The mechanism by which non-enveloped RNA viruses, such as the caliciviruses, escape the endosome, is poorly understood. The Caliciviridae are a family of viruses which include many important human and animal pathogens, most notably norovirus which causes winter vomiting disease. We used cryoEM and asymmetric three-dimensional reconstruction to investigate structural changes in the capsid of feline calicivirus (FCV) that occur upon virus binding to its cellular receptor; feline junctional adhesion molecule-A (fJAM-A). We discovered that following receptor engagement substantial conformational changes in the FCV capsid lead to the assembly of a portal-like structure at a unique three-fold symmetry axis. Atomic models of the major capsid protein, VP1, in the presence and absence of fJAM-A were calculated, revealing the conformational changes induced by the interaction. In the course of this analysis we discovered a large portal-like structure which assembles at a unique three-fold axis. The portal-like complex comprises 12 copies of the minor capsid protein VP2. We calculated an atomic model of VP2 and revealed structural changes in VP1 that lead to the formation of a pore in the capsid shell at the portal vertex. VP2 is encoded by all caliciviruses although despite being critical for the production of infectious virus, its function and structure were, until now, undetermined. We hypothesise that the VP2 portal-like complex is the method by which the virus escapes the endosome during virus entry, allowing delivery of the viral genome into the cytoplasm for replication to then ensue.

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