(A) Haploid S. cerevisiae have two mating types, a and α. Their mating with each other is initiated by the secretion of diffusible peptide pheromones that are recognized by G protein-coupled receptors (GPCRs): a-cells (magenta) secrete the lipidated peptide pheromone a-factor, which is recognized by the a-factor receptor expressed on α-cells, while α-cells (cyan) secrete the peptide pheromone α-factor, which is recognized by the α-factor receptor expressed by a-cells. (B) Mating pheromones (underlined) are encoded within precursor peptides by the MFA1 and MFA2 genes (for a-factor) and MFα1 and MFα2 genes (for α-factor). These peptides require several maturation steps before their secretion as biologically active molecules. (C) The modifications of initial products of the MFA1 and MFA2 genes that produce a-factor. Broadly, there are two stages of maturation, C-terminal modifications (S-thiol farnesylation, -AAX proteolysis (white bars), and carboxymethylation), followed by two steps of N-terminal proteolysis (two grey bars). The mature pheromone (black bar) is then exported from the cytosol through a dedicated ABC transporter, Ste6. See also Figure S1.