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Cystatin B (CSTB, Stefin B)

Cystatin B (CSTB), also known as stefin B, is an endogenous inhibitor of cysteine proteinases localized in the nukleus and the cytosol. Stefins are 11 kDa intracellular proteins, which are ubiquitously expressed in human tissues. They belong to the family of cystatins, which are endogenous cysteine protease inhibitors specific for the papain-family of cysteine proteinases and are classified as the IH clan in the MEROPS scheme. Stefin B is a globular protein of 98 amino acid residues which does not contain disulphide bonds (it has one cysteine at site 3) or carbohydrate groups. Cystatin B belongs to the cystatins superfamily which has primarily been explored with respect to its capacity to inhibit intracellular cysteine proteases leaking from lysosomes and has been implicated in several types of cancers, such as breast, lung and colorectal cancers, glio¬blastoma, squamous cell carcinoma of the head and neck, laryngeal, esophageal and hepatocellular carcinomas, and prostatic adenocarcinoma. The dysregulated expres¬sion of CSTB appears to be associated with tumorigenesis and may be mediated by a variety of cytokines and growth factors, including transforming growth factor-β (TGF β). In the nervous system, Cystatin B is found in neural stem cells, neurons, glia, astrocytes and neuroepithelium. Cystatin B can also interact with amyloid-β, and Cystatin B tetramers are specifically found to inhibit amyloid-β fibril growth. In rheumatoid arthritis, serum Cystatin B is reported to correlate with joint destruction and swelling.

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