The S-100 proteins (often styled without the hyphen, S100) are a family of low-molecular-weight proteins found in vertebrates and characterized by two calcium-binding sites that have helix-loop-helix ("EF-hand type") conformation. There are at least 21 different S-100 proteins. Their name is derived from the fact that these proteins are soluble in 100%, i.e. saturated ammonium sulfate at neutral pH. They are encoded by a family of genes whose symbols use the S100 prefix, for example, S100A1, S100A2, S100A3.
Most S100 proteins are homodimeric, consisting of two identical polypeptides, which are held together by non-covalent bonds. S100 proteins are structurally similar to calmodulin. On the other hand they differ from calmodulin on the other features. For instance, their expression pattern is cell-specific, i.e. they are expressed in particular cell types. Their expression depends on environmental factors. To contrast, calmodulin is a ubiquitous and universal intracellular Ca2+ receptor widely expressed in many cells.
S100 proteins have been implicated in a variety of intracellular and extracellular functions. S100 proteins are involved in regulation of protein phosphorylation, transcription factors, Ca2+ homeostasis, the dynamics of cytoskeleton constituents, enzyme activities, cell growth and differentiation, and the inflammatory response. S100A7 (psoriasin) and S100A15 have been found to act as cytokines in inflammation, particularly in autoimmune skin conditions such as psoriasis.
Several members of the S-100 protein family are useful as markers for certain tumors and epidermal differentiation. It can be found in melanomas, 100% of schwannomas, 100% of neurofibromas (weaker than schwannomas), 50% of malignant peripheral nerve sheath tumors (may be weak and/or focal), paraganglioma stromal cells, histiocytoma and clear cell sarcomas. Further, S100 proteins are markers for inflammatory diseases and can mediate inflammation and act as antimicrobials.
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Ronald Wolf, O. M. Zack Howard, Hui-Fang Dong, Christopher Voscopoulos, Karen Boeshans, Jason Winston, Rao Divi, Michele Gunsior, Paul Goldsmith, Bijan Ahvazi, Triantafyllos Chavakis, Joost J. Oppenheim and Stuart H. Yuspa (2010). "Chemotactic Activity of S100A7 (Psoriasin) Is Mediated by the Receptor for Advanced Glycation End Products and Potentiates Inflammation with Highly Homologous but Functionally Distinct S100A15.". The Journal of Immunology181 (2): 1499–1506.
Ronald Wolf, Francesca Mascia, Alif Dharamsi, O. M. Zack Howard, Christophe Cataisson, Val Bliskovski, Jason Winston, Lionel Feigenbaum, Ulrike Lichti, Thomas Ruzicka Triantafyllos Chavakis, and Stuart H. Yuspa. (2010). "Gene from a Psoriasis Susceptibility Locus Primes the Skin for Inflammation.". Science Translational Medicine2 (61): 61ra90. doi:10.1126/scitranslmed.3001108. PMID21148126.