Virion, virus particle

Vibrio (i) Curved, rod-shaped bacterial cell, (ii) Bacterium of the genus Vibrio. Virion Virus particle the virus nucleic acid surrounded by protein coat and in some cases other material. 

The structures of virions (virus particles) are quite diverse. Viruses vary widely in size, shape, and chemical composition. The... 

The virus genome consists of either RNA or DNA. The genome is surrounded by a coat of protein (and occasionally other material). When the virus genome is inside the coat it is called a virus particle or virion. 

The complete complex of nucleic acid and protein, packaged in the virus particle, is called the virus nucleocapsid. Although the virus structure just described is frequently the total structure of a virus particle, a number of animal viruses (and a few bacterial viruses) have more complex structures. These viruses are enveloped viruses, in which the nucleocapsid is enclosed in a membrane. Virus membranes are generally lipid bilayer membranes, but associated with these membranes are often virus-specific proteins. Inside the virion are often one or more virus-specific enzymes. Such enzymes usually play roles during the infection and replication process. 

Figure 5.4 Structure and manner of assembly of a simple virus, tobacco mosaic virus, (a) Electron micrograph at high resolution of a portion of the virus particle, (b) Assembly of the tobacco mosaic virion. The RNA assumes a helical configuration surrounded by the protein capsomeres. The center of the particle is hollow.

What is the function of the membrane in a virus particle We will discuss this in detail later but note that because of its location in the virion, the membrane is the structural component of the virus particle that interacts first with the cell. The specificity of virus infection, and some aspects of virus penetration, are controlled in part by characteristics of virus membranes. 

Complex viruses Some virions are even more complex, being composed of several separate parts, with separate shapes and symmetries. The most complicated viruses in terms of structure are some of the bacterial viruses, which possess not only icosahedral heads but helical tails. In some bacterial viruses, such as the T4 virus of Escherichia coli, the tail itself is a complex structure. For instance, T4 has almost 20 separate proteins in the tail, and the T4 head has several more proteins. In such complex viruses, assembly is also complex. For instance, in T4 the complete tail is formed as a subassembly, and then the tail is added to the DNA-containing head. Finally, tail fibers formed from another protein are added to make the mature, infectious virus particle. 

The virus genome We have stated that the virus genome consists of either DNA or RNA, never both. Viruses differ in size, amount, and character of their nucleic acid. Both single-stranded and doublestranded nucleic acid is found in viruses, and the amount of nucleic acid per virion may vary greatly from one virus type to another. In general, in enveloped viruses the nucleic acid constitutes only a small part of the mass of the virus particle (1-2 percent), whereas in nonenveloped viruses the percent of the particle which is nucleic acid is much larger, often 25-50 percent. 

The bacterial RNA viruses are all of quite small size, about 26 nm in size, and they are all icosahedral, with 180 copies of coat protein per virus particle. The complete nucleotide sequence of several RNA phages are known. In the RNA phage MS2, which infects Escherichia coli, the viral RNA is 3,569 nucleotides long. The virus RNA, although single stranded, has extensive regions of secondary and tertiary structure. The RNA strand in the virion has the plus (+) sense, acting directly as mRNA upon entry into the cell. 

Influenza virus particles are spheroidal and approximately 100 nm in diameter. The outer-membrane envelope contains 500 copies of hemagglutinin (HA) trimers and 100 copies of neuraminidase tetramers. The hemagglutinin constitutes the receptor sites for a-sialoside ligands. X-ray analyses show that the three sialic acid binding pockets reside 46 A apart, each trimer being separated on the virion surface by about 65-110 A [42],... 

Insertion sites are chosen in this expression strategy so that the peptide is displayed on the surface of the virus particle without interfering with the ability of the modihed CP to assemble into mature virions. As a result, this system has been developed mainly for viruses in which the topology of the capsid protein has been resolved to some degree of detail (Figure 4.1) (Fitchen et al., 1995 Johnson et al., 1997 Fernandez-Fernandez et al., 1998). 

The diminution in infectivity could be caused either by the production of virus particles having a lessened or abolished infectivity, or a decreased formation of infectious virions. The envelope glycoproteins of Semliki Forest virus and Sindbis virus synthesized in the presence of tunicamycin (and, thus, devoid of carbohydrate) are metabolically stable. They do not participate in the assembly of virus particles, although nucleocapsids are still formed under these conditions and are... 

The classification in Table 14.1 shows 16 groups of animal viruses and is based on (a) the nature of the nucleic acid, (b) the structural symmetry of the virus particle, (c) the presence of an envelope, and (d) the size of the virion. The classification is simplified from that of Fraenkel-Conrat (1974) and resembles that of Wildy (1971), but details of classifications differ as precise taxonomic relationships have not been established. 

---