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Tetraspanin, extracellular domain or large extracellular loop (LEL), CD9 family. 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". CD9 is found in virtually all tissues and is potentially involved in developmental processes. It associates with the tetraspanins CD81 and CD63, as well as with some integrin, and has been shown to be involved in a variety of activation, adhesion, and cell motility functions, as well as cell-cell interactions - such as during fertilization.
Jiyao W et al.(2023). "The conserved domain database in 2023.",