Molecular tectonic model of virus structural transitions: the putative cell entry states of poliovirus

Upon interacting with its receptor, poliovirus undergoes conformational cha nges that are implicated in cell entry, including the externalization of th e viral protein VP4 and the N terminus of VP1. We have determined the struc tures of native virions and of two putative cell entry intermediates, the 1 35S and 80S particles, at similar to 22-Angstrom resolution by cryo-electro n microscopy. The 135S and 80S particles are both similar to 4% larger than the virion. Pseudoatomic models were constructed by adjusting the beta-bar rel domains of the three capsid proteins VP1, VP2, and VP3 from their known positions in the virion to fit the 135S and 80S reconstructions. Domain mo vements of up to 9 Angstrom were detected, analogous to the shifting of tec tonic plates. These movements create gaps between adjacent subunits. The g aps at the sites ct-here VP1, VP3, and VP3 subunits meet are plausible cand idates for the emergence of VP4 and the N terminus of VP1. The implications of these observations are discussed for models in which the externalized c omponents form a transmembrane pore through which viral RNA enters the infe cted cell.