InterPro domain: IPR027214

This entry consists of a group of cysteine protease inhibitors (also known as cystatins). They can be classified as [ 1 ]:

• Type 1: intracellular cystatins, but may also appear in significant amounts in body fluids [ 2 ]. They are single-chain polypeptides of about 100 residues, which have neither disulphide bonds nor carbohydrate side chains. Type 1 cystatin includes cystatin-A/B.
• Type 2: extracellular secreted polypeptides synthesised with a 19-28 residue signal peptide. They are broadly distributed and found in most body fluids [ 3 ]. Type 2 cyctatin includes cystatin-C/D/M/F/SA/SN/G.
• Type 3: multidomain cystatins, including kininogens, known as kinin precursor proteins [ 3 ]. There are three different kininogens in mammals: H- (high molecular mass, IPR002395 ) 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: cystatin-like proteins.

This entry represents the type1, type2 and cystatin-like proteins.

1. Cystatins--Extra- and intracellular cysteine protease inhibitors: High-level secretion and uptake of cystatin C in human neuroblastoma cells. Biochimie 92, 1625-34
2. Isolation of six cysteine proteinase inhibitors from human urine. Their physicochemical and enzyme kinetic properties and concentrations in biological fluids. J. Biol. Chem. 261, 11282-9
3. Isolation of a human cDNA for alpha 2-thiol proteinase inhibitor and its identity with low molecular weight kininogen. Biochemistry 23, 5691-7