Multiple signals in the gut contract the mouse norovirus capsid to block antibody binding while enhancing receptor affinity.

Human norovirus is the leading cause of gastroenteritis world-wide, with no approved vaccine or antiviral treatment to mitigate infection. These plus-strand RNA viruses have T=3 icosahedral protein capsids with 90 pronounced protruding (P) domain dimers to which antibodies and cellular receptors bind. We previously demonstrated that bile binding to the capsid of mouse norovirus (MNV) causes several major conformational changes; the entire P domain rotates by ∼90° and contracts onto the shell, the P domain dimers rotate about each other, and the structural equilibrium of the epitopes at the top of the P domain shift towards the 'closed' conformation that favors receptor binding while blocking antibody binding. Here we demonstrate that MNV undergoes reversible conformational changes at pH 5.0 nearly identical to when bile binds. Notably, at low pH or when metals bind, a cluster of acidic resides in the G'H' loop interact, distort the G'H' loop, and this may drive C'D' loop movement towards the closed conformation. ELISA assays with infectious virus particles at low pH or in the presence of metals demonstrated that all tested antibodies do not bind to this contracted form, akin to what was observed with the MNV/bile complex. Therefore, low pH, cationic metals, and bile salts are physiological 'triggers' in the gut for P domain contraction and structural rearrangement that synergistically prime the virus for receptor binding while blocking antibody binding. Importance The protruding domains on the Calicivirus capsids are recognized by cell receptor(s) and antibodies. We demonstrated that mouse norovirus (MNV) P domains are highly mobile and bile causes contraction onto the shell surface while allosterically blocking antibody binding. We present the near atomic cryo-EM structures of infectious MNV at pH 5.0 and 7.5. Surprisingly, low pH is sufficient to cause the same conformational changes as when bile binds. A cluster of acidic residues on the G'H' loop was the most likely involved with the pH effects. These residues also bind divalent cations and had the same conformation as observed here at pH 5. Binding assays demonstrate that low pH and metals block antibody binding and therefore the G'H' loop might be driving the conformational changes. Therefore, low pH, cationic metals, and bile salts in the gut synergistically prime the virus for receptor binding while blocking antibody binding.