Flavivirus pre-membrane (prM) protein is important in assembly of viral particles. Its ectodomain is well conserved among flaviviruses. Previous work has shown that expression of this domain can trigger apoptosis in mammalian cells and that leucine to phenylalanine mutation of aminoacid residue 36 of yellow fever virus ectodomain can reduce induction of apoptosis. Here we investigated the effect of a similar mutation isoleucine to phenylalanine (M-I36F) in the prM ectodomain on the virus life cycle of Japanese Encephalitis (JEV) and West Nile (WNV) viruses. We demonstrated that viral particles production of the mutant M-I36F virus was impaired in mammalian cells, but not in mosquito cells. By using a virus-like particle (VLP) system, we were able to show that mutation M-I36F impaired late stages of virus life cycle. We also showed that the mutant virus M-I36F was attenuated in vivo in a mouse model of JEV and WNV infections and that mice infected with mutant virus produced neutralizing antibodies against JEV and WNV.
Electron microscopy analysis (collaboration with Ph. Roingeard, Université de Tours) shows that the M-I36F virus accumulates in the ER and ER-derived vesicles within the infected cells. Moreover, the overall aspect of the mutant viral particles strongly differed from that of the wt virus when produced in mammalian cells, but not in mosquito cells, demonstrating that its assembly is hampered in mammalian cells. We are currently studying the assembly and secretion steps of JEV and WNV viral particles in infected mammalian cells.