Functional characterization of syncytin-A, a newly murine endogenous virus envelope protein. Implication for its fusion mechanism.

Trophoblast fusion in placenta is an important event for preservation of a healthy pregnancy. This process takes place throughout the pregnancy and is crucial for the formation of syncytiotrophoblast layer. Syncytin-1 and syncytin-2 are strong candidate regulators of fusion from retroviral origin. Syncytin-A and syncytin-B are other candidates from retroviral origin in Muridae. The active role of syncytin in driving fusion of trophoblast has been identified, but its fusion mechanism is still unclear. As an intact retroviral envelope protein, syncytin-A shares similar structure profiling with other viral envelope fusion proteins, especially in the regions of N- and C-terminal heptad repeats (NHR and CHR, respectively). In this paper, we showed that SynA 1 + 2 of syncytin-A (residues 445-536, including predicted NHR, CHR, and a natural linker) could form trimer and exhibited significant alpha-helix structure and high thermo-stability. Limited proteolysis result identified a stable protease-resistant core of SynA 1 + 2, which was in good agreement with computational modeling data. NHR and CHR could interact with each other in vitro, too. Different from the previous studies, the disulfide-bonded linker was apparently vital to the stability of fusion core structure. By biological assays, NHR was shown to be inhibitive to cell-cell fusion, with IC(50) value about 5.4 microm, but CHR seemed to have no inhibitory activity even at 50 microm. From both biochemical and functional data, we first gave an explanation how syncytin-A mediated cell fusion. The insight into the mechanism of syncytin-A-mediated cell-cell fusion may provide a crucial clue to placental cytotrophoblast morphogenesis.