Cystatin superfamily (Cystatin_sf)
Type: Homologous_superfamily

The cystatin superfamily comprises cysteine protease inhibitors that play key regulatory roles in protein degradation processes. This superfamily includes cystatins, cathelicidins, staphopains, kininogens, latexins and secreted phosphoprotein 24 (Spp-24) . The progenitor of this superfamily was most probably intracellular and lacked a signal peptide and disulphide bridges, much like the extant Giardia cystatin. A primordial gene duplication produced two ancestral eukaryotic lineages, cystatins and stefins. Stefins remain encoded by a single or a small number of genes throughout the eukaryotes, whereas the cystatins have undergone a more complex and dynamic evolution through numerous gene and domain duplications . Members of this superfamily differ in amino acid sequence, but they share a conserved fold, consisting of a five-stranded anti-parallel β-sheet wrapped around a five-turn α-helix .

Cystatins are cysteine proteinase inhibitors belonging to MEROPS inhibitor family I25, clan IH . They mainly inhibit peptidases belonging to peptidase families C1 (papain family) and C13 (legumain family). The cystatin family includes:

The Type 1 cystatins, which are intracellular cystatins that are present in the cytosol of many cell types, but can also appear in body fluids at significant concentrations. They are single-chain polypeptides of about 100 residues, which have neither disulphide bonds nor carbohydrate side chains.

The Type 2 cystatins, which are mainly extracellular secreted polypeptides synthesised with a 19-28 residue signal peptide. They are broadly distributed and found in most body fluids.

The Type 3 cystatins, which are multidomain proteins. The mammalian representatives of this group are the kininogens. There are three different kininogens in mammals: H- (high molecular mass,

) and L- (low molecular mass) kininogen which are found in a number of species, and T-kininogen that is found only in rat.

Unclassified cystatins. These are cystatin-like proteins found in a range of organisms: plant phytocystatins, fetuin in mammals, insect cystatins and a puff adder venom cystatin which inhibits metalloproteases of the MEROPS peptidase family M12 (astacin/adamalysin). Also a number of the cystatins-like proteins have been shown to be devoid of inhibitory activity.

All true cystatins inhibit cysteine peptidases of the papain family (MEROPS peptidase family C1), and some also inhibit legumain family enzymes (MEROPS peptidase family C13). These peptidases play key roles in physiological processes, such as intracellular protein degradation (cathepsins B, H and L), are pivotal in the remodelling of bone (cathepsin K), and may be important in the control of antigen presentation (cathepsin S, mammalian legumain). Moreover, the activities of such peptidases are increased in pathophysiological conditions, such as cancer metastasis and inflammation. Additionally, such peptidases are essential for several pathogenic parasites and bacteria. Thus in animals cystatins not only have capacity to regulate normal body processes and perhaps cause disease when down-regulated, but in other organisms may also participate in defence against biotic and abiotic stress.