Cystatins A, B; Type I cystatins

Stefins are members of the cystatin superfamily of cysteine proteinase inhibitors localized in the cytosol and thought to protect cytoskeletal proteins from degradation by cysteine proteases released from lysosomes.

Type I or family 1 of the cystatin superfamily of cysteine protease inhibitors includes human stefins A and B and their homologues in other species such as cystatinsα and β in rat, bovine thymus stefin C, porcine thymus stefins D1 and D2, mouse stefin A (1–4), and others (see MEROPS subfamily I25A). The genes for human stefins A and B have been mapped to chromosomes 3q21 and 21q22.3, respectively. The lack of a signal sequence and disulfide bonds makes stefins distinct from other members of the cystatin superfamily. Stefins are single-chain proteins consisting of 98–103 amino acid residues, with a molecular mass of 11–12 kDa. Human stefin A is an acidic protein with pI values between 4.5 and 5.0, whereas...