The Unique Transmembrane Hairpin of Flavivirus Fusion Protein E Is Essential for Membrane Fusion

The fusion of enveloped viruses with cellular membranes is mediated by proteins that are anchored in the lipid bilayer of the virus and capable of triggered conformational changes necessary for driving fusion. The flavivirus envelope protein E is the only known viral fusion protein with a double membrane anchor, consisting of two antiparallel transmembrane helices (TM1 and TM2). TM1 functions as a stop-transfer sequence and TM2 as an internal signal sequence for the first nonstructural protein during polyprotein processing. The possible role of this peculiar C-terminal helical hairpin in membrane fusion has not been investigated so far. We addressed this question by studying TM mutants of tick-borne encephalitis virus (TBEV) recombinant subviral particles (RSPs), an established model system for flavivirus membrane fusion. The engineered mutations included the deletion of TM2, the replacement of both TM domains (TMDs) by those of the related Japanese encephalitis virus (JEV), and the use of chimeric TBEV-JEV membrane anchors. Using these mutant RSPs, we provide evidence that TM2 is not just a remnant of polyprotein processing but, together with TM1, plays an active role in fusion. None of the TM mutations, including the deletion of TM2, affected early steps of the fusion process, but TM interactions apparently contribute to the stability of the postfusion E trimer and the completion of the merger of the membranes. Our data provide evidence for both intratrimer and intertrimer interactions mediated by the TMDs of E and thus extend the existing models of flavivirus membrane fusion.