Surface structure of mouse mammary tumor virus

The structure of the envelope of mouse mammary tumor virus (MuMTV) was studied by using negative stain, thin sections and freeze-drying-shadowing. The surface of the viral membrane was found to contain a reticular structure composed mainly of hexagons and occasional pentagons. The spacing between the nearest corners was about 74 Å. The viral projections were attached to this reticulum and each projection was surrounded by five or six immediate neighbors. The average interprojection distance was 73.7 ± 10.3 Å. The projections have two distinguishable components, knobs at the distal ends and thin stalks that connect the knobs to the viral member-reticulum. The knobs measure 54.4 ± 10.3 Å in diameter and the average length of the projections was 94.9 ± 8.4 Å. The projection knobs were found to be composed of at least three subunits 15–25 Å in diameter. To explain the pattern of distribution of the surface projections in relation to the reticular structure, we propose that MuMTV contains two types of projections.Freeze-drying and shadowing revealed a highly regular hexagonal array of pits on the viral surface. Pits in fivefold symmetry were also observed. They were spaced 218 ± 43 Å apart and were about 80–100 Å in diameter.Considering the symmetry of the surface projections, reticular structure and pits, we conclude that the shape of the envelope of mouse mammary tumor virus is quasi-icosahedral or like a geodesic dome.