Viral Protein

The structural proteins of viruses have several important functions. Their major purpose is to facilitate transfer of the viral nucleic acid from one host cell to another. They serve to protect the viral genome against inactivation by nucleases, participate in the attachment of the virus particle to a susceptible cell, and provide the structural symmetry of the virus particle.

The proteins determine the antigenic characteristics of the virus. The host’s protective immune response is directed against antigenic determinants of proteins or glycoproteins exposed on the surface of the virus particle. Some surface proteins may also exhibit specific activities (eg, influenza virus hemagglutinin agglutinates red blood cells).

Some viruses carry protein enzymes inside the virions. The enzymes are present in very small amounts and are prob ably not important in the structure of the virus particles; how ever, they are essential for the initiation of the viral replicative cycle when the virion enters a host cell. Examples include an RNA polymerase carried by viruses with negative-sense RNA genomes (eg, orthomyxoviruses and rhabdoviruses) that is needed to copy the first mRNAs, and reverse transcriptase, an enzyme in retroviruses that makes a DNA copy of the viral RNA, an essential step in replication and transformation. At the extreme in this respect are the poxviruses, the cores of which contain a transcriptional system; many different enzymes are packaged in poxvirus particles.

Viral Nucleic Acid

Viruses contain a single kind of nucleic acid—either DNA or RNA—that encodes the genetic information necessary for replication of the virus. The genome may be single or double stranded, circular or linear, and segmented or nonsegmented. The type of nucleic acid, its polarity, and its size are major characteristics used for classifying viruses into families (see Table 1).

Table1. Families of Animal Viruses That Contain Members Able to Infect Humans

The size of the viral DNA genome ranges from 3.2 kbp (hepadnaviruses) to 375 kbp (poxviruses). The size of the viral RNA genome ranges from about 4 kb (picobirnaviruses) to 32 kb (coronaviruses).

All major DNA viral groups in Table 1 have genomes that are single molecules of DNA and have a linear or circular configuration.

Viral RNAs exist in several forms. The RNA may be a single linear molecule (eg, picornaviruses). For other viruses (eg, orthomyxoviruses), the genome consists of several segments of RNA that may be loosely associated within the virion. The isolated RNA of viruses with positive-sense genomes (ie, picornaviruses, togaviruses) is infectious, and the molecule functions as an mRNA within the infected cell. The isolated RNA of the negative-sense RNA viruses, such as rhabdoviruses and orthomyxoviruses, is not infectious. For these viral families, the virions carry an RNA polymerase that in the cell transcribes the genomic RNA molecules into several complementary RNA molecules, each of which may serve as an mRNA template.

The sequence and composition of nucleotides of each viral nucleic acid are distinctive. Many viral genomes have been sequenced. The sequences can reveal genetic relationships among isolates, including unexpected relationships between viruses not thought to be closely related. The number of genes in a virus can be estimated from the open reading frames deduced from the nucleic acid sequence.

Molecular techniques such as polymerase chain reaction assays and nucleic acid sequencing permit the study of transcription of the viral genome within the infected cell as well as comparison of the relatedness of different viruses. Viral nucleic acid may be characterized by its GC content, pro file based on use of restriction endonucleases, enzymes that cleave DNA at specific nucleotide sequences, and genome sequence. Comparison to nucleic acid or protein sequence databases can be used to classify viral agents.

Viral Lipid Envelopes

A number of different viruses contain lipid envelopes as part of their structure. The lipid is acquired when the viral nucleocapsid buds through a cellular membrane during maturation. Budding occurs only at sites where virus-specific proteins have been inserted into the host cell membrane. The budding process varies markedly depending on the replication strategy of the virus and the structure of the nucleocapsid. Bud ding by influenza virus is illustrated in Figure 1.

Fig1. Release of influenza virus by plasma membrane budding. First, viral envelope proteins (hemagglutinin and neuraminidase) are inserted into the host plasma membrane. Then the nucleocapsid approaches the inner surface of the membrane and binds to it. At the same time, viral proteins collect at the site, and host membrane proteins are excluded. Finally, the plasma membrane buds to simultaneously form the viral envelope and release the mature virion. (Reproduced with permission from Willey JM, Sherwood LM, Woolverton CJ: Prescott, Harley, and Klein’s Microbiology, 7th ed. McGraw-Hill, 2008. © McGraw-Hill Education.)

The phospholipid composition of a virion envelope is determined by the specific type of cell membrane involved in the budding process. For example, herpesviruses bud through the nuclear membrane of the host cell, and the phospholipid composition of the purified virus reflects the lipids of the nuclear membrane. The acquisition of a lipid-containing membrane is an integral step in virion morphogenesis in some viral groups.

Lipid-containing viruses are sensitive to treatment with ether and other organic solvents (see Table 1), indicating that disruption or loss of lipid results in loss of infectivity. Nonlipid-containing viruses are generally resistant to ether and detergents.

Viral Glycoproteins

Viral envelopes contain glycoproteins. In contrast to the lip ids in viral membranes, which are derived from the host cell, the envelope glycoproteins are virus encoded. However, the sugars added to viral glycoproteins typically reflect the host cell in which the virus is grown.

The surface glycoproteins of an enveloped virus attach the virus particle to a target cell by interacting with a cellular receptor. They are also often involved in the membrane fusion step of infection. The glycoproteins are also important viral antigens. As a result of their position at the outer surface of the virion, they are frequently involved in the interaction of the virus particle with neutralizing antibody. Extensive glycosylation of viral surface proteins may prevent effective neutralization of a virus particle by specific antibody. The three-dimensional structures of the externally exposed regions of some viral glycoproteins have been determined by x-ray crystallography (see Figure 2). Such studies provide insights into the antigenic structure and functional activities of viral glycoproteins.

Fig2. Influenza virus hemagglutinin (HA) and neuraminidase (NA) surface glycoproteins. A: Primary structures of HA and NA polypeptides. The cleavage of HA into HA1 and HA2 is necessary for virus to be infectious. HA1 and HA2 remain linked by a disulfide bond (S–S). No posttranslational cleavage occurs with NA. Carbohydrate attachment sites • are shown. The hydrophobic amino acids that anchor the proteins in the viral membrane are located near the carboxyl terminal of HA and the amino terminal of NA. B: Folding of the HA1 and HA2 polypeptides in an HA monomer. Five major antigenic sites (sites A–E) that undergo change are shown as shaded areas. The amino terminal of HA2 provides fusion activity (fusion peptide). The fusion particle is buried in the molecule until it is exposed by a conformational change induced by a low pH. C: Structure of the HA trimer as it occurs on a virus particle or the surface of infected cells. Some of the sites involved in antigenic variation are shown (A). Carboxyl terminal residues (C) protrude through the membrane. D: Structure of the NA tetramer. Each NA molecule has an active site on its upper surface. The amino terminal region (N) of the polypeptides anchors the complex in the membrane. (Redrawn with permission from [A, B] Murphy BR, Webster RG: Influenza viruses, pp. 1185 and 1186, and [C, D] Kingsbury DW: Orthomyxo-and paramyxoviruses and their replication, pp. 1163 and 1172. In Fields BN [editor-in-chief] Virology. Raven Press, 1985.)

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مواضيع ذات صلة

Survey of RNA-Containing Viruses

Survey of DNA-Containing Virus Families

Universal System of Virus Taxonomy

Human parainfluenza viruses (HPIV)

Complications of Influenza virus

Influenza virus

Chikungunya virus

West Nile virus

Transmission Cycle of Japanese encephalitis virus

Lab Diagnosis of Dengue virus

Pathogenesis of Dengue virus

Rotavirus