Mutational analysis of the Flavivirus fusion trigger

The entry of enveloped viruses into cells requires the function of viral surface proteins (fusion proteins) that mediate the fusion of the viral membrane with a cellular membrane. This function is dependent on a specific trigger, which allows the fusion protein to attach to the host membrane and causes it to undergo structural alterations that ultimately drive the merger of the membranes. In some cases the trigger is a specific interaction with a receptor molecule at the cell surface. In other cases the virus is first internalized by the cell and then fuses with the membrane of the endocytic vesicle in response to acidification. In low-pH-induced fusion, protonation of key residues in the viral fusion protein initiates a conformational change that drives the fusion reaction, but so far it is not known for any enveloped virus how individual protonation events induce this change. The flaviviruses are a genus of enveloped viruses that includes important human pathogens such as the yellow fever, dengue, West Nile, Japanese encephalitis, and tick-borne encephalitis (TBE) viruses. These viruses enter host cells by endocytosis and low-pH-induced fusion, and the structures of the pre- and postfusion conformations of the fusion proteins (envelope protein E) of dengue and TBE virus have been determined by X-ray crystallography. The goal of this project is to use structure-based mutagenesis to identify specific amino acids in the TBE virus E protein that are involved in pH sensing and induction of the conformational changes that lead to membrane fusion. The mechanistic details revealed in this study should provide new insights into how the entry of flaviviruses into cells is regulated.