or 
Receptor Family
Tetraspanins    CD63
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(Summary from Tetraspanins)

Tetraspanins are abundantly expressed plasma membrane proteins (of 25-50 kD)(Boucheix & Rubinstein 2001). They have 4 transmembrane domains and well conserved overall structure (Levy et al. 1998). Both N-terminus and C-terminus are intracellular, relatively short and do not have specific enzymatic activity. The first external loop (EC1) is short (10-25 aa) whereas the second (EC2) is large (80-130 aa). There are several key amino acid residues, mainly in the second external loop, which account for the various interaction of tetraspans with other molecules. Detailed sequence analysis of EC2 revealed that it is organized in two subdomains. The first subdomain, despite significant sequence divergence, appears to have a structurally conserved fold. A second subdomain, with an even higher heterogeneity, is extremely variable in size, secondary structure, and fold. The variable subdomain is inserted within the conserved subdomain and their relative topology is governed by the occurrence of key disulfide bridges, the number of which corresponds to distinct subtypes of tetraspanins (Hemler 2001). There are at least 28 distinct tetraspanin family members in mammals, 37 members in Drosophila, and 20 in C. elegans (Todres et al. 2000). The assumed functions of tetraspanins stem from their close association with various membrane proteins and receptors. The leading understanding of tetraspanins is of molecular facilitators, rather than independent receptors. A recent publication suggests however, that tetraspanins may be plasma membrane receptors in their own right. Pregnancy specific glycoprotein (PSG) 17 binds to CD9, a tetraspanin, and induces cytokines production by CD9 bearing macrophages (Waterhouse et al. 2002). The proposed downstream molecules to CD9 signaling were phosphatidylinositol 4-kinase and protein kinase C.
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