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Capsid
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{{Short description|Protein shell of a virus}} {{cs1 config|name-list-style=vanc}}[[Image:CMVschema.svg|thumb|right|Schematic of a [[cytomegalovirus]]]] [[File:Illustration of the Caspar-Klug model for viruses (or "Goldberg Polyhedra" or "Geodesic domes" or "Fullerenes").gif|thumb|right|Illustration of geometric model changing between two possible capsids. A similar change of size has been observed as the result of a single amino-acid mutation<ref>{{cite journal | vauthors = Asensio MA, Morella NM, Jakobson CM, Hartman EC, Glasgow JE, Sankaran B, Zwart PH, Tullman-Ercek D | display-authors = 6 | title = A Selection for Assembly Reveals That a Single Amino Acid Mutant of the Bacteriophage MS2 Coat Protein Forms a Smaller Virus-like Particle | journal = Nano Letters | volume = 16 | issue = 9 | pages = 5944β50 | date = September 2016 | pmid = 27549001 | doi = 10.1021/acs.nanolett.6b02948 | url = http://www.escholarship.org/uc/item/0hh6m30h | bibcode = 2016NanoL..16.5944A | osti = 1532201 | s2cid = 16706951 }}</ref>]] A '''capsid''' is the protein shell of a [[virus]], enclosing its [[genetic material]]. It consists of several [[oligomer]]ic (repeating) structural subunits made of [[protein]] called [[Protomer (structural biology)|protomer]]s. The observable 3-dimensional morphological subunits, which may or may not correspond to individual proteins, are called [[capsomere]]s. The proteins making up the capsid are called '''capsid proteins''' or '''viral coat proteins''' ('''VCP'''). The virus genomic component inside the capsid, along with occasionally present [[virus core protein]], is called the '''virus core'''. The capsid and core together are referred to as a '''nucleocapsid''' (cf. also [[virion]]). Capsids are broadly classified according to their structure. The majority of the viruses have capsids with either [[Helix|helical]] or [[icosahedral]]<ref>{{cite journal | vauthors = Lidmar J, Mirny L, Nelson DR | s2cid = 6023873 | title = Virus shapes and buckling transitions in spherical shells | journal = Physical Review E | volume = 68 | issue = 5 Pt 1 | pages = 051910 | date = November 2003 | pmid = 14682823 | doi = 10.1103/PhysRevE.68.051910 | arxiv = cond-mat/0306741 | bibcode = 2003PhRvE..68e1910L }}</ref><ref>{{cite journal | vauthors = Vernizzi G, Olvera de la Cruz M | title = Faceting ionic shells into icosahedra via electrostatics | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 104 | issue = 47 | pages = 18382β6 | date = November 2007 | pmid = 18003933 | pmc = 2141786 | doi = 10.1073/pnas.0703431104 | bibcode = 2007PNAS..10418382V | doi-access = free }}</ref> structure. Some viruses, such as [[bacteriophage]]s, have developed more complicated structures due to constraints of elasticity and electrostatics.<ref>{{cite journal | vauthors = Vernizzi G, Sknepnek R, Olvera de la Cruz M | title = Platonic and Archimedean geometries in multicomponent elastic membranes | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 108 | issue = 11 | pages = 4292β6 | date = March 2011 | pmid = 21368184 | pmc = 3060260 | doi = 10.1073/pnas.1012872108 | bibcode = 2011PNAS..108.4292V | doi-access = free }}</ref> The icosahedral shape, which has 20 equilateral triangular faces, approximates a [[sphere]], while the helical shape resembles the shape of a [[Spring (device)|spring]], taking the space of a cylinder but not being a cylinder itself.<ref>{{cite book|title=Introduction to Protein Structure| last1 = Branden | first1 = Carl | last2 = Tooze | first2 = John |year=1991|pages=161β162|isbn=978-0-8153-0270-4|publisher=Garland|location=New York}}</ref> The capsid faces may consist of one or more proteins. For example, the [[foot-and-mouth disease]] virus capsid has faces consisting of three proteins named VP1β3.<ref>{{cite web|url=http://www.web-books.com/MoBio/Free/Ch1E1.htm|title=Virus Structure (web-books.com)|access-date=2007-07-10|archive-date=2021-02-07|archive-url=https://web.archive.org/web/20210207140236/http://www.web-books.com/MoBio/Free/Ch1E1.htm|url-status=dead}}</ref> Some viruses are ''enveloped'', meaning that the capsid is coated with a lipid membrane known as the [[viral envelope]]. The envelope is acquired by the capsid from an intracellular membrane in the virus' host; examples include the inner nuclear membrane, the [[Golgi apparatus|Golgi]] membrane, and the cell's outer [[cell membrane|membrane]].<ref>{{cite book|title=Molecular Biology of the Cell|url=https://archive.org/details/molecularbiology00albe|url-access=registration|edition=4th|page=[https://archive.org/details/molecularbiology00albe/page/280 280]|year=1994|last1=Alberts|first1=Bruce|last2=Bray|first2=Dennis|last3=Lewis|first3=Julian|last4=Raff|first4=Martin|last5=Roberts|first5=Keith|last6=Watson|first6=James D. |author-link6=James D. Watson}}</ref> Once the virus has infected a cell and begins replicating itself, new capsid subunits are synthesized using the [[protein biosynthesis]] mechanism of the cell. In some viruses, including those with helical capsids and especially those with RNA genomes, the capsid proteins co-assemble with their genomes. In other viruses, especially more complex viruses with double-stranded DNA genomes, the capsid proteins assemble into empty precursor '''procapsids''' that include a specialized portal structure at one vertex. Through this portal, viral [[DNA]] is translocated into the capsid.<ref name="pmid16051846">{{cite journal | vauthors = Newcomb WW, Homa FL, Brown JC | title = Involvement of the portal at an early step in herpes simplex virus capsid assembly | journal = Journal of Virology | volume = 79 | issue = 16 | pages = 10540β6 | date = August 2005 | pmid = 16051846 | pmc = 1182615 | doi = 10.1128/JVI.79.16.10540-10546.2005 }}</ref> Structural analyses of major capsid protein (MCP) architectures have been used to categorise viruses into lineages. For example, the bacteriophage PRD1, the algal virus ''[[Chlorovirus|Paramecium bursaria Chlorella virus-1]]'' (PBCV-1), [[mimivirus]] and the mammalian [[Adenoviridae|adenovirus]] have been placed in the same lineage, whereas tailed, double-stranded DNA bacteriophages (''[[Caudovirales]]'') and herpesvirus belong to a second lineage.<ref>{{cite journal | vauthors = Krupovic M, Bamford DH | s2cid = 31542714 | title = Virus evolution: how far does the double beta-barrel viral lineage extend? | journal = Nature Reviews. Microbiology | volume = 6 | issue = 12 | pages = 941β8 | date = December 2008 | pmid = 19008892 | doi = 10.1038/nrmicro2033 }}</ref><ref name="pmid16505372">{{cite journal | vauthors = Forterre P | title = Three RNA cells for ribosomal lineages and three DNA viruses to replicate their genomes: a hypothesis for the origin of cellular domain | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 103 | issue = 10 | pages = 3669β74 | date = March 2006 | pmid = 16505372 | pmc = 1450140 | doi = 10.1073/pnas.0510333103 | bibcode = 2006PNAS..103.3669F | doi-access = free }}</ref><ref name="pmid16357204">{{cite journal | vauthors = Khayat R, Tang L, Larson ET, Lawrence CM, Young M, Johnson JE | title = Structure of an archaeal virus capsid protein reveals a common ancestry to eukaryotic and bacterial viruses | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 102 | issue = 52 | pages = 18944β9 | date = December 2005 | pmid = 16357204 | pmc = 1323162 | doi = 10.1073/pnas.0506383102 | doi-access = free }}</ref><ref name="pmid16338410">{{cite journal | vauthors = LaurinmΓ€ki PA, Huiskonen JT, Bamford DH, Butcher SJ | title = Membrane proteins modulate the bilayer curvature in the bacterial virus Bam35 | journal = Structure | volume = 13 | issue = 12 | pages = 1819β28 | date = December 2005 | pmid = 16338410 | doi = 10.1016/j.str.2005.08.020 | doi-access = free }}</ref>
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