Radical S-adenosyl methionine domain-containing protein 2 is a

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...

that in humans is encoded by the RSAD2

gene In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...

. RSAD2 is a multifunctional protein in viral processes that is an

interferon Interferons (IFNs, ) are a group of signaling proteins made and released by host cells in response to the presence of several viruses. In a typical scenario, a virus-infected cell will release interferons causing nearby cells to heighten the ...

stimulated gene. 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. The protein was previous called Virus inhibitory protein, endoplasmic reticulum-associated, interferon-inducible (Viperin). The name viperin has been rectified due to inappropriate use of it to describe prokaryotic enzymes producing nucleotide analogues. The enzymes across all domains of life are renamed SAM-dependent nucleotide dehydratase (SAND) using NC-IUBMB recommendations.

Function

Viperin is an

interferon-stimulated gene An interferon-stimulated gene (ISG) is a gene that can be expressed in response to stimulation by interferon. Interferons bind to receptors on the surface of a cell, initiating protein signaling pathways within the cell. This interaction leads to th ...

whose expression inhibits many DNA and

RNA viruses ''Orthornavirae'' is a kingdom of viruses that have genomes made of ribonucleic acid (RNA), those genomes encoding an RNA-dependent RNA polymerase (RdRp). The RdRp is used to transcribe the viral RNA genome into messenger RNA (mRNA) and to repli ...

including

CHIKV Chikungunya is an infection caused by the ''Chikungunya virus'' (CHIKV). Symptoms include fever and joint pains. These typically occur two to twelve days after exposure. Other symptoms may include headache, muscle pain, joint swelling, and a ra ...

HCMV ''Human betaherpesvirus 5'', also called human cytomegalovirus (HCMV), is species of virus in the genus ''Cytomegalovirus'', which in turn is a member of the viral family known as ''Herpesviridae'' or herpesviruses. It is also commonly called ...

, HCV, DENV, WNV, SINV,

influenza Influenza, commonly known as "the flu", is an infectious disease caused by influenza viruses. Symptoms range from mild to severe and often include fever, runny nose, sore throat, muscle pain, headache, coughing, and fatigue. These symptoms ...

, and

HIV The human immunodeficiency viruses (HIV) are two species of ''Lentivirus'' (a subgroup of retrovirus) that infect humans. Over time, they cause acquired immunodeficiency syndrome (AIDS), a condition in which progressive failure of the immune ...

. Initially identified as an

IFN-γ Interferon gamma (IFN-γ) is a dimerized soluble cytokine that is the only member of the type II class of interferons. The existence of this interferon, which early in its history was known as immune interferon, was described by E. F. Wheelock ...

induced antiviral protein in

human cytomegalovirus ''Human betaherpesvirus 5'', also called human cytomegalovirus (HCMV), is species of virus in the genus ''Cytomegalovirus'', which in turn is a member of the viral family known as ''Herpesviridae'' or herpesviruses. It is also commonly called ...

(HCMV) infected

macrophages Macrophages (abbreviated as M φ, MΦ or MP) ( el, large eaters, from Greek ''μακρός'' (') = large, ''φαγεῖν'' (') = to eat) are a type of white blood cell of the immune system that engulfs and digests pathogens, such as cancer ce ...

, it was reported that viperin could be induced by HCMV

glycoprotein Glycoproteins are proteins which contain oligosaccharide chains covalently attached to amino acid side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known as glycos ...

B in

fibroblasts A fibroblast is a type of biological cell that synthesizes the extracellular matrix and collagen, produces the structural framework ( stroma) for animal tissues, and plays a critical role in wound healing. Fibroblasts are the most common cells o ...

, 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. Viperin may also be induced and interact with HCMV viral proteins and relocate to

mitochondria A mitochondrion (; ) is an organelle found in the Cell (biology), cells of most Eukaryotes, such as animals, plants and Fungus, fungi. Mitochondria have a double lipid bilayer, membrane structure and use aerobic respiration to generate adenosi ...

in HCMV viral infected cells to enhance viral infectivity by disrupting cellular metabolism. Viperin is a radical SAM enzyme which is capable of producing the

chain terminator A chain is a serial assembly of connected pieces, called links, typically made of metal, with an overall character similar to that of a rope in that it is flexible and curved in compression but linear, rigid, and load-bearing in tension. A c ...

ddhCTP (3ʹ-deoxy-3′,4ʹdidehydro-CTP), which inhibits the viral RNA dependent RNA polymerase (RdRp). This activity appears to abolish metabolism of amino acids and mitochondrial respiration. In the inhibition of

influenza virus ''Orthomyxoviridae'' (from Greek ὀρθός, ''orthós'' 'straight' + μύξα, ''mýxa'' 'mucus') is a family of negative-sense RNA viruses. It includes seven genera: ''Alphainfluenzavirus'', ''Betainfluenzavirus'', '' Gammainfluenzavirus'', ...

budding and release, viperin is suggested to disrupt the

lipid rafts The plasma membranes of cells contain combinations of glycosphingolipids, cholesterol and protein receptors organised in glycolipoprotein lipid microdomains termed lipid rafts. Their existence in cellular membranes remains somewhat controversial. ...

on the cell's plasma membrane by binding to and decreasing the enzyme activities of

farnesyl diphosphate synthase Farnesyl diphosphate synthase may refer to: * Z-farnesyl_diphosphate_synthase, EC 2.5.1.68 * Dimethylallyltranstransferase Dimethylallyltranstransferase (DMATT), also known as farnesylpyrophosphate synthase (FPPS) or as farnesyldiphosphate sy ...

(FPPS), an essential enzyme in

isoprenoid The terpenoids, also known as isoprenoids, are a class of naturally occurring organic chemicals derived from the 5-carbon compound isoprene and its derivatives called terpenes, diterpenes, etc. While sometimes used interchangeably with "terpenes", ...

biosynthesis pathway. Viperin was suggested to inhibit the viral replication of HCV via its interaction with host protein hVAP-33 and

NS5A Nonstructural protein 5A (NS5A) is a zinc-binding and proline-rich hydrophilic phosphoprotein that plays a key role in Hepatitis C virus RNA replication. It appears to be a dimeric form without ''trans''-membrane helices. Structure NS5A is deriv ...

and disrupting the formation of the replication complex.

Structure

Human viperin is a single

polypeptide Peptides (, ) are short chains of amino acids linked by peptide bonds. Long chains of amino acids are called proteins. Chains of fewer than twenty amino acids are called oligopeptides, and include dipeptides, tripeptides, and tetrapeptides. A p ...

of 361

amino acids Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...

with a predicted molecular weight of 42 kDa. The

N-terminal The N-terminus (also known as the amino-terminus, NH2-terminus, N-terminal end or amine-terminus) is the start of a protein or polypeptide, referring to the free amine group (-NH2) located at the end of a polypeptide. Within a peptide, the ami ...

42 amino acids of viperin forms

amphipathic An amphiphile (from the Greek αμφις amphis, both, and φιλíα philia, love, friendship), or amphipath, is a chemical compound possessing both hydrophilic (''water-loving'', polar) and lipophilic (''fat-loving'') properties. Such a compoun ...

alpha-helix The alpha helix (α-helix) is a common motif in the secondary structure of proteins and is a right hand-helix conformation in which every backbone N−H group hydrogen bonds to the backbone C=O group of the amino acid located four residues ear ...

, which is relatively less conserved in different species and has a minor effect on the antiviral activity of viperin. The

N-terminal domain The N-terminus (also known as the amino-terminus, NH2-terminus, N-terminal end or amine-terminus) is the start of a protein or polypeptide, referring to the free amine group (-NH2) located at the end of a polypeptide. Within a peptide, the ami ...

of viperin is required for its localization to the ER and lipid droplets. Amino acids 77-209 of viperin constitute the radical

S-adenosyl methionine ''S''-Adenosyl methionine (SAM), also known under the commercial names of SAMe, SAM-e, or AdoMet, is a common cosubstrate involved in methyl group transfers, transsulfuration, and aminopropylation. Although these anabolic reactions occur throug ...

(SAM) domain, containing four conserved motifs. Motif 1 has three conserved

cysteine Cysteine (symbol Cys or C; ) is a semiessential proteinogenic amino acid with the formula . The thiol side chain in cysteine often participates in enzymatic reactions as a nucleophile. When present as a deprotonated catalytic residue, sometime ...

residues, CxxCxxC, which is the Fe-S binding motif and also essential for antiviral activity. The

C-terminal The C-terminus (also known as the carboxyl-terminus, carboxy-terminus, C-terminal tail, C-terminal end, or COOH-terminus) is the end of an amino acid chain (protein or polypeptide), terminated by a free carboxyl group (-COOH). When the protein is ...

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. When viperin is bound to SAM and

Cytidine triphosphate Cytidine triphosphate (CTP) is a pyrimidine nucleoside triphosphate. CTP, much like ATP, consists of a ribose sugar, and three phosphate groups. The major difference between the two molecules is the base used, which in CTP is cytosine. CTP is ...

(CTP) or

uridine triphosphate Uridine-5′-triphosphate (UTP) is a pyrimidine nucleoside triphosphate, consisting of the organic base uracil linked to the 1′ carbon of the ribose sugar, and esterified with tri-phosphoric acid at the 5′ position. Its main role is as substra ...

(UTP) is used as a substrate, different kinetic parameters are achieved. 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 Uracil () (symbol U or Ura) is one of the four nucleobases in the nucleic acid RNA. The others are adenine (A), cytosine (C), and guanine (G). In RNA, uracil binds to adenine via two hydrogen bonds. In DNA, the uracil nucleobase is replaced by ...

moiety Moiety may refer to: Chemistry * Moiety (chemistry), a part or functional group of a molecule ** Moiety conservation, conservation of a subgroup in a chemical species Anthropology * Moiety (kinship), either of two groups into which a society is ...

is less effective then the

cytosine Cytosine () ( symbol C or Cyt) is one of the four nucleobases found in DNA and RNA, along with adenine, guanine, and thymine (uracil in RNA). It is a pyrimidine derivative, with a heterocyclic aromatic ring and two substituents attached (an am ...

moiety at binding and ordering turns A and B.

Nucleotide Nucleotides are organic molecules consisting of a nucleoside and a phosphate. They serve as monomeric units of the nucleic acid polymers – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), both of which are essential biomolecules wi ...

-free viperin contains a (''βα'')₆ partial barrel and has a disordered N-terminal extension and a partially ordered C-terminal extension. When the C-terminal tail is ordered, a six-residue

α-helix The alpha helix (α-helix) is a common motif in the secondary structure of proteins and is a right hand-helix conformation in which every backbone N−H group hydrogen bonds to the backbone C=O group of the amino acid located four residues e ...

, 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 The endoplasmic reticulum (ER) is, in essence, the transportation system of the eukaryotic cell, and has many other important functions such as protein folding. It is a type of organelle made up of two subunits – rough endoplasmic reticulum ( ...

(ER) via its N-terminal domain, and also localized to

lipid droplet Lipid droplets, also referred to as lipid bodies, oil bodies or adiposomes, are lipid-rich cellular organelles that regulate the storage and hydrolysis of neutral lipids and are found largely in the adipose tissue. They also serve as a reservoir ...

, which are derived from the ER. However, it is also found in

in the HCMV infected

References

External links

* * {{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