FIG. 3.
Fit of VPs in the empty capsid of HRV14 at the correct size (gold) showing the Cα backbones of VP1-Δ (blue), VP2 (green), and VP3 (red). (A) Overall view with the two-, three-, and fivefold axes marked. The C terminus of VP3 that lies outside the density of the empty capsid is indicated with an open arrowhead. (B and C) Views of the two- and threefold axes. The N-terminal loop of VP2 encroaches on the region of low density at the twofold axis (indicated by arrowheads in panels A and B), so the N-terminal loop of VP2 has most probably been modified. In panel B, the grey lines represent the map simulated from the best-fit positions. The position of the N terminus of VP2 is marked by an N in panels A and C. The VPs on one icosahedral pentamer are shown in panel D for the native capsid and in panels E and F for the best-fit empty capsid. In panel F, the 20 N-terminal residues of VP3 have been removed to show that this would allow exit of the N terminus of VP1 as indicated in black. In three VP1 molecules, the 62nd residue is marked with an asterisk. (G and H) Schematic diagrams of the reorganization of the HRV14 capsid during release of the RNA. In the native capsid (G), the RNA is bound to Trp 2038 of VP2 and the pocket is filled with a pocket factor. The cellular receptor ICAM-1 binds in the canyon in a two-step process which expels the pocket factor and induces a hinge-type movement of VP1 (23) as the capsid undergoes a 4% expansion (H). Each VP1 protein is cantilevered up and away from the fivefold axis while it swivels around to open a 10-Å-diameter channel. The β-cylinders formed by the N termini of VP3 on each fivefold axis mostly remain in place. We postulate that at least one such β-cylinder must be modified to allow exit of the VP4 molecules, the N termini of VP1, and the RNA. We propose that the N termini of the VP1 molecules exit the capsid along channels which open up between neighboring VP1 molecules and become anchored in the membrane to retain the HRV14 close to the membrane. In association with VP4, which probably exits via the fivefold axis, they destroy the endosomal membrane and thereby releases the RNA into the cytosol (31). The N-terminal loop of VP2 is modified to detach the RNA bound to the conserved Trp 2038.