Abstract
Entry of enveloped viruses requires fusion of viral and cellular membranes, driven by conformational changes of viral glycoproteins. Crystal structures provide static pictures of pre- and post-fusion conformations of these proteins but the transition pathway remains elusive. Here, using several biophysical techniques, including analytical ultracentrifugation, circular dichroïsm, electron microscopy and small angle X-ray scattering, we have characterized the low-pH-induced fusogenic structural transition of a soluble form of vesicular stomatitis virus (VSV) glycoprotein G ectodomain (Gth, aa residues 1–422, the fragment that was previously crystallized). While the post-fusion trimer is the major species detected at low pH, the pre-fusion trimer is not detected in solution. Rather, at high pH, Gth is a flexible monomer that explores a large conformational space. The monomeric population exhibits a marked pH-dependence and adopts more elongated conformations when pH decreases. Furthermore, large relative movements of domains are detected in absence of significant secondary structure modification. Solution studies are complemented by electron micrographs of negatively stained viral particles in which monomeric ectodomains of G are observed at the viral surface at both pH 7.5 and pH 6.7. We propose that the monomers are intermediates during the conformational change and thus that VSV G trimers dissociate at the viral surface during the structural transition.
Highlights
Entry of enveloped viruses into host cells requires fusion of the viral envelope with the cellular membrane
There is a lack of information about G structural intermediates during the transition, in particular topological issues concerning the transition pathway, as the structural rearrangement cannot occur without breaking the threefold symmetry
We analyzed the structure of a soluble form of G ectodomain at several pH values to follow the structural transition
Summary
Entry of enveloped viruses into host cells requires fusion of the viral envelope with the cellular membrane. This step is mediated by virally encoded glycoproteins, anchored in the viral membrane by a transmembrane (TM) domain, that undergo large structural rearrangements following interaction with specific triggers (e.g. a low pH environment and/or cellular receptors). These conformational changes result in the exposure of hydrophobic motifs (socalled ‘‘fusion peptides’’ or ‘‘fusion loops’’), which interact with one or both of the participating membranes, resulting in their destabilization and fusion. Class III fusion proteins combine structural elements found in the two other classes [8,9,10,11]
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