Thiol protease which is required for parasite excystation and invasion of the proximal small intestine of the human host. To initiate infection, trophozoites emerge from a cyst in the host. Excystation is blocked by specific cysteine protease inhibitors. Vacuoles release it just prior to excystation. Expressed in replicating and encysting trophozoites without supplemental iron. Belongs to the peptidase C1 family.