Potentially reduced fusogenicity of syncytin-2 in New World monkeys.

Syncytin-2 is a membrane fusion protein involved in placenta development that is derived from the endogenous retrovirus envelope gene acquired in the common ancestral lineage of New World and Old World monkeys. It is known that syncytin-2 is conserved between apes and Old World monkeys, suggesting its functional importance; however, syncytin-2 of common marmosets (Callithrix jacchus) exhibits lower fusogenic activity than those of humans and Old World monkeys in human cell lines. To obtain insight into the functional diversity of syncytin-2 genes in primates, we examined the syncytin-2 gene in New World monkeys. We experimentally evaluated the cell fusion ability of syncytin-2 in humans, C. jacchus, and tufted capuchins (Sapajus apella). We found that the cell fusion ability of S. apella was lower than that of human syncytin-2. Chimeric syncytin-2 constructs revealed that the amino acid differences in the surface unit of S. apella syncytin-2 were responsible for the weak cell fusion activity. In addition, genomic sequence analyses of syncytin-2 revealed that the open reading frames (ORFs) of syncytin-2 were highly conserved in 7 apes and 22 Old World monkeys; however, the syncytin-2 ORFs of three out of 12 New World monkey species were truncated. Our results suggest that syncytin-2 in several New World monkeys may be of less importance than in Old World monkeys and apes, and other syncytin-like genes may be required for placental development in various New World monkey species.