This domain is the proregion of the cysteine protease staphopain. Like many papain type peptidases, staphopain is synthesised as an inactive precursor and cleavage of the proregion is required for activation. This proregion has a half-barrel or barrel-sandwich hybrid fold. The proregion blocks the active site cleft of the mature enzyme on one side of the nucleophilic cysteine [1]

1: Prostaphopain B structure: a comparison of proregion-mediated and staphostatin-mediated protease inhibition. Filipek R, Szczepanowski R, Sabat A, Potempa J, Bochtler M; Biochemistry. 2004;43:14306-14315. PMID:15518582