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The mating process between cells of the two haploid cell types of the yeast Saccharomyces cerevisiae provides a model system for the study of hormonally mediated intercellular interactions in a simple eukaryotic organism. The early events in the yeast mating process appear to be triggered by diffusible peptide pheromones, a-factor and a-factor, secreted by a cells and a cells, respectively (Thorner 1981).

These peptides would be expected, like other small secreted peptides, to be processed from larger precursor molecules (Herbert and Uhler 1982). This has been confirmed in the case of a-factor by analysis of the a-factor structural genes (Kurjan and Herskowitz 1982; Singh et al. 1983) and of the protein products (Brake et al. 1983; Julius et al. 1983, 1984a, b). The major a-factor structural gene encodes a primary translation product of 165 amino acids containing four mature a-factor-coding regions, each preceded by a short spacer peptide. Processing of the prepro-a-factor requires N-linked glycosylation of three sites in the leader region, proteolytic processing at Lys-Arg sequences in each spacer region by the product of the KEX2 gene, and proteolytic maturation at the amino terminus of each resulting peptide by dipeptidyl aminopeptidase A (the product of the STE13 gene) and at the carboxyl terminus by a carboxypeptidase-B-like enzyme.