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1.G.9 The Syncytin (Syncytin) Family

 Syncytin-1 of the endogenous defective retrovirus HERV-W, is a 'captive' retroviral envelope protein involved in placental morphogenesis (Mi et al., 2000). The viral protein has assumed an important function in mammalian physiology. Expression of recombinant syncytin induces formation of giant syncytia, and fusion of a human trophoblast cell line is mediated by syncytin. Thus, syncytin, a probable viral membrane fusion protein, mediates placental cytotrophoblast fusion in vivo, and thus is important for human placental morphogenesis (Mi et al., 2000). The ERVW-1 receptor is SLC1A5/ASCT-2/RDR/ATB0, a sodium-dependent neutral amino acid transporter B0 (2.A.23.3.3), and their interaction is mediated by connexin 43. Both proteins are essential for trophoblast cell fusion (Dunk et al., 2012).  Fusion in the placenta is facilitated by syncytin 1 and syncytin 2. These syncytins arose from retroviral sequences that entered the primate genome 25 million and more than 40 million years ago, respectively. Syncytins and their receptors are involved in fusion events during human reproduction and during tumorigenesis. (Soygur and Sati 2016).

References associated with 1.G.9 family:

Apellaniz B., Huarte N., Largo E. and Nieva JL. (2014). The three lives of viral fusion peptides. Chem Phys Lipids. 181:40-55. 24704587
Blond, J.L., D. Lavillette, V. Cheynet, O. Bouton, G. Oriol, S. Chapel-Fernandes, B. Mandrand, F. Mallet, and F.L. Cosset. (2000). An envelope glycoprotein of the human endogenous retrovirus HERV-W is expressed in the human placenta and fuses cells expressing the type D mammalian retrovirus receptor. J. Virol. 74: 3321-3329. 10708449
Blond, J.L., F. Besème, L. Duret, O. Bouton, F. Bedin, H. Perron, B. Mandrand, and F. Mallet. (1999). Molecular characterization and placental expression of HERV-W, a new human endogenous retrovirus family. J. Virol. 73: 1175-1185. 9882319
Dunk, C.E., A. Gellhaus, S. Drewlo, D. Baczyk, A.J. Pötgens, E. Winterhager, J.C. Kingdom, and S.J. Lye. (2012). The molecular role of connexin 43 in human trophoblast cell fusion. Biol Reprod 86: 115. 22238282
Lavillette, D., M. Marin, A. Ruggieri, F. Mallet, F.L. Cosset, and D. Kabat. (2002). The envelope glycoprotein of human endogenous retrovirus type W uses a divergent family of amino acid transporters/cell surface receptors. J. Virol. 76: 6442-6452. 12050356
Mi, S., X. Lee, X. Li, G.M. Veldman, H. Finnerty, L. Racie, E. LaVallie, X.Y. Tang, P. Edouard, S. Howes, J.C. Keith, Jr, and J.M. McCoy. (2000). Syncytin is a captive retroviral envelope protein involved in human placental morphogenesis. Nature 403: 785-789. 10693809
Soygur, B. and L. Sati. (2016). The role of syncytins in human reproduction and reproductive organ cancers. Reproduction 152: R167-178. 27486264
Sugimoto, J., M. Sugimoto, H. Bernstein, Y. Jinno, and D. Schust. (2013). A novel human endogenous retroviral protein inhibits cell-cell fusion. Sci Rep 3: 1462. 23492904