Double-stranded-RNA-dependent protein kinase and TAR RNA-binding protein form homo- and heterodimers in vivo

Dystonia 16 (DYT16) mutations in PACT cause dysregulated PKR activation and eIF2α signaling leading to a compromised stress response

Dystonia 16 (DYT16) is caused by mutations in PACT, the protein activator of interferon-induced double-stranded RNA-activated protein kinase (PKR). PKR regulates the integrated stress response (ISR) via phosphorylation of the translation initiation factor eIF2α. This post-translational modification attenuates general protein synthesis while concomitantly triggering enhanced translation of a few specific transcripts leading either to recovery and homeostasis or cellular apoptosis depending on the intensity and duration of stress signals. PKR plays a regulatory role in determining the cellular response to viral infections, oxidative stress, endoplasmic reticulum (ER) stress, and growth factor deprivation. In the absence of stress, both PACT and PKR are bound by their inhibitor transactivation RNA-binding protein (TRBP) thereby keeping PKR inactive. Under conditions of cellular stress these inhibitory interactions dissociate facilitating PACT-PACT interactions critical for PKR activation. While both PACT-TRBP and PKR-TRBP interactions are pro-survival, PACT-PACT and PACT-PKR interactions are pro-apoptotic. In this study we evaluate if five DYT16 substitution mutations alter PKR activation and ISR. Our results indicate that the mutant DYT16 proteins show stronger PACT-PACT interactions and enhanced PKR activation. In DYT16 patient derived lymphoblasts the enhanced PACT-PKR interactions and heightened PKR activation leads to a dysregulation of ISR and increased apoptosis. More importantly, this enhanced sensitivity to ER stress can be rescued by luteolin, which disrupts PACT-PKR interactions. Our results not only demonstrate the impact of DYT16 mutations on regulation of ISR and DYT16 etiology but indicate that therapeutic interventions could be possible after a further evaluation of such strategies.

Mutual Regulation of RNA Silencing and the IFN Response as an Antiviral Defense System in Mammalian Cells

RNA silencing is a posttranscriptional gene silencing mechanism directed by endogenous small non-coding RNAs called microRNAs (miRNAs). By contrast, the type-I interferon (IFN) response is an innate immune response induced by exogenous RNAs, such as viral RNAs. Endogenous and exogenous RNAs have typical structural features and are recognized accurately by specific RNA-binding proteins in each pathway. In mammalian cells, both RNA silencing and the IFN response are induced by double-stranded RNAs (dsRNAs) in the cytoplasm, but have long been considered two independent pathways. However, recent reports have shed light on crosstalk between the two pathways, which are mutually regulated by protein–protein interactions triggered by viral infection. This review provides brief overviews of RNA silencing and the IFN response and an outline of the molecular mechanism of their crosstalk and its biological implications. Crosstalk between RNA silencing and the IFN response may reveal a novel antiviral defense system that is regulated by miRNAs in mammalian cells.

Multiplex live single-cell transcriptional analysis demarcates cellular functional heterogeneity

A fundamental goal in the biological sciences is to determine how individual cells with varied gene expression profiles and diverse functional characteristics contribute to development, physiology, and disease. Here, we report a novel strategy to assess gene expression and cell physiology in single living cells. Our approach utilizes fluorescently labeled mRNA-specific anti-sense RNA probes and dsRNA-binding protein to identify the expression of specific genes in real-time at single-cell resolution via FRET. We use this technology to identify distinct myocardial subpopulations expressing the structural proteins myosin heavy chain α and myosin light chain 2a in real-time during early differentiation of human pluripotent stem cells. We combine this live-cell gene expression analysis with detailed physiologic phenotyping to capture the functional evolution of these early myocardial subpopulations during lineage specification and diversification. This live-cell mRNA imaging approach will have wide ranging application wherever heterogeneity plays an important biological role.

Protein kinase R and its cellular regulators in cancer: An active player or a surveillant?

Protein kinase R (PKR), originally known as an antiviral protein, senses various stresses as well as pathogen-driven double-stranded RNAs. Thereby activated PKR provokes diverse downstream events, including eIF2α phosphorylation and nuclear factor kappa-light-chain-enhancer of activated B cells activation. Consequently, PKR induces apoptosis and inflammation, both of which are highly important in cancer as much as its original antiviral role. Therefore, cellular proteins and RNAs should tightly control PKR activity. PKR and its regulators are often dysregulated in cancer and it is undoubted that such dysregulation contributes to tumorigenesis. However, PKR's precise role in cancer is still in debate, due to incomprehensible and even contradictory data. In this review, we introduce important cellular PKR regulators and discuss about their roles in cancer. Among them, we pay particular attention to nc886, a PKR repressor noncoding RNA that has been identified relatively recently, because its expression pattern in cancer can explain interesting yet obscure oncologic aspects of PKR. Based on nc886 and its regulation of PKR, we have proposed a tumor surveillance model, which reconciles contradictory data about PKR in cancer. This article is categorized under: Regulatory RNAs/RNAi/Riboswitches > Regulatory RNAs RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications.

The Noncoding RNA nc886 Regulates PKR Signaling and Cytokine Production in Human Cells

Protein kinase RNA-activated (PKR) is a cytoplasmic receptor for dsRNA, and as such is involved in detection of viral infection. On binding dsRNA, PKR dimerizes, autophosphorylates, and then phosphorylates its substrate, eukaryotic translation initiation factor 2 subunit α (eIF2α), causing inhibition of mRNA translation and shutdown of viral protein production. However, active PKR has also been found to be involved in the NF-κB signaling pathway by inducing phosphorylation of IκBα. PKR is regulated by the noncoding RNA nc886, which has altered expression in cancer. We have found that expression of nc886 is highly upregulated during activation of human CD4⁺ T cells. As has been described in other cell types, nc886 bound to PKR in human T cell lysates, preventing PKR phosphorylation by polyinosinic:polycytidylic acid or HIV trans-activation response element RNA in lysates of T cell lines or primary human CD4⁺ T cells. Using clonal human T cell lines, we found that nc886 expression was strictly required for IFN-γ and IL-2 expression and secretion after T cell activation but did not affect proliferation or activation-induced cell death. In stimulated human PBMCs, nc886 expression strongly correlated with IFN-γ expression. Although nc886 inhibited PKR activation by dsRNA, it was required for PKR phosphorylation during T cell stimulation, with subsequent NF-κB signaling and CREB phosphorylation. nc886 also regulated PKR phosphorylation during human monocyte-derived macrophage activation. We have therefore identified nc886 as a noncoding RNA marker of T cell activation and regulator of PKR-dependent signaling.

Virus Sensor RIG-I Represses RNA Interference by Interacting with TRBP through LGP2 in Mammalian Cells

Exogenous double-stranded RNAs (dsRNAs) similar to viral RNAs induce antiviral RNA silencing or RNA interference (RNAi) in plants or invertebrates, whereas interferon (IFN) response is induced through activation of virus sensor proteins including Toll like receptor 3 (TLR3) or retinoic acid-inducible gene I (RIG-I) like receptors (RLRs) in mammalian cells. Both RNA silencing and IFN response are triggered by dsRNAs. However, the relationship between these two pathways has remained unclear. Laboratory of genetics and physiology 2 (LGP2) is one of the RLRs, but its function has remained unclear. Recently, we reported that LGP2 regulates endogenous microRNA-mediated RNA silencing by interacting with an RNA silencing enhancer, TAR-RNA binding protein (TRBP). Here, we investigated the contribution of other RLRs, RIG-I and melanoma-differentiation-associated gene 5 (MDA5), in the regulation of RNA silencing. We found that RIG-I, but not MDA5, also represses short hairpin RNA (shRNA)-induced RNAi by type-I IFN. Our finding suggests that RIG-I, but not MDA5, interacts with TRBP indirectly through LGP2 to function as an RNAi modulator in mammalian cells.

LGP2 virus sensor regulates gene expression network mediated by TRBP-bound microRNAs

Here we show that laboratory of genetics and physiology 2 (LGP2) virus sensor protein regulates gene expression network of endogenous genes mediated by TAR-RNA binding protein (TRBP)-bound microRNAs (miRNAs). TRBP is an enhancer of RNA silencing, and functions to recruit precursor-miRNAs (pre-miRNAs) to Dicer that processes pre-miRNA into mature miRNA. Viral infection activates the antiviral innate immune response in mammalian cells. Retinoic acid-inducible gene I (RIG-I)-like receptors (RLRs), including RIG-I, melanoma-differentiation-associated gene 5 (MDA5), and LGP2, function as cytoplasmic virus sensor proteins during viral infection. RIG-I and MDA5 can distinguish between different types of RNA viruses to produce antiviral cytokines, including type I interferon. However, the role of LGP2 is controversial. We found that LGP2 bound to the double-stranded RNA binding sites of TRBP, resulting in inhibition of pre-miRNA binding and recruitment by TRBP. Furthermore, although it is unclear whether TRBP binds to specific pre-miRNA, we found that TRBP bound to particular pre-miRNAs with common structural characteristics. Thus, LGP2 represses specific miRNA activities by interacting with TRBP, resulting in selective regulation of target genes. Our findings show that a novel function of LGP2 is to modulate RNA silencing, indicating the crosstalk between RNA silencing and RLR signaling in mammalian cells.

Sequential steps in Tat trans-activation of HIV-1 mediated through cellular DNA, RNA, and protein binding factors

The regulation of HIV expression is controlled by the activity of the Long Terminal Repeat (LTR). Trans-activation by the virally encoded Tat protein is one of the main mechanisms of LTR activation. Tat binds to its target, TAR RNA, and cellular proteins that bind the LTR, Tat, or TAR RNA are important components of the trans-activation process. We will review the factors that have been characterized for a possible involvement in this mechanism. Whereas LTR binding proteins consist of Sp1 and TBP, a large number of factors that bind TAR RNA have been isolated. We have previously cloned two of them by RNA probe recognition: TRBP and La. We have shown that the in vitro and in vivo binding of TRBP to TAR RNA correlates with a constant expression of the protein during HIV-1 infection. Several proteins that interact with Tat have mainly positive, but some negative, effects on trans-activation. Genetic studies have defined that human chromosome 12 encodes a protein that will allow trans-activation in rodent cells. The binding and the functional data about these proteins suggest sequential steps for the Tat trans-activation mechanism. Each of these intracellular molecular events could be the target for molecular intervention against the virus.

Trans-acting translational regulatory RNA binding proteins

The canonical molecular machinery required for global mRNA translation and its control has been well defined, with distinct sets of proteins involved in the processes of translation initiation, elongation and termination. Additionally, noncanonical, trans-acting regulatory RNA-binding proteins (RBPs) are necessary to provide mRNA-specific translation, and these interact with 5' and 3' untranslated regions and coding regions of mRNA to regulate ribosome recruitment and transit. Recently it has also been demonstrated that trans-acting ribosomal proteins direct the translation of specific mRNAs. Importantly, it has been shown that subsets of RBPs often work in concert, forming distinct regulatory complexes upon different cellular perturbation, creating an RBP combinatorial code, which through the translation of specific subsets of mRNAs, dictate cell fate. With the development of new methodologies, a plethora of novel RNA binding proteins have recently been identified, although the function of many of these proteins within mRNA translation is unknown. In this review we will discuss these methodologies and their shortcomings when applied to the study of translation, which need to be addressed to enable a better understanding of trans-acting translational regulatory proteins. Moreover, we discuss the protein domains that are responsible for RNA binding as well as the RNA motifs to which they bind, and the role of trans-acting ribosomal proteins in directing the translation of specific mRNAs. This article is categorized under: RNA Interactions with Proteins and Other Molecules > RNA-Protein Complexes Translation > Translation Regulation Translation > Translation Mechanisms.

Stress-induced TRBP phosphorylation enhances its interaction with PKR to regulate cellular survival

Transactivation response element RNA-binding protein (TRBP or TARBP2) initially identified to play an important role in human immunodeficiency virus (HIV) replication also has emerged as a regulator of microRNA biogenesis. In addition, TRBP functions in signaling pathways by negatively regulating the interferon-induced double-stranded RNA (dsRNA)-activated protein kinase (PKR) during viral infections and cell stress. During cellular stress, PKR is activated and phosphorylates the α subunit of the eukaryotic translation factor eIF2, leading to the cessation of general protein synthesis. TRBP inhibits PKR activity by direct interaction as well as by binding to PKR’s two known activators, dsRNA and PACT, thus preventing their interaction with PKR. In this study, we demonstrate for the first time that TRBP is phosphorylated in response to oxidative stress and upon phosphorylation, inhibits PKR more efficiently promoting cell survival. These results establish that PKR regulation through stress-induced TRBP phosphorylation is an important mechanism ensuring cellular recovery and preventing apoptosis due to sustained PKR activation.

Loquacious-PD facilitates Drosophila Dicer-2 cleavage through interactions with the helicase domain and dsRNA

Loquacious-PD (Loqs-PD) is required for biogenesis of many endogenous siRNAs in Drosophila In vitro, Loqs-PD enhances the rate of dsRNA cleavage by Dicer-2 and also enables processing of substrates normally refractory to cleavage. Using purified components, and Loqs-PD truncations, we provide a mechanistic basis for Loqs-PD functions. Our studies indicate that the 22 amino acids at the C terminus of Loqs-PD, including an FDF-like motif, directly interact with the Hel2 subdomain of Dicer-2's helicase domain. This interaction is RNA-independent, but we find that modulation of Dicer-2 cleavage also requires dsRNA binding by Loqs-PD. Furthermore, while the first dsRNA-binding motif of Loqs-PD is dispensable for enhancing cleavage of optimal substrates, it is essential for enhancing cleavage of suboptimal substrates. Finally, our studies define a previously unrecognized Dicer interaction interface and suggest that Loqs-PD is well positioned to recruit substrates into the helicase domain of Dicer-2.

AGO2 Negatively Regulates Type I Interferon Signaling Pathway by Competition Binding IRF3 with CBP/p300

Viral infection triggers a series of signaling cascades and host innate immune responses, including interferon (IFN) production, which depends on coordinated activity of multiple transcription factors. IFN regulatory factor 3 (IRF3) and transcriptional coactivator CREB binding protein (CBP) and/or p300 are core factors that participate in transcriptional complex formation in the nucleus. In general, cells balance the production of IFNs through suppressive and stimulative mechanisms, but viral infections can disrupt such equilibrium. This study determined that H5N1 viral infection reduced the distribution of human argonaute 2 (AGO2) in A549 cell nucleus. AGO2 did not block phosphorylation, nuclear translocation, and DNA binding ability of IRF3 but inhibited its association with CBP. Therefore, this newly revealed mechanism shows that cellular response leads to transfer of AGO2 from cell nucleus and promotes IFN-β expression to increase host survival during viral infection.

Human Herpesvirus 6A Exhibits Restrictive Propagation with Limited Activation of the Protein Kinase R-eIF2α Stress Pathway

The eIF2α protein plays a critical role in the regulation of translation. The production of double-stranded RNA (dsRNA) during viral replication can activate protein kinase R (PKR), which phosphorylates eIF2α, leading to inhibition of the initial step of translation. Many viruses have evolved gene products targeting the PKR-eIF2a pathway, indicating its importance in antiviral defense. In the present study, we focused on alternations of PKR-eIF2a pathway during human herpesvirus 6A (HHV-6A) infection while monitoring viral gene expression and infectious viral yields. We have found increased phosphorylated PKR as well as phosphorylated eIF2α coincident with accumulation of the late gp82-105 viral protein. The level of total PKR was relatively constant, but it decreased by 144 h postinfection. The phosphorylation of eIF2a led to a moderate increase in activating transcription factor 4 (ATF4) accumulation, indicating moderate inhibition of protein translation during HHV-6A infection. The overexpression of PKR led to decreased viral propagation coincident with increased accumulation of phosphorylated PKR and phosphorylated eIF2a. Moreover, addition of a dominant negative PKR mutant resulted in a moderate increase in viral replication. HHV-6A exhibits relatively low efficiency of propagation of progeny virus secreted into the culture medium. This study suggests that the replicative strategy of HHV-6A involves a mild infection over a lengthy life cycle in culture, while preventing severe activation of the PKR-eIF2α pathway.

IMPORTANCE Human herpesvirus 6A (HHV-6A) and HHV-6B are common, widely prevalent viruses, causing from mild to severe disease. Our study focused on the PKR-eIF2α stress pathway, which limits viral replication. The HHV-6 genome carries multiple genes transcribed from the two strands, predicting accumulation of dsRNAs which can activate PKR and inhibition of protein synthesis. We report that HHV-6A induced the accumulation of phosphorylated PKR and phosphorylated eIF2α and a moderate increase of activating transcription factor 4 (ATF4), which is known to transcribe stress genes. Overexpression of PKR led to increased eIF2α phosphorylation and decreased viral replication, whereas overexpression of a dominant negative PKR mutant resulted in a moderate increase in viral replication. These results suggest that the HHV-6A replication strategy involves restricted activation of the PKR-eIF2α pathway, partial translation inhibition, and lower yields of infectious virus. In essence, HHV-6A limits its own replication due to the inability to bypass the eIF2α phosphorylation.

Integrated stress response of vertebrates is regulated by four eIF2α kinases

The integrated stress response (ISR) is a cytoprotective pathway initiated upon phosphorylation of the eukaryotic translation initiation factor 2 (eIF2α) residue designated serine-51, which is critical for translational control in response to various stress conditions. Four eIF2α kinases, namely heme-regulated inhibitor (HRI), protein kinase R (PKR), PKR-like endoplasmic reticulum kinase, (PERK) and general control non-depressible 2 (GCN2), have been identified thus far, and they are known to be activated by heme depletion, viral infection, endoplasmic reticulum stress, and amino acid starvation, respectively. Because eIF2α is phosphorylated under various stress conditions, the existence of an additional eIF2α kinase has been suggested. To validate the existence of the unidentified eIF2α kinase, we constructed an eIF2α kinase quadruple knockout cells (4KO cells) in which the four known eIF2α kinase genes were deleted using the CRISPR/Cas9-mediated genome editing. Phosphorylation of eIF2α was completely abolished in the 4KO cells by various stress stimulations. Our data suggests that the four known eIF2α kinases are sufficient for ISR and that there are no additional eIF2α kinases in vertebrates.

Staufen1 promotes HCV replication by inhibiting protein kinase R and transporting viral RNA to the site of translation and replication in the cells

Persistent hepatitis C virus (HCV) infection leads to chronic hepatitis C (CHC), which often progresses to liver cirrhosis (LC) and hepatocellular carcinoma (HCC). The molecular mechanisms that establish CHC and cause its subsequent development into LC and HCC are poorly understood. We have identified a cytoplasmic double-stranded RNA binding protein, Stau1, which is crucial for HCV replication. In this study, Stau1 specifically interacted with the variable-stem-loop region in the 3′ NTR and domain IIId of the HCV-IRES in the 5′ NTR, and promoted HCV replication and translation. Stau1 coimmunoprecipitates HCV NS5B and a cell factor, protein kinase R (PKR), which is critical for interferon-induced cellular antiviral and antiproliferative responses. Like Stau1, PKR displayed binding specificity to domain IIId of HCV-IRES. Stau1 binds to PKR and strongly inhibits PKR-autophosphorylation. We demonstrated that the transport of HCV RNA on the polysomes is Stau1-dependent, being mainly localized in the monosome fractions when Stau1 is downregulated and exclusively localized in the polysomes when Stau1 is overexpressed. Our findings suggest that HCV may appropriate Stau1 to its advantage to prevent PKR-mediated inhibition of eIF2α, which is required for the synthesis of HCV proteins for translocation of viral RNA genome to the polysomes for efficient translation and replication.

Regulation of PACT-Mediated Protein Kinase Activation by the OV20.0 Protein of Orf Virus

Double-stranded RNA (dsRNA)-activated protein kinase (PKR), a major component of the cellular antiviral system, is activated by the binding of either dsRNA or the cellular PKR activator, the PACT protein. The suppression of PKR activation is one of the main strategies that viruses employ to circumvent interferon signaling. Orf virus (ORFV), a parapoxvirus from the Poxviridae family, causes contagious pustular dermatitis in small ruminants. Previous studies have demonstrated that various OV20.0 isoforms, encoded by the OV20.0L gene, are able to inhibit PKR activation both by sequestering dsRNA and by physically interacting with PKR in vitro. Thus, this gene acts as a virulence factor of ORFV when tested using a mouse infection model. In the present study, the regions within OV20.0 that interact with dsRNA and with PKR have been mapped. Furthermore, this study demonstrates for the first time that OV20.0 is also able to interact with the dsRNA binding domain of PACT and that the presence of dsRNA strengthened the interaction of these two molecules. The presence of OV20.0 diminishes PKR phosphorylation when this is stimulated by PACT. Nevertheless, the association of OV20.0 with PKR, rather than with PACT, was found to be essential for reducing PACT-mediated PKR phosphorylation. These observations elucidate a new strategy whereby innate immunity can be evaded by ORFV.IMPORTANCE Our previous study indicated that ORFV's two OV20.0 isoforms act as a PKR antagonist via sequestering the PKR activator, dsRNA, and by interacting with PKR, leading to an inhibition of PKR activation (Y. Y. Tseng, F. Y. Lin, S. F. Cheng, D. Tscharke, S. Chulakasian, C. C. Chou, Y. F. Liu, W. S. Chang, M. L. Wong, and W. L. Hsu, J Virol 89:4966-4979, 2015, doi:10.1128/JVI.03714-14). In the current study, the possible mechanisms by which OV20.0 protein counteracts PKR activation were studied in depth. OV20.0 is able to bind PKR and its two activators, dsRNA and PACT. In addition, OV20.0 binds directly to the RNA binding domains (RBDs) of PKR, and this interaction does not require dsRNA. Moreover, OV20.0 interacts with or occupies the RBD2 and the kinase domain of PKR, which then prevents PACT binding to PKR. Finally, OV20.0 associates with PACT via the RBDs, which may reduce the ability of PACT to induce PKR activation. The findings in this study provide new concepts in relation to how ORFV modulates PKR activation.

Altered activation of protein kinase PKR and enhanced apoptosis in dystonia cells carrying a mutation in PKR activator protein PACT

PACT is a stress-modulated activator of the interferon-induced double-stranded RNA-activated protein kinase (PKR). Stress-induced phosphorylation of PACT is essential for PACT's association with PKR leading to PKR activation. PKR activation leads to phosphorylation of translation initiation factor eIF2α inhibition of protein synthesis and apoptosis. A recessively inherited form of early-onset dystonia DYT16 has been recently identified to arise due to a homozygous missense mutation P222L in PACT. To examine if the mutant P222L protein alters the stress-response pathway, we examined the ability of mutant P222L to interact with and activate PKR. Our results indicate that the substitution mutant P222L activates PKR more robustly and for longer duration albeit with slower kinetics in response to the endoplasmic reticulum stress. In addition, the affinity of PACT-PACT and PACT-PKR interactions is enhanced in dystonia patient lymphoblasts, thereby leading to intensified PKR activation and enhanced cellular death. P222L mutation also changes the affinity of PACT-TRBP interaction after cellular stress, thereby offering a mechanism for the delayed PKR activation in response to stress. Our results demonstrate the impact of a dystonia-causing substitution mutation on stress-induced cellular apoptosis.

Dissecting the roles of TRBP and PACT in double-stranded RNA recognition and processing of noncoding RNAs

HIV TAR RNA-binding protein (TRBP) and Protein Activator of PKR (PACT) are double-stranded (ds) RNA-binding proteins that participate in both small regulatory RNA biogenesis and the response to viral dsRNA. Despite considerable progress toward understanding the structure-function relationship of TRBP and PACT, their specific roles in these seemingly distinct cellular pathways remain unclear. Both proteins are composed of three copies of the double-stranded RNA-binding domain, two of which interact with dsRNA, while the C-terminal copy mediates protein-protein interactions. PACT and TRBP are found in a complex with the endonuclease Dicer and facilitate processing of immature microRNAs. Their precise contribution to the Dicing step has not yet been defined: possibilities include precursor recruitment, rearrangement of dsRNA within the complex, loading the processed microRNA into the RNA-induced silencing complex, and distinguishing different classes of small dsRNA. TRBP and PACT also interact with the viral dsRNA sensors retinoic acid-inducible gene I (RIG-I) and double-stranded RNA-activated protein kinase (PKR). Current models suggest that PACT enables RIG-I to detect a wider range of viral dsRNAs, while TRBP and PACT exert opposing regulatory effects on PKR. Here, the evidence that implicates TRBP and PACT in regulatory RNA processing and viral dsRNA sensing is reviewed and discussed in the context of their molecular structure. The broader implications of a link between microRNA biogenesis and the innate antiviral response pathway are also considered.

A critical role for PKR complexes with TRBP in Immunometabolic regulation and eIF2α phosphorylation in obesity

Aberrant stress and inflammatory responses are key factors in the pathogenesis of obesity and metabolic dysfunction, and the double-stranded RNA-dependent kinase (PKR) has been proposed to play an important role in integrating these pathways. Here, we report the formation of a complex between PKR and TAR RNA-binding protein (TRBP) during metabolic and obesity-induced stress, which is critical for the regulation of eukaryotic translation initiation factor 2 alpha (eIF2α) phosphorylation and c-Jun N-terminal kinase (JNK) activation. We show that TRBP phosphorylation is induced in the setting of metabolic stress, leading to PKR activation. Suppression of hepatic TRBP reduced inflammation, JNK activity, and eIF2α phosphorylation and improved systemic insulin resistance and glucose metabolism, while TRBP overexpression exacerbated the impairment in glucose homeostasis in obese mice. These data indicate that the association between PKR and TRBP integrates metabolism with translational control and inflammatory signaling and plays important roles in metabolic homeostasis and disease.

Deletion of human tarbp2 reveals cellular microRNA targets and cell-cycle function of TRBP

TRBP functions as both a Dicer cofactor and a PKR inhibitor. However, the role of TRBP in microRNA (miRNA) biogenesis is controversial and its regulation of PKR in mitosis remains unexplored. Here, we generate TRBP knockout cells and find altered Dicer-processing sites in a subset of miRNAs but no effect on Dicer stability, miRNA abundance, or Argonaute loading. By generating PACT, another Dicer interactor, and TRBP/PACT double knockout (KO) cells, we further show that TRBP and PACT do not functionally compensate for one another and that only TRBP contributes to Dicer processing. We also report that TRBP is hyperphosphorylated by JNK in M phase when PKR is activated by cellular double-stranded RNAs (dsRNAs). Hyperphosphorylation potentiates the inhibitory activity of TRBP on PKR, suppressing PKR in M-G1 transition. By generating human TRBP KO cells, our study clarifies the role of TRBP and unveils negative feedback regulation of PKR through TRBP phosphorylation.

miRNAs at the interface of cellular stress and disease

microRNAs (miRNAs) are a family of small, non-coding RNAs, which provides broad silencing activity of mRNA targets in a sequence-dependent fashion. This review explores the hypothesis that the miRNA machinery is intimately linked with the cellular stress pathway and apparatus. Stress signaling potentially alters the function of the miRNA-bioprocessing core components and decompensates regulation. In addition, dysregulation of miRNA activity renders the cell more prone to stress and emerges as a new pathway for age-related insults and diseases, such as neurodegeneration.

A KH-domain RNA-binding protein interacts with FIERY2/CTD phosphatase-like 1 and splicing factors and is important for pre-mRNA splicing in Arabidopsis

Eukaryotic genomes encode hundreds of RNA-binding proteins, yet the functions of most of these proteins are unknown. In a genetic study of stress signal transduction in Arabidopsis, we identified a K homology (KH)-domain RNA-binding protein, HOS5 (High Osmotic Stress Gene Expression 5), as required for stress gene regulation and stress tolerance. HOS5 was found to interact with FIERY2/RNA polymerase II (RNAP II) carboxyl terminal domain (CTD) phosphatase-like 1 (FRY2/CPL1) both in vitro and in vivo. This interaction is mediated by the first double-stranded RNA-binding domain of FRY2/CPL1 and the KH domains of HOS5. Interestingly, both HOS5 and FRY2/CPL1 also interact with two novel serine-arginine (SR)-rich splicing factors, RS40 and RS41, in nuclear speckles. Importantly, FRY2/CPL1 is required for the recruitment of HOS5. In fry2 mutants, HOS5 failed to be localized in nuclear speckles but was found mainly in the nucleoplasm. hos5 mutants were impaired in mRNA export and accumulated a significant amount of mRNA in the nuclei, particularly under salt stress conditions. Arabidopsis mutants of all these genes exhibit similar stress-sensitive phenotypes. RNA-seq analyses of these mutants detected significant intron retention in many stress-related genes under salt stress but not under normal conditions. Our study not only identified several novel regulators of pre-mRNA processing as important for plant stress response but also suggested that, in addition to RNAP II CTD that is a well-recognized platform for the recruitment of mRNA processing factors, FRY2/CPL1 may also recruit specific factors to regulate the co-transcriptional processing of certain transcripts to deal with environmental challenges.

Pre-mRNA processing, including 5′ capping, splicing, and 3′ polyadenylation, is critical for gene expression and is closely coupled with transcription. Phosphorylated carboxyl terminal domain (CTD) of RNA Polymerase II (RNAP II) serves as a platform for the recruitment of pre-mRNA processing factors, yet other components involved in the recruitment are less known. In a genetic study of stress signal transduction in Arabidopsis, we isolated a KH-domain RNA-binding protein HOS5 that plays important roles in stress gene regulation and stress tolerance. We found that HOS5 interacts with FIERY2/CTD phosphatase-like 1 (FRY2/CPL1) and they both also interact with two novel splicing factors, RS40 and RS41, in nuclear speckles. In fry2 mutants, HOS5 was unable to be recruited to nuclear speckles but rather was mainly localized in the nucleoplasm. Mutants in these genes have similar stress-sensitive phenotypes. Transcriptome analyses identified significant intron retention in many stress-related genes in these mutants under salt stress conditions. Our study reveals that, in addition to RNAP II, the CTD phosphatase may also recruit specific splicing factors and RNA binding proteins to regulate the co-transcriptional processing of certain transcripts to deal with environmental stresses.

NF90 in posttranscriptional gene regulation and microRNA biogenesis

Gene expression patterns are effectively regulated by turnover and translation regulatory (TTR) RNA-binding proteins (RBPs). The TTR-RBPs control gene expression at posttranscriptional levels, such as pre-mRNA splicing, mRNA cytoplasmic export, turnover, storage, and translation. Double-stranded RNA binding proteins (DSRBPs) are known to regulate many processes of cellular metabolism, including transcriptional control, translational control, mRNA processing and localization. Nuclear factor 90 (NF90), one of the DSRBPs, is abundantly expressed in vertebrate tissue and participates in many aspects of RNA metabolism. NF90 was originally purified as a component of a DNA binding complex which binds to the antigen recognition response element 2 in the interleukin 2 promoter. Recent studies have provided us with interesting insights into its possible physiological roles in RNA metabolism, including transcription, degradation, and translation. In addition, it was shown that NF90 regulates microRNA expression. In this review, we try to focus on the function of NF90 in posttranscriptional gene regulation and microRNA biogenesis.

The RNA-binding region of human TRBP interacts with microRNA precursors through two independent domains

MicroRNAs (miRNAs) are small non-coding RNAs that regulate gene expression through RNA interference. Human miRNAs are generated through a series of enzymatic processing steps. The precursor miRNA (pre-miRNA) is recognized and cleaved by a complex containing Dicer and several non-catalytic accessory proteins. HIV TAR element binding protein (TRBP) is a constituent of the Dicer complex, which augments complex stability and potentially functions in substrate recognition and product transfer to the RNA-induced silencing complex. Here we have analysed the interaction between the RNA-binding region of TRBP and an oncogenic human miRNA, miR-155, at different stages in the biogenesis pathway. We show that the region of TRBP that binds immature miRNAs comprises two independent double-stranded RNA-binding domains connected by a 60-residue flexible linker. No evidence of contact between the two double-stranded RNA-binding domains was observed either in the apo- or RNA-bound state. We establish that the RNA-binding region of TRBP interacts with both pre-miR-155 and the miR-155/miR-155* duplex through the same binding surfaces and with similar affinities, and that two protein molecules can simultaneously interact with each immature miRNA. These data suggest that TRBP could play a role before and after processing of pre-miRNAs by Dicer.

Toscana virus NSs protein promotes degradation of double-stranded RNA-dependent protein kinase

Toscana virus (TOSV), which is transmitted by Phlebotomus spp. sandflies, is a major etiologic agent of aseptic meningitis and encephalitis in the Mediterranean. Like other members of the genus Phlebovirus of the family Bunyaviridae, TOSV encodes a nonstructural protein (NSs) in its small RNA segment. Although the NSs of Rift Valley fever virus (RVFV) has been identified as an important virulence factor, which suppresses host general transcription, inhibits transcription from the beta interferon promoter, and promotes the proteasomal degradation of double-stranded RNA-dependent protein kinase (PKR), little is known about the functions of NSs proteins encoded by less-pathogenic members of this genus. In this study we report that TOSV is able to downregulate PKR with similar efficiency as RVFV, while infection with the other phleboviruses-i.e., Punta Toro virus, sandfly fever Sicilian virus, or Frijoles virus-has no effect on cellular PKR levels. In contrast to RVFV, however, cellular transcription remains unaffected during TOSV infection. TOSV NSs protein promotes the proteasome-dependent downregulation of PKR and is able to interact with kinase-inactive PKR in infected cells.

The multiple functions of TRBP, at the hub of cell responses to viruses, stress, and cancer

The TAR RNA binding protein (TRBP) has emerged as a key player in many cellular processes. First identified as a cellular protein that facilitates the replication of human immunodeficiency virus, TRBP has since been shown to inhibit the activation of protein kinase R (PKR), a protein involved in innate immune responses and the cellular response to stress. It also binds to the PKR activator PACT and regulates its function. TRBP also contributes to RNA interference as an integral part of the minimal RNA-induced silencing complex with Dicer and Argonaute proteins. Due to its multiple functions in the cell, TRBP is involved in oncogenesis when its sequence is mutated or its expression is deregulated. The depletion or overexpression of TRBP results in malignancy, suggesting that the balance of TRBP expression is key to normal cellular function. These studies show that TRBP is multifunctional and mediates cross talk between different pathways. Its activities at the molecular level impact the cellular function from normal development to cancer and the response to infections.

MicroRNAs: molecular features and role in cancer

microRNAs (miRNAs) are small noncoding endogenously produced RNAs that play key roles in controlling the expression of many cellular proteins. Once they are recruited and incorporated into a ribonucleoprotein complex miRISC, they can target specific mRNAs in a miRNA sequence-dependent process and interfere in the translation into proteins of the targeted mRNAs via several mechanisms. Consequently, miRNAs can regulate many cellular pathways and processes. Dysregulation of their physiological roles may largely contribute to disease. In particular, in cancer, miRNAs can be involved in the deregulation of the expression of important genes that play key roles in tumorigenesis, tumor development, and angiogenesis and have oncogenic or tumor suppressor roles. This review focuses on the biogenesis and maturation of miRNAs, their mechanisms of gene regulation, and the way their expression is deregulated in cancer. The involvement of miRNAs in several oncogenic pathways such as angiogenesis and apoptosis, and in the inter-cellular dialog mediated by miRNA-loaded exosomes as well as the development of new therapeutical strategies based on miRNAs will be discussed.

TRBP and eIF6 homologue in Marsupenaeus japonicus play crucial roles in antiviral response

Plants and invertebrates can suppress viral infection through RNA silencing, mediated by RNA-induced silencing complex (RISC). Trans-activation response RNA-binding protein (TRBP), consisting of three double-stranded RNA-binding domains, is a component of the RISC. In our previous paper, a TRBP homologue in Fenneropenaeus chinensis (Fc-TRBP) was reported to directly bind to eukaryotic initiation factor 6 (Fc-eIF6). In this study, we further characterized the function of TRBP and the involvement of TRBP and eIF6 in antiviral RNA interference (RNAi) pathway of shrimp. The double-stranded RNA binding domains (dsRBDs) B and C of the TRBP from Marsupenaeus japonicus (Mj-TRBP) were found to mediate the interaction of TRBP and eIF6. Gel-shift assays revealed that the N-terminal of Mj-TRBP dsRBD strongly binds to double-stranded RNA (dsRNA) and that the homodimer of the TRBP mediated by the C-terminal dsRBD increases the affinity to dsRNA. RNAi against either Mj-TRBP or Mj-eIF6 impairs the dsRNA-induced sequence-specific RNAi pathway and facilitates the proliferation of white spot syndrome virus (WSSV). These results further proved the important roles of TRBP and eIF6 in the antiviral response of shrimp.

RNA binding by a novel helical fold of b2 protein from wuhan nodavirus mediates the suppression of RNA interference and promotes b2 dimerization

Wuhan nodavirus (WhNV) is a newly identified member of the Nodaviridae family with a bipartite genome of positive-sense RNAs. A nonstructural protein encoded by subgenomic RNA3 of nodaviruses, B2, serves as a potent RNA silencing suppressor (RSS) by sequestering RNA duplexes. We have previously demonstrated that WhNV B2 blocks RNA silencing in cultured Drosophila cells. However, the molecular mechanism by which WhNV B2 functions remains unknown. Here, we successfully established an RNA silencing system in cells derived from Pieris rapae, a natural host of WhNV, by introducing into these cells double-stranded RNA (dsRNA)-expressing plasmids or chemically synthesized small interfering RNAs (siRNAs). Using this system, we revealed that the WhNV B2 protein inhibited Dicer-mediated dsRNA cleavage and the incorporation of siRNA into the RNA-induced silencing complex (RISC) by sequestering dsRNA and siRNA. Based on the modeled B2 3-dimensional structure, serial single alanine replacement mutations and N-terminal deletion analyses showed that the RNA-binding domain of B2 is formed by its helices α2 and α3, while helix α1 mediates B2 dimerization. Furthermore, positive feedback between RNA binding and B2 dimerization was uncovered by gel shift assay and far-Western blotting, revealing that B2 dimerization is required for its binding to RNA, whereas RNA binding to B2 in turn promotes its dimerization. All together, our findings uncovered a novel RNA-binding mode of WhNV B2 and provided evidence that the promotion effect of RNA binding on dimerization exists in a viral RSS protein.

Stress-induced phosphorylation of PACT reduces its interaction with TRBP and leads to PKR activation

PACT is a stress-modulated activator of interferon (IFN)-induced double-stranded (ds) RNA-activated protein kinase (PKR) and is an important regulator of PKR-dependent signaling pathways. Stress-induced phosphorylation of PACT is essential for PACT's association with PKR leading to PKR activation. PKR activation by PACT leads to phosphorylation of translation initiation factor eIF2α, inhibition of protein synthesis, and apoptosis. In addition to positive regulation by PACT, PKR activity in cells is also negatively regulated by TRBP. In this study, we demonstrate for the first time that stress-induced phosphorylation at serine 287 significantly increases PACT's ability to activate PKR by weakening PACT's interaction with TRBP. A non-phosphorylatable alanine substitution mutant at this position causes enhanced interaction of PACT with TRBP and leads to a loss of PKR activation. Furthermore, TRBP overexpression in cells is unable to block apoptosis induced by a phospho-mimetic, constitutively active PACT mutant. These results demonstrate for the first time that stress-induced PACT phosphorylation functions to free PACT from the inhibitory interaction with TRBP and also to enhance its interaction with PKR.

Structures of the first and second double-stranded RNA-binding domains of human TAR RNA-binding protein

The TAR RNA-binding Protein (TRBP) is a double-stranded RNA (dsRNA)-binding protein, which binds to Dicer and is required for the RNA interference pathway. TRBP consists of three dsRNA-binding domains (dsRBDs). The first and second dsRBDs (dsRBD1 and dsRBD2, respectively) have affinities for dsRNA, whereas the third dsRBD (dsRBD3) binds to Dicer. In this study, we prepared the single domain fragments of human TRBP corresponding to dsRBD1 and dsRBD2 and solved the crystal structure of dsRBD1 and the solution structure of dsRBD2. The two structures contain an α-β-β-β-α fold, which is common to the dsRBDs. The overall structures of dsRBD1 and dsRBD2 are similar to each other, except for a slight shift of the first α helix. The residues involved in dsRNA binding are conserved. We examined the small interfering RNA (siRNA)-binding properties of these dsRBDs by isothermal titration colorimetry measurements. The dsRBD1 and dsRBD2 fragments both bound to siRNA, with dissociation constants of 220 and 113 nM, respectively. In contrast, the full-length TRBP and its fragment with dsRBD1 and dsRBD2 exhibited much smaller dissociation constants (0.24 and 0.25 nM, respectively), indicating that the tandem dsRBDs bind simultaneously to one siRNA molecule. On the other hand, the loop between the first α helix and the first β strand of dsRBD2, but not dsRBD1, has a Trp residue, which forms hydrophobic and cation-π interactions with the surrounding residues. A circular dichroism analysis revealed that the thermal stability of dsRBD2 is higher than that of dsRBD1 and depends on the Trp residue.

Orchestration of the activation of protein kinase R by the RNA-binding motif

The protein kinase R (PKR) constitutes part of the host antiviral response. PKR activation is regulated by the N-terminus of protein, which encodes tandem RNA-binding motifs (RBMs). The full capabilities of RBMs from PKR and other proteins have surpassed the narrow specificities initially determined as merely binding double-stranded RNA. Recognition of the increased affinity of the RBM for additional RNA species has established an immunological distinction by which PKR can detect exogenous RNAs, as well as identified PKR-mediated expression of specific endogenous genes. Furthermore, as RBMs also mediate interactions with other proteins, including PKR itself, this motif connects PKR to the broader RNA metabolism. Given the fundamental importance of protein-RNA interactions, not only in the innate immune response to intracellular pathogens, but also to coordinate the cellular replication machinery, there is considerable interest in the mechanisms by which proteins recognize and respond to RNA. This review appraises our understanding of how PKR activity is modulated by the RBMs.

Analysis of PKR activation using analytical ultracentrifugation

Protein kinase R (PKR) is a central component of the interferon antiviral defense pathway. Upon binding to dsRNA, PKR undergoes autophosphorylation reactions that activate the kinase, resulting in the inhibition of protein synthesis in virally-infected cells. We have used analytical ultracentrifugation and related biophysical methods to quantitatively characterize the stoichiometries, affinities, and free energy couplings that govern the assembly of the macromolecular complexes in the PKR activation pathway. These studies demonstrate that PKR dimerization play a key role in enzymatic activation and support a model where the role of dsRNA is to bring two or more PKR monomers in close proximity to enhance dimerization.

Recognition of siRNA asymmetry by TAR RNA binding protein

The recognition of small interfering RNAs (siRNAs) by the RNA-induced silencing complex (RISC) and its precursor, the RISC loading complex (RLC), is a key step in the RNA interference pathway that controls the subsequent sequence-specific mRNA degradation. In Drosophila, selection of the guide strand has been shown to be mediated by RLC protein R2D2, which senses the relative hybridization stability between the two ends of the siRNA. A protein with similar function has yet to be conclusively identified in humans. We show here that human TAR RNA binding protein (TRBP) alone can bind siRNAs in vitro and sense their asymmetry. We also show that TRBP can bind 21-nucleotide single-stranded RNAs, though with far lower affinity than for double-stranded siRNA, and that TRBP cross-links preferentially to the 3'-ends of the guide strands of siRNAs. This suggests that TRBP binding depends both on the sequences of the siRNA strands and on the relative hybridization stability of the ends of the duplex. Together, these results demonstrate the importance of the siRNA-TRBP interaction in the selection of the siRNA guide strand in RNAi.

Essential role of PACT-mediated PKR activation in tunicamycin-induced apoptosis

Cellular stresses such as disruption of calcium homeostasis, inhibition of protein glycosylation, and reduction of disulfide bonds result in accumulation of misfolded proteins in the endoplasmic reticulum (ER) and lead to cell death by apoptosis. Tunicamycin, which is an inhibitor of protein glycosylation, induces ER stress and apoptosis. In this study, we examined the involvement of double-stranded RNA (dsRNA)-activated protein kinase (PKR) and its protein activator PACT in tunicamycin-induced apoptosis. We demonstrate for the first time that PACT is phosphorylated in response to tunicamycin and is responsible for PKR activation by direct interaction. Furthermore, PACT-induced PKR activation is essential for tunicamycin-induced apoptosis, since PACT as well as PKR null cells are markedly resistant to tunicamycin and show defective eIF2alpha phosphorylation and C/EBP homologous protein (CHOP, also known as GADD153) induction especially at low concentrations of tunicamycin. Reconstitution of PKR and PACT expression in the null cells renders them sensitive to tunicamycin, thus demonstrating that PACT-induced PKR activation plays an essential function in induction of apoptosis.

Human Dicer C-terminus functions as a 5-lipoxygenase binding domain

Dicer is a multidomain ribonuclease III enzyme involved in the biogenesis of microRNAs (miRNAs) in the vast majority of eukaryotes. In human, Dicer has been shown to interact with cellular proteins via its N-terminal domain. Here, we demonstrate the ability of Dicer C-terminus to interact with 5-lipoxygenase (5LO), an enzyme involved in the biosynthesis of inflammatory mediators, in vitro and in cultured human cells. Yeast two-hybrid and GST binding assays delineated the smallest 5-lipoxygenase binding domain (5LObd) of Dicer to its C-terminal 140 amino acids comprising the double-stranded RNA (dsRNA) binding domain (dsRBD). The Dicer 5LObd-5LO association was disrupted upon Ala substitution of Trp residues 13, 75 and 102 in 5LO, suggesting that the Dicer 5LObd may recognize 5LO via its N-terminal C2-like domain. Whereas a catalytically active 5LObd-containing Dicer fragment was found to enhance 5LO enzymatic activity in vitro, human 5LO modified the miRNA precursor processing activity of Dicer. Providing a link between miRNA-mediated regulation of gene expression and inflammation, our results suggest that the formation of miRNAs may be regulated by 5LO in leukocytes and cancer cells expressing this lipoxygenase.

dsRNA binding properties of RDE-4 and TRBP reflect their distinct roles in RNAi

Double-stranded RNA (dsRNA)-binding proteins facilitate Dicer functions in RNA interference. Caenorhabditis elegans RDE-4 facilitates cleavage of long dsRNA to small interfering RNA (siRNA), while human trans-activation response RNA-binding protein (TRBP) functions downstream to pass siRNA to the RNA-induced silencing complex. We show that these distinct in vivo roles are reflected in in vitro binding properties. RDE-4 preferentially binds long dsRNA, while TRBP binds siRNA with an affinity that is independent of dsRNA length. These properties are mechanistically based on the fact that RDE-4 binds cooperatively, via contributions from multiple domains, while TRBP binds noncooperatively. Our studies offer a paradigm for how dsRNA-binding proteins, which are not sequence specific, discern dsRNA length. Additionally, analyses of the ability of RDE-4 deletion constructs and RDE-4/TRBP chimeras to reconstitute Dicer activity suggest RDE-4 promotes activity using its dsRNA-binding motif 2 to bind dsRNA, its linker region to interact with Dicer, and its C-terminus for Dicer activation.

The C-terminal region (640-967) of Arabidopsis CPL1 interacts with the abiotic stress- and ABA-responsive transcription factors

Proteins in CPL1 family are unique to plants and contain a phosphatase catalytic domain and double-stranded RNA (dsRNA)-binding motifs (DRMs) in a single peptide. Though DRMs are important for the function of Arabidopsis CPL1 in vivo, the role of CPL1 DRM has been obscure. We have isolated two transcription factors, ANAC019 (At1g52890) and AtMYB3 (At1g22640), which specifically interact with the C-terminal region (640-967) of AtCPL1 containing two DRMs. Detailed interaction analysis indicated that AtMYB3 specifically interacted with the first DRM but not with the second DRM in CPL1 C-terminal fragment. GFP-fusion analysis indicated that AtMYB3 localized in nuclei-like CPL1, and its expression is induced by abiotic stress and ABA treatment. These results suggest that AtMYB3 function in abiotic stress signaling in concert with CPL1.

Double-stranded RNA-activated protein kinase PKR of fishes and amphibians: varying the number of double-stranded RNA binding domains and lineage-specific duplications

Background

Double-stranded (ds) RNA, generated during viral infection, binds and activates the mammalian anti-viral protein kinase PKR, which phosphorylates the translation initiation factor eIF2α leading to the general inhibition of protein synthesis. Although PKR-like activity has been described in fish cells, the responsible enzymes eluded molecular characterization until the recent discovery of goldfish and zebrafish PKZ, which contain Z-DNA-binding domains instead of dsRNA-binding domains (dsRBDs). Fish and amphibian PKR genes have not been described so far.

Results

Here we report the cloning and identification of 13 PKR genes from 8 teleost fish and amphibian species, including zebrafish, demonstrating the coexistence of PKR and PKZ in this latter species. Analyses of their genomic organization revealed up to three tandemly arrayed PKR genes, which are arranged in head-to-tail orientation. At least five duplications occurred independently in fish and amphibian lineages. Phylogenetic analyses reveal that the kinase domains of fish PKR genes are more closely related to those of fish PKZ than to the PKR kinase domains of other vertebrate species. The duplication leading to fish PKR and PKZ genes occurred early during teleost fish evolution after the divergence of the tetrapod lineage. While two dsRBDs are found in mammalian and amphibian PKR, one, two or three dsRBDs are present in fish PKR. In zebrafish, both PKR and PKZ were strongly upregulated after immunostimulation with some tissue-specific expression differences. Using genetic and biochemical assays we demonstrate that both zebrafish PKR and PKZ can phosphorylate eIF2α in yeast.

Conclusion

Considering the important role for PKR in host defense against viruses, the independent duplication and fixation of PKR genes in different lineages probably provided selective advantages by leading to the recognition of an extended spectrum of viral nucleic acid structures, including both dsRNA and Z-DNA/RNA, and perhaps by altering sensitivity to viral PKR inhibitors. Further implications of our findings for the evolution of the PKR family and for studying PKR/PKZ interactions with viral gene products and their roles in viral infections are discussed.

In vitro reconstitution of the human RISC-loading complex

Targeted gene silencing by RNAi requires the RNA-induced silencing complex (RISC), whose core component is the protein Argonaute (Ago) bound to a microRNA (miRNA) or an siRNA. In humans, Ago2 is loaded with miRNAs by the action of a specialized assembly called the RISC-loading complex (RLC), comprising the proteins Ago2, Dicer, and TRBP. Here we show that the human RLC assembles spontaneously in vitro from purified components. No cofactors or chaperones are required for the complex to form. The reconstituted RLC, containing one copy of each protein, has the dicing, slicing, guide-strand selection, and Ago2-loading activities observed for the endogenous RLC. Furthermore, once Ago2 is loaded with an miRNA, it tends to dissociate from the rest of the complex. These results lay the groundwork for future structural and functional dissection of RISC loading in humans.

Interaction of human tRNA-dihydrouridine synthase-2 with interferon-induced protein kinase PKR

PKR is an interferon (IFN)-induced protein kinase, which is involved in regulation of antiviral innate immunity, stress signaling, cell proliferation and programmed cell death. Although a low amount of PKR is expressed ubiquitously in all cell types in the absence of IFNs, PKR expression is induced at transcriptional level by IFN. PKR's enzymatic activity is activated by its binding to one of its activators. Double-stranded (ds) RNA, protein activator PACT and heparin are the three known activators of PKR. Activation of PKR in cells leads to a general block in protein synthesis due to phosphorylation of eIF2α on serine 51 by PKR. PKR activation is regulated very tightly in mammalian cells and a prolonged activation of PKR leads to apoptosis. Thus, positive and negative regulation of PKR activation is important for cell viability and function. The studies presented here describe human dihydrouridine synthase-2 (hDUS2) as a novel regulator of PKR. We originally identified hDUS2 as a protein interacting with PACT in a yeast two-hybrid screen. Further characterization revealed that hDUS2 also interacts with PKR through its dsRNA binding/dimerization domain and inhibits its kinase activity. Our results suggest that hDUS2 may act as a novel inhibitor of PKR in cells.

Impact of protein kinase PKR in cell biology: from antiviral to antiproliferative action

The double-stranded RNA-dependent protein kinase PKR is a critical mediator of the antiproliferative and antiviral effects exerted by interferons. Not only is PKR an effector molecule on the cellular response to double-stranded RNA, but it also integrates signals in response to Toll-like receptor activation, growth factors, and diverse cellular stresses. In this review, we provide a detailed picture on how signaling downstream of PKR unfolds and what are the ultimate consequences for the cell fate. PKR activation affects both transcription and translation. PKR phosphorylation of the alpha subunit of eukaryotic initiation factor 2 results in a blockade on translation initiation. However, PKR cannot avoid the translation of some cellular and viral mRNAs bearing special features in their 5' untranslated regions. In addition, PKR affects diverse transcriptional factors such as interferon regulatory factor 1, STATs, p53, activating transcription factor 3, and NF-kappaB. In particular, how PKR triggers a cascade of events involving IKK phosphorylation of IkappaB and NF-kappaB nuclear translocation has been intensively studied. At the cellular and organism levels PKR exerts antiproliferative effects, and it is a key antiviral agent. A point of convergence in both effects is that PKR activation results in apoptosis induction. The extent and strength of the antiviral action of PKR are clearly understood by the findings that unrelated viral proteins of animal viruses have evolved to inhibit PKR action by using diverse strategies. The case for the pathological consequences of the antiproliferative action of PKR is less understood, but therapeutic strategies aimed at targeting PKR are beginning to offer promising results.

Translational control of retroviruses

All replication-competent retroviruses contain three main reading frames, gag, pol and env, which are used for the synthesis of structural proteins, enzymes and envelope proteins respectively. Complex retroviruses, such as lentiviruses, also code for regulatory and accessory proteins that have essential roles in viral replication. The concerted expression of these genes ensures the efficient polypeptide production required for the assembly and release of new infectious progeny virions. Retroviral protein synthesis takes place in the cytoplasm and depends exclusively on the translational machinery of the host infected cell. Therefore, not surprisingly, retroviruses have developed RNA structures and strategies to promote robust and efficient expression of viral proteins in a competitive cellular environment.

Protein-protein interactions in the allosteric regulation of protein kinases

Protein-protein interactions involving the catalytic domain of protein kinases are likely to be generally important in the regulation of signal transduction pathways, but are rather sparsely represented in crystal structures. Recently determined structures of the kinase domains of the mitogen-activated protein kinase Fus3, the RNA-dependent kinase PKR, the epidermal growth factor receptor and Ca(2+)/calmodulin-dependent protein kinase II have revealed unexpected and distinct mechanisms by which interactions with the catalytic domain can modulate kinase activity.

Binding and nuclear relocalization of protein kinase R by human cytomegalovirus TRS1

The human cytomegalovirus (HCMV) TRS1 and IRS1 genes block the phosphorylation of the alpha subunit of eukaryotic initiation factor 2 (eIF2alpha) and the consequent shutoff of cellular protein synthesis that occur during infection with vaccinia virus (VV) deleted of the double-stranded RNA binding protein gene E3L (VVDeltaE3L). To further define the underlying mechanism, we first evaluated the effect of pTRS1 on protein kinase R (PKR), the double-stranded RNA (dsRNA)-dependent eIF2alpha kinase. Immunoblot analyses revealed that pTRS1 expression in the context of a VVDeltaE3L recombinant decreased levels of PKR in the cytoplasm and increased its levels in the nucleus of infected cells, an effect not seen with wild-type VV or a VVDeltaE3L recombinant virus expressing E3L. This effect of pTRS1 was confirmed by visualizing the nuclear relocalization of PKR-EGFP expressed by transient transfection. PKR present in both the nuclear and cytoplasmic fractions was nonphosphorylated, indicating that it was unactivated when TRS1 was present. PKR also accumulated in the nucleus during HCMV infection as determined by indirect immunofluorescence and immunoblot analysis. Binding assays revealed that pTRS1 interacted with PKR in mammalian cells and in vitro. This interaction required the same carboxy-terminal region of pTRS1 that is necessary to rescue VVDeltaE3L replication in HeLa cells. The carboxy terminus of pIRS1 was also required for rescue of VVDeltaE3L and for mediating an interaction of pIRS1 with PKR. These results suggest that these HCMV genes directly interact with PKR and inhibit its activation by sequestering it in the nucleus, away from both its activator, cytoplasmic dsRNA, and its substrate, eIF2alpha.

The evolution and diversification of Dicers in plants

Most multicellular organisms regulate developmental transitions by microRNAs, which are generated by an enzyme, Dicer. Insects and fungi have two Dicer-like genes, and many animals have only one, yet the plant, Arabidopsis, has four. Examining the poplar and rice genomes revealed that they contain five and six Dicer-like genes, respectively. Analysis of these genes suggests that plants require a basic set of four Dicer types which were present before the divergence of mono- and dicotyledonous plants ( approximately 200 million years ago), but after the divergence of plants from green algae. A fifth type of Dicer seems to have evolved in monocots.

Mechanistic link between PKR dimerization, autophosphorylation, and eIF2alpha substrate recognition

The antiviral protein kinase PKR inhibits protein synthesis by phosphorylating the translation initiation factor eIF2alpha on Ser51. Binding of double-stranded RNA to the regulatory domains of PKR promotes dimerization, autophosphorylation, and the functional activation of the kinase. Herein, we identify mutations that activate PKR in the absence of its regulatory domains and map the mutations to a recently identified dimerization surface on the kinase catalytic domain. Mutations of other residues on this surface block PKR autophosphorylation and eIF2alpha phosphorylation, while mutating Thr446, an autophosphorylation site within the catalytic-domain activation segment, impairs eIF2alpha phosphorylation and viral pseudosubstrate binding. Mutational analysis of catalytic-domain residues preferentially conserved in the eIF2alpha kinase family identifies helix alphaG as critical for the specific recognition of eIF2alpha. We propose an ordered mechanism of PKR activation in which catalytic-domain dimerization triggers Thr446 autophosphorylation and specific eIF2alpha substrate recognition.