Noroviruses, members of the Calicivirus family, are the major cause of epidemic gastroenteritis in humans, causing ∼20 million cases annually. 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 ...
Noroviruses, members of the Calicivirus family, are the major cause of epidemic gastroenteritis in humans, causing ∼20 million cases annually. 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. In the case of mouse norovirus (MNV), bile salts have been shown to enhance receptor (CD300lf) binding to the P domain. We previously demonstrated that the P domains of several genotypes are markedly flexible and 'float' over the shell, but the role of this flexibility was unclear. Recently, we demonstrated that bile causes a 90° rotation and collapse of the P domain on to the shell surface. Since bile binds distal to the P/shell interface, it was not at all clear how it could cause such dramatic changes. Here we present the near-atomic resolution cryo-EM structure of the protruding MNV complexed with a neutralizing Fab. Combined with previous results, we show here that bile salts cause allosteric conformational changes in the P domain that block antibody recognition to the top of the P domain. In addition, bile also causes a major rearrangement of the P domain dimers that are likely responsible for the bile-induced collapse of the P domain onto the shell. In the contracted shell conformation, antibodies to the P1 and shell domains are not expected to bind. Therefore, at the site of infection in the gut, the host's own bile allows the virus to escape antibody-mediated neutralization while enhancing cell attachment. Importance: The major feature of the Calicivirus capsids are the 90 protruding domains (P domains) that are the site of cell receptor(s) attachment and antibody epitopes. We previously demonstrated that these P domains are highly mobile and that bile causes these 'floating' P domains in mouse norovirus (MNV) to contract onto the shell surface. Here, we present the near atomic cryo-EM structure of the isolated MNV P domain complexed with a neutralizing Fab fragment. Together, the data shows that bile causes two sets of changes. First, bile causes allosteric conformational changes in the epitopes at the top of the P domain that block antibody binding. Second, bile causes the P domain dimer subunits to rotate relative to each other, causing contraction of the P domain that buries epitopes at the base of the P and shell domains. Collectively, MNV uses the host's own metabolites to enhance cell receptor binding while simultaneously blocking antibody recognition.
Organizational Affiliation:
University of Texas Medical Branch at Galveston, Department of Biochemistry and Molecular Biology, Galveston, Texas, USA.
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