The membrane-proximal “stem” region increases the stability of the flavivirus e protein postfusion trimer and modulates its structure

The flavivirus fusion protein E contains a “stem ” region which is hypothesized to be crucial for driving fusion. This sequence element connects the ectodomain to the membrane anchor, and its structure in the trimeric postfusion conformation is still poorly defined. Using E trimers of tick-borne encephalitis virus with stem truncations of different lengths, we show that the N-terminal part of the stem increases trimer stability and also modulates the trimer structure outside the stem interaction site. Flaviviruses, members of a genus in the family Flaviviridae,comprise a number of medically relevant viruses, such as den-gue virus, yellow fever virus, Japanese encephalitis virus, West Nile virus, and tick-borne encephalitis virus (TBEV) (1). These small enveloped viruses infect cells by receptor-mediated endocy-tosis and acidic-pH-induced membrane fusion in endosomes (2, 3). The fusion reaction is mediated by the major envelope protein E, a class II fusion protein (4), which contains three domains (DI, DII, and DIII), a membrane-proximal region (the so-called “stem”), and the carboxy-terminal transmembrane anchor (Fig. 1A). The current flavivirus fusion model (Fig. 1A to C) is based o