This endogenous retroviral envelope protein has retained its original fusogenic properties and participates in trophoblast fusion and the formation of a syncytium during placenta morphogenesis. May recognize and induce fusion through binding of SLC1A4 and SLC1A5 (By similarity). Endogenous envelope proteins may have kept, lost or modified their original function during evolution. Retroviral envelope proteins mediate receptor recognition and membrane fusion during early infection. The surface protein (SU) mediates receptor recognition, while the transmembrane protein (TM) acts as a class I viral fusion protein. The protein may have at least 3 conformational states: pre-fusion native state, pre-hairpin intermediate state, and post-fusion hairpin state. During viral and target cell membrane fusion, the coiled coil regions (heptad repeats) assume a trimer-of-hairpins structure, positioning the fusion peptide in close proximity to the C-terminal region of the ectodomain. The formation of this structure appears to drive apposition and subsequent fusion of membranes (By similarity). The mature envelope protein (Env) consists of a trimer of SU-TM heterodimers attached probably by a labile interchain disulfide bond. Interacts with the C-type lectin CD209/DC-SIGN (By similarity). The surface protein is not anchored to the membrane, but localizes to the extracellular surface through its binding to TM. The cytoplasmic region is essential for the fusiogenic function. The 17 amino acids long immunosuppressive region is present in many retroviral envelope proteins. Synthetic peptides derived from this relatively conserved sequence inhibit immune function in vitro and in vivo (By similarity). Specific enzymatic cleavages in vivo yield mature proteins. Envelope glycoproteins are synthesized as an inactive precursor that is heavily N-glycosylated and processed likely by furin in the Golgi to yield the mature SU and TM proteins. The cleavage site between SU and TM requires the minimal sequence [KR]-X-[KR]-R (By similarity). The CXXC motif is highly conserved across a broad range of retroviral envelope proteins. It is thought to participate in the formation of a labile disulfide bond possibly with the CX6CC motif present in the transmembrane protein (By similarity). Ortholog of the human HERV-W_7q21.2 envelope protein. The genome contains a high percentage of proviral-like elements, also called endogenous retroviruses (ERVs) that are the genomic traces of ancient infections of the germline by exogenous retroviruses. Although most of these elements are defective, some have conserved a functional envelope (env) gene, most probably diverted by the host for its benefit. Belongs to the gamma type-C retroviral envelope protein family. HERV class-I W env subfamily.