Structure of a Pestivirus Envelope Glycoprotein E2 Clarifies Its Role in Cell Entry

Enveloped viruses have developed various adroit mechanisms to invade their host cells. This process requires one or more viral envelope glycoprotein to achieve cell attachment and membrane fusion. Members of the Flaviviridae such as flaviviruses possess only one envelope glycoprotein, E, whereas pestiviruses and hepacivirus encode two glycoproteins, E1 and E2. Although E2 is involved in cell attachment, it has been unclear which protein is responsible for membrane fusion. We report the crystal structures of the homodimeric glycoprotein E2 from the pestivirus bovine viral diarrhea virus 1 (BVDV1) at both neutral and low pH. Unexpectedly, BVDV1 E2 does not have a class II fusion protein fold, and at low pH the N-terminal domain is disordered, similarly to the intermediate postfusion state of E2 from sindbis virus, an alphavirus. Our results suggest that the pestivirus and possibly the hepacivirus fusion machinery are unlike any previously observed. [Display omitted] ► Structure of the major antigenically dominant protein of BVDV ► The overall fold of BVDV E2 shows no similarity to the class II fusion proteins ► At low pH, BVDV E2 N-terminal domain is disordered ► Entry mechanism of BVDV is probably applicable to hepatitis C virus Stuart and colleagues have determined the atomic structure of the ectodomain of bovine viral diarrhea virus E2 glycoprotein, the major, antigenically dominant protein on the virus surface. The structure was expected to resemble the fusion molecules found on the surface of viruses such as dengue virus, but it is unlike anything previously seen. E2 itself is not, in fact, the fusion protein but binds the cell receptor and directs fusion via a pH-dependent conformational switch.