Logo of nihpaAbout Author manuscriptsSubmit a manuscriptHHS Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
Nat Rev Immunol. Author manuscript; available in PMC 2009 Jan 1.
Published in final edited form as:
PMCID: PMC2522268
NIHMSID: NIHMS56475

Interferon-inducible antiviral effectors

Preface

Since the discovery of interferons (IFNs), considerable progress has been made in describing the nature of the cytokines themselves, the signalling components that direct the cell response and their antiviral activities. Gene targeting studies have distinguished four effector pathways of the IFN-mediated antiviral response: the Mx GTPase pathway, the 2′-5′ oligoadenylate-synthetase-directed ribonuclease L pathway, the protein kinase R pathway and the ISG15 ubiquitin-like pathway. These effector pathways individually block viral transcription, degrade viral RNA, inhibit translation, and modify protein function to control all steps of viral replication. Ongoing research continues to expose additional activities for the effector proteins and has revealed unanticipated functions of the antiviral response.

Introduction

Interferon (IFN) was discovered more than 50 years ago as an agent that inhibited the replication of influenza virus1. The IFN family of cytokines are now recognized as key components of the innate immune response and the first line of defence against virus infection. Accordingly, IFNs are currently used therapeutically, with the most noteworthy example, to combat Hepatitis C viral (HCV) infection, but also against a range of other disorders, including numerous malignancies and multiple sclerosis (recently reviewed2).

Three classes of IFN have been identified, designated types I to III, which are classified according to the receptor complex they signal through (Figure 1). Type II IFN consists of the single IFNγ gene product that binds the IFNGR receptor complex. IFNγ mediates broad immune responses to pathogens other than viruses. The more recently described type III IFNs include three IFNλ gene products that signal via the combined IFNLR1 and interleukin-10 receptor 2 (IL-10R2) receptors. To date little is known about the type III IFNs, although they are known to regulate the antiviral response and have been proposed to be the ancestral type I IFNs3. Type I IFNs, which in humans comprise 13 IFNα subtypes, IFNβ, IFNκ, IFNε, IFNo, IFNτ and IFNδ, engage the ubiquitously expressed IFNα receptor (IFNAR) complex that is composed of the two components, IFNAR1 and IFNAR2. The function of type I IFNs is well characterized and they are known to be essential for mounting a robust host response against viral infection. Accordingly, IFNAR-deficient mice have increased susceptibility to numerous viruses but maintain resistance to other microbial pathogens, such as Listeria monocytogenes4, 5. Similarly, humans with genetic defects in components of IFN signalling (STAT1, TYK2 or UNC93B) die of viral disease, with the defect in type I IFN (rather than IFNγ) signalling having the more significant role6-9.

Figure 1
IFN-signalling

Binding of type I IFNs to IFNAR, with ensuing signal transduction, leads to the induction of more than 300 IFN-stimulated genes (ISGs)10. However, relatively few of these ISGs have been directly implicated in instigating the antiviral state. Instead, many of the gene products encode pattern-recognition receptors (PRRs) that detect viral molecules and modulate signalling pathways, or transcription factors that form an amplification loop resulting in increased IFN production and protection from virus spread to limit disease. ISGs that encode promising candidates with direct antiviral activity include proteins that catalyse cytoskeletal remodelling, that induce apoptosis and that regulate post-transcriptional events, such as splicing, mRNA editing, RNA degradation and the multiple steps of protein translation, as well as subsequent post-translational modification. Indeed, several such proteins, ISG15, the GTPase Mx1, ribonuclease L (RNaseL) and protein kinase R (PKR) have been validated as antiviral effectors in studies of gene knockout mice. Mice with mutations or deficiency in key steps in the pathways triggered by these proteins have increased susceptibility to virus infection.

In this Review, we summarize our current understanding of the role of these four antiviral proteins. However, it bears mentioning that this isn't the complete antiviral repertoire. Additional ISGs with likely significant roles in antiviral activities are the deaminases ADAR1 and APOBECS, the exonuclease ISG20, members of the tripartite motif proteins such as PML, the putative S-adenosyl-L-methione Viperin, and the highly induced translation regulators IFIT1 and IFIT2. All have been reported to function as antiviral proteins but await better description for their relative importance using the appropriate knockout models. In addition, responses elicited by IFN-induced microRNA are emerging as regulators of viral infection11.

ISG15, the Mx proteins, the 2′-5′ oligoadenylate synthetase (OAS)-directed RNaseL pathway and PKR represent a gradient of IFN responsiveness: ISG15 is one of the most highly induced ISGs and when coupled to protein substrates modulates pleiotropic cellular activities; Mx proteins are also highly induced by IFN then self-assemble into oligomers that are constitutively active; OAS proteins are expressed at low levels, then considerably induced by type I IFNs that are subsequently activated by viral RNA; and PKR is constitutively expressed as an inactive kinase that is activated by viral double-stranded RNA (dsRNA), then further induced by IFN. This variable responsiveness to IFN underlies the function of each protein as solely an IFN effector, or as determinants of innate immunity and PRRs to enhance the IFN response.

ISG15

One of the most prominent ISGs induced during viral infection and the ensuing IFN response is the 17 kDa ISG15. Although the gene was cloned over 20 years ago12, an antiviral function for ISG15 has only recently been established, and considerable work is still required to detail all of its actions and to resolve contradictory findings.

ISG15 was identified soon after the landmark discovery of ubiquitin, and was immediately recognised as a ubiquitin homologue (Figure 2)13. Protein ubiquitylation regulates many aspects of immunity, including intracellular signal transduction, for instance via activation of NF-κB, as well as acquired immune functions, such as initiating tolerance (reviewed by Liu et al.14). Given the importance of ubiquitylation in the immune response, it is perhaps not surprising that there is a tailored IFN-regulated ubiquitin-like protein response. This response, as mediated by ISGs, has been coined ISGylation.

Figure 2
Domain structure of antiviral proteins

ISG15 is expressed as a 165 amino acid precursor that is subsequently processed to expose the C-terminal sequence LRLRGG. The equivalent diglycine residues within this motif on ubiquitin are adenlylated and conjugated by a thiolester bond to sequential cystine residues on the E1-activating, E2-carrier and E3-ligase enzymes, before being transferred to lysine residues on protein substrates. As the ubiquitin E1 enzyme (UBE1) is unable to form a thiolester bond with ISG15, ISGylation was initially thought to require a parallel and distinct pathway15. However, as the counterpart enzymes that catalyse ISGylation are identified, it is becoming apparent that there is direct interplay between these two pathways. The enzyme UBE1L (ubiquitin-activating enzyme E1-like) was shown to be the specific ISG15-activating enzyme16. Challenging this specificity, two E2 ubiquitin carrier enzymes, UBCH6 (also abbreviated UBE2E1) and UBCH8 (also abbreviated UBE2L6) were shown to also serve as ISG15 carriers17, 18. Ablation of UBCH8 by RNAi suggested this as the principal ISG15 E2 carrier in HeLa cells19. Finally, two E3 ubiquitin ligases, HERC5 (HECT domain and RLD 5) and TRIM25 (Tripartite Motif Protein 25), have been identified to also conjugate ISG15 to protein substrates, via their respective HECT (Homologous to the E6-Ap Carboxyl Terminus) or RING (Really Interesting New Gene) domains20, 21. Appropriately, all enzymes identified in the ISGylation pathway are coordinately induced by IFN (Figure 3). As for ubiquitylation, ISGylation is reversible and a number of enzymes that catalyse the hydrolysis of ISG15 (termed deISGylation) have been identified, including the ubiquitin-specific protease-18 (USP18, also abbreviated UBP43), USP2, USP5, USP13 and USP1422, 23.

Figure 3
Mechanism of action of ISG15

At least 158 putative ISG15 target proteins have been identified to date24-26. Many of these substrates are important in the IFN response, including the signalling components JAK1 and STAT1, PRRs such as RIG-I (retinoic-acid-inducible gene I), and the antiviral effector proteins MxA, PKR and RNaseL24. Unlike ubiquitylation, ISGylation does not drive protein degradation (regulated by K48-linked ubiquitin), but rather parallels ubiquitin's activating effects (mediated by K63-linkage). Accordingly, ISG15 has been reported to prevent virus-mediated degradation of the IFN regulatory factor 3 (IRF3), thereby enhancing induction of IFNβ27. ISGylation has also been shown to modulate the function of enzymes. An instance of this is the increased affinity of the Eukaryotic Translation Initiation Factor 4E family member 4EHP for the 5′ cap structure of RNA28. Conversely, conjugation of ISG15 to the Protein Phosphatase 1B (PPM1B) suppressed enzyme activity, thereby enhancing NF-κB signalling29.

In addition to its intracellular role, ISG15 is secreted in large amounts and has been shown to act as a cytokine to modulate immune responses30. The mechanism by which extracellular ISG15 functions is unresolved. Ubiquitin is also secreted from cells with immunomodulatory effects that are not understood31. However, extrinsic ubiquitylation has been claimed through analysis of surface proteins on spermatozoa during post-testicular maturation32. Conceivably, secreted ISG15 may also function in extrinsic ISGylation. This intriguing possibility could be tested by the treatment of Ube1l-/- mice with ISG15.

Crucial for designation as an antiviral protein, mice ablated for Isg15 have increased susceptibility to a number of viruses, including the influenza A and B viruses, Sindbis virus (SV), and both herpes simplex virus 1 (HSV-1) and murine γ–herpesvirus33, 34. Supporting this, chimeric SV expressing ISG15 are protected from the otherwise lethality of the wild-type viral infection in Ifnar1-/- mice. Compellingly, this rescue effect was dependent on the integrity of the conserved LRLRGG residues at the C-terminus of ISG1535. Confounding these reports, a similar chimeric ISG15-SV did not rescue the same Ifnar1-/- mouse, as reported by another laboratory. However, the recombinant SV construct did provide modest protection in vitro36. Ablation of the deISGylation enzyme, USP18, in mice increased resistance to virus infection, notably VSV. However, the expected reciprocal sensitivity to VSV is not apparent in either the Isg15-/- or Ube1l-/- mice37. However, Isg15-/- mouse embryonic fibroblasts were more susceptible to VSV infection, although this sensitivity was lost after IFN treatment, suggesting circulating IFNs in vivo may obscure some viral resistance mechanisms27. Other in vitro experiments support a role for ISG15 in mediating resistance to Ebola virus, via ISGylation of the E3 ubiquitin ligase NEDD4 (Neural Precursor Cell Expressed, Developmentally Downregulated-4), thereby preventing subsequent ubiquitylation38. Similarly, HIV-1 Gag and Tsg101 ubiquitylation is inhibited by ISG1539. Both NEDD and Gag/Tsg101 ubiquitylation mediate virion release from cells. Further corroboration of an antiviral role for ISG15 comes from the identification of viral proteins that have evolved to target different steps in ISGylation. The Nonstructural protein-1 (NS1) of the influenza B virus binds ISG15 to block ISGylation16, 40. Finally, a number of viral proteases from SARS coronavirus, crimean-congo hemorrhagic fever virus, equine arteritis virus, porcine respiratory and reproductive syndrome virus, and SV have been identified to mediate deISGylation41-43.

As mentioned, one of the substrates modified by ISG15 are the Mx proteins24.

Mx GTPases

IFNs induce the expression of several guanine-hydrolysing proteins. This class of protein is involved in scission to mediate vesicle budding, organogenesis and cytokinesis. There is evidence that four families within this protein class: the p47 guanylate-binding proteins (GBPs), the p65 GBPs, the very large inducible GTPases (VLIGs) and the Mx proteins, afford resistance to pathogens44. However, of these only the Mx proteins have a well-characterized antiviral role and show a strict dependence on type I and III IFN for their expression45.

The Mx family of GTPases, which comprises MxA and MxB in humans and Mx1 and Mx2 in mice, were first identified as antiviral proteins by the observation that the sensitivity of many inbred mouse strains to orthomyxomavirus was solely due to arbitrary mutations within the Mx locus on chromosome 1646-48. This sensitivity could be rescued by restoration of Mx1 expression49. Strikingly, constitutive expression of the human equivalent of mouse Mx1, MxA, in IFNAR-deficient mice confers full resistance to an otherwise fatal infection with Thogoto virus (THOV), LaCrosse Virus (LACV) and Semliki Forest virus (SFV)49.

The two human Mx proteins are encoded on chromosome 21, in a region syngeneic to the Mx region on mouse chromosome 1650, 51. The human proteins are cytoplasmic, as is the murine Mx2. However, the murine Mx1 is nuclear. This differential distribution is thought to allow each protein to target viruses that replicate in either cell compartment52. Only human MxA has demonstrated antiviral activity, and this is directed against both nuclear and cytosolic viruses.

Considerable evidence now shows that mouse Mx proteins confer viral resistance. Viruses that are susceptible to the activities of Mx proteins include orthomyxomaviruses, paramyxomaviruses, rhabdoviruses, togaviruses and bunyaviruses. Similarly, human MxA has been shown to inhibit all infectious genera of the Bunyaviridae family (orthobunyavirus, hantavirus, phlebovirus and dugbe virus)53. Members of other virus families, such as the clinically significant Coxsackie virus (from the Picornaviridae) and Hepatitis B virus (HBV) (Hepadnaviridae) are also susceptible to MxA54, 55. In addition, genetic studies of human populations have shown that a polymorphism in MxA correlates with increased susceptibility to Hepatitis C virus (HCV)56 and HBV57, as well as to measles virus, with the latter associated with higher rates of subacute sclerosing panencephalitis58. Appropriately, Mx proteins are expressed in cells of the periphery, for example in hepatocytes, endothelial cells and immune cells, including peripheral blood monocytes, plasmacytoid dendritic cells and myeloid cells59.

The Mx proteins have a large (relative to many GTPases) N-terminal GTPase domain, a central interacting domain (CID) and a C-terminal leucine zipper (LZ) (Figure 2). Both the CID and LZ domains are required to recognize target viral structures. The main viral target appears to be viral nucleocapsid-like structures60. By virtue of their location at the smooth endoplasmic reticulum, Mx proteins can effectively police exocytic events and mediate vesicle trafficking to trap essential viral components, and in so doing, they prevent viral replication at early time points (Figure 4)61. Also, both MxA and Mx1 associate with subunits of the influenza virus polymerase (PB2 and nucleocapsid protein) to block transcription62. This is a potent antiviral measure, which effectively prevents the generation of viral Mx escape mutants. As a result, few viral countermeasures against Mx proteins have been identified.

Figure 4
Mechanism of action of MxA

Most viral escape mechanisms that are described target IFN signalling: for example, highly virulent strains of influenza virus increase their replicative fitness to effectively out run the IFN response63. More directly, the HBV precore/core protein has been reported to interact with the MXA promoter to prevent MXA gene expression64. Also, West Nile virus (WNV) produces, what appear to be, decoy cytoplasmic membrane structures to hide crucial replication components65.

In contrast to ISG15 and Mx, the OAS and RNaseL pathways and PKR are present ubiquitiously at constitutive levels but can be amplified by exposure to IFNs.

The OAS and RNaseL pathway

Initially identified as IFN-induced proteins that generate low-molecular weight inhibitors of cell-free protein synthesis, the OAS proteins are distinguished by their capacity to synthesize 2′, 5′-linked phosphodiester bonds to polymerize ATP into oligomers of adenosine66, 67. These unique 2′, 5′-oligomers specifically activate the latent form of RNaseL leading to RNA degradation68. In this way, OAS in combination with RNaseL constitutes an antiviral RNA decay pathway. As the OAS proteins are constitutively expressed at low levels they can act as PRRs for the detection of viral dsRNA in the cytosol65. RNA degraded by RNaseL is able to activate the other cytoplasmic PRRs RIG-I and MDA5 (melanoma differentiation-associated gene 5) resulting in IFN gene induction. This accounts for the observation that RNaseL-deficient cells show markedly decreased IFNβ production due to reduced signalling via these PRRs69.

The four OAS genes identified in humans, termed OAS1, OAS2, OAS3 and OASL (OAS-like), have been mapped to chromosome 12 (chromosome 5 in mice) (reviewed by Hovanessian and Justesen70). OAS1 has two spliced forms in humans (eight in mice) that produce two, 40 and 46 kDa, proteins that differ at their C-termini by 18 and 54 amino acids, respectively. OAS2 produces four alternatively spliced transcripts that encode two proteins of 69 and 71 kDa. OAS3 encodes a single transcript that produces a 100 kDa protein. These proteins have considerable homology to each other, with OAS1, OAS2 and OAS3 encoding one, two and three, respectively, ‘OAS’ domains (Figure 2). The most distinctive of the OAS proteins is OASL. Two OASL transcripts are expressed producing two proteins of 30 and 59 kDa. The higher molecular weight OASL contains a putative nucleolar localization signal (RKVKEKIRRTR) at its C-terminus that, probably, accounts for its unique (from the other OAS isoforms) distribution in the cell. The OASL protein also has an OAS domain, however, mutations at key residues disable the catalytic function of this human protein. Interestingly, one of the two mouse homologues retains its 2′, 5′-polymerase activity. In addition to the OAS domain, OASL has a unique 160 amino acid C-terminus that encodes a ubiquitin-like domain that is homologous to ISG15. Accordingly, OASL becomes conjugated (ISGylation) to cellular proteins following the treatment of cells with type I IFNs71.

There appears to be differential expression and induction of each form of the human OAS proteins72. Also, each of the three functional OAS proteins has unique biological functions. A tripeptide motif (CFK) within the OAS domains of OAS1 and OAS2 mediate oligomerization, so the catalytically active form of these enzymes is a tetramer and dimer, for OAS1 and OAS2, respectively73. This tripeptide motif is not conserved in the OAS domains of OAS3 and OASL and therefore these proteins function as monomers. The polymerization of OAS monomers influences their processivity, with OAS3 synthesizing dimeric molecules of 2′, 5′-linked oligomers, whereas OAS1 and OAS2 are capable of synthesizing trimeric and tetrameric oligomers74, 75. The dimeric 2′, 5′-linked oligomers are not efficient activators of RNaseL76 and, consequently, are thought to regulate alternative processes, with one report suggesting a role in gene expression by regulating DNA topoisomerase I77.

The 2′, 5′-dependent RNaseL is expressed as an 80 kDa protein with two kinase-like domains (PUG and STYKc) and eight ankyrin repeats (Figure 2) (reviewed by Silverman78). The enzyme is constitutively expressed as an inactive monomer. Autoinhibition of the enzyme is relieved upon binding of 2′, 5′-oligomers (generated by OAS proteins) to the ankyrin repeats, and subsequent homodimerization79. The active dimeric enzyme then degrades ssRNA (Figure 5)80, 81. Due to their perceived common action via RNaseL, the antiviral function of the OAS proteins has been investigated using RNaseL-deficient mice82. These mice show increased susceptibility to RNA viruses from the Picornaviridae, Reoviridae, Togaviridae, Paramyxoviridae, Orthomyxoviridae, Flaviviridae and Retroviridae families78. An antiviral role for RNaseL against DNA viruses is less directly established, although as these viruses produce dsRNA replicative intermediates they can induce 2′, 5′-oligomers. However, ensuing activation of RNaseL is less commonly observed, presumably because of virally encoded inhibitory factors. A case in point is the E3L protein from Vaccinia virus (VV)83. The direct importance of OAS proteins in the antiviral response in humans is highlighted by genetic studies showing that polymorphisms within a splice-acceptor site of the OAS1 gene (producing two isoforms of the enzyme with different activities) significantly correlate with the antiviral response to the yellow fever vaccine in immunization trials84.

Figure 5
The OAS-RNaseL antiviral pathway

It has recently become apparent that the OAS proteins have additional antiviral functions that are independent of RNaseL activity. The precise mechanisms of RNaseL independence remain to be elucidated. Nevertheless, single nucleotide polymorphisms (SNPs) at a splice enhancer site in OASL have been correlated with susceptibility to WNV85. Intriguingly, the capacity to accept GTP has suggested a potential role for OAS in RNA splicing, whereby the enzyme generates a 2′, 5′-phosphodiester bond between the G at the 5′ end of an intron, and the A of a 3′ splice signal in the splicing intermediate structure86.

PKR

Similar to OAS, PKR (also known as EIF2αK2) was initially identified as a regulator of the antiviral response through studies of protein synthesis in cell-free lysates from IFN and dsRNA-treated cells66, 87. PKR belongs to a small family of protein kinases that respond to environmental stresses to regulate protein synthesis (the other members are EIF2αK1 (eukaryotic translation initiation factor 2α kinase 1; also known as HRI), EIF2αK3 (also known as PERK) and EIF2αK4 (also known as GCN2)). Members of this kinase family phosphorylate the translation initiation factor EIF2α at serine residue 51, resulting in sequestration of the limiting guanine nucleotide exchange factor EIF2β88. This prevents recycling of GDP, halting translation, to allow the cell to reconfigure gene expression. Much of the antiviral and antiproliferative activities of PKR can be attributed to its phosphorylation of EIF2α. Moreover, structural determination of the complex of EIF2α and PKR argues against the existence of alternative substrates89. However, there is extensive biological and biochemical evidence for alternative PKR targets, although the consequences of PKR phosphoregulation of other proteins have not been well characterized. As well as directly regulating proteins by phosphorylation, PKR evokes cellular responses by modulating cell-signalling pathways (discussed below).

PKR is constitutively expressed in all tissues at a basal level and is induced by type I and III IFNs90. Under normal circumstances, PKR is maintained as an inactive monomer, through steric hindrance of the kinase domain by the N-terminus of the protein (Figure 2)91, 92. This repression is released by activating ligands, including viral RNAs, polyanionic molecules such as heparin93 or ceramide94, and protein activators95 that elicit a conformational shift that permits the binding of ATP to the C-terminal kinase domain. The kinase domain consists of two lobes that separately regulate the interaction between protein monomers and the substrate. The active enzyme consists of a homodimer oriented in a parallel, back-to-back arrangement, with the active sites of the enzyme facing outwards89. Dimerization elicits, and requires autophosphorylation at several key residues96-98. Activation elicits phosphorylation of the EIF2α to halt translation (Figure 6).

Figure 6
Mechanism of action of PKR

Direct activation of PKR has been demonstrated with various RNAs by virtue of the two RNA-binding motifs (RBMs) in the N-terminal of PKR. All RBMs tested bind dsRNA independent of sequence, but recognize a specific higher ordered structure. Accordingly, PKR, like the OAS/RNaseL pathway, functions as a PRR. Although RBMs have been shown to bind just 16 base pairs of RNA, longer RNA moieties are required to engage both of the RBMs in PKR to activate the kinase99. Consequently, dsRNA that is longer than 30 base pairs activates PKR most effectively. Alternatively, single-stranded RNAs of 47 bases that have limited ternary structure, activate the kinase if they carry 5′-triphosphates100. As cellular RNA transcripts predominantly have 5′-monophosphates, this equips PKR to specifically target viral RNAs.

Other pathogen-associated molecules, such as lipopolysaccharide (LPS), which is a ligand of Toll-like receptor 4 (TLR4), can also activate PKR101, but this is likely indirect, via another protein. Accordingly, three protein activators of PKR have been identified. PKR interacts with the tumour-necrosis factor (TNF)-receptor-associated factor (TRAF) family of adaptor molecules that are integral to TLR signalling pathways102. The protein activator of PKR PACT responds to stress-inducing molecules, such as hydrogen peroxide, ceramide and cytokines (including IFNγ, IL-3 and TNF)95. The consequences of activation of PKR by PACT in an antiviral context await further characterization of PACT-deficient mice. Finally, PKR can be activated by caspase-mediated (caspase-3, caspase-7 and caspase-8) cleavage of its inhibitory N-terminus, to generate a constitutively active, truncated, kinase domain103, 104.

The role of PKR has been investigated in mice using transgenic models. Deletion mutations have targeted both functional domains of the enzyme105, 106 and transgenic mice that are defective in PKR activity have been generated by expression of a trans-dominant negative mutant of human PKR that is defective in kinase activity (K296R)107. Finally, transgenic mice overexpressing wild-type human PKR have also been produced108. These transgenic mice have impaired antiviral responses with, for example, increased susceptibility to otherwise innocuous infections with VSV109, 110, influenza virus and bunyawera virus111. Experiments in PKR-deficient mouse embryonic fibroblasts show that PKR is involved in protection against infection with a number of RNA viruses, including HCV112, Hepatitis D virus113, WNV114, HIV-1115, SV116, encephalomyocarditis virus (EMCV)117, and the Foot-and-mouth virus (FMDV)118, as well as some DNA viruses such as HSV-1119. As for MxA and OAS1, genetic analysis of human populations show that polymorphisms in PKR correlate with the outcome of HCV infection120.

Additional immune functions of IFN-induced effectors

Studies of the ISG15, OAS/RNaseL and PKR pathways suggest additional functions for each of these proteins. The primary, activating enzyme in ISGylation, UBE1L, was recognized as being deleted in almost all small cell lung cancers, and so the gene product was speculated to be a tumour suppressor121, 122. Given the specificity of UBE1L in ISGylation, this presents an intriguing possible mechanism by which IFN could regulate proliferation.

RNaseL-deficient mice have an enlarged thymus and spleen due to suppressed apoptosis82. The significance of this ability of RNaseL to induce apoptosis has been exemplified through genetic analysis that identified a polymorphism (R462Q) that was associated with reduced enzyme activity and increased incidence of prostate cancer123. Intriguingly, this SNP was subsequently associated with a putative oncolytic xenotropic murine leukaemia-related virus124. Other nonviral functions of RNaseL are implied in experiments that show delayed skin-graft rejection in RNaseL-deficient mice125. A polymorphism identified in OAS1 has also been associated with type I diabetes, which is consistent with a viral aetiology for this disease126, 127.

Given that PKR modulates several signalling pathways, a wider function than determined solely by regulation of translation would be expected. As mentioned, PKR is required for TLR4-mediated apoptosis in macrophages. Although PKR-mediated apoptosis is in part attributable to inhibition of translation through EIF2α phosphorylation, alternative signalling via IRF3 is also important101. PKR also promotes degradation of the inhibitor IκB, thereby activating the potent transcription factor NF-κB128. Regulation of NF-κB accounts for the diminished NADPH oxidase 2 (NOS2) and IFNβ expression in PKR-deficient cells129. Additionally, PKR was shown to be required for LPS-induced STAT inflammatory signalling130. There is also a defect in IFN-induced phosphorylation of serine 727 in STAT1131, which is shown to be necessary for the basal expression of caspase-3. Accordingly, PKR-deficient fibroblasts are variably resistant to apoptosis induced by different stimuli, including dsRNA, LPS and TNF132. Conversely, over expression of PKR in NIH3T3 fibroblasts sensitises them to apoptosis103. PKR may also influence the adaptive immune response by negatively regulating CD8+ T-cell function, as PKR-deficient mice have increased contact hypersensitivity responses and stimulus-dependent T-cell proliferation133. PKR has also been implicated in IgE class switching in B cells. This points to a mechanism by which viral infection may induce IgE-mediated disorders, such as allergy and asthma134, 135. Surprisingly, these signalling events are not wholly mediated by direct phosphorylation by PKR, suggesting that the kinase acts as a scaffold to bridge signalling pathways from alternative PRRs136. The mechanisms underlying these links to adaptive immunity are intriguing but remain to be explained.

Concluding remarks and future directions

The analyses of mice with targeted deletions in ISG15, Mx1, PKR and RNaseL have helped to elucidate specific roles for these enzymes in the response to virus. Further details of the mechanisms for each of these effectors are still to be elucidated. Less well characterized is the ISG15 pathway. Considerable work is still required to decipher the processes of ISGylation, to characterize protein substrates and to detail the consequence for the antiviral response. Also, the precise function of the Mx proteins remains uncertain. The contribution of alternative PKR substrates (beside EIF2α) to the immune response are poorly explored, and questions remain about specific roles for PKR in regulating inflammatory responses and whether this requires the kinase function or indicates a role as an adaptor protein. The precise roles of the different OAS proteins, especially relating to RNaseL-independent antiviral effects, are ill-defined. These alternative functions will be better addressed in vivo by the analysis of mice with more subtle targeted mutations, and in vitro by detailed biochemical analyses (for example, by identification of specific phosphorylation sites on substrates for PKR) and by more detailed structural investigation of the mechanism of activation of each protein.

Strategies used by viruses to escape these antiviral pathways are also informative and a large number of viral countermeasures to block ISG15, PKR, RNaseL and Mx have been reported (reviewed78, 137, 138). It should be noted however, that many of the mechanisms attributed to virus-evolved avoidance of antiviral effectors and enhanced virulence have not been rigorously proven. An exception to this is the mechanism developed by the HSV-1 protein γ1ICP34.5 for targeting PKR. In this case, a virus that has been attenuated by deletion of ICP34.5 only replicates as efficiently as wild-type virus when it infects PKR-deficient mice. Restoration of virulence was shown to depend on ICP34.5 by using both host and viral mutants139. Although this approach is not trivial to replicate, it remains the most valid method of defining effective viral avoidance of IFN activity.

Another important development currently underway is genetic studies that catalogue SNPs in each of these genetic loci within human populations and other animal species. Correlation of genetic polymorphisms in putative antiviral genes with susceptibility to virus, as well as incidence of other immune responses, is emerging as a powerful means to confirm gene function and measure their contribution to disease.

Finally, the most interesting developments in this field are likely to come from further insights into the broader role of each of these antiviral proteins, particularly in mediating adaptive immunity. While the mechanisms underlying these links to adaptive immunity remain to be explained, progress in this area has great potential to realize the goal of manipulating immunity to enhance resistance to pathogens and disease or, alternatively, diminish deleterious autoimmune responses. This will probably also require parallel advancement in our understanding of the specificities of type I and III IFN signalling.

Acknowledgments

Work in our laboratory is supported by grants from the US National Institutes of Health (P01 CA062220 and R01 AI034039) and the Australian National Health and Medical Research Council (436814).

Glossary

Pattern-recognition receptors
(PRRs). A host receptor that can sense pathogen-associated molecular patterns and initiate signalling cascades that lead to an innate immune response. These can be membrane bound (e.g. TLRs) or soluble cytoplasmic receptors (e.g. RIG-I, MDA5 and NLRs).
plasmacytoid dendritic cells
(pDCs). A unique type of dendritic cell with a morphology that resembles that of a plasmablast. These cells are also known as interferon (IFN)-producing cells because they are the main source of type I IFNs during viral infections.
suppressors of cytokine signalling
(SOCS). Intracellular proteins that are cytokine-induced negative regulators of cytokine signalling.
TNF-receptor associated factors
(TRAFs). A family of conserved scaffold proteins that link receptors of the TNF and Toll/IL-1 receptor family to various protein kinases (e.g. IRAK1). These proteins encode RING domains that, by association with E3 ubiquitin ligases, catalyze polyubiquitylation of proteins (e.g. IKKs to activate the transcription factors NF-κB) to transduce signals within the cell.

References

1. Isaacs A, Lindenmann J. Virus Interference. I. The Interferon. Proceedings of the Royal Society of London. Series B, Biological Sciences. 1957;147:258–267. [PubMed]The first description of interferon, initiating the field.
2. Borden EC, et al. Interferons at age 50: past, current and future impact on biomedicine. Nat Rev Drug Discov. 2007;6:975–90. [PubMed]
3. Levraud JP, et al. Identification of the zebrafish IFN receptor: implications for the origin of the vertebrate IFN system. J Immunol. 2007;178:4385–94. [PubMed]Suggests type III IFN as the ancestral antiviral system of vertebrates, thereby possibly clarifying the evolution of this complex antiviral system.
4. O'Connell RM, et al. Type I interferon production enhances susceptibility to Listeria monocytogenes infection. J Exp Med. 2004;200:437–45. [PMC free article] [PubMed]
5. Muller U, et al. Functional role of type I and type II interferons in antiviral defense. Science. 1994;264:1918–21. [PubMed]Describes generation of the IFNAR1-/- mouse (a second IFNAR-/- animal was independently produced and described soon after this) and profound susceptibility to virus infection.
6. Casrouge A, et al. Herpes simplex virus encephalitis in human UNC-93B deficiency. Science. 2006;314:308–12. [PubMed]
7. Dupuis S, et al. Impaired response to interferon-alpha/beta and lethal viral disease in human STAT1 deficiency. Nat Genet. 2003;33:388–91. [PubMed]
8. Jouanguy E, et al. Human primary immunodeficiencies of type I interferons. Biochimie. 2007;89:878–83. [PubMed]
9. Minegishi Y, et al. Human tyrosine kinase 2 deficiency reveals its requisite roles in multiple cytokine signals involved in innate and acquired immunity. Immunity. 2006;25:745–55. [PubMed]
10. Der SD, Zhou A, Williams BR, Silverman RH. Identification of genes differentially regulated by interferon alpha, beta, or gamma using oligonucleotide arrays. Proc Natl Acad Sci U S A. 1998;95:15623–8. [PMC free article] [PubMed]This paper was the first description of the complexity of the IFN-regulated transcriptome.
11. Pedersen IM, et al. Interferon modulation of cellular microRNAs as an antiviral mechanism. Nature. 2007;449:919–22. [PMC free article] [PubMed]The first report showing IFN regulates a number of cellular miRNAs that target HCV replication, contributing to the antiviral effects of IFN.
12. Blomstrom DC, Fahey D, Kutny R, Korant BD, Knight E., Jr Molecular characterization of the interferon-induced 15-kDa protein. Molecular cloning and nucleotide and amino acid sequence. J Biol Chem. 1986;261:8811–6. [PubMed]
13. Loeb KR, Haas AL. The interferon-inducible 15-kDa ubiquitin homolog conjugates to intracellular proteins. J Biol Chem. 1992;267:7806–13. [PubMed]Confirmed ISG15 functions as a ubiquitin homolog participating in post-transcriptional modification of proteins to modulate the IFN response.
14. Liu YC, Penninger J, Karin M. Immunity by ubiquitylation: a reversible process of modification. Nat Rev Immunol. 2005;5:941–52. [PubMed]
15. Narasimhan J, Potter JL, Haas AL. Conjugation of the 15-kDa interferon-induced ubiquitin homolog is distinct from that of ubiquitin. J Biol Chem. 1996;271:324–30. [PubMed]
16. Yuan W, Krug RM. Influenza B virus NS1 protein inhibits conjugation of the interferon (IFN)-induced ubiquitin-like ISG15 protein. Embo J. 2001;20:362–71. [PMC free article] [PubMed]
17. Takeuchi T, Iwahara S, Saeki Y, Sasajima H, Yokosawa H. Link between the Ubiquitin Conjugation System and the ISG15 Conjugation System: ISG15 Conjugation to the UbcH6 Ubiquitin E2 Enzyme. J Biochem (Tokyo) 2005;138:711–9. [PubMed]
18. Zhao C, et al. The UbcH8 ubiquitin E2 enzyme is also the E2 enzyme for ISG15, an IFN-alpha/beta-induced ubiquitin-like protein. Proc Natl Acad Sci U S A. 2004;101:7578–82. [PMC free article] [PubMed]This paper overturned the accepted paradigm that ubiquitin and ubiquitin-like pathways were catalysed by separate specific enzymes. Consequently, it was recognised that these two pathways (ubiquitylation and ISGylation) compete for shared components and so may be antithetic.
19. Kim KI, Giannakopoulos NV, Virgin HW, Zhang DE. Interferon-inducible ubiquitin E2, Ubc8, is a conjugating enzyme for protein ISGylation. Mol Cell Biol. 2004;24:9592–600. [PMC free article] [PubMed]
20. Wong JJ, Pung YF, Sze NS, Chin KC. HERC5 is an IFN-induced HECT-type E3 protein ligase that mediates type I IFN-induced ISGylation of protein targets. Proc Natl Acad Sci U S A. 2006;103:10735–40. [PMC free article] [PubMed]
21. Zou W, Zhang DE. The interferon-inducible ubiquitin-protein isopeptide ligase (E3) EFP also functions as an ISG15 E3 ligase. J Biol Chem. 2006;281:3989–94. [PubMed]
22. Catic A, et al. Screen for ISG15-crossreactive deubiquitinases. PLoS ONE. 2007;2:e679. [PMC free article] [PubMed]
23. Malakhov MP, Malakhova OA, Kim KI, Ritchie KJ, Zhang DE. UBP43 (USP18) specifically removes ISG15 from conjugated proteins. J Biol Chem. 2002;277:9976–81. [PubMed]
24. Zhao C, Denison C, Huibregtse JM, Gygi S, Krug RM. Human ISG15 conjugation targets both IFN-induced and constitutively expressed proteins functioning in diverse cellular pathways. Proc Natl Acad Sci U S A. 2005;102:10200–5. [PMC free article] [PubMed]
25. Giannakopoulos NV, et al. Proteomic identification of proteins conjugated to ISG15 in mouse and human cells. Biochem Biophys Res Commun. 2005;336:496–506. [PubMed]
26. Takeuchi T, Yokosawa H. Detection and analysis of protein ISGylation. Methods Mol Biol. 2008;446:139–49. [PubMed]
27. Lu G, et al. ISG15 enhances the innate antiviral response by inhibition of IRF-3 degradation. Cell Mol Biol (Noisy-le-grand) 2006;52:29–41. [PubMed]
28. Okumura F, Zou W, Zhang DE. ISG15 modification of the eIF4E cognate 4EHP enhances cap structure-binding activity of 4EHP. Genes Dev. 2007;21:255–60. [PMC free article] [PubMed]
29. Takeuchi T, Kobayashi T, Tamura S, Yokosawa H. Negative regulation of protein phosphatase 2Cbeta by ISG15 conjugation. FEBS Lett. 2006;580:4521–6. [PubMed]
30. D'Cunha J, et al. In vitro and in vivo secretion of human ISG15, an IFN-induced immunomodulatory cytokine. J Immunol. 1996;157:4100–8. [PubMed]
31. Majetschak M, et al. Extracellular ubiquitin inhibits the TNF-alpha response to endotoxin in peripheral blood mononuclear cells and regulates endotoxin hyporesponsiveness in critical illness. Blood. 2003;101:1882–90. [PubMed]
32. Sakai N, Sawada H, Yokosawa H. Extracellular ubiquitin system implicated in fertilization of the ascidian, Halocynthia roretzi: isolation and characterization. Dev Biol. 2003;264:299–307. [PubMed]
33. Lenschow DJ, et al. Identification of interferon-stimulated gene 15 as an antiviral molecule during Sindbis virus infection in vivo. J Virol. 2005;79:13974–83. [PMC free article] [PubMed]
34. Osiak A, Utermohlen O, Niendorf S, Horak I, Knobeloch KP. ISG15, an interferon-stimulated ubiquitin-like protein, is not essential for STAT1 signaling and responses against vesicular stomatitis and lymphocytic choriomeningitis virus. Mol Cell Biol. 2005;25:6338–45. [PMC free article] [PubMed]
35. Lenschow DJ, et al. IFN-stimulated gene 15 functions as a critical antiviral molecule against influenza, herpes, and Sindbis viruses. Proc Natl Acad Sci U S A. 2007;104:1371–6. [PMC free article] [PubMed]Recognised ISG15 as an antiviral effector in vivo using Isg15-/- mice. This manuscript validated an earlier strategy used by the same lab (reported in reference number 33) using chimeric SV infection of Ifnar1-/-.
36. Zhang Y, Burke CW, Ryman KD, Klimstra WB. Identification and characterization of interferon-induced proteins that inhibit alphavirus replication. J Virol. 2007;81:11246–55. [PMC free article] [PubMed]
37. Kim KI, et al. Ube1L and protein ISGylation are not essential for alpha/beta interferon signaling. Mol Cell Biol. 2006;26:472–9. [PMC free article] [PubMed]
38. Malakhova OA, Zhang DE. ISG15 inhibits Nedd4 ubiquitin E3 activity and enhances the innate antiviral response. J Biol Chem. 2008;283:8783–7. [PMC free article] [PubMed]
39. Okumura A, Lu G, Pitha-Rowe I, Pitha PM. Innate antiviral response targets HIV-1 release by the induction of ubiquitin-like protein ISG15. Proc Natl Acad Sci U S A. 2006;103:1440–5. [PMC free article] [PubMed]
40. Chang YG, et al. Different Roles for Two Ubiquitin-like Domains of ISG15 in Protein Modification. J Biol Chem. 2008;283:13370–7. [PubMed]
41. Arguello MD, Hiscott J. Ub surprised: viral ovarian tumor domain proteases remove ubiquitin and ISG15 conjugates. Cell Host Microbe. 2007;2:367–9. [PubMed]
42. Frias-Staheli N, et al. Ovarian tumor domain-containing viral proteases evade ubiquitin- and ISG15-dependent innate immune responses. Cell Host Microbe. 2007;2:404–16. [PMC free article] [PubMed]
43. Lindner HA, et al. Selectivity in ISG15 and ubiquitin recognition by the SARS coronavirus papain-like protease. Arch Biochem Biophys. 2007;466:8–14. [PubMed]
44. MacMicking JD. IFN-inducible GTPases and immunity to intracellular pathogens. Trends Immunol. 2004;25:601–9. [PubMed]
45. Haller O, Arnheiter H, Lindenmann J, Gresser I. Host gene influences sensitivity to interferon action selectively for influenza virus. Nature. 1980;283:660–2. [PubMed]
46. Haller O, Arnheiter H, Gresser I, Lindenmann J. Genetically determined, interferon-dependent resistance to influenza virus in mice. J Exp Med. 1979;149:601–12. [PMC free article] [PubMed]
47. Lindenmann J. Resistance of mice to mouse adapted myxoviruses. Virology. 1962;16:203–204. [PubMed]This manuscript showed that the inbreed mouse strain A2G was resistant to mouse-adapted influenza, which was lethal to other inbreed mouse strains. This was later shown (in reference number 49) to be dependent on a single dominant locus named Mx1.
48. Lindenmann J. Inheritance of resistance to influenza virus in mice. Proc Soc Exp Biol Med. 1964;116:506–509. [PubMed]
49. Arnheiter H, Skuntz S, Noteborn M, Chang S, Meier E. Transgenic mice with intracellular immunity to influenza virus. Cell. 1990;62:51–61. [PubMed]
50. Aebi M, et al. cDNA structures and regulation of two interferon-induced human Mx proteins. Mol Cell Biol. 1989;9:5062–72. [PMC free article] [PubMed]
51. Horisberger MA, et al. cDNA cloning and assignment to chromosome 21 of IFI-78K gene, the human equivalent of murine Mx gene. Somat Cell Mol Genet. 1988;14:123–31. [PubMed]
52. Haller O, Frese M, Rost D, Nuttall PA, Kochs G. Tick-borne thogoto virus infection in mice is inhibited by the orthomyxovirus resistance gene product Mx1. J Virol. 1995;69:2596–601. [PMC free article] [PubMed]
53. Andersson I, et al. Human MxA protein inhibits the replication of Crimean-Congo hemorrhagic fever virus. J Virol. 2004;78:4323–9. [PMC free article] [PubMed]
54. Chieux V, et al. Inhibition of coxsackievirus B4 replication in stably transfected cells expressing human MxA protein. Virology. 2001;283:84–92. [PubMed]
55. Gordien E, et al. Inhibition of hepatitis B virus replication by the interferon-inducible MxA protein. J Virol. 2001;75:2684–91. [PMC free article] [PubMed]
56. Hijikata M, Ohta Y, Mishiro S. Identification of a single nucleotide polymorphism in the MxA gene promoter (G/T at nt -88) correlated with the response of hepatitis C patients to interferon. Intervirology. 2000;43:124–7. [PubMed]
57. Suzuki F, et al. Single nucleotide polymorphism of the MxA gene promoter influences the response to interferon monotherapy in patients with hepatitis C viral infection. J Viral Hepat. 2004;11:271–6. [PubMed]
58. Torisu H, et al. Functional MxA promoter polymorphism associated with subacute sclerosing panencephalitis. Neurology. 2004;62:457–60. [PubMed]
59. Fernandez M, et al. In vivo and in vitro induction of MxA protein in peripheral blood mononuclear cells from patients chronically infected with hepatitis C virus. J Infect Dis. 1999;180:262–7. [PubMed]
60. Kochs G, Haller O. Interferon-induced human MxA GTPase blocks nuclear import of Thogoto virus nucleocapsids. Proc Natl Acad Sci U S A. 1999;96:2082–6. [PMC free article] [PubMed]This report reveals a novel trapping mechanism as the basis for the antiviral activity of MxA.
61. Accola MA, Huang B, Al Masri A, McNiven MA. The antiviral dynamin family member, MxA, tubulates lipids and localizes to the smooth endoplasmic reticulum. J Biol Chem. 2002;277:21829–35. [PubMed]
62. Turan K, et al. Nuclear MxA proteins form a complex with influenza virus NP and inhibit the transcription of the engineered influenza virus genome. Nucleic Acids Res. 2004;32:643–52. [PMC free article] [PubMed]
63. Grimm D, et al. Replication fitness determines high virulence of influenza A virus in mice carrying functional Mx1 resistance gene. Proc Natl Acad Sci U S A. 2007;104:6806–11. [PMC free article] [PubMed]
64. Fernandez M, Quiroga JA, Carreno V. Hepatitis B virus downregulates the human interferon-inducible MxA promoter through direct interaction of precore/core proteins. J Gen Virol. 2003;84:2073–82. [PubMed]
65. Hoenen A, Liu W, Kochs G, Khromykh AA, Mackenzie JM. West Nile virus-induced cytoplasmic membrane structures provide partial protection against the interferon-induced antiviral MxA protein. J Gen Virol. 2007;88:3013–7. [PubMed]
66. Kerr IM, Brown RE, Hovanessian AG. Nature of inhibitor of cell-free protein synthesis formed in response to interferon and double-stranded RNA. Nature. 1977;268:540–2. [PubMed]
67. Rebouillat D, Hovanessian AG. The human 2′,5′-oligoadenylate synthetase family: interferon-induced proteins with unique enzymatic properties. J Interferon Cytokine Res. 1999;19:295–308. [PubMed]
68. Clemens MJ, Vaquero CM. Inhibition of protein synthesis by double-stranded RNA in reticulocyte lysates: evidence for activation of an endoribonuclease. Biochem Biophys Res Commun. 1978;83:59–68. [PubMed]
69. Malathi K, Dong B, Gale M, Jr, Silverman RH. Small self-RNA generated by RNase L amplifies antiviral innate immunity. Nature. 2007;448:816–9. [PMC free article] [PubMed]This study identified a novel amplification mechanism for IFN production via activation of the 2′-5′ oligoadenylate-RNaseL pathway.
70. Hovanessian AG, Justesen J. The human 2′-5′oligoadenylate synthetase family: unique interferon-inducible enzymes catalyzing 2′-5′ instead of 3′-5′ phosphodiester bond formation. Biochimie. 2007;89:779–88. [PubMed]
71. Andersen JB, Strandbygard DJ, Hartmann R, Justesen J. Interaction between the 2′-5′ oligoadenylate synthetase-like protein p59 OASL and the transcriptional repressor methyl CpG-binding protein 1. Eur J Biochem. 2004;271:628–36. [PubMed]
72. Marie I, Svab J, Robert N, Galabru J, Hovanessian AG. Differential expression and distinct structure of 69- and 100-kDa forms of 2-5A synthetase in human cells treated with interferon. J Biol Chem. 1990;265:18601–7. [PubMed]
73. Ghosh A, Sarkar SN, Guo W, Bandyopadhyay S, Sen GC. Enzymatic activity of 2′-5′-oligoadenylate synthetase is impaired by specific mutations that affect oligomerization of the protein. J Biol Chem. 1997;272:33220–6. [PubMed]
74. Hartmann R, Justesen J, Sarkar SN, Sen GC, Yee VC. Crystal structure of the 2′-specific and double-stranded RNA-activated interferon-induced antiviral protein 2′-5′-oligoadenylate synthetase. Mol Cell. 2003;12:1173–85. [PubMed]The first structure of the 2′-5′-oligoadenylate synthetase.
75. Sarkar SN, Pal S, Sen GC. Crisscross enzymatic reaction between the two molecules in the active dimeric P69 form of the 2′-5′ oligodenylate synthetase. J Biol Chem. 2002;277:44760–4. [PubMed]
76. Dong B, et al. Intrinsic molecular activities of the interferon-induced 2-5A-dependent RNase. J Biol Chem. 1994;269:14153–8. [PubMed]
77. Castora FJ, Erickson CE, Kovacs T, Lesiak K, Torrence PF. 2′,5′-oligoadenylates inhibit relaxation of supercoiled DNA by calf thymus DNA topoisomerase I. J Interferon Res. 1991;11:143–9. [PubMed]
78. Silverman RH. Viral encounters with 2′,5′-oligoadenylate synthetase and RNase L during the interferon antiviral response. J Virol. 2007;81:12720–9. [PMC free article] [PubMed]
79. Nakanishi M, Goto Y, Kitade Y. 2-5A induces a conformational change in the ankyrin-repeat domain of RNase L. Proteins. 2005;60:131–8. [PubMed]
80. Floyd-Smith G, Slattery E, Lengyel P. Interferon action: RNA cleavage pattern of a (2′-5′)oligoadenylate--dependent endonuclease. Science. 1981;212:1030–2. [PubMed]
81. Wreschner DH, McCauley JW, Skehel JJ, Kerr IM. Interferon action--sequence specificity of the ppp(A2′p)nA-dependent ribonuclease. Nature. 1981;289:414–7. [PubMed]
82. Zhou A, et al. Interferon action and apoptosis are defective in mice devoid of 2′,5′-oligoadenylate-dependent RNase L. Embo J. 1997;16:6355–63. [PMC free article] [PubMed]Describes the RNaseL-/- mouse and confirmed the antiviral effect of this protein in vivo.
83. Xiang Y, et al. Blockade of interferon induction and action by the E3L double-stranded RNA binding proteins of vaccinia virus. J Virol. 2002;76:5251–9. [PMC free article] [PubMed]
84. Bonnevie-Nielsen V, et al. Variation in antiviral 2′,5′-oligoadenylate synthetase (2′5′AS) enzyme activity is controlled by a single-nucleotide polymorphism at a splice-acceptor site in the OAS1 gene. Am J Hum Genet. 2005;76:623–33. [PMC free article] [PubMed]
85. Yakub I, et al. Single nucleotide polymorphisms in genes for 2′-5′-oligoadenylate synthetase and RNase L inpatients hospitalized with West Nile virus infection. J Infect Dis. 2005;192:1741–8. [PubMed]
86. Sperling J, et al. Possible involvement of (2′5′)oligoadenylate synthetase activity in pre-mRNA splicing. Proc Natl Acad Sci U S A. 1991;88:10377–81. [PMC free article] [PubMed]
87. Meurs E, et al. Molecular cloning and characterization of the human double-stranded RNA-activated protein kinase induced by interferon. Cell. 1990;62:379–90. [PubMed]
88. Roberts WK, Hovanessian A, Brown RE, Clemens MJ, Kerr IM. Interferon-mediated protein kinase and low-molecular-weight inhibitor of protein synthesis. Nature. 1976;264:477–80. [PubMed]The initial identification of PKR and OAS-dependent RNaseL pathways.
89. Dar AC, Dever TE, Sicheri F. Higher-order substrate recognition of eIF2alpha by the RNA-dependent protein kinase PKR. Cell. 2005;122:887–900. [PubMed]This paper together with reference number 96 presents the first molecular structure of a PKR-eIF2alpha complex.
90. Ank N, et al. Lambda interferon (IFN-lambda), a type III IFN, is induced by viruses and IFNs and displays potent antiviral activity against select virus infections in vivo. J Virol. 2006;80:4501–9. [PMC free article] [PubMed]
91. Gelev V, et al. Mapping of the auto-inhibitory interactions of protein kinase R by nuclear magnetic resonance. J Mol Biol. 2006;364:352–63. [PMC free article] [PubMed]
92. Nanduri S, Rahman F, Williams BR, Qin J. A dynamically tuned double-stranded RNA binding mechanism for the activation of antiviral kinase PKR. Embo J. 2000;19:5567–74. [PMC free article] [PubMed]
93. George CX, Thomis DC, McCormack SJ, Svahn CM, Samuel CE. Characterization of the heparin-mediated activation of PKR, the interferon-inducible RNA-dependent protein kinase. Virology. 1996;221:180–8. [PubMed]
94. Ruvolo PP, Gao F, Blalock WL, Deng X, May WS. Ceramide regulates protein synthesis by a novel mechanism involving the cellular PKR activator RAX. J Biol Chem. 2001;276:11754–8. [PubMed]
95. Patel RC, Sen GC. PACT, a protein activator of the interferon-induced protein kinase, PKR. Embo J. 1998;17:4379–90. [PMC free article] [PubMed]This paper introduces protein activators of PKR.
96. Dey M, et al. Mechanistic link between PKR dimerization, autophosphorylation, and eIF2alpha substrate recognition. Cell. 2005;122:901–13. [PubMed]
97. Romano PR, et al. Autophosphorylation in the activation loop is required for full kinase activity in vivo of human and yeast eukaryotic initiation factor 2alpha kinases PKR and GCN2. Mol Cell Biol. 1998;18:2282–97. [PMC free article] [PubMed]
98. Taylor DR, et al. Autophosphorylation sites participate in the activation of the double-stranded-RNA-activated protein kinase PKR. Mol Cell Biol. 1996;16:6295–302. [PMC free article] [PubMed]
99. Nanduri S, Carpick BW, Yang Y, Williams BR, Qin J. Structure of the double-stranded RNA-binding domain of the protein kinase PKR reveals the molecular basis of its dsRNA-mediated activation. Embo J. 1998;17:5458–65. [PMC free article] [PubMed]
100. Nallagatla SR, et al. 5′-triphosphate-dependent activation of PKR by RNAs with short stem-loops. Science. 2007;318:1455–8. [PubMed]
101. Hsu LC, et al. The protein kinase PKR is required for macrophage apoptosis after activation of Toll-like receptor 4. Nature. 2004;428:341–5. [PubMed]
102. Gil J, et al. TRAF family proteins link PKR with NF-kappa B activation. Mol Cell Biol. 2004;24:4502–12. [PMC free article] [PubMed]
103. Gil J, Esteban M. The interferon-induced protein kinase (PKR), triggers apoptosis through FADD-mediated activation of caspase 8 in a manner independent of Fas and TNF-alpha receptors. Oncogene. 2000;19:3665–74. [PubMed]
104. Saelens X, Kalai M, Vandenabeele P. Translation inhibition in apoptosis: caspase-dependent PKR activation and eIF2-alpha phosphorylation. J Biol Chem. 2001;276:41620–8. [PubMed]
105. Abraham N, et al. Characterization of transgenic mice with targeted disruption of the catalytic domain of the double-stranded RNA-dependent protein kinase, PKR. J Biol Chem. 1999;274:5953–62. [PubMed]
106. Yang YL, et al. Deficient signaling in mice devoid of double-stranded RNA-dependent protein kinase. Embo J. 1995;14:6095–106. [PMC free article] [PubMed]The first report of the phenotype of the PKR knockout mouse.
107. Scheuner D, et al. The double-stranded RNA-activated protein kinase mediates viral-induced encephalitis. Virology. 2003;317:263–74. [PubMed]
108. Ladiges W, et al. Expression of human PKR protein kinase in transgenic mice. J Interferon Cytokine Res. 2002;22:329–34. [PubMed]
109. Balachandran S, et al. Essential role for the dsRNA-dependent protein kinase PKR in innate immunity to viral infection. Immunity. 2000;13:129–41. [PubMed]
110. Stojdl DF, et al. The murine double-stranded RNA-dependent protein kinase PKR is required for resistance to vesicular stomatitis virus. J Virol. 2000;74:9580–5. [PMC free article] [PubMed]
111. Bergmann M, et al. Influenza virus NS1 protein counteracts PKR-mediated inhibition of replication. J Virol. 2000;74:6203–6. [PMC free article] [PubMed]
112. Noguchi T, et al. Effects of mutation in hepatitis C virus nonstructural protein 5A on interferon resistance mediated by inhibition of PKR kinase activity in mammalian cells. Microbiol Immunol. 2001;45:829–40. [PubMed]
113. Chen CW, et al. The double-stranded RNA-activated kinase, PKR, can phosphorylate hepatitis D virus small delta antigen at functional serine and threonine residues. J Biol Chem. 2002;277:33058–67. [PubMed]
114. Samuel MA, et al. PKR and RNase L contribute to protection against lethal West Nile Virus infection by controlling early viral spread in the periphery and replication in neurons. J Virol. 2006;80:7009–19. [PMC free article] [PubMed]
115. Nagai K, et al. Induction of CD4 expression and human immunodeficiency virus type 1 replication by mutants of the interferon-inducible protein kinase PKR. J Virol. 1997;71:1718–25. [PMC free article] [PubMed]
116. Gorchakov R, Frolova E, Williams BR, Rice CM, Frolov I. PKR-dependent and -independent mechanisms are involved in translational shutoff during Sindbis virus infection. J Virol. 2004;78:8455–67. [PMC free article] [PubMed]
117. Yeung MC, Chang DL, Camantigue RE, Lau AS. Inhibitory role of the host apoptogenic gene PKR in the establishment of persistent infection by encephalomyocarditis virus in U937 cells. Proc Natl Acad Sci U S A. 1999;96:11860–5. [PMC free article] [PubMed]
118. Chinsangaram J, Koster M, Grubman MJ. Inhibition of L-deleted foot-and-mouth disease virus replication by alpha/beta interferon involves double-stranded RNA-dependent protein kinase. J Virol. 2001;75:5498–503. [PMC free article] [PubMed]
119. Al-khatib K, Williams BR, Silverman RH, Halford W, Carr DJ. The murine double-stranded RNA-dependent protein kinase PKR and the murine 2′,5′-oligoadenylate synthetase-dependent RNase L are required for IFN-beta-mediated resistance against herpes simplex virus type 1 in primary trigeminal ganglion culture. Virology. 2003;313:126–35. [PubMed]
120. Knapp S, et al. Polymorphisms in interferon-induced genes and the outcome of hepatitis C virus infection: roles of MxA, OAS-1 and PKR. Genes Immun. 2003;4:411–9. [PubMed]
121. Carritt B, et al. A gene from human chromosome region 3p21 with reduced expression in small cell lung cancer. Cancer Res. 1992;52:1536–41. [PubMed]
122. Kok K, et al. A gene in the chromosomal region 3p21 with greatly reduced expression in lung cancer is similar to the gene for ubiquitin-activating enzyme. Proc Natl Acad Sci U S A. 1993;90:6071–5. [PMC free article] [PubMed]
123. Carpten J, et al. Germline mutations in the ribonuclease L gene in families showing linkage with HPC1. Nat Genet. 2002;30:181–4. [PubMed]
124. Urisman A, et al. Identification of a novel Gammaretrovirus in prostate tumors of patients homozygous for R462Q RNASEL variant. PLoS Pathog. 2006;2:e25. [PMC free article] [PubMed]
125. Silverman RH, et al. Skin allograft rejection is suppressed in mice lacking the antiviral enzyme, 2′,5′-oligoadenylate-dependent RNase L. Viral Immunol. 2002;15:77–83. [PubMed]
126. Field LL, et al. OAS1 splice site polymorphism controlling antiviral enzyme activity influences susceptibility to type 1 diabetes. Diabetes. 2005;54:1588–91. [PubMed]
127. Bonnevie-Nielsen V, Larsen ML, Frifelt JJ, Michelsen B, Lernmark A. Association of IDDM and attenuated response of 2′,5′-oligoadenylate synthetase to yellow fever vaccine. Diabetes. 1989;38:1636–42. [PubMed]
128. Kumar A, Haque J, Lacoste J, Hiscott J, Williams BR. Double-stranded RNA-dependent protein kinase activates transcription factor NF-kappa B by phosphorylating I kappa B. Proc Natl Acad Sci U S A. 1994;91:6288–92. [PMC free article] [PubMed]
129. Uetani K, et al. Central role of double-stranded RNA-activated protein kinase in microbial induction of nitric oxide synthase. J Immunol. 2000;165:988–96. [PubMed]
130. Lee JH, et al. Double-stranded RNA-activated protein kinase is required for the LPS-induced activation of STAT1 inflammatory signaling in rat brain glial cells. Glia. 2005;50:66–79. [PubMed]
131. Karehed K, Dimberg A, Dahl S, Nilsson K, Oberg F. IFN-gamma-induced upregulation of Fcgamma-receptor-I during activation of monocytic cells requires the PKR and NFkappaB pathways. Mol Immunol. 2007;44:615–24. [PubMed]
132. Lee SB, Esteban M. The interferon-induced double-stranded RNA-activated protein kinase induces apoptosis. Virology. 1994;199:491–6. [PubMed]The authors provide the first evidence for a role for PKR in regulating apoptosis.
133. Kadereit S, et al. Negative regulation of CD8+ T cell function by the IFN-induced and double-stranded RNA-activated kinase PKR. J Immunol. 2000;165:6896–901. [PubMed]
134. Hardenberg G, et al. Specific TLR ligands regulate APRIL secretion by dendritic cells in a PKR-dependent manner. Eur J Immunol. 2007;37:2900–11. [PubMed]
135. Rager KJ, et al. Activation of antiviral protein kinase leads to immunoglobulin E class switching in human B cells. J Virol. 1998;72:1171–6. [PMC free article] [PubMed]
136. Bonnet MC, Weil R, Dam E, Hovanessian AG, Meurs EF. PKR stimulates NF-kappaB irrespective of its kinase function by interacting with the IkappaB kinase complex. Mol Cell Biol. 2000;20:4532–42. [PMC free article] [PubMed]
137. Haller O, Kochs G, Weber F. Interferon, Mx, and viral countermeasures. Cytokine Growth Factor Rev. 2007;18:425–33. [PubMed]
138. Langland JO, Cameron JM, Heck MC, Jancovich JK, Jacobs BL. Inhibition of PKR by RNA and DNA viruses. Virus Res. 2006;119:100–10. [PubMed]
139. Leib DA, Machalek MA, Williams BR, Silverman RH, Virgin HW. Specific phenotypic restoration of an attenuated virus by knockout of a host resistance gene. Proc Natl Acad Sci U S A. 2000;97:6097–101. [PMC free article] [PubMed]This paper establishes the key steps required for defining the role of host genes in resistance to viral infection.
PubReader format: click here to try

Formats:

Related citations in PubMed

See reviews...See all...

Cited by other articles in PMC

See all...

Links

Recent Activity

Your browsing activity is empty.

Activity recording is turned off.

Turn recording back on

See more...