FIGURE 5-43 Virion structures. (a) Helical tobacco mosaic virus. (b, left) Diagram of the structure of poliovirus, a small icosahedral virus, made of 20 equilateral triangular faces, one of which is outlined in red. Each face is composed of three outlined structural elements called capsomeres. The numbers show how five capsomeres associate at the 12 vertices of the icosahedron. (b, right) Space-filling model of poliovirus based on x-ray crystallography. The model is color-coded according to distance from the center of the virion, red furthest, blue closest. The virion binds to host cell receptors (not shown), which are long narrow cell surface proteins that enter the blue “canyons” around each vertex. (c) Bacteriophage T4. (d) Influenza virus, an example of an enveloped virus.

[Part (a): Omikron/Science Source. Part (b) data from D. J. Filman et al., 1989. EMBO J. 8:1567, PDB ID 2plv. Part (c) Department of Microbiology, Biozentrum, University of Basel/Science Source. Part (d) James Cavallini/Science Source.]