Involved in the N-terminal endoproteolytic cleavage of the P2 precursor of the a-factor mating pheromone. Capable of proteolysing the established mammalian insulin-degrading enzymes (IDEs) substrates amyloid-beta peptide and insulin B-chain. Binds 1 zinc ion per subunit. Inhibited by chelating agents like EDTA, TPEN and 1,1-phenanthroline, as well as NEM, free cysteine and DTT. Optimum pH is 8.1. Optimum temperature is 37 degrees Celsius. Expressed in both haploid and diploid yeast cells. Belongs to the peptidase M16 family. It is uncertain whether Met-1 or Met-53 is the initiator. Met-53 is probably the physiologically relevant initiator methionine, as Met-1 is dispensable for the expression of functional STE23, whereas Met-53 is not.