A process for preparing ripe proteins from fusion proteins, synthetized in pro- or eukaryotic cells

A fusion protein with the amino acid sequence: (Ym...Y2-Y1)-(Pro)p-(X1-X2...Xn) in which (Ym...Y2-Y1)-(Pro)p is the pro-sequence and the rest is the ripe protein, m is an integer greater than 2, and Y is an arbitrary amino acid, p is O ifX1 or X2 is Pro, and 1 if X1 or X2 is different from Pro, X is an arbitrary amino acid, and n is integer equal to or greater than 4, is converted into ripe proteins by enzymatic cleavage, which is carried out by stepwise cleavage off of the amino acid groups Ym...Y2 if p = 1 or X1 = Pro, or the groups Ym...Y2-Y1 if X2 = Pro, by means of an aminopeptidase, and then the two amino acids Y1-Pro if P = 1 are cleaved off enzymatically in one or two steps in a manner known per se, and similarly Y1 alone is cleaved off if X1 = Pro. The preferred aminopeptidase is leucine aminopeptidase. The process enables removal in vitro of the formyl methionine or the methionine group and optionally the pro-sequence from a protein synthetized in pro- or eukaryotic cells by genetic engineering, and thus achievement of a desired ripe protein, such as human growth hormone or human proinsulin.