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). The effects of individually silenced N-glycosylation sites and non-synonymous single-nucleotide polymorphisms on the fusogenic function of human syncytin-2 have been demonstrated (Cui et al. 2016).Syncytins have bee reviewed by Hernández and Podbilewicz 2017.