Cryoelectron microscopy of mouse mammary tumor virus

Abstract

Cryoelectron microscopy of Mouse mammary tumor virus, a Betaretrovirus, provided information about glycoprotein structure and core formation. The virions showed the broad range of diameters typical of retroviruses. Betaretroviruses assemble cytoplasmically, so the broad size range cannot reflect the use of the plasma membrane as a platform for assembly.

FIG. 1.

Two fields of MMTV virions. The range of particle size and core size is illustrated. Large black arrows indicate two cores within one virion. The inset shows a contrast transfer function corrected image taken closer to focus. Small black arrows indicate the two radially arranged densities in the glycoprotein. Scale bar, 200 nm.

FIG. 2.

(A) The radial density profile of the membrane region of mature MMTV. Radial density profiles for eight MMTV virions were aligned and averaged. The two largest peaks correspond to the inner (left) and outer (right) leaflets of the bilayer. Outside the membrane, the glycoprotein consists of two distinct densities. The inset shows the schematic glycoprotein structure (black) with the lipid bilayer (grey), indicating the distances in nanometers between features. (B) A top view of the viral spike obtained by aligning, averaging, and applying threefold symmetry to images of 542 negatively stained spikes. Scale bar, 5 nm.

FIG. 3.

The size distribution of mature HIV-1 (2) and MMTV virions. A histogram of the diameters to the outside of the lipid bilayer for all virions (grey bars) and for the subset of virions for which single cores were clearly distinguished (black bars) is shown.

FIG. 4.

The relationship between virus size and core size. The mean of the long and short axial lengths of the core were plotted against the virus diameter. There is no significant correlation.