Differential Evolutionary Fate of an Ancestral Primate Endogenous Retrovirus Envelope Gene, the EnvV Syncytin, Captured for a Function in Placentation
Figure 3
Cell–cell fusion assay for the primate EnvV proteins.
(A) Env-expressing vectors and rationale of the assay. Each of the 24 cloned envV genes was introduced into the phCMV expression vector, between the beta-globin intron and polyadenylation (pA) sequences. Cells were transfected and stained with May-Grünwald-Giemsa 24 h after transfection. The fusion index is defined as [(N-S)/T]×100, where N is the number of nuclei in the syncytia, S is the number of syncytia, and T is the total number of nuclei counted. (B) 293T cells transfected with expression vectors for no protein (none), and for the human, macaque and marmoset EnvV2 (V2 hum, V2 mac and V2 mar, respectively), displaying large multinucleated syncytia 24 h later, only upon transfection with the latter two. (C) Histogram showing the fusion index of the indicated series of primate envV1 (upper) and envV2 (lower) genes in 293T cells transfected with the corresponding expression vectors. The primate phylogeny is illustrated below, with the Old World monkeys (OWM) and New World monkeys (NWM) indicated. None, no protein; hum, human; cpz, chimpanzee; gor, gorilla; gib, gibbon; mac, macaque; bab, baboon; agm, African green monkey; lan, langur; mar, marmoset; tam, tamarin; sak, saki. (D) Same as (C) but with feline G355.5 cells transfected with the envV2 expression vectors.