Picornavirus uncoating intermediate captured in atomic detail

Abstract

It remains largely mysterious how the genomes of non-enveloped eukaryotic viruses are transferred across a membrane into the host cell. Picornaviruses are simple models for such viruses, and initiate this uncoating process through particle expansion, which reveals channels through which internal capsid proteins and the viral genome presumably exit the particle, although this has not been clearly seen until now. Here we present the atomic structure of an uncoating intermediate for the major human picornavirus pathogen CAV16, which reveals VP1 partly extruded from the capsid, poised to embed in the host membrane. Together with previous low-resolution results, we are able to propose a detailed hypothesis for the ordered egress of the internal proteins, using two distinct sets of channels through the capsid, and suggest a structural link to the condensed RNA within the particle, which may be involved in triggering RNA release.

Cryo-EM study of slow bee paralysis virus at low pH reveals iflavirus genome release mechanism

“…These two regions are separated by a spherical shell 90-80 Å in diameter with a lower RNA density. The SBPV RNA does not appear to form specific contacts with the capsid, similar to the previously studied A particles of enteroviruses (22,39). No particles in the process of RNA release were observed in the cryo-micrographs of low-pH SBPV.…”

“…The A particles contain two types of pores located at twofold and between twofold and fivefold symmetry axes of the capsids, have N termini of VP1 subunits exposed at the virion surface, and spontaneously release VP4 subunits (46,47). The formation of enterovirus A particles is characterized by the movement of α3 helixes from the VP2 subunits away from the icosahedral twofold axis, which results in the formation of a 9 × 20-Å pore (22,39). In contrast, the α3 helices of VP2 subunits remain tightly associated in the SBPV empty capsid (Fig.…”

Antibody-induced uncoating of human rhinovirus B14

“…As a result, the tail participates in forming the aforementioned pores at twofold axes and probably contributes to stabilize the particle. Such structural change of the VP2 C-terminal tail has not been previously observed in other A-particle or empty particle-like structures of EVs (13,27).…”

“…In addition, because of the rigid body movements and the disordering of residues 3172-3181 in the VP3 GH loop (the loop that links β-strands G and H), pores are opened up near the quasi-threefold axes (Fig. 1), which probably allow for the externalization of VP1 N-terminal residues as previously suggested (26,27).…”

“…Unlike ICAM-1, binding of C5 Fab to the virus at physiological temperatures does not lead to the formation of an A-particle or A-particle-like intermediate state, which exhibits an expanded capsid that contains the viral RNA (11,27). This observation indicates that a higher activation energy barrier probably exists in the process of C5-Fab-induced virus uncoating than that required for ICAM-1-induced virus uncoating.…”