Kexin, subtilisin-like protease (proprotein convertase); calcium-dependent serine protease involved in activation of proproteins of secretory pathway; originally discovered based on failure of mutant to make active killer toxin in presence of wild-type killer plasmid; required for the secretion of toxin by plasmid-carrying strains; also required for normal mating by alpha strains and meiotic sporulation in all strains