View source for Sputnik virophage

{{Short description|Subviral agent}}
{{Virusbox
| image = Sputnik virofago.jpg
| image_alt =
| image_caption =
| parent = Sputnikvirus
| species = Mimivirus-dependent virus Sputnik
| synonyms =
| synonyms_ref =
| subdivision_ranks = Member virus
| subdivision_ref =
| subdivision =
* Sputnik 1
* Sputnik 2
* Sputnik 3
}}
[[File:Giant Mimivirus with satellite Sputnik virophages.png|thumb|224px| A [[mimivirus]] with two satellite Sputnik virophages (arrows) <ref>Duponchel, S. and Fischer, M.G. (2019) "Viva lavidaviruses! Five features of virophages that parasitize giant DNA viruses". ''PLoS pathogens'', '''15'''(3). {{doi|10.1371/journal.ppat.1007592}}. [[File:CC-BY icon.svg|50px]] Material was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].</ref>]]

'''''Mimivirus-dependent virus Sputnik'''''  (from [[Russian language|Russian]] {{lang|ru|спутник}} "satellite") is a [[subviral agent]] that reproduces in [[amoeba]] cells that are already infected by a certain helper virus; Sputnik uses the [[helper virus]]'s machinery for reproduction and inhibits replication of the helper virus. It is known as a [[virophage]], in analogy to the term ''[[bacteriophage]]''.<ref name="scola"/>

Viruses like Sputnik that depend on co-infection of the [[Host (biology)|host]] cell by helper viruses are known as [[satellite (biology)|satellite viruses]]. At its discovery in a Paris water-cooling tower in 2008, Sputnik was the first known satellite virus that inhibited replication of its helper virus and thus acted as a parasite of that virus. In analogy, it was called a ''[[virophage]]''.<ref name="scola"/>

Sputnik virophages were found infecting giant viruses of ''[[Mimiviridae]]'' group A. However, they are able to grow in amoebae infected by ''Mimiviridae'' of any of the groups A, B, and C.<ref name="MG_Zamilon" />

==Virology==

Sputnik was first isolated in 2008 from a sample obtained from humans; it was harvested from the contact lens fluid of an individual with [[keratitis]].<ref>{{cite journal|title=Provirophages and transpovirons as the diverse mobilome of giant viruses|journal=Proc Natl Acad Sci U S A|volume=109|issue=44|pages=18078–83|year=2012|pmid=23071316|doi=10.1073/pnas.1208835109|author=Desnues C|pmc=3497776|bibcode=2012PNAS..10918078D|doi-access=free}}</ref> Naturally however, the Sputnik virophage has been found to multiply inside species of the opportunistically pathogenic protozoan ''[[Acanthamoeba]]'', but only if that amoeba is infected with the large [[mamavirus]]. Sputnik harnesses the mamavirus proteins to rapidly produce new copies of itself.<ref name="scola">{{cite journal|title=The virophage as a unique parasite of the giant mimivirus |journal=Nature |volume=455 |issue=7205 |pages= 100–4|year=2008 |pmid= 18690211|doi=10.1038/nature07218 |author=Bernard La Scola|display-authors=etal|bibcode=2008Natur.455..100L |s2cid=4422249 }}</ref><ref name="EdYong" />

[[Mamavirus]] is formally known as ''[[Acanthamoeba|Acanthamoeba polyphaga]]'' mimivirus (APMV) and is a close relative of the previously known [[mimivirus]]. The mimivirus is a giant in the viral world; it has more genes than many bacteria and performs functions that normally occur only in cellular organisms. The mamavirus is even larger than the mimivirus, but the two are very similar in that they form large viral factories and complex viral particles.<ref name=xie>{{cite web|author=Xie, Yun|url=https://arstechnica.com/science/news/2008/09/sputnik-the-virophage-the-parasite-of-a-giant-virus.ars|title=Sputnik the virophage: a virus gets a virus|date=September 2008|work=ARS technica}}</ref> There are conditions in which Sputnik cannot produce new virions within these viruses however.  It has been observed that when Mimivirus is cultured with germ-free amoeba, bald virions are produced that lack the surface fibers that are characteristic of this virus.  For reasons unknown, Sputnik is unable to replicate and produce new virions in these bald viruses.<ref>{{cite journal|title=Mimivirus shows dramatic genome reduction after intraamoebal culture|journal=Proc Natl Acad Sci U S A|volume=108|issue=25|pages=10296–301|year=2011|pmid=21646533|doi=10.1073/pnas.1101118108|author=Boyer M|pmc=3121840|bibcode=2011PNAS..10810296B|doi-access=free}}</ref>  Virophage growth is deleterious to APMV and results in the production of abortive forms and abnormal capsid assembly of APMV. In one of the experiments done by inoculating ''[[Acanthamoeba|Acanthamoeba polyphaga]]'' with water containing an original strain of APMV, it was discovered that several capsid layers accumulate asymmetrically on one side of the viral particle causing the virus to become ineffective. Sputnik decreased the yield of infective viral particle by 70% and also reduced the amoeba [[lysis]] by threefold at 24h.<ref name="scola"/>

Sputnik has a circular double stranded [[DNA]] [[genome]] consisting of 18,343 base pairs.<ref name=xie/> It contains genes able to infect all three domains of life: [[Eukarya]], [[Archaea]] and [[Bacteria]]. Of the twenty-one predicted protein-coding genes, three are apparently derived from APMV itself, one is a homologue of an archaeal virus, and four others are homologues of proteins in bacteriophages and eukaryotic viruses. The fact that three of these genes are derived from APMV indicates that Sputnik is able to participate in gene-transfer processes and mediate lateral gene transfer between giant viruses.<ref>{{Cite journal|title = Structural Studies of the Sputnik Virophage|journal = Journal of Virology|date = 2010-01-01|issn = 0022-538X|pmc = 2798384|pmid = 19889775|pages = 894–897|volume = 84|issue = 2|doi = 10.1128/JVI.01957-09|first1 = Siyang|last1 = Sun|first2 = Bernard|last2 = La Scola|first3 = Valorie D.|last3 = Bowman|first4 = Christopher M.|last4 = Ryan|first5 = Julian P.|last5 = Whitelegge|first6 = Didier|last6 = Raoult|first7 = Michael G.|last7 = Rossmann}}</ref> Thirteen are [[Orphan gene|ORFans]], that is they do not have any detectable homologues in current sequence databases. The Sputnik genome has a high [[AT-content]] (73%), similar to that of APMV. It contains 16 predicted [[hairpin loop]]s, all but two of which fall between ORFs.<ref>{{citation |title=Mimivirus and its Virophage |author=Claverie J-M, Abergel C |journal=[[Annual Review of Genetics]] |year=2009 |volume=43 |pages=49–66 |doi=10.1146/annurev-genet-102108-134255 |pmid = 19653859}}</ref>

Several other homologues such as those of a [[Helicase-primase complex|helicase-primase]], a packaging [[ATPase]], an insertion sequence transposase [[DNA-binding subunit]], and a Zn-ribbon protein, were detected in the [[Global Ocean Sampling Expedition|Global Ocean Survey]] environmental data set, suggesting that [[virophage]]s could be a currently unknown family of viruses.{{cn|date=November 2022}}

Sputnik was found to contain genes that were shared by APMV. These genes could have been acquired by Sputnik after the association of APMV with the host and then interaction between the virophage and the viral host. Recombination within the viral factory might have resulted in the exchange of genes. Sputnik is one of the most convincing pieces of evidence for gene mixing and matching between viruses.{{cn|date=November 2022}}

The presence of these genes homologous to the mimivirus in Sputnik suggests that gene transfer between Sputnik and the mimivirus can occur during the infection of ''Acanthamoeba''. Therefore, it is hypothesized that the virophage could be a source of vehicle mediating [[lateral gene transfer]] between giant viruses, which constitute a significant part of the [[DNA virus]] population in marine environments. Moreover, the presence of three APMV genes in Sputnik implies that gene transfer between a virophage and a giant virus is crucial to viral evolution.<ref>{{cite web|url=http://forms.asm.org/microbe/index.asp?bid=61386|title=Biggest Known Virus Yields First-Ever Virophage|date=November 2008|work=Microbe Magazine|url-status=dead|archive-url=https://web.archive.org/web/20110722193614/http://forms.asm.org/microbe/index.asp?bid=61386|archive-date=2011-07-22}}</ref>

In 2016, for classification of Sputnik-like viruses, including Zamilon, genus ''[[Sputnikvirus]]'' in the family ''[[Lavidaviridae]]'' has been established by the [[International Committee on Taxonomy of Viruses]].<ref name=Krupovic>{{citation|title=A classification system for virophages and satellite viruses |vauthors=Krupovic M, Kuhn JH, Fischer MG |journal=[[Archives of Virology]] |year=2016 |volume=161 |issue=1 |pages=233–47 |doi=10.1007/s00705-015-2622-9 |pmid=26446887|doi-access=free |hdl=11858/00-001M-0000-0028-DC34-F |hdl-access=free }}</ref>

==Structure==

The Sputnik virophage has a capsid 74&nbsp;nm in diameter, with [[icosahedral]] symmetry.<ref name=Krupovic /> Within each asymmetric unit of the structure, there are 4 and 1/3 [[Hexon protein|hexon]] capsomers. On the 3-fold axis lies one hexon which gives rise to the 1/3 hexon in each asymmetric unit.<ref name="Zhang X 2012 18431–6">{{cite journal|title=Structure of Sputnik, a virophage, at 3.5-Å resolution|journal=Proc Natl Acad Sci U S A|volume=109|issue=45|pages=18431–6|year=2012|pmid=23091035|doi=10.1073/pnas.1211702109|author=Zhang X|pmc=3494952|bibcode=2012PNAS..10918431Z|doi-access=free}}</ref>  There are flexible, mushroom-like fibers that protrude from each hexamer.<ref name=Sun10>{{cite journal|title=Structural Studies of the Sputnik Virophage|journal=Journal of Virology|volume=84|issue=2|pages= 894–7|year=2010 |pmid= 19889775|doi=10.1128/JVI.01957-09|author=Siyang Sun|pmc=2798384}}</ref>  Each asymmetric unit also houses 1/5 of a penton which lies on each 5-fold axis.<ref name="Zhang X 2012 18431–6"/>  In the middle of the pentamers are cavities that may allow for the entry or exit of DNA.<ref name=Sun10/>  Sputnik has a triangulation number of 27 with 260 hexamers and 12 pentamers.  This virus does not contain a lipid membrane which goes against what has been previously reported.<ref name="Zhang X 2012 18431–6"/>

==Other viruses of Sputnik genus==
Genus '''''Sputnikvirus''''' has two species, which can be divided further into strains:
* Species '''Mimivirus-dependent virus Sputnik'''. All three Sputnik virophages share more than 99% of their DNA and can grow with viruses of any Mimiviridae group A, B, and C.<ref name="MG_Zamilon" />
** Sputnik 1 was discovered in 2008. Its host is [[Mamavirus]].{{cn|date=November 2022}}
** Sputnik 2 was discovered in 2012. It can infect ''[[Acanthamoeba polyphaga]]'' [[Lentille virus]] (Mimiviridae group A).<ref name="MG_Zamilon" /><ref name="EdYong">Ed Yong: [http://www.the-scientist.com/?articles.view/articleNo/32840/title/A-Parasite-s-Parasites/ A Parasite’s Parasites], in: The Scientist, October 15, 2012</ref>
** Sputnik 3 was discovered in 2013. It was isolated with Mimivirus reporter (which is not its natural viral host).<ref>Gaia M, Pagnier I, Campocasso A, Fournous G, Raoult D, et al: [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3626643/ Broad spectrum of mimiviridae virophage allows its isolation using a mimivirus reporter]. PLoS One 8: e61912 (2013) [[doi: 10.1371/journal.pone.0061912]]</ref><ref name="MG_Zamilon" />
* Species [[Zamilon virophage|Mimivirus-dependent virus Zamilon]]. Cannot infect group A due to a [[MIMIVIRE]] defense system.<ref name=Mimivire>{{citation  |title=MIMIVIRE is a defence system in mimivirus that confers resistance to virophage |vauthors= Levasseur A, Bekliz M, Chabrière E, Pontarotti P, La Scola B, Raoult D |journal=[[Nature (journal)|Nature]] |year=2016 |volume= 531|issue= 7593 |pages=249–252|doi=10.1038/nature17146 |pmid=26934229|bibcode= 2016Natur.531..249L |s2cid= 4382855 }}</ref>
** Zamilon 1 was discovered in 2013 in Tunisia.
** Zamilon 2 was discovered in 2015 in North America.

==Other virophages==
{{main|Virophage}}
In March 2011, two additional virophages were described: the [[Mavirus virophage]] which preys on the giant [[Cafeteria roenbergensis virus|''Cafeteria roenbergensis'' virus]],<ref>{{cite journal|title=A virophage at the origin of large DNA transposons |journal=Science |volume=332 |issue=6026 |pages= 231–4|year=2011 |pmid= 21385722|doi=10.1126/science.1199412 |author=Matthias G. Fischer and Curtis A. Suttle|bibcode=2011Sci...332..231F |s2cid=206530677 }}</ref><ref name="MG_Zamilon">Morgan Gaia et al.: [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3991649/ Zamilon, a Novel Virophage with ''Mimiviridae'' Host Specificity], in: PLoS One. 2014; 9(4): e94923. Published online 2014 Apr 18. [[doi: 10.1371/journal.pone.0094923]]</ref> and the [[Organic Lake virophage]] (OLV), found in the salty [[Organic Lake]] in Antarctica, and which preys on [[phycodnavirus]]es that attack [[algae]].<ref>Virginia Gewin: [http://www.nature.com/news/2011/110328/full/news.2011.188.html 'Virus-eater' discovered in Antarctic lake], in: Nature, 28 March 2011, [[doi: 10.1038/news.2011.188]]</ref><ref>{{cite journal|title=Virophage control of antarctic algal host–virus dynamics |journal=Proc Natl Acad Sci U S A|volume=108 |issue=15 |pages= 6163–8|year=2011 |pmid= 21444812|doi=10.1073/pnas.1018221108 |author=Sheree Yau|pmc=3076838|display-authors=etal|bibcode=2011PNAS..108.6163Y|doi-access=free}}</ref><ref name="EdYong" /> [[Zamilon virophage]] was the first one found infecting a member of Mimiviridae group C (i. e. [[Mont1]] virus), being able to grow also in Mimiviridae group B, but not group A.<ref name="MG_Zamilon" />

All host viruses of the known virophages belong to the group of [[nucleocytoplasmic large DNA viruses]]. Studies have been done to show similarities among the various virophages. Homologous genes among virophages include putative FtsK-HerA family DNA packaging ATPase (ATPase), putative DNA helicase/primase (HEL/PRIM), putative cysteine protease (PRSC), putative MCP, and putative minor capsid protein (mCP). These genes are also referred to as conserved core genes although there is sometimes no or very little sequence similarity among these virophages.<ref>{{cite journal|title=Diversity of virophages in metagenomic datasets.|journal=Journal of Virology|year=2013|pmid=23408616|doi=10.1128/JVI.03398-12|author=Jinglie Zhou |display-authors=etal |volume=87|issue=8|pages=4225–36|pmc=3624350}}</ref>

==See also==
{{Portal|Viruses}}
* [[Satellite (biology)]]
* [[Mamavirus]]
* [[Zamilon virophage]]

==References==
{{Reflist}}

==External links==
* [https://viralzone.expasy.org/670?outline=all_by_species Sputnik virophage] at [[ViralZone]]
* [http://www.pdbe.org/emsearch/sputnik%20AND%20virophage 3D macromolecular structures of the Sputnik virophage] at the [[EM Data Bank]]
{{Refbegin}}
* {{cite web |title=''Sputnik virophage'' |work=NCBI Taxonomy Browser |url=https://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?mode=Info&id=543939 |id=543939}}
{{Refend}}

{{Taxonbar|from=Q1955796}}

[[Category:DNA viruses]]
[[Category:Virophages]]