Читать книгу Basic Virology - Martinez J. Hewlett - Страница 93

A naked capsid defines the outer extent of bacterial, plant, and many animal viruses, but other types of viruses have a more complex structure in which the capsid is surrounded by a lipid envelope. This envelope is made up of a lipid bilayer that is derived from the cell in which the virus replicates and from virus‐encoded membrane‐associated proteins. The presence or absence of a lipid envelope (described as enveloped or naked, respectively) is another important defining property of different groups of animal viruses.

Figure 5.3 Crystallographic structure of a simple icosahedral virus. (a) The structure of Desmodium yellow mottle virus as determined by x‐ray crystallography to 2.7‐Å resolution. This virus is a member of the tymovirus group and consists of a single positive‐strand RNA genome about 6300 nucleotides long. The virion is 25–30 nm in diameter and is made up of 180 copies of a single capsid protein that self‐associates in two basic ways: in groups of 5 to form the 12 pentons, and in groups of six to form the 20 hexamers. Two views are shown: Panels at left are looking down at a fivefold axis of symmetry, and the right‐hand panels look at the threefold and twofold axes. Note that the individual capsomers arrange themselves in groups of five at vertices of the icosahedra, and in groups of six on the icosahedral faces. Since there are 12 vertices and 20 faces, this yields the 180 capsomers that make up the structure. The axes are outlined in the lower panels.

Source: Courtesy of S. Larson and A. McPherson, University of California, Irvine.

(b) Schematic diagram of the vertices and faces of a regular icosahedron showing the axes of symmetry. Arrangements of the capsomers described in (a) are also shown.

Figure 5.4 The structure of a simple icosahedral virus. (a) A space‐filling model of the capsid of Desmodium yellow mottle virus as determined by x‐ray crystallography to 2.7‐Å resolution. (b) The assembly of the single capsid protein into 12 pentons and 20 hexons to form the capsid.

Source: Courtesy of S. Larson and A. McPherson, University of California, Irvine.

(c) The structure of the RNA genome inside the capsid as determined by x‐ray crystallography.

The shape of a given type of virus is determined by the shape of the virus capsid and really does not depend on whether or not the virus is enveloped. This is because for most viruses, the lipid envelope is amorphous and deforms readily upon preparation for visualization using the electron microscope.