Figure 3.

Electron cryo-tomograms of influenza virions at pH 5.5. (A) 5.3 nm-thick section through the reconstructed tomographic density for an X31 influenza A virion at pH 5.5 after 5 min acidification. The glycoprotein fringe is evident on the exterior. Most particles bear an ∼8–12 nm-thick viral envelope. Density beneath the exterior spikes (red arrowheads), which bores through the viral envelope (white arrowheads), may be HA and NA transmembrane anchors. HA (green arrowhead) and NA (orange arrowhead) ectodomains are distinguishable. Inset: HA spike density, cropped from the tomogram is in good agreement with the HA ectodomain crystal structure; anchors are modeled into the structure (Wilson et al, 1981). The spike is tethered to three layers of the virus envelope (postulated strata: black arrowhead outer leaflet, white, inner leaflet, grey, M1 matrix protein layer). Microscope defocus 3 μm. Scalebar 50 nm. (B) At early timepoints, DOPC liposomes do not appear to be deformed by virions even when they are in close proximity. Sample acidified at pH 5.5 for 2.5 min. Microscope defocus 3 μm. Scalebar 50 nm.