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 because only a limited amount of DNA or RNA, and therefore a limited number of genes, can fit into a virion capsid.

Nature has found two basic ways of arranging the multiple capsid protein subunits and the viral genome into a virion. In some viruses, multiple copies of a single capsid 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 structure, such as tobacco mosaic virus, have a rodlike shape (Figure 1a). 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 (Figure 1b). During infection, some icosahedral viruses interact with host-cell receptors via clefts in between the capsid subunits. Others interact via long fiberlike proteins extending from the vertices of the icosahedron.

Fig1. 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.]

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 the viral DNA then passes down the tail into the cytoplasm of the host cell (Figure 1c).

In some viruses, a symmetrically arranged nucleocapsid composed of the viral genome associated with multiple copies of one or a few proteins is covered by an external mem brane, or envelope, which consists mainly of a phospholipid bilayer but also contains one or more types of virus-encoded glycoproteins (Figure 1d). 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 will discuss shortly.

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مواضيع ذات صلة

HIV Infections in Humans: Laboratory Diagnosis

HIV Infections in Humans :Clinical Findings

HIV Infections in Humans : Pathogenesis and Pathology

Animal Lentivirus Systems

Classification of Lentiviruses

Viral DNA Is Integrated into the Host-Cell Genome in Some Nonlytic Viral Growth Cycles

Lytic Viral Growth Cycles Lead to Death of Host Cells

Structure and Composition of Lentiviruses

HERPESVIRUSES

POLYOMAVIRUSES

Human T-Lymphotropic Viruses

Replication of Retroviruses