a branched peptide, a therapeutic protein, an enzyme, a polypeptide subunit of a multisubunit protein, a transmembrane protein, a cell surface protein, a methylated peptide or protein, an acylated peptide or protein, a lipidated peptide or protein, a phosphorylated peptide or protein, and a glycosylated peptide or protein. In certain embodiments, n is 2. In certain embodiments, n is 4.

0009In some embodiments, the transamidase is a sortase. In certain embodiments, the sortase is sortase A. In certain embodiments, the sortase is sortase B.

0010According to one aspect, the present invention provides a compound of formula: <chemistry id="chem0007" num="0007"><img file="EP2391714B2_D0007.tif" /></chemistry> wherein the transamidase recognition sequence, A¹, and R¹ are as defined above and described herein and X is -O-. In certain embodiments, R¹ is methyl.

0011In one embodiment, the present invention provides a compound of formula: <chemistry id="chem0008" num="0008"><img file="EP2391714B2_D0008.tif" /></chemistry> wherein A¹, X, R¹, and R² are as defined above and described herein. In certain embodiments, the compound is of formula: <chemistry id="chem0009" num="0009"><img file="EP2391714B2_D0009.tif" /></chemistry>

0012In another aspect, the disclosure provides a compound obtainable by performing a transamidase-mediated reaction in which the compound described herein is reacted with a nucleophile, wherein either (i) A1 is not a polypeptide or (ii) the nucleophile is not a polypeptide.

0013Also disclosed herein are kits comprising an acyl donor compound as described herein. In some embodiments, the kit further comprises a transamidase. In some embodiments, the kit further comprises a sortase. In some embodiments, the kit provides a nucleophilic acyl acceptor as described herein.

0014Also disclosed herein are improved methods for removing sortase from a reaction system in which sortase has been used to perform a transamidation reaction. See Example 1 for exemplary description.

0015Also disclosed herein are methods and compositions related to use of multiple sortases, recognizing different motifs, for multiple labeling of polypeptides and other applications.

0016Also disclosed herein are masked transamidase recognition sequences and compounds containing them and methods of use thereof, e.g., for modifying polypeptides. Also disclosed herein are methods and compositions relating to use of transamidase-mediated, e.g., sortase-mediated, transpeptidation for the site-specific attachment of a moiety, e.g., a non-polypeptide polymer, e.g., polyethylene glycol or a derivative thereof, to a protein, e.g., a clinically useful protein, to enhance at least one property of the protein, such as extend the circulating half-life, while maintaining high biological activity, as well as covalently joining the N- and C- termini of the protein, e.g. to increase thermal stability. The methods and compositions disclosed herein may combine both of these properties by exploiting two distinct transamidase, e.g., sortase, enzymes and engineering a suitable compound (sometimes referred to herein as a "probe") that allows for site-specific modification (e.g., PEGylation) as well as allowing covalent closure in a second transamidase-mediated reaction. Also disclosed herein is a circularized version of a polypeptide, wherein the original C- and N-termini of the polypeptide are joined to each other via a linker peptide, wherein the linker peptide comprises a transamidase recognition sequence. The polypeptide may be a four-helix bundle polypeptide, e.g., a four-helix bundle cytokine. In some embodiments the polypeptide is a secreted polypeptide. In some embodiments the polypeptide binds to a cellular receptor. In some embodiments, the C-terminus, N-terminus, or both, of the polypeptide are not involved in receptor binding.

BRIEF DESCRIPTION OF THE DRAWING

0017<ul id="ul0004" list-style="none" compact="compact"><li><figref idref="f0001 f0002"><b>Figure 1</b></figref>. (a) Optimization of a two-step protocol for lipid ligation followed by removal of the sortase enzyme. Conditions: 77 µM eGFP-LPETG-His₆, 150 µM sortase A, 2 mM nucleophile, 1% (w/v) detergent, 50 mM Tris, pH 7.5, 150 mM NaCl, 10 mM CaCl2, 3 h at 37 °C. After transpeptidation, Ni-NTA resin was added as a slurry in 1 M NaCl and 40 mM imidazole and incubated for 2 h at room temperature. Abbreviations: OG = n-octyl glucoside, DM = n-dodecyl maltoside, DOC = deoxycholate. The asterisks indicate samples not treated with Ni-NTA resin. (b) ESI-MS spectrum of eGFP-LPET-1-C22 ligation product.</li><li><figref idref="f0003"><b>Figure 2</b></figref>. Lipid attachment through sortase-catalyzed transpeptidation: (a) optimized procedure for lipid coupling followed by removal of His6-tagged proteins with Ni-NTA resin, (b) SDS-PAGE analysis of lipid-modified eGFP following transpeptidation and depletion of His6-tagged proteins ("Input" = eGFP-LPETG-His₆ in the absence of sortase and nucleophile; the asterisks indicate samples incubated with Ni-NTA resin and 300 mM imidazole to block binding of His₆-tagged proteins).</li><li><figref idref="f0004"><b>Figure 3</b></figref><b>.</b> Association of lipid-modified eGFP with HeLa cells. Cells were incubated with lipid-modified eGFP (2.5 µg/mL) for 1 h in serum-free medium and then analyzed by flow cytometry. Numbers in parentheses represent the fold enhancement in mean cellular fluorescence relative to that of eGFP-GGG. Histograms for eGFP-GGG (black) and eGFP-1-ad (gray) are overlapping.</li><li><figref idref="f0005 f0006"><b>Figure 4</b></figref><b>.</b> Subcellular distribution of eGFP-1-C22 and eGFP-2-chol in U373 and HeLa cells. Cells were incubated with lipid-modified eGFP (2.5 µg/mL) at 37 °C in serum-free medium and then imaged live using spinning disk confocal microscopy. Key: (a) U373 cells, 1 h of incubation, (b) U373 cells, 5 h of incubation, (c) HeLa cells, 1.25 h of incubation, (d) HeLa cells, 5 h of incubation.</li><li><figref idref="f0007"><b>Figure 5</b></figref><b>.</b> eGFP-1-C22 is able to access early endosomal compartments as determined by partial colocalization with transferrin-Alexa 647. U373 cells were incubated with eGFP-1-C22 (2.5 µg/mL) for 5 h in serum-free medium followed by the addition of transferrin-Alexa 647 (100 µg/mL) during the final 15 min. White arrows indicate vesicles with positive staining for both eGFP-1-C22 and transferrin-Alexa 647. Significant colocalization between eGFP-2-chol and transferrin-Alexa 647 was not observed.</li><li><figref idref="f0007"><b>Figure 6</b></figref><b>.</b> Scheme 1. Site-Specific Lipid Attachment through Sortase-Mediated Transpeptidation.</li><li><figref idref="f0008"><b>Figure 7</b></figref><b>.</b> Scheme 2. Synthesis of Lipid-Modified Triglycine Nucleophiles.</li><li><figref idref="f0009"><b>Figure 8</b></figref><b>.</b> Schematic representation of N-terminal labeling method using protein substrates with N-terminal glycine residues and ligating them to synthetic peptides containing the LPXTG motif or structural analogs of this sequence element.</li><li><figref idref="f0010"><b>Figure 9</b></figref><b>.</b> (a) Fluorescence gel scan showing robust labeling of CtxB polypeptide containing 3 or 5 glycines. (b) ESI-MS showing labeling of CtxB polypeptide containing 5 glycines.</li><li><figref idref="f0011"><b>Figure 10</b></figref><b>.</b> ESI-MS showing labeling of eGFP and UCH-L3 polypeptides bearing 5 or 1 glycine, respectively.</li><li><figref idref="f0012 f0013"><b>Figure 11</b></figref><b>.</b> Characterization of lipidated triglycine nucleophiles. (a) LC-ESI-MS total ion current chromatogram for lipidated triglycine nucleophiles. (b) RP-HPLC chromatogram (280 nm) for purified <b>2-chol.</b> (c) ESI-MS data for lipidated triglycine nucleophiles.</li><li><figref idref="f0014 f0015"><b>Figure 12</b></figref><b>.</b> (a) Reconstructed ESI-MS spectra for modified eGFP. (b) MALDI-TOF MS spectrum of eGFP-2-chol.</li><li><figref idref="f0015"><b>Figure 13</b></figref><b>.</b> Negative control experiment demonstrating that the formation of the eGFP-LPETG-<b>1-C22</b> transpeptidation product requires sortase. Conditions: 77 µM eGFP-LPETG-His₆, 150 µM sortase A, 2 mM nucleophile, 1% (w/v) n-dodecyl maltoside, 50 mM Tris pH 7.5, 150 mM NaCl, 10 mM CaCl₂, 5 h at 37 °C. Samples analyzed by SDSPAGE.</li><li><figref idref="f0016"><b>Figure 14</b></figref><b>.</b>(<b>a</b>) Standard curve for estimating the yield of lipid-modified eGFP. (<b>b</b>) Absorbance of lipid-modified eGFP and estimated yield.</li><li><figref idref="f0017"><b>Figure 15</b></figref><b>.</b> Control experiment demonstrating minor increases in cellular fluorescence as a result of incubation with eGFP-GGG in combination with free lipidated triglycine nucleophiles.</li><li><figref idref="f0018 f0019"><b>Figure 16</b></figref><b>.</b> (a) Preparation of compounds 1 and 2 (FITC-LPRT-OMe and biotin-LPRT-OMe); (b) Purification of compound 1. (c) Purification of compound 2.</li><li><figref idref="f0020 f0021"><b>Figure 17</b></figref><b>.</b> ESI-MS showing quantitative labeling of CtxB constructs containing 3 or 5 glycine residues.</li><li><figref idref="f0022"><b>Figure 18</b></figref><b>.</b> Streptavidin immunoblot showing labeling of biotinylated derivative GGGGG-CtxB with biotinylated LPRT derivative.</li><li><figref idref="f0023 f0028"><b>Figures 19-22</b></figref> present an overview of biological function of sortase in bacterial cell wall anchoring and examples of C-terminal labeling using sortase.</li><li><figref idref="f0029"><b>Figure 23</b></figref> shows a schematic overview of N-terminal labeling using sortase.</li><li><figref idref="f0030"><b>Figure 24</b></figref> schematically depicts control of reaction equilibrium using disclosed LPXTG mimetics</li><li><figref idref="f0031"><b>Figure 25</b></figref> schematically shows transpeptidation reactions involving N and C termini.</li><li><figref idref="f0032 f0033"><b>Figure 26</b></figref> illustrates cyclization of Cre recombinase. A, G-Cre-LPETG-His6 (50 µm) was incubated with sortase A (50 µm) in the presence or absence of fluorescent GGG-TMR (10 mm) in sortase reaction buffer (50 mm Tris, pH 7.5, 150 mm NaCl, 10 mm CaCl2) for 21.5 h at 37 °C. SDS-PAGE revealed the expected C-terminal transpeptidation product when GGG-TMR was included, whereas omission of the triglycine nucleophile resulted in clean conversion to a unique protein species with a lower apparent molecular weight. Schematic representations to the right of the gel indicate the topology of the protein species produced by transpeptidation. B, ESI-MS of linear G-Cre-LPETG-His6 and circular Cre formed by intramolecular transpeptidation. C, MS/MS spectrum of a tryptic fragment of circular Cre showing the ligation of the N-terminal residues (GEFAPK) to the C-terminal LPET motif. Expected masses for y and b ions are listed above and below the peptide sequence. Ions that were positively identified in the MS/MS spectrum are highlighted in blue or red. Only the most prominent daughter ions have been labeled in the MS/MS spectrum.</li><li><figref idref="f0034"><b>Figure 27</b></figref><b>.</b> Cyclization of Cre is reversible.A, G-Cre-LPETG-His6 (50 µm) was circularized by treatment with sortase A (50 µm) in sortase reaction buffer (50 mm Tris, pH 7.5, 150 mm NaCl, 10 mm CaCl2) for 21.5 h at 37 °C (lane 1). This reaction mixture was then treated with 10 mm GGG-TMR (lane 3) or simply incubated for an additional 24 h at 37 °C (lane 2). All reactions were analyzed by SDS-PAGE with visualization by Coomassie staining or in-gel fluorescence. For comparison, a C-terminal labeling reaction performed using 10 mm GGG-TMR without prior cyclization of the Cre substrate (lane 4) and a sample of linear G-Cre-LPETG-His6 incubated without sortase A or nucleophile (lane 5) are included. B, molecular model of Cre recombinase monomer (generated from PDB code 1kbu) (29) showing the proximity relationship between the N and C termini. The N-terminal glycine residue is highlighted in red, and the C-terminal LPET residues are shown in green.</li><li><figref idref="f0035 f0036"><b>Figure 28</b></figref><b>.</b> Circularization of eGFP. A, molecular model of eGFP (generated from PDB code 1gfl) (31) showing the proximity relationship between the N and C termini. The N-terminal glycine residue is highlighted in red, and the C-terminal LPET residues are shown in green. B, G5-eGFP-LPETG-His6 (50 µm) was circularized by treatment with sortase A (50 µm) in sortase reaction buffer (50 mm Tris, pH 7.5, 150 mm NaCl, 10 mm CaCl2) for 24 h at 37 °C. Schematic representations to the right of the gel indicate the topology of the protein species produced by transpeptidation. C, circular eGFP (C) recovers fluorescence more rapidly than linear G5-eGFP-LPETG-His6 (L) following thermal denaturation for 5 min at 90 °C. SDS-PAGE analysis confirmed the purity and concentration of the circular eGFP (C) and linear G5-eGFP-LPETG-His6 (L) samples used for refolding.</li><li><figref idref="f0037"><b>Figure 29</b></figref><b>.</b> UCHL3 with an internally positioned LPETG motif is circularized by sortase A.A, molecular model of human UCHL3 (generated from PDB code 1xd3) (30) showing the active site crossover loop bearing an LPETG substitution (LPET residues shown in green and Gly residue shown in red). The N-terminal glycine residue that serves as the nucleophile for intramolecular transpeptidation is highlighted in red. B, UCHL3 (30 µm) bearing an LPETG substitution in the active site crossover loop was incubated with sortase A (150 µm) in the absence or presence of GGG nucleophile (90 mm) in sortase reaction buffer (50 mm Tris, pH 7.5, 150 mm NaCl, 10 mm CaCl2) for the indicated times at 37 °C and analyzed by SDS-PAGE, followed by Coomassie staining. Schematic representations to the right of the gel indicate the topology of the protein species produced by transpeptidation.</li><li><figref idref="f0038"><b>Figure 30</b></figref><b>.</b> Positions of the N and C termini in the p97 hexamer are suitable for intermolecular cross-linking through sortase-catalyzed transpeptidation.A, molecular model of p97 trimer (generated from PDB code 3cf1) (28) showing the relative position of p97 monomers in the hexameric ring. The visible N and C termini from the published p97 trimer structure are indicated in red and green, respectively. B, molecular model of G-His6-p97-LPSTG-XX showing the proximity relationship between N and C termini in adjacent p97 monomers. N- and C-terminal residues not visible in the published crystal structure have been modeled onto the existing structure. N-terminal glycine residues are shown in red, and the C-terminal LPST residues are shown in green. For clarity, the C-terminal domains of the outer monomers are hidden, as is the N-terminal domain of the central monomer. C, G-His6-p97-LPSTG-XX (1.5 mg/ml) was incubated with sortase A (30 µm) in sortase reaction buffer (50 mm Tris, pH 7.5, 150 mm NaCl, 10 mm CaCl2) for the times indicated at 37 °C. After 22 h, diglycine (GG) was added (100 mm final concentration), resulting in disappearance of the covalent oligomers (lane 5). For comparison, a control reaction containing 100 mm diglycine peptide from the outset of the experiment is shown in lane 6.</li><li><figref idref="f0039"><b>Figure 31</b></figref><b>.</b> Overview of orthogonal labeling strategies using multiple sortases with distinct recognition motifs (recognition motifs described in <nplcit id="ncit0003" npl-type="s"><text>Pallen, M. J.; Lam, A. C.; Antonio, M.; Dunbar, K. Trends in Microbiology 2001, 9(3), 97-101</text></nplcit>).</li><li><figref idref="f0040"><b>Figure 32</b></figref><b>.</b> N-terminal labeling using SrtA_staph. (a) SrtA_staph catalyzes a transacylation reaction using labeled LPRT methyl esters as substrates. The labeled LPRT fragment is transferred to proteins containing N-terminal glycines in a site-specific fashion. (b) FITC (<b>1</b>) and biotin (<b>2</b>) LPRT methyl esters for N-terminal transacylation. (c) CtxB derivatives (50 µM) were treated with 500 µM <b>1</b> and 50 µM SrtA_staph for 2 h at 37 °C in 50 mM Tris (pH 7.5), 150 mM NaCl, and 10 mM CaCl₂. Reactions were analyzed by SDS-PAGE with visualization by coomassie staining and fluorescent gel scanning.</li><li><figref idref="f0041 f0042"><b>Figure 33</b></figref><b>.</b> Site-specific N- and C-terminal labeling using multiple sortases. (a) Tetramethylrhodamine-labeled dialanine nucleophile (3) for SrtAstrep-mediated transpeptidation. (b) Strategy for the installation of discrete labels at both termini of the same protein using Δ59-SrtAstaph and SrtAstrep. (c, d) SDS-PAGE characterization with fluorescent gel scanning of dual-labeled (c) eGFP and (d) UCHL3.</li><li><figref idref="f0043"><b>Figure 34</b></figref><b>.</b> Scheme for site-specific N- and C-terminal labeling using multiple sortases. Starting materials, intermediates, and products may be analyzed using ESI-MS.</li><li><figref idref="f0044 f0045"><b>Figure 35</b></figref><b>.</b> Site-specific labeling at the N and C termini of eGFP and UCHL3. (<b>a</b>) ESI-MS spectra for all intermediates generated during the double labeling procedure. From top to bottom, this includes the eGFP starting material (m/z = 31080 Da), the intermediate formed after C-terminal modification with <b>3</b> mediated by SrtA_strep (m/z = 29470 Da), the product of thrombin cleavage (m/z = 28855 Da), crude dual labeled eGFP (m/z = 29726 Da), and dual labeled eGFP after anion exchange chromatography (m/z = 29725 Da). (<b>b</b>) ESI-MS spectra for all intermediates generated during double labeling of UCHL3. From top to bottom, this includes the UCHL3 starting material (m/z = 29252 Da), the intermediate formed after C-terminal modification with <b>3</b> mediated by SrtA_strep (m/z = 29050 Da), the product of thrombin cleavage (m/z = 28458 Da), crude dual labeled UCHL3 (m/z = 29412 Da), and dual labeled UCHL3 after anion exchange chromatography (m/z = 29412 Da). The MTSET reagent used to quench SrtA_strep also modifies the active site cysteine of UCHL3 creating an extra +118 Da signal in the mass spectrum. This modification is easily removed from the final product by brief treatment with DTT.</li><li><figref idref="f0046"><b>Figure 36</b></figref> shows a ribbon structure of a representative four helix bundle cytokine (erythropoietin) and its receptor, showing that the N and C termini are located away from the receptor binding domain and in close proximity to each other.</li><li><figref idref="f0047"><b>Figure 37</b></figref> shows a scheme for C terminal modification or circularization.</li><li><figref idref="f0048"><b>Figure 38</b></figref> shows a nucleotide sequence (SEQ ID NO: 1) encoding an IFN alpha2 variant and corresponding encoded protein (SEQ ID NO: 2). In blue and bold print at the N-terminus of SEQ ID NO: 2 is the initiating methioinine (which gets clipped off when the coding sequence is translated in bacteria), followed by the two glycines used by the transamidase for cyclization. These are not present in the delGG variant described herein. In black (normal text) is the interferon alpha sequence from the normal human interferon alpha 2 protein. This is followed by two glycines in green (underlined), and the SrtAstaph recognition sequence LPETGG in red italics. Finally, the rest of the sequence (GSHHHHHH) in blue (immediately following the LPETGG) includes the 6His tag for purification. It should be noted that the IFN alpha sequence lacks the signal sequence for insertion into the ER and secretion that is found in the naturally occurring version prior to cleavage and secretion. This sequence is dispensable when a secreted eukaryotic, e.g., mammalian, polypeptide is produced in prokaryotes, e.g., in bacteria, or in an in vitro translation system. Sequences in SEQ ID NO: 1 are shown using the same notation as in SEQ ID NO: 2, i.e., blue and bold print denotes start codon (encoding methionine) followed by codons for two glycines; black (normal text) denotes IFN alpha2 coding sequence; followed by green (underlined) codons encoding two glycines; sequence encoding SrtAstaph recognition sequence is in red italics, foolowed by sequence in blue encoding GSHHHHHH.</li><li><figref idref="f0049"><b>Figure 39A</b></figref> shows a Coomassie gel of purified Interferon α (IFNα) variants. The IFNα variant contain two additional G residues at the C terminus that were introduced during cloning and are not depicted in the legend. Cyclization results in removal of GGHis6 so that the circularized version contains an additional GGLPETGG relative to native IFNα positioned as follows: -Xaa_C-GGLPETGG-Xaa_N-, where Xaa_C and Xaa_N represent the C-terminal and N-terminal amino acids, respectively, of native IFNα.</li><li><figref idref="f0050"><b>Figure 39B</b></figref> shows MS/MS identification of the junction peptide for the ciruclar interferon alpha.</li><li><figref idref="f0051"><b>Figure 40</b></figref> shows results of a Daudi cell proliferation inhibition assay demonstrating that IFNα variants retain biological activity equivalent to the original protein. IFNα variants are as shown in <figref idref="f0049 f0050">Figure 39</figref>. Legend: standard: commercially available IFNα; +GG: GG-IFNα-LPETGG-His6; circular: (see description of <figref idref="f0049 f0050">Figure 39</figref>); delGG: IFNα - LPETGG-His6; PEG: IFNα -LPETGGGK-PEG</li><li><figref idref="f0051"><b>Figure 41</b></figref> shows serum half life ELISA results for PEGylated vs. linear IFNα demonstrating that PEGylation significantly prolongs the half-life. The linear IFNα variant tested in this experiment is the delGG variant (which comprises an LPETGG-His6 moiety at the C-terminus).</li><li><figref idref="f0052"><b>Figure 42</b></figref> shows Tm's for a thermal denaturation assay performed on IFNα variants with lysozyme as a control and shows that the circular variant exhibits significantly enhanced thermal stability relative to the other variants.</li><li><figref idref="f0053"><b>Figure 43</b></figref> shows a scheme for scheme for double transpeptidation using two sortases and a probe that contains a masked sortase recognition sequence (masking achieved by using phosphothreonine in the LPXTG motif).</li><li><figref idref="f0054"><b>Figure 44</b></figref> shows mass spectrometry results showing sequential transpeptidation using a probe containing a masked sortase recognition sequence. Upper panel: IFNαLPETAAGKGLPEtGG-His6; Middle panel: IFNαLPETAAGKGLPETGG-His6; IFNαLPETAAGKGLPETGGG</li><li><figref idref="f0055"><b>Figure 45</b></figref> shows a scheme for ligating a first protein or protein domain to a second protein or protein domain using two sortase-mediated reactions.</li><li><figref idref="f0056"><b>Figure 46</b></figref> shows a scheme for using transamidase-mediated reaction for detecting cell-cell interactions.</li><li><figref idref="f0057 f0058"><b>Figure 47</b></figref> shows (A) exemplary C-terminal probe. (B) exemplary N-terminal probe. (C) exemplary C-terminal masked probe. Representative R1 groups: (1) PEG; (2) biotin; (3) tetramethylrhodamine.</li></ul>

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS

0018In simplest terms, the transpeptidation reaction results in the ligation of species containing a transamidase recognition sequence with those bearing one or more N-terminal glycine residues. In certain embodiments, the recognition sequence is an LPXTG motif. However, a drawback of using such recognition sequences as acyl donors is that the transfer of an LPXT unit to a nucleophilic acyl acceptor releases a stoichiometric amount of a peptide fragment containing an N-terminal glycine. Such by-products compete with the desired acyl acceptor and hinder the progress of the ligation reaction. Moreover, hydrolytic cleavage of an LPXTG peptide, although a relatively slow process, can exacerbate competition with the protein nucleophile as the reaction proceeds. Indeed, initial attempts to perform N-terminal protein labeling with peptides containing all five residues (LPXTG) yielded disappointing results as useful levels of ligation could only be obtained using concentrations > 5mM of the LPXTG-containing peptide.

0019Applicants have found that the substitution of the C-terminal residue of the recognition sequence with a moiety exhibiting poor nucleophilicity once released from the transamidase provides for a more efficient ligation. Exemplary embodiments are described herein.

0020The present invention provides methods for efficiently linking an acyl donor with a nucleophilic acyl acceptor. The development of these methods is significant as they are widely applicable to many acyl donors and a multitude of different acyl acceptors. In certain embodiments, the processes are useful for ligating proteins and/or peptides to one another, ligating synthetic peptides to recombinant proteins, linking a reporting molecule to a protein or peptide, joining a nucleic acid to a protein or peptide, conjugating a protein or peptide to a solid support or polymer, and linking a protein or peptide to a label. Such products and processes save cost and time associated with ligation product synthesis and are useful for conveniently linking an acyl donor to an acyl acceptor. In certain embodiments, the ligation products and purified products are useful in diagnostic procedures (<i>e.g.</i>, an antibody that specifically binds to a cancer cell epitope is joined to a radioisotope and the conjugate is administered to a patient to detect the presence or absence of cancer cells). In certain embodiments, the ligation products and purified products are useful in therapeutic procedures (<i>e.g.</i>, an antibody that specifically binds to a cancer cell epitope is joined to a toxin such as ricin A and the conjugate is administered to a patient to selectively treat the cancer). In certain embodiments, the ligation products and purified products are useful in research methods (<i>e.g.</i>, a NH₂-CH₂-derivatized fluorophore and sortase are contacted with fixed cells that express a protein linked to a sortase recognition sequence and the location of the protein is detected by a fluorescence imaging technique).

0021Methods for ligation described herein are catalyzed by a transamidase. A transamidase is an enzyme that can form a peptide linkage (<i>i.e.</i>, amide linkage) between an acyl donor compound and a nucleophilic acyl acceptor containing a NH₂-CH₂-moiety. Sortases are enzymes having transamidase activity and have been isolated from Gram-positive bacteria. They have, as part of their cell wall structure, peptidoglycan as well as polysaccharides and/or teichoic acids. Gram- positive bacteria include the following genera: <i>Actinomyces, Bacillus, Bifidobacterium, Cellulomonas, Clostridium, Corynebacterium, Micrococcus, Mycobacterium, Nocardia, Staphylococcus, Streptococcus and Streptomyces.</i>

0022In certain embodiments, the present invention provides a ligation method comprising the step of contacting an acyl donor compound of formula: <chemistry id="chem0010" num="0010"><img file="EP2391714B2_D0010.tif" /></chemistry> wherein <ul id="ul0005" list-style="none" compact="compact"><li>the transamidase recognition sequence is any peptide motif recognized by a transamidase enzyme;</li><li>X is -O-, or -S-;</li><li>A¹ is acyl, substituted or unsubstituted aliphatic, substituted or unsubstituted heteroaliphatic, substituted or unsubstituted aryl, substituted or unsubstituted heteroaryl, a peptide, a protein, a polynucleotide, a carbohydrate, a tag, a metal atom, a contrast agent, a catalyst, a non-polypeptide polymer, a recognition element, a small molecule, a lipid, or a label;</li><li>R¹ is substituted or unsubstituted aliphatic, substituted or unsubstituted heteroaliphatic, substituted or unsubstituted aryl, or substituted or unsubstituted heteroaryl; and</li></ul> with a nucleophilic compound of formula: <chemistry id="chem0011" num="0011"><img file="EP2391714B2_D0011.tif" /></chemistry> wherein <ul id="ul0006" list-style="none" compact="compact"><li>B¹ is acyl, substituted or unsubstituted aliphatic, substituted or unsubstituted heteroaliphatic, substituted or unsubstituted aryl, substituted or unsubstituted heteroaryl, a peptide, a protein, a polynucleotide, a carbohydrate, a tag, a metal atom, a contrast agent, a catalyst, a non-polypeptide polymer, a recognition element, a small molecule, a lipid, or a label; and</li><li>n is an integer from 0 to 100, inclusive;</li></ul> in the presence of a transamidase enzyme under suitable conditions to form a compound of formula: <chemistry id="chem0012" num="0012"><img file="EP2391714B2_D0012.tif" /></chemistry> wherein the recognition sequence is LPXT, wherein the X of the recognition sequence is any natural or unnatural amino acid, preferably wherein X is the amino acid D, E, A, N, Q, K, or R; or wherein the recognition sequence is selected from LPXT, SPXT, LAXT, LSXT, NPXT, VPXT, IPXT and YPXR wherein the X of the recognition sequence is a natural or unnatural amino acid; or wherein the recognition sequence is selected from: LPKT, LPIT, LPDT, SPKT, LAET, LAAT, LAET, LAST, LAET, LPLT, LSRT, LPET, VPDT, IPQT, YPRR, LPMT, LPLT, LAFT, LPQT, NSKT, NPQT, NAKT, and NPQS; or wherein the recognition sequence is X₁PX₂X₃, where X₁ is leucine, isoleucine, valine or methionine, X₂ is any amino acid, X₃ is threonine, serine or alanine, P is proline, preferably wherein the transamidase is sortase A; or wherein the recognition sequence NPX₁TX₂, where X₁ is glutamine or lysine, X₂ is asparagine or glycine, N is asparagine, P is proline and T is threonine, preferably wherein the transamidase is sortase B.

0023One of ordinary skill will appreciate that the C-terminal amino acid of the transamidase recognition sequence is omitted. That is, an acyl group <chemistry id="chem0013" num="0013"><img file="EP2391714B2_D0013.tif" /></chemistry> replaces the C-terminal amino acid of the transamidase recognition sequence. In some embodiments, the acyl group is <chemistry id="chem0014" num="0014"><img file="EP2391714B2_D0014.tif" /></chemistry> In some embodiments, the acyl group is <chemistry id="chem0015" num="0015"><img file="EP2391714B2_D0015.tif" /></chemistry>

0024In some embodiments, the transamidase recognition sequence is LPXT, wherein X is a standard or non-standard amino acid. In some embodiments, X is selected from D, E, A, N, Q, K, or R. In some embodiments, the recognition sequence is selected from LPXT, LPXT, SPXT, LAXT, LSXT, NPXT, VPXT, IPXT, and YPXR. In some embodiments X is selected to match a naturally occurring transamidase recognition sequence. In some embodiments, the transamidase recognition sequence is selected from: LPKT, LPIT, LPDT, SPKT, LAET, LAAT, LAET, LAST, LAET, LPLT, LSRT, LPET, VPDT, IPQT, YPRR, LPMT, LPLT, LAFT, LPQT, NSKT, NPQT, NAKT, and NPQS. In some embodiments, e.g., in certain embodiments in which sortase A is used (see below), the transamidase recognition motif comprises the amino acid sequence X₁PX₂X₃, where X₁ is leucine, isolucine, valine or methionine; X₂ is any amino acid; X₃ is threonine, serine or alanine; P is proline and G is glycine. In specific embodiments, as noted above X₁, is leucine and X₃ is threonine. In certain embodiments, X₂ is aspartate, glutamate, alanine, glutamine, lysine or methionine. In certain embodiments, e.g., where sortase B is utilized, the recognition sequence often comprises the amino acid sequence NPX₁TX₂, where X₁ is glutamine or lysine; X₂ is asparagine or glycine; N is asparagine; P is proline and T is threonine. The invention encompasses the recognition that selection of X may be based at least in part in order to confer desired properties on the compound containing the recognition motif. In some embodiments, X is selected to modify a property of the compound that contains the recognition motif, such as to increase or decrease solubility in a particular solvent. In some embodiments, X is selected to be compatible with reaction conditions to be used in synthesizing a compound comprising the recognition motif, e.g., to be unreactive towards reactants used in the synthesis.

0025In some embodiments, X is -O-. In some embodiments, X is -S-.

0026In certain embodiments, R¹ is substituted aliphatic. In certain embodiments, R¹ is unsubstituted aliphatic. In some embodiments, R¹ is substituted C_1-12 aliphatic. In some embodiments, R¹ is unsubstituted C_1-12 aliphatic. In some embodiments, R¹ is substituted C_1-6 aliphatic. In some embodiments, R¹ is unsubstituted C_1-6 aliphatic. In some embodiments, R¹ is C_1-3 aliphatic. In some embodiments, R¹ is butyl. In some embodiments, R¹ is <i>n</i>-butyl. In some embodiments, R¹ is isobutyl. In some embodiments, R¹ is propyl. In some embodiments, R¹ is <i>n</i>-propyl. In some embodiments, R¹ is isopropyl. In some embodiments, R¹ is ethyl. In some embodiments, R¹ is methyl.

0027In certain embodiments, R¹ is substituted aryl. In certain embodiments, R¹ is unsubstituted aryl. In certain embodiments, R¹ is substituted phenyl. In certain embodiments, R¹ is unsubstituted phenyl.

0028It will be appreciated that a variety of functional groups, motifs, macromolecules, etc., can comprise part of the acyl donor as the A¹ moieity. In some embodiments, A¹ comprises a protein. In some embodiments, A¹ comprises a peptide. In some embodiments, A¹ comprises an element selected from the group consisting of an antibody, an antibody chain, an antibody fragment, an antibody epitope, a recombinant protein, a peptide comprising one or more D-amino acids, a branched peptide, a therapeutic protein, an enzyme, a polypeptide subunit of a multisubunit protein, a transmembrane protein, a cell surface protein, a methylated peptide or protein, an acylated peptide or protein, a lipidated peptide or protein, a phosphorylated peptide or protein, and a glycosylated peptide or protein. In some embodiments, A¹ is an amino acid. In some embodiments, A¹ comprises a protein. In some embodiments, A¹ comprises a peptide. In some embodiments, A¹ comprises an antibody. In some embodiments, A¹ comprises an antibody fragment. In some embodiments, A¹ comprises an antibody epitope. In some embodiments, A¹ comprises green fluorescent protein.

0029In some embodiments, A¹ comprises a lipid. In some embodiments, A¹ comprises a saturated hydrocarbon. In some embodiments, A¹ comprises a partially unsaturated hydrocarbon. In some embodiments, A¹ comprises a C_1-40 hydrocarbon chain. In some embodiments, A¹ comprises a C_1-30 hydrocarbon chain. In some embodiments, A¹ comprises a C_1-20 hydrocarbon chain. In some embodiments, A¹ comprises a C_12-40 hydrocarbon chain.

0030In some embodiments, A¹ comprises a substituted or unsubstituted C_1-20 aliphatic group. In some embodiments, A¹ comprises an adamantyl group.

0031In certain embodiments, A¹ comprises a small molecule. In certain embodiments, A¹ comprises a bioactive small molecule. In certain embodiments, A¹ comprises a FDA-approved drug. In certain embodiments, A¹ comprises a cytotoxic agent. In certain embodiments, A¹ comprises a steroid. In certain embodiments, A¹ comprises a steroid analog.

0032In certain embodiments, A¹ comprises a label. In some embodiments, A¹ comprises a fluorescent label. In certain embodiments, A¹ comprises a radiolabel. In certain embodiments, A¹ comprises a chemiluminescent label. In certain embodiments, A¹ comprises a phosphorescent label.

0033In certain embodiments, A¹ comprises biotin. In certain embodiments, A¹ comprises streptavidin. In certain embodiments, A¹ comprises fluorescein.

0034One of ordinary skill in the art will appreciate that A¹ and B¹ groups of the present invention are interchangable. That is, a given group A¹ on an acyl donor could also be used as a group B¹ on a nucleophilic acyl acceptor, and vice versa. Thus, all embodiments and variants of A¹ as described herein are also contemplated as B¹ groups.

0035In certain embodiments, n is an integer from 0 to 50, inclusive. In certain embodiments, n is an integer from 0 to 20, inclusive. In certain embodiments, n is 0. In certain embodiments, n is 1. In certain embodiments, n is 2. In certain embodiments, n is 3. In certain embodiments, n is 4. In certain embodiments, n is 5. In certain embodiments, n is 6.

0036In some embodiments, the present invention provides a ligation method comprising the step of contacting an acyl donor compound of formula: <chemistry id="chem0016" num="0016"><img file="EP2391714B2_D0016.tif" /></chemistry> wherein each of A¹, X, and R¹ is as defined above; and R² is a natural or unnatural amino acid side chain; with a nucleophilic compound of formula: <chemistry id="chem0017" num="0017"><img file="EP2391714B2_D0017.tif" /></chemistry> wherein B¹ and n are as defined above; in the presence of a transamidase enzyme under suitable conditions to form a compound of formula: <chemistry id="chem0018" num="0018"><img file="EP2391714B2_D0018.tif" /></chemistry>

0037In some embodiments, R² is a natural amino acid side chain. In some embodiments, R² is the side chain group of aspartic acid, glutamic acid, alanine, asparagine, glutamine, lysine, or arginine. In some embodiments, R² is the side chain group of arginine. In some embodiments, R² is the side chain group of glutamic acid. In some embodiments, R² is the side chain group of lysine. In some embodiments, R² is the side chain group of aspartic acid. In some embodiments, R² is the side chain group of alanine. In some embodiments, R² is the side chain group of asparagine. In some embodiments, R² is the side chain group of glutamine.

0038In certain embodiments, R² is an unnatural amino acid side chain.

0039In some embodiments, the acyl donor compound is of formula: <chemistry id="chem0019" num="0019"><img file="EP2391714B2_D0019.tif" /></chemistry>

0040In some embodiments, the acyl donor compound is of formula: <chemistry id="chem0020" num="0020"><img file="EP2391714B2_D0020.tif" /></chemistry>

0041In some embodiments, the acyl donor compound is of formula: <chemistry id="chem0021" num="0021"><img file="EP2391714B2_D0021.tif" /></chemistry>

0042In some embodiments, the acyl donor compound is selected from: <chemistry id="chem0022" num="0022"><img file="EP2391714B2_D0022.tif" /></chemistry> or <chemistry id="chem0023" num="0023"><img file="EP2391714B2_D0023.tif" /></chemistry>

0043In certain embodiments, the transamidase is a sortase. Enzymes identified as "sortases" from Gram-positive bacteria cleave and translocate proteins to proteoglycan moieties in intact cell walls. Among the sortases that have been isolated from <i>Staphylococcus aureus</i>, are sortase A (Srt A) and sortase B (Srt B). Thus, in certain embodiments, a transamidase used in accordance with the present invention is a sortase A, e.g., from S. aureus. In certain embodiments, a transamidase is a sortase B, e.g., from S. aureus.

0044Sortases have been classified into 4 classes, designated A , B, C, and D, based on sequence alignment and phylogenetic analysis of 61 sortases from Gram positive bacterial genomes (<nplcit id="ncit0004" npl-type="s"><text>Dramsi S, Trieu-Cuot P, Bierne H, Sorting sortases: a nomenclature proposal for the various sortases of Gram-positive bacteria. Res Microbiol.156(3):289-97, 2005</text></nplcit>. These classes correspond to the following subfamilies, into which sortases have also been classified by Comfort and Clubb (<nplcit id="ncit0005" npl-type="s"><text>Comfort D, Clubb RT. A comparative genome analysis identifies distinct sorting pathways in gram-positive bacteria. Infect Immun., 72(5):2710-22, 2004</text></nplcit>): Class A (Subfamily 1), Class B (Subfamily 2), Class C (Subfamily 3), Class D (Subfamilies 4 and 5). The aforementioned references disclose numerous sortases and recognition motifs. See also <nplcit id="ncit0006" npl-type="s"><text>Pallen, M. J.; Lam, A. C.; Antonio, M.; Dunbar, K. TRENDS in Microbiology, 2001, 9(3), 97-101</text></nplcit>. Those skilled in the art will readily be able to assign a sortase to the correct class based on its sequence and/or other characteristics such as those described in Drami, et al., supra. The term "sortase A" is used herein to refer to a class A sortase, usually named SrtA in any particular bacterial species, e.g., SrtA from S. aureus. Likewise "sortase B" is used herein to refer to a class B sortase, usually named SrtB in any particular bacterial species, e.g., SrtB from S. aureus. The invention encompasses embodiments relating to a sortase A from any bacterial species or strain. The invention encompasses embodiments relating to a sortase B from any bacterial species or strain. The invention encompasses embodiments relating to a class C sortase from any bacterial species or strain. The invention encompasses embodiments relating to a class D sortase from any bacterial species or strain.

0045Amino acid sequences of Srt A and Srt B and the nucleotide sequences that encode them are disclosed in a number of references cited herein and which are incorporated herein by reference. The amino acid sequences of S. aureus SrtA and SrtB are homologous, sharing, for example, 22% sequence identity and 37% sequence similarity. The amino acid sequence of a sortase-transamidase from <i>Staphylococcus aureus</i> also has substantial homology with sequences of enzymes from other Gram-positive bacteria, and such transamidases can be utilized in the ligation processes described herein. For example, for SrtA there is about a 31 % sequence identity (and about 44% sequence similarity) with best alignment over the entire sequenced region of the <i>S. pyogenes</i> open reading frame. There is about a 28% sequence identity with best alignment over the entire sequenced region of the <i>A</i>. <i>naeslundii</i> open reading frame. It will be appreciated that different bacterial strains may exhibit differerences in sequence of a particular polypeptide, and the sequences herein are exemplary.

0046In certain embodiments a transamidase bearing 18% or more sequence identity, 20% or more sequence identity, or 30% or more sequence identity with the <i>S. pyogenes, A. naeslundii, S. mutans, E. faecalis</i> or <i>B. subtilis</i> open reading frame encoding a sortase can be screened, and enzymes having transamidase activity comparable to Srt A or Srt B from S. <i>aureas</i> can be utilized (e. g. , comparable activity sometimes is 10% of Srt A or Srt B activity or more).

0047Thus in some embodiments of the invention the sortase is a sortase A (SrtA). SrtA recognizes the motif LPXTG, with common recognition motifs being, e.g., LPKTG, LPATG, LPNTG.

0048It will be appreciated that the terms "recognition motif" and "recognition sequence", with respect to sequences recognized by a transamidase, are used interchangably. The term "transamidase recognition sequence" is sometimes abbreviated "TRS" herein.

0049In some embodiments of the invention the sortase is a sortase B (SrtB), e.g., a sortase B of S. aureus, B. anthracis, or L. monocytogenes. Motifs recognized by sortases of the B class (SrtB) often fall within the consensus sequences NPXTX, e.g., NP[Q/K]-[T/s]-[N/G/s], such as NPQTN or NPKTG. For example, sortase B of S. aureus or B. anthracis cleaves the NPQTN or NPKTG motif of IsdC in the respective bacteria (see, e.g., <nplcit id="ncit0007" npl-type="s"><text>Marraffini, L. and Schneewind, O., Journal of Bacteriology, 189(17), p. 6425-6436, 2007</text></nplcit>). Other recognition motifs found in putative substrates of class B sortases are NSKTA, NPQTG, NAKTN, and NPQSS. For example, SrtB from L. monocytogenes recognizes certain motifs lacking P at position 2 and/or lacking Q or K at position 3, such as NAKTN and NPQSS (<nplcit id="ncit0008" npl-type="s"><text>Mariscotti JF, Garcia-Del Portillo F, Pucciarelli MG. The listeria monocytogenes sortase-B recognizes varied amino acids at position two of the sorting motif. J Biol Chem. 2009 Jan 7</text></nplcit>. [Epub ahead of print])

0050In some embodiments, the sortase is a class C sortase. Class C sortases may utilize LPXTG as a recognition motif.

0051In some embodiments, the sortase is a class D sortase. Sortases in this class are predicted to recognize motifs with a consensus sequence NA-[E/A/S/H]-TG (Comfort D, supra). Class D sortases have been found, e.g., in Streptomyces spp., Corynebacterium spp., Tropheryma whipplei, Thermobifida fusca, and Bifidobacterium longhum. LPXTA or LAXTG may serve as a recognition sequence for class D sortases, e.g., of subfamilies 4 and 5, respectively subfamily-4 and subfamily-5 enzymes process the motifs LPXTA and LAXTG, respectively). For example, B. anthracis Sortase C, which is a class D sortase, has been shown to specifically cleave the LPNTA motif in B. anthracis BasI and BasH (Marrafini, supra).

0052See Barnett and Scott for description of a sortase from that recognizes QVPTGV motif (<nplcit id="ncit0009" npl-type="s"><text>Barnett, TC and Scott, JR, Differential Recognition of Surface Proteins in Streptococcus pyogenes by Two Sortase Gene Homologs. Journal of Bacteriology, Vol. 184, No. 8, p. 2181-2191, 2002</text></nplcit>).

0053The invention contemplates use of sortases found in any gram positive organism, such as those mentioned herein and/or in the references (including databases) cited herein. The invention also contemplates use of sortases found in gram negative bacteria, e.g., <i>Colwellia psychrerythraea, Microbulbifer degradans, Bradyrhizobium japonicum, Shewanella oneidensis,</i> and <i>Shewanella putrefaciens.</i> They recognize sequence motifs LP[Q/K]T[A/S]T.

0054The term "transamidase recognition sequence" may refer to a masked or unmasked transamidase recognition sequence. A unmasked transamidase recognition sequence can be recognized by a transamidase. An unmasked transamidase recognition sequence may have been previously masked, e.g., as described herein. In some embodiments, a "masked transamidase recognition sequence" is a sequence that is not recognized by a transamidase but that can be readily modified ("unmasked") such that the resulting sequence is recognized by a transamidase. For example, in some embodiments at least one amino acid of a masked transamidase recognition sequence has a side chain that comprises a moiety that inhibits, e.g., substantially prevents, recognition of the sequence by a transamidase of interest, wherein removal of the moiety allows the transamidase to recognize the sequence. Masking may, for example, reduce recognition by at least 80%, 90%, 95%, or more (e.g., to undetectable levels) in certain embodiments. The masked recognition sequence can be unmasked by treatment with a phosphatase, thus allowing it to be used in a SrtA-catalyzed transamidation reaction. The invention provides novel masked transamidase recognition sequences, and compounds comprising them. In certain embodiments, the transamidase recognition sequence comprises the formula: <chemistry id="chem0024" num="0024"><img file="EP2391714B2_D0024.tif" /></chemistry> wherein <ul id="ul0007" list-style="none" compact="compact"><li>R² is the side chain of any natural or unnatural amino acid, optionally modified or substituted; and</li><li>R^B is hydrogen; phosphate; sulfate; an oxygen-protecting group; substituted or unsubstituted acyl; branched or unbranched, substituted or unsubstituted, cyclic or acyclic aliphatic; branched or unbranched, substituted or unsubstituted, cyclic or acyclic heteroaliphatic; branched or unbranched, substituted or unsubstituted, cyclic or acyclic aryl; or branched or unbranched, substituted or unsubstituted, cyclic or acyclic heteroaryl.</li></ul>

0055In certain embodiments, the transamidase recognition sequence comprises the formula: <chemistry id="chem0025" num="0025"><img file="EP2391714B2_D0025.tif" /></chemistry>

0056A masked transamidase recognition sequence may be unmasked by a chemical or biological process that does not adversely affect other moieties of the protein or the protein itself. In certain embodiments, a phosphatase is used to remove a phosphate group masking a recognition sequence. In certain embodiments, a sulfatase is used to remove a sulfate group. In certain embodiments, an esterase or amidase is used to remove an acyl group. In certain embodiments, a glycosidase, e.g., an N-glycosidase, is used to remove a carbohydrate moiety. In certain embodiments, a photocleavable moiety is removed by exposing the protein or peptide to light. In some embodiments, mild alkaline hydrolysis is used to remove a masking moiety. In certain embodiments, a protecting group is removed using reagents and/or conditions typically used to remove the protecting group. See, <i>e.g.</i>, <nplcit id="ncit0010" npl-type="b"><text>Wuts et al., Greene's Protective Groups in Organic Synthesis (Wiley-Interscience; 4 edition; October 30, 2006</text></nplcit>).

0057The invention provides compounds, sometimes referred to herein as "probes" that comprise a novel masked transamidase recognition sequence and are of use in a transamidase-mediated reaction. Optionally, the probe further comprises one or more additional amino acids either N- or C-terminal to the masked transamidase recognition sequence. Such additional amino acid(s) may be modified with, e.g., carbohydrates, lipids, phosphate groups, labels, tags, other polymers (for example, a non-polypeptide polymer such as polyethylene glycol), and acetyl groups. The invention further provides methods of ligation using an inventive probe. In some aspects, inventive methods comprise N-terminal and/or C-terminal modification of a polypeptide using an inventive probe. In some embodiments, both N- and C-termini are modified (dual modification). In some aspects, the invention provides methods of circularizing a polypeptide using an inventive probe. The invention further provides methods of joining two or more polypeptides using an inventive probe, thereby resulting in insertion of a non-genetically encoded peptide element between the two polypeptides. The non-genetically encoded peptide element in some embodiments comprises a modifying moiety, e.g., an acyl, substituted or unsubstituted aliphatic, substituted or unsubstituted heteroaliphatic, substituted or unsubstituted aryl, substituted or unsubstituted heteroaryl, a peptide, a protein, a polynucleotide, a carbohydrate, a tag, a particle, a metal atom, a contrast agent, a catalyst, a non-polypeptide polymer (e.g,. a polyethylene glycol), a recognition element, a small molecule, a lipid, or a label.

0058Disclosed herein are methods of using inventive transamidase recognition sequences, and probes containing them, for a variety of purposes. In some embodiments, a probe comprising a masked transamidase recognition sequence is used for C-terminal labeling, which in some embodiments is followed by unmasking and cyclization (see, e.g., <figref idref="f0053">Figure 43</figref> and discussion below). In some embodiments, a probe comprising a masked transamidase recognition sequence is used for transamidase-mediated C-terminal labeling, followed by unmasking, followed by a second transamidase-mediated reaction. The second transamidase-mediated reaction may append a second polypeptide at the C-terminus of the oligopeptide probe. (See, e.g., <figref idref="f0054">Figure 44</figref>, wherein the second transamidase-mediated reaction appends a GGG sequence to an IFNalpha variant that has undergone a first transamidase-mediated reaction.) Optionally, the first transamidase-mediated reaction installs a modifying group near the C-terminus, as shown in <figref idref="f0041 f0042">Figure 33</figref>, wherein the modifying group is denoted as "Label". The first and second polypeptides are often each produced by translation of an RNA, e.g., recombinantly produced. The resulting product comprises two polypeptides joined by a non-genetically encoded element.

0059For C-terminal labeling, e.g., as shown in <figref idref="f0053">Figure 43</figref> (upper reaction) or <figref idref="f0054">Figure 44</figref> (step 1), the disclosed method may use an oligopeptide with a nucleophilic amino N-terminus to attack the acyl enzyme and form an amide bond between the protein to be labeled and the oligopeptide. The oligopeptide may have any sequence of natural and/or unnatural amino acids and one or more of the amino acids may be modified or labeled. To give a few examples, the label may include a fluorescent tag, a phosphorescent tag, biotin, poly(histidine) tag, or a radioactive label. The oligopeptide may also be modified with carbohydrates, lipids, phosphate groups, labels, tags, other polymers (for example, a non-polypeptide polymer such as polyethylene glycol), and acetyl groups. Any biological or chemical modification of an amino acid or protein may be incorporated into the oligopeptide to be ligated using a transamidase. The oligopeptide may include a second transamidase recognition sequence or a masked transamidase recognition sequence. The oligopeptide may be of the formula: <chemistry id="chem0026" num="0026"><img file="EP2391714B2_D0026.tif" /></chemistry> wherein <ul id="ul0008" list-style="none" compact="compact"><li>j is an integer between 0 and 10, inclusive;</li><li>k is an integer between 0 and 10, inclusive;</li><li>X is -O-, -NR-, or -S-;</li><li>R is hydrogen; substituted or unsubstituted aliphatic; or substituted or unsubstituted heteroaliphatic;</li><li>R¹ is hydrogen; branched or unbranched, substituted or unsubstituted, cyclic or acyclic aliphatic, branched or unbranched, substituted or unsubstituted, cyclic or acyclic heteroaliphatic, branched or unbranched, substituted or unsubstituted, cyclic or acyclic aryl, or branched or unbranched, substituted or unsubstituted, cyclic or acyclic heteroaryl;</li><li>each occurrence of R³ is independently the side chain of a natural or unnatural amino acid;</li><li>R⁴ is the side chain of a natural or unnatural amino acid, optionally modified or substituted; substituted or unsubstituted acyl; branched or unbranched, substituted or unsubstituted, cyclic or acyclic aliphatic; branched or unbranched, substituted or unsubstituted, cyclic or acyclic heteroaliphatic; substituted or unsubstituted aryl; or substituted or unsubstituted heteroaryl; and</li><li>each occurrence of R⁵ is independently the side chain of a natural or unnatural amino acid.</li></ul>

0060The oligopeptide may be of the formula: <chemistry id="chem0027" num="0027"><img file="EP2391714B2_D0027.tif" /></chemistry> wherein <ul id="ul0009" list-style="none" compact="compact"><li>transamidase recognition sequence is a sequence recognized by a transamidase;</li><li>j is an integer between 0 and 10, inclusive;</li><li>k is an integer between 0 and 10, inclusive;</li><li>m is an integer between 0 and 10, inclusive;</li><li>p is 1 or 2;</li><li>X is -O-, -NR-, or -S-;</li><li>R is hydrogen; substituted or unsubstituted aliphatic; or substituted or unsubstituted heteroaliphatic;</li><li>each occurrence of R¹ is independently hydrogen; branched or unbranched, substituted or unsubstituted, cyclic or acyclic aliphatic, branched or unbranched, substituted or unsubstituted, cyclic or acyclic heteroaliphatic, branched or unbranched, substituted or unsubstituted, cyclic or acyclic aryl, or branched or unbranched, substituted or unsubstituted, cyclic or acyclic heteroaryl;</li><li>each occurrence of R³ is independently the side chain of a natural or unnatural amino acid, optionally modified or substituted; substituted or unsubstituted acyl; branched or unbranched, substituted or unsubstituted, cyclic or acyclic aliphatic; branched or unbranched, substituted or unsubstituted, cyclic or acyclic heteroaliphatic; substituted or unsubstituted aryl; or substituted or unsubstituted heteroaryl;</li><li>R⁴ is the side chain of a natural or unnatural amino acid, optionally modified or substituted; substituted or unsubstituted acyl; branched or unbranched, substituted or unsubstituted, cyclic or acyclic aliphatic; branched or unbranched, substituted or unsubstituted, cyclic or acyclic heteroaliphatic; substituted or unsubstituted aryl; or substituted or unsubstituted heteroaryl;</li><li>each occurrence of R⁵ is independently the side chain of a natural or unnatural amino acid, optionally modified or substituted; substituted or unsubstituted acyl; branched or unbranched, substituted or unsubstituted, cyclic or acyclic aliphatic; branched or unbranched, substituted or unsubstituted, cyclic or acyclic heteroaliphatic; substituted or unsubstituted aryl; or substituted or unsubstituted heteroaryl; and</li><li>each occurrence of R⁶ is independently the side chain of a natural or unnatural amino acid, wherein X is NR and R is H, R¹ is not CH₂CO₂H; and wherein the transamidase recognition sequence is LPXT, wherein X is a natural or unnatural amino acid, preferably wherein X is selected from D, E, A, N, Q, K, or R; or wherein the transamidase recognition sequence is LPET; or wherein the transamidase recognition sequence is LPX₁TX₂, where X₁ is selected from any natural or unnatural amino acid, preferably D, E, A, N, Q, K, or R; X₂ is alanine or glycine; L is leucine; P is proline and T is threonine. In certain embodiments, for recognition sequence of SrtA_staph is LPX₁TA. In other embodiments, for recognition sequence of SrtA_staph is LPX₁TG. The disclosure encompasses the recognition that selection of X may be based at least in part in order to confer desired properties on the compound containing the recognition motif. X may be selected to modify a property of the compound that contains the recognition motif, such as to increase or decrease solubility in a particular solvent. X may be selected to be compatible with reaction conditions to be used in synthesizing a compound comprising the recognition motif, <i>e.g.</i>, to be unreactive towards reactants used in the synthesis.</li><li>j may be 0. j may be 1. j may be 2. j may be 3. j may be 4. j may be 5.</li><li>k may be 0. k may be 1. k may be 2. k may be 3. k may be 4. k may be 5.</li><li>m may be 0. m may be 1. m may be 2. m may be 3. m may be 4. m may be 5.</li><li>X may be -O-. X may be -NH-.</li><li>R¹ may be substituted hydrogen. R¹ may be substituted aliphatic. R¹ may be unsubstituted aliphatic. R¹ may be substituted C_1-12 aliphatic. R¹ may be unsubstituted C_1-12 aliphatic. R¹ may be substituted C_1-6 aliphatic. R¹ may be unsubstituted C_1-6 aliphatic. R¹ may be C_1-3 aliphatic. R¹ may be C_1-6 alkyl. R¹ may be butyl. R¹ may be <i>n</i>-butyl. R¹ may be isobutyl. R¹ may be propyl. R¹ may be <i>n</i>-propyl. R¹ may be isopropyl. R¹ may be ethyl. R¹ may be methyl. R¹ may be substituted aryl. R¹ may be unsubstituted aryl. R¹ may be substituted phenyl. R¹ may be unsubstituted phenyl.</li><li>-XR¹ may be -OCH₃. -XR¹ may be -OH. -XR¹ may be -NH₂.</li><li>R³ may be the side chain of a natural amino acid. R³ may be hydrogen. R³ may be methyl.</li><li>R⁴ may be a modified side chain of a natural amino acid. R⁴ may be a modified side chain of serine, threonine, lysine, aspartate, glutamate, arginine, asparagine, glutamine, or tyrosine.</li><li>R⁴ may be of the formula: <chemistry id="chem0028" num="0028"><img file="EP2391714B2_D0028.tif" /></chemistry> wherein R^D is hydrogen; substituted or unsubstituted acyl; branched or unbranched, substituted or unsubstituted, cyclic or acyclic aliphatic, branched or unbranched, substituted or unsubstituted, cyclic or acyclic heteroaliphatic, branched or unbranched, substituted or unsubstituted, cyclic or acyclic aryl, or branched or unbranched, substituted or unsubstituted, cyclic or acyclic heteroaryl.</li><li>R⁴ may be of the formula: <chemistry id="chem0029" num="0029"><img file="EP2391714B2_D0029.tif" /></chemistry> wherein R^D is substituted or unsubstituted acyl; branched or unbranched, substituted or unsubstituted, cyclic or acyclic aliphatic, branched or unbranched, substituted or unsubstituted, cyclic or acyclic heteroaliphatic, branched or unbranched, substituted or unsubstituted, cyclic or acyclic aryl, or branched or unbranched, substituted or unsubstituted, cyclic or acyclic heteroaryl.</li><li>R⁴ may be of the formula: <chemistry id="chem0030" num="0030"><img file="EP2391714B2_D0030.tif" /></chemistry> wherein R^D is substituted or unsubstituted acyl; branched or unbranched, substituted or unsubstituted, cyclic or acyclic aliphatic, branched or unbranched, substituted or unsubstituted, cyclic or acyclic heteroaliphatic, branched or unbranched, substituted or unsubstituted, cyclic or acyclic aryl, or branched or unbranched, substituted or unsubstituted, cyclic or acyclic heteroaryl.</li><li>R⁴ may be of the formula: <chemistry id="chem0031" num="0031"><img file="EP2391714B2_D0031.tif" /></chemistry> wherein R^D is substituted or unsubstituted acyl; branched or unbranched, substituted or unsubstituted, cyclic or acyclic aliphatic, branched or unbranched, substituted or unsubstituted, cyclic or acyclic heteroaliphatic, branched or unbranched, substituted or unsubstituted, cyclic or acyclic aryl, or branched or unbranched, substituted or unsubstituted, cyclic or acyclic heteroaryl.</li><li>R⁴ may be of the formula: <chemistry id="chem0032" num="0032"><img file="EP2391714B2_D0032.tif" /></chemistry> wherein R^D is hydrogen; substituted or unsubstituted acyl; branched or unbranched, substituted or unsubstituted, cyclic or acyclic aliphatic, branched or unbranched, substituted or unsubstituted, cyclic or acyclic heteroaliphatic, branched or unbranched, substituted or unsubstituted, cyclic or acyclic aryl, or branched or unbranched, substituted or unsubstituted, cyclic or acyclic heteroaryl.</li><li>R⁴ may be of the formula: <chemistry id="chem0033" num="0033"><img file="EP2391714B2_D0033.tif" /></chemistry> wherein R^D is hydrogen; substituted or unsubstituted acyl; branched or unbranched, substituted or unsubstituted, cyclic or acyclic aliphatic, branched or unbranched, substituted or unsubstituted, cyclic or acyclic heteroaliphatic, branched or unbranched, substituted or unsubstituted, cyclic or acyclic aryl, or branched or unbranched, substituted or unsubstituted, cyclic or acyclic heteroaryl.</li><li>R⁴ may be of the formula: <chemistry id="chem0034" num="0034"><img file="EP2391714B2_D0034.tif" /></chemistry> wherein R^D is hydrogen; substituted or unsubstituted acyl; branched or unbranched, substituted or unsubstituted, cyclic or acyclic aliphatic, branched or unbranched, substituted or unsubstituted, cyclic or acyclic heteroaliphatic, branched or unbranched, substituted or unsubstituted, cyclic or acyclic aryl, or branched or unbranched, substituted or unsubstituted, cyclic or acyclic heteroaryl.</li><li>R⁴ may be of the formula: <chemistry id="chem0035" num="0035"><img file="EP2391714B2_D0035.tif" /></chemistry> wherein R^D is hydrogen; substituted or unsubstituted acyl; branched or unbranched, substituted or unsubstituted, cyclic or acyclic aliphatic, branched or unbranched, substituted or unsubstituted, cyclic or acyclic heteroaliphatic, branched or unbranched, substituted or unsubstituted, cyclic or acyclic aryl, or branched or unbranched, substituted or unsubstituted, cyclic or acyclic heteroaryl.</li><li>R^D may be of the formula: <chemistry id="chem0036" num="0036"><img file="EP2391714B2_D0036.tif" /></chemistry> wherein i is an integer between 1 and 100, inclusive.</li><li>R⁵ may be the side chain of a natural amino acid. R⁵ may be hydrogen. R⁵ may be methyl.</li><li>R⁶ may be the side chain of a natural amino acid. R⁶ may be hydrogen. R⁶ may be methyl.</li><li>m may be 0, and -XR¹ may be -OCH₃. m may be 1, R⁶ may be methyl, and -XR¹ may be -OH. m may be 1, R⁶ may be hydrogen, and -XR¹ may be -OH.</li></ul>

0061The oligopeptide for c-terminal labeling may be of the formula: <chemistry id="chem0037" num="0037"><img file="EP2391714B2_D0037.tif" /></chemistry> wherein <ul id="ul0010" list-style="none" compact="compact"><li>R^D is hydrogen; substituted or unsubstituted acyl; branched or unbranched, substituted or unsubstituted, cyclic or acyclic aliphatic, branched or unbranched, substituted or unsubstituted, cyclic or acyclic heteroaliphatic, branched or unbranched, substituted or unsubstituted, cyclic or acyclic aryl, or branched or unbranched, substituted or unsubstituted, cyclic or acyclic heteroaryl. In certain embodiments, R^D is of the formula: <chemistry id="chem0038" num="0038"><img file="EP2391714B2_D0038.tif" /></chemistry> wherein i is an integer between 1 and 100, inclusive.</li><li>R^D may be of the formula: <chemistry id="chem0039" num="0039"><img file="EP2391714B2_D0039.tif" /></chemistry></li><li>R^D may be of the formula: <chemistry id="chem0040" num="0040"><img file="EP2391714B2_D0040.tif" /></chemistry></li></ul>

0062For N-terminal labeling, e.g., as shown in <figref idref="f0030">Figure 24</figref>, disclosed methods use an oligopeptide with a transamidase recognition sequence that forms an acyl enzyme. The oligopeptide may have any sequence of natural and/or unnatural amino acids and one or more of the amino acids may be modified or labeled in various embodiments. To give a few examples, the label may include a fluorescent tag, a phosphorescent tag, biotin, poly(histidine) tag, or a radioactive label. The oligopeptide may also be modified with carbohydrates, lipids, phosphate groups, labels, tags, other polymers (for example, a non-polypeptide polymer such as polyethylene glycol), and acetyl groups. Any biological or chemical modification of an amino acid or protein may be incorporated into the oligopeptide to be ligated using a transamidase. The oligopeptide may include a second transamidase recognition sequence or a masked transamidase recognition sequence. In certain embodiments, the oligopeptide is of the formula: <chemistry id="chem0041" num="0041"><img file="EP2391714B2_D0041.tif" /></chemistry> wherein <ul id="ul0011" list-style="none" compact="compact"><li>transamidase recognition sequence is a sequence recognized by a transamidase as defined herein, j, k, m, p, X, R¹, R², R³, R⁴, R⁵ and R⁶ are as defined herein;</li><li>j may be 0. j may be 1. j may be 2. j may be 3. j may be 4. j may be 5.</li><li>k may be 0. k may be 1. k may be 2. k may be 3. k may be 4. k may be 5.</li><li>m may be 0. m may be 1. m may be 2. m may be 3. m may be 4. m may be 5.</li><li>p may be 1. p may be 2.</li><li>X may be -O-. X may be -NH-.</li><li>R¹ may be hydrogen. R¹ may be substituted aliphatic. R¹ may be unsubstituted aliphatic. R¹ may be substituted C_1-12 aliphatic. R¹ may be unsubstituted C_1-12 aliphatic. R¹ may be substituted C_1-6 aliphatic. R¹ may be unsubstituted C_1-6 aliphatic. R¹ may be C_1-3 aliphatic. R¹ may be C_1-6 alkyl. R¹ may be butyl. R¹ may be <i>n</i>-butyl. R¹ may be isobutyl. R¹ may be propyl. R¹ may be <i>n</i>-propyl. R¹ may be isopropyl. R¹ may be ethyl. R¹ may be methyl. R¹ may be substituted aryl. R¹ may be unsubstituted aryl. R¹ may be substituted phenyl. R¹ may be unsubstituted phenyl.</li><li>-XR¹ may be -OCH₃. -XR¹ may be -OH. -XR¹ may be -NH₂.</li><li>R³ may be the side chain of a natural amino acid. R³ may be hydrogen. R³ may be methyl.</li><li>R⁴ may be a modified side chain of a natural amino acid. R⁴ may be a modified side chain of serine, threonine, lysine, aspartate, glutamate, arginine, asparagine, glutamine, or tyrosine.</li><li>R⁴ may be of the formula: <chemistry id="chem0042" num="0042"><img file="EP2391714B2_D0042.tif" /></chemistry> wherein R^D is hydrogen; substituted or unsubstituted acyl; branched or unbranched, substituted or unsubstituted, cyclic or acyclic aliphatic, branched or unbranched, substituted or unsubstituted, cyclic or acyclic heteroaliphatic, branched or unbranched, substituted or unsubstituted, cyclic or acyclic aryl, or branched or unbranched, substituted or unsubstituted, cyclic or acyclic heteroaryl.</li><li>R⁴ is of the formula: <chemistry id="chem0043" num="0043"><img file="EP2391714B2_D0043.tif" /></chemistry> wherein R^D is substituted or unsubstituted acyl; branched or unbranched, substituted or unsubstituted, cyclic or acyclic aliphatic, branched or unbranched, substituted or unsubstituted, cyclic or acyclic heteroaliphatic, branched or unbranched, substituted or unsubstituted, cyclic or acyclic aryl, or branched or unbranched, substituted or unsubstituted, cyclic or acyclic heteroaryl.</li><li>R⁴ may be of the formula: <chemistry id="chem0044" num="0044"><img file="EP2391714B2_D0044.tif" /></chemistry> wherein R^D is substituted or unsubstituted acyl; branched or unbranched, substituted or unsubstituted, cyclic or acyclic aliphatic, branched or unbranched, substituted or unsubstituted, cyclic or acyclic heteroaliphatic, branched or unbranched, substituted or unsubstituted, cyclic or acyclic aryl, or branched or unbranched, substituted or unsubstituted, cyclic or acyclic heteroaryl.</li><li>R⁴ may be of the formula: <chemistry id="chem0045" num="0045"><img file="EP2391714B2_D0045.tif" /></chemistry> wherein R^D is substituted or unsubstituted acyl; branched or unbranched, substituted or unsubstituted, cyclic or acyclic aliphatic, branched or unbranched, substituted or unsubstituted, cyclic or acyclic heteroaliphatic, branched or unbranched, substituted or unsubstituted, cyclic or acyclic aryl, or branched or unbranched, substituted or unsubstituted, cyclic or acyclic heteroaryl.</li><li>R⁴ may be of the formula: <chemistry id="chem0046" num="0046"><img file="EP2391714B2_D0046.tif" /></chemistry> wherein R^D is hydrogen; substituted or unsubstituted acyl; branched or unbranched, substituted or unsubstituted, cyclic or acyclic aliphatic, branched or unbranched, substituted or unsubstituted, cyclic or acyclic heteroaliphatic, branched or unbranched, substituted or unsubstituted, cyclic or acyclic aryl, or branched or unbranched, substituted or unsubstituted, cyclic or acyclic heteroaryl.</li><li>R⁴ may be of the formula: <chemistry id="chem0047" num="0047"><img file="EP2391714B2_D0047.tif" /></chemistry> wherein R^D is hydrogen; substituted or unsubstituted acyl; branched or unbranched, substituted or unsubstituted, cyclic or acyclic aliphatic, branched or unbranched, substituted or unsubstituted, cyclic or acyclic heteroaliphatic, branched or unbranched, substituted or unsubstituted, cyclic or acyclic aryl, or branched or unbranched, substituted or unsubstituted, cyclic or acyclic heteroaryl.</li><li>R⁴ may be of the formula: <chemistry id="chem0048" num="0048"><img file="EP2391714B2_D0048.tif" /></chemistry> wherein R^D is hydrogen; substituted or unsubstituted acyl; branched or unbranched, substituted or unsubstituted, cyclic or acyclic aliphatic, branched or unbranched, substituted or unsubstituted, cyclic or acyclic heteroaliphatic, branched or unbranched, substituted or unsubstituted, cyclic or acyclic aryl, or branched or unbranched, substituted or unsubstituted, cyclic or acyclic heteroaryl.</li><li>R⁴ may be of the formula: <chemistry id="chem0049" num="0049"><img file="EP2391714B2_D0049.tif" /></chemistry> wherein R^D is hydrogen; substituted or unsubstituted acyl; branched or unbranched, substituted or unsubstituted, cyclic or acyclic aliphatic, branched or unbranched, substituted or unsubstituted, cyclic or acyclic heteroaliphatic, branched or unbranched, substituted or unsubstituted, cyclic or acyclic aryl, or branched or unbranched, substituted or unsubstituted, cyclic or acyclic heteroaryl.</li><li>R^D may be of the formula: <chemistry id="chem0050" num="0050"><img file="EP2391714B2_D0050.tif" /></chemistry> wherein i is an integer between 1 and 100, inclusive.</li><li>R⁵ may be the side chain of a natural amino acid. R⁵ may be hydrogen. R⁵ may be methyl.</li><li>R⁶ may be the side chain of a natural amino acid. R⁶ may be hydrogen. R⁶ may be methyl.</li><li>m may be 0, and -XR¹ may be -OCH₃. m may be 1, R⁶ may be methyl, and -XR¹ may be -OH. m may be 1, R⁶ may be hydrogen, and -XR¹ may be -OH.</li></ul>

0063The oligopeptide for c-terminal labeling may be of the formula: <chemistry id="chem0051" num="0051"><img file="EP2391714B2_D0051.tif" /></chemistry> wherein j, k, p, m, X, R¹, R³, R⁴, R⁵, and R⁶ are as defined herein.

0064A probe for use in N-terminal labeling may comprise a modified transamidase sequence that comprises an ester group in place of a C-terminal amino acid residue. In some embodiments, a probe for use in n-terminal labeling comprises an amino acid residue at the C-terminus, e.g., G, GG, GGG, etc. In some embodiments, a nucleophile located at the N-terminus of a polypeptide to be labeled comprises at least 2 G residues, e.g., 2, 3, 4, 5, or more G residues.

0065Transamidase fragments having transamidation activity can be utilized in the methods described herein. Such fragments can be identified by producing transamidase fragments by known recombinant techniques or proteolytic techniques, for example, and determining the rate of protein or peptide ligation. The fragment sometimes consists of about 80% of the full-length transamidase amino acid sequence, and sometimes about 70%, about 60%, about 50%, about 40% or about 30% of the full- length transamidase amino acid sequence such as that of S. aureus Sortase A (GenBank Accession number AAD48437). In some embodiments, the fragment lacks an N-terminal portion of the full-length sequence, e.g., the fragment lacks the N-terminal portion extending to the end of the membrane anchor sequence. In some embodiments the fragment comprises the C-terminus of a full-length transamidase amino acid sequence. In some embodiments, a catalytic core region from a sortase is utilized, e.g., a region is from about position 60 to about position 206 of SrtA, e.g., S. aureus SrtA, or about from position 82 to about position 249 of SrtAstrep.

0066Transamidases from other organisms also can be utilized in the processes described herein. Such transamidases often are encoded by nucleotide sequences substantially identical or similar to the nucleotide sequences that encode Srt A and Srt B. A similar or substantially identical nucleotide sequence may include modifications to the native sequence, such as substitutions, deletions, or insertions of one or more nucleotides. Included are nucleotide sequences that sometimes are 55%, 60%, 65%, 70%, 75%, 80%, or 85% or more identical to a native nucleotide sequence, and often are 90% or 95% or more identical to the native nucleotide sequence (each identity percentage can include a 1%, 2%, 3% or 4% variance). One test for determining whether two nucleic acids are substantially identical is to determine the percentage of identical nucleotide sequences shared between the nucleic acids.

0067Calculations of sequence identity can be performed as follows. Sequences are aligned for optimal comparison purposes and gaps can be introduced in one or both of a first and a second nucleic acid sequence for optimal alignment. Also, non-homologous sequences can be disregarded for comparison purposes. The length of a reference sequence aligned for comparison purposes sometimes is 30% or more, 40% or more, 50% or more, often 60% or more, and more often 70%, 80%, 90%, 100% of the length of the reference sequence. The nucleotides at corresponding nucleotide positions then are compared among the two sequences. When a position in the first sequence is occupied by the same nucleotide as the corresponding position in the second sequence, the nucleotides are deemed to be identical at that position. The percent identity between the two sequences is a function of the number of identical positions shared by the sequences, taking into account the number of gaps, and the length of each gap, introduced for optimal alignment of the two sequences.

0068Comparison of sequences and determination of percent identity between two sequences can be accomplished using a mathematical algorithm. Percent identity between two nucleotide sequences can be determined using the algorithm of <nplcit id="ncit0011" npl-type="s"><text>Meyers &amp; Miller, CABIOS 4: 11 17 (1989</text></nplcit>), which has been incorporated into the ALIGN program (version 2.0), using a PAM120 weight residue table, a gap length penalty of 12 and a gap penalty of 4. Percent identity between two nucleotide sequences can be determined using the GAP program in the GCG software package (available at www.gcg.com), using a NWSgapdna. CMP matrix and a gap weight of 40, 50, 60, 70, or 80 and a length weight of 1, 2, 3, 4, 5, or 6. A set of parameters often used is a Blossum 62 scoring matrix with a gap open penalty of 12, a gap extend penalty of 4, and a frame shift gap penalty of 5.

0069Another manner for determining if two nucleic acids are substantially identical is to assess whether a polynucleotide homologous to one nucleic acid will hybridize to the other nucleic acid under stringent conditions. As use herein, the term"stringent conditions"refers to conditions for hybridization and washing. Stringent conditions are known to those skilled in the art and can be found in <nplcit id="ncit0012" npl-type="b"><text>Ausubel et al. Current Protocols in Molecular Biology (John Wiley &amp; Sons, Inc., New York, 1999</text></nplcit>). Aqueous and non-aqueous methods are described in that reference and either can be used. An example of stringent conditions is hybridization in 6X sodium chloride/sodium citrate (SSC) at about 45°C, followed by one or more washes in 0.2X SSC, 0.1% SDS at 50°C. Another example of stringent conditions are hybridization in 6X sodium chloride/sodium citrate (SSC) at about 45°C, followed by one or more washes in 0.2X SSC, 0.1% SDS at 55°C. A further example of stringent conditions is hybridization in 6X sodium chloride/sodium citrate (SSC) at about 45°C, followed by one or more washes in 0.2X SSC, 0. 1% SDS at 60°C. Often, stringent conditions are hybridization in 6X sodium chloride/sodium citrate (SSC) at about 45°C, followed by one or more washes in 0.2X SSC, 0.1% SDS at 65°C. Also, stringency conditions include hybridization in 0. 5M sodium phosphate, 7% SDS at 65°C, followed by one or more washes at 0.2X SSC, 1% SDS at 65°C.

0070A variant sequence can depart from a native amino acid sequence in different manners. Amino acid substitutions may be made on the basis of similarity in polarity, charge, solubility, hydrophobicity, hydrophilicity, helix-forming properties and/or amphipathic properties and the resulting variants are screened for antimicrobial activity. For example, negatively charged amino acids include aspartic acid and glutamic acid; positively charged amino acids include lysine and arginine; and amino acids with uncharged polar head groups having similar hydrophilicity values include leucine, isoleucine, valine, glycine, alanine, asparagine, glutamine, serine, threonine, phenylalanine, and tyrosine. In certain embodiments, conservative substitutions may be made, according to Table A. Amino acids in the same block in the second column and in the same line in the third column may be substituted for one another other in a conservative substitution. Certain conservative substitutions are substituting an amino acid in one row of the third column corresponding to a block in the second column with an amino acid from another row of the third column within the same block in the second column. <tables id="tabl0001" num="0001"><table frame="all"><title><u>Table A</u></title><tgroup cols="3"><colspec colnum="1" colname="col1" colwidth="18mm" /><colspec colnum="2" colname="col2" colwidth="30mm" /><colspec colnum="3" colname="col3" colwidth="16mm" /><tbody><row><entry morerows="5" align="center" valign="middle">Aliphatic</entry><entry morerows="1" align="center" valign="middle">Non-polar</entry><entry align="center" valign="middle">G A P</entry></row><row><entry align="center" valign="middle">I L V</entry></row><row><entry morerows="1" align="center" valign="middle">Polar - uncharged</entry><entry align="center" valign="middle">C S T M</entry></row><row><entry align="center" valign="middle">N Q</entry></row><row><entry morerows="1" align="center" valign="middle">Polar - charged</entry><entry align="center" valign="middle">D E</entry></row><row><entry align="center" valign="middle">K R</entry></row><row><entry align="center" valign="middle">Aromatic</entry><entry align="center" valign="middle" /><entry align="center" valign="middle">H F W Y</entry></row></tbody></tgroup></table></tables>

0071In certain embodiments homologous substitution may occur, which is a substitution or replacement of like amino acids, such as basic for basic, acidic for acidic, polar for polar amino acids, and hydrophobic for hydrophobic, for example. Non-homologous substitutions can be introduced to a native sequence, such as from one class of residue to another (e. g. , a non-hydrophobic to a hydrophobic amino acid), or substituting a naturally occurring amino acid with an unnatural amino acids or non- classical amino acid replacements.

0072Srt A and Srt B nucleotide sequences may be used as "query sequences" to perform a search against public databases to identify related sequences. Such searches can be performed using the NBLAST and XBLAST programs (version 2.0) of <nplcit id="ncit0013" npl-type="s"><text>Altschul, et al., J. Mol. Biol. 215: 403 410 (1990</text></nplcit>). BLAST nucleotide searches can be performed with the NBLAST program, score = 100, wordlength = 12 to obtain homologous nucleotide sequences. To obtain gapped alignments for comparison purposes, gapped BLAST can be utilized as described in <nplcit id="ncit0014" npl-type="s"><text>Altschul, et al., Nucleic Acids Res. 25 (17): 3389-3402 (1997</text></nplcit>). When utilizing BLAST and Gapped BLAST programs, default parameters of the respective programs (<i>e.g.</i>, XBLAST and NBLAST) can be used (see www.ncbi.nlm.nih.gov).

0073Different transamidases may specifically recognize different recognition sequences. In some embodiments, acyl donors utilized in the ligation processes described herein include or are modified with an appropriate sortase recognition motif. One or more appropriate sortase recognition sequences can be added to an acyl donor not having one by known synthetic and recombinant techniques. In certain embodiments, 2, 3, 4, 5, 6, 7, 8, 9, or 10 or more sortase or transamidase recognition sequences are incorporated in an acyl donor.

0074In certain embodiments where Srt A is utilized as a transamidase enzyme, the acyl donor comprises the amino acid sequence X₁PX₂X₃G, where X₁ is leucine, isolucine, valine or methionine; X₂ is any amino acid; X₃ is threonine, serine or alanine; P is proline and G is glycine; wherein the G is omitted and replaced with an acyl group as described herein. In some embodiments, X₁ is leucine and X₃ is threonine. In certain embodiments, X₂ is aspartate, glutamate, alanine, glutamine, lysine or methionine. In some embodiments, the acyl donor comprises the sequence <chemistry id="chem0052" num="0052"><img file="EP2391714B2_D0052.tif" /></chemistry> wherein X is any amino acid and R¹ is as described herein.

0075In certain embodiments where Srt B is utilized as a transamidase enzyme, the acyl donor comprises the amino acid sequence NPX₁TX₂, where X₁ is glutamine or lysine; X₂ is asparagine or glycine; N is asparagine; P is proline and T is threonine.

0076Transamidases utilized in the inventive ligation methods described herein sometimes are isolated and incorporated into a kit. Any convenient method known can be utilized to isolate the sortase or transamidase. In certain embodiments, the transamidase is produced with a N-terminal or C- terminal amino acid sequence that facilitates purification. For example, the transamidase sometimes is produced with a C-terminal or N-terminal polyhistidine sequence, often six histidines, which have affinity and bind to nickel-derivitized solid supports. In some embodiments, a transamidase used in accordance with the present invention further comprises an affinity tag. Examples of affinity tags include , e.g., His-tag (His6), FLAG, Streptag, HA tag, Myc tag, maltose binding protein, SNUT tag, NusA, T7, thioredoxin, GST, etc. (See, e.g., <nplcit id="ncit0015" npl-type="s"><text>Arnau J, Lauritzen C, Petersen GE, Pedersen J., "Current strategies for the use of affinity tags and tag removal for the purification of recombinant proteins." Protein Expr Purif., 48(1):1-13, 2006</text></nplcit>).

0077In some embodiments, a kit includes a plasmid having a transamidase-encoding nucleotide sequence that the user uses to produce the sortase or transamidase in a cell culture or cell free translation system, and often, then purifies the sortase or transamidase according to instructions provided with the kit.

0078In the ligation methods described herein, the transamidase, acyl donor, and nucleophilic acyl acceptor are contacted with one another under suitable conditions to effect ligation of the acyl donor to the acyl acceptor. As used herein, the term "contacting" refers to placing the components of the process in close proximity to one another and allowing the molecules to collide by diffusion. Contacting these components with one another can be accomplished by adding them to one body of fluid and/or in one reaction vessel, for example. The components in the system may be mixed in a variety of manners, such as by oscillating a vessel, subjecting a vessel to a vortex generating apparatus, repeated mixing with a pipette or pipettes, or by passing fluid containing one assay component over a surface having another assay component immobilized thereon, for example. The components may be added in any order to the system.

0079The ligation may be performed in any convenient vessel (e.g., tubes such as microfuge tubes, flask, dish), microtiter plates (<i>e.g.</i>, 96-well or 384-well plates), glass slides, silicon chips, filters, or any solid or semisolid support having surface (optionally coated) having molecules immobilized thereon and optionally oriented in an array (see, <i>e.g.</i>, <patcit id="pcit0009" dnum="US6261776B"><text>U. S. Patent No. 6,261,776</text></patcit> and <nplcit id="ncit0016" npl-type="s"><text>Fodor, Nature 364: 555-556 (1993</text></nplcit>)), and microfluidic devices (see, <i>e.g.</i>, <patcit id="pcit0010" dnum="US6440722B"><text>U. S. Patent Nos. 6,440,722</text></patcit>; <patcit id="pcit0011" dnum="US6429025B"><text>6,429,025</text></patcit>; <patcit id="pcit0012" dnum="US6379974B"><text>6,379,974</text></patcit>; and <patcit id="pcit0013" dnum="US6316781B"><text>6,316,781</text></patcit>). The system can include attendant equipment such as signal detectors, robotic platforms, and pipette dispensers.

0080The reaction mixture often is cell free and often does not include bacterial cell wall components or intact bacterial cell walls. In some embodiments, the system includes one or more cells or cell wall components, often non-bacterial cells or non-Gram-positive bacterial cells. In such embodiments, one or more components often are expressed by one or more recombinant nucleotide sequences in a cell, which nucleotide sequences are integrated into the cell genome or non-integrated (<i>e.g.</i>, in a plasmid). Cells in such systems often are maintained <i>in vivo,</i> sometimes <i>ex vivo,</i> and sometimes <i>in vitro.</i>

0081The reaction mixture is maintained at any convenient temperature at which the ligation reaction can be performed. In some embodiments, the ligation is performed at a temperature ranging from about 15 °C to about 50 °C. In some embodiments, the ligation is performed at a temperature ranging from about 23 °C to about 37 °C. In certain embodiments, the temperature is room temperature (<i>e.g.</i>, about 25°C). The temperature can be optimized by repetitively performing the same ligation procedure at different temperatures and determining ligation rates. Any convenient assay volume and component ratio is utilized. In certain embodiments, a component ratio of 1:1000 or greater transamidase enzyme to acyl donor is utilized, or a ratio of 1:1000 or greater transamidase enzyme to acyl acceptor is utilized. In specific embodiments, ratios of enzyme to acyl donor or enzyme to acyl acceptor is about 1:1, including 1:2 or greater, 1:3 or greater, 1:4 or greater, 1:5 or greater, 1:6 or greater, 1:7 or greater, 1:8 or greater, and 1:9 or greater.

0082In some embodiments, the acyl donor is present at a concentration ranging from about 10 µM to about 10 mM. In some embodiments, the acyl donor is present at a concentration ranging from about 100 µM to about 1 mM. In some embodiments, the acyl donor is present at a concentration ranging from about 100 µM to about 5 mM. In some embodiments, the acyl donor is present at a concentration ranging from about 200 µM to about 1 mM. In some embodiments, the acyl donor is present at a concentration ranging from about 200 µM to about 800 µM. In some embodiments, the acyl donor is present at a concentration ranging from about 400 µM to about 600 µM.

0083In certain embodiments, the nucleophilic acyl acceptor is present at a concentration ranging from about 1 µM to about 500 µM. In certain embodiments, the nucleophilic acyl acceptor is present at a concentration ranging from about 15 µM to about 150 µM. In certain embodiments, the nucleophilic acyl acceptor is present at a concentration ranging from about 25 µM to about 100 µM. In certain embodiments, the nucleophilic acyl acceptor is present at a concentration ranging from about 40 µM to about 60 µM.

0084In certain embodiments, the transamidase is present at a concentration ranging from about 1 µM to about 500 µM. In certain embodiments, the transamidase is present at a concentration ranging from about 15 µM to about 150 µM. In certain embodiments, the transamidase is present at a concentration ranging from about 25 µM to about 100 µM. In certain embodiments, the transamidase is present at a concentration ranging from about 40 µM to about 60 µM.

0085In certain embodiments, the ligation method is performed in a reaction mixture comprising an aqueous environment. Water with an appropriate buffer and/or salt content often may be utilized. An alcohol or organic solvent may be included in certain embodiments. The amount of an organic solvent often does not appreciably esterify a protein or peptide in the ligation process (<i>e.g.</i>, esterified protein or peptide often increase only by 5% or less upon addition of an alcohol or organic solvent). Alcohol and/or organic solvent contents sometimes are 20% or less, 15% or less, 10% or less or 5% or less, and in embodiments where a greater amount of an alcohol or organic solvent is utilized, 30% or less, 40% or less, 50% or less, 60% or less, 70% or less, or 80% or less alcohol or organic solvent is present. In certain embodiments, the system includes only an alcohol or an organic solvent, with only limited amounts of water if it is present.

0086In some embodiments, suitable ligation conditions comprise a buffer. One of ordinary skill in the art will be familiar with a variety of buffers that could be used in accordance with the present invention. In some embodiments, the buffer solution comprises calcium ions. In certain embodiments, the buffer solution does not contain substances that precipitate calcium ions. In some embodiments, the buffer solution does not include phosphate ions. In some embodiments, the buffer solution does not contain chelating agents.

0087In some embodiments, suitable ligation conditions comprise pH in the range of 6 to 8.5. In some embodiments, suitable ligation conditions comprise pH in the range of 6 to 8. In some embodiments, suitable ligation conditions comprise pH in the range of 6 to 7.5. In some embodiments, suitable ligation conditions comprise pH in the range of 6.5 to 8.5. In some embodiments, suitable ligation conditions comprise pH in the range of 7 to 8.5. In some embodiments, suitable ligation conditions comprise pH in the range of 7.5 to 8.5. In some embodiments, suitable ligation conditions comprise pH in the range of 7.0 to 8.5. In some embodiments, suitable ligation conditions comprise pH in the range of 7.3 to 7.8.

0088One or more components for ligation or a ligation product may be immobilized to a solid support. The attachment between an assay component and the solid support may be covalent or non-covalent (<i>e.g.</i>, <patcit id="pcit0014" dnum="US6022688A"><text>U. S. Patent No. 6,022,688</text></patcit> for non-covalent attachments). The solid support may be one or more surfaces of the system, such as one or more surfaces in each well of a microtiter plate, a surface of a glass slide or silicon wafer, Biacore chip, a surface of a particle, e.g., a bead (<i>e.g.</i>, <nplcit id="ncit0017" npl-type="s"><text>Lam, Nature 354: 82-84 (1991</text></nplcit>)) that is optionally linked to another solid support, or a channel in a microfluidic device, for example. Types of solid supports, linker molecules for covalent and non-covalent attachments to solid supports, and methods for immobilizing nucleic acids and other molecules to solid supports are known (<i>e.g.</i>, <patcit id="pcit0015" dnum="US6261776B"><text>U. S. Patent Nos. 6,261,776</text></patcit>; <patcit id="pcit0016" dnum="US5900481A"><text>5,900,481</text></patcit>; <patcit id="pcit0017" dnum="US6133436A"><text>6,133,436</text></patcit>; and <patcit id="pcit0018" dnum="US6022A"><text>6,022</text></patcit>, <patcit id="pcit0019" dnum="US688A"><text>688</text></patcit>; and WIPO publication<patcit id="pcit0020" dnum="WO0118234A"><text> WO 01/18234</text></patcit>). Any material may be used, e.g., plastic (e.g., polystyrene), metal, glass, cellulose, gels (e.g., formed at least in part from organic polymers such as PDMS), etc. In some embodiments the solid support is semi-solid and/or gel-like, deformable, flexible, or the like.

0089Any protein or peptide may be utilized as an acyl donor or nucleophilic acyl acceptor in the ligation process described herein. The protein or peptide often is isolated when utilized in a cell-free system. The protein or peptide sometimes is a subregion of a protein, such as in the N-terminus, C-terminus, extracellular region, intracellular region, transmembrane region, active site (<i>e.g.</i>, nucleotide binding region or a substrate binding region), a domain (<i>e.g.</i>, an SH2 or SH3 domain) or a post-translationally modified region (<i>e.g.</i>, phosphorylated, glycosylated or ubiquinated region), for example. Peptides often are 50 amino acids or fewer in length (<i>e.g.</i>, 45, 40, 35, 30, 25, 20, or 15 amino acids or fewer in length) and proteins sometimes are 100 or fewer amino acids in length, or 200, 300, 400, 500, 600, 700, or 900 or fewer amino acids in length. The protein or peptide includes the modification moiety or a portion thereof (<i>e.g.</i>, the glycosyl group or a portion thereof). In certain embodiments, the protein is a signal transduction factor, cell proliferation factor, apoptosis factor, angiogenesis factor, or cell interaction factor. Examples of cell interaction factors include but are not limited to cadherins (<i>e.g.</i>, cadherins E, N, BR, P, R, and M; desmocollins; desmogleins; and protocadherins); connexins; integrins; proteoglycans ; immunoglobulins (<i>e.g.</i>, ALCAM, NCAM-1 (CD56), CD44, intercellular adhesion molecules (<i>e.g.</i>, ICAM-1 and ICAM-2), LFA-1, LFA-2, LFA-3, LECAM-1, VLA-4, ELAM and N-<BR> CAM); selectins (<i>e.g.</i>, L-selectin (CD62L), E-selectin (CD62e), and P-selectin (CD62P)) ; agrin; CD34; and a cell surface protein that is cyclically internalized or internalized in response to ligand binding.

0090Examples of signal transduction factors include but are not limited to protein kinases (<i>e.g.</i>, mitogen activated protein (MAP) kinase and protein kinases that directly or indirectly phosphorylate it, Janus kinase (JAK1), cyclin dependent kinases, epidermal growth factor (EGF) receptor, platelet-derived growth factor (PDGF) receptor, fibroblast-derived growth factor receptor (FGF), insulin receptor and insulin-like growth factor (IGF) receptor); protein phosphatases (<i>e.g.</i>, PTP1B, PP2A and PP2C); GDP/GTP binding proteins (<i>e.g.</i>, Ras, Raf, ARF, Ran and Rho); GTPase activating proteins (GAFs); guanine nucleotide exchange factors (GEFs); proteases (<i>e.g.</i>, caspase 3, 8 and 9), ubiquitin ligases (<i>e.g.</i>, MDM2, an E3 ubiquitin ligase), acetylation and methylation proteins (<i>e.g.</i>, p300/CBP, a histone acetyl transferase) and tumor suppressors (<i>e.g.</i>, p53, which is activated by factors such as oxygen tension, oncogene signaling, DNA damage and metabolite depletion). The protein sometimes is a nucleic acid- associated protein (<i>e.g.</i>, histone, transcription factor, activator, repressor, co-regulator, polymerase or origin recognition (ORC) protein), which directly binds to a nucleic acid or binds to another protein bound to a nucleic acid. The protein sometimes is useful as a detectable label, such as a green or blue fluorescent protein. In some embodiments a protein is a therapeutically useful protein. Certain proteins of interest are described further in the section entitled "Polypeptides" below.

0091In certain embodiments, the acyl donor comprises an antibody or antibody chain (heavy or light chain) or a portion thereof comprising an immunoglobulin domain (e.g., constant domain or variable domain). In some embodiments, antibodies are IgG, IgM, IgA, or IgE. The antibody may be of any class, subclass, or isotype in various embodiments of the invention. In some embodiments the antibody is of a class or isotype that is secreted. See, e.g., <nplcit id="ncit0018" npl-type="b"><text>Murphy, K., et al., Janeway's Immunobiology, Garland Science; 7th edition (November 27, 2007</text></nplcit>). In some embodiments, antibodies are polyclonal or monoclonal, and may be chimeric, humanized or bispecific. Polyclonal and monoclonal antibodies that bind specific antigens are commercially available, and methods for generating such antibodies are known. In general, polyclonal antibodies are produced by injecting an isolated antigen into a suitable animal (<i>e.g.</i>, a goat or rabbit); collecting blood and/or other tissues from the animal containing antibodies specific for the antigen and purifying the antibody. Methods for generating monoclonal antibodies, in general, include injecting an animal with an isolated antigen (<i>e.g.</i>, often a mouse or a rat); isolating splenocytes from the animal; fusing the splenocytes with myeloma cells to form hybridomas; isolating the hybridomas and selecting hybridomas that produce monoclonal antibodies which specifically bind the antigen (<i>e.g.</i>, <nplcit id="ncit0019" npl-type="s"><text>Kohler &amp; Milstein, Nature 256: 495 497 (1975</text></nplcit>) and <nplcit id="ncit0020" npl-type="s"><text>StGroth &amp; Scheidegger, J Immunol Methods 5: 1 21 (1980</text></nplcit>)). Examples of monoclonal antibodies are anti MDM 2 antibodies, anti-p53 antibodies (pAB421, DO 1, and an antibody that binds phosphoryl-serl5), anti-dsDNA antibodies and anti-BrdU antibodies, described hereafter.

0092Methods for generating chimeric and humanized antibodies also are known (see, <i>e.g.</i>, <patcit id="pcit0021" dnum="US5530101A"><text>U. S. patent No. 5,530,101 (Queen, et al.</text></patcit>), <patcit id="pcit0022" dnum="US5707622A"><text>U. S. patent No. 5,707,622 (Fung, et al.</text></patcit>) and <patcit id="pcit0023" dnum="US5994524A"><text>U. S. Patent Nos. 5,994,524</text></patcit> and <patcit id="pcit0024" dnum="US6245894B"><text>6,245,894 (Matsushima, et al.</text></patcit>)), which generally involve transplanting an antibody variable region from one species (<i>e.g.</i>, mouse) into an antibody constant domain of another species (<i>e.g.</i>, human). Antigen-binding regions of antibodies (<i>e.g.</i>, Fab regions) include a light chain and a heavy chain, and the variable region is composed of regions from the light chain and the heavy chain.

0093Given that the variable region of an antibody is formed from six complementarity-determining regions (CDRs) in the heavy and light chain variable regions, one or more CDRs from one antibody can be substituted (<i>e.g.</i>, grafted) with a CDR of another antibody to generate chimeric antibodies. Also, humanized antibodies are generated by introducing amino acid substitutions that render the resulting antibody less immunogenic when administered to humans.

0094In some embodiments, the acyl donor comprises an antibody fragment, such as a Fab, Fab', F(ab)'2, Dab, Fv or single-chain Fv (ScFv) fragment, and recombinant methods for generating antibody fragments are known (<i>e.g.</i>, <patcit id="pcit0025" dnum="US6099842A"><text>U. S. Patent Nos. 6,099,842</text></patcit> and <patcit id="pcit0026" dnum="US5990296A"><text>5,990,296</text></patcit> and <patcit id="pcit0027" dnum="WO2001036486A"><text>WO 2001/036486</text></patcit>). In general, single-chain antibody fragments are constructed by joining a heavy chain variable region with a light chain variable region by a polypeptide linker (<i>e.g.</i>, the linker is attached at the C-terminus or N- terminus of each chain), and such fragments often exhibit specificities and affinities for an antigen similar to the original monoclonal antibodies. Bifunctional antibodies sometimes are constructed by engineering two different binding specificities into a single antibody chain and sometimes are constructed by joining two Fab'regions together, where each Fab'region is from a different antibody (<i>e.g.</i>, <patcit id="pcit0028" dnum="US6342221B"><text>U.S. Patent No. 6,342,221</text></patcit>). Antibody fragments often comprise engineered regions such as CDR-grafted or humanized fragments. In certain embodiments the binding partner is an intact immunoglobulin, and in other embodiments the binding partner is a Fab monomer or a Fab dimer.

0095Acyl donors comprising proteins and peptides may chemically synthesized using known techniques (<i>e.g.</i>, <nplcit id="ncit0021" npl-type="b"><text>Creighton, 1983 Proteins. New York, N. Y.: W. H. Freeman and Company</text></nplcit>; and <nplcit id="ncit0022" npl-type="s"><text>Hunkapiller et al., (1984) Nature July 12-18; 310 (5973): 105-11</text></nplcit>). For example, a peptide can be synthesized by a peptide synthesizer. If desired, non-classical amino acids or chemical amino acid analogs can be introduced as a substitution or addition into the fragment sequence. Non-classical amino acids include but are not limited to D-isomers of the common amino acids, 2, 4-diaminobutyric acid, a-amino isobutyric acid, 4-aminobutyric acid, Abu, 2-amino butyric acid, g-Abu, e-Ahx, 6-amino hexanoic acid, Aib, 2-amino isobutyric acid, 3-amino propionic acid, ornithine, norleucine, norvaline, hydroxyproline, sarcosine, citrulline, homocitrulline, cysteic acid, t-butylglycine, t-butylalanine, phenylglycine, cyclohexylalanine, b-alanine, fluoroamino acids, designer amino acids such as b-methyl amino acids, Ca-methyl amino acids, Na-methyl amino acids, and amino acid analogs in general. Each amino acid in the peptide often is L (levorotary) and sometimes is D (dextrorotary). Proteins often are produced by known recombinant methods, or sometimes are purified from natural sources.

0096Native protein and peptide sequences sometimes are modified. For example, conservative amino acid modifications may be introduced at one or more positions in the amino acid sequences of target polypeptides. A "conservative amino acid substitution" is one in which the amino acid is replaced by another amino acid having a similar structure and/or chemical function. Families of amino acid residues having similar structures and functions are well known. These families include amino acids with basic side chains (<i>e.g.</i>, lysine, arginine, histidine), acidic side chains (<i>e.g.</i>, aspartic acid, glutamic acid), uncharged polar side chains (<i>e.g.</i>, glycine, asparagine, glutamine, serine, threonine, tyrosine, cysteine), nonpolar side chains (<i>e.g.</i>, alanine, valine, leucine, isoleucine, proline, phenylalanine, methionine, tryptophan), beta-branched side chains (<i>e.g.</i>, threonine, valine, isoleucine) and aromatic side chains (<i>e.g.</i>, tyrosine, phenylalanine, tryptophan, histidine). Also, essential and non- essential amino acids may be replaced. A "non-essential" amino acid is one that can be altered without abolishing or substantially altering the biological function of a target polypeptide, whereas altering an "essential" amino acid abolishes or substantially alters the biological function of a target polypeptide. Amino acids that are conserved among target polypeptides typically are essential amino acids.

0097Proteins or peptides of an acyl donor may exist as chimeric or fusion polypeptides. As used herein, a "chimeric polypeptide" or "fusion polypeptide" includes a protein or peptide linked to a different polypeptide. The different polypeptide can be fused to the N-terminus or C-terminus of the target polypeptide. Fusion polypeptides can include a moiety having high affinity for a ligand. For example, the fusion polypeptide can be a GST-target fusion polypeptide in which the protein or peptide sequences are fused to the C-terminus of the GST sequences, or a polyhistidine-target fusion polypeptide in which the protein or peptide is fused at the N-or C-terminus to a string of histidine residues (<i>e.g.</i>, sometimes three to six histidines). Such fusion polypeptides can facilitate purification of recombinant protein or peptide. Expression vectors are commercially available that already encode a fusion moiety, and a nucleotide sequence encoding the peptide or polypeptide can be cloned into an expression vector such that the fusion moiety is linked in-frame to the target polypeptide. Further, the fusion polypeptide can be a protein or peptide containing a heterologous signal sequence at its N- terminus. In certain host cells (<i>e.g.</i>, mammalian host cells), expression, secretion, cellular internalization, and cellular localization of a target polypeptide can be increased through use of a heterologous signal sequence. Fusion polypeptides sometimes include all or a part of a serum polypeptide (<i>e.g.</i>, an IgG constant region or human serum albumin).

0098Suitable systems for producing polypeptides in prokaryotic and eukaryotic systems are well known in the art and are of use in the invention. One of skill in the art will select an appropriate system (e.g., appropriate cell type and appropriate vector for inserting a nucleic acid encoding the protein). In some embodiments, a host cell is transformed to contain a vector that encodes a polypeptide whereby the polypeptide is produced in the cell. A host cell can be prokaryotic or eukaryotic cell, e.g., bacterial, fungal, plant, or animal (e.g., insect or mammalian). Exemplary host cells include bacterial cells (e.g., Gram-negative bacteria such as E. coli or Gram-positive bacteria such as B. subtilis or Lactococcus lactis), insect cells (e.g., Sf9), mammalian cells (e.g., CHO cells, COS cells, SP2/0 and NS/0 myeloma cells, human embryonic kidney (e.g., HEK 293) cells, baby hamster kidney (BHK) cell, human B cells, seed plant cells, and Ascomycete cells (e.g., Neurospora, Aspergillus and yeast cells; e.g., yeast of the genera Saccharomyces, Pichia, Hansenula, Schizosaccharomyces, Kluyveromyces, Yarrowia, and Candida). Exemplary yeast species include S. cerevisiae, Hansenula polymorpha, Kluyveromyces lactis, Pichia pastoris, Schizosaccharomyces pombe, and Yarrowia lipolytica.

0099For a glycosylated protein, a cell type capable of performing such glycosylation can be selected. For production in eukaryotic cells, e.g., mammalian cells, yeast cells, insect cells (e.g., baculovirus-based), plant cells, or transgenic non-human animals or plants, a suitable signal sequence may be included at the N-terminus. In such instances, the nucleophilic residue(s), e.g., G or GG, and optional masking residues (e.g., a sequence comprising a protease cleavage site), are located C-terminal to the signal sequence, so that they are located at the N-terminus of the mature, secreted polypeptide. In some embodiments, a secretion signal sequence whose cleavage generates an N-terminal nucleophilic residue (e.g., an A or G) is used if a polypeptide is produced in eukaryotic, e.g., mammalian, cells. For example, cleavage of the H-2kb secretion sequence generates an N-terminal glycine. The protein sequence of the H-2kb secretion signal sequence is MVPCTLLLLLAAALAPTQTRA-GG (SEQ ID NO: 3). It gets cleaved at the dash to leave the GG at the N-terminus). An exemplary nucleotide sequence encoding this sequence is: atggtaccgt gcacgctgct cctgctgttg g cggccgccc tggctccgac tcagacccgc gcg (SEQ ID NO: 4). In some embodiments, a secretion signal sequence is selected such that, when cleaved, it leaves a masked nucleophilic residue near the N-terminus.

0100In certain embodiments, the acyl donor or acyl acceptor is modified by a process or with a moiety not typically incorporated into a protein during translation. In specific embodiments, the acyl donor or acyl acceptor comprises one or more moieties selected from an alkyl moiety (<i>e.g.</i>, methyl moiety), an alkanoyl moiety (<i>e.g.</i>, an acetyl group (<i>e.g.</i>, an acetylated histone)), an alkanoic acid or alkanoate moiety (<i>e.g.</i>, a fatty acid), a glyceryl moiety (<i>e.g.</i>, a lipid), a phosphoryl moiety, a glycosyl moiety (<i>e.g.</i>, N-linked or O-linked carbohydrate chains) or an ubiquitin moiety. Further, any of numerous chemical modifications may be carried out by known techniques, including but not limited to specific chemical cleavage by cyanogen bromide, trypsin, chymotrypsin, papain, V8 protease, NaBH₄; acetylation, formylation, oxidation, reduction; metabolic synthesis in the presence of tunicamycin; and the like. The N-terminal and/or C-terminal ends may be processed (<i>e.g.</i>, the N-terminal methionine may not be present due to prokaryotic expression of the protein or peptide) and chemical moieties may be attached to the amino acid backbone. Proteins and peptides sometimes are modified with a detectable label, such as an enzymatic, fluorescent, isotopic or affinity label to allow for detection and isolation of the polypeptide.

0101When no transamidase or sortase recognition sequence is present in a acyl donor or acyl acceptor to be ligated, the acyl donor or acyl acceptor is modified to include an appropriate recognition sequence. When a recognition sequence is present in a native protein or peptide amino acid sequence, the recognition sequence often is removed unless it is located near the N-terminus or C-terminus. A recognition sequence can be removed in a native amino acid sequence by synthesizing it without the recognition sequence or by modifying some and/or all amino acids in the nucleotide sequence encoding the amino acid recognition sequence by known recombinant techniques. In some embodiments, when a recognition sequence and non-native sequence useful for purification are introduced to the native protein or peptide amino acid sequence, the recognition sequence is incorporated closer such that the non-native sequence useful for purification is cleaved from the acyl donor during ligation. In such embodiments, the transamidase also is modified with the non-native sequence and the ligated product is purified away from the reactants when contacted with a solid support that binds the non-native sequence. In embodiments where the acyl donor is modified with a detectable label or homing sequence for a detectable label, such sequences often are incorporated closer to the N-terminus than the recognition sequence so they are not cleaved from the protein or peptide in the ligation process.

0102In certain embodiments, the present invention provides an acyl donor compound of formula: <chemistry id="chem0053" num="0053"><img file="EP2391714B2_D0053.tif" /></chemistry> wherein the transamidase recognition sequence, X, R¹, and A¹ are as defined in appended claim 5.

0103In certain embodiments, the present invention provides an acyl donor compound of formula: <chemistry id="chem0054" num="0054"><img file="EP2391714B2_D0054.tif" /></chemistry> wherein X, R¹, R², and A¹ are as defined in appended claim 6.

0104As described above, the transamidase catalyzes formation of an amide linkage between a NH₂-CH₂-moiety, which is joined to and/or is in the acyl acceptor, and an acyl moiety in the acyl donor. Suitable NH₂-CH₂-moieties are known and can be determined by performing the linkage processes described herein in a routine manner. Where an acyl acceptor does not include a suitable NH₂-CH₂-moiety, one or more NH₂-CH₂-moieties are joined to the molecule. Methods for joining one or more NH₂-CH₂-moieties to a molecule of interest are known and can be developed using techniques known in the art. In some embodiments, NH₂-CH₂-moieties utilized in the methods described herein are present as one or more glycine amino acids in attached to a B¹ group. In certain embodiments, between one and seven glycines are present in or are incorporated into/onto a B¹ group, and in specific embodiments, a B¹ group is derivitized with three glycines.

0105The nucleophilic acyl acceptor can be any molecule that leads to a useful ligated conjugate. In certain embodiments, the nucleophilic acyl acceptor comprises a protein or peptide. In some embodiments, the nucleophilic acyl acceptor comprises an antibody epitope, an antibody, a recombinant protein, a synthetic peptide or polypeptide, a peptide comprising one or more D-amino acids, a peptide comprising all D-amino acids, a peptide comprising one or more unnatural or non-classical amino acids (<i>e. g.</i>, ornithine), a peptide mimetic, or a branched peptide. In some embodiments, the nucleophilic acyl acceptor comprises a peptide that confers enhanced cell penetrance to the acyl donor (<i>e.g.</i>, a greater amount of the acyl donor compound conjugated to the acyl acceptor is translocated across a cell membrane in a certain time frame as compared to the acyl donor not conjugated to the acyl acceptor), which is referred to herein as a "protein transduction domain (PTD)" peptide or "transduction peptide." Any PTD can be conjugated to a protein or peptide using the methods described herein. PTD peptides are known, and include amino acid subsequences from HIV-tat (<i>e.g.</i>, U. S. Patent No. <patcit id="pcit0029" dnum="US6316003B"><text>6,316,003</text></patcit>), sequences from a phage display library (<i>e.g.</i>, <patcit id="pcit0030" dnum="US20030104622A"><text>U. S. 20030104622</text></patcit>) and sequences rich in amino acids having positively charged side chains (<i>e.g</i>., guanidino-, amidino-and amino-containing side chains; <i>e.g.</i>, <patcit id="pcit0031" dnum="US6593292B"><text>U. S. Patent No. 6,593,292</text></patcit>). The PTD peptide sometimes is branched. The appended peptides promote cellular uptake and in some cases, cell-type specificity. Organelle-specific PTDs have also been generated, providing a means to target specific subcellular sites. See, e.g., <nplcit id="ncit0023" npl-type="s"><text>Stewart KM, Horton KL, Kelley SO. Cell-penetrating peptides as delivery vehicles for biology and medicine. Org Biomol Chem. 6(13):2242-55, 2008</text></nplcit>. Also of interest are moieties that enhance stability of a second moiety such as a polypeptide, when conjugated thereto. For example, such moieties may increase the half-life of a compound in vivo. Examples include non-polypeptide polymers such as polyethylene glycol and derivatives thereof. In some embodiments a polymer is biocompatible. In some embodiments a polymer is biodegradable.

0106In some embodiments, the nucleophilic acyl acceptor comprises a detectable moiety. Any known and convenient detectable moiety can be utilized. In certain embodiments, avidin, streptavidin, a fluorescent molecule, or a radioisotope is linked to the acyl donor. In some embodiments, biotin or another vitamin, such as thiamine or folate, is linked to the acyl donor. Examples of detectable moieties include various enzymes, prosthetic groups, fluorescent materials, luminescent materials, bioluminescent materials, and radioisotopes. Examples of suitable enzymes include horseradish peroxidase, alkaline phosphatase, p-galactosidase, or acetylcholinesterase; examples of suitable prosthetic group complexes include streptavidin/biotin and avidin/biotin; examples of suitable fluorescent materials include umbelliferone, fluorescein, fluorescein isothiocyanate, rhodamine, dichlorotriazinylamine fluorescein, dansyl chloride or phycoerythrin; an example of a luminescent material includes luminol; examples of bioluminescent materials include luciferase, luciferin, and aequorin, and examples of suitable radioisotopes include ¹²⁵I, ¹³¹1, ³⁵S or ³H. The radioisotope sometimes is selected based upon its appropriate use in a nuclear medicinal procedure, such as Be-7, Mg-28, Co-57, Zn-65, Cu-67, Ge-68, Sr-82, Rb-83, Tc-95m, Tc-96, Pd-103, Cd-109, and Xe-127, to name but a few.

0107Conjugates between an acyl donor and a detectable label are useful in diagnostic procedures. Diagnostic procedures include, for example, nuclear medicinal procedures for locating diseased locations of a subject, and procedures for detecting specific components or pathogens in a biological sample from a subject. The conjugates also are useful as research tools. For example, the conjugates are useful in flow cytometry techniques and for detecting a cellular location of a specific protein or peptide in a cell. Thus, in certain embodiments, a NH₂-CH₂-derivitized fluorophore and a transamidase are contacted with cells that express a protein linked to a sortase recognition sequence. The transamidase, often a sortase, joins the fluorophore to the protein and allows detection of the cell, protein in the cell, or protein on the cell surface. In certain embodiments, the transamidase is expressed by a nucleotide sequence that encodes it in the cell (<i>e.g.</i>, the nucleotide sequence sometimes is in a plasmid), and in other embodiments useful for detecting a protein on a cell surface or in a fixed cell, exogenous transamidase protein, often isolated protein, is contacted with the cell.

0108The location of a protein in a cell sometimes is detected by a fluorescence imaging technique in cells fixed to a solid support. Imaging techniques include, for example, using a standard light microscope or a confocal microscope (<i>e.g.</i>, <patcit id="pcit0032" dnum="US5283433A"><text>U. S. Patent Nos. 5,283,433</text></patcit> and <patcit id="pcit0033" dnum="US5296703A"><text>5,296,703 (Tsien</text></patcit>)). Appropriate light microscopes are commercially available and are useful for probing cells in two dimensions (<i>i</i>. <i>e</i>., the height of a cell often is not resolved), and confocal microscopy is useful for probing cells in three-dimensions. Many microscopy techniques are useful for determining the location of a protein in a cell (<i>e.g.</i>, in the nucleus, cytoplasm, plasma cell membrane, nucleolus, mitochondria, vacuoles, endoplasmic reticulum or Golgi apparatus). Some microscopic techniques are useful for determining the location of molecular antigens in groups of cells, tissue samples, and organs. Cellular locations often are visualized by counter-staining for subcellular organelles.

0109In some embodiments, cells expressing the protein are subjected to a known flow cytometry procedure, such as flow microfluorimetry (FMF) and fluorescence activated cell sorting (FACS); <patcit id="pcit0034" dnum="US6090919A"><text>U. S. Patent Nos. 6,090,919 (Cormack, et al.</text></patcit>) ; <patcit id="pcit0035" dnum="US6461813B"><text>6,461,813 (Lorens</text></patcit>); and <patcit id="pcit0036" dnum="US6455263B"><text>6,455,263 (Payan</text></patcit>)). For use in these procedures, the protein often is expressed on the cell surface.

0110In certain embodiments, the nucleophilic acyl acceptor comprises a polymer or a small molecule. Polymers can be useful for enhancing protein or peptide solubility, stability and circulating time, and/or for decreasing immunogenicity when the protein or peptide is administered to a subject. In some embodiments, the polymer is a water soluble polymer such as polyethylene glycol, ethylene glycol/propylene glycol copolymers, carboxymethylcellulose, dextran, polyvinyl alcohol and the like, for example. The acyl donor may include one, two, three or more attached polymer moieties after ligation. The polymer may be of any molecular weight, and may be branched or unbranched. For polyethylene glycol, the molecular weight often is between about 1 kDa and about 100 kDa (the term "about" indicating that in preparations of polyethylene glycol, some molecules will weigh more, some less, than the stated molecular weight). Other sizes may be used, depending on a desired therapeutic profile (<i>e.g.</i>, the duration of sustained release desired, the effects if any on biological activity, the ease in handling, the degree or lack of antigenicity and other known effects of the polyethylene glycol to a therapeutic protein or analog).

0111Any small molecule derivatized with or having a NH₂-CH₂-moiety can be linked to an acyl donor having a transamidase recognition site. In certain embodiments, the small molecule is known to be bioactive. Such small molecules can be obtained using any of the numerous approaches in combinatorial library methods known in the art, including: biological libraries; peptoid libraries (libraries of molecules having the functionalities of peptides, but with a novel, non-peptide backbone which are resistant to enzymatic degradation but which nevertheless remain bioactive (see, <i>e.g.</i>, <nplcit id="ncit0024" npl-type="s"><text>Zuckermann et al., J. Med. Chem. 37 : 2678-85 (1994</text></nplcit>)); spatially addressable parallel solid phase or solution phase libraries; synthetic library methods requiring deconvolution; "one-bead one-compound" library methods; and synthetic library methods using affinity chromatography selection. Biological library and peptoid library approaches are typically limited to peptide libraries, while the other approaches are applicable to peptide, non-peptide oligomer or small molecule libraries of compounds (<nplcit id="ncit0025" npl-type="s"><text>Lam, Anticancer Drug Des. 12: 145, (1997</text></nplcit>)). Examples of methods for synthesizing molecular libraries are described, for example, in <nplcit id="ncit0026" npl-type="s"><text>DeWitt et al., Proc. Natl. Acad. Sci. U. S. A. 90: 6909 (1993</text></nplcit>); <nplcit id="ncit0027" npl-type="s"><text>Erb et al., Proc. Natl. Acad. Sci. USA 91: 11422 (1994</text></nplcit>); <nplcit id="ncit0028" npl-type="s"><text>Zuckermann et al., J. Med. Chem. 37: 2678 (1994</text></nplcit>); <nplcit id="ncit0029" npl-type="s"><text>Cho et al., Science 261: 1303 (1993</text></nplcit>); <nplcit id="ncit0030" npl-type="s"><text>Carrell et al., Angew. Chem. Int. Ed. Engl. 33: 2059 (1994</text></nplcit>); <nplcit id="ncit0031" npl-type="s"><text>Carell et al., Angew. Chem. Int. Ed. Engl. 33: 2061 (1994</text></nplcit>); and in <nplcit id="ncit0032" npl-type="s"><text>Gallop et al., J. Med. Chem. 37: 1233 (1994</text></nplcit>). Libraries of compounds may be presented in solution (<i>e.g.</i>, <nplcit id="ncit0033" npl-type="s"><text>Houghten, Biotechniques 13: 412-421 (1992</text></nplcit>)), or on beads (<nplcit id="ncit0034" npl-type="s"><text>Lam, Nature 354: 82-84 (1991</text></nplcit>)), chips (<nplcit id="ncit0035" npl-type="s"><text>Fodor, Nature 364: 555-556 (1993</text></nplcit>)), bacteria or spores (Ladner, United States Patent No. <patcit id="pcit0037" dnum="US5223409A"><text>5,223,409</text></patcit>), plasmids (<nplcit id="ncit0036" npl-type="s"><text>Cull et al., Proc. Natl. Acad. Sci. USA 89: 1865-1869 (1992</text></nplcit>)). Small molecules include, but are not limited to, peptides, peptidomimetics (<i>e.g.</i>, peptoids), amino acids, amino acid analogs, polynucleotides, polynucleotide analogs, nucleotides, nucleotide analogs, organic or inorganic compounds (<i>i.e.</i>, including heteroorganic and organometallic compounds) having a molecular weight less than about 10,000 grams per mole, organic or inorganic compounds having a molecular weight less than about 5,000 grams per mole, organic or inorganic compounds having a molecular weight less than about 1,000 grams per mole, organic or inorganic compounds having a molecular weight less than about 500 grams per mole, and salts, esters, and other pharmaceutically acceptable forms of such compounds. In certain embodiments, the small molecule folate, spermine or puromycin is a component of an acyl donor. In other embodiments, the small molecule is a modification moiety described above, such as a phosphoryl moiety, ubiquitin moiety or a glycosyl moiety, for example.

0112In some embodiments, the acyl acceptor comprises is a nucleic acid, such as a deoxyribonucleic acid, a ribonucleic acid, nucleic acid derivatives, and a modified nucleic acid. The nucleic acid may comprise or consist of DNA nucleotide sequences (<i>e.g.</i>, genomic DNA (gDNA) and complementary DNA (cDNA)) or RNA nucleotide sequences (<i>e.g.</i>, mRNA, tRNA, and rRNA). The nucleic acid sometimes is about 8 to about 50 nucleotides in length, about 8 to about 35 nucleotides in length, and sometimes from about 10 to about 25 nucleotides in length. Nucleic acids often are 40 or fewer nucleotides in length, and sometimes are 35 or fewer, 30 or fewer, 25 or fewer, 20 or fewer, and 15 or fewer nucleotides in length. Synthetic oligonucleotides can be synthesized using standard methods and equipment, such as by using an ABI3900 High Throughput DNA Synthesizer, which is available from Applied Biosystems (Foster City, CA).

0113A nucleic acid sometimes is an analog or derivative nucleic acid, which can include backbone/linkage modifications (<i>e.g.</i>, peptide nucleic acid (PNA) or phosphothioate linkages) and/or nucleobase modifications. Examples of such modifications are set forth in U. S. Patent No. <patcit id="pcit0038" dnum="US6455308B"><text>6,455,308 (Freier et al.</text></patcit>) ; in <patcit id="pcit0039" dnum="US4469863A"><text>U. S. Patent Nos. 4,469,863</text></patcit>; <patcit id="pcit0040" dnum="US5536821A"><text>5,536,821</text></patcit>; <patcit id="pcit0041" dnum="US5541306A"><text>5,541,306</text></patcit>; <patcit id="pcit0042" dnum="US5637683A"><text>5,637,683</text></patcit>; <patcit id="pcit0043" dnum="US5637684A"><text>5,637,684</text></patcit>; <patcit id="pcit0044" dnum="US5700922A"><text>5,700,922</text></patcit>; <patcit id="pcit0045" dnum="US5717083A"><text>5,717,083</text></patcit>; <patcit id="pcit0046" dnum="US5719262A"><text>5,719,262</text></patcit>; <patcit id="pcit0047" dnum="US5739308A"><text>5,739,308</text></patcit>; <patcit id="pcit0048" dnum="US5773601A"><text>5,773,601</text></patcit>; <patcit id="pcit0049" dnum="US5886165A"><text>5,886,165</text></patcit>; <patcit id="pcit0050" dnum="US5929A"><text>5,929</text></patcit>, <patcit id="pcit0051" dnum="US226A"><text>226</text></patcit>; <patcit id="pcit0052" dnum="US5977296A"><text>5,977,296</text></patcit>; <patcit id="pcit0053" dnum="US6140482A"><text>6,140,482</text></patcit>; and in WIPO publications <patcit id="pcit0054" dnum="WO0056746A"><text>WO 00/56746</text></patcit> and <patcit id="pcit0055" dnum="WO0114398A"><text>WO 01/14398</text></patcit>. Methods for synthesizing oligonucleotides comprising such analogs or derivatives are disclosed, for example, in the patent publications cited above, and in <patcit id="pcit0056" dnum="US6455308B"><text>U. S. Patent Nos. 6,455,308</text></patcit>; <patcit id="pcit0057" dnum="US5614A"><text>5,614</text></patcit>, <patcit id="pcit0058" dnum="US622A"><text>622</text></patcit>; <patcit id="pcit0059" dnum="US5739314A"><text>5,739,314</text></patcit>; <patcit id="pcit0060" dnum="US5955599A"><text>5,955,599</text></patcit>; <patcit id="pcit0061" dnum="US5962674A"><text>5,962,674</text></patcit>; <patcit id="pcit0062" dnum="US6117992A"><text>6,117,992</text></patcit>; and in <patcit id="pcit0063" dnum="WO0075372A"><text>WO 00/75372</text></patcit>. Nucleic acids may be modified by chemical linkages, moieties, or conjugates that enhance activity, cellular distribution, or cellular uptake of nucleic acid, and examples of modifications in modified nucleic acids are in <patcit id="pcit0064" dnum="US6455308B"><text>U. S. Patent Nos. 6,455,308 (Freier</text></patcit>), <patcit id="pcit0065" dnum="US6455307B"><text>6,455,307 (McKay et al.</text></patcit>), <patcit id="pcit0066" dnum="US6451602B"><text>6,451,602 (Popoff et al.</text></patcit>), and <patcit id="pcit0067" dnum="US6451538B"><text>6,451,538 (Cowsert</text></patcit>).

0114In some embodiements, the acyl acceptor comprises a toxin. Any toxin may be selected, and often is selected for high cytotoxic activity. Proteins or peptides ligated to a toxin often are useful as therapeutics. For example, a protein or peptide antibody or receptor that specifically binds to a cancer cell when linked to a toxin such as ricin is useful for treating cancer in subjects. In certain embodiments, the toxin is selected from the group consisting of abrin, ricin A, pseudomonas exotoxin and diphtheria toxin.

0115In some embodiments, the acyl acceptor comprises a solid support. The solid support often is derivitized with multiple NH₂-CH₂-moieties, such as triglycine moieties. The solid support may be any described herein. In certain embodiments, the solid support is a glass slide, a glass bead, an agarose bead, a magnetic bead, a silicon wafer or a resin. In specific embodiments, a resin such as EAH Sepharose is derivitized with triglycine moieties using a FMOC/EDC derivitization procedure. In certain embodiments the solid support is a particle, e.g., a microparticle (e.g. a particle having an average diameter or smallest cross-sectional length greater than about 1 micron) or nanoparticle (average diameter or smallest cross-sectional length equal to or less than about 1 micron), liposome, etc.

0116In some embodiments, the acyl acceptor comprises a phage that expresses a NH₂-CH₂-moiety on the surface. In some embodiments, the phage expresses a protein or peptide comprising one or more N- terminal glycines, sometimes three or five glycines at the N-terminus, and the phage expressing such a protein is contacted with a protein or peptide containing a transamidase recognition motif and a transamidase, thereby producing a phage/protein or phage/peptide conjugate. In some embodiments, a protein or peptide expressed at the phage surface comprises the transamidase recognition motif, and the phage is contacted with an acyl acceptor comprising a NH₂-CH₂-moiety and a transamidase, thereby producing a conjugate between the phage and the acyl acceptor. Methods for displaying a wide variety of peptides or proteins at the surface of a phage are known. The protein or peptide often is expressed as a fusion with a bacteriophage coat protein (<nplcit id="ncit0037" npl-type="s"><text>Scott &amp; Smith, Science 249: 386-390 (1990</text></nplcit>); <nplcit id="ncit0038" npl-type="s"><text>Devlin, Science 249: 404-406 (1990</text></nplcit>); <nplcit id="ncit0039" npl-type="s"><text>Cwirla et al., Proc. Natl. Acad. Sci. 87: 6378-6382 (1990</text></nplcit>); <nplcit id="ncit0040" npl-type="s"><text>Felici, J. Mol. Biol. 222: 301-310 (1991</text></nplcit>)). Methods also are available for linking the test polypeptide to the N-terminus or the C-terminus of the phage coat protein. The original phage display system was disclosed, for example, in <patcit id="pcit0068" dnum="US5096815A"><text>U. S. Patent Nos. 5,096,815</text></patcit> and <patcit id="pcit0069" dnum="US5198346A"><text>5,198,346</text></patcit>. This system used the filamentous phage M13, which required that the cloned protein be generated in E. coli and required translocation of the cloned protein across the E. coli inner membrane. Lytic bacteriophage vectors, such as lambda, T4 and T7 are more practical since they are independent of E. coli secretion. T7 is commercially available and described in <patcit id="pcit0070" dnum="US5223409A"><text>U. S. Patent Nos. 5,223,409</text></patcit>; <patcit id="pcit0071" dnum="US5403484A"><text>5,403,484</text></patcit>; <patcit id="pcit0072" dnum="US5571698A"><text>5,571,698</text></patcit> ; and <patcit id="pcit0073" dnum="US5766905A"><text>5,766,905</text></patcit>. A protein or peptide comprising a NH₂-CH₂-moiety or a transamidase recognition sequence can be expressed on the surface of any other phage or virus, including but not limited to, murine leukemia virus (MLV), mouse mammary tumor virus (MMTV), Rous sarcoma virus (RSV), Fujinami sarcoma virus (FuSV), Moloney murine leukemia virus (Mo-MLV), FBR murine osteosarcoma virus (FBR MSV), Moloney murine sarcoma virus (Mo-MSV), Abelson murine leukemia virus (A-MLV), Avian myelocytomatosis virus- 29 (MC29), and Avian erythroblastosis virus (AEV), human immunodeficiency virus (HIV), simian immunodeficiency virus (SIV), visna/maedi virus (VMV), caprine arthritis-encephalitis virus (CAEV), equine infectious anemia virus (EIAV), feline immunodeficiency virus (FIV), bovine immunodeficiency virus (BIV), a hepatitis virus (<i>e.g.</i>, hepatitis B or C), rhinovirus, herpes-zoster virus (VZV), herpes simplex virus (<i>e.g.</i>, HSV-1 or HSV-2), cytomegalovirus (CMV), vaccinia virus, influenza virus, encephalitis virus, hantavirus, arbovirus, West Nile virus, human papilloma virus (HPV), Epstein-Barr virus, and respiratory syncytial virus.

0117In some embodiments, the acyl donor or acyl acceptor comprises a cleavable moiety, e.g., A₁ or B₁ comprises a cleavable moiety. In some embodiments the acyl donor or acyl acceptor comprises a cleavage site for a protease (e.g., a polypeptide sequence that is recognized by a protease and cleaved). In such embodiments, a cleavage reagent, e.g., a protease, may be used to cleave the product of a transamidation reaction. The cleavable moiety may be positioned at any desired location, e.g., between a sortase recognition sequence and a moiety of interest such as a tag, label, etc. A number of proteases that cleave polypeptides at particular sequences are known in the art. Examples of sequences having peptidase activity are known (e. g., cysteine-type endopeptidases, aspartic-type endopeptidases, elastases, metalloendopeptidases, mitochondrial inner membrane peptidases, serine-type endopeptidases, threonine endopeptidases, thrombin and other proteases involved in the coagulation and/or clotting cascade or other biological cascades. In some embodiments the cleavable moiety is photocleavable. In some embodiments the cleavage site is located at a position 1,2, 3,4, 5,6, 7, 8,9 or 10 amino acids away from an end of the recognition sequence, and sometimes at a position 15 or fewer, 20 or fewer, 50 or fewer or 100 or fewer amino acids away from the an end of the recognition sequence or, in some embodiments, from an N- or C-terminal end of a polypeptide comprising a recognition sequence or that acts as a nuclephile in a method described herein. In some embodiments, the acyl donor or acyl acceptor comprises a linker sequence, also referred to as a "spacer". A linker sequencemay be, e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 25 or fewer, 30 or fewer, 40 or fewer, 50 or fewer, 60 or fewer, 70 or fewer, 80 or fewer, 90 or fewer or 100 or fewer amino acids in length. The linker may be positioned at any desired location, e.g., between a sortase recognition sequence and a moiety of interest such as a tag, label, etc.

0118Also disclosed herein are kits comprising one or more acyl donor compounds as described above. The kit may comprise a compound of formula: <chemistry id="chem0055" num="0055"><img file="EP2391714B2_D0055.tif" /></chemistry> wherein the transamidase recognition sequence, X, R¹, and A¹ are as defined above. The kit may comprise a compound of formula: <chemistry id="chem0056" num="0056"><img file="EP2391714B2_D0056.tif" /></chemistry> wherein X, R¹, R², and A¹ are as defined above. The kit may comprise a compound comprising a masked transamidase recognition sequence, e.g., any of the inventive probes described herein. The kit may further comprise a transamidase. The kit may further comprise a sortase. The kit may further comprise sortase A, e.g., SrtA_staph, SrtA_strep, or both. The kit may comprise an enzyme or other reagent for unmasking a masked transamidase recognition sequence. The kit may further comprise instructions for ligation with a nucleophilic compound.

0119The kit may provide a nucleophilic compound of formula: <chemistry id="chem0057" num="0057"><img file="EP2391714B2_D0057.tif" /></chemistry> wherein B¹ and n are as defined above.

0120Other techniques, ligands, antibodies, and enzymes are known in the art and may be used in accordance with the present disclosure, including those described by <nplcit id="ncit0041" npl-type="b"><text>Hornbeck, P., Curr Protoc Immunol., Enzyme-Linked Immunosorbent Assays, 2001 May; Chapter 2, Unit 2.1</text></nplcit>; <nplcit id="ncit0042" npl-type="b"><text>Ausubel et al. Current Protocols in Molecular Biology (John Wiley &amp; Sons, Inc., New York, 1999</text></nplcit>); <nplcit id="ncit0043" npl-type="b"><text>Molecular Cloning: A Laboratory Manual, 2nd Ed., ed. by Sambrook, Fritsch, and Maniatis (Cold Spring Harbor Laboratory Press: 1989</text></nplcit>); <nplcit id="ncit0044" npl-type="b"><text>Antibodies: A Laboratory Manual, E. Harlow and D. Lane, ed., Cold Spring Harbor Laboratory (Cold Spring Harbor, NY, 1988</text></nplcit>).

Transamidase-Mediated Protein Circularization

0121Disclosed herein is the recognition that transamidases can be used to circularize polypeptides with high efficiency. Disclosed herein is a method of circularizing a polypeptide comprising contacting the polypeptide with a transamidase, e.g., sortase, wherein the polypeptide comprises one or more nucleophilic residue(s) at its N-terminus and a transamidase recognition sequence (e.g., LPXTG) at or near its C-terminus, under suitable conditions such that circularization occurs. The nucleophilic residue at the N-terminus is one that can be used by the transamidase and the transamidase recognition sequence is one that can be recognized by the transamidase. For example, if the transamidase is S. aureus SrtA, the N-terminus can comprise one or more G residues (e.g., 1, 2, 3, 4, 5, etc.), and the transamidase recognition sequence can comprise LPXTG. The resulting circular polypeptide comprises a transamidase recognition sequence positioned between the original C- and N-termini of the polypeptide. Remarkably, it has been discovered that circularization can proceed with high efficiency, thus making the transamidase-mediated circularization reaction a feasible means of producing circular proteins with yield and purity compatible with the needs of large-scale and/or commercially feasible production. At least 30%, 40%, 50%, 60%, 70%, 80%, 90%, or more of the starting polypeptide may be converted to circular form.

0122Disclosed herein is a circular variant of a polypeptide of interest, wherein the original N- and C-terminal amino acids of the polypeptide of interest are joined together via a linker oligopeptide that comprises or consists of a transamidase recognition sequence. For example, the circular polypeptide may comprise the sequence Xaa_C-(Xaa)_k- <img file="EP2391714B2_D0058.tif" /><img file="EP2391714B2_D0059.tif" /> -(Xaa)_m -Xaa_N, where"Xaa_C" and "Xaa_N" may represent the amino acids present at the C- and N-termini of the original linear polypeptide, "Xaa" may be any amino acid (wherein each Xaa may be a natural or unnatural amino acid and may optionally comprises a modified or substituted side chain, e.g., as defined elsewhere herein), and k and m may independently be integers between 0 and 15. k and m may each independently be between 0 and 5 (e.g., 0, 1, or 2), or between 0 and 10. k+m may be between 0 and 5, between 0 and 10, or between 0 and 20. The transamidase recognition sequence may be LPXTG, e.g., LPETG. The circular polypeptide may comprise Xaa_C-LPXTG-Xaa_N; Xaa_C-LPXTGG-Xaa_N; Xaa_C-GLPXTGG-Xaa_N; Xaa_C-GGLPXTGG-Xaa_N. X may be E.

0123Also disclosed herein is means for purifying a circularized polypeptide away from a starting linear polypeptide. The method may comprise (i) providing a polypeptide of interest that comprises one or more nucleophilic residue(s) at its N-terminus and a sequence comprising a transamidase recognition sequence (e.g., LPXTG) and a peptide that serves as an affinity tag at or near its C-terminus; and (ii) contacting the polypeptide of interest with a transamidase under conditions suitable for transamidation reaction to occur. For example, the sequence at or near the C-terminus of the polypeptide of interest can comprise His6, e.g., LPXTGG-His6. The transamidase may also comprisesa tag, which may be the same or different from the tag present in the polypeptide of interest. In accordance with the method disclosed herein, transamidase-mediated circularization removes a portion of the C-terminal sequence comprising the tag. The reaction mixture can then be contacted with a moiety that binds to the tag(s). The circular polypeptide will not be bound and can be conveniently separated from the starting linear polypeptide and/or transamidase. For example, the reaction mixture can be contacted with a resin that binds to the tag (e.g., by passing the material through a column containing the resin). The circular polypeptide does not bind to the resin, flows through the column, and can thus be easily purified.

0124Disclsoed herein are purified preparations of a circularized polypeptide as described above, wherein the circularized polypeptide is at least 50%, 60%, 70%, 80%, 90%, 95%, 96%, 97%, 98%, 99%, or more of the polypeptide material in the preparation by dry weight. The circularized polypeptide may be at least 50%, 60%, 70%, 80%, 90%, 95%, 96%, 97%, 98%, 99%, or more of the material in the preparation by dry weight (optionally excluding counterions that may be present). A polypeptide preparation amy comprise at least 10 mg, at least 100 mg, or at least 1 g of polypeptide by dry weight.

0125A circularized polypeptide disclosed herein may display at least one enhanced property relative to a non-circular form. The polypeptide may display increased stability, e.g., increased resistance to exoprotease(s), increased thermal stability, increased circulating time in vivo, increased biological half-life, less propensity to aggregate, and/or increased rate of refolding.

Dual Modification Using Different Sortases

0126Disclosed herein are methods for modifying a polypeptide at both the N- and C-termini. The disclosed methods may make use of two transamidases, e.g., sortases, that recognize distinct transamidase recognition sequences. A first transamidase may be able to recognize a first TRS but is not able to significantly recognize a second TRS, while the second transamidase is able to recognize the second TRS but is not able to significantly recognize the first TRS. The transamidase reactions may be carried out sequentially (in either order) or simultaneously. The disclosed methods may make use of two transamidases, e.g., sortases, wherein the first transamidase is able to recognize a first TRS but is not able to recognize a second TRS, and the second transmidase is able to recognize both the first TRS and a second TRS. For example, Streptococcus pyogenes Sortase A (SrtAstrep) will accept di-alanine based nucleophiles. This sortase will efficiently cleave LPXTA between theronine and alanine and install modified alanine-based nucleophiles. SrtAstrep will also recognize and cleave LPXTG motifs, albeit with reduced efficiency. SrtAstaph will not significantly cleave LPXTA motifs or accept alanine based nucleophiles. A polypeptide may be contacted with SrtAstrep and an alanine-containing nucleophile comprising the desired modifying moiety. The polypeptide comprises a TRS that can recognized by SrtA strep at or near its C-terminus and one or more amino acids capable of serving as a nucleophile for a SrtAstaph-mediated reaction at or near its N-terminus (e.g., (G)n, where n is between 1 and 10, e.g., between 1 and 5). This leads to the formation of an LPXTA sequence at the site of ligation, a motif refractory to cleavage by SrtA_staph. This allows SrtA_staph to act on the N terminus without affecting the C-terminal modification installed with SrtA_strep. The nucleophilic amino acid(s) may be masked, so that they are not recognized by SrtA_strep. For example, a sequence that can be cleaved by a protease can be positioned so that cleavage exposes the nucleophilic amino acid(s). In the absence of such cleavage, the nucleophilic residue cannot participate in a sortase-mediated reaction. A sequence that contains two amino acids in suitable context for cleavage by a protease is referred to as a "protease recognition site" herein. The protease recognition sequence serves to mask the nucleophilic amino acid. Additional amino acids may be present N-terminal of the protease recognition site. The protease recognition site may be a thrombin recognition site. The thrombin recognition site may comprise the sequence LVPRG, LVPRGG or LVPRGS, wherein thrombin cleaves between R and G. PreScission Protease, a genetically engineered fusion protein consisting of human rhinovirus 3C protease and GST, may be used. LEVLFQ/GP is the standard recognition site. Alternative sites can be cleaved. LE-P4-LFQ/GP (where P₄=Val Ala or Thr) can be used. Other proteases that cleave between a first amino acid and a second amino acid, wherein the second amino acid can serve as a nucleophile for a transamidase, can be used. One of skill in the art will be able to select an appropriate protease to cleave C-terminal to a nucleophilic amino acid of interest. The protease and/or cleavage conditions can be selected such that minimal or no undesired cleavage within the polypeptide occurs. The cleaving conditions and agent (e.g., protease) may be selected consistent with maintaining stability of the engineered protein except with respect to the desired cleavage. After cleavage, the protease may be removed or the protein isolated from the reaction mixture in which cleavage was performed. The protease can comprise a tag for convenient removal after cleavage. The protease may be immobilized, e.g., on a solid or semi-solid support, e.g., particles. Following a transamidase-mediated ligation at the C-terminus, the polypeptide is contacted with the protease to expose the N-terminal nucleophilic amino acid. The polypeptide is contacted with a second transamidase and an appropriate probe for N-terminal modification. An exemplary strategy for dual-terminus labeling is outlined in <figref idref="f0041">Figures 33b</figref> and <figref idref="f0043">34</figref>. Also disclosed herein is the use of chemical agents for cleavage to expose a nucleophilic amino acid. Polypeptides comprising a masked nucleophilic amino acid usable by a transamidase and appropriate cleavable masking sequence (often positioned near the N-terminus of the polypeptide) and a transamidase recognition sequence at or near the C-terminus are also disclosed and have a variety of uses.

Site-Specific Modification and Circularization of Polypeptides

0127Disclosed herein are methods for both circularizing and site-specifically modifying a polypeptide. Modification may be performed using an oligonucleotide probe comprising a TRS, which may be a masked TRS. The TRS comprises a side chain comprising a modifying moiety of interest. <figref idref="f0053">Figure 43</figref> shows a schematic diagram of an approach according to this aspect of the disclosure. A polypeptide is equipped with a nucleophilic residue at or near the N-terminus and a first TRS at or near its C-terminus. The nucleophilic residue may be masked, e.g., with a protease recognition sequence that, when cleaved, exposes a nucleophilic residue. The polypeptide is contacted with a transamidase and an inventive oligonucleotide probe comprising a second TRS (which may be masked and/or may be different in sequence to the first TRS) and an amino acid having a modifying moiety (e.g., a label) attached thereto. The transamidase appends the oligonucleotide probe at the C-terminus of the polypeptide. Where the second TRS (in the probe) is a masked TRS, the polypeptide may be contacted with an agent or condition (phosphatase, light, etc.) that unmasks the TRS. Where the nucleophilic residue at the N terminus is masked, the polypeptide may be also contacted with an agent that unmasks it (e.g., a protease). The polypeptide is contacted with a second transamidase, which catalyzes a second transamidase reaction, which circularizes the polypeptide. The sequence of the polypeptide between the original N- and C-terminal amino acids can be unchanged relative to the original sequence. The N- and C-termini of the polypeptide are joined in the circularized version by a short linker peptide that comprises at least one TRS. The linker peptide may comprise two TRSs. It should be noted that the probe depicted in <figref idref="f0053">Fig. 43</figref> is exemplary. The G residues in the GKG sequence may be omitted, or other amino acids are used. A probe may comprise two or more lysine residues (or other residues having a readily modifiable side chain), wherein at least two moieties are attached to the side chains. The probe may comprise two or more different modifying moieties, which are optionally attached to different amino acids.

0128A polypeptide circularized using a disclosed probe mat comprise the following sequence: ---lpetAAGKGLPETgg---- wherein lpet in lower case is from the C-terminus of the polypeptide. The AAGKGLPET is from the probe. The gg in lower case is from the N-terminus of the polypeptide. It will be understood that the lpet and gg are often not present in an original naturally occurring polypeptide but are present in a modified polypeptide for purposes of serving as a TRS and nucleophile, respectively, in transamidase-mediated reactions. Thus, a circularized polypeptide may comprise the sequence (Xaa)_C---TRS1-SP-TRS2---(Xaa)_N, wherein TRS1 is a first TRS, TRS2 is a second TRS, SP is a spacer peptide comprising sequences from the probe (e.g.,GKG), and each --- represents one or more optionally present amino acid residues. A circularized version of an original (e.g., naturally occurring) polypeptide may comprise the sequence (Xaa)_ClpetAAGKGLPETgg(Xaa)_N, wherein (Xaa)_C is from the C-terminus of the original polypeptide, and (Xaa)_N is from the N-terminus of the original polypeptide. A moiety, e.g., a PEG, may be attached to the lysine side chain. One or more of the G residues in the probe (and thus the circularized polypeptide) may be omitted.

Joining Two or More Polypeptides or Polypeptide Domains

0129Disclosed herein are methods of covalently linking two or more polypeptides using a disclosed olignucleotide probe, which may comprise a TRS, e.g., a masked TRS. <figref idref="f0055">Figure 45</figref> is a schematic diagram showing certain embodiments of the approach. The probe may comprise a moiety of interest such as PEG, a detectable label, etc. At least one of the polypeptides may comprise a targeting or delivery domain. A delivery domain facilitates delivery of a compound, e.g,. a polypeptide, to a site of interest, e.g., a site in the body. A targeting domain (which may function as a delivery domain) specifically binds to a molecule of interest, such as a receptor. For example, a targeting domain can be an an antibody, antibody chain, antibody fragment, e.g., a single-chain antibody or an antigen-binding portion thereof. One of the polypeptides may comprise an Fc domain of an antibody, e.g., an IgG1 antibody. An Fc domain may be dimeric. Without limitation, an Fc domain can confer one or more desired properties. For example, an Fc domain can prolong the half-life of a polypeptide. An Fc domain can function as a targeting/delivery domain. For example, the neonatal constant region fragment (Fc) receptor (FcRn), which is responsible for IgG transport across the intestinal epithelium in newborn rodents, is expressed in epithelial cells in adult humans and non-human primates, and FcRn-mediated transport is functional in the lung of non-human primates. This transport system can be used to deliver a polypeptide of interest when it is conjugated to the Fc domain of IgG1. See, e.g., <nplcit id="ncit0045" npl-type="s"><text>Bitonti AJ, et al., "Pulmonary delivery of an erythropoietin Fc fusion protein in non-human primates through an immunoglobulin transport pathway. Proc Natl Acad Sci U S A 101(26):9763-8, 2004</text></nplcit>. See also <patcit id="pcit0074" dnum="US20070178112A"><text>U.S. Pat. Pub. No. 2007/0178112</text></patcit>. One of the polypeptides may comprise albumin, e.g., human serum albumin. A targeting or delivery domain may comprise a ligand for a receptor, wherein the receptor is expressed at the surface of a target cell.

Selected Moieties of Interest

0130Without limiting the disclosure in any way, a moiety may comprise a biocompatible polymer, e.g., a polyalkylene glycol, e.g., a polyethylene glycol (PEG), a polypropylene glycol, or a derivative thereof. In some aspects, the disclosed method offers a number of advantages relative to various other available methods for modifying a polypeptide with a polymer, e.g., other PEGyation methods. PEG is discussed herein for exemplary purposes. Without limitation, the methods permit installation of a single PEG chain (or a small number of chains, e.g., 2, 3, 4) at a defined location and avoid PEGylation of residues within the native polypeptide sequence, which could reduce activity, e.g., potency. Methods disclosed herein provide means to modify a polypeptide at a defined position without introducing a cysteine residue into the polypeptide or appending a cysteine residue thereto. PEG with an average molecular weight of about 5 kD may be used. A PEG with an average molecular weight of 10 kD may be used. PEG with an average molecular weight of about 15 kD, 20 kD, 25 kD, 30kD, 40 kD, 45 kD, 50kD, 55 kD, or more may be used. A linear PEG may be used. A branched PEG, forked PEG, or comb structure PEG may be used. A polypeptide PEGylated using method disclosed herein may display increased circulating time without causing a significant reduction in potency. In contrast, standard, random PEGylation approaches often result in considerable reduction in potency (Jev<img file="EP2391714B2_D0060.tif" />evar, S, et al., <nplcit id="ncit0046" npl-type="s"><text>PEGylation of Therapeutic Proteins, Biotechnol. J. 5, 113-128, 2010</text></nplcit> discusses certain PEGylated pharmaceuticals, PEG reagents, etc).

Selected Polypeptides of Interest

0131Without limiting the disclosure in any way, this section discusses certain polypeptides of interest. Disclosed herein is the recognition that certain polypeptides are particularly amenable to the circularization approaches described herein. A polypeptide to be circularized is one whose N- and C-terminus are located within close proximity to each other in the native structure. For example, the termini may be located no more than 15 Angstroms (Å) apart. The termini may be located no more than 20 Å apart. The termini may be located no more than 25 Å apart. The termini may be located more than, e.g., about 15Å apart in the native structure, the polypeptide is modified to extend either the N- or C-terminus, or both, with a flexible spacer peptide so that the termini can be in closer proximity to each otherA polypeptide to be circularized and, optionally, labeled as described herein (e.g., PEGylated) may be characterized in that its N- and C-termini are positioned such that they are not required for the activity of the polypeptide and/or for interaction of the polypeptide with a biological target of the polypeptide. For example, if the polyptide is one that naturally interacts with a receptor, the N- and C-termini are positioned such that they are not required for such interaction, e.g., they are positioned away from the receptor binding domain. Tthe polypeptide of interest may comprise or consist of a polypeptide that is at least 80%, or at least 90%, e.g., at least 95%, 86%, 97%, 98%, 99%, 99.5%, or 100% identical to a naturally occurring polypeptide. The polypeptide may have no more than 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 amino acid differences relative to a naturally occurring sequence. Tthe naturally occurring polypeptide may be a mammalian polypeptide, e.g., of human origin. The naturally occurring polypeptide may be a cytokine, e.g., a type I cytokine. The polypeptide of interest may be a four-helix bundle protein, e.g., a four-helix bundle cytokine. Exemplary four-helix bundle cytokines include, e.g., certain interferons (e.g., a type I intereferon, e.g., IFN-α), interleukins (e.g., IL-2, IL -3, IL-4, IL-5, IL-6, IL-7, IL-12), and colony stimulating factors (e.g., G-CSF, GM-CSF, M-CSF). The IFN can be, e.g., interferon alpha 2a or interferon alpha 2b. See, e.g., <nplcit id="ncit0047" npl-type="s"><text>Mott HR and Campbell ID. "Four-helix bundle growth factors and their receptors: protein-protein interactions." Curr Opin Struct Biol. 1995 Feb;5(1):114-21</text></nplcit>; <nplcit id="ncit0048" npl-type="s"><text>Chaiken IM, Williams WV. "Identifying structure-function relationships in four-helix bundle cytokines: towards de novo mimetics design." Trends Biotechnol. 1996 Oct;14(10):369-75</text></nplcit>; <nplcit id="ncit0049" npl-type="s"><text>Klaus W, et al., "The three-dimensional high resolution structure of human interferon alpha-2a determined by heteronuclear NMR spectroscopy in solution". J Mol Biol., 274(4):661-75, 1997</text></nplcit>, for further discussion of certain of these cytokines.

0132The cytokine may have a similar structure to one or more of the afore-mentioned cytokines. For example, the cytokine can be an IL-6 class cytokine such as leukemia inhibitory factor (LIF) or oncostatin M. The cytokine may be one that in nature binds to a receptor that comprises a GP130 signal transducing subunit. Other four-helix bundle proteins of interest include growth hormone (GH), prolactin (PRL), and placental lactogen. The polypeptide of interest may be an erythropoiesis stimulating agent, e.g., erythropoietin (EPO), which is also a four-helix bundle cytokine. An erythropoiesis stimulating agent may be an EPO variant, e.g., darbepoetin alfa, also termed novel erythropoiesis stimulating protein (NESP), which is engineered to contain five N-linked carbohydrate chains (two more than recombinant HuEPO). The polypeptide may comprise five helices. For example, the polypeptide can be an interferon beta, e.g., interferon beta-la or interferon beta-lb, which (as will be appreciated) is often classified as a four-helix bundle cytokine.

0133A polypeptide of interest may be IL-9, IL-10, IL-11, IL-13, or IL-15. For example, See, e.g., <nplcit id="ncit0050" npl-type="s"><text>Hunter, CA, Nature Reviews Immunology 5, 521-531, 2005</text></nplcit>, for discussion of certain cytokines. See also <nplcit id="ncit0051" npl-type="b"><text>Paul, WE (ed.), Fundamental Immunology, Lippincott Williams &amp; Wilkins; 6th ed., 2008</text></nplcit>.

0134A polypeptide may be one that is approved by the US Food &amp; Drug Administration (or an equivalent regulatory authority such as the European Medicines Evaluation Agency) for use in treating a disease or disorder in humans. Such polypeptide may or may not be one for which a PEGylated version has been tested in clinical trials and/or has been approved for marketing.

0135A polypeptide of interest may be a neurotrophic factor, i.e., a factor that promotes survival, development and/or function of neural lineage cells (which term as used herein includes neural progenitor cells, neurons, and glial cells, e.g., astrocytes, oligodendrocytes, microglia). For example, the factor could promote neurite outgrowth. The polypeptide may be ciliary neurotrophic factor (CNTF; a four-helix bundle protein) or an analog thereof such as Axokine, which is a modified version of human Ciliary neurotrophic factor with a 15 amino acid truncation of the C terminus and two amino acid substitutions, which is three to five times more potent than CNTF in in vitro and in vivo assays and has improved stability properties.

0136The polypeptide of interest may be one that forms homodimers or heterodimers, (or homo- or heterooligomers comprising more than two subunits, such as tetramers). The homodimer, heterodimer, or oligomer structure may be such that a terminus of a first subunit is in close proximity to a terminus of a second subunit. For example, an N-terminus of a first subunit is in close proximity to a C-terminus of a second subunit. The homodimer, heterodimer, or oligomer structure may be such that a terminus of a first subunit and a terminus of a second subunit are not involved in interaction with a receptor, so that the termini can be joined via a non-genetically encoded peptide element without significantly affecting biological activity. An inventive ligation method is used to join termini of two subunits of a homodimer, heterodimer, or oligomer via a non-genetically encoded peptide element, which optionally comprises a moiety attached to a side chain thereof, thereby producing a dimer (or oligomer) in which at least two subunits are covalently joined. For example, the neurotrophins nerve growth factor (NGF); brain-derived neurotrophic factor (BDNF); neurotrophin 3 (NT3); and neurotrophin 4 (NT4) are dimeric molecules which share approximately 50% sequence identity and exist in dimeric forms (<nplcit id="ncit0052" npl-type="s"><text>Robinson RC, et al., "Structure of the brain-derived neurotrophic factor/neurotrophin 3 heterodimer.", Biochemistry. 34(13):4139-46, 1995</text></nplcit>; <nplcit id="ncit0053" npl-type="s"><text>Robinson RC, et al., "The structures of the neurotrophin 4 homodimer and the brain-derived neurotrophic factor/neurotrophin 4 heterodimer reveal a common Trk-binding site." Protein Sci. 8(12):2589-97, 1999</text></nplcit>). The dimeric polypeptide may be a cytokine, e.g., an interleukin.Other polypeptides of interest are enzymes, e.g., enzymes that are important in metabolism or other physiological processes. As known in the art, deficiencies of enzymes or other proteins can lead to a variety of disease. Such diseases include diseases associated with defects in carbohydrate metabolism, amino acid metabolism, organic acid metabolism, porphyrin metabolism, purine or pyrimidine metabolism, lysosomal storage disorders, blood clotting, etc. Examples include Fabry disease, Gaucher disease, Pompe disease, adenosine deaminase deficiency, asparaginase deficiency, porphyria, hemophilia, and hereditary angioedema. A polypeptide may be a clotting or coagulation factor,(e.g., factor VII, VIIa, VIII or IX). In A polypeptide is an enzyme that plays a role in carbohydrate metabolism, amino acid metabolism, organic acid metabolism, porphyrin metabolism, purine or pyrimidine metabolism, and/or lysosomal storage, wherein exogenous administration of the enzyme at least in part alleviates the disease.

0137A polypeptide may comprise a receptor or receptor fragment (e.g., extracellular domain). A receptor may be a TNFα receptor.

0138A polypeptide may comprise urate oxidase.

0139One of skill in the art will be aware of the sequences of polypeptides. Without limitation, sequences of certain polypeptides of interest are found in, e.g.,<patcit id="pcit0075" dnum="US20050107591A"><text>US 2005/0107591</text></patcit>;<patcit id="pcit0076" dnum="US20070020235A"><text>US 2007/0020235</text></patcit>;<patcit id="pcit0077" dnum="US20070224665A"><text>US 2007/0224665</text></patcit>;<patcit id="pcit0078" dnum="US20060194735A"><text>US 2006/0194735</text></patcit>;<patcit id="pcit0079" dnum="US20060198824A"><text>US 2006/0198824</text></patcit>. A polypeptide of interest may be listed in Table B. Disclosed herein is the application of the inventive techniques to any of the polypeptides described. Disclosed herein are modified versions of any of these polypeptides, wherein a modified version comprises (i) one or more nucleophilic residues such as glycine at the N-terminus (e.g., between 1 and 10 residues) and, optionally, a cleavage recognition sequence, e.g., a protease cleavage recognition sequence that masks the nucleophilic residue(s); (ii) a TRS at or near the C-terminus; (iii) both (i) and (ii). Such modified polypeptides are useful, e.g., as substrates for transamidase-mediated ligation as described herein.

0140One of skill in the art will be aware that certain polypeptides, e.g., secreted eukaryotic (e.g., mammalian) polypeptides, often undergo intracellular processing (e.g., cleavage of a secretion signal prior to secretion and/or removal of other portion(s) that are not required for biological activity), to generate a mature form. <tables id="tabl0002" num="0002"><table frame="all"><title>Table B: Protein Sequences</title><tgroup cols="2"><colspec colnum="1" colname="col1" colwidth="53mm" /><colspec colnum="2" colname="col2" colwidth="113mm" /><tbody><row rowsep="0"><entry morerows="2" rowsep="1" valign="bottom">Tissue plasminogen activator (1rtf)</entry><entry>Chain A: TTCCGLRQY (SEQ ID NO: 5)</entry></row><row rowsep="0"><entry>Chain B:</entry></row><row><entry><img file="EP2391714B2_D0061.tif" /></entry></row><row rowsep="0"><entry morerows="3" rowsep="1" valign="bottom">Factor IX</entry><entry>Chain A:</entry></row><row rowsep="0"><entry><img file="EP2391714B2_D0062.tif" /></entry></row><row rowsep="0"><entry>Chain B:</entry></row><row><entry><img file="EP2391714B2_D0063.tif" /></entry></row><row><entry valign="bottom">glucocerebrosidase</entry><entry><img file="EP2391714B2_D0064.tif" /></entry></row><row><entry valign="bottom">alpha galactosidase A</entry><entry><img file="EP2391714B2_D0065.tif" /></entry></row><row><entry valign="bottom">arylsulfatase-A (iduronidase, α-L-)</entry><entry><img file="EP2391714B2_D0066.tif" /></entry></row><row rowsep="0"><entry valign="bottom">arylsulfatase B (N-acetylgalactos-amine-4-sulfatase) (1fsu)</entry><entry><img file="EP2391714B2_D0067.tif" /></entry></row><row><entry valign="bottom" /><entry><img file="EP2391714B2_D0068.tif" /></entry></row><row><entry valign="bottom">beta-hexosaminidase A (2gjx)</entry><entry><img file="EP2391714B2_D0069.tif" /></entry></row><row rowsep="0"><entry morerows="6" valign="bottom">Hexosaminidase A and B (2gjx)</entry><entry>CHAIN A:</entry></row><row rowsep="0"><entry><img file="EP2391714B2_D0070.tif" /></entry></row><row rowsep="0"><entry>Chain B:</entry></row><row rowsep="0"><entry><img file="EP2391714B2_D0071.tif" /></entry></row><row rowsep="0"><entry>Chain C:</entry></row><row rowsep="0"><entry><img file="EP2391714B2_D0072.tif" /></entry></row><row rowsep="0"><entry>Chain D:</entry></row><row><entry valign="bottom" /><entry><img file="EP2391714B2_D0073.tif" /></entry></row><row><entry valign="bottom">phenylalanine hydroxylase (PAH) (1j8u)</entry><entry><img file="EP2391714B2_D0074.tif" /></entry></row><row><entry valign="bottom">Cathepsin A</entry><entry><img file="EP2391714B2_D0075.tif" /></entry></row><row><entry valign="bottom">G-CSF</entry><entry><img file="EP2391714B2_D0076.tif" /></entry></row><row><entry valign="bottom">GM-CSF</entry><entry><img file="EP2391714B2_D0077.tif" /></entry></row><row><entry valign="bottom">Interferon alfa-2</entry><entry><img file="EP2391714B2_D0078.tif" /></entry></row><row><entry valign="bottom">Interferon beta-1</entry><entry><img file="EP2391714B2_D0079.tif" /></entry></row><row><entry valign="bottom">Interferon gamma-1b</entry><entry><img file="EP2391714B2_D0080.tif" /></entry></row><row><entry valign="bottom">IL-2 (1M47)</entry><entry><img file="EP2391714B2_D0081.tif" /></entry></row><row><entry valign="bottom">IL-1 (2nvh)</entry><entry><img file="EP2391714B2_D0082.tif" /></entry></row><row rowsep="0"><entry valign="bottom">TNF-alpha (4tsv)</entry><entry><img file="EP2391714B2_D0083.tif" /></entry></row><row><entry valign="bottom" /><entry><img file="EP2391714B2_D0084.tif" /></entry></row><row><entry valign="bottom">TNF-beta (lymphotoxin) (1tnr)</entry><entry><img file="EP2391714B2_D0085.tif" /></entry></row><row><entry valign="bottom">Erythropoietin</entry><entry><img file="EP2391714B2_D0086.tif" /></entry></row><row rowsep="0"><entry morerows="1" rowsep="1" valign="bottom">Insulin</entry><entry>Chain A: GIVEQCCTSICSLYQLENYCN (SEQ ID NO: 29)</entry></row><row><entry>Chain B: FVNQHLCGSHLVEALYLVCGERGFFYTPK (SEQ ID NO: 30)</entry></row><row><entry valign="bottom">Growth hormone (GH) (Somatotropin) (1huw)</entry><entry><img file="EP2391714B2_D0087.tif" /></entry></row><row><entry valign="bottom">Follicle-stimulating hormone (FSH)</entry><entry><img file="EP2391714B2_D0088.tif" /></entry></row><row><entry valign="bottom">Leptin (1ax8)</entry><entry><img file="EP2391714B2_D0089.tif" /></entry></row><row><entry valign="bottom">Insulin-like growth factor (or somatomedin) (1wqj)</entry><entry><img file="EP2391714B2_D0090.tif" /></entry></row><row rowsep="0"><entry morerows="5" rowsep="1" valign="bottom">Adiponectin (1c28)</entry><entry>Chain A:</entry></row><row rowsep="0"><entry><img file="EP2391714B2_D0091.tif" /></entry></row><row rowsep="0"><entry>Chain B:</entry></row><row rowsep="0"><entry><img file="EP2391714B2_D0092.tif" /></entry></row><row rowsep="0"><entry>Chain C:</entry></row><row><entry><img file="EP2391714B2_D0093.tif" /></entry></row><row rowsep="0"><entry morerows="1" valign="bottom">Factor VIII (aka antihemophilic factor) (2r7e)</entry><entry>Chain A:</entry></row><row rowsep="0"><entry><img file="EP2391714B2_D0094.tif" /></entry></row><row><entry morerows="1" valign="bottom" /><entry rowsep="0">Chain B:</entry></row><row><entry><img file="EP2391714B2_D0095.tif" /></entry></row><row rowsep="0"><entry morerows="3" rowsep="1" valign="bottom">Human serum albumin (1ao6)</entry><entry>Chain A:</entry></row><row rowsep="0"><entry><img file="EP2391714B2_D0096.tif" /></entry></row><row rowsep="0"><entry>Chain B:</entry></row><row><entry><img file="EP2391714B2_D0097.tif" /></entry></row><row rowsep="0"><entry morerows="3" rowsep="1" valign="bottom">Hemoglobin (1bz0)</entry><entry>Chain A:</entry></row><row rowsep="0"><entry><img file="EP2391714B2_D0098.tif" /></entry></row><row rowsep="0"><entry>Chain B:</entry></row><row><entry><img file="EP2391714B2_D0099.tif" /></entry></row></tbody></tgroup></table></tables>

0141It will be appreciated that considerable structure/function information is available regarding many of the afore-mentioned polypeptides, as well as sequences from different mammalian species, that can be used to design variants of the naturally occurring sequence that retain significant biological activity (e.g., at least 25%, 75%, 90% or more of the activity of the naturally occurring polypeptide). For example, crystal structures or NMR structures of a number of these polypeptides, in some instances in a complex with the corresponding receptor, are available. In addition, it will be understood that, if the naturally occurring N- and C-termini are not located in close proximity to each other in the native structure, a naturally occurring sequence can be extended at the N- and/or C-termini, e.g., with a flexible peptide spacer so that the termini can come into close proximity.

Pharmaceutical Compositions

0142Disclosed herein are pharmaceutical compositions comprising any of the polypeptides described herein, i.e., a polypeptide that has been modified using an inventive ligation composition or method. The polypeptide may be circular. The polypeptide may be PEGylated in a site-specific manne. The polypeptide may be circular and PEGylated, e.g., the polypeptide is circularized via a linker peptide that comprises a PEG moiety attached to at least one side chain.

0143A pharmaceutical composition may comprise a variety of pharmaceutically acceptable carriers. Pharmaceutically acceptable carriers are well known in the art and include, for example, aqueous solutions such as water, 5% dextrose, or physiologically buffered saline or other solvents or vehicles such as glycols, glycerol, oils such as olive oil, or injectable organic esters that are suitable for administration to a human or non-human subject. See, e.g., <nplcit id="ncit0054" npl-type="b"><text>Remington: The Science and Practice of Pharmacy, 21st edition; Lippincott Williams &amp; Wilkins, 2005</text></nplcit>. A pharmaceutically acceptable carrier or composition may be sterile. A pharmaceutical composition can comprise, in addition to the active agent, physiologically acceptable compounds that act, for example, as bulking agents, fillers, solubilizers, stabilizers, osmotic agents, uptake enhancers, etc. Physiologically acceptable compounds include, for example, carbohydrates, such as glucose, sucrose, lactose; dextrans; polyols such as mannitol; antioxidants, such as ascorbic acid or glutathione; preservatives; chelating agents; buffers; or other stabilizers or excipients. The choice of a pharmaceutically acceptable carrier(s) and/or physiologically acceptable compound(s) can depend for example, on the nature of the active agent, e.g., solubility, compatibility (meaning that the substances can be present together in the composition without interacting in a manner that would substantially reduce the pharmaceutical efficacy of the pharmaceutical composition under ordinary use situations) and/or route of administration of the composition. The pharmaceutical composition could be in the form of a liquid, gel, lotion, tablet, capsule, ointment, cream, transdermal patch, etc. A pharmaceutical composition can be administered to a subject by various routes including, for example, parenteral administration. Exemplary routes of administration include intravenous administration; respiratory administration (e.g., by inhalation), intramuscular administration, nasal administration, intraperitoneal administration, oral administration, subcutaneous administration and topical administration. For oral administration, the compounds can be formulated with pharmaceutically acceptable carriers as tablets, pills, dragees, capsules, liquids, gels, syrups, slurries, suspensions, etc. A compound may be administered directly to a target tissue. Direct administration could be accomplished, e.g., by injection or by implanting a sustained release implant within the tissue. Of course a sustained release implant could be implanted at any suitable site. A sustained release implant may be particularly suitable for prophylactic treatment of subjects at risk of developing a recurrent cancer. A sustained release implant may deliver therapeutic levels of the active agent for at least 30 days, e.g., at least 60 days, e.g., up to 3 months, 6 months, or more. One skilled in the art would select an effective dose and administration regimen taking into consideration factors such as the patient's weight and general health, the particular condition being treated, etc. Exemplary doses may be selected using in vitro studies, tested in animal models, and/or in human clinical trials as standard in the art.

0144The pharmaceutical composition may be delivered by means of a microparticle or nanoparticle or a liposome or other delivery vehicle or matrix. A number of biocompatible synthetic or naturally occurring polymeric materials are known in the art to be of use for drug delivery purposes. Examples include polylactide-co-glycolide, polycaprolactone, polyanhydride, cellulose derivatives, and copolymers or blends thereof. Liposomes, for example, which consist of phospholipids or other lipids, are nontoxic, physiologically acceptable and metabolizable carriers that are relatively simple to make and administer.

0145A compound, e.g., a disclosed polypeptide (e.g., a polypeptide produced at least in part using a disclosed ligation method) may be delivered in an effective amount, by which is meant an amount sufficient to achieve a biological response of interest, e.g., reducing one or more symptoms or manifestations of a disease or condition. The exact amount required will vary from subject to subject, depending on factors such as the species, age, weight, sex, and general condition of the subject, the severity of the disease or disorder, the particular compound and its activity, its mode of administration, concurrent therapies, and the like. A compound, e.g., a polypeptide, may be formulated in unit dosage unit form for ease of administration and uniformity of dosage, which term as used herein refers to a physically discrete unit of agent appropriate for the patient to be treated. It will be understood, however, that the total daily dosage will be decided by the attending physician within the scope of sound medical judgment, e.g., when administering a PEGylated polypeptide, information available regarding a suitable dose of the unPEGylated version, optionally in conjunction with in vitro activity data, can be used as a guideline in selecting an appropriate dose for preclinical testing and/or for clinical use.

0146The pharmaceutical compositions can be used to treat a wide variety of different diseases and disorders. A pharmaceutical composition may be used, e.g., to treat any disease or condition for which the unmodified polypeptide is of use. Thus the disclosure provides methods of treatment comprising administering a disclosed polypeptide to a subject in need thereof. The subject is typically a mammalian subject, e.g., a human. The subject may be a non-human animal that serves as a model for a disease or disorder that affects humans. The animal model may be used, e.g., in preclinical studies, e.g., to assess efficacy and/or determine a suitable dose.

0147A polypeptide may be administered prophylactically, e.g., to a subject who does not exhibit signs or symptoms of the disease or disorder (but may be at increased risk of developing the disorder or is expected to develop the disease or disorder). A polypeptide may be administered to a subject who has developed one or more signs or symptoms of the disease or sorder, e.g., the subject has been diagnose as having the disease or disorder. Optionally, the method comprises diagnosing the subject as having a disease or disorder for which the polypeptide is an appropriate treatment. For example, interferons have a variety of uses, e.g., in the treatment of autoimmune diseases (e.g., multiple sclerosis) and infectious diseases (e.g., viral infections such as those caused by viruses belonging to the Flaviviridae family, e.g., HBV, HCV; bacterial infections, fungal infections, parasites). Exemplary viruses include, but are not limited to, viruses of the Flaviviridae family, such as, for example, Hepatitis C Virus, Yellow Fever Virus, West Nile Virus, Japanese Encephalitis Virus, Dengue Virus, and Bovine Viral Diarrhea Virus; viruses of the Hepadnaviridae family, such as, for example, Hepatitis B Virus; viruses of the Picornaviridae family, such as, for example, Encephalomyocarditis Virus, Human Rhinovirus, and Hepatitis A Virus; viruses of the Retroviridae family, such as, for example, Human Immunodeficiency Virus, Simian Immunodeficiency Virus, Human T-Lymphotropic Virus, and Rous Sarcoma Virus; viruses of the Coronaviridae family, such as, for example, SARS coronavirus; viruses of the Rhabdoviridae family, such as, for example, Rabies Virus and Vesicular Stomatitis Virus, viruses of the Paramyxoviridae family, such as, for example, Respiratory Syncytial Virus and Parainfluenza Virus, viruses of the Papillomaviridae family, such as, for example, Human Papillomavirus, and viruses of the Herpesviridae family, such as, for example, Herpes Simplex Virus.

0148Interferon therapy is used (often in combination with chemotherapy and radiation) as a treatment for many cancers, which term is used herein to encompass solid tumors (carcinomas, sarcomas), and leukemias. The tumor may be an adenocarcinoma. The tumor may be a sarcoma. The tumor may affect an organ or organ system selected from breast, lymph node, prostate, kidney, bladder, lung, liver, gastrointestinal tract, colon, testis, stomach, pancreas, thyroid, skin, ovary, uterus, cervix, skin, nerve, bone, and nervous system (e.g., brain). An interferon may be used for treating a hematological malignancy, e.g., a leukemia or a lymphoma, .e.g., hairy cell leukemia, chronic myeloid leukemia, nodular lymphoma, cutaneous T-cell lymphoma. An IFN, e.g., IFN-α2b, may be used to treat a melanoma.

0149Erythropoiesis stimulating agents such as EPO are of use to treat anemia, which may result from a variety of causes. For example, the anemia may be an anemia of chronic disease, anemia associated with medications (e.g., cancer chemotherapy), radiation, renal disease (e.g., diabetes), infectious diseases, or blood loss. Colony stimulating factors such as G-CSF, GM-CSF, and/or M-CSF may be used to treat leukopenia, e.g., neutropenia and/or lymphopenia, which may result, e.g., from medications (e.g., cancer chemotherapy), radiation, infectious disease, or blood loss.

0150Neurotrophic factor polypeptides may be used, e.g., to treat neurodegenerative diseases (e.g., amyotrophic lateral sclerosis, Huntington disease, Alzheimer disease, Parkinson disease), central or peripheral nervous system injury.

0151Growth hormone may be used, e.g., to treat children's growth disorders and adult growth hormone deficiency.

0152Interleukins are of use to modulate the immune response for a wide variety of purposes, e.g., to stimulate an immune response against an infectious agent or cancer. An interleukin may stimulate immune system cells and/or increases the intensity and/or duration of innate and/or adaptive immune responses. As known in the art, certain interleukins help to limit the intensity and/or duration of innate and/or adaptive immune responses. Administration of such interleukins may be of use in treatment of autoimmune diseases, sepsis, or other conditions in which an aberrant or overactivated immune response can be deleterious.

0153Autoimmune disorders include type I diabetes (e.g., juvenile onset diabetes), multiple sclerosis, scleroderma, ankylosing spondylitis, sarcoid, pemphigus vulgaris, myasthenia gravis, systemic lupus erythemotasus, rheumatoid arthritis, juvenile arthritis, Behcet's syndrome, Reiter's disease, Berger's disease, dermatomyositis, Wegener's granulomatosis, autoimmune myocarditis, anti-glomerular basement membrane disease (including Goodpasture's syndrome), dilated cardiomyopathy, thyroiditis (e.g., Hashimoto's thyroiditis, Graves' disease), and Guillane-Barre syndrome.

0154Diseases caused by gram- positive or gram-negative bacteria, mycobacteria, fungi such as Candida or Aspergillus, helminths, etc., may be of interest. Exemplary bacteria and fungi include those falling within the following groups Actinomycetales (e.g., Corynebacterium, Mycobacterium, Norcardia), Aspergillosis, Bacillaceae (e.g., Anthrax, Clostridium), Bacteroidaceae, Blastomycosis, Bordetella, Borrelia, Brucellosis, Candidiasis, Campylobacter, Coccidioidomycosis, Cryptococcosis, Dermatocycoses, Enterobacteriaceae (Klebsiella, Salmonella, Serratia, Yersinia), Erysipelothrix, Helicobacter, Legionella, Leptospires Listeria, Mycoplasmatales, Neisseriaceae (e.g., Acinetobacter, Menigococci), Pasteurellacea (e.g., Actinobacillus, Heamophilus, Pasteurella), Pseudomonas, Rickettsiaceae, Chlamydiaceae, Treponema, and Staphylococci.

0155A modified, e.g., PEGylated and/or circularized polypeptide mat exhibit increase efficacy relative to an unmodified form and/or requires a lower dose or less frequent administration (greater dosing interval) to achieve equivalent efficacy and/or exhibits reduced toxicity (reduced side effects, greater tolerability, greater safety) and/or can be administered by a more convenient or preferable route of administration.

Use of Transamidases for Reporting on Cell-Cell Interactions

0156There is a dearth of tools that report on cell-cell interactions. Currently available split-GFP reporters only detect long-lasting interactions (GFP Reconstitution Across Synaptic Partners (GRASP) defines cell contacts and synapses in living nervous systems. <nplcit id="ncit0055" npl-type="s"><text>Feinberg EH, Vanhoven MK, Bendesky A, Wang G, Fetter RD, Shen K, Bargmann CI. Neuron. 2008 Feb 7;57(3):353-63</text></nplcit>.). We have previously shown that type II membrane proteins bearing the requisite sortase cleavage site at their C-terminus and expressed at the living cell surface are susceptible to sortase-mediated labeling when incubated with a soluble recombinant sortase and a suitable probe (<nplcit id="ncit0056" npl-type="s"><text>Popp et al. Nature Chemical Biology, 2007</text></nplcit>). The disclosure provides tools and methods for reporting on (detecting, optionally quantifying) transient cell-cell interactions, where one cell bearing a sortase substrate comes into close proximity to another cell bearing a membrane anchored version of a transamidase, e.g., a SrtA, e.g., either SrtA_strep or SrtA_staph. This would allow for acyl-enzyme formation between the sortase and substrate on opposite cells and when incubated with a functionalized probe (either based on an AA nucleophile or a GG nucleophile for SrtA_strepand SrtA_staph, respectively) will ultimately result in labeling of the cell bearing the sortase substrate. In this way a long-lived, e.g., indelible, mark for a possibly transient cell-cell interaction is imparted on the cell bearing the sortase substrate, given that the interaction is long-lived enough for the transacylation reaction to occur (<figref idref="f0056">Fig. 46</figref>). In its natural context, sortases are embedded in the bacterial membrane through a transmembrane anchor at the N-terminus. We can thus express sortase at the mammalian cell surface by fusing the catalytic domain of sortase to a mammalian type II membrane protein bearing a signal-anchor sequence for ER insertion. This would result in insertion into the plasma membrane and exposure of the sortase catalytic domain to the extracellular space. The label may comprise a detectable moiety and/or a tag that can be used for isolating the cells. This approach can be applied in vitro (in cell culture) or in vivo. Potential applications include, e.g., examining interactions of immune system cells (e.g., lymphocytes, macrophages) with other immune or non-immune system cells, examining cell-cell interactions in the nervous system or during development, cell migration, etc. This approach can be used to report on cell interactions with noncellular materials as well, by attaching a sortase substrate thereto.

Immunoglobulins and Transgenic Non-human Animals

0157Disclosed herein is a transgenic, non-human animal comprising cells whose genome comprises a DNA segment comprising a portion that encodes a polypeptide of interest and a portion that encodes a sortase recognition motif. "Transgenic animal" is used consistently with usage in the art and may refer to a non-human animal having a non-endogenous (i.e., heterologous) nucleic acid sequence (transgene) stably integrated into its DNA (i.e., in the genomic sequence of most or all of its cells, including somatic cells and cells of the germ line) or, may present as an extrachromosomal element in at least some of its cells. A transgenic animal may be a "knockout" animal, in which a gene has been inactivated, e.g., by at least partially deleting the gene or insertion of a heterologous sequence. It will be understood that the term "transgenic" applies both to the original genetically modified animal and to its descendants.

0158The animal may be of a type in which genetic modification, e.g., generating transgenic organisms, has been performed and in which methods for performing genetic modification, e.g., generating transgenic organisms, without undue experimentation are known in the art. The transgenic non-human animal may be a mammal. The mammal may be ovine, porcine, or bovine. The transgenic non-human mammal may be a rodent, e.g., a mouse or rat. The transgenic non-human animal may be an avian, e.g., a chicken. Also disclosed herein are offspring of the transgenic animal, wherein the offspring are obtained by crossing the transgenic animal with an animal having a phenotype or genetic modification of interest.

0159Also disclosed herein is a cell derived from the transgenic animal. The cell may be of any cell type. The cell may be an immune system cell (e.g., a B or T lineage cell, neutrophil or neutrophil precursor), etc. "Derived from" refers to cells that are obtained from an animal and descendants of such cells. The cells may be subjected to genetic modification or other manipulation. The cells may be immortalized.

0160Transgenic animals disclosed herein have a number of applications. They may be sources of sortase substrates and/or of cells that express sortase substrates comprising a sortase recognition motif. An application of particular interest is the production of antibodies or antibody chains that comprise a sortase recognition motif. Such antibodies may be conveniently labeled or attached to a solid support, e.g., an array, e.g., a glass slide. The animal may be immunized or otherwise contacted with (e.g., exposed to), an antigen of interest (e.g., an allergen, an infectious agent or an extract, portion, or product thereof such as a polypeptide, etc.), may be genetically predisposed to develop an autoimmune disease or immunodeficiency, may be infected with a pathogenic or non-pathogenic infectious agent, etc. Immune system cells, e.g., B-lineage cells, may be obtained from the animal. Hybridomas may be generated from the B-lineage cells. Disclosed herein are collections ('panels') of antibodies and hybridomas. See, e.g., USSN <patcit id="pcit0080" dnum="US10324114B"><text>10/ 324,114</text></patcit> (<patcit id="pcit0081" dnum="US20030224490A"><text>US Pub. No. 20030224490</text></patcit>) for information regarding production of monoclonal antibodies, hybridomas, and various other methods that may be employed in the present invention.

0161The transgenic non-human animal, e.g., transgenic mouse, may be an animal in which at least one endogenous immunoglobulin locus has been at least in part replaced by a human immunoglobulin gene locus. The transgenic animal may be fully reconstituted with human immunoglobulin loci, and, optionally, the endogenous immunoglobulin loci are functionally inactivated, e.g., through targeted deletion, insertion, etc., making it possible to generate fully human antibodies in such animals. Such transgenic mice have been generated by introducing yeast artificial chromosomes (YACs) containing human heavy-chain and κ and λ light-chain immunoglobulin genes, into mice whose endogenous heavy-chain and κ loci were functionally inactivated by targeted deletion (<nplcit id="ncit0057" npl-type="s"><text>Jakobovits, A., et al., "From XenoMouse technology to panitumumab, the first fully human antibody product from transgenic mice." Nature Biotechnology 25, 1134 - 1143 (2007</text></nplcit>)). It will be appreciated that a variety of other approaches could have been used to generate such mice or mice having similar properties.

0162Disclosed herein are transgenic plants having genetic modifications as described herein for animals (e.g., such plants may have at least part of an immunoglobulin gene inserted into their genome). It will be understood that appropriate modifications may be made to achieve expression in plants.

Definitions

0163Definitions of specific functional groups and chemical terms are described in more detail below. For purposes of this invention, the chemical elements are identified in accordance with the Periodic Table of the Elements, CAS version, <nplcit id="ncit0058" npl-type="b"><text>Handbook of Chemistry and Physics, 75th Ed., inside cover, and specific functional groups are generally defined as described therein. Additionally, general principles of organic chemistry, as well as specific functional moieties and reactivity</text></nplcit>, are described in <nplcit id="ncit0059" npl-type="b"><text>Organic Chemistry, Thomas Sorrell, University Science Books, Sausalito: 1999</text></nplcit>.

0164Certain compounds of the present invention may exist in particular geometric or stereoisomeric forms. The present invention contemplates all such compounds, including <i>cis-</i> and <i>trans</i>-isomers, <i>E</i>- and <i>Z</i>-isomers, <i>R</i>- and <i>S</i>-enantiomers, diastereomers, (D)-isomers, (L)-isomers, (-)- and (+)-isomers, racemic mixtures thereof, and other mixtures thereof, as falling within the scope of the invention. Additional asymmetric carbon atoms may be present in a substituent such as an alkyl group. All such isomers, as well as mixtures thereof, are intended to be included in this invention.

0165If, for instance, a particular enantiomer of a compound of the present invention is desired, it may be prepared by asymmetric synthesis, chiral chromatography, or by derivation with a chiral auxiliary, where the resulting diastereomeric mixture is separated and the auxiliary group cleaved to provide the pure desired enantiomers. Alternatively, where the molecule contains a basic functional group, such as amino, or an acidic functional group, such as carboxyl, diastereomeric salts are formed with an appropriate optically-active acid or base, followed by resolution of the diastereomers thus formed by fractional crystallization or chromatographic means well known in the art, and subsequent recovery of the pure enantiomers.

0166Isomeric mixtures containing any of a variety of isomer ratios may be utilized in accordance with the present invention. For example, where only two isomers are combined, mixtures containing 50:50, 60:40, 70:30, 80:20, 90:10, 95:5, 96:4, 97:3, 98:2, 99:1, or 100:0 isomer ratios are all contemplated by the present invention. Those of ordinary skill in the art will readily appreciate that analogous ratios are contemplated for more complex isomer mixtures.

0167It will be appreciated that the compounds, as described herein, may be substituted with any number of substituents or functional moieties. In general, the term "substituted" whether preceded by the term "optionally" or not, and substituents contained in formulas of this invention, refer to the replacement of hydrogen radicals in a given structure with the radical of a specified substituent. When more than one position in any given structure may be substituted with more than one substituent selected from a specified group, the substituent may be either the same or different at every position. As used herein, the term "substituted" is contemplated to include all permissible substituents of organic compounds. In a broad aspect, the permissible substituents include acyclic and cyclic, branched and unbranched, carbocyclic and heterocyclic, aromatic and non-aromatic substituents of organic compounds. For purposes of this invention, heteroatoms such as nitrogen may have hydrogen substituents and/or any permissible substituents of organic compounds described herein which satisfy the valencies of the heteroatoms. Furthermore, this invention is not intended to be limited in any manner by the permissible substituents of organic compounds. Combinations of substituents and variables envisioned by this invention are preferably those that result in the formation of stable compounds. The term "stable", as used herein, preferably refers to compounds which possess stability sufficient to allow manufacture and which maintain the integrity of the compound for a sufficient period of time to be detected and preferably for a sufficient period of time to be useful for the purposes detailed herein.

0168The term <i>acyl</i> as used herein refers to a moiety that includes a carbonyl group oro a group having the general formula -C(=O)R, where R is alkyl, alkenyl, alkynyl, aryl, carbocylic, heterocyclic, or aromatic heterocyclic. An example of an acyl group is acetyl.

0169The term <i>aliphatic,</i> as used herein, includes both saturated and unsaturated, straight chain (<i>i.e.</i>, unbranched), branched, acyclic, cyclic, or polycyclic aliphatic hydrocarbons, which are optionally substituted with one or more functional groups. As will be appreciated by one of ordinary skill in the art, "aliphatic" is intended herein to include, but is not limited to, alkyl, alkenyl, alkynyl, cycloalkyl, cycloalkenyl, and cycloalkynyl moieties. Thus, as used herein, the term "alkyl" includes straight, branched and cyclic alkyl groups. An analogous convention applies to other generic terms such as "alkenyl", "alkynyl", and the like. Furthermore, as used herein, the terms "alkyl", "alkenyl", "alkynyl", and the like encompass both substituted and unsubstituted groups. In certain embodiments, as used herein, "lower alkyl" is used to indicate those alkyl groups (cyclic, acyclic, substituted, unsubstituted, branched or unbranched) having 1-6 carbon atoms.

0170The term <i>alkyl</i> as used herein refers to saturated, straight- or branched-chain hydrocarbon radicals derived from a hydrocarbon moiety containing between one and twenty carbon atoms by removal of a single hydrogen atom. In some embodiments, the alkyl group employed in the invention contains 1-12 carbon atoms. In another embodiment, the alkyl group employed contains 1-8 carbon atoms. In still other embodiments, the alkyl group contains 1-6 carbon atoms. In yet another embodiment, the alkyl group contains 1-4 carbons. Examples of alkyl radicals include, but are not limited to, methyl, ethyl, n-propyl, isopropyl, n-butyl, iso-butyl, sec-butyl, sec-pentyl, iso-pentyl, tert-butyl, n-pentyl, neopentyl, n-hexyl, sec-hexyl, n-heptyl, n-octyl, n-decyl, n-undecyl, dodecyl, and the like, which may bear one or more substituents.

0171In general, the terms <i>aryl</i> and <i>heteroaryl,</i> as used herein, refer to stable mono- or polycyclic, heterocyclic, polycyclic, and polyheterocyclic unsaturated moieties having preferably 3-14 carbon atoms, each of which may be substituted or unsubstituted. Substituents include, but are not limited to, any of the previously mentioned substituents, <i>i.e.</i>, the substituents recited for aliphatic moieties, or for other moieties as disclosed herein, resulting in the formation of a stable compound. In certain embodiments of the present invention, <i>aryl</i> refers to a mono- or bicyclic carbocyclic ring system having one or two aromatic rings including, but not limited to, phenyl, naphthyl, tetrahydronaphthyl, indanyl, indenyl, and the like. In certain embodiments of the present invention, the term <i>heteroaryl,</i> as used herein, refers to a cyclic aromatic radical having from five to ten ring atoms of which one ring atom is selected from the group consisting of S, O, and N; zero, one, or two ring atoms are additional heteroatoms independently selected from the group consisting of S, O, and N; and the remaining ring atoms are carbon, the radical being joined to the rest of the molecule via any of the ring atoms, such as, for example, pyridyl, pyrazinyl, pyrimidinyl, pyrrolyl, pyrazolyl, imidazolyl, thiazolyl, oxazolyl, isooxazolyl, thiadiazolyl,oxadiazolyl, thiophenyl, furanyl, quinolinyl, isoquinolinyl, and the like.

0172It will be appreciated that <i>aryl</i> and <i>heteroaryl</i> groups can be unsubstituted or substituted, wherein substitution includes replacement of one, two, three, or more of the hydrogen atoms thereon independently with any one or more of the following moieties including, but not limited to: aliphatic; heteroaliphatic; aryl; heteroaryl; arylalkyl; heteroarylalkyl; alkoxy; aryloxy; heteroalkoxy; heteroaryloxy; alkylthio; arylthio; heteroalkylthio; heteroarylthio; -F; -Cl; -Br; -I; -OH; -NO₂; -CN; -CF₃; -CH₂CF₃; -CHCl₂; - CH₂OH; -CH₂CH₂OH; -CH₂NH₂; -CH₂SO₂CH₃; -C(O)R_x; -CO₂(R_x); -CON(R_x)₂; -OC(O)R_x; -OCO₂R_x; -OCON(R_x)₂; -N(R_x)₂; -S(O)₂R_x; -NR_x(CO)R_x, wherein each occurrence of R_x independently includes, but is not limited to, aliphatic, heteroaliphatic, aryl, heteroaryl, arylalkyl, or heteroarylalkyl, wherein any of the aliphatic, heteroaliphatic, arylalkyl, or heteroarylalkyl substituents described above and herein may be substituted or unsubstituted, branched or unbranched, cyclic or acyclic, and wherein any of the aryl or heteroaryl substituents described above and herein may be substituted or unsubstituted. Additional examples of generally applicable substituents are illustrated by the specific embodiments shown in the Examples that are described herein.

0173The term <i>carboxylic acid</i> as used herein refers to a group of formula -CO₂H.

0174The terms <i>halo</i> and <i>halogen</i> as used herein refer to an atom selected from the group consisting of fluorine, chlorine, bromine, and iodine.

0175The term <i>heteroaliphatic,</i> as used herein, refers to aliphatic moieties that contain one or more oxygen, sulfur, nitrogen, phosphorus, or silicon atoms, <i>e.g.</i>, in place of carbon atoms. Heteroaliphatic moieties may be branched, unbranched, cyclic or acyclic and include saturated and unsaturated heterocycles such as morpholino, pyrrolidinyl, <i>etc.</i> In certain embodiments, heteroaliphatic moieties are substituted by independent replacement of one or more of the hydrogen atoms thereon with one or more moieties including, but not limited to aliphatic; heteroaliphatic; aryl; heteroaryl; arylalkyl; heteroarylalkyl; alkoxy; aryloxy; heteroalkoxy; heteroaryloxy; alkylthio; arylthio; heteroalkylthio; heteroarylthio; -F; -Cl; -Br; -I; -OH; -NO₂; -CN; -CF₃; -CH₂CF₃; -CHCl₂; -CH₂OH; -CH₂CH₂OH; -CH₂NH₂; - CH₂SO₂CH₃; -C(O)R_x; -CO₂(R_x); -CON(R_x)₂; -OC(O)R_x; -OCO₂R_x; -OCON(R_x)₂; -N(R_x)₂; - S(O)₂R_x; -NR_x(CO)R_x, wherein each occurrence of R_x independently includes, but is not limited to, aliphatic, heteroaliphatic, aryl, heteroaryl, arylalkyl, or heteroarylalkyl, wherein any of the aliphatic, heteroaliphatic, arylalkyl, or heteroarylalkyl substituents described above and herein may be substituted or unsubstituted, branched or unbranched, cyclic or acyclic, and wherein any of the aryl or heteroaryl substituents described above and herein may be substituted or unsubstituted.

0176The term <i>heterocyclic,</i> as used herein, refers to an aromatic or non-aromatic, partially unsaturated or fully saturated, 3- to 10-membered ring system, which includes single rings of 3 to 8 atoms in size and bi- and tri-cyclic ring systems which may include aromatic five- or six-membered aryl or aromatic heterocyclic groups fused to a non-aromatic ring. These heterocyclic rings include those having from one to three heteroatoms independently selected from the group consisting of oxygen, sulfur, and nitrogen, in which the nitrogen and sulfur heteroatoms may optionally be oxidized and the nitrogen heteroatom may optionally be quaternized. In certain embodiments, the term heterocyclic refers to a non-aromatic 5-, 6-, or 7-membered ring or a polycyclic group wherein at least one ring atom is a heteroatom selected from the group consisting of O, S, and N (wherein the nitrogen and sulfur heteroatoms may be optionally oxidized), including, but not limited to, a bi- or tri-cyclic group, comprising fused six-membered rings having between one and three heteroatoms independently selected from the group consisting of the oxygen, sulfur, and nitrogen, wherein (i) each 5-membered ring has 0 to 2 double bonds, each 6-membered ring has 0 to 2 double bonds, and each 7-membered ring has 0 to 3 double bonds, (ii) the nitrogen and sulfur heteroatoms may be optionally oxidized, (iii) the nitrogen heteroatom may optionally be quaternized, and (iv) any of the above heterocyclic rings may be fused to an aryl or heteroaryl ring.

0177The term <i>aromatic heterocyclic,</i> as used herein, refers to a cyclic aromatic radical having from five to ten ring atoms of which one ring atom is selected from the group consisting of sulfur, oxygen, and nitrogen; zero, one, or two ring atoms are additional heteroatoms independently selected from the group consisting of sulfur, oxygen, and nitrogen; and the remaining ring atoms are carbon, the radical being joined to the rest of the molecule via any of the ring atoms, such as, for example, pyridyl, pyrazinyl, pyrimidinyl, pyrrolyl, pyrazolyl, imidazolyl, thiazolyl, oxazolyl, isooxazolyl, thiadiazolyl, oxadiazolyl, thiophenyl, furanyl, quinolinyl, isoquinolinyl, and the like. Aromatic heterocyclic groups can be unsubstituted or substituted with substituents selected from the group consisting of branched and unbranched alkyl, alkenyl, alkynyl, haloalkyl, alkoxy, thioalkoxy, amino, alkylamino, dialkylamino, trialkylamino, acylamino, cyano, hydroxy, halo, mercapto, nitro, carboxyaldehyde, carboxy, alkoxycarbonyl, and carboxamide.

0178Specific heterocyclic and aromatic heterocyclic groups that may be included in the compounds of the invention include: 3-methyl-4-(3-methylphenyl)piperazine, 3 methylpiperidine, 4-(bis-(4-fluorophenyl)methyl)piperazine, 4-(diphenylmethyl)piperazine, 4-(ethoxycarbonyl)piperazine, 4-(ethoxycarbonylmethyl)piperazine, 4-(phenylmethyl)piperazine, 4-(1-phenylethyl)piperazine, 4-(1,1-dimethylethoxycarbonyl)piperazine, 4-(2-(bis-(2-propenyl) amino)ethyl)piperazine, 4-(2-(diethylamino)ethyl)piperazine, 4-(2-chlorophenyl)piperazine, 4-(2-cyanophenyl)piperazine, 4-(2-ethoxyphenyl)piperazine, 4-(2-ethylphenyl)piperazine, 4-(2-fluorophenyl)piperazine, 4-(2-hydroxyethyl)piperazine, 4-(2-methoxyethyl)piperazine, 4-(2-methoxyphenyl)piperazine, 4-(2-methylphenyl)piperazine, 4-(2-methylthiophenyl) piperazine, 4-(2-nitrophenyl)piperazine, 4-(2-nitrophenyl)piperazine, 4-(2-phenylethyl)piperazine, 4-(2-pyridyl)piperazine, 4-(2-pyrimidinyl)piperazine, 4-(2,3-dimethylphenyl)piperazine, 4-(2,4-difluorophenyl) piperazine, 4-(2,4-dimethoxyphenyl)piperazine, 4-(2,4-dimethylphenyl)piperazine, 4-(2,5-dimethylphenyl)piperazine, 4-(2,6-dimethylphenyl)piperazine, 4-(3-chlorophenyl)piperazine, 4-(3-methylphenyl)piperazine, 4-(3-trifluoromethylphenyl)piperazine, 4-(3,4-dichlorophenyl)piperazine, 4-3,4-dimethoxyphenyl)piperazine, 4-(3,4-dimethylphenyl)piperazine, 4-(3,4-methylenedioxyphenyl)piperazine, 4-(3,4,5-trimethoxyphenyl)piperazine, 4-(3,5-dichlorophenyl)piperazine, 4-(3,5-dimethoxyphenyl)piperazine, 4-(4-(phenylmethoxy)phenyl)piperazine, 4-(4-(3,1-dimethylethyl)phenylmethyl)piperazine, 4-(4-chloro-3-trifluoromethylphenyl)piperazine, 4-(4-chlorophenyl)-3-methylpiperazine, 4-(4-chlorophenyl)piperazine, 4-(4-chlorophenyl)piperazine, 4-(4-chlorophenylmethyl)piperazine, 4-(4-fluorophenyl)piperazine, 4-(4-methoxyphenyl)piperazine, 4-(4-methylphenyl)piperazine, 4-(4-nitrophenyl)piperazine, 4-(4-trifluoromethylphenyl)piperazine, 4-cyclohexylpiperazine, 4-ethylpiperazine, 4-hydroxy-4-(4-chlorophenyl)methylpiperidine, 4-hydroxy-4-phenylpiperidine, 4-hydroxypyrrolidine, 4-methylpiperazine, 4-phenylpiperazine, 4-piperidinylpiperazine, 4-(2-furanyl)carbonyl)piperazine, 4-((1,3-dioxolan-5-yl)methyl)piperazine, 6-fluoro-1,2,3,4-tetrahydro-2-methylquinoline, 1,4-diazacylcloheptane, 2,3-dihydroindolyl, 3,3-dimethylpiperidine, 4,4-ethylenedioxypiperidine, 1,2,3,4-tetrahydroisoquinoline, 1,2,3,4-tetrahydroquinoline, azacyclooctane, decahydroquinoline, piperazine, piperidine, pyrrolidine, thiomorpholine, and triazole.

0179The terms <i>substituted,</i> whether preceded by the term "optionally" or not, and <i>substituent,</i> as used herein, refer to the ability, as appreciated by one skilled in this art, to change one functional group for another functional group provided that the valency of all atoms is maintained. When more than one position in any given structure may be substituted with more than one substituent selected from a specified group, the substituent may be either the same or different at every position. The substituents may also be further substituted (<i>e.g.</i>, an aryl group substituent may have another substituent off it, such as another aryl group, which is further substituted with fluorine at one or more positions).

0180The term <i>arylalkyl</i> refers to alkyl groups in which a hydrogen atom has been replaced with an aryl group. Such groups include, without limitation, benzyl, cinnamyl, and dihyrocinnamyl.

0181The term <i>heteroatom</i> means one or more of oxygen, sulfur, nitrogen, phosphorus, or silicon (including, any oxidized form of nitrogen, sulfur, phosphorus, or silicon; the quaternized form of any basic nitrogen or; a substitutable nitrogen of a heterocyclic ring, for example N (as in 3,4-dihydro-2<i>H</i>-pyrrolyl), NH (as in pyrrolidinyl) or NR⁺ (as in N-substituted pyrrolidinyl)).

0182The term <i>unsaturated,</i> as used herein, means that a moiety has one or more units of unsaturation.

0183As used herein, the term <i>partially unsaturated</i> refers to a ring moiety that includes at least one double or triple bond. The term "partially unsaturated" is intended to encompass rings having multiple sites of unsaturation, but is not intended to include aryl or heteroaryl moieties, as herein defined.

0184Additionally, unless otherwise stated, structures depicted herein are also meant to include compounds that differ only in the presence of one or more isotopically enriched positions of the compound. For example, compounds having the present structures including the replacement of hydrogen by deuterium or tritium, or the replacement of a carbon by a ¹³C- or ¹⁴C-enriched carbon are within the scope of this invention. Such compounds are useful, for example, as analytical tools, as probes in biological assays, or as therapeutic agents in accordance with the present invention.

0185One of ordinary skill in the art will appreciate that the synthetic methods, as described herein, utilize a variety of protecting groups. By the term "protecting group," as used herein, it is meant that a particular functional moiety, <i>e.g.</i>, O, S, or N, is masked or blocked, permitting, if desired, a reaction to be carried out selectively at another reactive site in a multifunctional compound. Suitable protecting groups are well known in the art and include those described in detail in <nplcit id="ncit0060" npl-type="b"><text>Protecting Groups in Organic Synthesis, T. W. Greene and P. G. M. Wuts, 3rd edition, John Wiley &amp; Sons, 1999</text></nplcit>, the entirety of which is incorporated herein by reference. In certain embodiments, a protecting group reacts selectively in good yield to give a protected substrate that is stable to the projected reactions; the protecting group is preferably selectively removable by readily available, preferably nontoxic reagents that do not attack the other functional groups; the protecting group forms a separable derivative (more preferably without the generation of new stereogenic centers); and the protecting group will preferably have a minimum of additional functionality to avoid further sites of reaction. As detailed herein, oxygen, sulfur, nitrogen, and carbon protecting groups may be utilized. By way of non-limiting example, hydroxyl protecting groups include methyl, methoxylmethyl (MOM), methylthiomethyl (MTM), <i>t</i>-butylthiomethyl, (phenyldimethylsilyl)methoxymethyl (SMOM), benzyloxymethyl (BOM), <i>p-</i>methoxybenzyloxymethyl (PMBM), (4-methoxyphenoxy)methyl (<i>p</i>-AOM), guaiacolmethyl (GUM), <i>t</i>-butoxymethyl, 4-pentenyloxymethyl (POM), siloxymethyl, 2-methoxyethoxymethyl (MEM), 2,2,2-trichloroethoxymethyl, bis(2-chloroethoxy)methyl, 2-(trimethylsilyl)ethoxymethyl (SEMOR), tetrahydropyranyl (THP), 3-bromotetrahydropyranyl, tetrahydrothiopyranyl, 1-methoxycyclohexyl, 4-methoxytetrahydropyranyl (MTHP), 4-methoxytetrahydrothiopyranyl, 4-methoxytetrahydrothiopyranyl S,S-dioxide, 1-[(2-chloro-4-methyl)phenyl]-4-methoxypiperidin-4-yl (CTMP), 1,4-dioxan-2-yl, tetrahydrofuranyl, tetrahydrothiofuranyl, 2,3,3a,4,5,6,7,7a-octahydro-7,8,8-trimethyl-4,7-methanobenzofuran-2-yl, 1-ethoxyethyl, 1-(2-chloroethoxy)ethyl, 1-methyl-1-methoxyethyl, 1-methyl-1-benzyloxyethyl, 1-methyl-1-benzyloxy-2-fluoroethyl, 2,2,2-trichloroethyl, 2-trimethylsilylethyl, 2-(phenylselenyl)ethyl, <i>t-</i>butyl, allyl, <i>p</i>-chlorophenyl, <i>p</i>-methoxyphenyl, 2,4-dinitrophenyl, benzyl, <i>p</i>-methoxybenzyl, 3,4-dimethoxybenzyl, <i>o</i>-nitrobenzyl, <i>p</i>-nitrobenzyl, <i>p</i>-halobenzyl, 2,6-dichlorobenzyl, <i>p-</i>cyanobenzyl, <i>p</i>-phenylbenzyl, 2-picolyl, 4-picolyl, 3-methyl-2-picolyl <i>N</i>-oxido, diphenylmethyl, <i>p,p</i>'-dinitrobenzhydryl, 5-dibenzosuberyl, triphenylmethyl, α-naphthyldiphenylmethyl, <i>p</i>-methoxyphenyldiphenylmethyl, di(<i>p-</i>methoxyphenyl)phenylmethyl, tri(<i>p</i>-methoxyphenyl)methyl, 4-(4'-bromophenacyloxyphenyl)diphenylmethyl, 4,4',4"-tris(4,5-dichlorophthalimidophenyl)methyl, 4,4',4"-tris(levulinoyloxyphenyl)methyl, 4,4',4"-tris(benzoyloxyphenyl)methyl, 3-(imidazol-1-yl)bis(4',4"-dimethoxyphenyl)methyl, 1,1-bis(4-methoxyphenyl)-1'-pyrenylmethyl, 9-anthryl, 9-(9-phenyl)xanthenyl, 9-(9-phenyl-10-oxo)anthryl, 1,3-benzodithiolan-2-yl, benzisothiazolyl S,S-dioxido, trimethylsilyl (TMS), triethylsilyl (TES), triisopropylsilyl (TIPS), dimethylisopropylsilyl (IPDMS), diethylisopropylsilyl (DEIPS), dimethylthexylsilyl, <i>t</i>-butyldimethylsilyl (TBDMS), <i>t-</i>butyldiphenylsilyl (TBDPS), tribenzylsilyl, tri-<i>p</i>-xylylsilyl, triphenylsilyl, diphenylmethylsilyl (DPMS), <i>t</i>-butylmethoxyphenylsilyl (TBMPS), formate, benzoylformate, acetate, chloroacetate, dichloroacetate, trichloroacetate, trifluoroacetate, methoxyacetate, triphenylmethoxyacetate, phenoxyacetate, <i>p</i>-chlorophenoxyacetate, 3-phenylpropionate, 4-oxopentanoate (levulinate), 4,4-(ethylenedithio)pentanoate (levulinoyldithioacetal), pivaloate, adamantoate, crotonate, 4-methoxycrotonate, benzoate, <i>p</i>-phenylbenzoate, 2,4,6-trimethylbenzoate (mesitoate), alkyl methyl carbonate, 9-fluorenylmethyl carbonate (Fmoc), alkyl ethyl carbonate, alkyl 2,2,2-trichloroethyl carbonate (Troc), 2-(trimethylsilyl)ethyl carbonate (TMSEC), 2-(phenylsulfonyl) ethyl carbonate (Psec), 2-(triphenylphosphonio) ethyl carbonate (Peoc), alkyl isobutyl carbonate, alkyl vinyl carbonate alkyl allyl carbonate, alkyl <i>p</i>-nitrophenyl carbonate, alkyl benzyl carbonate, alkyl <i>p</i>-methoxybenzyl carbonate, alkyl 3,4-dimethoxybenzyl carbonate, alkyl <i>o</i>-nitrobenzyl carbonate, alkyl <i>p</i>-nitrobenzyl carbonate, alkyl <i>S</i>-benzyl thiocarbonate, 4-ethoxy-1-napththyl carbonate, methyl dithiocarbonate, 2-iodobenzoate, 4-azidobutyrate, 4-nitro-4-methylpentanoate, <i>o-</i>(dibromomethyl)benzoate, 2-formylbenzenesulfonate, 2-(methylthiomethoxy)ethyl, 4-(methylthiomethoxy)butyrate, 2-(methylthiomethoxymethyl)benzoate, 2,6-dichloro-4-methylphenoxyacetate, 2,6-dichloro-4-(1,1,3,3-tetramethylbutyl)phenoxyacetate, 2,4-bis(1,1-dimethylpropyl)phenoxyacetate, chlorodiphenylacetate, isobutyrate, monosuccinoate, (<i>E</i>)-2-methyl-2-butenoate, <i>o</i>-(methoxycarbonyl)benzoate, α-naphthoate, nitrate, alkyl <i>N,N,N',N'-</i>tetramethylphosphorodiamidate, alkyl <i>N</i>-phenylcarbamate, borate, dimethylphosphinothioyl, alkyl 2,4-dinitrophenylsulfenate, sulfate, methanesulfonate (mesylate), benzylsulfonate, and tosylate (Ts). For protecting 1,2- or 1,3-diols, the protecting groups include methylene acetal, ethylidene acetal, 1-<i>t</i>-butylethylidene ketal, 1-phenylethylidene ketal, (4-methoxyphenyl)ethylidene acetal, 2,2,2-trichloroethylidene acetal, acetonide, cyclopentylidene ketal, cyclohexylidene ketal, cycloheptylidene ketal, benzylidene acetal, <i>p-</i>methoxybenzylidene acetal, 2,4-dimethoxybenzylidene ketal, 3,4-dimethoxybenzylidene acetal, 2-nitrobenzylidene acetal, methoxymethylene acetal, ethoxymethylene acetal, dimethoxymethylene ortho ester, 1-methoxyethylidene ortho ester, 1-ethoxyethylidine ortho ester, 1,2-dimethoxyethylidene ortho ester, α-methoxybenzylidene ortho ester, <i>1-</i>(<i>N,N-</i>dimethylamino)ethylidene derivative, α-(<i>N,N</i>'-dimethylamino)benzylidene derivative, 2-oxacyclopentylidene ortho ester, di-<i>t</i>-butylsilylene group (DTBS), 1,3-(1,1,3,3-tetraisopropyldisiloxanylidene) derivative (TIPDS), tetra-<i>t</i>-butoxydisiloxane-1,3-diylidene derivative (TBDS), cyclic carbonates, cyclic boronates, ethyl boronate, and phenyl boronate. Amino-protecting groups include methyl carbamate, ethyl carbamante, 9-fluorenylmethyl carbamate (Fmoc), 9-(2-sulfo)fluorenylmethyl carbamate, 9-(2,7-dibromo)fluoroenylmethyl carbamate, 2,7-di-<i>t</i>-butyl-[9-(10,10-dioxo-10,10,10,10-tetrahydrothioxanthyl)]methyl carbamate (DBD-Tmoc), 4-methoxyphenacyl carbamate (Phenoc), 2,2,2-trichloroethyl carbamate (Troc), 2-trimethylsilylethyl carbamate (Teoc), 2-phenylethyl carbamate (hZ), 1-(1-adamantyl)-1-methylethyl carbamate (Adpoc), 1,1-dimethyl-2-haloethyl carbamate, 1,1-dimethyl-2,2-dibromoethyl carbamate (DB-<i>t</i>-BOC), 1,1-dimethyl-2,2,2-trichloroethyl carbamate (TCBOC), 1-methyl-1-(4-biphenylyl)ethyl carbamate (Bpoc), 1-(3,5-di-<i>t-</i>butylphenyl)-1-methylethyl carbamate (<i>t</i>-Bumeoc), 2-(2'- and 4'-pyridyl)ethyl carbamate (Pyoc), 2-(<i>N,N</i>-dicyclohexylcarboxamido)ethyl carbamate, <i>t</i>-butyl carbamate (BOC), 1-adamantyl carbamate (Adoc), vinyl carbamate (Voc), allyl carbamate (Alloc), 1-isopropylallyl carbamate (Ipaoc), cinnamyl carbamate (Coc), 4-nitrocinnamyl carbamate (Noc), 8-quinolyl carbamate, <i>N</i>-hydroxypiperidinyl carbamate, alkyldithio carbamate, benzyl carbamate (Cbz), <i>p</i>-methoxybenzyl carbamate (Moz), <i>p</i>-nitobenzyl carbamate, <i>p-</i>bromobenzyl carbamate, <i>p</i>-chlorobenzyl carbamate, 2,4-dichlorobenzyl carbamate, 4-methylsulfinylbenzyl carbamate (Msz), 9-anthrylmethyl carbamate, diphenylmethyl carbamate, 2-methylthioethyl carbamate, 2-methylsulfonylethyl carbamate, 2-(<i>p-</i>toluenesulfonyl)ethyl carbamate, [2-(1,3-dithianyl)]methyl carbamate (Dmoc), 4-methylthiophenyl carbamate (Mtpc), 2,4-dimethylthiophenyl carbamate (Bmpc), 2-phosphonioethyl carbamate (Peoc), 2-triphenylphosphonioisopropyl carbamate (Ppoc), 1,1-dimethyl-2-cyanoethyl carbamate, <i>m</i>-chloro-<i>p</i>-acyloxybenzyl carbamate, <i>p-</i>(dihydroxyboryl)benzyl carbamate, 5-benzisoxazolylmethyl carbamate, 2-(trifluoromethyl)-6-chromonylmethyl carbamate (Tcroc), <i>m</i>-nitrophenyl carbamate, 3,5-dimethoxybenzyl carbamate, <i>o</i>-nitrobenzyl carbamate, 3,4-dimethoxy-6-nitrobenzyl carbamate, phenyl(<i>o-</i>nitrophenyl)methyl carbamate, phenothiazinyl-(10)-carbonyl derivative, <i>N'-p-</i>toluenesulfonylaminocarbonyl derivative, <i>N'</i>-phenylaminothiocarbonyl derivative, <i>t</i>-amyl carbamate, <i>S</i>-benzyl thiocarbamate, <i>p</i>-cyanobenzyl carbamate, cyclobutyl carbamate, cyclohexyl carbamate, cyclopentyl carbamate, cyclopropylmethyl carbamate, <i>p-</i>decyloxybenzyl carbamate, 2,2-dimethoxycarbonylvinyl carbamate, <i>o-</i>(<i>N,N-</i>dimethylcarboxamido)benzyl carbamate, 1,1-dimethyl-3-(<i>N,N</i>-dimethylcarboxamido)propyl carbamate, 1,1-dimethylpropynyl carbamate, di(2-pyridyl)methyl carbamate, 2-furanylmethyl carbamate, 2-iodoethyl carbamate, isoborynl carbamate, isobutyl carbamate, isonicotinyl carbamate, <i>p</i>-(<i>p</i>'-methoxyphenylazo)benzyl carbamate, 1-methylcyclobutyl carbamate, 1-methylcyclohexyl carbamate, 1-methyl-1-cyclopropylmethyl carbamate, 1-methyl-1-(3,5-dimethoxyphenyl)ethyl carbamate, 1-methyl-1-(<i>p</i>-phenylazophenyl)ethyl carbamate, 1-methyl-1-phenylethyl carbamate, 1-methyl-1-(4-pyridyl)ethyl carbamate, phenyl carbamate, <i>p</i>-(phenylazo)benzyl carbamate, 2,4,6-tri-<i>t</i>-butylphenyl carbamate, 4-(trimethylammonium)benzyl carbamate, 2,4,6-trimethylbenzyl carbamate, formamide, acetamide, chloroacetamide, trichloroacetamide, trifluoroacetamide, phenylacetamide, 3-phenylpropanamide, picolinamide, 3-pyridylcarboxamide, <i>N</i>-benzoylphenylalanyl derivative, benzamide, <i>p</i>-phenylbenzamide, <i>o</i>-nitophenylacetamide, <i>o</i>-nitrophenoxyacetamide, acetoacetamide, (<i>N'</i>-dithiobenzyloxycarbonylamino)acetamide, 3-(<i>p-</i>hydroxyphenyl)propanamide, 3-(<i>o</i>-nitrophenyl)propanamide, 2-methyl-2-(<i>o-</i>nitrophenoxy)propanamide, 2-methyl-2-(<i>o</i>-phenylazophenoxy)propanamide, 4-chlorobutanamide, 3-methyl-3-nitrobutanamide, <i>o</i>-nitrocinnamide, <i>N</i>-acetylmethionine derivative, <i>o</i>-nitrobenzamide, <i>o</i>-(benzoyloxymethyl)benzamide, 4,5-diphenyl-3-oxazolin-2-one, <i>N</i>-phthalimide, <i>N</i>-dithiasuccinimide (Dts), <i>N</i>-2,3-diphenylmaleimide, <i>N</i>-2,5-dimethylpyrrole, <i>N</i>-1,1,4,4-tetramethyldisilylazacyclopentane adduct (STABASE), 5-substituted 1,3-dimethyl-1,3,5-triazacyclohexan-2-one, 5-substituted 1,3-dibenzyl-1,3,5-triazacyclohexan-2-one, 1-substituted 3,5-dinitro-4-pyridone, <i>N</i>-methylamine, <i>N</i>-allylamine, <i>N</i>-[2-(trimethylsilyl)ethoxy]methylamine (SEM), <i>N</i>-3-acetoxypropylamine, <i>N</i>-(1-isopropyl-4-nitro-2-oxo-3-pyroolin-3-yl)amine, quaternary ammonium salts, <i>N</i>-benzylamine, <i>N</i>-di(4-methoxyphenyl)methylamine, <i>N</i>-5-dibenzosuberylamine, <i>N</i>-triphenylmethylamine (Tr), <i>N-</i>[(4-methoxyphenyl)diphenylmethyl]amine (MMTr), <i>N</i>-9-phenylfluorenylamine (PhF), <i>N</i>-2,7-dichloro-9-fluorenylmethyleneamine, <i>N</i>-ferrocenylmethylamino (Fcm), <i>N</i>-2-picolylamino <i>N'-</i>oxide, <i>N</i>-1,1-dimethylthiomethyleneamine, <i>N</i>-benzylideneamine, <i>N-p-</i>methoxybenzylideneamine, <i>N</i>-diphenylmethyleneamine, <i>N</i>-[(2-pyridyl)mesityl]methyleneamine, <i>N</i>-(<i>N',N'</i>-dimethylaminomethylene)amine, <i>N,N'-</i>isopropylidenediamine, <i>N</i>-<i>p</i>-nitrobenzylideneamine, <i>N</i>-salicylideneamine, <i>N</i>-5-chlorosalicylideneamine, <i>N</i>-(5-chloro-2-hydroxyphenyl)phenylmethyleneamine, <i>N-</i>cyclohexylideneamine, <i>N</i>-(5,5-dimethyl-3-oxo-1-cyclohexenyl)amine, <i>N</i>-borane derivative, <i>N</i>-diphenylborinic acid derivative, <i>N</i>-[phenyl(pentacarbonylchromium- or tungsten)carbonyl]amine, <i>N</i>-copper chelate, <i>N</i>-zinc chelate, <i>N</i>-nitroamine, <i>N</i>-nitrosoamine, amine <i>N</i>-oxide, diphenylphosphinamide (Dpp), dimethylthiophosphinamide (Mpt), diphenylthiophosphinamide (Ppt), dialkyl phosphoramidates, dibenzyl phosphoramidate, diphenyl phosphoramidate, benzenesulfenamide, <i>o</i>-nitrobenzenesulfenamide (Nps), 2,4-dinitrobenzenesulfenamide, pentachlorobenzenesulfenamide, 2-nitro-4-methoxybenzenesulfenamide, triphenylmethylsulfenamide, 3-nitropyridinesulfenamide (Npys), <i>p</i>-toluenesulfonamide (Ts), benzenesulfonamide, 2,3,6,-trimethyl-4-methoxybenzenesulfonamide (Mtr), 2,4,6-trimethoxybenzenesulfonamide (Mtb), 2,6-dimethyl-4-methoxybenzenesulfonamide (Pme), 2,3,5,6-tetramethyl-4-methoxybenzenesulfonamide (Mte), 4-methoxybenzenesulfonamide (Mbs), 2,4,6-trimethylbenzenesulfonamide (Mts), 2,6-dimethoxy-4-methylbenzenesulfonamide (iMds), 2,2,5,7,8-pentamethylchroman-6-sulfonamide (Pmc), methanesulfonamide (Ms), β-trimethylsilylethanesulfonamide (SES), 9-anthracenesulfonamide, 4-(4',8'-dimethoxynaphthylmethyl)benzenesulfonamide (DNMBS), benzylsulfonamide, trifluoromethylsulfonamide, and phenacylsulfonamide. Exemplary protecting groups are detailed herein, however, it will be appreciated that the present invention is not intended to be limited to these protecting groups; rather, a variety of additional equivalent protecting groups can be readily identified using the above criteria and utilized in the method of the present invention. Additionally, a variety of protecting groups are described by Greene and Wuts (<i>supra</i>).

0186"Polypeptide", "peptide", or "protein": According to the present invention, a "polypeptide", "peptide", or "protein" comprises a string of at least three amino acids linked together by peptide bonds. The terms "polypeptide", "peptide", and "protein", may be used interchangeably. Peptide may refer to an individual peptide or a collection of peptides. Inventive peptides preferably contain only natural amino acids, although non natural amino acids (<i>i.e.</i>, compounds that do not occur in nature but that can be incorporated into a polypeptide chain) and/or amino acid analogs as are known in the art may alternatively be employed. Also, one or more of the amino acids in a peptide may be modified, for example, by the addition of a chemical entity such as a carbohydrate group, a phosphate group, a farnesyl group, an isofarnesyl group, a fatty acid group, a linker for conjugation, functionalization, or other modification, etc. In one embodiment, the modifications of the peptide lead to a more stable peptide (<i>e.g.</i>, greater half-life <i>in vivo</i>). These modifications may include cyclization of the peptide, the incorporation of D-amino acids, <i>etc.</i> None of the modifications should substantially interfere with the desired biological activity of the peptide.

0187"Carbohydrate": The term "carbohydrate" refers to a sugar or polymer of sugars. The terms "saccharide", "polysaccharide", "carbohydrate", and "oligosaccharide", may be used interchangeably. Most carbohydrates are aldehydes or ketones with many hydroxyl groups, usually one on each carbon atom of the molecule. Carbohydrates generally have the molecular formula C_nH_2nO_n. A carbohydrate may be a monosaccharide, a disaccharide, trisaccharide, oligosaccharide, or polysaccharide. The most basic carbohydrate is a monosaccharide, such as glucose, sucrose, galactose, mannose, ribose, arabinose, xylose, and fructose. Disaccharides are two joined monosaccharides. Exemplary disaccharides include sucrose, maltose, cellobiose, and lactose. Typically, an oligosaccharide includes between three and six monosaccharide units (<i>e.g.</i>, raffinose, stachyose), and polysaccharides include six or more monosaccharide units. Exemplary polysaccharides include starch, glycogen, and cellulose. Carbohydrates may contain modified saccharide units such as 2'-deoxyribose wherein a hydroxyl group is removed, 2'-fluororibose wherein a hydroxyl group is replace with a fluorine, or N-acetylglucosamine, a nitrogen-containing form of glucose. (<i>e.g.</i>, 2'-fluororibose, deoxyribose, and hexose). Carbohydrates may exist in many different forms, for example, conformers, cyclic forms, acyclic forms, stereoisomers, tautomers, anomers, and isomers.

0188"Polysaccharide", "carbohydrate" or "oligosaccharide": The terms "polysaccharide", "carbohydrate", or "oligosaccharide" refer to a polymer of sugars. The terms "polysaccharide", "carbohydrate", and "oligosaccharide", may be used interchangeably. Typically, a polysaccharide comprises at least two sugars. The polymer may include natural sugars (<i>e.g.</i>, glucose, fructose, galactose, mannose, arabinose, ribose, and xylose) and/or modified sugars (<i>e.g.</i>, 2'-fluororibose, 2'-deoxyribose, and hexose). "Sugar alcohols" include glycerol (glycertil, propane-1,2,3-triol), erythritol, threitol, ribitol (adonitol), arabinitol, xylitol, allitol, altritol, galactitol, glucitol (sorbitol), mannitol, or iditol.

0189As used herein, the phrase "natural amino acid side-chain" refers to the side-chain group of any of the 20 amino acids naturally occuring in proteins. Such natural amino acids include the nonpolar, or hydrophobic amino acids, glycine, alanine, valine, leucine, isoleucine, methionine, phenylalanine, tryptophan, and proline. Cysteine is sometimes classified as nonpolar or hydrophobic and other times as polar. Natural amino acids also include polar, or hydrophilic amino acids, such as tyrosine, serine, threonine, aspartic acid (also known as aspartate, when charged), glutamic acid (also known as glutamate, when charged), asparagine, and glutamine. Certain polar, or hydrophilic, amino acids have charged side-chains. Such charged amino acids include lysine, arginine, and histidine. One of ordinary skill in the art would recognize that protection of a polar or hydrophilic amino acid side-chain can render that amino acid nonpolar. For example, a suitably protected tyrosine hydroxyl group can render that tyroine nonpolar and hydrophobic by virtue of protecting the hydroxyl group.

0190As used herein, the phrase "unnatural amino acid side-chain" refers to the side-chain group of amino acids not included in the list of 20 amino acids naturally occuring in proteins, as described above. Such amino acids include the D-isomer of any of the 20 naturally occuring amino acids. Unnatural amino acids also include homoserine, ornithine, norleucine, and thyroxine. Other unnatural amino acids side-chains are well known to one of ordinary skill in the art and include unnatural aliphatic side chains. Other unnatural amino acids include modified amino acids, including those that are N-alkylated, cyclized, phosphorylated, acetylated, amidated, azidylated, labelled, and the like. In some embodiments, an unnatural amino acid is a D-isomer. In some embodiments, an unnatural amino acid is a L-isomer.

0191"Small molecule": As used herein, the term "small molecule" is used to refer to molecules, whether naturally-occurring or artificially created (<i>e.g.</i>, via chemical synthesis) that have a relatively low molecular weight. Typically, a small molecule is an organic compound (<i>i.e.</i>, it contains carbon). The small molecule may contain multiple carbon-carbon bonds, stereocenters, and other functional groups (<i>e.g.</i>, amines, hydroxyl, carbonyls, heterocyclic rings, <i>etc</i>.). In some embodiments, small molecules are monomeric and have a molecular weight of less than about 1500 g/mol. In certain embodiments, the molecular weight of the small molecule is less than about 1000 g/mol or less than about 500 g/mol. Preferred small molecules are biologically active in that they produce a biological effect in animals, preferably mammals, more preferably humans. Small molecules include, but are not limited to, radionuclides and imaging agents. In certain embodiments, the small molecule is a drug. Preferably, though not necessarily, the drug is one that has already been deemed safe and effective for use in humans or animals by the appropriate governmental agency or regulatory body. For example, drugs approved for human use are listed by the FDA under 21 C.F.R. §§ 330.5, 331 through 361, and 440 through 460; drugs for veterinary use are listed by the FDA under 21 C.F.R. §§ 500 through 589. All listed drugs are considered acceptable for use in accordance with the present invention.

0192"Polynucleotide" is used herein consistently with usage in the art and is used interchangeably with "nucleic acid" or "oligonucleotide", with "oligonucleotide" typically being used to refer to short (e.g, 50 residues or less) polynucleotides. The polynucleotide may include natural nucleosides found in RNA and/or DNA, nucleoside analogs (e.g., 2-aminoadenosine, 2-thiothymidine, inosine, pyrrolo-pyrimidine, 3-methyl adenosine, C5-propynylcytidine, C5-propynyluridine, C5-bromouridine, C5-fluorouridine, C5-iodouridine, C5-methylcytidine, 7-deazaadenosine, 7-deazaguanosine, 8-oxoadenosine, 8-oxoguanosine, O(6)-methylguanine, and 2-thiocytidine), and/or nucleosides comprising chemically or biologically modified bases, (e.g., methylated bases) and/or modified sugars (e.g., 2'-fluororibose, ribose, 2'-deoxyribose, arabinose, and hexose). Polynucleotides containing modified backbones (relative to those found in RNA or DNA) or non-naturally occurring internucleoside linkages can also be used in the present invention. A polynucleotide may be single-stranded or double-stranded. It should be noted that where the present invention discloses a single-stranded polynucleotide, the perfect complement of the polynucleotide, and a double-stranded polynucleotide comprising both the single-stranded polynucleotide and its perfect complement, are also disclosed. Polynucleotide sequences are listed in a 5' to 3' direction unless otherwise indicated.

0193As used herein, the term "tag" refers to a moiety appended to another entity that imparts a characteristic or property otherwise not present in the un-tagged entity. In some embodiments, the tag is an affinity tag, an epitope tag, a fluorescent tag, etc. Examples of fluorescent tags include GFP and other fluorescent proteins mentioned above. Affinity tags can facilitate the purification or solubilization of fusion proteins. Examples of affinity tags include maltose binding protein (MBP), glutathione-S-transferase (GST), thioredoxin, polyhistidine (also known as <i>hexa histidine, 6xHis,</i> and by the trademarked name <i>HIS-TAG</i>®)<i>,</i> etc. Examples of epitope tags, which facilitate recognition by antibodies, include c-myc, FLAG (FLAG octapeptides), HA (hemagglutinin), etc.

0194As used herein, the term "catalyst" refers to a substance the presence of which increases the rate and/or extent of a chemical reaction, while not being consumed or undergoing a permanent chemical change itself.

0195The terms "label" and "labeling moiety" entity refer to agents that can be visualized, for example following binding to another entity. The detectable agent or moiety may be selected such that it generates a signal which can be measured and whose intensity is related to (<i>e.g.</i>, proportional to) the amount of bound entity. A wide variety of systems for labeling and/or detecting proteins and peptides are known in the art. Labeled proteins and peptides can be prepared by incorporation of, or conjugation to, a label that is detectable by spectroscopic, photochemical, biochemical, immunochemical, electrical, optical, chemical or other means. A label or labeling moiety may be directly detectable (<i>i.e.</i>, it does not require any further reaction or manipulation to be detectable, <i>e.g.</i>, a fluorophore is directly detectable) or it may be indirectly detectable (<i>i.e.</i>, it is made detectable through reaction or binding with another entity that is detectable, <i>e.g.</i>, a hapten is detectable by immunostaining after reaction with an appropriate antibody comprising a reporter such as a fluorophore). Suitable detectable agents include, but are not limited to, radionuclides, fluorophores, chemiluminescent agents, microparticles, enzymes, colorimetric labels, magnetic labels, haptens, Molecular Beacons, aptamer beacons, and the like.

0196As used herein and in the claims, the singular forms "a", "an", and "the" include the plural reference unless the context clearly indicates otherwise. Thus, for example, a reference to "a compound" includes a plurality of such compounds.

0197It will be understood that wherein the claims or description use the word "is" (<i>e.g.</i>, as in "A¹ is xxx" or "recognition motif is xxx"), the invention encompasses embodiments in which A¹ (or recognition motif) "consists of xxx".

0198As used herein, the term "isolated" often refers to having a specific activity of at least tenfold greater than the sortase-transamidase activity present in a crude extract, lysate, or other state from which proteins have not been removed and also in substantial isolation from proteins found in association with sortase-transamidase in the cell. An "isolated" or "purified" polypeptide or protein is substantially free of cellular material or other contaminating proteins from the cell or tissue source from which the protein is derived, or substantially free from chemical precursors or other chemicals when chemically synthesized. In one embodiment, the term "substantially free" refers to preparing a target polypeptide having less than about 30%, 20%, 10% and sometimes 5% (by dry weight), of non-target polypeptide (also referred to herein as a"contaminating protein"), or of chemical precursors or non-target chemicals. When the target polypeptide or a biologically active portion thereof is recombinantly produced, it also often is substantially free of culture medium, where culture medium represents less than about 20%, sometimes less than about 10%, and often less than about 5% of the volume of the polypeptide preparation. In certain embodiments, isolated or purified target polypeptide preparations are 0. 01 milligrams or more. In certain embodiments, isolated or purified target polypeptide preparations are 0.1 milligrams or more. In certain embodiments, isolated or purified target polypeptide preparations are 1.0 milligrams or more and 10 milligrams or more in dry weight.

Exemplification

Example 1: Lipid Modification using Sortase

<i>Overview</i>

0199Protein-lipid conjugates and methods for their construction are valuable tools for studying the role of post-translational lipid modification in controlling protein function and localization. Lipid attachment also provides a means for manipulating the properties of peptides and proteins that normally do not possess these types of modifications. Highlighting the utility of synthetic lipoproteins, Bertozzi and co-workers recently reported the synthesis of GFP-containing GPI-anchor mimetics as a means for probing the role of individual sugar residues in this complex membrane anchor (1, 2). Numerous reports have also shown that the addition of lipids and fatty acids can endow proteins and peptides with cell-penetrating capabilities (3-7), modulate immunogenicity (8, 9), and provide anchors for incorporation into liposomes (10, 11).

0200With respect to full-size protein substrates, the attachment of lipids in site-specific fashion remains a significant bioconjugation challenge. Only a relatively limited set of chemical and chemoenzymatic strategies are available for this purpose. These include the use of enzymes such as myristoyl (12, 13) and prenyl (14-16) transferases, wherein the user is limited to the natural substrates of these enzymes or closely related analogoues. In the case of chemical labeling, a common strategy is cysteine derivatization through alkylation (17, 18) or disulfide formation (19, 20), an approach that requires site-directed mutagenesis to ensure that protein substrates contain only a single reactive cysteine residue. Among the available strategies for lipid attachment, expressed protein ligation (EPL) (1, 2, 21-25) has proven to be the most versatile. For example, this method has been used to prepare GFP bearing a range of lipid modifications (1, 2, 24) and to generate lipidated Ras (25) and Rab (22) proteins. However, EPL mandates the expression of protein substrates as fusions with large intein domains, a necessity that can reduce protein expression yields and may require careful selection of the intein domain to prevent premature intein cleavage and a corresponding drop in the yield of ligation products (26-29).

0201With the goal of complementing existing strategies for lipoprotein synthesis, we have developed a general method for the site-specific installation of lipids and fatty acids using sortase-catalyzed transpeptidation (Scheme 1). As mentioned above, sortase A from Staphylococcus aureus recognizes a five amino acid sequence (LPXTG) near the C-terminus of its natural protein targets (39-41). The active site cysteine of sortase cleaves between the threonine and glycine residues of the recognition site to produce a thioester intermediate which is then attacked by the amino terminus of an oligoglycine nucleophile, resulting in the formation of a new amide bond. The method is extremely versatile, stemming from the remarkable tolerance of the enzyme for substituents C-terminal to the oligoglycine unit. Synthetic nucleophiles, containing 1-5 glycine residues, have been decorated with a range of substituents including fluorophores (33, 34) photoaffinity probes (34), peptide nucleic acids (36), polymers (35), solid supports (32, 35, 42) or other polypeptides (33, 38) allowing the site-specific ligation of these moieties to peptide and protein substrates. Here we describe the synthesis of lipid-modified oligoglycine nucleophiles compatible with the sortase-mediated transpeptidation reaction and demonstrate their high-yielding ligation to a protein substrate. Lipoproteins prepared using this procedure were shown to strongly associate with mammalian cells in a lipid tail-dependent fashion and were found to localize to the plasma membrane and endosomes.

<i>Materials and Instrumentation</i>

0202<b>Chemicals.</b> Unless otherwise noted, all chemicals were obtained from commercial sources and used without further purification. Rink amide resin (100-200 mesh, 0.7 mmol/g) was obtained from Advanced Chemtech. Fmoc-Gly-OH, Fmoc-Lys(Mtt)-OH, and Fmoc-Trp(Boc)-OH were obtained from EMD Biosciences/Novabiochem. Triglycine peptide was purchased from Sigma (G1377). Water used in biological procedures or as a reaction solvent was purified using a MilliQ purification system (Millipore). DriSolv^® anhydrous CH₂Cl₂ and DriSolv^® anhydrous MeCN were purchased from EMD Chemicals. Redistilled, anhydrous N,N'-diisopropylethylamine (DIPEA) was obtained from Sigma-Aldrich.

0203<b>NMR. ¹</b>H and ¹³C spectra were measured with a Bruker AVANCE-400 (400 MHz) spectrometer at the MIT Department of Chemistry Instrumentation Facility. ¹H NMR chemical shifts are reported as δ in units of parts per million (ppm) relative to chloroform-d (δ 7.24, singlet). Multiplicities are reported as follows: s (singlet), d (doublet), or m (multiplet). Coupling constants are reported as a J value in Hertz (Hz). The number of protons (n) for a given resonance is indicated as nH, and is based on spectral integration values. ¹³C NMR chemical shifts are reported as δ in units of parts per million (ppm) relative to chloroform-d (δ 77.23, triplet).

0204<b>Mass Spectrometry.</b> Electrospray Ionization (ESI) mass spectra were obtained at the MIT Department of Chemistry Instrumentation Facility. Matrix assisted laser desorption-ionization time-of-flight mass spectrometry (MALDI-TOF MS) was performed on a MALDI micro MX™ system (Micromass^® MS Technologies, USA). Samples were co-crystallized using a sinapinic acid solution (10 mg/mL in 70:30 MeCN:H₂O with 0.1% TFA). LC-ESI-MS analysis was performed using a Micromass LCT mass spectrometer (Micromass^® MS Technologies, USA) and a Paradigm MG4 HPLC system equipped with a HTC PAL autosampler (Michrom BioResources, USA) and a Waters Symmetry 5 µm C8 column (2.1 x 50 mm, MeCN:H₂O (0.1% formic acid) gradient mobile phase, 150 µL/min).

0205<b>HPLC/FPLC.</b> HPLC purifications were achieved using an Agilent 1100 Series HPLC system equipped with a Waters Delta Pak 5 µm, 100 Å C4 column (3.9 x 150 mm, MeCN:H₂O gradient mobile phase containing 0.1% trifluoroacetic acid, 1 mL/min), a Waters Delta Pak 15 µm, 100 Å C18 column (7.8 x 300 mm, 2-propanol:H₂O gradient mobile phase, 2 mL/min), or a Waters Cosmosil 5PE column (8 x 250 mm, 2propanol:H₂O gradient mobile phase, 1 mL/min) as indicated below. Size exclusion chromatography was performed on a Pharmacia AKTA Purifier system equipped with a HiLoad 16/60 Superdex 75 column (Amersham).

0206<b>UV-Vis Spectrocopy.</b> UV-Vis spectroscopy was performed on a Nanodrop ND-1000 spectrophotometer (Thermo Scientific, USA).

0207<b>Flow Cytometry.</b> Flow cytometry was performed on a LSR-II system (BD Biosciences).

0208<b>Spinning Disc Confocal Microscopy.</b> Fluorescence microscopy was performed on a Nikon spinning disc confocal microscope with MetaMorph 7 software. Multidimensional acquisition was used to collect images in the 488, 647, and phase contrast channels from the same focal plane.

<i>Design, Synthesis, and Characterization of Lipid-Modified Triglycine Nucleophiles</i>

0209Most nucleophiles compatible with sortase-mediated transpeptidation that have been identified to date have the simple structural requirement of a stretch of glycine residues with a free amino terminus. While successful transpeptidation has been reported with nucleophiles containing anywhere from one to five glycines, maximum reaction rates are obtained when two or more glycines are present.(32, 38). It should be noted that primary alkylamines have been shown to participate in the transpeptidation reaction, (31, 35) though in general these appeared to be less efficient than oligoglycine derivatives. With these considerations in mind, we synthesized a panel of lipid-modified triglycine nucleophiles using solid-phase synthesis (Scheme 2).

0210Resin-bound precursor <b>1</b> was prepared on Rink amide resin using standard Fmoc chemistry. An orthogonally protected lysine residue was incorporated to allow selective liberation and subsequent acylation of the ε-amino group. Following removal of the 4-methyltrityl (Mtt) group, the lysine side chain was acylated with a range of hydrophobic carboxylic acids, including naturally occurring post-translational modifications such as myristic (<b>1</b>-C14) and palmitic (<b>1</b>-C16) acid, as well as modifications not found in eukaryotic cells (<b>1</b>-ad). The couplings of C18, C20, C22, and C24 acids were performed at 50°C to maintain the solubility of all the reaction components. Following Fmoc deprotection and resin cleavage, the identity of nucleophiles derived from <b>1</b> was confirmed by LC-ESI-MS (<figref idref="f0012 f0013">Figure 11</figref>, and see below). All peptides were obtained with good purity and did not require extensive purification prior to transpeptidation.

0211In addition to purely aliphatic substituents, we also prepared a cholesterol-modified triglycine nucleophile (<b>2</b>-chol, Scheme 2). Peptide <b>2</b> was prepared on Rink amide resin and then cleaved from the solid support, leaving the N-terminal Fmoc group intact. A tryptophan residue was included in this peptide as a UV-active chromophore to aid in purification by HPLC. Peptide <b>2</b> was then reacted with cholesterol NHS-carbonate <b>3</b>, followed by the addition of piperidine to remove the Fmoc group. The crude material was then purified by RP-HPLC to yield <b>2</b>-chol in 22% yield.

0212Further details regarding synthesis and characterization of the lipidated triglycine nucleophiles are as follows: <b>Resin-Bound Intermediate 1.</b> A glass solid-phase reaction vessel containing a fritted glass filter and a Teflon stopcock was loaded with Rink amide resin (1.0 g, 0.70 mmol). The resin was first washed/swollen extensively with N-methyl-2-pyrrolidone (NMP) by gentle agitation with a wrist action shaker. The Fmoc protecting group was then removed by treatment with ∼30 mL of 80:20 NMP/piperidine for 20 min at RT. The resin was washed with ∼30 mL of NMP (3x, 3-5 min per wash). Amino acid building blocks were then coupled as follows: Fmoc-protected amino acid (1.75 mmol, 2.5 equivalents relative to initial resin loading), benzotriazole-1-yl-oxy-tris-pyrrolidino-phosphonium hexafluorophosphate (PyBOP) (911 mg, 1.75 mmol), N-hydroxybenzotriazole (HOBt) (236 mg, 1.75 mmol), and N,N'-diisopropylethylamine (DIPEA) (914 µL, 5.25 mmol) were dissolved in NMP to a final volume of 8.75 mL (200 mM final concentration of amino acid building block). This solution was mixed until all reagents had dissolved, and then added to the deprotected Rink amide resin. Couplings were incubated for 16-48 h at RT. The resin was then washed with ∼30 mL of NMP (3x, 3-5 min per wash). The extent of coupling was assessed by Kaiser test. In the event that the coupling was incomplete, the above procedure was repeated. Fmoc removal was then achieved by exposing the resin to ∼30 mL of 80:20 NMP/piperidine for 20 min at RT, followed by additional washing with ∼30 mL of NMP (3x, 3-5 min per wash). Cycles of amino acid coupling and Fmoc deprotection were then repeated to complete the synthesis of <b>1</b>. The resin was then washed with ∼ 30 mL of CH₂Cl₂ (5x, 3-5 min per wash) and dried.

0213<b>1-ad, 1-C12, and 1-C14 (General Procedure).</b> A 3.0 mL fritted polypropylene syringe equipped with a capped hypodermic needle was loaded with dry resin <b>1</b> (50 mg). The resin was washed with 2.5 mL of CH₂Cl₂ (3x, 3-5 min per wash). Next, the 4methyltrityl (Mtt) protecting group was removed by treatment with 2.5 mL of 94:5:1 CH₂Cl₂/TIPS/TFA at RT (5x, 5 min each) followed by washing with 2.5 mL of NMP (3x, 3-5 min per wash). The resin was then treated with a solution of the appropriate carboxylic acid (R₁-COOH, 0.12 mmol), PyBOP (65 mg, 0.12 mmol), HOBt (16 mg, 0.12 mmol), and DIPEA (65 µL, 0.38 mmol) in 1.0 mL of NMP. The reaction was incubated at RT for 21 h followed by washing with 2.5 mL of NMP (3x, 3-5 min per wash). Fmoc removal was achieved by treatment with 2.5 mL of 80:20 NMP/piperidine for 20 min at RT followed by washing with 2.5 mL of NMP (3x, 3-5 min per wash) and 2.5 mL of CH₂Cl₂ (3x, 3-5 min per wash). The peptide was cleaved from the resin with 2.5 mL of 95:3:2 TFA/TIPS/H₂O (4x, ∼30 min each) and the combined cleavage solutions were concentrated in vacuo. <b>1-ad</b> was then precipitated from ether and dried under vacuum. Crude <b>1-C12</b> and crude <b>1-C14</b> were washed with hexanes (1 mL, 2x) and then dried under vacuum. The identity of all peptides was confirmed by LC-ESI-MS analysis (<figref idref="f0012 f0013">Figure 11</figref>). Peptides were used without further purification and the yield was not determined.

0214<b>1-C16.</b> A 3.0 mL fritted polypropylene syringe equipped with a capped hypodermic needle was loaded with dry resin <b>1</b> (200 mg). The resin was washed with 2.5 mL of CH₂Cl₂ (3x, 3-5 min per wash). Next, the 4-methyltrityl (Mtt) protecting group was removed by treatment with 2.5 mL of 94:5:1 CH₂Cl₂/TIPS/TFA at RT (5x, 5 min each). The resin was then washed with 2.5 mL of NMP (3x, 3-5 min per wash). The resin was then treated with a solution of palmitic acid (103 mg, 0.40 mmol), PyBOP (208 mg, 0.40 mmol), HOBt (54 mg, 0.4 mmol), and DIPEA (207 µL, 1.2 mmol) in 1.6 mL of NMP. The reaction was incubated at RT overnight. The resin was then washed with 2.5 mL of NMP (3x, 3-5 min per wash). Fmoc removal was achieved by treatment with 2.5 mL of 80:20 NMP/piperidine for 20 min at RT followed by washing with 2.5 mL of NMP (3x, 3-5 min per wash) and 2.5 mL of CH₂Cl₂ (3x, 3-5 min per wash). The peptide was cleaved from the resin with 2.5 mL of 95:3:2 TFA/TIPS/H₂O (4x, ∼30 min each) and the combined cleavage solutions were dried under vacuum. The identity of <b>1-C16</b> was confirmed by LC-ESI-MS analysis (<figref idref="f0012 f0013">Figure 11</figref>). This material was used without further purification and yield was not determined.

0215<b>1-C18, 1-C20, 1-C22, and 1-C24 (General Procedure).</b> A 3.0 mL fritted polypropylene syringe equipped with a capped hypodermic needle was loaded with dry resin <b>1</b> (50 mg). The resin was washed with 2.5 mL of CH₂Cl₂ (3x, 3-5 min per wash). Next, the 4-methyltrityl (Mtt) protecting group was removed by treatment with 2.5 mL of 94:5:1 CH₂Cl₂/TIPS/TFA at RT (5x, 5 min each) followed by washing with 2.5 mL of NMP (3x, 3-5 min per wash). The resin was then carefully transferred to a 1.5 mL microcentrifuge tube. The resin was treated with a solution of the appropriate carboxylic acid (R₂-COOH, 0.12 mmol), PyBOP (65 mg, 0.12 mmol), HOBt (16 mg, 0.12 mmol), and DIPEA (65 µL, 0.38 mmol) in 1.0 mL of NMP. The reaction was incubated at 50 °C for 5 h. The reaction was then centrifuged, and 0.5 mL of the supernatant was discarded. An additional 0.5 mL of NMP was then added followed again by centrifugation and removal of 0.5 mL of the supernatant. This process was repeated two additional times. The resin slurry was carefully transferred to a 3.0 mL fritted polypropylene syringe equipped with a capped hypodermic needle and washed with 2.5 mL of NMP (3x, 3-5 min per wash). Fmoc removal was achieved by treatment with 2.5 mL of 80:20 NMP/piperidine for 20 min at RT followed by washing with 2.5 mL of NMP (3x, 3-5 min per wash) and 2.5 mL of CH₂Cl₂ (3x, 3-5 min per wash). The peptide was cleaved from the resin with 2.5 mL of 95:3:2 TFA/TIPS/H₂O (3x, ∼30 min each) and the combined cleavage solutions were concentrated in vacuo. <b>1-C18, 1-C20, 1-C22,</b> and <b>1-C24</b> were precipitated from cold ether and dried under vacuum. The identity of all peptides was confirmed by LC-ESIMS analysis (<figref idref="f0012 f0013">Figure 11</figref>). Peptides were used without further purification and yield was not determined.

0216<b>Cholesterol NHS-carbonate (3).</b> An oven dried round bottom flask equipped with an argon inlet was charged with cholesterol (500 mg, 1.29 mmol) and N,N'-disuccinimidyl carbonate (DSC) (665 mg, 2.59 mmol) in 15 mL of 1:1:1 CH₂Cl₂/MeCN/DIPEA (all anhydrous). The reaction was stirred at RT for 24 h. The reaction mixture was then partitioned between brine and CH₂Cl₂. The aqueous layer was extracted two additional times with CH₂Cl₂. The combined organic layers were then dried over MgSO₄, filtered, and concentrated. The remaining residue was then purified by flash chromatography (95:5 CH₂Cl₂/EtOAc) to yield <b>3</b> as a white solid (274 mg, 40%). ¹H NMR (400 MHz, CDCl₃): δ, 5.39 (d, 1H, J = 4.4 Hz), 4.57 (m, 1H), 2.81 (s, 4H), 2.46 (m, 2H), 2.05-0.80 (m, 26H), 1.00 (s, 3H), 0.89 (d, 3H, J = 6.4 Hz), 0.84 (d, 3H, J = 6.4 Hz), 0.83 (d, 3H, J = 6.8 Hz), 0.65 (s, 3H) . ¹³C NMR (100 MHz, CDCl₃): δ, 169.0, 151.0, 138.7, 123.9, 82.4, 56.8, 56.3, 50.1, 42.5, 39.9, 39.7, 37.8, 36.9, 36.7, 36.4, 36.0, 32.1, 32.0, 28.4, 28.2, 27.6, 25.7, 24.5, 24.0, 23.0, 22.8, 21.2, 19.4, 18.9, 12.0. HRMS (ESI+) calculated for C₃₂H₄₉NO₅Na ([M+Na]⁺) 550.3503, found 550.3383.

0217<b>Fmoc-Gly-Gly-Gly-Trp-Lys-CONH₂ (2)</b>. Intermediate <b>2</b> was synthesized on Rink amide resin following the procedure described above for the preparation of <b>1</b>. Peptide <b>2</b> was cleaved from the resin with 95:3:2 TFA/TIPS/H₂O (3x, ∼30 min each). The combined cleavage solutions were concentrated and <b>2</b> was precipitated from ether and dried. The identity of <b>2</b> was confirmed by LC-ESI-MS. Yield not determined. LRMS (ESI+) calculated for C₃₈H₄₅N₈O₇ ([M+H]⁺) 725.3, found 725.4.

0218<b>2-chol.</b> To a 1.5 mL microcentrifuge tube was added <b>3</b> (10.5 mg dissolved in 50 µL of CHCl₃, 19.9 µmol), <b>2</b> (7.2 mg dissolved in 50 µL of NMP, 9.9 µmol), and anhydrous DIPEA (5.00 µL, 28.7 µmol). After overnight incubation at RT, piperidine (25 µL) was added and the reaction was incubated for an additional 2 h at RT. The reaction was then diluted with 0.5 mL of 2-propanol and fractionated by RP-HPLC [semi-preparative C18 column, 2-propanol:H₂O gradient mobile phase, 2 mL/min, 50% 2-propanol → 90% 2-propanol (0-20 min)]. Fractions containing <b>2-chol</b> were pooled, concentrated, and further purified by RP-HPLC [semi-preparative PE column, 2-propanol:H₂O gradient mobile phase, 1 mL/min, 50% 2-propanol → 90% 2-propanol (0-20 min)] to yield <b>2-chol</b> (2 mg, 22%). The identity of <b>2-chol</b> was confirmed by LC-ESI-MS (<figref idref="f0012 f0013">Figure 11</figref>).

<i>Sortase-Mediated Ligation of Triglycine Nucleophiles to a Protein Substrate</i>

0219With a series of lipidated nucleophiles in hand, we proceeded to explore their coupling to a protein substrate using sortase. A model protein consisting of eGFP equipped with a C-terminal LPETG sequence followed by a His₆ affinity handle was expressed and purified from Escherichia coli. Recombinant sortase A containing an N-terminal His₆ tag was produced as described (40). At the outset of these studies, we decided to incorporate a procedure for removing the sortase enzyme after the reaction, similar to a purification scheme proposed by Parthasarathy et al. (35) but with important improvements. The configuration of His₆ tags on both sortase and the eGFP substrate provided a simple means for doing so, where Ni-NTA resin was added following transpeptidation to retrieve the sortase enzyme as well as any unreacted eGFP-LPETG-His₆. In this scenario, the transpeptidation product should no longer be His₆-tagged because residues C-terminal to the LPETG motif are lost in the course of transpeptidation, and thus, this material will not be bound by Ni-NTA.

0220eGFP-LPETG-His₆ cloning and expression were performed as follows: The gene encoding eGFP was PCR amplified from the commercially available pEGFP-N1 plasmid (Clontech) using forward primer 5'-CGCGCGCCATGGTGAGCAAGGGCGAGGAG-3' and reverse primer 5' CGCGCGGATCCCGACCAGTTTCAGGAAGCTTGTACAGCTCGTCCATGCCG-3'. The PCR product was digested with NcoI and BamHI and ligated into pET28a+ (Novagen). This plasmid was then transformed into E. coli BL-21. In a typical experiment, cells were grown in two 2 L batches of sterile LB containing kanamycin (30 µg/mL) to an optical density of ∼0.6-0.9 at 600 nm. Cells were induced with IPTG (1 mM) for 3 h at 37°C. Cells were harvested by centrifugation and the pellet was stored overnight at -20°C. The pellet was thawed and resuspended in 60 mL of 10 mM Tris pH 8.0, 100 mM phosphate, 300 mM NaCl, and 20 mM imidazole containing a protease inhibitor cocktail (complete mini tablet, EDTA-free, Roche) and treated with 240 µL of DNAse I (10 mg/mL in PBS), 480 µL of lysozyme (50 mg/mL in PBS), and 60 µL of MgCl₂ (1 M in PBS). The lysis reaction was incubated for 1 h at 4°C. The cells were then sonicated and centrifuged to remove insoluble material. The pellet was then treated with an additional 20 mL of 10 mM Tris pH 8.0, 100 mM phosphate, 300 mM NaCl, and 20 mM imidazole, briefly sonicated, and centrifuged. The combined supernatants were applied to a Ni-NTA column consisting of 7.5 mL of commercial Ni-NTA slurry (Qiagen) equilibrated with 10 mM Tris pH 8.0, 100 mM phosphate, 300 mM NaCl, and 20 mM imidazole. The column was washed with three 45 mL portion of 10 mM Tris pH 8.0, 100 mM phosphate, 300 mM NaCl, and 20 mM imidazole. eGFP-LPETG-His₆ was eluted with ∼10 mL of 10 mM Tris pH 8.0, 100 mM phosphate, 300 mM NaCl, and 300 mM imidazole. This material was concentrated and further purified by size exclusion chromatography on a HiLoad 16/60 Superdex 75 column (Amersham), eluting with 20 mM Tris pH 8.0, 150 mM NaCl at a flow rate of 1 mL/min. Fractions containing eGFP-LPETG-His₆ were pooled, concentrated, and stored at -80°C.

0221Immediately prior to sortase-catalyzed transpeptidation, the eGFP-LPETG-His₆ stock was thawed and again purified by affinity chromatography over commercial Ni-NTA resin. After binding eGFP-LPETG-His₆ to the resin, the column was washed with three portions of 20 mM Tris pH 8.0, 150 mM NaCl, and 20 mM imidazole. The protein was eluted with 20 mM Tris pH 8.0, 150 mM NaCl, and 300 mM imidazole. This material was buffer exchanged into 20 mM Tris pH 8.0, 150 mM NaCl using a NAP™5 Sephadex™ column (GE Healthcare) and concentrated. The concentration was estimated by UV-Vis spectroscopy using the absorbance of eGFP at 488 nm (extinction coefficient 55,900 M^-1cm^-1) (43).

0222As shown in <figref idref="f0001">Figure <u>1</u>a</figref>, exposing eGFP-LPETG-His₆ to 150 µM sortase and 2 mM triglycine (GGG) in Tris buffer (pH 7.5) containing 10 mM CaCl₂ resulted in the appearance of a lower molecular weight species just below the band corresponding to sortase (lane 2). Excess sortase was used to drive the reaction to completion in a reasonable time frame. Near-quantitative conversion of the input eGFP-LPETG-His₆ protein (lane 1) was observed over the course of a 3 h incubation at 37 °C. A slurry of Ni-NTA resin containing 1 M NaCl and 40 mM imidazole was then added, and the mixture was incubated for 2 h. In further detail, removal of His₆-tagged proteins following transpeptidation was achieved with Ni-NTA resin treated in the following way: 500 µL of commercial Ni-NTA resin (Qiagen) was treated with 1 mL of 40 mM Tris pH 8.0, 1 M NaCl, and 40 mM imidazole. The mixture was then centrifuged and the supernatant was removed. This process was repeated two additional times. The resin was then suspended to a final volume of 500 µL with 40 mM Tris pH 8.0, 1 M NaCl, and 40 mM imidazole. This slurry was used to treat samples depicted in <figref idref="f0001">Figure 1a</figref> lanes 3-11 and <figref idref="f0003">Figure 2b</figref> lanes 3-13. For control experiments (<figref idref="f0003">Figure 2b</figref>, lanes 1-2), the above procedure was performed on a separate batch of commercial Ni-NTA resin using 40 mM Tris pH 8.0, 1 M NaCl, and 600 mM imidazole to block binding of His₆-tagged proteins.

0223The addition of imidazole and NaCl at the specified concentrations contributed importantly to preventing nonspecific binding of the transpeptidation products. After filtration to remove Ni-NTA, the filtrate was analyzed by SDS-PAGE. A single polypeptide was observed with no evidence for the presence of residual sortase (lane 3). Characterization by ESI-MS confirmed that the protein observed in lane 3 was indeed the desired ligation product (<figref idref="f0014 f0015">Figure 12</figref>, see below). Also, judging from the intensity of the band in lane 3 as compared to the corresponding eGFP bands in lanes 1 and 2, we were confident that very little material was lost over the course of the two-step transpeptidation/sortase-depletion protocol.

0224Initial attempts to ligate one of the lipid-modified nucleophiles (<b>1</b>-C22) using the conditions described above yielded only small amounts of the putative transpeptidation product as well as an unidentified higher molecular weight species (<figref idref="f0001">Figure <u>1</u>a</figref>, lane 4). The high molecular weight byproduct may correspond to the acyl enyme intermediate formed between sortase A and the eGFP substrate. This assignment implies that the affinity of the His6 tag on the N-terminus of sortase is impaired in the acyl enzyme intermediate because this species is not removed during depletion of His6-tagged proteins using Ni-NTA resin. Further characterization of this byproduct was not performed.

0225Not surprisingly, the solubility of <b>1</b>-C22 in aqueous buffer was extremely poor, and we suspected that this may limit the amount of nucleophile available in solution. To improve solubility, we examined the effect of adding mild detergents to the reaction mixture. As shown in <figref idref="f0001 f0002">Figure <u>1</u></figref>a (lane 8), the addition of 1% (w/v) <i>n</i>-dodecyl maltoside afforded a significant increase in the level of transpeptidation product relative to that of the other detergents tested. This material was further characterized by ESI-MS and found to consist exclusively of the desired lipid-modified eGFP derivative (<figref idref="f0002">Figure 1b</figref>).

0226Using 1% (w/v) <i>n</i>-dodecyl maltoside to maintain nucleophile solubility, we were able to successfully ligate the entire panel of lipid-modified triglycine nucleophiles to our eGFP model substrate (<figref idref="f0003">Figure 2</figref>). In all cases, a single major protein that migrated at a lower apparent molecular weight than unreacted eGFP-LPETG-His₆ (lane 1) was observed in the filtrate of reactions using lipidated nucleophiles (lanes 5-13). As expected, these samples also showed complete removal of the sortase enzyme. Coupling reactions using only triglycine (lanes 2 and 3) served as positive controls for transpeptidation. A reaction lacking an oligoglycine nucleophile resulted in slow hydrolysis of the acyl enzyme intermediate and the formation of a higher molecular weight byproduct (43) (lane 4). As an additional negative control, we confirmed that the formation of transpeptidation products was dependent on the presence of sortase (<figref idref="f0015">Figure 13</figref>). The identity of all eGFP conjugates was ultimately established by mass spectrometry (Table 1 and <figref idref="f0014 f0015">Figure 12</figref>). <tables id="tabl0003" num="0003"><table frame="all"><title><b>Table 1.</b> Mass Spectral Characterization and Yield (%) of eGFP-Lipid Conjugates</title><tgroup cols="4"><colspec colnum="1" colname="col1" colwidth="34mm" /><colspec colnum="2" colname="col2" colwidth="30mm" /><colspec colnum="3" colname="col3" colwidth="28mm" /><colspec colnum="4" colname="col4" colwidth="26mm" /><thead><row><entry align="center" valign="top"><b>eGFP conjugate</b></entry><entry align="center" valign="top"><b>calcd mass (Da)</b></entry><entry align="center" valign="top"><b>obsd mass (Da)</b></entry><entry align="center" valign="top"><b>yield (%)</b></entry></row></thead><tbody><row><entry>eGFP-GGG</entry><entry align="center">27534</entry><entry align="center">27531</entry><entry align="center">93</entry></row><row><entry>eGFP-<b>1</b>-ad</entry><entry align="center">27837</entry><entry align="center">27835</entry><entry align="center">87</entry></row><row><entry>eGFP-<b>1</b>-C12</entry><entry align="center">27843</entry><entry align="center">27842</entry><entry align="center">87</entry></row><row><entry>eGFP-<b>1</b>-C14</entry><entry align="center">27871</entry><entry align="center">27870</entry><entry align="center">72</entry></row><row><entry>eGFP-<b>1</b>-C16</entry><entry align="center">27899</entry><entry align="center">27898</entry><entry align="center">64</entry></row><row><entry>eGFP-<b>1</b>-C18</entry><entry align="center">27927</entry><entry align="center">27926</entry><entry align="center">57</entry></row><row><entry>eGFP-<b>1</b>-C20</entry><entry align="center">27955</entry><entry align="center">27954</entry><entry align="center">66</entry></row><row><entry>eGFP-<b>1</b>-C22</entry><entry align="center">27983</entry><entry align="center">27982</entry><entry align="center">83</entry></row><row><entry>eGFP-<b>1</b>-C24</entry><entry align="center">28011</entry><entry align="center">28010</entry><entry align="center">84</entry></row><row><entry>eGFP-<b>2</b>-chol</entry><entry align="center">28260</entry><entry align="center">28257</entry><entry align="center">87</entry></row></tbody></tgroup></table></tables>

0227The eGFP chromophore provided a convenient method for quantifying the yield of the eGFP conjugates. By comparing the absorbance at 488 nm of the conjugates relative to the input eGFP-LPETG-His₆ protein, we estimated that the lipid-modified protein conjugates were all obtained in excellent (∼60-90%) yield (Table 1 and <figref idref="f0016">Figure 14</figref>). A reproducible drop in reaction yield for nucleophiles containing fatty acids of intermediate length (<b>1</b>-C16, <b>1</b>-C18, <b>1</b>-C20) was observed. Whether this reflects a drop in the efficiency of the actual transpeptidation reaction or limited solubility of the resulting lipoprotein conjugates, which could contribute to losses during the sortase depletion step, is presently unclear. It should be noted, however, that the purity of these eGFP conjugates, as determined by both SDS-PAGE (<figref idref="f0003">Figure 2</figref>) and ESI-MS (<figref idref="f0014 f0015">Figure 12</figref>), was not compromised.

0228Further details regarding lipid modification of GFP-LPETG-His₆, removal of His₆-tagged proteins, acetone precipitation of eGFP conjugates and characterization by ESI-MS, and MALDI-TOF MS Characterization of eGFP-2-chol., and estimation of % yield of eGFP conjugates are as follows:

0229<b>Lipid modification of GFP-LPETG-His6 and removal of His6-tagged proteins (General Procedure).</b> Lipidated triglycine nucleophile (0.5 µL of a 100 mM solution in DMSO) was combined with 2.5 µL of 10x sortase reaction buffer (500 mM Tris pH 7.5, 100 mM CaCl2, 1.5 M NaCl) and 1.25 µL of 20% (w/v) n-dodecyl maltoside. This mixture was repeatedly heated in a 90 °C heat block and incubated in a sonicating water bath to disperse and solubilize the lipidated nucleophile. After cooling to room temperature, eGFP-LPETG-His6 (12.4 µL of a 110 µM solution in 20 mM Tris pH 8.0, 150 mM NaCl) and sortase A (8.35 µL of a 448 µM solution in 50 mM Tris pH 8.0, 150 mM NaCl, 10% glycerol (v/v)) were added. The reaction mixtures were incubated at 37 °C for 5 h in the dark and then treated with 50 µL of the appropriate Ni-NTA slurry described above and incubated for an additional 2 h at room temperature. The mixtures were then filtered (Spin-X^® Centrifugal Tube Filters 0.22 µm, Corning) to yield solutions containing lipid-modified eGFP. Samples were analyzed by SDS-PAGE with visualization by Coomassie blue staining.

0230<b>Acetone Precipitation of eGFP conjugates and Characterization by ESI-MS.</b> 20 µL of lipid-modified eGFP solution was treated with 1 mL of acetone and centrifuged at maximum speed for 3 minutes. The supernatant was removed and the pellet was washed with 1 mL of acetone. The sample was again centrifuged and the pellet was dissolved in 100 µL of 0.1% formic acid. Samples were then analyzed by LC-ESI-MS (<figref idref="f0014">Figure 12a</figref>).

0231<b>MALDI-TOF MS Characterization of eGFP-2-chol. eGFP-2-chol</b> was desalted by RP-HPLC [C4 column, MeCN:H₂O gradient mobile phase with 0.1% TFA, 1 mL/min, 5% MeCN → 50% MeCN (0-15 min), 50% MeCN → 80% MeCN (15-18 min)]. The fraction containing eGFP-<b>2-chol</b> was analyzed by MALDI-TOF-MS (<figref idref="f0015">Figure 12b</figref>).

0232<b>Estimation of % Yield of eGFP Conjugates.</b> The yield of lipid modified eGFP was estimated using the absorbance of eGFP at 488 nm. As a standard, a sample of unmodified eGFP-LPETG-His₆ was incubated in the absence of sortase A and lipidated nucleophile (<figref idref="f0003">Figure 2b</figref>, lane 1) and treated with Ni-NTA resin containing excess imidazole to prevent protein binding. After filtration to remove Ni-NTA, the absorbance of this sample at 488 nm was measured and assumed to represent 100% recovery. Serial dilutions of this sample were measured in triplicate and used to construct a standard curve (<figref idref="f0016">Figure 14</figref>).

<i>Interaction of Lipid-Modified eGFP with Mammalian Cells</i>

0233The presence of a hydrophobic tail on eGFP should render it capable of spontaneous association with biological membranes (1, 2, 24, 44). As an initial investigation into the effect of the identity of the lipid tail on the ability of these lipoproteins to associate with cells, we incubated HeLa cells with lipid-modified eGFP (2.5 µg/mL) in serum-free medium for 1 h at 37 °C. After washing, the cells were analyzed by flow cytometry (<figref idref="f0004">Figure 3</figref>). In general, we observed a steady increase in cellular fluorescence with increasing length of the lipid tail. In the case of eGFP-<b>1</b>-C22, we recorded a 500-fold enhancement in mean cellular fluorescence relative to that of eGFP-GGG. Interestingly, eGFP-<b>1</b>-C24 consistently yielded lower association than eGFP-<b>1</b>-C22. For all samples cell viability was >94% as determined by exclusion of propidium iodide, indicative of a lack of cytotoxicity of the lipidated eGFP derivatives. Also, because the eGFP preparations contained low levels of free lipidated nucleophile (∼1 µM in culture medium), we verified that this contaminant did not contribute to the observed increase in cellular fluorescence. eGFP-GGG in combination with <b>1</b> µM <b>1</b>-C22, <b>1</b>-C24, or <b>2</b>-chol produced only modest increases in fluorescence, <5% of the magnitude observed for the corresponding covalently modified eGFP conjugates (<figref idref="f0017">Figure 15</figref>). In this control experiment, cells were seeded into 6-well polystyrene cell culture plates at a density of ∼100,000 cells per well and incubated overnight. The media was then removed and replaced with 2 mL of serum-free DME containing 2.5 µg/mL eGFP-GGG and 1 µM lipidated triglycine nucleophile (<b>1-C22</b>, <b>1-C24</b>, or <b>2-chol</b>). Cells were incubated for 1 h at 37 °C. Next, the cells were washed three times with ∼4 mL of cold PBS containing 1% FBS. The cells were lifted from the culture plate by treatment with 1 mL of 1 mM EDTA in PBS and pelleted at 250 x g for 5 min. The cells were then resuspended in 200 µL of PBS containing 1% FBS and propidium iodide (0.5 µM) and analyzed by flow cytometry (<figref idref="f0017">Figure 15</figref>).

0234A preliminary evaluation of the subcellular localization of eGFP-<b>1</b>-C22 and eGFP-<b>2</b>-chol was performed using spinning disk confocal microscopy. HeLa cells and U373 cells were incubated with 2.5 µg/mL lipoprotein in serum-free medium, followed by washing and live cell imaging. For both cell types we observed staining of the cell surface as well as the appearance of internal fluorescent structures (<figref idref="f0005 f0006">Figure 4</figref>). For all cells, filamentous membrane extensions, presumably filopodia, were evident in the fluorescent micrographs, indicating the presence of the eGFP lipoproteins on the plasma membrane. In U373 cells, distinct vesicular structures were observed at early (1 h) and later (5 h) time points, with a more pronounced presence of vesicles observed for eGFP-<b>1</b>-C22 than for eGFP-<b>2</b>-chol. In general, less internalization was observed for HeLa cells, though punctate internal staining could be observed with eGFP-<b>1</b>-C22 after a 5 h incubation in these cells. In the case of U373, we were also able to verify that eGFP-<b>1</b>-C22 had access to early/recycling endosomal compartments as determined by partial colocalization with transferrin (<figref idref="f0007">Figure 5</figref>) (6, 45). Colocalization between eGFP-<b>2</b>-chol and transferrin was not observed (<figref idref="f0007">Figure 5</figref>) for this cell line. Taken together, these results demonstrate that synthetic lipoproteins prepared using sortase-mediated transpeptidation can interact strongly with the plasma membrane and access endosomal compartments. The range of staining observed for our eGFP conjugates suggests that interactions with cells could be fine-tuned, and studies are under way to characterize the differences in the behavior of protein conjugates bearing different lipid anchors, specifically looking for differences in subcellular distribution and routes of endocytosis.

0235Further details regarding materials and methods used in the experiments described in this section are as follows:

0236<b>1000x stock solutions of lipid-modified eGFP.</b> Solutions of lipid-modified eGFP obtained by transpeptidation and depletion of His₆-tagged proteins were used without further purification for cell membrane association studies. Prior to addition to cells, the concentration of eGFP in each sample was measured by UV-Vis (488 nm), and all solutions were adjusted to a final eGFP concentration of 2.5 mg/mL. Dilutions were made with a mock reaction mixture lacking eGFP, sortase A, and lipidated triglycine nucleophile to ensure that all samples contained the same amount of residual components (n-dodecyl maltoside, imidazole, CaCl₂, etc.) from the sortase labeling procedure.

0237<b>Cell culture and membrane association of lipid-modified eGFP.</b> HeLa cells were maintained in DME media supplemented with 10% fetal bovine serum (FBS), penicillin (50 units/mL), and streptomycin sulfate (50 µg/mL). Cells were incubated in a 5% CO₂ humidified incubator at 37°C. One day prior to treatment with lipid-modified eGFP, cells were seeded into 6-well polystyrene cell culture plates at a density of ∼100,000 cells per well and incubated overnight. The media was then removed and replaced with 2 mL of serum-free DME containing 2.5 µg/mL modified eGFP (lipid modified proteins were delivered as 1000x stock solutions, see above). Cells were then incubated for 1 h at 37°C. Next, the cells were washed three times with ∼4 mL of cold PBS containing 1% FBS. The cells were lifted from the culture plate by treatment with 1 mL of 1 mM EDTA in PBS and pelleted at 250 x g for 5 min. The cells were then resuspended in 200 µL of PBS containing 1% FBS and propidium iodide (0.5 µM) and analyzed by flow cytometry.

0238<b>Spinning Disc Confocal Microscopy.</b> HeLa and U373 cells were maintained in DME media supplemented with 10% fetal bovine serum (FBS), penicillin (50 units/mL), and streptomycin sulfate (50 µg/mL). Cells were incubated in a 5% CO2 humidified incubator at 37°C. One day prior to treatment with lipid-modified eGFP, cells were seeded into Lab-Tek™ II chambered coverglass slides (Nalge Nunc International) and allowed to adhere overnight. The media was then removed and replaced with 200 µL of serum-free DME containing 2.5 µg/mL modified eGFP (lipid modified proteins were delivered as 1000x stock solutions, see above). Cells were then incubated for 1-5 h at 37°C. Next, the cells were washed three times with ∼0.5 mL of cold PBS containing 1% FBS. Cells were then covered with 200 uL of PBS containing 1% FBS and imaged. For transferrin colocalization, U373 were incubated with 200 µL of serum-free DME containing 2.5 µg/mL modified eGFP for 5 h at 37°C. Transferrin-Alexa 647 (Invitrogen, 100 µg/mL final concentration) was added during the final 15 minutes of this incubation. Cells were then washed as described above and imaged.

<i>Discussion of Example 1</i>

0239In summary, we have developed a general strategy using sortase-mediated transpeptidation as a means to install hydrophobic lipid modifications onto protein substrates in site-specific fashion. The ease of use of this method stands out as a significant asset. Protein substrates require, e.g., only a five amino acid extension (such as LPETG) to serve as sortase substrates, a modest insertion that is not expected to impede the function of most proteins and should also have minimal impact on the expression yield of these polypeptides. Recombinant proteins containing the LPETG motif are also stable and unreactive until activated by the sortase enzyme. This affords a distinct advantage over the expressed protein ligation approach where premature cleavage of intein fusion proteins has been documented, leading to a reduction in the yield of final ligation products. The requisite lipid-modified nucleophiles compatible with sortase-mediated transpeptidation also have a very straightforward structural requirement of a short stretch of glycine residues and are prepared by standard solid-phase synthesis. Therefore, both natural and non-natural hydrophobic modifications can be easily incorporated using this system.

0240While the requirement for the sortase enzyme itself could be perceived as a disadvantage of the transpeptidation strategy, it should be noted that we routinely express batches with yields of >50 mg/L, and stock solutions of the enzyme can be stored for several months with no apparent degradation in enzymatic activity. Moreover, in this work we provided a convenient method for removing sortase following transpeptidation. The ready availability of sortase and ease of removal following transpeptidation are important features as the enzyme is relatively inefficient and excess sortase is typically employed in the art to drive the labeling reactions to high levels of conversion in a reasonable period of time.

0241In this initial work, we used eGFP as our model protein substrate due to its natural fluorescence and verified that eGFP proteins lipidated through sortase-mediated transpeptidation strongly associate with mammalian cells. We envision further applications of this chemical modification strategy to include anchorage of proteins to cellular membranes that do not normally occur on the cell surface to create artificial cell surface receptors. Additionally, because the transpeptidation strategy is able to attach extremely hydrophobic modifications, it could provide a new method for interfacing proteins with materials that possess poor water solubility, such as carbon nanotubes.

<i>References</i>

0242<ol id="ol0001" compact="compact" ol-style=""><li>1. <nplcit id="ncit0061" npl-type="s"><text>Paulick, M. G., Forstner, M. B., Groves, J. T., and Bertozzi, C. R. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 20332-20337</text></nplcit>.</li><li>2. <nplcit id="ncit0062" npl-type="s"><text>Paulick, M. G., Wise, A. R., Forstner, M. B., Groves, J. T., and Bertozzi, C. R. J. Am. Chem. Soc. 2007, 129, 11543-11550</text></nplcit>.</li><li>3. <nplcit id="ncit0063" npl-type="s"><text>Martin, S. E., and Peterson, B. R. Bioconjugate Chem. 2003, 14, 67-74</text></nplcit>.</li><li>4. <nplcit id="ncit0064" npl-type="s"><text>Boonyarattanakalin, S., Athavankar, S., Sun, Q., and Peterson, B. R. J. Am. Chem. Soc. 2006, 128, 4917-4917</text></nplcit>.</li><li>5. <nplcit id="ncit0065" npl-type="s"><text>Covic, L., Gresser, A. L., Talavera, J., Swift, S., and Kuliopulos, A. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 643-648</text></nplcit>.</li><li>6. <nplcit id="ncit0066" npl-type="s"><text>Sun, Q., Cai, S., and Peterson, B. R. J. Am. Chem. Soc. 2008, 130- 10064-10065</text></nplcit>.</li><li>7. <nplcit id="ncit0067" npl-type="s"><text>Nelson, A. R., Borland, L., Allbritton, N. L., and Sims, C. E. Biochemistry 2007, 46, 14771- 14781</text></nplcit>.</li><li>8. <nplcit id="ncit0068" npl-type="s"><text>Moyle, P. M., and Toth, I. Curr. Med. Chem. 2008, 15, 506-516</text></nplcit>.</li><li>9. <nplcit id="ncit0069" npl-type="s"><text>BenMohamed, L., Wechsler, S. L., and Nesburn, A. B. Lancet Infect. Dis. 2002, 2, 425-431</text></nplcit>.</li><li>10. <nplcit id="ncit0070" npl-type="s"><text>Reulen, S. W., Brusselaars, W. W., Langereis, S., Mulder, W. J., Breurken, M., and Merkx, M. Bioconjugate Chem. 2007, 18, 590-596</text></nplcit>.</li><li>11. <nplcit id="ncit0071" npl-type="s"><text>Torchilin, V. P. Nat. Rev. Drug Discovery 2005, 4, 145-160</text></nplcit>.</li><li>12. <nplcit id="ncit0072" npl-type="s"><text>Ha, V. L., Thomas, G. M., Stauffer, S., and Randazzo, P. A. Methods Enzymol. 2005, 404, 164-174</text></nplcit>.</li><li>13. <nplcit id="ncit0073" npl-type="s"><text>Heal, W. P., Wickramasinghe, S. R., Bowyer, P. W., Holder, A. A., Smith, D. F., Leatherbarrow, R. J., and Tate, E. W. Chem. Commun. 2008, 480-482</text></nplcit>.</li><li>14. <nplcit id="ncit0074" npl-type="s"><text>Kalinin, A., Thoma, N. H., Iakovenko, A., Heinemann, I., Rostkova, E., Constantinescu, A. T., and Alexandrov, K. Protein Expression Purif. 2001, 22, 84-91</text></nplcit>.</li><li>15. <nplcit id="ncit0075" npl-type="s"><text>Gauchet, C., Labadie, G. R., and Poulter, C. D. J. Am. Chem. Soc. 2006, 128, 9274-9275</text></nplcit>.</li><li>16. <nplcit id="ncit0076" npl-type="s"><text>Duckworth, B. P., Zhang, Z. Y., Hosokawa, A., and Distefano, M. D. ChemBioChem 2007, 8, 98- 105</text></nplcit>.</li><li>17. <nplcit id="ncit0077" npl-type="s"><text>Breydo, L., Sun, Y., Makarava, N., Lee, C. I., Novitskaia, V., Bocharova, O., Kao, J. P., and Baskakov, I. V. Biochemistry 2007, 46, 852-861</text></nplcit>.</li><li>18. <nplcit id="ncit0078" npl-type="s"><text>Bader, B., Kuhn, K., Owen, D. J., Waldmann, H., Wittinghofer, A., and Kuhlmann, J. Nature 2000, 403, 223-226</text></nplcit>.</li><li>19. <nplcit id="ncit0079" npl-type="s"><text>Eberl, H., Tittmann, P., and Glockshuber, R. J. Biol. Chem. 2004, 279, 25058-25065</text></nplcit>.</li><li>20. <nplcit id="ncit0080" npl-type="s"><text>Hicks, M. R., Gill, A. C., Bath, I. K., Rullay, A. K., Sylvester, I. D., Crout, D. H., and Pinheiro, T. J. FEBS J. 2006, 273, 1285-1299</text></nplcit>.</li><li>21. <nplcit id="ncit0081" npl-type="s"><text>Muir, T. W. Annu. Rev. Biochem. 2003, 72, 249-289</text></nplcit>.</li><li>22. <nplcit id="ncit0082" npl-type="s"><text>Alexandrov, K., Heinemann, I., Durek, T., Sidorovitch, V., Goody, R. S., and Waldmann, H. J. Am. Chem. Soc. 2002 124 5648-5649</text></nplcit>.</li><li>23.<nplcit id="ncit0083" npl-type="s"><text> Pellois, J. P., and Muir, T. W. Angew. Chem., Int. Ed. 2005, 44, 5713-5717</text></nplcit>.</li><li>24. <nplcit id="ncit0084" npl-type="s"><text>Grogan, M. J., Kaizuka, Y., Conrad, R. M., Groves, J. T., and Bertozzi, C. R. J. Am. Chem. Soc. 2005, 127, 14383-14387</text></nplcit>.</li><li>25. <nplcit id="ncit0085" npl-type="s"><text>Gottlieb, D., Grunwald, C., Nowak, C., Kuhlmann, J., and Waldmann, H. Chem. Commun. 2006, 260-262</text></nplcit>.</li><li>26. <nplcit id="ncit0086" npl-type="s"><text>Luka, Z., and Wagner, C. Protein Expression Purif. 2003, 28, 280-286</text></nplcit>.</li><li>27. <nplcit id="ncit0087" npl-type="s"><text>Zhao, W., Zhang, Y., Cui, C., Li, Q., and Wang, J. Protein Sci. 2008, 17, 736-747</text></nplcit>.</li><li>28. <nplcit id="ncit0088" npl-type="s"><text>Cui, C., Zhao, W., Chen, J., Wang, J., and Li, Q. Protein Expression Purif. 2006, 50, 74-81</text></nplcit>.</li><li>29.<nplcit id="ncit0089" npl-type="s"><text> Tan, L. P., Lue, R. Y., Chen, G. Y., and Yao, S. Q. Bioorg. Med. Chem. Lett. 2004, 14, 6067 6070</text></nplcit>.</li><li>30. <nplcit id="ncit0090" npl-type="s"><text>Pritz, S., Kraetke, O., Klose, A., Klose, J., Rothemund, S., Fechner, K., Bienert, M., and Beyermann, M. Angew. Chem., Int. Ed. 2008, 47, 3642-3645</text></nplcit>.</li><li>31. <nplcit id="ncit0091" npl-type="s"><text>Samantaray, S., Marathe, U., Dasgupta, S., Nandicoori, V. K., and Roy, R. P. J. Am. Chem. Soc. 2008,130, 2132-2133</text></nplcit>.</li><li>32. <nplcit id="ncit0092" npl-type="s"><text>Chan, L., Cross, H. F., She, J. K., Cavalli, G., Martins, H. F. P., and Neylon, C. PLoS ONE 2007, 2, e1164</text></nplcit>.</li><li>33. <nplcit id="ncit0093" npl-type="s"><text>Tanaka, T., Yamamoto, T., Tsukiji, S., and Nagamune, T. ChemBioChem 2008, 9, 802-807</text></nplcit>.</li><li>34. <nplcit id="ncit0094" npl-type="s"><text>Popp, M. W., Antos, J. M., Grotenbreg, G. M., Spooner, E., and Ploegh, H. L. Nat. Chem. Biol. 2007, 3, 707-708</text></nplcit>.</li><li>35. <nplcit id="ncit0095" npl-type="s"><text>Parthasarathy, R., Subramanian, S., and Boder, E. T. Bioconjugate Chem. 2007, 18, 469-476</text></nplcit>.</li><li>36. <nplcit id="ncit0096" npl-type="s"><text>Pritz, S., Wolf, Y., Kraetke, O., Klose, J., Bienert, M., and Beyermann, M. J. Org. Chem. 2007, 72, 3909-3912</text></nplcit>.</li><li>37. <nplcit id="ncit0097" npl-type="s"><text>Mao, H. Protein Expression Purif. 2004, 37, 253-263</text></nplcit>.</li><li>38. <nplcit id="ncit0098" npl-type="s"><text>Mao, H., Hart, S. A., Schink, A., and Pollok, B. A. J. Am. Chem. Soc. 2004, 126, 2670-2671</text></nplcit>.</li><li>39. <nplcit id="ncit0099" npl-type="s"><text>Kruger, R. G., Otvos, B., Frankel, B. A., Bentley, M., Dostal, P., and McCafferty, D. G. Biochemistry 2004, 43, 1541-1551</text></nplcit>.</li><li>40. <nplcit id="ncit0100" npl-type="s"><text>Ton-That, H., Liu, G., Mazmanian, S. K., Faull, K. F., and Schneewind, O. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 12424-12429</text></nplcit>.</li><li>41. <nplcit id="ncit0101" npl-type="s"><text>Ton-That, H., Mazmanian, S. K., Faull, K. F., and Schneewind, O. J. Biol. Chem. 2000, 275, 9876-9881</text></nplcit>.</li><li>42. <nplcit id="ncit0102" npl-type="s"><text>Clow, F., Fraser, J. D., and Proft, T. Biotechnol. Lett. 2008, 30, 1603-1607</text></nplcit>.</li><li>43.<nplcit id="ncit0103" npl-type="s"><text> Tsien, R. Y. Annual Review of Biochemistry 1998, 67, 509-544</text></nplcit>.</li><li>44. <nplcit id="ncit0104" npl-type="s"><text>Zacharias, D. A., Violin, J. D., Newton, A. C., and Tsien, R. Y. Science 2002, 296, 913- 916</text></nplcit>.</li><li>45. <nplcit id="ncit0105" npl-type="s"><text>Sheff, D., Pelletier, L., O'Connell, C. B., Warren, G., and Mellman, I. J. Cell Biol. 2002, 156, 797-804</text></nplcit>.</li></ol>

0243Please note that methods and references described in Exampes herein are applicable to other Eexamples and aspects of the invention.

Example 2: Modification of N-Terminal Glycine Residues using Structural Mimetics of the Sortase A Recognition Sequence

0244The transpeptidation reaction catalyzed by sortase A has emerged as a general method for derivatizing proteins with a variety of modifications. Yet, in its present incarnation, it appears that the practical application of this approach has been limited to the protein C-terminus. Target proteins are engineered to contain the sortase A recognition motif (LPXTG) near their C-termini. When incubated with synthetic peptides containing one or more N-terminal glycine residues and recombinant sortase A, these artificial sortase substrates undergo a transacylation reaction resulting in the exchange of residues C-terminal to the threonine residue with the synthetic oligoglycine peptide. The use of excess oligoglycine drives this equilibrium process, and in most cases quantitative protein labeling can be achieved.

0245In simple terms, the transpeptidation reaction results in the ligation of species containing an LPXTG motif with those bearing one or more N-terminal glycine residues. We therefore became interested in whether a similar method for N-terminal labeling could be achieved by preparing protein substrates with N-terminal glycine residues and ligating them to synthetic peptides containing the LPXTG motif or structural analogs of this recognition element (<figref idref="f0009">Figure 8</figref>). We herein describe the successful development of a general strategy for site-specific labeling of proteins with one or more N-terminal glycine residues. This technique allows for the installation of modifications suitable for a range of proteomic and biological applications.

0246As noted above, synthetic peptides containing the LPXTG motif have been shown to function as substrates for sortase A, generating the acyl enzyme intermediate necessary for transpeptidation. However, at the outset of our studies we were concerned that peptides containing the entire LPXTG sequence could create a potential complication for N-terminal protein labeling. As outlined in <figref idref="f0009">Figure 8a</figref>, each successful transfer of an LPXT unit to the target protein would release a stoichiometric amount of a peptide fragment containing an N-terminal glycine. This by-product would compete with the protein nucleophile for the same acyl enzyme intermediate, thereby limiting the efficiency of the labeling reaction. Moreover, hydrolytic cleavage of an LPXTG peptide, although a relatively slow process, would exacerbate competition with the protein nucleophile as the reaction proceeded. Indeed, preliminary attempts in our laboratory to perform N-terminal protein labeling with peptides containing all five residues (LPXTG) yielded disappointing results and useful levels of protein modification could only be obtained using high concentrations (>5 mM) of the LPXTG-containing peptide (data not shown). We reasoned that a simple solution to this problem could be to substitute the glycine in the LPXTG motif with a moiety that would exhibit poor nucleophilicity once released by sortase (<figref idref="f0009">Figure 8b</figref>). With this design principle in mind, we synthesized fluorescein (<b>1</b>) and biotin (<b>2</b>) derivatives of an LPRT peptide where the glycine residue was replaced by a simple methyl ester. Compounds <b>1</b> and <b>2</b> were prepared through a combination of solid-phase and solution-phase transformations and purified to homogeneity by reversed phase-HPLC (see <figref idref="f0018 f0019">Figure 16</figref>). Arginine was placed in the X position to enhance water solubility and because a side chain that would be unreactive toward reagents used to modify the N-terminus of the LPRT peptide.

0247With <b>1</b> and <b>2</b> in hand we expressed a series of model protein substrates containing N-terminal glycine residues. It should be noted that recombinant protein bearing N-terminal glycine residues have been shown to function as nucleophiles for sortase-mediated transpeptidation, though only in the context of ligation to other proteins bearing the LPXTG motif. For our studies, three variants of the cholera toxin B-subunit (CtxB) were prepared containing 1, 3, or 5 N-terminal glycine residues. To investigate selectivity for glycine, a final construct containing an N-terminal alanine residue was also prepared. Following some brief optimization, we were able to arrive at conditions that yielded quantitative labeling of constructs containing 3 or 5 glycines using 500 µM <b>1</b> with only trace labeling of the alanine-containing control. As shown in <figref idref="f0010">Figure 9a</figref>, a fluorescence gel scan revealed robust labeling of CtxB if 3 or 5 glycines were present. No background labeling was observed in the absence of sortase A. For CtxB constructs containing 3 or 5 glycines residues, quantitative labeling was evident after 2 h at 37 °C as determined by ESI-MS (<figref idref="f0010">Figure 9b</figref>, and <figref idref="f0020 f0021">Figure 17</figref>). Similar labeling experiments with biotinylated derivative <b>2</b> GGGGG-CtxB yielded identical results and the resulting conjugates could be detected by streptavidin immunoblot (<figref idref="f0022">Figure 18</figref>) and conjugates were characterized using ESI-MS. Conditions used for experiment shown in <figref idref="f0022">Fig. 18</figref> were: 33 µM G₅-CtxB, 50 mM SrtAstaph, 500 µM <b>2</b>, 50 mM Tris pH 7.5, 150 mM NaCl, 10 mM CaCl2, 2h at 37 degrees C. In all cases, residual labeling of the sortase enzyme itself could be detected. This was expected because the transpeptidation reaction proceeds through a covalent acyl enzyme intermediate.

0248N-terminal transpeptidation was easily extended to two additional protein substrates (<figref idref="f0011">Figure 10</figref>). eGFP bearing five glycines and the ubiquitin specific hydrolase UCH-L3 containing a single glycine were labeled efficiently as determined by ESI-MS. It should be noted that the utility of this labeling method for recombinant proteins expressed in <i>E. coli</i> is facilitated by natural processing of newly synthesized polypeptides by endogenous methionine aminopeptidase. When the initiator N-formylmethionine (N-fmet) is followed by small residues such as glycine, the removal of N-fmet is quantitative. Therefore, preparation of substrates for N-terminal transpeptidation requires only basic cloning techniques, and in most cases would only require a single residue substitution.

Reference Example 3: Highly Efficient Sortase-Catalyzed Protein Circularization

0249This Example demonstrates that sortase-catalyzed transpeptidation can be used for efficient synthesis of circular and oligomeric proteins. This method has general applicability, as illustrated by successful intramolecular reactions with three structurally unrelated proteins. In addition to circularization of individual protein units, the multiprotein complex AAA-ATPase p97/VCP/CDC48, with six identical subunits containing the LP<i>X</i>TG motif and an N-terminal glycine, was found to preferentially react in daisy chain fashion to yield linear protein fusions (<nplcit id="ncit0106" npl-type="s"><text>Antos, JM, et al. "A straight path to circular proteins" J Biol Chem. ;284(23):16028-36, 2009</text></nplcit>).

Synthesis of Triglycine Tetramethylrhodamine Peptide.

Materials and Methods

Cloning and Protein Expression

0250Recombinant sortase A (residues 26-206) containing an N-terminal hexahistidine tag was produced in <i>Escherichia coli</i> as described previously (8). Purified sortase A was stored in 10% glycerol, 50 mM Tris, pH 8.0, 150 NaCl at -80 °C until further use.

0251G-Cre-LPETG-His₆ was cloned into the pTriEx-1.1 Neo expression vector (Novagen) using standard molecular biology techniques. The construct contains two point mutations (M117V and E340Q) and a flexible spacer (GGGGSGGGGS) inserted before the LPETG sortase recognition site. G-Cre-LPETG-His₆ was expressed and purified using procedures similar to those reported previously for HTNCre (24). G-Cre-LPETG-His₆ was first transformed into Tuner (DE3) pLacI cells (Novagen), and a starter culture was grown in sterile LB media supplemented with 1% (w/v) glucose, chloramphenicol (34 µg/ml), and ampicillin (100 µg/ml). This culture was used to inoculate a large scale culture of sterile LB containing chloramphenicol (34 µg/ml) and ampicillin (100 µg/ml). G-Cre-LPETG-His₆ was expressed after a 3-h induction with isopropyl 1-thio-β-D-galactopyranoside (0.5 mM) at 37 °C. Cells were resuspended in 10 mM Tris, 100 mM phosphate, 300 mM NaCl, and 20 mM imidazole, pH 8.0. The suspension was adjusted to 50 µg/ml DNase I, 460 µg/ml lysozyme, and 1 mM MgCl₂ and incubated at 4 °C for 1.5 h. The suspension was then sonicated and centrifuged. The clarified lysate was then treated with Ni-NTA-agarose (Qiagen) for 1 h at 4 °C. The resin was washed with 12 column volumes of 10 mM Tris, 100 mM phosphate, 300 mM NaCl, and 20 mM imidazole, pH 8.0, followed by 4 column volumes of 10 mM Tris, 100 mM phosphate, 300 mM NaCl, and 30 mM imidazole, pH 8.0. The protein was then eluted with 10 mM Tris, 100 mM phosphate, 300 mM NaCl, and 300 mM imidazole, pH 8.0. The purified protein was then dialyzed first against 20 mM Tris, pH 7.5, 500 mM NaCl followed by 50% glycerol, 20 mM Tris, pH 7.5, 500 mM NaCl. G-Cre-LPETG-His₆ was then passed through a 0.22-µm filter to remove minor precipitation and stored at 4 °C.

0252G₅-eGFP-LPETG-His₆ was prepared from a previously reported eGFP construct lacking the five N-terminal glycine residues using a QuickChange® II site-directed mutagenesis kit (Stratagene) and produced in <i>E. coli</i> using reported procedures (11). Purified G₅-eGFP-LPETG-His₆ was buffer exchanged into 20 mM Tris, pH 8.0, 150 mM NaCl and stored at 4 °C. UCHL3 with the sortase recognition sequence (LPETG) substituted for amino acids 159-163 was cloned and produced in <i>E</i>. <i>coli</i> as described previously (25).

0253Human p97 (806 amino acids) was PCR-amplified and cloned via the NdeI and HindIII restriction sites into a pET28a+ expression vector (Novagen) to yield the G-His₆-p97 construct. G-His₆-p97-LPSTG-<i>XX</i> was generated by introducing two point mutations (G782L and Q785T) and a stop codon at position 791 using QuickChange® mutagenesis (Stratagene). Recombinant p97 was expressed at 30 °C in <i>E</i>. <i>coli</i> after induction for 3 h with 0.5 mM isopropyl 1-thio-β-D-galactopyranoside. Cells were resuspended in buffer A (50 mM Tris, pH 8.0, 300 mM NaCl, 5% glycerol, 20 mM imidazole, and 7.1 mM β-mercaptoethanol), adjusted to 15 µg/ml lysozyme and 10 µg/ml DNase I, and lysed by two passes through a French pressure cell at 1200 p.s.i. After centrifugation for 30 min at 40,000 × <i>g</i>, the supernatant was bound to nickel-Sepharose resin (GE Healthcare). After washing the resin with 20 column volumes of buffer A, p97 was eluted with buffer A containing 250 mM imidazole. Hexameric rings of p97 were further purified on a Superdex 200 HR 16/60 column (GE Healthcare) using 25 mM Tris, pH 8.0, 150 mM KCl, 2.5 mM MgCl₂, 5% glycerol as the mobile phase. The purified protein was snap-frozen and stored at -80 °C.

Circularization and Intermolecular Transpeptidation

0254Transpeptidation reactions were performed by combining the necessary proteins/reagents at the specified concentrations in the presence of sortase reaction buffer (50 mM Tris, pH 7.5, 150 mM NaCl, 10 mM CaCl₂) and incubating at 37 °C for the times indicated. Diglycine and triglycine (GGG) peptides were purchased from Sigma. Reactions were halted by the addition of reducing Laemmli sample buffer and analyzed by SDS-PAGE. Gels were visualized by staining with Coomassie Blue. Fluorescence was visualized on a Typhoon 9200 Imager (GE Healthcare). Crude reactions were also diluted into either 0.1% formic acid or water for ESI-MS analysis. ESI-MS was performed on a Micromass LCT mass spectrometer (Micromass® MS Technologies) and a Paradigm MG4 HPLC system equipped with an HTC PAL autosampler (Michrom BioResources) and a Waters symmetry 5-µm C8 column (2.1 × 50 mm, MeCN:H₂O (0.1% formic acid) gradient mobile phase, 150 µl/min).

Purification and Refolding of eGFP

0255G₅-eGFP-LPETG-His₆ (50 µM) was circularized by treatment with sortase A (50 µM) in sortase reaction buffer (50 mM Tris, pH 7.5, 150 mM NaCl, 10 mM CaCl₂) for 24 h at 37 °C. The reaction was run on a 750-µl scale. The entire reaction was then diluted into 10 ml of 20 mM Tris, 500 mM NaCl, and 20 mM imidazole, pH 8.0. This solution was then applied to a column consisting of 2 ml of Ni-NTA-agarose (Qiagen) pre-equilibrated with 20 mM Tris, 500 mM NaCl, and 20 mM imidazole, pH 8.0. The flow-through was then concentrated and buffer exchanged in 20 mM Tris, 150 mM NaCl, pH 8.0, using a NAP™ 5 Sephadex™ column (GE Healthcare). The concentrations of circular eGFP and linear G₅-eGFP-LPETG-His₆ were estimated by UV-visible spectroscopy using the absorbance of eGFP at 488 nm (extinction coefficient 55,900 M^-1 cm^-1) (<u>26</u>). Circular and linear eGFP (40 µl of 18 µM solutions) was placed in 1.5-ml microcentrifuge tubes and denatured by heating to 90 °C for 5 min. Samples were then incubated at room temperature in the dark for the times indicated. Fluorescent images were acquired using a UV gel documentation system (UVP Laboratory Products).

Reaction of Cyclic UCHL3 with Activity-based Ubiquitin Probe

0256UCHL3 (30 µM) was incubated with sortase A (150 µM) in sortase reaction buffer (50 mM Tris, pH 7.5, 150 mM NaCl, 10 mM CaCl₂) in the presence or absence of 90 mM GGG peptide (Sigma) on a 25-µl scale at 37 °C for 3 h. Ten microliters was withdrawn and diluted with 10 µl of labeling buffer (100 mM Tris, pH 7.5, 150 mM NaCl). Hemagglutinin epitope-tagged ubiquitin vinyl methyl ester (4 µg) was added as well as 1 mM dithiothreitol and incubated at room temperature for 1 h. Reactions were then separated on an SDS-polyacrylamide gel and visualized by Coomassie staining or α-HA immunoblot. Hemagglutinin epitope-tagged ubiquitin vinyl methyl ester was prepared following published protocols.

MS/MS Sequencing of Proteolytic Fragments from Circular Proteins

0257Prior to MS/MS analysis, circular eGFP and Cre were separated from sortase A by RP-HPLC using an Agilent 1100 Series HPLC system equipped with a Waters Delta Pak 5 µm, 100 Å C18 column (3.9 × 150 mm, MeCN:H₂O gradient mobile phase containing 0.1% trifluoroacetic acid, 1 ml/min). Fractions containing the circular proteins were pooled and subjected to trypsin digestion. Crude transpeptidation reactions containing circular UCHL3 were separated by SDS-PAGE followed by Coomassie staining. The band corresponding to circular UCHL3 was excised and digested with Glu-C. Crude transpeptidation reactions containing dimeric p97 were separated by SDS-PAGE followed by Coomassie staining. The transpeptidation reaction used for this purpose was incubated for only 2 h and therefore contains less oligomerization than that seen after an overnight incubation. The band corresponding to dimeric p97 was excised and digested with chymotrypsin. For all protein substrates, the peptides generated from proteolytic digestion were extracted and concentrated for analysis by RP-HPLC and tandem mass spectrometry. RP-HPLC was carried out on a Waters NanoAcquity HPLC system with a flow rate of 250 nl/min and mobile phases of 0.1% formic acid in water and 0.1% formic acid in acetonitrile. The gradient used was isocratic 1% acetonitrile for 1 min followed by 2% acetonitrile per min to 40% acetonitrile. The analytical column was 0.075 µm × 10 cm with the tip pulled to 0.005 µm and self-packed with 3 µm Jupiter C18 (Phenomenex). The column was interfaced to a Thermo LTQ linear ion trap mass spectrometer in a nanospray configuration, and data were collected in full scan mode followed by MS/MS analysis in a data-dependent manner. The mass spectral data were data base searched using SEQUEST.

Construction of Molecular Models

0258Molecular models were generated from published crystal structures (PDB codes <u>1kbu</u>, <u>1gfl</u>, <u>1xd3</u>, and <u>3cf1</u>). N- and C-terminal residues were added using Coot 0.5. Protein termini were repositioned using the Auto Sculpting function in MacPyMOL (DeLano Scientific LLC). Residues visible in the published crystal structures were not moved during positioning of the extended N and C termini. All protein images in this study were generated using MacPyMOL.

Results

0259<i>Cre Recombinase.</i> We first noticed the presence of a circular protein product when installing a C-terminal modification onto a nonfunctional mutant of Cre recombinase containing a single N-terminal glycine residue and an LPETG sequence near the C terminus. The LPETG motif was separated from the native protein by a flexible amino acid linker (GGGGSGGGGS). Whereas installation of the label at the Cre C terminus proceeded efficiently when a triglycine nucleophile containing tetramethylrhodamine (GGG-TMR) was included, we observed a product that migrated more rapidly on SDS-PAGE when nucleophile was omitted from the reaction mixture (<figref idref="f0032 f0033">Fig. 26</figref>). Hydrolysis of the sortase acyl enzyme is known to proceed slowly in the absence of glycine nucleophiles. However, when reaction mixtures were analyzed by ESI-MS, we consistently observed a protein species that differed from the mass expected for hydrolysis by approximately -18 Da (<figref idref="f0003"><u>Fig. 2</u></figref><i>B</i>). This mass was consistent with intramolecular nucleophilic attack, suggesting that the single N-terminal glycine residue was serving as the nucleophile in this transformation. Ultimately, MS/MS on tryptic digests of this species showed unequivocally that it consisted of a covalently closed circular product of Cre, with the N-terminal glycine fused exactly at the LPETG cleavage site in the expected position (<figref idref="f0033">Fig. 26C</figref>). Recognizing that the LPETG motif is maintained in the cyclized Cre product, we suspected that sortase should be capable of cleaving the circular protein at this site, thus producing an equilibrium between circular and linear forms of Cre. To demonstrate this point, Cre was first incubated with sortase in the presence or absence of triglycine nucleophile (<figref idref="f0034">Fig. <i>27A</i></figref>). A portion of the cyclized reaction mixture (<figref idref="f0034">Fig. <i>27A</i></figref>, <i>lane 1</i>) was then treated with a large molar excess of triglycine nucleophile or left alone for a further 24 h (<figref idref="f0034">Fig. <i>27A</i></figref>, <i>lanes 2</i> and <i>3</i>). Remarkably, upon treatment with exogenous nucleophile, the pre-cyclized material yielded a reaction mixture that was nearly identical to the result obtained when nucleophile was included from the very beginning of the experiment (<figref idref="f0034">Fig. 27A</figref>, compare lanes 3 and 4). This result provided further evidence that cyclized Cre indeed contains the expected LPETG motif at the site of covalent closure. In addition, it suggested that hydrolysis of the acyl enzyme intermediate does not effectively compete during cyclization, because the hydrolyzed material should be unable to participate in the transpeptidation reaction. The circularization reaction observed for Cre proceeded with remarkable efficiency. Conversion was estimated to be >90% by SDS-PAGE. By taking an existing crystal structure of the Cre protein and modeling in those residues not visible in the structure, it was clear that the N and C termini were located in sufficiently close proximity to permit closure without significant perturbation of the native structure (<figref idref="f0034">Fig. 27B</figref>). We assume that these regions possess considerable flexibility because they are not resolved in the crystal structure.

0260<b>eGFP.</b> Having verified the cyclization of Cre recombinase, we sought to explore the generality of this technique. To this end we generated a derivative of eGFP containing the LPETG sequence and five N-terminal glycine residues. This construct was of particular interest because inspection of the x-ray crystal structure revealed that the N and C termini were positioned on the same end of the β-barrel, suggesting that this substrate should be ideal for cyclization (<figref idref="f0035">Fig. <i>28A</i></figref>). Furthermore, in one of the earliest reports on the use of sortase for protein engineering, a similar eGFP substrate was described and reported to cyclize in the presence of sortase. In this instance, cyclization only proceeded in modest yield, and the putative cyclized product was produced as a mixture with higher molecular weight species assigned as oligomers of eGFP formed by intermolecular transpeptidation. Thus, to explore potential complications caused by intermolecular reactions, we studied the reaction of our eGFP construct in the presence of sortase.

0261In our hands, we observed clean conversion to a lower molecular weight species (>90% estimated conversion) with little to no evidence for oligomerization (<figref idref="f0035">Fig. <i>28B</i></figref>). A higher molecular weight polypeptide was observed at early time points and may represent a covalent eGFP dimer that is generated transiently over the course of the reaction. Higher molecular species, however, were only observed in trace quantities in the final reaction mixture. As in the case of Cre, evidence for circularization was provided by mass spectral characterization of the intact circular protein and MS/MS sequencing of tryptic peptides. As an additional control to demonstrate that the N-terminal glycine residue was the only nucleophile participating in intramolecular transpeptidation, we analyzed the behavior of an eGFP derivative that lacked an N-terminal glycine. In this case, ESI-MS revealed products consistent with hydrolysis of the acyl enzyme intermediate, rather than intramolecular nucleophilic attack.

0262Circularization has been shown to confer unique properties onto proteins when compared with the linear form. In the case of GFP circularized using intein-based methods, these properties include a reduced rate of unfolding when exposed to denaturants, as well as an enhanced rate of refolding following denaturation. We observed a similar phenomenon for eGFP circularized using sortase (<figref idref="f0036">Fig. <i>28C</i></figref>). Circular eGFP was first separated from residual sortase A using Ni-NTA resin. This material retained fluorescence suggesting that covalent ligation of the N and C termini had minimal impact on the structure of this substrate. Circular and linear eGFP were then subjected to simple thermal denaturation, followed by recovery at room temperature. As shown in <figref idref="f0035 f0036">Fig. 28</figref><i>C</i>, circular eGFP regained fluorescence more rapidly than linear eGFP.

0263<b>UCHL3.</b> Even an internally positioned LP<i>X</i>TG motif was sufficient to effectuate a circularization reaction. We installed a sortase recognition site in the crossover loop of the ubiquitin C-terminal hydrolase UCHL3, and we demonstrated that the continuity of the polypeptide backbone can be disrupted with concomitant installation of a covalent modification that reports on the accuracy of cleavage and transpeptidation. This reaction proceeds without complete loss of activity of UCHL3, indicating that even the cleaved form of UCHL3 retains its structural integrity to a significant degree. This UCHL3 construct was prepared with an N-terminal glycine residue, and examination of the crystal structure of UCHL3 clearly showed the close apposition of the N terminus and the crossover loop, suggesting that cyclization to yield a circular fragment containing the N-terminal portion of UCHL3 should be readily observable (<figref idref="f0037">Fig. <i>29A</i></figref>).

0264As expected, in the absence of added nucleophile, the N-terminal glycine serves as a highly efficient nucleophile to yield a circular fragment that contains the N-terminal portion of UCHL3 (<figref idref="f0037">Fig. <i>29B</i></figref>). The identity of the circular polypeptide was confirmed by MS/MS of the peptide containing the expected fusion of the N-terminal glycine residue with the new C terminus released from the crossover loop. Cyclization was efficiently blocked if a high concentration of triglycine (GGG) was included in the reaction, generating instead the N-terminal fragment of UCHL3 transacylated onto the triglycine nucleophile (<figref idref="f0037">Fig. <i>29B</i></figref>, <i>lane 9</i>). Cyclization could also be reversed by adding an excess of triglycine to reaction mixtures preincubated with sortase to allow cyclization. This reopening reaction was observed by both SDS-PAGE and ESI-MS.

0265To test the functional properties of cyclic UCHL3, we incubated reaction mixtures with an activity-based probe consisting of ubiquitin equipped with an electrophilic vinyl methyl ester moiety at the C terminus. Probes of this nature are able to specifically alkylate active site cysteine residues in ubiquitin-specific hydrolases such as UCHL3. Following circularization, the active site cysteine (Cys-95) of UCHL3 is located in the circular N-terminal fragment, and indeed we observed covalent labeling of this fragment with a corresponding shift in apparent molecular weight consistent with the attachment of ubiquitin. This result suggests that despite cleavage of the polypeptide backbone, the circular N-terminal fragment of UCHL3 and the C-terminal portion released during transpeptidation remain associated and preserve the affinity of UCHL3 for ubiquitin. This result is consistent with previous observations from our laboratory demonstrating that covalent closure of the UCHL3 crossover loop is dispensable for enzyme activity.

0266<b>p97.</b> The examples described above in this Example concern single chain proteins whose termini are sufficiently close to allow covalent closure by means of the sortase-mediated transacylation reaction. Similar proximity relationships between protein termini should also be present on separate polypeptides that assemble into defined oligomeric structures. As an example, we examined p97, a hexameric AAA-ATPase. We generated a derivative of p97 (G-His₆-p97-LPSTG-<i>XX</i>) containing an LPSTG motif near the C terminus, and a hexahistidine tag capped by two serine residues and a single glycine at the N terminus. The structure of a p97 trimer in the presence of ADP has been solved at 3.5 Å resolution, with several residues from the N and C termini not visible (<figref idref="f0038">Fig. <i>30A</i></figref>). When all the residues present in our modified version of p97 were modeled onto the published trimer of p97, it was evident that the N and C termini of adjacent p97 units were sufficiently close to permit covalent cross-linking (<figref idref="f0038">Fig. <i>30B</i></figref>). G-His₆-p97-LPSTG-<i>XX</i> was expressed in <i>E. coli</i> and yielded the hexameric p97 ring, as assessed by gel filtration. As expected, this derivative of p97 was an excellent substrate for transpeptidation at its C terminus, allowing efficient installation of a label when incubated in the presence of sortase and GGG-TMR. In contrast, a variant of p97 lacking the LPSTG sequence showed no labeling. When G-His₆-p97-LPSTG-<i>XX</i> was treated with sortase A in the absence of added nucleophile, we observed formation of an SDS-resistant ladder of polypeptides, as would be expected for intermolecular cross-linking of p97 monomers (<figref idref="f0038">Fig. <i>30C</i></figref>). We were confident that these species arise from head-to-tail ligation of p97 because introduction of excess diglycine peptide after oligomerization caused collapse of the higher molecular weight structures back to monomeric p97 (<figref idref="f0038">Fig. <i>30C</i></figref>, <i>lane 5</i>). This suggested that the higher order aggregates are held together by newly formed LPSTG units formed from the C-terminal LPST residues of one p97 monomer and the N-terminal glycine residue of a neighboring monomer. The banding pattern observed for reopening was also nearly identical to that seen when diglycine was included from the very beginning of the experiment, a scenario where installation of diglycine at the C terminus of each p97 subunit is presumed to be the major reaction pathway (<figref idref="f0038">Fig. <i>30C</i></figref><i>, lane 6</i>). We have also been able to identify peptides consistent with intermolecular cross-linking of p97 subunits by MS/MS.

0267In this Example we have explored transpeptidation reactions using four structurally diverse protein substrates. Cyclization has been confirmed for three proteins, including an example (UCHL3) utilizing an LP<i>X</i>TG sequence positioned in a flexible internal loop rather than near the C terminus of the protein. Cyclization and oligomerization via sortase-mediated transpeptidation have been previously suggested to occur for an eGFP construct modified in a manner similar to that used here (<u>16</u>), and for a by-product from a protein purification system where the substrate circularized appears to be sortase A itself (<u>20</u>). In both cases, the identity of the circular products was not rigorously confirmed. Our data identify the circular or oligomeric products unambiguously by MS/MS for all substrates studied. We also find that our eGFP derivative strongly favors cyclization over oligomerization, showing little evidence for the formation of higher order structures that might be expected by the head-to-tail ligation of termini from separate eGFP monomers. Subtle differences in the structure of the eGFP constructs cannot be overlooked as a potential cause for the observed results. For example, our eGFP is extended at the N terminus by only five glycine residues, whereas the construct studied by Parthasarathy <i>et al.</i>(<u>16</u>) contains an additional 17 residues, including 3 N-terminal glycines. Future work will be required to thoroughly characterize the effect of distance relationships between protein termini on favoring intra- <i>versus</i> intermolecular transpeptidation. With respect to protein cyclization, sortase-mediated circularization is efficient despite the potential for competing reaction pathways. In the absence of added oligoglycine nucleophile, these include hydrolysis of the acyl enzyme intermediate, reattachment of the C-terminal protein fragment that is lost upon initial cleavage of the protein substrate by sortase, or, as mentioned above, oligomerization of protein monomers in head-to-tail fashion. Even when oligoglycine nucleophile is added with the intent of blocking the cyclization pathway, millimolar concentrations are necessary to efficiently compete with cyclization. One factor that certainly must contribute to this observed preference for cyclization is the distance between protein termini. Inspection of the data base of PDB shows that nearly one-third of proteins with known structures have their termini in rather close apposition (within 20 Å) (<u>40</u>). The LP<i>X</i>TG sequence itself spans roughly 15 Å in an extended conformation, suggesting that circularization via sortase-catalyzed transpeptidation might be amenable to a significant fraction of proteins using the LP<i>X</i>TG sequence alone to bridge the gap between N and C termini. Larger distances could simply be covered by inserting flexible amino acid spacers at either termini. We also consider it likely that the circularized version of a protein will show more restricted mobility in the segment that corresponds to the newly established LP<i>X</i>TG connection between its termini. This fact alone may render the circular product a comparatively worse substrate for sortase and therefore assist in driving the transpeptidation reaction toward cyclization. As evidence for this point, we have observed previously that sortase fails to cleave LP<i>X</i>TG motifs placed in structured loops of class I major histocompatibility complex molecules.

0268The sortase-catalyzed approach also provides additional levels of control over the ensuing transpeptidation reaction. This may be particularly useful for oligomeric species, such as the p97 example described here. Specifically, our modified p97 protein (G-His₆-p97-LPSTG-<i>XX</i>) is produced in a form that is by itself unreactive. This allows protein expression and the subsequent assembly and purification of the hexamer to be completed first, without complications caused by premature covalent oligomerization. Cross-linking is then induced by the addition of sortase after the individual subunits have been correctly positioned in the hexameric ring. The extent of transpeptidation can be further controlled by inclusion of synthetic oligoglycine nucleophiles, either during the transpeptidation reaction or after transpeptidation is complete. The latter scenario even allows cyclization to be completely reversed. Incubating circular protein products with sortase in the presence of an oligoglycine nucleophile restores linearity to the protein product, because in the course of the initial cyclization reaction, the LP<i>X</i>TG motif is restored. An equilibrium between closed and open forms is thus established and can be driven toward the linear state by adding a large excess of the oligoglycine nucleophile.

Reference Example 4: Dual Modification Using Distinct Sortases

Materials and Methods.

0269<b>SrtAstrep.</b> The expression plasmid for SrtAstrep (residues 82-249) including an N-terminal His6 tag has been described.³ The construct was transformed into <i>E. coli</i> BL-21. Cells were grown in 2 L of sterile LB containing kanamycin (30 µg/mL) to an optical density of ∼0.7 at 600 nm. Cells were induced with IPTG (1 mM) for 3 h at 37 °C. Cells were harvested by centrifugation and the pellet was stored overnight at -20 °C. The pellet was thawed and resuspended in 70 mL of 50 mM Tris pH 8.0, 150 mM NaCl, 20 mM imidazole and 10% glycerol. Cells were then treated with 300 L of DNAse I (10 mg/mL in PBS), 500 L of lysozyme (50 mg/mL in PBS), and 10 L of MgCl2 (1 M in PBS). The lysis reaction was incubated for 1 h at 4 °C. The cells were then sonicated and centrifuged to remove insoluble material. The clarified lysate was then applied to a Ni-NTA column consisting of 5.0 mL of commercial Ni-NTA slurry (Qiagen) equilibrated with 50 mM Tris pH 8.0, 150 mM NaCl, 20 mM imidazole, and 10% glycerol. The column was washed with 80 mL of 50 mM Tris pH 8.0, 150 mM NaCl, 20 mM imidazole, and 10% glycerol. Protein was eluted with five 5 mL portions of 50 mM Tris pH 8.0, 150 mM NaCl, 300 mM imidazole, and 10% glycerol. Fractions containing SrtAstrep were pooled and further purified by size exclusion chromatography on a HiLoad 16/60 Superdex 75 column (Amersham), eluting with 20 mM Tris pH 8.0, 150 mM NaCl at a flow rate of 1 mL/min. Fractions containing SrtAstrep were pooled and subjected to a second round of Ni-affinity chromatography. Purified SrtAstrep was then dialyzed against 50 mM Tris pH 8.0, 150 mM NaCl, and 10% glycerol. These solutions were stored at -80 °C until further use. Protein concentration was estimated by Bradford assay.

0270<b>SrtAstaph.</b> Recombinant SrtAstaph (residues 26-206) containing an N-terminal His6 tag was produced in <i>E. coli</i> as previously described.⁴ SrtAstaph does not contain an N-terminal glycine residue (retains initiator methionine). Purified SrtAstaph was stored in 10% (w/v) glycerol, 50 mM Tris pH 8.0, 150 NaCl at -80 °C until further use. Protein concentration was estimated by Bradford assay.

0271<b>Δ59-SrtAstaph.</b> Recombinant Δ59-SrtAstaph (residues 60-206) containing an N-terminal His6 tag was cloned into pET28a+. 59-SrtAstaph does not contain an N-terminal glycine residue (retains initiator methionine). Expression of Δ59-SrtAstaph was achieved following the protocol described above for SrtAstrep. Purification by size exclusion chromatography was not necessary because Δ59-SrtAstaph was sufficiently pure following Ni-affinity chromatography.

0272<b>CtxB.</b> The template for construction of G1-CtxB, G3-CtxB, G5-CtxB, and AG4-CtxB consisted of the B-subunit of cholera toxin fused at its N terminus to the signal peptide sequence of <i>E. coli</i> heat labile enterotoxin LTIIb. ⁵ This targets the expressed protein to the periplasm where the signal peptide is removed. Glycine and/or alanine residues were inserted between the signal sequence and CtxB via Quickchange® II Site-Directed Mutagenesis (Stratagene). Plasmids were transformed into <i>E. coli</i> BL21. Cells were grown in 1 L of sterile LB containing chloroamphenicol (34 µg/mL) to an optical density of ∼0.5-1.0 at 600 nm. Cells were induced with arabinose (0.25% w/v) for 3 h at 37 °C. Cells were harvested by centrifugation and the pellet was stored overnight at -20 °C. The pellet was thawed and resuspended in 30 mL of 50 mM Tris pH 8.0 and 300 mM NaCl. This suspension was then treated with 3 mL of polymixin B solution (5 mg/mL freshly made in water). This was mixture was gently stirred at room temperature for 1 h and then centrifuged. The clarified lysate was treated with 2.5 mL of Ni-NTA slurry (Qiagen). CtxB has a naturally affinity for Ni-NTA although it does not possess a His6 tag. The Ni-NTA mixture was incubated at 4 °C for 1 h and then poured into a fritted plastic column (Bio-Rad). The resin was washed with 40 mL of 50 mM Tris pH 8.0 and 300 mM NaCl. Protein was eluted with two 10 mL portions of 50 mM Tris pH 8.0, 300 mM NaCl, and 300 mM imidazole. Purified CtxB was buffer exchanged into 20 mM Tris pH 8.0 and 150 mM NaCl. Solutions were stored at 4 °C. Protein concentration was estimated by Bradford assay.

0273<b>G5-eGFP and dual labeling eGFP substrate.</b> G5-eGFP (containing a C-terminal His6 tag) and the eGFP dual labeling substrate (containing an N-terminal thrombin cleavage site and a C-terminal LPETG motif followed by a His6 tag) and were prepared in pET28a+ (Novagen) using a Quickchange® II Site-Directed Mutagenesis Kit (Stratagene). The template plasmid used for mutagenesis has been described.⁶ Plasmids were then transformed into <i>E. coli</i> BL-21. In a typical experiment, cells were grown in sterile LB containing kanamycin (30 µg/mL) to an optical density of ∼0.6-0.9 at 600 nm. Cells were induced with IPTG (1 mM) for 3 h at 37 °C. Cells were harvested by centrifugation and the pellet was stored overnight at -20 °C. The pellet was thawed and resuspended in 20 mM Tris pH 8.0, 150 mM NaCl, 20 mM imidazole and 1% NP-40. The cell suspension was then lysed by French press and centrifuged. The clarified lysate was then applied to a Ni-NTA column consisting of 5.0 mL of commercial Ni-NTA slurry (Qiagen) equilibrated with 20 mM Tris pH 8.0, 150 mM NaCl, 20 mM imidazole and 1% NP-40. The column was washed with 40 mL of 20 mM Tris pH 8.0, 150 mM NaCl, 20 mM imidazole and 1% NP-40, followed by 40 mL of 20 mM Tris pH 8.0, 150 mM NaCl, and 20 mM imidazole. Protein was eluted with 20 mM Tris pH 8.0, 150 mM NaCl, and 300 mM imidazole until the characteristic green color was fully removed from the column. This material was concentrated and further purified by size exclusion chromatography on a HiLoad 16/60 Superdex 75 column (Amersham), eluting with 20 mM Tris pH 8.0, 150 mM NaCl at a flow rate of 1 mL/min. Fractions containing eGFP were pooled and subjected to a second round of Ni-affinity chromatography. Purified eGFP was then buffer exchanged into 20 mM Tris pH 8.0, 150 mM NaCl using a PD-10 Sephadex™ column (GE Healthcare), concentrated, and treated with glycerol (10% v/v final concentration). These solutions were stored at -80 °C until further use. Protein concentration was estimated by UV-vis spectroscopy using the absorbance of eGFP at 488 nm (extinction coefficient 55,900 M^-1cm^-1).⁷

0274<b>UCHL3 and dual labeling UCHL3 substrate.</b> UCHL3 containing a single N-terminal glycine residue was produced in <i>E</i>. <i>coli</i> as described previously.⁸ This construct (in pET28a+, Novagen) was then used to prepare the dual labeling UCHL3 substrate. Synthetic 5'-phosphorylated oligonucleotide duplexes containing appropriate sticky ends were designed to achieve insertion of an N-terminal thrombin site and a C-terminal LPETG sequence separated from UCHL3 by a GGGGSGGGGS spacer in two sequential cloning steps. Duplexes were annealed before ligation into the parent vector. The C-terminal insertion was performed first using the PstI and XhoI restriction sites. The result of using the XhoI site was the addition of a His6-tag after the LPETG sequence. The N-terminal insertion was then achieved using the XbaI and NdeI restriction sites. This plasmid was then transformed into <i>E</i>. <i>coli</i> BL21. Cells were grown in sterile LB containing kanamycin (30 µg/mL) to an optical density of ∼0.6-0.9 at 600 nm.

0275Cells were induced with IPTG (1 mM) for 3 h at 37 °C. Bacteria were then harvested by centrifugation and the pellet was stored overnight at -20 °C. The pellet was thawed and resuspended in 20 mM Tris pH 8.0, 150 mM NaCl, 20 mM imidazole and 1% NP-40. The cell suspension was then lysed by French press and centrifuged. The clarified lysate was then applied to a Ni-NTA column consisting of 5.0 mL of commercial Ni-NTA slurry (Qiagen) equilibrated with 20 mM Tris pH 8.0, 150 mM NaCl, 20 mM imidazole and 1% NP-40. The column was washed with 40 mL of 20 mM Tris pH 8.0, 150 mM NaCl, 20 mM imidazole and 1% NP-40, followed by 40 mL of 20 mM Tris pH 8.0, 150 mM NaCl, and 20 mM imidazole. Protein was eluted with 20 mM Tris pH 8.0, 150 mM NaCl, and 300 mM imidazole. This material was then purified by anion-exchange chromatography on a Mono Q 5/50 GL column (Amersham) [Buffer A (50 mM Tris pH 7.5, 5 mM DTT, 0.5 mM EDTA), Buffer B (50 mM Tris pH 7.5, 5 mM DTT, 0.5 mM EDTA, 500 mM NaCl), 1.5 mL/min, gradient: 100% Buffer A (0-15 mL), 0% Buffer B → 50% Buffer B (15-45 mL), 50% Buffer B (45-50 mL)]. Fractions containing UCHL3 were pooled and further purified by size exclusion chromatography on a HiLoad 16/60 Superdex 75 column (Amersham), eluting with 20 mM Tris pH 8.0, 150 mM NaCl at a flow rate of 1 mL/min. Fractions containing UCHL3 were pooled and subjected to a final purification step by anion-exchange chromatography on a Mono Q 5/50 GL column (Amersham) [Buffer A (50 mM phosphate pH 6.0), Buffer B (50 mM phosphate pH 6.0, 500 mM NaCl), 1.5 mL/min, gradient: 100% Buffer A (0-15 mL), 0% Buffer B → 50% Buffer B (15-45 mL), 50% Buffer B (45-50 mL)]. The dual labeling UCHL3 substrate was buffer exchanged into 20 mM Tris pH 8.0, 150 NaCl and protein concentration was estimated by Bradford assay.

0276<b>N-terminal labeling.</b> N-terminal transpeptidation reactions were performed by combining the necessary proteins/reagents at the specified concentrations in the presence of SrtAstaph or Δ59-SrtAstaph in sortase reaction buffer (50 mM Tris pH 7.5, 150 mM NaCl, 10 mM CaCl2) and incubating at 37 °C for the times indicated. Reactions were either diluted with 2x reducing Laemmli sample buffer for SDSPAGE analysis or diluted with water (∼50 fold) for ESI-MS analysis. Gels were visualized by staining with coomassie blue. Fluorescence was visualized on a Typhoon 9200 Imager (GE Healthcare). For detection of biotinylation, proteins were separated by SDS-PAGE and transferred to a nitrocellulose membrane. The membrane was then probed with a streptavidin-horseradish peroxidase conjugate (GE Healthcare) and visualized by chemiluminescence. ESI-MS was performed on a Micromass LCT mass spectrometer (Micromass® MS Technologies, USA) and a Paradigm MG4 HPLC system equipped with a HTC PAL autosampler (Michrom BioResources, USA) and a Waters Symmetry 5 m C8 column (2.1 x 50 mm, MeCN:H2O (0.1% formic acid) gradient mobile phase, 150 L/min).

0277<i>Dual labeling of eGFP</i>. Immediately prior to starting the dual labeling sequence, the eGFP stock was thawed and again purified by affinity chromatography over commercial Ni-NTA resin. After binding eGFP to the resin, the column was washed with 20 mM Tris pH 8.0, 150 mM NaCl, and 20 mM imidazole. The protein was eluted with 20 mM Tris pH 8.0, 150 mM NaCl, and 300 mM imidazole. This material was buffer exchanged into 20 mM Tris pH 8.0, 150 mM NaCl using a NAP™ 5 Sephadex™ column (GE Healthcare) and concentrated. The concentration was estimated to be 84 M by UV-vis spectroscopy using eGFP absorbance at 488 nm (extinction coefficient 55,900 M^-1cm^-1).⁷

0278<i>C-terminal modification of eGFP with <b>3</b> and SrtAstrep.</i> 400 L of the freshly purified eGFP solution was then treated with SrtAstrep (87 L of a 140 M stock solution) and <b>3</b> (4.9 L of a 100 mM stock solution) [<b>Note</b>: SrtAstrep does not require Ca²⁺ for activity].³ The reaction was incubated for 7 h at 37 °C. ESIMS analysis of the crude reaction mixture revealed excellent conversion to the desired product (Supporting Figure S6a). The reaction was then treated with [2-(trimethylammonium)ethly] methane thiosulfonate bromide (MTSET) (2.5 L of a 500 mM solution in 1:1 DMSO/H2O) for 10 min at room temperature to quench SrtAstrep. The entire reaction was then diluted with 5 mL of 20 mM Tris pH 8.0, 500 mM NaCl, and 20 mM imidazole. This solution was then passed over a 1.5 mL column of Ni-NTA that been equilibrated with 20 mM Tris pH 8.0, 500 mM NaCl, and 20 mM imidazole. The column was then washed with 1.5 mL of 20 mM Tris pH 8.0, 500 mM NaCl, and 20 mM imidazole. His6-tagged SrtAstrep was bound by Ni-NTA while the eGFP product (which lost its His6 tag during the course of transpeptidation) was not retained. The eGFP solution was then concentrated and passed over a PD-10 Sephadex™ desalting column (equilibrated with 20 mM Tris pH 8.0, 150 mM NaCl) to remove excess <b>3.</b> This material was concentrated to ∼800 L and subjected to thrombin cleavage.

0279<i>Thrombin cleavage of eGFP</i>. All 800 uL of the solution described above was combined with 100 L of 10x cleavage buffer and 100 L of thrombin agarose beads (from Thrombin CleanCleave™ Kit, Sigma). This mixture was incubated for 1 h at 37 °C, and then checked by ESI-MS to ensure quantitative cleavage (<figref idref="f0044 f0045">Figure 35</figref>). The reaction was then filtered to remove the thrombin beads.

0280<i>N-terminal labeling of eGFP with <b>1</b> and 59-SrtAstaph.</i> 389 L of the thrombin cleaved material was combined with <b>1</b> (25 L of a 10 mM DMSO solution), 59-SrtAstaph (36 L of a 700 M stock solution), and 10x sortase reaction buffer (50 L of 500 mM Tris pH 8.0, 1.5 M NaCl, 100 mM CaCl2). The reaction was incubated for 75 min at 37 °C. ESI-MS of the crude reaction mixture showed clean formation of the dual labeled product as the major reaction product (Supporting Figure S6a). The reaction was then diluted with 5 mL of 20 mM Tris pH 8.0, 500 mM NaCl, and 20 mM imidazole. This solution was passed over a 1.5 mL column of Ni-NTA that been equilibrated with 20 mM Tris pH 8.0, 500 mM NaCl, and 20 mM imidazole in order to remove His6-tagged 59-SrtAstaph. 2.5 mL of this eluate was then TM passed over a PD-10 Sephadex desalting column (equilibrated with 20 mM Tris pH 8.0). This material was then purified by anion-exchange chromatography on a Mono Q 5/50 GL column (Amersham) [Buffer A (20 mM Tris pH 8.0), Buffer B (20 mM Tris pH 8.0, 1 M NaCl), 1.5 mL/min, gradient: 100% Buffer A (0-15 mL), 0% Buffer B → 50% Buffer B (15-45 mL), 50% Buffer B (45-50 mL)]. Fractions containing dual labeled eGFP were pooled and analyzed by SDS-PAGE and ESI-MS. Coomassie stained gels were imaged using a CanoScan 8600F scanner. Protein purity was estimated from these images using ImageJ 1.42q densitometry software.

Dual labeling of UCHL3

0281<i>C-terminal modification of UCHL3 with <b>3</b> and SrtAstrep.</i> UCHL3 (350 L of a 65 M stock solution) was treated with SrtAstrep (76 L of a 140 M stock solution) and <b>3</b> (4.3 L of a 100 mM stock solution) [<b>Note:</b> SrtAstrep does not require Ca²⁺ for activity].³ The reaction was incubated for 15 h at 37 °C. ESIMS analysis of the crude reaction mixture revealed excellent conversion to the desired product (Supporting Figure S6b). The entire reaction was then diluted with 5 mL of 20 mM Tris pH 8.0, 500 mM NaCl, and 20 mM imidazole. This solution was then treated with [2-(trimethylammonium)ethly] methane thiosulfonate bromide (MTSET) (5.0 L of a 500 mM solution in 1:1 DMSO/H2O) for 10 min at room temperature to quench SrtAstrep. [<b>Note:</b> UCHL3 contains an active site cysteine residue and is therefore modified by MTSET. The resulting modification is disulfide linked, and is easily removed by treatment with DTT following completion of the dual labeling procedure]. The diluted reaction solution was then passed over a 1.5 mL column of Ni-NTA that been equilibrated with 20 mM Tris pH 8.0, 500 mM NaCl, and 20 mM imidazole. The column was then washed with 1.5 mL of 20 mM Tris pH 8.0, 500 mM NaCl, and 20 mM imidazole. The TM UCHL3 solution was then concentrated and passed over a PD-10 Sephadex desalting column (equilibrated with 20 mM Tris pH 8.0, 150 mM NaCl) to remove excess <b>3.</b> This material was concentrated to 1 mL and subjected to thrombin cleavage.

0282<i>Thrombin cleavage of UCHL3.</i> All 1 mL of the solution described above was combined with 100 L of 10x cleavage buffer and 100 L of thrombin agarose beads (from Thrombin CleanCleave™ Kit, Sigma). This mixture was incubated for 1 h at 37 degrees C, and then checked by ESI-MS to ensure quantitative cleavage (<figref idref="f0044 f0045">Figure 35</figref>). The reaction was then filtered to remove the thrombin beads.

0283<i>N-terminal labeling 3 with <b>1</b> and 59-SrtAstaph.</i> 778 L of the thrombin cleaved material was combined with <b>1</b> (50 L of a 10 mM DMSO solution), 59-SrtAstaph (72 L of a 700 M stock solution), and 10x sortase reaction buffer (100 L of 500 mM Tris pH 8.0, 1.5 M NaCl, 100 mM CaCl2). The reaction was incubated for 60 min at 37 °C. ESI-MS of the crude reaction mixture showed clean formation of the dual labeled product as the major reaction product (Supporting Figure S6b). The reaction was then diluted with 5 mL of 20 mM Tris pH 8.0, 500 mM NaCl, and 20 mM imidazole. This solution was passed over a 1.5 mL column of Ni-NTA that been equilibrated with 20 mM Tris pH 8.0, 500 mM NaCl, and 20 mM imidazole in order to remove His6-tagged 59-SrtAstaph. The column was then washed with 2.0 mL of 20 mM Tris pH 8.0, 500 mM NaCl, and 20 mM imidazole. The eluate was TM then desalted using PD-10 Sephadex columns (equilibrated with 20 mM Tris pH 8.0). This material was then purified by anion-exchange chromatography on a Mono Q 5/50 GL column (Amersham) [Buffer A (20 mM Tris pH 8.0), Buffer B (20 mM Tris pH 8.0, 1 M NaCl), 1.5 mL/min, gradient: 100% Buffer A (0-15 mL), 0% Buffer B → 50% Buffer B (15-45 mL), 50% Buffer B (45-50 mL)]. Fractions containing dual labeled UCHL3 were pooled and analyzed by SDS-PAGE and ESI-MS. Prior to ESIMS, dual labeled UCHL3 was treated with 10 mM DTT for 10 min at RT to remove the MTSET modification on the active site cysteine residue. Coomassie stained gels were imaged using a CanoScan 8600F scanner. Protein purity was estimated from these images using ImageJ 1.42q densitometry software.

<u>Results</u>

0284We synthesized FITC (<b>1</b>) and biotin (<b>2</b>) derivatives of an LPRT peptide in which the glycine of the normal LPXTG motif was replaced by a methyl ester (<figref idref="f0040">Figure 32</figref>; see also Example 2). With the ability of SrtA_staph to append labels at either terminus, as described herein (see, e.g., Example 2), we pursued the possibility of installing two modifications within the same protein. Attempts to execute this type of transformation using SrtA_staph alone failed to yield desired levels of product, due to intramolecular transpeptidation between N-terminal glycines and the C-terminal LPXTG motif that occurred in most cases. Therefore, we considered the possibility of using a second, distinct sortase. We initially sought to use sortase B (SrtB) from either <i>Staph. aureus</i> or <i>Bacillus anthracis</i> as enzymes with recognition sequences (NPQTN and NPKTG, respectively) orthogonal to that of SrtA_staph.(9, 10). Both SrtB enzymes were easily produced in <i>Escherichia coli</i> and purified to homogeneity. We reproduced the reported in vitro enzyme activity using a FRET-based assay to measure cleavage of short peptides substrates <u>(9, 10)</u>. However, we did not obtain transpeptidation with either SrtB on protein substrates modified with the appropriate recognition sequences on a time scale or with yields that compare favorably with SrtA_staph (data not shown).

0285We ultimately arrived at a more successful orthogonal strategy using SrtA_strep, which recognizes the same LPXTG sequence used by SrtA_staph but can accept alanine-based nucleophiles <u>(11)</u> This leads to the formation of an LPXTA sequence at the site of ligation, a motif refractory to cleavage by SrtA_staph.<u>(12)</u> This allows SrtA_staph to act on the N terminus without affecting the C-terminal modification installed with SrtA_strep. An exemplary strategy for dual-terminus labeling is outlined in <figref idref="f0041">Figures 33b</figref> and <figref idref="f0043">34</figref>. We first synthesized a tetramethylrhodamine-labeled peptide (<b>3</b>) containing two N-terminal alanine residues to serve as the nucleophile for SrtA_strep-mediated protein ligation (<figref idref="f0041">Figure 33a</figref>). We prepared two model substrates (eGFP and UCHL3) containing masked N-terminal glycines that are exposed only upon thrombin cleavage. Masking was required because SrtA_strep was observed to ligate both glycine and alanine nucleophiles (data not shown). Substrates also contained an LPXTG motif at the C terminus to allow a first round of labeling with SrtA_strep. For both eGFP and UCHL3, C-terminal labeling using <b>3</b> and SrtA_strep resulted in >90% conversion to the desired adduct, as revealed by ESI-MS (<figref idref="f0044 f0045">Figure 35</figref>). SrtA_strep was quenched by the addition of MTSET followed by removal of His₆-tagged SrtA_strep using Ni-NTA. Residual <b>3</b> was then removed using a disposable desalting column. Thrombin cleavage proceeded in quantitative fashion using commercial thrombin agarose resin (<figref idref="f0044 f0045">Figure 35</figref>). The exposed N-terminal glycines were then labeled by treatment with 500 µM <b>1</b> and 50 µM Δ59-SrtA_staph<u>(13)</u> for ∼1 h at 37 °C. ESI-MS of crude reaction mixtures showed the dual-labeled material as the major component, with only minor amounts of byproduct (<figref idref="f0044 f0045">Figure 35</figref>). A final separation by anion-exchange chromatography yielded dual-labeled eGFP and UCHL3 with excellent purity, as determined by both SDS-PAGE and ESI-MS (<figref idref="f0043">Figure 34</figref> and <figref idref="f0044 f0045">Figure 35</figref>). In the case of UCHL3, we observed some additional low-intensity bands in the fluorescent gel scan (<figref idref="f0041 f0042">Figure 33</figref>). However, quantitative densitometric analysis of coomassie-stained gels indicated purity in excess of 95% for both dual-labeled eGFP and UCHL3.

References for Example 4

0286<ol id="ol0002" compact="compact" ol-style=""><li>(1) <nplcit id="ncit0107" npl-type="s"><text>Zumbuehl, A.; Jeannerat, D.; Martin, S. E.; Sohrmann, M.; Stano, P.; Vigassy, T.; Clark, D. D.; Hussey, S. L.; Peter, M.; Peterson, B. R.; Pretsch, E.; Walde, P.; Carreira, E. M. Angew Chem Int Ed Engl 2004, 43, 5181-5</text></nplcit>.</li><li>(2) <nplcit id="ncit0108" npl-type="s"><text>Kottani, R.; Valiulin, R. A.; Kutateladze, A. G. Proc Natl Acad Sci USA 2006, 103, 13917-21</text></nplcit>.</li><li>(3)<nplcit id="ncit0109" npl-type="s"><text> Race, P. R.; Bentley, M. L.; Melvin, J. A.; Crow, A.; Hughes, R. K.; Smith, W. D.; Sessions, R. B.; Kehoe, M. A.; McCafferty, D. G.; Banfield, M. J. J Biol Chem 2009, 284, 6924-33</text></nplcit>.</li><li>(4) <nplcit id="ncit0110" npl-type="s"><text>Ton-That, H.; Liu, G.; Mazmanian, S. K.; Faull, K. F.; Schneewind, O. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 12424-9</text></nplcit>.</li><li>(5) <nplcit id="ncit0111" npl-type="s"><text>Jobling, M. G.; Palmer, L. M.; Erbe, J. L.; Holmes, R. K. Plasmid 1997, 38, 158-73</text></nplcit>.</li><li>(6)<nplcit id="ncit0112" npl-type="s"><text> Antos, J. M.; Miller, G. M.; Grotenbreg, G. M.; Ploegh, H. L. J Am Chem Soc 2008, 130, 16338-43</text></nplcit>.</li><li>(7) <nplcit id="ncit0113" npl-type="s"><text>Tsien, R. Y. Annu. Rev. Biochem. 1998, 67, 509-544</text></nplcit>.</li><li>(8) <nplcit id="ncit0114" npl-type="s"><text>Popp, M. W.; Artavanis-Tsakonas, K.; Ploegh, H. L. Journal of Biological Chemistry 2008, 284, 3593-3602</text></nplcit>.</li><li>(9) <nplcit id="ncit0115" npl-type="s"><text>Maresso, A. W., Chapa, T. J. and Schneewind, O. J. Bacteriol. 2006, 188, 8145-8152</text></nplcit></li><li>(10) <nplcit id="ncit0116" npl-type="s"><text>Mazmanian, S. K., Ton-That, H., Su, K. and Schneewind, O. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 2293-2298</text></nplcit></li><li>(11) <nplcit id="ncit0117" npl-type="s"><text>Race, P. R., Bentley, M. L., Melvin, J. A., Crow, A., Hughes, R. K., Smith, W. D., Sessions, R. B., Kehoe, M. A., McCafferty, D. G. and Banfield, M. J. J. Biol. Chem. 2009, 284, 6924-6933</text></nplcit></li><li>(12) <nplcit id="ncit0118" npl-type="s"><text>Kruger, R. G., Otvos, B., Frankel, B. A., Bentley, M., Dostal, P. and McCafferty, D. G. Biochemistry 2004, 43, 1541-1551</text></nplcit></li><li>(13) Δ59-SrtA_staph is a truncated form of SrtA_staph that has identical reactivity</li></ol>

Example 5: Site-Specific PEGylation Using SrtA_staph

0287This example describes PEGylation of the four-helix bundle cytokine interferon α at the C-terminus using sortase and demonstrates that the PEGylated version retains activity and has enhanced in vivo circulating time. We cloned into the pET28a+ vector (Novagen) a version of interferon alpha lacking the leader sequence and fused to LPETGG, the SrtA_staph recognition site, separated from the body of the protein by two glycines (<figref idref="f0049">Fig 39A</figref>). For purification, we added a 6-His tag following the SrtA_staph cleavage site; this tag is lost upon transpeptidation. We expressed this protein in Rosetta-gami(DE3)pLysS cells (Novagen) and purified soluble protein by Ni-NTA IMAC chromatography (Qiagen), followed by size exclusion chromatography on a Superdex 75 column (GE) at pH 5.0.

0288We constructed a Gly₃K-PEG(10kDa) nucleophile compatible with SrtA_staph. The Gly₃K peptide scaffold was constructed by standard Fluorenylmethoxycarbonyl (Fmoc) solid phase peptide chemistry on rink amide resin. Fmoc protected peptide was liberated from the resin by treatment with 95% TFA/3%TIPS/2%H₂O, precipitated with cold ether and lyophilized. Methyl-capped 10 kDa PEG succinimidyl ester (1 equivalent, Nanocs) was mixed with peptide (2equivalents), 1 equivalent N-hydroxysuccinimide (NHS, Sigma) and 1 equivalent 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC, Pierce) in <i>N-</i>Methylpyrrolidone (NMP, Fluka) for 24 hours at room temperature, followed by precipitation in cold ether. The resulting solid was resuspended in 20% piperidine/NMP for 30 minutes, followed by re-precipitation in cold ether. This material was resuspended in H₂O and dialyzed extensively against H₂O to remove free peptide.

0289Sortase labeling was conducted as described previously (see, e.g., other Examples herein). PEGylated protein was purified by cation exchange chromatography on a Mono-S column (GE) at pH 5.0 or 4.5. Protein concentrations were measured by the Bradford method.

0290To compare <i>in-vitro</i> activity of interferon alpha conjugates, a standard Daudi cell proliferation inhibition assay was used. The site-specifically PEGylated interferon alpha conjugate is extremely potent in inhibiting Daudi cell proliferation (<figref idref="f0051">Fig. 40</figref>, Table C), despite conjugation to the large 10kDa PEG chain. The PEG moiety, positioned away from the receptor binding site prolongs the time the interferon alpha conjugate spends in circulation, as assessed by ELISA performed on mice injected with 10µg of interferon alpha, followed by retro-orbital collection of serum at different time points (<figref idref="f0051">Fig. 41</figref>). Thus, by PEGylating interferon alpha at a site distant from the receptor binding site, we can extend the circulating half-life with minimal sacrifice in potency. <tables id="tabl0004" num="0004"><table frame="all"><title><b>Table C</b>*: Inhibition of Daudi cell proliferation by IFNα and IFNα variants</title><tgroup cols="3"><colspec colnum="1" colname="col1" colwidth="36mm" /><colspec colnum="2" colname="col2" colwidth="33mm" /><colspec colnum="3" colname="col3" colwidth="40mm" /><thead><row><entry valign="top"><b>IFNα2 Variant</b></entry><entry valign="top"><b>IC50 (pg/mL)</b></entry><entry valign="top"><b>LogIC50 Standard Error</b></entry></row></thead><tbody><row><entry>Standard</entry><entry align="right">15.51</entry><entry align="right">0.0378</entry></row><row><entry>+GG</entry><entry align="right">9.969</entry><entry align="right">0.0553</entry></row><row><entry>Circular</entry><entry align="right">4.918</entry><entry align="right">0.0639</entry></row><row><entry>delGG</entry><entry align="right">7.098</entry><entry align="right">0.0411</entry></row><row><entry>PEGylated</entry><entry align="right">21.67</entry><entry align="right">0.0488</entry></row></tbody></tgroup><tgroup cols="3" rowsep="0"><colspec colnum="1" colname="col1" colwidth="36mm" /><colspec colnum="2" colname="col2" colwidth="33mm" /><colspec colnum="3" colname="col3" colwidth="40mm" /><tbody><row><entry namest="col1" nameend="col3" align="justify">* See <figref idref="f0049 f0050">Figs. 39</figref> and <figref idref="f0051">40</figref> legend for details regarding the <b>IFNα2</b> sequences</entry></row></tbody></tgroup></table></tables>

0291As described herein, we have extended the sortase methodology to N-terminal labeling. Since the N-terminus lies in close proximity to the C-terminus in four-helix bundle cytokines, we expect PEG conjugation using N-terminal probes (e.g., <figref idref="f0057 f0058">Fig. 47</figref>) to yield similar results.

Reference Example 6:

Sortase-Mediated Circularization of IFNα

0292We have shown that proteins bearing an LPXTG motif at the C-terminus and an N-terminal glycine can be covalently cyclized after incubation with sortase A, provided that the N and C terminus are in close apposition to one another, as they are in the four helix bundle cytokines. We constructed an interferon alpha variant by inserting two glycine residues between the initiating methionine and the body of the interferon alpha sequence used for PEGylation (<figref idref="f0049">Fig. 39A</figref>). When incubated with sortase, this protein cyclizes, as shown unambiguously by MS/MS sequence identification of the junction peptide between the N and C-termini following chymotrypsin digestion (<figref idref="f0050">Fig. 39B</figref>). The circular interferon alpha was purified by cation exchange chromatography and tested in the cell based interferon alpha activity assay. Again, the engineered protein is as potent as the recombinant standard (<figref idref="f0051">Fig 40</figref>, Table C).

0293To test the thermal stability of the circular protein, we turned to a dye-based thermal denaturation assay, Thermofluor. When denatured, proteins expose hydrophobic residues that bind to and increase fluorescence of the dye Sypro Orange (Invitrogen). The circular interferon alpha has a significantly higher T_m than any other variant tested (<figref idref="f0052">Fig 42</figref>). We conclude that covalently clamping the termini of interferon alpha together stabilizes the protein by preventing premature fraying of the ends, without sacrificing activity.

Reference Example 7:

Circular Site-specfically PEGylated Proteins

0294Combining the insertion of a non-genetically encoded PEG moiety at the terminus of interferon alpha and subsequently joining the resulting termini will impart both increased circulating half-life and thermal stability. This is accomplished by two subsequent transacylation steps (<figref idref="f0053">Fig. 43</figref>), the first joining a peptide bearing the PEG moiety as well as another distinct sortase recognition element to the candidate protein. The second transacylation step is accomplished by a different sortase and results in covalent closure of the protein. However, because the sortase reactions are equilibrium reactions and the sortases SrtA_strep and SrtA_staph are orthogonal in one direction only (SrtA_strep cleaves both LPXTG and LPXTA sites but SrtA_staph attacks only LPXTG), we sought to render the sortase cleavage site on the nucleophile refractory to cleavage during the initial installation of the probe. To that end, we have generated a nucleophile bearing an LPXtG sequence, where t is a phosphorylated threonine residue. Based on the crystal structures of SrtA_staph and SrtA_strep, phosphothreonine will not fit into the substrate recognition groove. Thus, the first transpeptidation step can be accomplished without premature cleavage of the SrtA_staph site on the PEG nucleophile. The resulting conjugate can then be dephosphorylated and incubated with SrtA_staph (which will not cleave the LPETAA junction between the protein and nucleophile) to join the termini.

0295Our experiments demonstrate the feasibility of this scheme. We installed a peptide bearing an N-terminal AA sequence (compatible with SrtA_strep) followed by a lysine for conjugating to a non-genetically encoded moiety of choice, an LPEtG SrtA_staph cleavage site, and a hexahistidine epitope tag onto interferon alpha (<figref idref="f0053">Fig. 43</figref>). The reaction product was confirmed by LC ESI-MS. We then dephosphorylated the protein by incubating with Antarctic phosphatase (NEB), exposing the SrtA_staph cleavage site. This site is competent to undergo the second transacylation step; we installed a triglycine nucleophile onto this site by incubating with SrtA_staph and Gly₃ peptide (Sigma) (<figref idref="f0054">Fig. 44</figref>). Thus, we can accomplish two successive transacylation steps by controlling the accessibility of two orthogonal sortase enzymes to their respective cleavage sites.

Reference Example 8: Sortase-Mediated Installation of a Non-Genetically Encoded Element Between Two Proteins

0296The strategy described in Example 7 can be used to install a non-genetically encoded element between two proteins or protein domains as shown, e.g., in <figref idref="f0055">Fig. 45</figref>. A specific example is the construction of a cytokine variant joined to the Fc region of an antibody, with a non-genetically encoded moiety in the intervening region. Genetic fusions of cytokines to Fc regions can be delivered non-invasively into the bloodstream by Fc receptor bearing epithelial cells in the lungs, after which the half-life of the molecule is prolonged. Implicit in this mechanism is the requirement for an Fc region that is able to bind to its cognate Fc receptor, and thus chemical conjugation of a molecule to the C-terminus or an internal lysine of the Fc portion would likely decrease binding capacity. Using two transacylation steps, we will install a PEG moiety onto the C-terminus of the cytokine portion, followed by installation of an unhindered Fc region. Such a conjugate may show even greater gains in half-life relative to the genetic Fc fusion, as kidney filtration of the molecule is predicted to be further reduced. Other applications include installing a detectable label, e.g., for diagnostic purposes or to monitor the distribution or elimination of the molecule in vivo.