The nucleic acid of a virion is enclosed within a protein coat, or capsid, composed of multiple copies of one protein or a few different proteins, each of which is encoded by a single viral gene. Because of this structure, a virus is able to encode all the information for making a relatively large capsid in a small number of genes. This efficient use of genetic information is important, since only a limited amount of RNA or DNA, and therefore a limited number of genes, can fit into a virion capsid. A capsid plus the enclosed nucleic acid is called a nucleocapsid.
Nature has found two basic ways of arranging the multiple capsid protein subunits and the viral genome into a nu-cleocapsid. In some viruses, multiple copies of a single coat protein form a helical structure that encloses and protects the viral RNA or DNA, which runs in a helical groove within the protein tube. Viruses with such a helical nucleocapsid, such as tobacco mosaic virus, have a rodlike shape. The other major structural type is based on the icosahedron, a solid, approximately spherical object built of 20 identical faces, each of which is an equilateral triangle.
The number and arrangement of coat proteins in icosa-hedral, or quasi-spherical, viruses differ somewhat depending on their size. In small viruses of this type, each of the 20 triangular faces is constructed of three identical capsid protein subunits, making a total of 60 subunits per capsid. All the protein subunits are in equivalent contact with one another (Figure 4-37a). In large quasi-spherical viruses, each face of the icosahedron is composed of more than three sub-units. As a result, the contacts between subunits not at the vertices are quasi-equivalent (Figure 4-37b). Models of several quasi-spherical viruses, based on cryoelectron microscopy, are shown in Figure 4-37. In the smaller viruses (e.g., poliovirus), clefts that encircle each of the vertices of the icosahedral structure interact with receptors on the surface of host cells during infection. In the larger viruses (e.g., ade-novirus), long fiberlike proteins extending from the nucleo-capsid interact with cell-surface receptors on host cells.
In many DNA bacteriophages, the viral DNA is located within an icosahedral "head" that is attached to a rodlike "tail." During infection, viral proteins at the tip of the tail bind to host-cell receptors, and then the viral DNA passes down the tail into the cytoplasm of the host cell.
In some viruses, the symmetrically arranged nucleocap-sid is covered by an external membrane, or envelope, which
(a) Small icosahedral viruses
(a) Small icosahedral viruses
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