Sugimoto, J.; Schust, D. J.; Nakagawa, S.; Hiyoshi, M.; Saito, M.; Sugimoto, M.; Nagamatsu, T.; Takahashi, H.; Sotomaru, Y.; Jinno, Y.; Kudo, Y.

Cell-cell fusion is a fundamental biological process required for tissue morphogenesis, regeneration, and viral infection. In mammals, trophoblast fusion is indispensable for placental development and is mediated by endogenous retrovirus (ERV)-derived fusogenic proteins known as syncytins. Although several ERV-derived proteins promote membrane fusion, little is known about evolutionarily conserved mechanisms that negatively regulate this process. We identified a previously uncharacterized murine ERV envelope-derived protein that potently suppresses syncytin-mediated membrane fusion and designated it mouse suppressyn-like 1 (mSUPYNL1). Unlike human suppressyn (hSUPYN), which inhibits fusion through receptor interference, mSUPYNL1 suppressed both murine and human syncytin-mediated fusion independently of receptor usage. Mechanistically, mSUPYNL1 directly associates with the surface (SU) subunit of multiple syncytin envelope glycoproteins, revealing a previously unrecognized receptor-independent mechanism for regulating membrane fusion. Unexpectedly, this activity extended beyond endogenous fusogens. mSUPYNL1 also directly associates with the SU glycoprotein (gp46) of Human T-cell Leukemia Virus type 1 (HTLV-1), markedly suppressing virus-induced syncytium formation, whereas human suppressyn showed no detectable antiviral activity in this assay. These findings identify mSUPYNL1 as a broad-spectrum inhibitor of envelope glycoprotein-mediated membrane fusion. To define its physiological context, we generated mSUPYNL1 knockout mice and characterized gene expression by in situ hybridization, immunoblotting, and reporter knock-in analyses. In contrast to the placenta-restricted expression of hSUPYN, mSUPYNL1 was broadly expressed, with its most prominent localization in decidual stromal and vascular endothelial cells of the pregnant uterus, as well as in hematopoietic tissues including the spleen and thymus. Together, our findings uncover a previously unknown class of ERV-derived fusion suppressors that function through direct association with viral envelope glycoproteins rather than receptor interference. These results expand current concepts of ERV protein domestication by demonstrating that endogenous retroviral envelope proteins have evolved not only to promote membrane fusion but also to restrain it, providing a mechanistic link between placental development, antiviral defense, and host evolutionary adaptation.