This endogenous retroviral envelope protein has retained its original fusogenic properties (PubMed:15644441, PubMed:17762178, PubMed:18077339, PubMed:19564597, PubMed:27589388). Together with Synb, participates in trophoblast fusion and the formation of a syncytium during placenta morphogenesis (PubMed:19564597). Syna is essential for placental development and is specifically required for formation of syncytiotrophoblast layer I (SynT-I) (PubMed:19564597). Promotes muscle myoblast fusion (PubMed:27589388). Does not have immunosuppressive activity (PubMed:18077339). The mature protein consists of a trimer of SU-TM heterodimers (Probable). The SU-TM heterodimers are attached by a labile interchain disulfide bond (By similarity). The surface protein is not anchored to the membrane, but localizes to the extracellular surface through its binding to TM. Highly expressed in placenta where it localizes to syncytiotrophoblasts of the labyrinthine zona (PubMed:15644441). Specifically localizes to syncytiotrophoblast layer I (SynT-I) (PubMed:18448564). Also detected at very low levels in hippocampus, brain, testis and ovary (PubMed:15644441). Expressed in the placental labyrinth from stage 8.5 dpc onwards. 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. 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. Isomerization of the intersubunit disulfide bond to an SU intrachain disulfide bond is thought to occur upon receptor recognition in order to allow membrane fusion. Embryonic lethal with no survival beyond stage 14.5 dpc (PubMed:19564597). Embryos show retarded growth but otherwise have no significant morphological defects (PubMed:19564597). Placental development is abnormal with significantly reduced vascularization of extraembryonic tissues (PubMed:19564597). In the placental labyrinth, there is an expansion of trophoblast cells which reduces available space for fetal blood vessels (PubMed:19564597). Trophoblast cells fail to fuse and form syncytiotrophoblast layer I (SynT-I), however development of syncytiotrophoblast layer II (SynT-II) is not significantly affected (PubMed:19564597). Double knockouts of Syna and Synb are embryonic lethal at stage 9.5 dpc to 10.5 dpc, indicating a more severe phenotype than the Syna single knockout (PubMed:22032925). The mouse 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.