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You are watching: The protein coat that surrounds the nucleic acid of a virus is called a

Baron S, editor. Medical Microbiology. Fourth edition. Galveston (TX): university of Texas medical Branch at Galveston; 1996.


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Structure and Function

Viruses are small obligate intracellular parasites, which by meaning containeither a RNA or DNA genome surrounded by a protective, virus-coded protein coat.Viruses might be regarded as mobile hereditary elements, most most likely of cellularorigin and also characterized by a long co-evolution the virus and host. Forpropagation viruses rely on dedicated host cells offering the complexmetabolic and also biosynthetic machine of eukaryotic or prokaryotic cells. Acomplete virus bit is dubbed a virion. The main duty of the virion isto deliver its DNA or RNA genome into the hold cell so that the genome have the right to beexpressed (transcribed and translated) by the hold cell. The famous genome, oftenwith associated simple proteins, is packaged within a symmetric protein capsid.The nucleic acid-associated protein, referred to as nucleoprotein, together with thegenome, forms the nucleocapsid. In covering viruses, the nucleocapsid issurrounded by a lipid bilayer obtained from the modified hold cell membrane andstudded with an outer layer the virus envelope glycoproteins.


Classification of Viruses

Morphology: Viruses space grouped on the basis of size and shape,chemical composition and also structure that the genome, and also mode the replication.Helical morphology is seen in nucleocapsids of many filamentous and also pleomorphicviruses. Helical nucleocapsids consists a helical array of capsid proteins(protomers) wrapped roughly a helical filament of main point acid. Icosahedralmorphology is properties of the nucleocapsids the many“spherical” viruses. The number and arrangement the thecapsomeres (morphologic subunits the the icosahedron) are advantageous inidentification and also classification. Numerous viruses also have an external envelope.

Chemical Composition and also Mode of Replication: The genome the a virusmay consist of DNA or RNA, which might be solitary stranded (ss) or dual stranded(ds), linear or circular. The whole genome might occupy one of two people one main point acidmolecule (monopartite genome) or several nucleic mountain segments (multipartitegenome). The different varieties of genome necessitate various replicationstrategies.


Nomenclature

Aside from physical data, genome structure and mode of replication space criteriaapplied in the classification and nomenclature the viruses, including thechemical composition and configuration that the nucleic acid, even if it is the genomeis monopartite or multipartite. The genomic RNA strand that single-stranded RNAviruses is called sense (positive sense, add to sense) in orientation if that canserve as mRNA, and also antisense (negative sense, minus sense) if a complementarystrand synthesized by a famous RNA transcriptase serves as mRNA. Likewise consideredin viral classification is the site of capsid assembly and, in envelopedviruses, the website of envelopment.


Structure and also Function

Viruses space inert external the host cell. Little viruses, e.g., polio and also tobaccomosaic virus, can also be crystallized. Viruses space unable to create energy. Asobligate intracellular parasites, during replication, they completely depend on thecomplicated biochemical machinery of eukaryotic bio or prokaryotes cells. The mainpurpose that a virus is to deliver its genome right into the organize cell to permit itsexpression (transcription and also translation) by the organize cell.

A completely assembled contagious virus is referred to as a virion. The simplest virions consistof two simple components: nucleic acid (single- or double-stranded RNA or DNA) and aprotein coat, the capsid, which functions as a covering to defend the viral genomefrom nucleases and which throughout infection attaches the virion to specific receptorsexposed ~ above the prospective hold cell. Capsid proteins are coded because that by the virusgenome. Since of its limited size (Table41-1) the genome codes for just a couple of structural proteins (besidesnon-structural regulatory proteins affiliated in virus replication). Capsids areformed as single or twin protein shells and also consist of only one or a fewstructural protein species. Therefore, multiple protein duplicates must me assemble toform the constant three-dimensional capsid structure. Me assembly that viruscapsids adheres to two an easy patterns: helical symmetry, in i beg your pardon the protein subunitsand the nucleic acid room arranged in a helix, and also icosahedral symmetry, in i beg your pardon theprotein subunits assemble right into a symmetric covering that consist of the nucleicacid-containing core.


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Some virus family members have an additional covering, dubbed the envelope, i beg your pardon isusually obtained in part from modified organize cell membranes. Viral envelopes consistof a lipid bilayer that closely surrounds a shell of virus-encodedmembrane-associated proteins. The exterior the the double layer is studded withvirus-coded, glycosylated (trans-) membrane proteins. Therefore, sheathe virusesoften exhibition a fringe the glycoprotein spikes or knobs, likewise called peplomers. Inviruses that obtain their envelope through budding through the plasma or anotherintracellular cell membrane, the lipid ingredient of the viral envelope closelyreflects that of the certain host membrane. The external capsid and also the envelopeproteins of viruses space glycosylated and important in identify the host selection andantigenic ingredient of the virion. In enhancement to virus-specified envelopeproteins, budding viruses bring also particular host cabinet proteins together integralconstituents that the viral envelope. Virus envelopes deserve to be considered an additionalprotective coat. Larger viruses frequently have a facility architecture consists of bothhelical and isometric symmetries confined to various structural components. Smallviruses, e.g., hepatitis B virus or the members of the picornavirus or parvovirusfamily, space orders the magnitude much more resistant than room the larger facility viruses,e.g. Members that the herpes or retrovirus families.


Classification that Viruses

Viruses are classified ~ above the basis of morphology, chemical composition, and also mode ofreplication. The viruses the infect humans are at this time grouped right into 21 families,reflecting only a small component of the spectrum the the multitude of different viruseswhose host ranges prolong from vertebrates to protozoa and from plants and also fungi tobacteria.


Helical Symmetry

In the replication of viruses with helical symmetry, similar proteinsubunits (protomers) self-assemble right into a helical range surrounding thenucleic acid, which complies with a comparable spiral path. Together nucleocapsids formrigid, highly elongated rods or functional filaments; in one of two people case, detailsof the capsid framework are regularly discernible through electron microscopy. Inaddition to classification as flexible or rigid and also as naked or enveloped,helical nucleocapsids are defined by length, width, key of thehelix, and variety of protomers every helical turn. The many extensivelystudied helical virus is tobacco mosaic virus (Fig. 41-1). Plenty of important structural functions ofthis tree virus have been detect by x-ray diffraction studies. Figure 41-2 mirrors Sendai virus, anenveloped virus with helical nucleocapsid symmetry, a member of theparamyxovirus family (see Ch.30).


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Figure 41-1

The helical structure of the strict tobacco mosaic virusrod. Around 5 percent the the length of the virion is depicted.Individual 17,400-Da protein subunits (protomers) assemble in ahelix through an axial repeat of 6.9 nm (49 subunits every threeturns). Each (more...)


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Figure 41-2

Fragments of flexible helical nucleocapsids (NC) the Sendaivirus, a paramyxovirus, room seen either in ~ the protectiveenvelope (E) or free, ~ rupture of the envelope. The undamaged nucleocapsid is around 1,000 nm long and also 17 nm indiameter; its pitch (more...)


Icosahedral Symmetry

An icosahedron is a polyhedron having 20 equilateral triangular faces and 12vertices (Fig. 41-3). Lines throughopposite vertices specify axes of fivefold rotational symmetry: allstructural attributes of the polyhedron repeat five times in ~ each360° that rotation around any that the fivefold axes. Lines with thecenters the opposite triangle faces form axes that threefold rotationalsymmetry; twofold rotational symmetry axes are formed by currently throughmidpoints of the contrary edges. One icosaheron (polyhedral or spherical) withfivefold, threefold, and also twofold axes the rotational the opposite (Fig. 41-3) is identified as having actually 532symmetry (read as 5,3,2).


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Figure 41-3

Icosahedral models seen, left come right, ~ above fivefold,threefold, and twofold axes of rotational symmetry. These axes are perpendicular to the aircraft of the page and also passthrough the centers of every figure. Both polyhedral (upper) andspherical (lower) forms (more...)


Viruses were very first found to have actually 532 the opposite by x-ray diffraction studiesand ultimately by electron microscopy through negative-staining techniques.In many icosahedral viruses, the protomers, i.e. The structural polypeptidechains, room arranged in oligomeric clusters called capsomeres, which arereadily delineated by an unfavorable staining electron microscopy and kind theclosed capsid shell (Fig. 41-4 a/b). The arrangement of capsomeres right into an icosahedral shell (compare Fig. 41-4 v the upper best modelin Fig. 41-3) permits theclassification of together viruses by capsomere number and also pattern. Thisrequires the to know of the nearest pair of crest capsomeres (calledpenton: those v which the fivefold the opposite axes pass) and thedistribution that capsomeres between them.


Figure 41-4

Adenovirus after an unfavorable stain electron microscopy. (A) The capsid reveals the usual isometric shell comprised from20 equilateral triangular faces. The 252 capsomeres, 12 pentonsand the 240 hollow hexon capsomeres space arranged in a T= 25 the opposite (more...)


In the adenovirus design in Figure41-4, one of the penton capsomeres is arbitrary assigned theindices h = 0, k = 0 (origin), whereby h and k room theindicated axes that the skinny (60°) network of capsomeres. The network axesare created by present of the closest-packed neighboring capsomeres. Inadenoviruses, the h and also k axes also coincide with the edge of thetriangular faces. Any 2nd neighboring vertex capsomere has indices h= 5, k = 0 (or h = 0, k = 5).The capsomere number (C) deserve to be determined to it is in 252 from the h and also kindices and also the equation: C = 10(h2 +hk+ k2) + 2. This symmetry and also number ofcapsomeres is common of every members the the adenovirus family.


Virus core Structure

Except in helical nucleocapsids, small is known around the packaging ororganization the the viral genome within the core. Little virions room simplenucleocapsids include 1 to 2 protein species. The larger viruses containin a main point the nucleic mountain genome complexed with basic protein(s) andprotected by a single- or dual layered capsid (consisting of more than onespecies the protein) or by one envelope (Fig.41-5).


Figure 41-5

Two-dimensional chart of HIV-1 correlating (immuno-)electron microscopic findings v the recent nomenclature forthe structural components in a 2-letter code and also with themolecular weights that the virus structural (glyco-)proteins. SU represents (more...)


RNA Virus Genomes

RNA viruses, comprising 70% of all viruses, differ remarkably in genomestructure (Fig. 41-6). Because ofthe error rate of the enzymes connected in RNA replication, this virusesusually show much higher mutation prices than execute the DNA viruses. Mutationrates that 10-4 result in the constant generation of virus variantswhich show an excellent adaptability to brand-new hosts. The famous RNA may besingle-stranded (ss) or double-stranded (ds), and the genome might occupy asingle RNA segment or be spread on 2 or an ext separate segments(segmented genomes). In addition, the RNA strand the a single-stranded genomemay be one of two people a sense strand (plus strand), i beg your pardon can function as messengerRNA (mRNA), or an antisense strand (minus strand), i beg your pardon is complementary tothe feeling strand and cannot role as mRNA protein translate into (see Ch. 42). Feeling viral RNA alonecan replicate if injected right into cells, because it can duty as mRNA andinitiate translate in of virus-encoded proteins. Antisense RNA, on the otherhand, has no translational role and cannot per se develop viralcomponents.


Figure 41-6

Schemes of 21 virus families infecting humans showing anumber the distinctive criteria: visibility of an envelope or(double-) capsid and also internal nucleic mountain genome. +, sense strand; -, antisense strand; ±,dsRNA or DNA; 0, circular DNA; C, number (more...)


DsRNA viruses, e.g., members the the reovirus family, contain 10, 11 or 12separate genome segments coding for 3 enzymes involved in RNA replication, 3major capsid proteins and also a number of smaller structure proteins. Eachsegment is composed of a complementary sense and also antisense strand that ishydrogen bonded right into a direct ds molecule. The replication of this virusesis complex; just the sense RNA strands space released indigenous the infectingvirion come initiate replication.

The retrovirus genome comprises 2 identical, plus-sense ssRNA molecules,each monomer 7–11 kb in size, that room noncovalently attached over ashort terminal region. Retroviruses save on computer 2 envelope proteins encoded bythe env-gene, 4–6 nonglycosylated main point proteins and 3non-structural sensible proteins (reverse transcriptase, integrase,protease: RT, IN, PR) mentioned by the gag-gene (Fig. 41-5). The RT transcribes the famous ssRNA intodouble-stranded, circular proviral DNA. This DNA, mediated by the viralintegrase, becomes covalently bonded into the DNA that the hold cell to makepossible the subsequent transcription of the feeling strands the eventuallygive climb to retrovirus progeny. After ~ assembly and budding, retrovirusesshow structural and also functional maturation. In immature virions thestructural proteins of the core are existing as a large precursor proteinshell. After proteolytic processing by the viral protease the proteins ofthe tires virion room rearranged and type the thick isometric or cone-shapedcore typical of the mature virion, and the bit becomes infectious.


DNA Virus Genomes

Most DNA viruses (Fig. 41-6) containa solitary genome of straight dsDNA. The papovaviruses, consisting of the polyoma-and papillomaviruses, however, have circular DNA genomes, about 5.1 and 7.8kb pairs in size. DsDNA serves together a template both because that mRNA and forself-transcription. 3 or 2 structure proteins comprise the papovaviruscapsid: in addition, 5-6 nonstructural proteins room encoded that arefunctional in virus transcription, DNA replication and also celltransformation.

Single-stranded linear DNA, 4–6 kb in size, is uncovered with themembers of the Parvovirus family that comprises the parvo-, the erythro- andthe dependoviruses. The virion includes 2–4 structural proteinspecies which space differently obtained from the very same gene product (see Ch. 64). The adeno-associatedvirus (AAV, a dependovirus) is incapable of producing progeny virions exceptin the visibility of helper viruses (adenovirus or herpesvirus). It istherefore said to it is in replication defective.

Circular single-stranded DNA of just 1.7 to 2.3 kb is discovered in members that theCircovirus family which make up the the smallest autonomously propagatedviruses. The isometric capsid actions 17 nm and is created of 2 proteinspecies only.


Virus Classification

On the communication of shared properties viruses space grouped at various hierarchicallevels of order, family, subfamily, genus and also species. More than 30,000 differentvirus isolates are known today and also grouped in more than 3,600 species, in 164 generaand 71 families. Famous morphology provides the basis because that grouping viruses intofamilies. A virus family members may consist of members that replicate just in vertebrates,only in invertebrates, just in plants, or only in bacteria. Particular families containviruses that replicate in much more than one of these hosts. This section comes to onlythe 21 families and also genera of clinical importance.

Besides physical properties, several factors pertaining to the setting of replicationplay a role in classification: the construction of the nucleic acid (ss or ds,linear or circular), whether the genome is composed of one molecule the nucleic mountain oris segmented, and whether the strand of ss RNA is feeling or antisense. Alsoconsidered in group is the site of famous capsid assembly and, in envelopedviruses, the website of nucleocapsid envelopment.Table 41-1 list the significant chemical and also morphologic properties of thefamilies that viruses that cause condition in humans.

The use of Latinized names ending in -viridae because that virus families and also ending in -virusfor famous genera has gained broad acceptance. The name of subfamilies end in-virinae. Vernacular names continue to be used to define the viruses in ~ agenus. In this text, Latinized endings because that families and subfamilies usually space notused. Table 41-2 mirrors the currentclassification of medically far-ranging viruses.


In the beforehand days the virology, viruses were called according to usual pathogenicproperties, e.g. Body organ tropism and/or modes of transmission, and also often likewise aftertheir discoverers. Indigenous the early on 1950s until the mid-1960s, once many new viruseswere being discovered, that was well-known to compose virus names by using sigla(abbreviations obtained from a couple of or early letters). Therefore the surname Picornaviridaeis derived from pico (small) and also RNA; the surname Reoviridae is acquired fromrespiratory, enteric, and also orphan viruses due to the fact that the agents were uncovered in bothrespiratory and also enteric specimens and were not associated to various other classified viruses;Papovaviridae is from papilloma, polyoma, and also vacuolating certified dealer (simian virus 40); retrovirus is from reverse transcriptase; Hepadnaviridaeis from the replication that the virus in hepatocytes and their DNA genomes, as seenin hepatitis B virus. Hepatitis A virus is classified currently in the familyPicornaviridae, genus Hepatovirus. Although the present rules for nomenclature donot prohibit the advent of brand-new sigla, they require that the siglum bemeaningful to workers in the field and also be known by worldwide studygroups.

The names of the other family members that save viruses pathogenic for human beings arederived as follows: Adenoviridae (adeno, “gland”; refers to theadenoid organization from which the viruses were first isolated); Astroviridae (astronmeans star); Arenaviridae (arena “sand”) explains the sandyappearance that the virion. Bunyaviridae (from Bunyamwera, the ar in Africa wherethe form strain to be isolated); Calicivirus (calix, “cup” or“goblet” native the cup-shaped depressions on the famous surfaces);Coronaviridae (corona, “crown”) describes the illustration of thepeplomers protruding from the viral surface; Filoviridae (from the Latin filum,“thread” or “filament”) defines themorphology of these viruses. Herpesviridae (herpes, “creeping”)describes the nature the the lesions; Orthomyxoviridae (ortho,“true,” to add myxo “mucus,” a problem forwhich the viruses have actually an affinity; Paramyxoviridae derived from para,“closely resembling” and also myxo; Parvoviridae (parvus means,“small”); Poxviridae (pock means,“pustule”); Rhabdoviridae (rhabdo, “rod”describes the shape of the viruses and also Togaviridae (toga,“cloak”) describes the tight famous envelope.

Several viruses of clinical importance still remain unclassified. Some space difficultor impossible to propagate in standard laboratory organize systems and thus cannot beobtained in adequate quantity come permit much more precise characterization. Hepatitis Evirus, the Norwalk virus and similar agents (see Ch. 65) that reason nonbacterial gastroenteritis in humansare now assigned come the calicivirus family.

The deadly transmissible dementias in humans and also other animals (scrapie in sheep andgoat; bovine spongiform encephalopathy in cattle, transmissible mink encephalopathy;Kuru, Creutzfeldt-Jakob disease, and also Gerstmann-Straussler-Scheinker syndrome inhumans) (see Ch. 71) are caused by theaccumulation that non-soluble amyloid fibrils in the central nervous systems. Theagents resulting in transmissible subacute spongiform encephalopathies have been linkedto viroids or virinos (i.e. Tree pathogens consisting of naked, but an extremely stablecircular RNA molecules of about 3-400 bases in size, or contagious genomes enwrappedinto a hold cell coat) since of your resistance come chemical and also physical agents.According come an alternative theory, the term “prion” has beencoined to point to vital nonviral infectious reason for these fatalencephalopathies—prion standing because that self-replicating proteinaceous agentdevoid of demonstrable main point acid. Some of the transmissible amyloidoses show afamilial pattern and also can be explained by identified mutations which render a primarysoluble glycoprotein insoluble, which consequently leads come the pathognomonicaccumulation the amyloid fibers and also plaques. The pathogenesis of the sporadicamyloidoses, however, is tho a issue of highly ambitious research.


Caspar DLD: design principles in virus particleconstruction. In Horsfall FL, Tamm ns (eds): Viral and Rickettsial infections inMan. 4th Ed. JB Lippincott, Philadelphia, 1975 .
Mattern CFT: the contrary in virus architecture. InNayak DP (ed): molecule Biology of animal Viruses. Marcel Dekker, new York,1977 .

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Murphy FA, Fauquet CM, Bishop DHL, et al. (eds):Virus Taxonomy: sixth Report that the worldwide Committee ~ above Taxonomy ofViruses. Springer-Verlag, new York, 1995 .