Secretion and processing of insulin precursors in yeast

A series of dibasic insulin precursors including proinsulin was expressed and secreted from Saccharomyces cerevisiae. Recombinant plasmids were constructed to encode fusion proteins consisting of a modified mating factor alpha 1 leader sequence and an insulin precursor. The leader sequence serves to direct the fusion protein into the secretory pathway of the cell and to expose it to the Lys-Arg processing enzyme system. The secreted peptides were purified from the fermentation broth and characterized by sequencing and amino acid analysis. Processing at one or both dibasic sequences was shown in proinsulin and in other insulin precursors containing a short spacer peptide in place of the C peptide. In contrast, no processing was observed in the absence of a spacer peptide in the insulin precursor molecule, e.g., B-Lys-Arg-A (where A and B are the A and B chain of human proinsulin, respectively). This type of single-chain insulin precursors isolated from such constructions could be enzymatically converted into insulin by treatment with trypsin and carboxypeptidase B. The above results suggest that the C-peptide region of proinsulin serves to direct the trypsin-like converting enzyme to process at the two dibasic sequences. We propose that in hormone precursors in general the spacer peptides serve to expose dibasic sequences for processing.