Yeast KEX1 gene encodes a putative protease with a carboxypeptidase B-like function involved in killer toxin and α-factor precursor processing

The yeast KEX1 gene product has homology to yeast carboxypeptidase Y. A mutant replacing serine at the putative active site of the KEX1 protein abolished activity in vivo. A probable site of processing by the KEX1 product is the C-terminus of the α-subunit of killer toxin, where toxin is followed in the precursor by 2 basic residues. Processing involves endoproteolysis following these basic residues and trimming of their C-terminal by a carboxypeptidase. Consistent with the KEX1 product being this carboxypeptidase is its role in α-factor pheromone production. In wild-type yeast, KEX1 is not essential for α-factor production, as the final pheromone repeat needs no C-terminal processing. However, in a mutant in which α-factor production requires a carboxypeptidase, pheromone production is KEX1 -dependent.