FIG 6.

Lower-resolution maps validate the atomic model. To verify that the newly built conformation of the GH loop of VP1 is real and not an artifact of high-frequency noise, we displayed the pseudoatomic model, superimposed on two independent lower-resolution reconstructions of poliovirus 135S particles (EMDB entries 1133 and 1144) that were previously solved by Bubeck et al. at 10-Å resolution (17). Comfortably above the noise level, both 135S maps show extra density features (which are not present in 160S particles), located atop the VP2 beta barrel (arrow) and bridging across the canyon (open arrowhead). Importantly, these density features were more pronounced in the 1144 population of particles, though the two populations were produced using the same protocol and processed identically using PFT2 (42) and EM3DR2 (http://people.chem.byu.edu/belnap/). Observe that the 1144 map (middle panel) has a density projection that covers most of the newly built GH loop of VP1 (red). In contrast, coverage of the GH loop is poor in the 1133 map (left panel). For comparison (right), the 160S and 73S conformations of the VP1 GH loop (red), the VP1 C terminus (gray), and the tip of the EF loop of VP2 (gray) all clearly lie outside the isocontour surface in both maps.