Tetraspanin, extracellular domain or large extracellular loop (LEL), CD81_like subfamily. Tetraspanins are trans-membrane proteins with 4 trans-membrane segments. Both the N- and C-termini lie on the intracellular side of the membrane. This alignment model spans the extracellular domain between the 3rd and 4th trans-membrane segment. Tetraspanins are involved in diverse processes and their various functions may relate to their ability to act as molecular facilitators. Tetraspanins associate laterally with one another and cluster dynamically with numerous parnter domains in membrane microdomains, forming a network of multimolecular complexes, the "tetraspanin web". CD81, also referred to as Target for anti-proliferative antigen-1, TAPA-1, is found in virtually all tissues, may be involved in regulation of cell growth and has been described as a member of the CD19/CD21/Leu-13 signal transduction complex identified on B cells (the B-Cell co-receptor).
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