The Multispanning Membrane Protein Ste24p Catalyzes CAAX Proteolysis and NH₂-terminal Processing of the Yeast a-Factor Precursor

Saccharomyces cerevisiae Ste24p is a multispanning membrane protein implicated in the CAAX proteolysis step that occurs during biogenesis of the prenylated a-factor mating pheromone. Whether Ste24p acts directly as a CAAX protease or indirectly to activate a downstream protease has not yet been established. In this study, we demonstrate that purified, detergent-solubilized Ste24p directly mediates CAAX proteolysis in a zinc-dependent manner. We also show that Ste24p mediates a separate proteolytic step, the first NH ₂-terminal cleavage in a-factor maturation. These results establish that Ste24p functions both as a bona fide COOH-terminal CAAX protease and as an a-factor NH₂-terminal protease. Importantly, this study is the first to directly demonstrate that a eukaryotic multispanning membrane protein can possess intrinsic proteolytic activity.