In vitro Reconstitution of Syncytin-2 and MFSD2A Reveals their Functional Tethering in Membrane Fusion Initiation.

Syncytin-2, a human endogenous retrovirus-derived fusogen, is essential for placental syncytiotrophoblast formation by mediating trophoblast cell-cell fusion. While Syncytin-2 shares structural similarities with viral envelope proteins, the molecular mechanisms underlying its receptor recognition and membrane fusion remain poorly characterized, largely due to the lack of in vitro reconstitution systems. Here, we established an in vitro reconstitution system to investigate the initial steps of Syncytin-2-mediated membrane fusion. We purified Syncytin-2 and its receptor MFSD2A and confirmed their interaction through co-immunoprecipitation and pull-down assays. Both proteins were successfully reconstituted into liposomes and exhibited specific tethering dependent on the receptor-fusogen interaction, which is the first committed step of fusion. This study provides an in vitro reconstitution of a placental fusogen-receptor pair, establishing a powerful platform for dissecting the molecular mechanism of syncytin-mediated cell-cell fusion and shedding light on understanding placenta development at the molecular level.