Ste24 protease

This chapter examines the activity, specificity and structural chemistry of ste24protease. The STE24 gene product plays two distinct proteolytic roles in the a-factor biogenesis pathway, one inC-terminal modification and another in N-terminal cleavage. Consistent with its proposed proteolytic functions, Ste24p possesses a consensus HEXXH zinc-metalloprotease motif. A notable feature of Ste24p that sets it apart from most other zinc metalloproteases is that it contains multiple membrane spans. Reconstitution studies in vitro utilizing crude Ste24p-containing membranes have confirmed that active Ste24p is indeed membrane associated and is required for dual roles in -factor processing. The proof for whether Ste24p acts directly as the -factor protease or indirectly to activate a downstream protease has awaited the purification of Ste24p. Ste24p is a 52 kDa integral membrane protein that resides in the membrane of the endoplasmic reticulum, along with two other CaaX-processing components, Rce lp and Ste 14p. Hydropathy analysis predicts that Ste24p may span the ER membrane seven times. Protease protection studies with microsomes containing N- or C-terminally epitope-tagged versions of Ste24p have demonstrated that the N-terminus of Ste24p is luminally disposed while the C-terminus is cytosolically disposed.