Residues SFQ ( 173 - 175 ) in the large extracellular loop of CD 9 are required for gamete fusion

Sperm-egg binding and fusion initiate the development of a new organism, but the molecular mechanisms of gamete adhesion, gamete membrane fusion and associated signaling are still poorly understood. Recently, one egg surface protein, CD9, was shown to be essential for gamete fusion. The fertility of CD9-deficient female mice is severely reduced because membrane fusion ability is lost in CD9-deficient eggs (Kaji et al., 2000; Le Naour et al., 2000; Miyado et al., 2000). CD9 belongs to a family of proteins called tetraspanins, which contain four transmembrane domains, one small extracellular loop (EC1) and one large extracellular loop (EC2), and cytoplasmic amino and carboxyl termini (Boucheix and Rubinstein, 2001). Tetraspanins are reported to function in a variety of cell activities including cell adhesion, motility, proliferation, differentiation and signaling (Maecker et al., 1997; Lagaudriere-Gesbert et al., 1997). Immunoprecipitation and other studies suggest that tetraspanins in the plasma membrane are associated with each other and with several other cell surface molecules, including a subset of β1 integrins and Ig superfamily members, to form a tetraspanin web (Nakamura et al., 1995; Berditchevski et al., 1996; Rubinstein et al., 1996; Serru et al., 1999; Charrin et al., 2001; Stipp et al., 2001; Maecker et al., 1997; Boucheix and Rubinstein, 2001). Tetraspanins may organize specific cell surface molecules to form functional macromolecular complexes on the surface of the cell that express the tetraspanin (Maecker et al., 1997; Boucheix and Rubinstein, 2001). Additionally, it is possible that tetraspanins are ligands for receptors on other cells, as reported for the tetraspanin CD81 (Kelic et al., 2001). Another egg surface protein, the integrin α6β1, was previously proposed as the receptor for sperm on mouse eggs (Almeida et al., 1995). It was thought that α6β1 binds to the disintegrin domain of fertilin β on the sperm surface (Chen and Sampson, 1999; Chen et al., 1999; Evans, 2001). Because it was found that α6β1 and CD9 could be coimmunoprecipitated from mouse eggs, it was suggested that CD9 plays its crucial role in gamete fusion through involvement in a fertilin β-α6β1/CD9 intergamete complex (Miyado et al., 2000). This view, however, is challenged by two other reports. One shows that fertilin βdeficient sperm fuse with eggs at 50% of the wild-type level (Cho et al., 1998). The other shows that α6β1-deficient eggs fuse with sperm at the same level as the wild-type eggs (Miller et al., 2000). These two reports indicate that there must be other molecules on the cell surface of gametes that can act in spermegg fusion. 1995 Development 129, 1995-2002 (2002) Printed in Great Britain © The Company of Biologists Limited 2002 DEV3606