Specific mutations near the amino terminus of double-stranded RNA-dependent protein kinase (PKR) differentially affect its double-stranded RNA binding and dimerization properties

Induction of the human protein P56 by interferon, double-stranded RNA, or virus infection

P56 is the most abundant protein induced by interferon (IFN) treatment of human cells. To facilitate studies on its induction pattern and cellular functions, we expressed recombinant P56 as a hexahistidine-tagged protein in Escherichia coli and purified it to apparent homogeneity using affinity chromatography. A polyclonal antibody raised against this recombinant protein was used to show that P56 is primarily a cytoplasmic protein. Cellular expression of P56 by transfection did not inhibit the replication of vesicular stomatitis virus and encephalomyocarditis virus. P56 synthesis was rapidly induced by IFN-beta, and the protein had a half-life of 6 h. IFN-gamma or poly(A)(+) could not induce the protein, but poly(I)-poly(C) or an 85-bp synthetic double-stranded RNA efficiently induced it. Similarly, infection of GRE cells, which are devoid of type I IFN genes, by vesicular stomatitis virus, encephalomyocarditis virus, or Sendai virus caused P56 induction. Surprisingly, Sendai virus could also induce P56 in the mutant cell line P2.1, which cannot respond to either IFN-alpha/beta or double-stranded RNA. Induction of P56 in the P2.1 cells and the parental U4C cells by virus infection was preceded by activation of IRF-3 as judged by its translocation to the nucleus from the cytoplasm.

Characterization of the interaction between the interferon-induced protein P56 and the Int6 protein encoded by a locus of insertion of the mouse mammary tumor virus

For determining cellular functions of the interferon-inducible human cytoplasmic protein P56, we undertook a Saccharomyces cerevisiae two-hybrid screen that identified Int6 as a P56-interacting protein. That the interaction also occurs in human cells was confirmed by coimmunoprecipitation and the observed cytoplasmic displacement of nuclear Int6 upon coexpression of P56. Because Int6 has been claimed to be both a cytoplasmic and a nuclear protein, we investigated the structural basis of this discrepancy. By mutational analyses, we showed that the Int6 protein contains a bipartite nuclear localization signal and a nuclear export signal at the far end of the amino terminus. The 20 amino-terminal residues of Int6, when they were attached to a different nuclear protein, were sufficient to translocate that protein to the cytoplasm. Within this region, replacement of any of the three leucine residues with alanine destroyed the function of the export signal. The specific domain of P56 that is required for its interaction with Int6 was mapped using the yeast two-hybrid assay and a mammalian coimmunoprecipitation assay. Both assays demonstrated that the C-terminal region of P56 containing three specific tetratricopeptide motifs is required for this interaction. In contrast, removal of an internal domain of P56 enhanced the interaction, as quantified by the two-hybrid assay.

Alternative splice variants of the human PKR protein kinase possessing different 5'-untranslated regions: expression in untreated and interferon-treated cells and translational activity

The double-stranded RNA-dependent protein kinase PKR is an interferon-inducible enzyme that possesses antiviral and antiproliferative activities. We examined expression of PKR transcripts in human placenta tissue and cultured human amnion U cells. Alternative exon 2 structures were identified and characterized that possess different functional activities. Cloning and sequence analyses of 5'-RACE cDNAs from human placenta established a linkage between exon 1 and three alternative exon 2 structures that constitute, together with part of exon 3, the 5'-untranslated region of the PKR mRNA. The alternative splice variants of exon 2 were designated Ex2alpha (83 nucleotides), Ex2beta (167 nucleotides), and Ex2gamma (401 nucleotides). All three exon 2 variants were present in placenta tissue. However, only the Ex2alpha and Ex2beta forms were detectable in the amnion U cell line. Nuclease protection analysis revealed that the Ex2beta form was slightly more abundant than the Ex2alpha form, in both placenta tissue and U cells. Interferon treatment of U cells increased the level of both Ex2alpha and Ex2beta RNA by approximately 5-fold. The translational activities, measured in a luciferase reporter assay, of RNA transcripts possessing the Ex2alpha and Ex2beta forms of the PKR 5'-UTR were comparable to each other and more efficient than those with the Ex2gamma form.

PKR; a sentinel kinase for cellular stress

The double stranded RNA (dsRNA)-activated protein kinase PKR is a ubiquitously expressed serine/threonine protein kinase that is induced by interferon and activated by dsRNA, cytokine, growth factor and stress signals. It is essential for cells to respond adequately to different stresses including growth factor deprivation, products of the inflammatory response (TNF) and bacterial (lipopolysaccharide) and viral (dsRNA) products. As a vital component of the cellular antiviral response pathway, PKR is autophosphorylated and activated on binding to dsRNA. This results in inhibition of protein synthesis via the phosphorylation of eIF2alpha and also induces transcription of inflammatory genes by PKR-dependent signaling of the activation of different transcription factors. Along with RNaseL, PKR constitutes the antiviral arm of a group of mammalian stress response proteins that have counterparts in yeast. What began as adaptation to amino acid deprivation and sensing unfolded proteins in the endoplasmic reticulum has evolved into a family of sophisticated mammalian stress response proteins able to mediate cellular responses to both physical and biological stress.

DRBP76, a double-stranded RNA-binding nuclear protein, is phosphorylated by the interferon-induced protein kinase, PKR

The interferon-induced double-stranded RNA-activated protein kinase PKR is the prototype of a class of double-stranded (dsRNA)-binding proteins (DRBPs) which share a dsRNA-binding motif conserved from Drosophila to humans. Here we report the purification of DRBP76, a new human member of this class of proteins. Sequence from the amino terminus of DRBP76 matched that of the M phase-specific protein, MPP4. DRBP76 was also cloned by the yeast two-hybrid screening of a cDNA library using a mutant PKR as bait. Analysis of the cDNA sequence revealed that it is the full-length version of MPP4, has a bipartite nuclear localization signal, two motifs that can mediate interactions with both dsRNA and PKR, five epitopes for potential M phase-specific phosphorylation, two potential sites for phosphorylation by cyclin-dependent kinases, a RG2 motif present in many RNA-binding proteins and predicts a protein of 76 kDa. DsRNA and PKR interactions of DRBP76 were confirmed by analysis of in vitro translated and purified native proteins. Cellular expression of an epitope-tagged DRBP76 demonstrated its nuclear localization, and its co-immunoprecipitation with PKR demonstrated that the two proteins interact in vivo. Finally, purified DRBP76 was shown to be a substrate of PKR in vitro, indicating that this protein's cellular activities may be regulated by PKR-mediated phosphorylation.

PKR, apoptosis and cancer

The double-stranded (ds) RNA-regulated serine/threonine protein kinase, PKR, is an interferon-inducible enzyme of widespread occurrence in mammalian cells. PKR is activated by dsRNA via a mechanism involving autophosphorylation. Once activated, the enzyme phosphorylates the alpha-subunit of protein synthesis initiation factor eIF2, thereby inhibiting translation. Accumulating data suggest that PKR has additional substrates, and that the kinase may also regulate gene transcription and signal transduction pathways. Although PKR plays an important role in mediating the antiviral effects of interferons, PKR is also implicated in regulating cell proliferation in uninfected cells and may have a tumor suppressor function under normal conditions. Studies of human malignancies and tumor cell lines suggest that, in general, patients bearing tumors with a higher PKR content have a more favorable prognosis. However, in human breast carcinoma cells, dysregulation of PKR may be associated with the establishment or maintenance of the transformed state.

Mechanism of interferon action: identification of essential positions within the novel 15-base-pair KCS element required for transcriptional activation of the RNA-dependent protein kinase pkr gene

RNA-dependent protein kinase PKR is an important regulator of gene expression in interferon (IFN)-treated and virus-infected cells. The 50-kb gene encoding human PKR kinase (pkr) is inducible by IFN. Transfection analyses, using chloramphenicol acetyltransferase (CAT) as the reporter in constructs possessing various 5'-flanking fragments of the human pkr gene, led to the identification of a functional TATA-less promoter that directed IFN-inducible transcription. Sequence determination and mutational analysis of the pkr promoter region revealed, in addition to a functional copy of the IFN-stimulated response element (ISRE) responsible for inducibility by type I IFN, a novel 15-bp element required for optimal promoter activity mediated by the ISRE. This element (5' GGGAAGGCGGAGTCC 3'), designated KCS for kinase-conserved sequence, is exactly conserved between the human and mouse pkr promoters in sequence and position relative to the ISRE. We have now carried out an extensive mutational analysis of the 15-bp KCS element. Site-directed mutagenesis was performed, whereby every base pair position within the KCS element was replaced by each of the other three alternatives. Forty-five substitution mutants were analyzed for promoter activity by transient transfection analysis of untreated and IFN-treated human cells. The results establish 5' NNRRRGG(C,A,T)GGRGYYN 3', where R stands for purine and Y stands for pyrimidine, as the consensus sequence for the KCS element, both for basal and for IFN-inducible promoter activity. KCS-binding proteins were detected by electrophoretic mobility shift analysis (EMSA). Competition EMSA established that constitutively expressed nuclear proteins bound the KCS element selectively; KCS protein binding activity correlated with promoter activity in the transient transfection reporter assay.

Inhibition of double-stranded RNA-dependent protein kinase PKR by vaccinia virus E3: role of complex formation and the E3 N-terminal domain

The human double-stranded RNA (dsRNA)-dependent protein kinase PKR inhibits protein synthesis by phosphorylating translation initiation factor 2alpha (eIF2alpha). Vaccinia virus E3L encodes a dsRNA binding protein that inhibits PKR in virus-infected cells, presumably by sequestering dsRNA activators. Expression of PKR in Saccharomyces cerevisiae inhibits protein synthesis by phosphorylation of eIF2alpha, dependent on its two dsRNA binding motifs (DRBMs). We found that expression of E3 in yeast overcomes the lethal effect of PKR in a manner requiring key residues (Lys-167 and Arg-168) needed for dsRNA binding by E3 in vitro. Unexpectedly, the N-terminal half of E3, and residue Trp-66 in particular, also is required for anti-PKR function. Because the E3 N-terminal region does not contribute to dsRNA binding in vitro, it appears that sequestering dsRNA is not the sole function of E3 needed for inhibition of PKR. This conclusion was supported by the fact that E3 activity was antagonized, not augmented, by overexpressing the catalytically defective PKR-K296R protein containing functional DRBMs. Coimmunoprecipitation experiments showed that a majority of PKR in yeast extracts was in a complex with E3, whose formation was completely dependent on the dsRNA binding activity of E3 and enhanced by the N-terminal half of E3. In yeast two-hybrid assays and in vitro protein binding experiments, segments of E3 and PKR containing their respective DRBMs interacted in a manner requiring E3 residues Lys-167 and Arg-168. We also detected interactions between PKR and the N-terminal half of E3 in the yeast two-hybrid and lambda repressor dimerization assays. In the latter case, the N-terminal half of E3 interacted with the kinase domain of PKR, dependent on E3 residue Trp-66. We propose that effective inhibition of PKR in yeast requires formation of an E3-PKR-dsRNA complex, in which the N-terminal half of E3 physically interacts with the protein kinase domain of PKR.

Requirement of PKR dimerization mediated by specific hydrophobic residues for its activation by double-stranded RNA and its antigrowth effects in yeast

The roles of protein dimerization and double-stranded RNA (dsRNA) binding in the biochemical and cellular activities of PKR, the dsRNA-dependent protein kinase, were investigated. We have previously shown that both properties of the protein are mediated by the same domain. Here we show that dimerization is mediated by hydrophobic residues present on one side of an amphipathic alpha-helical structure within this domain. Appropriate substitution mutations of residues on that side produced mutants with increased or decreased dimerization activities. Using these mutants, we demonstrated that dimerization is not essential for dsRNA binding. However, enhancing dimerization artificially, by providing an extraneous dimerization domain, increased dsRNA binding of both wild-type and mutant proteins. In vitro, the dimerization-defective mutants could not be activated by dsRNA but were activated normally by heparin. In Saccharomyces cerevisiae, unlike wild-type PKR, these mutants could not inhibit cell growth and the dsRNA-binding domain of the dimerization-defective mutants could not prevent the antigrowth effect of wild-type PKR. These results demonstrate the biological importance of the dimerization properties of PKR.

The vaccinia virus E3L gene product interacts with both the regulatory and the substrate binding regions of PKR: implications for PKR autoregulation

The vaccinia virus E3L gene product, pE3, is a dsRNA binding protein that prevents activation of the interferon-induced, dsRNA-activated protein kinase, PKR. Activation of PKR, which results in phosphorylation of the translation initiation factor, eIF2alpha, leads to the inhibition of protein synthesis, a process involved in defense against virus infection. The E3L gene product has a conserved dsRNA binding domain (DRBD) in its carboxyl-terminal region and has been shown to function in vitro by sequestration of dsRNA. We have utilized in vitro binding assays and the yeast two-hybrid system to demonstrate direct interactions of pE3 with PKR. By these methods, we demonstrate that pE3 interacts with two distinct regions in PKR, the amino-terminal (amino acids 1-99) located in the regulatory domain and the carboxyl-terminal (amino acids 367-523) located in the catalytic domain. The amino-terminal region of PKR that interacts with pE3 contains a conserved DRBD, suggesting that PKR can form nonfunctional heterodimers with pE3, analogous to those seen with other dsRNA binding proteins. Interaction of pE3 with the amino-terminal region of PKR is enhanced by dsRNA. In contrast, dsRNA reduces the interaction of pE3 with the carboxyl-terminal region of PKR. Competition experiments demonstrate that the carboxyl-terminal region of PKR, to which pE3 binds, overlaps the region with which eIF2alpha and the pseudosubstrate pK3 interact, suggesting that pE3 may also prevent PKR activation by masking the substrate binding domain. Like pE3, the amino-terminal region of PKR also interacts with the carboxyl-terminal domain of PKR. These interactions increase our understanding of the mechanisms by which pE3 downregulates PKR. In addition, the PKR-PKR interactions observed leads us to suggest a novel autoregulatory mechanism for activation of PKR in which dsRNA binding to the DRBD(s) induces a conformational change that results in release of the amino terminal region from the substrate binding domain, allowing access to eIF2alpha and its subsequent phosphorylation.

Inhibition of the double-stranded RNA-dependent protein kinase PKR by mammalian ribosomes

Previous evidence has shown that the majority of the interferon-inducible, double-stranded RNA-dependent protein kinase PKR is associated with ribosomes in vivo. Here we show that ribosomes are inhibitory for PKR activity since they compete with dsRNA for binding to PKR, inhibit the activation of the protein kinase by dsRNA, and prevent the phosphorylation of the PKR substrate eIF2alpha. We suggest that ribosomes constitute a reservoir of inactive PKR and that the protein kinase must be displaced from the ribosome by dsRNA in order to become activated.

Structure of the double-stranded RNA-binding domain of the protein kinase PKR reveals the molecular basis of its dsRNA-mediated activation

Protein kinase PKR is an interferon-induced enzyme that plays a key role in the control of viral infections and cellular homeostasis. Compared with other known kinases, PKR is activated by a distinct mechanism that involves double-stranded RNA (dsRNA) binding in its N-terminal region in an RNA sequence-independent fashion. We report here the solution structure of the 20 kDa dsRNA-binding domain (dsRBD) of human PKR, which provides the first three-dimensional insight into the mechanism of its dsRNA-mediated activation. The structure of dsRBD exhibits a dumb-bell shape comprising two tandem linked dsRNA-binding motifs (dsRBMs) both with an alpha-beta-beta-beta-alpha fold. The structure, combined with previous mutational and biochemical data, reveals a highly conserved RNA-binding site on each dsRBM and suggests a novel mode of protein-RNA recognition. The central linker is highly flexible, which may enable the two dsRBMs to wrap around the RNA duplex for cooperative and high-affinity binding, leading to the overall change of PKR conformation and its activation.

Control of PKR protein kinase by hepatitis C virus nonstructural 5A protein: molecular mechanisms of kinase regulation

The PKR protein kinase is a critical component of the cellular antiviral and antiproliferative responses induced by interferons. Recent evidence indicates that the nonstructural 5A (NS5A) protein of hepatitis C virus (HCV) can repress PKR function in vivo, possibly allowing HCV to escape the antiviral effects of interferon. NS5A presents a unique tool by which to study the molecular mechanisms of PKR regulation in that mutations within a region of NS5A, termed the interferon sensitivity-determining region (ISDR), are associated with sensitivity of HCV to the antiviral effects of interferon. In this study, we investigated the mechanisms of NS5A-mediated PKR regulation and the effect of ISDR mutations on this regulatory process. We observed that the NS5A ISDR, though necessary, was not sufficient for PKR interactions; we found that an additional 26 amino acids (aa) carboxyl to the ISDR were required for NS5A-PKR complex formation. Conversely, we localized NS5A binding to within PKR aa 244 to 296, recently recognized as a PKR dimerization domain. Consistent with this observation, we found that NS5A from interferon-resistant HCV genotype 1b disrupted kinase dimerization in vivo. NS5A-mediated disruption of PKR dimerization resulted in repression of PKR function and inhibition of PKR-mediated eIF-2alpha phosphorylation. Introduction of multiple ISDR mutations abrogated the ability of NS5A to bind to PKR in mammalian cells and to inhibit PKR in a yeast functional assay. These results indicate that mutations within the PKR-binding region of NS5A, including those within the ISDR, can disrupt the NS5A-PKR interaction, possibly rendering HCV sensitive to the antiviral effects of interferon. We propose a model of PKR regulation by NS5A which may have implications for therapeutic strategies against HCV.

PACT, a protein activator of the interferon-induced protein kinase, PKR

PKR, a latent protein kinase, mediates the antiviral actions of interferon. It is also involved in cellular signal transduction, apoptosis, growth regulation and differentiation. Although in virus-infected cells, viral double-stranded (ds) RNA can serve as a PKR activator, cellular activators have remained obscure. Here, we report the cloning of PACT, a cellular protein activator of PKR. PACT heterodimerized with PKR and activated it in vitro in the absence of dsRNA. In mammalian cells, overexpression of PACT caused PKR activation and, in yeast, co-expression of PACT enhanced the anti-growth effect of PKR. Thus, PACT has the hallmarks of a direct activator of PKR.

RNA binding and modulation of PKR activity

PKR is an RNA-dependent protein kinase that is induced in mammalian cells by interferon treatment. It is present in a latent or inactive form in mammalian cells and is activated by very low concentrations of double-stranded (ds) RNA. Activated PKR phosphorylates eIF2, an essential initiation factor of protein synthesis, as well as other substrates including histone IIA, a 90-kDa protein from rabbit reticulocytes, the inhibitor, IkappaB, of the transcription factor, NF-kappaB, and the HIV-1 Tat protein. PKR interacts with several cellular and viral products and these interactions modulate its activation by dsRNA. Here we describe methods that are used to study the activation or inhibition of PKR by RNA modulators. Specifically, we detail (1) the purification of PKR from interferon-treated mammalian cells, (2) functional assays for PKR activation and inhibition in vitro, using purified enzyme or crude cell lysates, and (3) assays allowing evaluation of the binding of dsRNA and single-stranded RNA to PKR.

Mutations in the double-stranded RNA-activated protein kinase insert region that uncouple catalysis from eIF2alpha binding

The interferon-induced, double-stranded RNA (dsRNA)-activated protein kinase, PKR, inhibits protein synthesis via phosphorylation of the alpha subunit of the translation initiation factor eIF2. A kinase insert region N-terminal of PKR kinase subdomain V, which is conserved among eIF2alpha kinases, has been proposed to determine substrate specificity of these kinases. To investigate the function of this kinase insert region, selective PKR mutants were generated, and kinase activities and eIF2alpha affinities were analyzed in vitro. The in vivo function was investigated by growth inhibitory assays in yeast and translational assays in COS cells. Among the 13 mutations, 5 lost kinase activity and 3 exhibited less than 30% of wild-type eIF2alpha binding activity. The deletion of the conserved sequence (amino acids 362-370) resulted in a protein that had no kinase activity and only about 25% of wild-type eIF2alpha binding, suggesting that this sequence is not only required for PKR kinase activity but also is important for substrate interaction. It was determined that the hydrophobicity of the conserved sequence of PKR is required for kinase activity but is not crucial for eIF2alpha binding. The amino acid residue Glu-367 in the conserved motif was shown to be directly involved in substrate binding but was not important for kinase activity. These results suggest that the activation of PKR is not a prerequisite for its binding to the substrate and that the conserved motif in subdomain V contributes to the interaction of PKR and eIF2alpha.

Double-stranded RNA-independent dimerization of interferon-induced protein kinase PKR and inhibition of dimerization by the cellular P58IPK inhibitor

The interferon (IFN)-induced, double-stranded RNA-activated protein kinase (PKR) mediates the antiviral and antiproliferative actions of IFN, in part, via its translational inhibitory properties. Previous studies have demonstrated that PKR forms dimers and that dimerization is likely to be required for activation and/or function. In the present study we used multiple approaches to examine the modulation of PKR dimerization. Deletion analysis with the lambda repressor fusion system identified a previously unrecognized site involved in PKR dimerization. This site comprised amino acids (aa) 244 to 296, which span part of the third basic region of PKR and the catalytic subdomains I and II. Using the yeast two-hybrid system and far-Western analysis, we verified the importance of this region for dimerization. Furthermore, coexpression of the 52-aa region alone inhibited the formation of full-length PKR dimers in the lambda repressor fusion and two-hybrid systems. Importantly, coexpression of aa 244 to 296 exerted a dominant-negative effect on wild-type kinase activity in a functional assay. Due to its role as a mediator of IFN-induced antiviral resistance, PKR is a target of viral and cellular inhibitors. Curiously, PKR aa 244 to 296 contain the binding site for a select group of specific inhibitors, including the cellular protein P58IPK. We demonstrated, utilizing both the yeast and lambda systems, that P58IPK, a member of the tetratricopeptide repeat protein family, can block kinase activity by preventing PKR dimerization. In contrast, a nonfunctional form of P58IPK lacking a TPR motif did not inhibit kinase activity or perturb PKR dimers. These results highlight a potential mechanism of PKR inhibition and define a novel class of PKR inhibitors. Finally, the data document the first known example of inhibition of protein kinase dimerization by a cellular protein inhibitor. On the basis of these results we propose a model for the regulation of PKR dimerization.

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

The human double-stranded RNA-dependent protein kinase (PKR) is an important component of the interferon response to virus infection. The activation of PKR is accompanied by autophosphorylation at multiple sites, including one in the N-terminal regulatory region (Thr-258) that is required for full kinase activity. Several protein kinases are activated by phosphorylation in the region between kinase subdomains VII and VIII, referred to as the activation loop. We show that Thr-446 and Thr-451 in the PKR activation loop are required in vivo and in vitro for high-level kinase activity. Mutation of either residue to Ala impaired translational control by PKR in yeast cells and COS1 cells and led to tumor formation in mice. These mutations also impaired autophosphorylation and eukaryotic initiation factor 2 subunit alpha (eIF2alpha) phosphorylation by PKR in vitro. Whereas the Ala-446 substitution substantially reduced PKR function, the mutant kinase containing Ala-451 was completely inactive. PKR specifically phosphorylated Thr-446 and Thr-451 in synthetic peptides in vitro, and mass spectrometry analysis of PKR phosphopeptides confirmed that Thr-446 is an autophosphorylation site in vivo. Substitution of Glu-490 in subdomain X of PKR partially restored kinase activity when combined with the Ala-451 mutation. This finding suggests that the interaction between subdomain X and the activation loop, described previously for MAP kinase, is a regulatory feature conserved in PKR. We found that the yeast eIF2alpha kinase GCN2 autophosphorylates at Thr-882 and Thr-887, located in the activation loop at exactly the same positions as Thr-446 and Thr-451 in PKR. Thr-887 was more critically required than was Thr-882 for GCN2 kinase activity, paralleling the relative importance of Thr-446 and Thr-451 in PKR. These results indicate striking similarities between GCN2 and PKR in the importance of autophosphorylation and the conserved Thr residues in the activation loop.

Molecular mechanisms of interferon resistance mediated by viral-directed inhibition of PKR, the interferon-induced protein kinase

The interferon (IFN)-induced cellular antiviral response is the first line of defense against viral infection within an animal host. In order to establish a productive infection, eukaryotic viruses must first overcome the IFN-induced blocks imposed on viral replication. The double-stranded RNA-activated protein kinase (PKR) is a key component mediating the antiviral actions of IFN. This IFN-induced protein kinase can restrict viral replication through its ability to phosphorylate the protein synthesis initiation factor eukaryotic initiation factor-2 alpha-subunit and reduce levels of viral protein synthesis. Viruses, therefore, must block the function of PKR in order to avoid these deleterious antiviral effects associated with PKR activity. Indeed, many viruses have developed effective measures to repress PKR activity during infection. This review will focus primarily on an overview of the different molecular mechanisms employed by these viruses to meet a common goal: the inhibition of PKR function, uncompromised viral protein synthesis, and unrestricted virus replication. The past few years have seen exciting new advances in this area. Rather unexpectedly, this area of research has benefited from the use of the yeast system to study PKR. Other recent advances include studies on PKR regulation by the herpes simplex viruses and data from our laboratory on the medically important hepatitis C viruses. We speculate that IFN is ineffective as a therapeutic agent against hepatitis C virus because the virus can effectively repress PKR function. Finally, we will discuss briefly the future directions of this PKR field.

Double-stranded RNA-dependent protein kinase (PKR) is regulated by reovirus structural proteins

Reovirus sigma3 is a virion outer shell protein that also binds dsRNA and stimulates translation by blocking activation of the dsRNA-dependent protein kinase, PKR. Purified sigma3 was shown by gel shift assay to bind specifically to RNA duplexes of minimal length 32-45 base pairs. PKR binding to dsRNA was prevented by sigma3, and translation inhibition of luciferase reporter by PKR expression in transfected cells was reversed by sigma3. Association of sigma3 with its outer capsid partner mu1/mu1C eliminated dsRNA binding and prevented restoration of protein synthesis. Analyses of sigma3 mutants demonstrated a direct correlation between dsRNA binding and reversal of the down-regulation of translation by PKR. In infected cells, sigma3 was stable but dsRNA binding decreased, presumably due to mu1/mu1C complex formation. The results suggest a functional transition from early inhibition of PKR activation by sigma3 to its association with mu1/mu1C in capsid structures.