Figure 2.

Syncytin family expressed in human trophoblast. Syncytin-1 and syncytin-2 are the env gene products of HERV-W and HERV-FRD, respectively. The coding genes are also known as ERVW-1 and ERVFRD-1, respectively. It has been shown that both syncytin-1 and -2 promote cell-cell fusion in placenta development. These two syncytins have all domains required for the fusion process; a fusion core made of heptad repeats (HR1 and HR2), CX(6)C motif, and homotrimerization interface. ERVV-1, ERVV-2, and ERV3-1 are little studied but are expressed in syncytiotrophoblast from early human embryos and in in vitro differentiated placental trophoblast (TB). ERVRH48-1 is known as suppressyn or SUPYN, expressed in unfused cytotrophoblast cells. SUPYN inhibits TB fusion in vitro, because it lacks all major domains and presumably acts as a competitor for other syncytins. ERVMER34-1 may inhibit promote fusion, too. ERVK13-1 encodes a long non-coding RNA transcript of unknown function. The domains are retrieved from NCBI's Conserved Domain Database (CDD) (Marchler-Bauer et al. 2017).