Viperin

{{Infobox_gene}} '''Radical S-adenosyl methionine domain-containing protein 2''' is a protein that in humans is encoded by the RSAD2 gene. RSAD2 is a multifunctional protein in viral processes that is an interferon stimulated gene.<ref>{{cite journal | vauthors = Seo JY, Yaneva R, Cresswell P | title = Viperin: a multifunctional, interferon-inducible protein that regulates virus replication | journal = Cell Host & Microbe | volume = 10 | issue = 6 | pages = 534–9 | date = December 2011 | pmid = 22177558 | pmc = 3246677 | doi = 10.1016/j.chom.2011.11.004 }}</ref> It has been reported that viperin could be induced by either IFN-dependent or IFN-independent pathways and certain viruses may use viperin to increase their infectivity.<ref name="Mattijssen_2012">{{cite journal | vauthors = Mattijssen S, Pruijn GJ | title = Viperin, a key player in the antiviral response | journal = Microbes and Infection | volume = 14 | issue = 5 | pages = 419–26 | date = May 2012 | pmid = 22182524 | doi = 10.1016/j.micinf.2011.11.015 | doi-access = free | hdl = 2066/94161 | hdl-access = free }}</ref><ref name="Helbig_2014">{{cite journal | vauthors = Helbig KJ, Beard MR | title = The role of viperin in the innate antiviral response | journal = Journal of Molecular Biology | volume = 426 | issue = 6 | pages = 1210–9 | date = March 2014 | pmid = 24157441 | doi = 10.1016/j.jmb.2013.10.019 | series = Antiviral Innate Immunity (Part II) | doi-access = free | hdl = 2440/94032 | hdl-access = free }}</ref>

The protein was previously called '''v'''irus '''i'''nhibitory '''p'''rotein, '''e'''ndoplasmic '''r'''eticulum-associated, '''i'''nterferon-inducible ('''viperin'''). The name viperin has been rectified due to inappropriate use of it to describe homologous prokaryotic enzymes producing nucleotide analogues.<ref name = "Ji_2022">{{cite journal | vauthors = Ji Y, Wei L, Da A, Stark H, Hagedoorn PL, Ciofi-Baffoni S, Cowley SA, Louro RO, Todorovic S, Mroginski MA, Nicolet Y, Roessler MM, Le Brun NE, Piccioli M, James WS, Hagen WR, Ebrahimi KH | display-authors = 6 | title = Radical-SAM dependent nucleotide dehydratase (SAND), rectification of the names of an ancient iron-sulfur enzyme using NC-IUBMB recommendations | journal = Frontiers in Molecular Biosciences | volume = 9 | article-number = 1032220 | date = 21 October 2022 | pmid = 36387278 | doi = 10.3389/fmolb.2022.1032220 | pmc = 9642334 | doi-access = free }}</ref> The enzymes across all domains of life are renamed SAM-dependent nucleotide dehydratase (SAND) using NC-IUBMB recommendations.<ref name = "Ji_2022" />

== Function == Viperin is an interferon-stimulated gene whose expression inhibits many DNA and RNA viruses including CHIKV, HCMV, HCV, DENV, WNV, SINV, influenza, and HIV.<ref name="Mattijssen_2012" /> Initially identified as an IFN-γ induced antiviral protein in human cytomegalovirus (HCMV) infected macrophages, it was reported that viperin could be induced by HCMV glycoprotein B in fibroblasts, but inhibits HCMV viral infection and down-regulates viral structural proteins. The reason why virus protein would induce viperin against itself is still not clear; however, the viral induced redistribution of viperin may reflect the mechanism of virus evading its antiviral activities.<ref>{{cite journal | vauthors = Chin KC, Cresswell P | title = Viperin (cig5), an IFN-inducible antiviral protein directly induced by human cytomegalovirus | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 98 | issue = 26 | pages = 15125–15130 | date = December 2001 | pmid = 11752458 | pmc = 64994 | doi = 10.1073/pnas.011593298 | doi-access = free | bibcode = 2001PNAS...9815125C }}</ref> Viperin may also be induced and interact with HCMV viral proteins and relocate to mitochondria in HCMV viral infected cells to enhance viral infectivity by disrupting cellular metabolism.<ref name="pmid215276752">{{cite journal | vauthors = Seo JY, Yaneva R, Hinson ER, Cresswell P | title = Human cytomegalovirus directly induces the antiviral protein viperin to enhance infectivity | journal = Science | volume = 332 | issue = 6033 | pages = 1093–1097 | date = May 2011 | pmid = 21527675 | doi = 10.1126/science.1202007 | s2cid = 22293459 | bibcode = 2011Sci...332.1093S | doi-access = free }}</ref>

Viperin is a radical SAM enzyme which is capable of producing the chain terminator ddhCTP (3ʹ-deoxy-3′,4ʹdidehydro-CTP), which inhibits the viral RNA dependent RNA polymerase (RdRp).<ref name="Gizzi Grove Arnold Jose 2018 pp. 610–614">{{cite journal | vauthors = Gizzi AS, Grove TL, Arnold JJ, Jose J, Jangra RK, Garforth SJ, Du Q, Cahill SM, Dulyaninova NG, Love JD, Chandran K, Bresnick AR, Cameron CE, Almo SC | display-authors = 6 | title = A naturally occurring antiviral ribonucleotide encoded by the human genome | journal = Nature | volume = 558 | issue = 7711 | pages = 610–614 | date = June 2018 | pmid = 29925952 | pmc = 6026066 | doi = 10.1038/s41586-018-0238-4 | publisher = Springer Science and Business Media LLC | bibcode = 2018Natur.558..610G }}</ref> ddhCTP also appears to abolish metabolism of amino acids and mitochondrial respiration.<ref name=":2">{{cite journal | vauthors = Honarmand Ebrahimi K, Vowles J, Browne C, McCullagh J, James WS | title = ddhCTP produced by the radical-SAM activity of RSAD2 (viperin) inhibits the NAD⁺ -dependent activity of enzymes to modulate metabolism | journal = FEBS Letters | volume = 594 | issue = 10 | pages = 1631–1644 | date = May 2020 | pmid = 32232843 | doi = 10.1002/1873-3468.13778 | doi-access = free }}</ref>

In the inhibition of influenza virus budding and release, viperin is suggested to disrupt the lipid rafts on the cell's plasma membrane by binding to and decreasing the enzyme activities of farnesyl diphosphate synthase (FPPS), an essential enzyme in isoprenoid biosynthesis pathway.<ref>{{cite journal | vauthors = Wang X, Hinson ER, Cresswell P | title = The interferon-inducible protein viperin inhibits influenza virus release by perturbing lipid rafts | journal = Cell Host & Microbe | volume = 2 | issue = 2 | pages = 96–105 | date = August 2007 | pmid = 18005724 | doi = 10.1016/j.chom.2007.06.009 | doi-access = free }}</ref> Viperin was suggested to inhibit the viral replication of HCV via its interaction with host hVAP-33 and viral NS5A and disrupting the formation of the replication complex.<ref>{{cite journal | vauthors = Helbig KJ, Eyre NS, Yip E, Narayana S, Li K, Fiches G, McCartney EM, Jangra RK, Lemon SM, Beard MR | display-authors = 6 | title = The antiviral protein viperin inhibits hepatitis C virus replication via interaction with nonstructural protein 5A | journal = Hepatology | volume = 54 | issue = 5 | pages = 1506–1517 | date = November 2011 | pmid = 22045669 | pmc = 3207276 | doi = 10.1002/hep.24542 }}</ref>

== Structure == Human viperin is a single polypeptide of 361 amino acids with a predicted molecular weight of 42 kDa. The N-terminal 42 amino acids of viperin forms amphipathic alpha-helix, which is relatively less conserved in different species and has a minor effect on the antiviral activity of viperin. The N-terminal domain of viperin is required for its localization to the ER and lipid droplets.<ref name="pmid199201762">{{cite journal | vauthors = Hinson ER, Cresswell P | title = The antiviral protein, viperin, localizes to lipid droplets via its N-terminal amphipathic alpha-helix | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 106 | issue = 48 | pages = 20452–20457 | date = December 2009 | pmid = 19920176 | pmc = 2778571 | doi = 10.1073/pnas.0911679106 | doi-access = free | bibcode = 2009PNAS..10620452H }}</ref> Amino acids 77-209 of viperin constitute the radical S-adenosyl methionine (SAM) domain, containing four conserved motifs. Motif 1 has three conserved cysteine residues, CxxCxxC, which is the Fe-S binding motif and also essential for antiviral activity.<ref name="pmid215276752" /> The C-terminal 218-361 amino acids of viperin are highly conserved in different species and essential for viperin dimerization. The C-terminal tail appears to be critical for the antiviral activities against HCV since a C-terminal flag tagged of viperin lost its antiviral activity.<ref>{{cite journal | vauthors = Jiang D, Guo H, Xu C, Chang J, Gu B, Wang L, Block TM, Guo JT | display-authors = 6 | title = Identification of three interferon-inducible cellular enzymes that inhibit the replication of hepatitis C virus | journal = Journal of Virology | volume = 82 | issue = 4 | pages = 1665–1678 | date = February 2008 | pmid = 18077728 | pmc = 2258705 | doi = 10.1128/JVI.02113-07 }}</ref>

When viperin is bound to SAM and Cytidine triphosphate (CTP) or uridine triphosphate (UTP) is used as a substrate, different kinetic parameters are achieved.<ref name="Fenwick Su Dong Lin pp. 652–662">{{cite journal | vauthors = Fenwick MK, Su D, Dong M, Lin H, Ealick SE | title = Structural Basis of the Substrate Selectivity of Viperin | journal = Biochemistry | volume = 59 | issue = 5 | pages = 652–662 | date = February 2020 | pmid = 31917549 | pmc = 7920147 | doi = 10.1021/acs.biochem.9b00741 | publisher = American Chemical Society (ACS) }}</ref> It is predicted that the CTP substrate binds much more tightly with viperin because of the low K_m value of the substrate. However, the overall structure of both UTP- and CTP-bound compounds are similar. The difference being that the uracil moiety is less effective than the cytosine moiety at binding and ordering turns A and B. Nucleotide-free viperin contains a (''βα'')₆ partial barrel and has a disordered N-terminal extension and a partially ordered C-terminal extension.<ref name="Fenwick Li Cresswell Modis p=201705402">{{cite journal | vauthors = Fenwick MK, Li Y, Cresswell P, Modis Y, Ealick SE | title = Structural studies of viperin, an antiviral radical SAM enzyme | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 114 | issue = 26 | pages = 6806–6811 | date = June 2017 | pmid = 28607080 | pmc = 5495270 | doi = 10.1073/pnas.1705402114 | publisher = Proceedings of the National Academy of Sciences | doi-access = free | bibcode = 2017PNAS..114.6806F }}</ref> When the C-terminal tail is ordered, a six-residue α-helix, an eight-residue P-loop (that binds the γ-phosphate of CTP), and a 3₁₀-helix are revealed.

== Cellular localization == Viperin is normally localized to the endoplasmic reticulum (ER) via its N-terminal domain, and also localized to lipid droplet, which are derived from the ER.<ref name="pmid199201762" /> However, it is also found in mitochondria in the HCMV infected fibroblasts.<ref name="pmid215276752" />

== References == {{Reflist}}

== External links == * {{PDBe-KB2|Q8CBB9|Mouse Radical S-adenosyl methionine domain-containing protein 2}} * {{cite journal | vauthors = Bernheim A, Millman A, Ofir G, Meitav G, Avraham C, Shomar H, Rosenberg MM, Tal N, Melamed S, Amitai G, Sorek R | display-authors = 6 | title = Prokaryotic viperins produce diverse antiviral molecules | journal = Nature | volume = 589 | issue = 7840 | pages = 120–124 | date = January 2021 | pmid = 32937646 | pmc = 7610908 | doi = 10.1038/s41586-020-2762-2 | s2cid = 221769131 | bibcode = 2021Natur.589..120B }}