User:Tylernovsak/Viperin

Viperin (Virus inhibitory protein, endoplasmic reticulum-associated, interferon-inducible), also known as RSAD2 (radical SAM domain-containing 2), is a multifunctional protein in viral processes, which could be induced in a variety of cell types by different cellular factors, such as type I, II, III interferon, DNA, RNA viral proteins, poly (I: C) and polysaccharide. It has been reported that viperin could be induced in either IFN-dependent or IFN-independent pathway and it has been found that viruses use viperin to increase their infectivity.

Function
When mammalian cells are infected, pathways that recognize viral infections are triggered and causes the innate defense to limit viral production. This starts an antiviral response know as the induction of interferon (IFN). There are three types of IFN, type I IFNs (IFN-α and IFN-β), type II IFN (IFN-γ) and type III IFNs (IFN-λ1, IFN-λ2 and IFN-λ3), but only type 1 helps the immune system fight against viruses. Type 1 causes the expression of interferon-stimulated genes (ISGs), that further help to stop viral replication. ISGs have been defined as genes that are either induce, or regulate by IFNs. These IFNs cause expression of ISGs when the body is fighting infection or they are providing immunity.

Viperin is a cellular protein which could inhibit many DNA and RNA viruses such as CHIKV, HCMV, HCV, DENV, WNV, SINV, influenza, HIV LAI strain, and so on. Initially identified as an IFN-γ induced antiviral protein in human cytomegalovirus (HCMV) infected macrophages, viperin is reported that it could be induced by HCMV glycoprotein B in fibroblasts but inhibits HCMV viral infection and down-regulates viral structural proteins, which is essential for viral assembling and maturation. The mechanism of how the virus protein induces viperin against itself is still not clear. However, the viral induced redistribution of Viperin is also found in HCMV infected cells, which may reflect the mechanism of virus evading antiviral activities of Viperin. Viperin could also be induced, and then interact with HCMV viral proteins and relocate to mitochondria in HCMV viral infected cells, and finally enhance viral infectivity by the disrupted cellular metabolism.

In the inhibition of influenza virus budding and release, viperin could disrupt the lipid rafts on cell plasma membrane by decreasing the enzyme activities of farnesyl diphosphate synthase (FPPS), which is an essential enzyme in isoprenoid biosynthesis pathway. Besides, viperin can also inhibit the viral replication of HCV via the interaction with host protein hVAP-33 and NS5A and disruption of the formation of the replication complex.

Viperin is a radical SAM enzyme which is capable of producing inhibitory ddhCTP (3ʹ-deoxy-3′,4ʹdidehydro-CTP), which is an elongation inhibitor, especially for the viral RNA dependent RNA polymerase (RdRp). RSAD2 can abolish metabolism of amino acids and mitochondrial respiration, but it is not known how RSAD2 can have such a broad cellular function. Due to fungal homologue of the enzyme and cell biological/biochemical assays, it has been suggested that ddhCTP is a chain terminator of the RNA-dependent RNA polymerases28 (RdRps) of a number of flaviviruses, but was later challenged. However, the data is consistent with ddhCTP being the product of the radical-SAM activity of viperin destroying nucleotides and inhibiting mitochondrial activity.

Structure
Human Viperin consists of 361 amino acids, and it is a single polypeptide chain with a predicted 42kDa molecular mass. The first 42 residues of human viperin is the N-terminal amphipathic alpha-helix, which is relatively less conserved in different species and has a minor effect on the antiviral ability of viperin against HCV, WNV and DENV. The N-terminal domain of viperin is also required for the viperin localization in ER and the lipid droplets. The residues 77-209 constitute the radical S-adenosylmethionine (SAM) domain, in which there are four conserved motifs. Motif 1 contains three conserved cysteine residues, CxxCxxC, which is the Fe-S binding motif and also essential for the antiviral activities against HCV and HCMV infections. The 218-361 residues constitute the C-terminal domain of viperin, which is highly conserved in different species of viperin and essential for viperin dimerization. The last residue 361 of the C-terminal, a tryptophan, is essential for the antiviral activities against HCV since a C-terminal flag tagged of viperin lost its antiviral activities. Viperin forms homodimers in ER, and the over expression of viperin could lead the formation of a crystalloid ER.

When viperin is bound to SAM and cytidine triphosphate (CTP) or uridine triphophate (UTP) is used as a substrate, different kinetic parameters are achieved. These kinetic parameters are achieved by reaction to the different substrates. It is predicted that CTP substrate binds much more tightly with viperin because of the low Km value the substrate has. However, the overall structure of both UTP- and CTP-bound compounds are similar. The difference being uracil moiety is less effective then the cytosine moiety at binding and ordering turns A and B. Nucleotide-free viperin contains a (βα)6 partial barrel and has disordered N-terminal extension and a partially ordered C-terminal extension. When the C-terminal tail is put into order, a six-residue α-helix, an eight-residue P-loop that binds the γ-phosphate of CTP and a 310-helix is introduced.

Cellular localization
Viperin is normally localized in endoplasmic reticulum (ER) via its N-terminal domain, and also localized to lipid droplet, which is derived from the ER. However, it is also found in mitochondria in the HCMV infected fibroblasts by a viral mediated mechanism. HCV NS proteins localize to the cytologist face of the ER through the N-terminal amphitheater α-helix and viperin follows this path as well.