PRINCIPLES OF ANIMAL VIROLOGY
VIRUSES OF DOMESTIC ANIMALS
Viral Diseases by Domestic Animal SpeciesUnder Construction Structure and Composition of Viruses Viral Morphology The virion (infectious virus particle) of the simplest viruses consists of a single molecule of nucleic acid surrounded by a protein coat, the capsid. In some of the more complex viruses the capsid surrounds a core, which consists of one or more proteins surrounding the viral nucleic acid; the capsid and associated nucleic acid then constitute the nucleocapsid. For some viruses the capsid is surrounded by a lipoprotein envelope.
The capsid is composed of a defined number of morphological units called capsomers, which are held together by noncovalent bonds. Within an infected cell, the capsomers self-assemble to form the capsid. The manner of assembly is strictly defined by the nature of the bonds formed between individual capsomers, which imparts symmetry to the capsid. Only two kinds of symmetry have been recognized, cubical (icosahedral) and helical.
The cubic symmetry found in viruses is invariably that of an icosahedron, one of the five classical "Platonic solids" of geometry; it has 12 vertices (corners), 30 edges, and 20 faces, each an equilateral triangle. It has axes of two-, three-, and fivefold rotational symmetry, passing through its edges, faces, and vertices, respectively. The icosahedron is the optimum solution to the problem of constructing, from repeating subunits, a strong structure to enclose a maximum volume. Before icosahedrons were discovered in viruses, the same principles were applied by the architect Buckminster Fuller to the construction of icosahedral buildings ("geodesic domes"). An object with icosahedral symmetry need not appear angular in outline; the virions of many animal viruses with icosahedral symmetry appear spherical with a bumpy surface.
Only certain arrangements of the capsomers can fit into the faces, edges, and vertices of the viral icosahedron. The capsomers on the faces and edges of adenovirus particles, for example, bond to six neighboring capsomers and are called hexamers; those at the vertices bond to five neighbors and are called pentamers. In virions of some viruses both hexamers and pentamers consist of the same polypeptide(s); in those of other viruses they are formed from different polypeptides.
The examination by negative staining electron microscopy of quite crude preparations such as skin scrapings or clarified fecal samples can provide immediate morphologic identification of a virus, allowing assignment to a particular family. Combined with the clinical picture, electron microscopy is an important method of rapid diagnosis in diarrheal and skin diseases.High-Resolution Structure. The recent demonstration by X-ray crystallography of the structure of the capsids of several picornaviruses (for example, foot-and-mouth disease virus), the canine parvovirus, and simian virus 40 (SV40) at near atomic resolution has provided a remarkable insight into capsid organization and assembly, the location of neutralizing epitopes, and aspects of virus attachment and penetration into cells. Among several picornaviruses examined, the amino acids of each of the three larger structural proteins are packaged so as to have a wedge-shaped eight-stranded antiparallel ß-barrel domain. The outer contour of the virion depends on the packing of these domains and on the way that the loops project from the framework. The capsomers canine parvovirus consist of an unusually large wedge-shaped protein with a ß-barrel core, hence the ability to form a 250-A shell from only 60 subunits of a single protein. In virions of foot-and-mouth disease virus there are 60 large disordered protrusions at the surface, each corresponding to a copy of the major antigenic site. Ultrastructural studies have also provided some insight into how virons bind to cell receptors. In this connection the words "receptor" and "ligand" are often used in imprecise ways. In this section we use the term receptor to designate the specific molecule or structure on the surface of the cell membrane that is recognized by a specific extracellular molecule or structure that binds to it. The ligand is the receptor-binding molecule of the virus. For example, the hernagglutinin of influenza virus is the ligand which binds to the cellular receptor, a glycoconjugate terminating in an N-acetylneuraminic acid. In some picornaviruses there is a deep canyon or pit encircling each fivefold axis. The amino acids within the canyon are far more conserved than residues elsewhere on the surface; it has been suggested that these conserved residues constitute at least part of the ligands that bind to the host cell receptors. Foot-and-mouth disease virus binds to the cell receptor via critical residues within highly flexible and sequence-variable superficial structures on the virion. Helical Symmetry The capsomers and nucleic acid genomes of several RNA viruses selfassemble as a cylindrical nucleocapsid which has helical symmetry, the viral RNA being coiled within the nucleocapsid. Each capsomer of such viruses consists of a single polypeptide molecule.
The plant viruses whose nucleocapsids have helical symmetry are rodshaped and nonenveloped. However, in all animal viruses with helical nucleocapsids these are wound into a secondary coil and enclosed within a lipoprotein envelope.Viral Envelopes Virions acquire envelopes during maturation, by a process known as budding from cellular membranes. The lipids of the viral envelope are derived directly from the cell, but the proteins in the envelope are viruscoded. One kind of viral envelope protein structure is the glycoprotein peplomer (peplos = envelope) or spike, which can often be seen in electron micrographs as projections from the surface of the envelope. Another kind of envelope protein, matrix protein, is nonglycosylated and is found as a layer on the inside of the envelope of virions of several families; matrix protein provides added rigidity to the virion. For example, the envelope of rhabdoviruses with its projecting peplomers is closely applied to a layer of matrix protein which in turn interfaces with a helical nucleocapsid. Other enveloped viruses, including arenaviruses, bunyaviruses, and coronaviruses, have no matrix protein.
Envelopes are not restricted to
viruses of helical symmetry; icosahedral viruses belonging to several families (African
swine fever virus, herpesviruses, togaviruses, flaviviruses, and retroviruses) have
envelopes. The infectivity of most enveloped viruses depends on the integrity of the
envelope, but some poxviruses have an envelope which is not necessary for infectivity.
CHEMICAL COMPOSITION OF VIRIONS
Viruses are distinguished from all other forms of life by their simple chemical composition, which includes a genome comprising one or a few molecules of either DNA or RNA, a small number of proteins which form the capsid or are present within the virion as enzymes or regulatory proteins, and, in the case of enveloped viruses, a lipoprotein bilayer with associated glycoprotein peplomers and sometimes a matrix protein.
Viral Nucleic Acid
All viral genomes are haploid, that is, they contain only one copy of each gene, except for retrovirus genomes, which are diploid. Viral DNA or RNA can be double-stranded (ds) or single-stranded (ss). The genomes of representative members of most viral families have now been completely sequenced.
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