Functional refolding of the penetration protein on a non-enveloped virus

Non-enveloped viruses must create a transient membrane lesion to initiate infection by transferring their genomes into a target cell(1). Rotaviruses offer a particularly favorable opportunity to visualize the mechanism for subviral particle delivery, the principal function of their outer-layer protein, VP4(2–4). We show here by electron cryomicroscopy (cryo-EM) that VP4, activated by cleavage to VP8* and VP5*, rearranges on the virion surface from an “upright” to a “reversed” conformation. The reversed structure projects an initially buried, “foot” domain outward into the host cell membrane to which the virion has attached. Analysis of cryo-tomograms of virus particles entering cells is consistent with this picture. We have stabilized with a disulfide mutant a likely intermediate in this transition. The results define molecular mechanisms for the first steps in viral membrane penetration and suggest similarities with mechanisms postulated for other viruses.