A Rotavirus Spike Protein Conformational Intermediate Binds Lipid Bilayers

During rotavirus entry, a virion penetrates a host cell membrane, sheds its outer capsid proteins, and releases a transcriptionally active subviral particle into the cytoplasm. VP5*, the rotavirus protein believed to interact with the membrane bilayer, is a tryptic cleavage product of the outer capsid spike protein, VP4. When a rotavirus particle uncoats, VP5* folds back, in a rearrangement that resembles the fusogenic conformational changes in enveloped-virus fusion proteins. We present direct experimental evidence that this rearrangement leads to membrane binding. VP5* does not associate with liposomes when mounted as part of the trypsin-primed spikes on intact virions, nor does it do so after it has folded back into a stably trimeric, low-energy state. But it does bind liposomes when they are added to virions before uncoating, and VP5* rearrangement is then triggered by addition of EDTA. The presence of liposomes during the rearrangement enhances the otherwise inefficient VP5* conformational change. A VP5* fragment, VP5CT, produced from monomeric recombinant VP4 by successive treatments with chymotrypsin and trypsin, also binds liposomes only when the proteolysis proceeds in their presence. A monoclonal antibody that neutralizes infectivity by blocking a postattachment entry event also blocks VP5* liposome association. We propose that VP5* binds lipid bilayers in an intermediate conformational state, analogous to the extended intermediate conformation of enveloped-virus fusion proteins.