CD81 (TAPA-1, target of anti-proliferative antibody-1) is a member of the tetraspanin family, is expressed on virtually all nucleated cells, but above all on germinal center B cells. CD81 forms complexes with other tetraspanin proteins, integrins, coreceptors, MHC class I and II molecules, and influences adhesion, morphology, activation, proliferation and differentiation of B, T cells. In muscles, CD81 promotes cell fusion and myotube maintenance. CD81 has been also identified as a receptor for the hepatitis C virus. CD81 is a cell-surface proteins that are characterized by the presence of four hydrophobic domains. The proteins mediate signal transduction events that play a role in the regulation of cell development, activation, growth and motility.CD81 appears to promote muscle cell fusion and support myotube maintenance. Antibodies to CD81 have anti-proliferative effects on different lymphoid cell lines, particularly those derived from large cell lymphomas. CD81 is also localized in the tumor-suppressor gene region and is a candidate gene for malignancies.
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