▲ FIGURE 4-37 Structures of quasi-spherical (icosahedral) viruses. The actual shape of the protein subunits in these viruses is not a flat triangle as illustrated in the schematic diagrams, but the overall effect when the subunits are assembled is of a roughly spherical structure with triangular faces. The three-dimensional models are all shown at the same magnification. (a) In the simplest and smallest quasi-spherical viruses, three identical capsid protein subunits form each triangular face (red) of the icosahedron (schematic). The subunits meet in fivefold symmetry at each vertex. Models of three such viruses are shown: poliovirus, a human RNA virus; cowpea mosaic virus (CPMV), a plant RNA virus; and simian virus 40 (SV40), a monkey DNA virus. (b) In some larger viruses of this type, each triangular face is composed of six subunits. The subunits at the vertices maintain fivefold symmetry, but those making up the surfaces in between exhibit sixfold symmetry. A model of adenovirus, a human DNA virus, illustrates how much larger it is than the viruses in part (a) and shows the fibers (green) that bind to receptors on host cells. [See P L. Stewart et al., 1997, EMBO J. 16:1189. Models of CPMV, poliovirus, and SV40 courtesy of T S. Baker; model of adenovirus courtesy of P L. Stewart.]

(b) A large icosahedral virus

▲ EXPERIMENTAL FIGURE 4-38 Viral protein spikes protrude from the surface of an influenza virus virion.

Influenza viruses are surrounded by an envelope consisting of a phospholipid bilayer and embedded viral proteins. The large spikes seen in this electron micrograph of a negatively stained influenza virion are composed of neuraminidase, a tetrameric protein, or hemagglutinin, a trimeric protein (see Figure 3-7). Inside is the helical nucleocapsid. [Courtesy of A. Helenius and J. White.]

consists mainly of a phospholipid bilayer but also contains one or two types of virus-encoded glycoproteins (Figure 4-38). The phospholipids in the viral envelope are similar to those in the plasma membrane of an infected host cell. The viral envelope is, in fact, derived by budding from that membrane, but contains mainly viral glycoproteins, as we discuss shortly.

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