DDX3 RNA helicase is required for HIV-1 Tat function

Development and characterization of a double-fluorescent HIV-1 reporter cellular model to tackle the Rev-dependent export pathway

The Rev-dependent nuclear export of unspliced and singly-spliced transcripts of human immunodeficiency virus type 1 (HIV-1) constitutes a critical yet poorly characterized post-transcriptional event essential for effective viral replication. In this study, we engineered a dual-fluorescent HIV-1-based cellular reporter system to elucidate the mechanisms underpinning Rev-dependent export. By generating multiple stably integrated inducible cellular clones, we ensured the expression of two distinct fluorescent proteins, mKO2, and ECFP, from unspliced (Rev dependent) and multiply spliced (Rev independent) HIV-1 transcripts, respectively. Utilizing flow cytometry, we performed quantitative analyses of dual-fluorescent cell populations. The developed tool enables precise assessment of the Rev-dependent export, and we validated it using known inhibitors of this pathway (leptomycin D), as well as targeted depletion of MATR3, an essential cofactor of Rev, and CRNKL1, a repressor of unspliced HIV-1 RNA export.IMPORTANCEThe developed dual-fluorescent reporter system represents a powerful and handy tool for the identification and characterization of novel molecular players involved in the Rev-dependent export pathway. This system not only holds promise for advancing our understanding of human immunodeficiency virus type 1 (HIV-1) biology but also serves as an invaluable platform for high-throughput drug screening aimed at targeting post-transcriptional HIV-1 RNA processes, particularly nuclear export. Consequently, this study offers significant implications for the development of novel therapeutic strategies to eradicate the virus.

Human microRNA miR-197-3p positively regulates HIV-1 virion infectivity through its target DDX52 by stabilizing Vif protein expression

MicroRNAs are a part of the integral regulatory mechanisms found in eukaryotic cells that help in maintaining cellular homeostasis by modulating the expression of target genes. However, during stress conditions like viral infection, the expression profile of the microRNAs change, thereby directly modulating the expression of viral genes and/or indirectly targeting the virus by regulating the host genes. The present study intends to identify previously uncharacterized cellular microRNAs, which are significantly modulated upon HIV-1 infection. With the available microarray data of five independent studies in the NCBI GEO database, 10 common yet functionally uncharacterized microRNAs that are deregulated during HIV-1 infection in humans were identified. Their expression profiles were validated in HIV-1 infected human peripheral blood mononuclear cells and a CD4+T cell line. Among them, miR-197-3p showed significant upregulation during HIV-1 infection in all the cell types tested and was selected for further characterization. We then found that miR-197-3p increases progeny virion infectivity through restricting the expression of DDX52. Interestingly, DDX52 showed a negative impact on virion infectivity by downregulating the HIV-1 viral infectivity factor (Vif) at the protein level. Mechanistically, our study also revealed that Vif, DDX52, and APOBEC3G form a complex, which might be responsible for Vif downregulation by proteasomal degradation. Taken together, our results demonstrate that miR-197-3p is a positive regulator of HIV-1 infectivity as it enhances the progeny virion infectivity by targeting DDX52, which is a negative regulator of Vif.

Viral Subversion of the Chromosome Region Maintenance 1 Export Pathway and Its Consequences for the Cell Host

In eukaryotic cells, the spatial distribution between cytoplasm and nucleus is essential for cell homeostasis. This dynamic distribution is selectively regulated by the nuclear pore complex (NPC), which allows the passive or energy-dependent transport of proteins between these two compartments. Viruses possess many strategies to hijack nucleocytoplasmic shuttling for the benefit of their viral replication. Here, we review how viruses interfere with the karyopherin CRM1 that controls the nuclear export of protein cargoes. We analyze the fact that the viral hijacking of CRM1 provokes are-localization of numerous cellular factors in a suitable place for specific steps of viral replication. While CRM1 emerges as a critical partner for viruses, it also takes part in antiviral and inflammatory response regulation. This review also addresses how CRM1 hijacking affects it and the benefits of CRM1 inhibitors as antiviral treatments.

Cytoplasmic ribonucleoprotein complexes, RNA helicases and coronavirus infection

RNA metabolism in the eukaryotic cell includes the formation of ribonucleoprotein complexes (RNPs) that, depending on their protein components, have a different function. Cytoplasmic RNPs, such as stress granules (SGs) or P-bodies (PBs) are quite relevant during infections modulating viral and cellular RNA expression and as key players in the host cell antiviral response. RNA helicases are abundant components of RNPs and could have a significant effect on viral infection. This review focuses in the role that RNPs and RNA helicases have during coronavirus (CoVs) infection. CoVs are emerging highly pathogenic viruses with a large single-stranded RNA genome. During CoV infection, a complex network of RNA-protein interactions in different RNP structures is established. In general, RNA helicases and RNPs have an antiviral function, but there is limited knowledge on whether the viral protein interactions with cell components are mediators of this antiviral effect or are part of the CoV antiviral counteraction mechanism. Additional data is needed to elucidate the role of these RNA-protein interactions during CoV infection and their potential contribution to viral replication or pathogenesis.

Identification and characterization of DEAD-box RNA helicase DDX3 in rainbow trout (Oncorhynchus mykiss) and its relationship with infectious hematopoietic necrosis virus infection

DDX3, a member of the DEAD-box RNA helicase family and has highly conserved ATP-dependent RNA helicase activity, has important roles in RNA metabolism and innate anti-viral immune responses. In this study, five transcript variants of the DDX3 gene were cloned and characterized from rainbow trout (Oncorhynchus mykiss). These five transcript variants of DDX3 encoded proteins were 74.2 kDa (686 aa), 76.4 kDa (709 aa), 77.8 kDa (711 aa), 78.0 kDa (718 aa), and 78.8 kDa (729 aa) and the predicted isoelectric points were 6.91, 7.63, 7.63, 7.18, and 7.23, respectively. All rainbow trout DDX3 proteins contained two conserved RecA-like domains that were similar to the DDX3 protein reported in mammals. Phylogenetic analysis showed that the five cloned rainbow trout DDX3 were separate from mammals but clustered with fish, especially Northern pike (Esox lucius) and Nile tilapia (Oreochromis niloticus). RT-qPCR analysis showed that the DDX3 gene was broadly expressed in all tissues studied. The expression of DDX3 after infectious hematopoietic necrosis virus (IHNV) infection increased gradually after the early stage of IHNV infection, decreased gradually with the proliferation of IHNV in vivo (liver, spleen, and kidney), and was significantly decreased after the in vitro infection of epithelioma papulosum cyprini (EPC) and rainbow trout gonad cell line-2 (RTG-2) cell lines. We also found that rainbow trout DDX3 was significantly increased by a time-dependent mechanism after the poly I:C treatment of EPC and RTG cells; however no significant changes were observed with lipopolysaccharide (LPS) treatment. Knockdown of DDX3 by siRNA showed significantly increased IHNV replication in infected RTG cells. This study suggests that DDX3 has an important role in host defense against IHNV infection and these results may provide new insights into IHNV pathogenesis and antiviral drug research.

DEAD-ly Affairs: The Roles of DEAD-Box Proteins on HIV-1 Viral RNA Metabolism

In order to ensure viral gene expression, Human Immunodeficiency virus type-1 (HIV-1) recruits numerous host proteins that promote optimal RNA metabolism of the HIV-1 viral RNAs (vRNAs), such as the proteins of the DEAD-box family. The DEAD-box family of RNA helicases regulates multiple steps of RNA metabolism and processing, including transcription, splicing, nucleocytoplasmic export, trafficking, translation and turnover, mediated by their ATP-dependent RNA unwinding ability. In this review, we provide an overview of the functions and role of all DEAD-box family protein members thus far described to influence various aspects of HIV-1 vRNA metabolism. We describe the molecular mechanisms by which HIV-1 hijacks these host proteins to promote its gene expression and we discuss the implications of these interactions during viral infection, their possible roles in the maintenance of viral latency and in inducing cell death. We also speculate on the emerging potential of pharmacological inhibitors of DEAD-box proteins as novel therapeutics to control the HIV-1 pandemic.

DDX5: an expectable treater for viral infection- a literature review

Background and Objective

DEAD-box protein (DDX)5 plays important roles in multiple aspects of cellular processes that require modulating RNA structure. Alongside the canonical role of DDX5 in RNA metabolism, many reports have shown that DDX5 influences viral infection by directly interacting with viral proteins. However, the functional role of DDX5 in virus-associated cancers, as well as the identity of DDX5 in virus infection-associated signaling pathways, has remained largely unexplained. Here, we further explore the precise functions of DDX5 and its potential targets for antiviral treatment.

Methods

We searched the PubMed and PMC databases to identify studies on role of DDXs, especially DDX5, during various viral infection published up to May 2022.

Key Content and Findings

DDX5 functions as both a viral infection helper and inhibitor, which depends on virus type. DDXs proteins have been identified to play roles on multiple aspects covering RNA metabolism and function.

Conclusions

DDX5 influences viral pathogenesis by participating in viral replication and multiple viral infection-related signaling pathways, it also plays a double-edge sword role under different viral infection conditions. Deep investigation into the mechanism of DDX5 modulating immune response in host cells revealed that it holds highly potential usage for future antiviral therapy. We reviewed current studies to provide a comprehensive update of the role of DDX5 in viral infection.

Host cellular RNA helicases regulate SARS-CoV-2 infection

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has the largest RNA genome, approximately 30 kb, among RNA viruses. The DDX DEAD box RNA helicase is a multifunctional protein involved in all aspects of RNA metabolism. Therefore, host RNA helicases may regulate and maintain such a large viral RNA genome. In this study, I investigated the potential role of several host cellular RNA helicases in SARS-CoV-2 infection. Notably, DDX21 knockdown markedly accumulated intracellular viral RNA and viral production, as well as viral infectivity of SARS-CoV-2, indicating that DDX21 strongly restricts the SARS-CoV-2 infection. In addition, MOV10 RNA helicase also suppressed the SARS-CoV-2 infection. In contrast, DDX1, DDX5, and DDX6 RNA helicases were required for SARS-CoV-2 replication. Indeed, SARS-CoV-2 infection dispersed the P-body formation of DDX6 and MOV10 RNA helicases as well as XRN1 exonuclease, while the viral infection did not induce stress granule formation. Accordingly, the SARS-CoV-2 nucleocapsid (N) protein interacted with DDX1, DDX3, DDX5, DDX6, DDX21, and MOV10 and disrupted the P-body formation, suggesting that SARS-CoV-2 N hijacks DDX6 to carry out viral replication. Conversely, DDX21 and MOV10 restricted SARS-CoV-2 infection through an interaction of SARS-CoV-2 N with host cellular RNA helicases. Altogether, host cellular RNA helicases seem to regulate the SARS-CoV-2 infection.

IMPORTANCE SARS-CoV-2 has a large RNA genome, of approximately 30 kb. To regulate and maintain such a large viral RNA genome, host RNA helicases may be involved in SARS-CoV-2 replication. In this study, I have demonstrated that DDX21 and MOV10 RNA helicases limit viral infection and replication. In contrast, DDX1, DDX5, and DDX6 are required for SARS-CoV-2 infection. Interestingly, SARS-CoV-2 infection disrupted P-body formation and attenuated or suppressed stress granule formation. Thus, SARS-CoV-2 seems to hijack host cellular RNA helicases to play a proviral role by facilitating viral infection and replication and by suppressing the host innate immune system.

The TSN1 Binding Protein RH31 Is a Component of Stress Granules and Participates in Regulation of Salt-Stress Tolerance in Arabidopsis

Tudor staphylococcal nucleases (TSNs) are evolutionarily conserved RNA binding proteins, which include redundant TSN1 and TSN2 in Arabidopsis. It has been showed TSNs are the components of stress granules (SGs) and regulate plant growth under salt stress. In this study, we find a binding protein of TSN1, RH31, which is a DEAD-box RNA helicase (RH). Subcellular localization studies show that RH31 is mainly located in the nucleus, but under salinity, it translocates to the cytoplasm where it accumulates in cytoplasmic granules. After cycloheximide (CHX) treatment which can block the formation of SGs by interfering with mRNP homeostasis, these cytoplasmic granules disappeared. More importantly, RH31 co-localizes with SGs marker protein RBP47. RH31 deletion results in salt-hypersensitive phenotype, while RH31 overexpression causes more resistant to salt stress. In summary, we demonstrate that RH31, the TSN1 binding protein, is a component of plant SGs and participates in regulation of salt-stress tolerance in Arabidopsis.

RNA helicases required for viral propagation in humans

RNA viruses cause many routine illnesses, such as the common cold and the flu. Recently, more deadly diseases have emerged from this family of viruses. The hepatitis C virus has had a devastating impact worldwide. Despite the cures developed in the U.S. and Europe, economically disadvantaged countries remain afflicted by HCV infection due to the high cost of these medications. More recently, COVID-19 has swept across the world, killing millions and disrupting economies and lifestyles; the virus responsible for this pandemic is a coronavirus. Our understanding of HCV and SARS CoV-2 replication is still in its infancy. Helicases play a critical role in the replication, transcription and translation of viruses. These key enzymes need extensive study not only as an essential player in the viral lifecycle, but also as targets for antiviral therapeutics. In this review, we highlight the current knowledge for RNA helicases of high importance to human health.

RNA Helicase DDX3: A Double-Edged Sword for Viral Replication and Immune Signaling

DDX3 is a cellular ATP-dependent RNA helicase involved in different aspects of RNA metabolism ranging from transcription to translation and therefore, DDX3 participates in the regulation of key cellular processes including cell cycle progression, apoptosis, cancer and the antiviral immune response leading to type-I interferon production. DDX3 has also been described as an essential cellular factor for the replication of different viruses, including important human threats such HIV-1 or HCV, and different small molecules targeting DDX3 activity have been developed. Indeed, increasing evidence suggests that DDX3 can be considered not only a promising but also a viable target for anticancer and antiviral treatments. In this review, we summarize distinct functional aspects of DDX3 focusing on its participation as a double-edged sword in the host immune response and in the replication cycle of different viruses.

Targeting host DEAD-box RNA helicase DDX3X for treating viral infections

DDX3X or DDX3, a member of the DEAD (asp, glu, ala, asp) box RNA helicase family of proteins, is a multifunctional protein, which is usurped by several viruses and is vital to their production. To date, 18 species of virus from 12 genera have been demonstrated to be dependent on DDX3 for virulence. In addition, DDX3 has been shown to function within 7 of 10 subcellular regions that are involved in the metabolism of viruses. As such, due to its direct interaction with viral components across most or all stages of viral life cycles, DDX3 can be considered an excellent host target for pan-antiviral drug therapy and has been reported to be a possible broad-spectrum antiviral target. Along these lines, it has been demonstrated that treatment of virally infected cells with small molecule inhibitors of DDX3 blunts virion productions. On the other hand, DDX3 bolsters an innate immune response and viruses have evolved capacities to sequester or block DDX3, which dampens an innate immune response. Thus, enhancing DDX3 production or co-targeting direct viral products that interfere with DDX3's modulation of innate immunity would also diminish virion production. Here we review the evidence that supports the hypothesis that modulating DDX3's agonistic and antagonistic functions during viral infections could have an important impact on safely and efficiently subduing a broad-spectrum of viral infections.

Molecular identification of duck DDX3X and its potential role in response to Tembusu virus

ATP-dependent DEAD (Asp-Glu-Ala-Asp)-box RNA helicases not only regulate RNA metabolism, but also are involved in host antiviral innate immune responses. It is important to investigate the orthologs of this protein family to broaden our understanding of innate immunity and promote protective strategies against viral infections in ducks. In the current study, duck DDX3X (duDDX3X) was first cloned, which consists of 1959 bp encoding a protein of 652 amino acids. duDDX3X has the typical structure of this family, including nine motifs, DEAD and HELICc domains. The amino acid sequence of duDDX3X shares a high similarity with the DDX3Xs of avian and mammalian. Quantitative real-time PCR indicated that duDDX3X was ubiquitously expressed in nearly all tissues. Overexpression of duDDX3X could activate interferon (IFN)-β and enhance the RIG-I-induced IFN-β yield in duck embryo fibroblast cells. However, duDDX3X had no significant effect on the expression of proinflammatory cytokines such as IL-1β, IL-6, and CXCL-8. Tembusu virus (TMUV) infection significantly downregulated duDDX3X. Overexpression and siRNA interference studies showed that duDDX3X inhibited the replication of TMUV through IFN-β at the early stages of infection. Collectively, our results indicated that duDDX3X could positively modulate type I interferon and play an essential role in response to TMUV infection. This study will contribute to a better understanding of duDDX3X in the innate immune system of ducks and lay a solid foundation for further studies of duDDX3X in antiviral immunity.

DEAD-box RNA Helicase DDX3: Functional Properties and Development of DDX3 Inhibitors as Antiviral and Anticancer Drugs

This short review is focused on enzymatic properties of human ATP-dependent RNA helicase DDX3 and the development of antiviral and anticancer drugs targeting cellular helicases. DDX3 belongs to the DEAD-box proteins, a large family of RNA helicases that participate in all aspects of cellular processes, such as cell cycle progression, apoptosis, innate immune response, viral replication, and tumorigenesis. DDX3 has a variety of functions in the life cycle of different viruses. DDX3 helicase is required to facilitate both the Rev-mediated export of unspliced/partially spliced human immunodeficiency virus (HIV) RNA from nucleus and Tat-dependent translation of viral genes. DDX3 silencing blocks the replication of HIV, HCV, and some other viruses. On the other hand, DDX displays antiviral effect against Dengue virus and hepatitis B virus through the stimulation of interferon beta production. The role of DDX3 in different types of cancer is rather controversial. DDX3 acts as an oncogene in one type of cancer, but demonstrates tumor suppressor properties in other types. The human DDX3 helicase is now considered as a new attractive target for the development of novel pharmaceutical drugs. The most interesting inhibitors of DDX3 helicase and the mechanisms of their actions as antiviral or anticancer drugs are discussed in this short review.

From the magic bullet to the magic target: exploiting the diverse roles of DDX3X in viral infections and tumorigenesis

DDX3X is an ATPase/RNA helicase of the DEAD-box family and one of the most multifaceted helicases known up to date, acting in RNA metabolism, cell cycle control, apoptosis, stress response and innate immunity. Depending on the virus or the viral cycle stage, DDX3X can act either in a proviral fashion or as an antiviral factor. Similarly, in different cancer types, it can act either as an oncogene or a tumor-suppressor gene. Accumulating evidence indicated that DDX3X can be considered a promising target for anticancer and antiviral chemotherapy, but also that its exploitation requires a deeper understanding of the molecular mechanisms underlying its dual role in cancer and viral infections. In this Review, we will summarize the known roles of DDX3X in different tumor types and viral infections, and the different inhibitors available, illustrating the possible advantages and potential caveats of their use as anticancer and antiviral drugs.

HIV-1 Vpr and p21 restrict LINE-1 mobility

Long interspersed element-1 (LINE-1, L1) composes ∼17% of the human genome. However, genetic interactions between L1 and human immunodeficiency virus type 1 (HIV-1) remain poorly understood. In this study, we found that HIV-1 suppresses L1 retrotransposition. Notably, HIV-1 Vpr strongly inhibited retrotransposition without inhibiting L1 promoter activity. Since Vpr is known to regulate host cell cycle, we examined the possibility whether Vpr suppresses L1 retrotransposition in a cell cycle dependent manner. We showed that the inhibitory effect of a mutant Vpr (H71R), which is unable to arrest the cell cycle, was significantly relieved compared with that of wild-type Vpr, suggesting that Vpr suppresses L1 mobility in a cell cycle dependent manner. Furthermore, a host cell cycle regulator p21Waf1 strongly suppressed L1 retrotransposition. The N-terminal kinase inhibitory domain (KID) of p21 was required for this inhibitory effect. Another KID-containing host cell cycle regulator p27Kip1 also strongly suppressed L1 retrotransposition. We showed that Vpr and p21 coimmunoprecipitated with L1 ORF2p and they suppressed the L1 reverse transcriptase activity in LEAP assay, suggesting that Vpr and p21 inhibit ORF2p-mediated reverse transcription. Altogether, our results suggest that viral and host cell cycle regulatory machinery limit L1 mobility in cultured cells.

DEAD-box helicases: the Yin and Yang roles in viral infections

Viruses hijack the host cell machinery and recruit host proteins to aid their replication. Several host proteins also play vital roles in inhibiting viral replication. Emerging class of host proteins central to both of these processes are the DEAD-box helicases: a highly conserved family of ATP-dependent RNA helicases, bearing a common D-E-A-D (Asp-Glu-Ala-Asp) motif. They play key roles in numerous cellular processes, including transcription, splicing, miRNA biogenesis and translation. Though their sequences are highly conserved, these helicases have quite diverse roles in the cell. Interestingly, often these helicases display contradictory actions in terms of the support and/or clearance of invading viruses. Increasing evidence highlights the importance of these enzymes, however, little is known about the structural basis of viral RNA recognition by the members of the DEAD-box family. This review summarizes the current knowledge in the field for selected DEAD-box helicases and highlights their diverse actions upon viral invasion of the host cell. We anticipate that through a better understanding of how these helicases are being utilized by viral RNAs and proteins to aid viral replication, it will be possible to address the urgent need to develop novel therapeutic approaches to combat viral infections.

DDX5 RNA Helicases: Emerging Roles in Viral Infection

Asp-Glu-Ala-Asp (DEAD)-box polypeptide 5 (DDX5), also called p68, is a prototypical member of the large ATP-dependent RNA helicases family and is known to participate in all aspects of RNA metabolism ranging from transcription to translation, RNA decay, and miRNA processing. The roles of DDX5 in cell cycle regulation, tumorigenesis, apoptosis, cancer development, adipogenesis, Wnt-β-catenin signaling, and viral infection have been established. Several RNA viruses have been reported to hijack DDX5 to facilitate various steps of their replication cycles. Furthermore, DDX5 can be bounded by the viral proteins of some viruses with unknown functions. Interestingly, an antiviral function of DDX5 has been reported during hepatitis B virus and myxoma virus infection. Thus, the precise roles of this apparently multifaceted protein remain largely obscure. Here, we provide a rapid and critical overview of the structure and functions of DDX5 with a particular emphasis on its role during virus infection.

A Survey of DDX21 Activity During Rev/RRE Complex Formation

HIV-1 requires a specialized nuclear export pathway to transport unspliced and partially spliced viral transcripts to the cytoplasm. Central to this pathway is the viral protein Rev, which binds to the Rev response element in stem IIB located on unspliced viral transcripts and subsequently oligomerizes in a cooperative manner. Previous work identified a number of cellular DEAD-box helicases as in vivo binding partners of Rev, and siRNA experiments indicated a functional role for many in the HIV replication cycle. Two DEAD-box proteins, DDX1 and DDX3, had previously been shown to play a role in HIV pathogenesis. In this study, another protein identified in that screen, DDX21, is tested for protein and RNA binding and subsequent enzymatic activities in the context of the Rev/RRE pathway. We found that DDX21 can bind to the RRE with high affinity, and this binding stimulates ATPase activity with an enzymatic efficiency similar to DDX1. Furthermore, DDX21 is both an ATP-dependent and ATP-independent helicase, and both ATPase and ATP-dependent helicase activities are inhibited by Rev in a dose-dependent manner, although ATP-independent helicase activity is not. A conserved binding interaction between DDX protein's DEAD domain and Rev was identified, with Rev's nuclear diffusion inhibitory signal motif playing a significant role in binding. Finally, DDX21 was shown to enhance Rev binding to the RRE in a manner similar to that previously described for DDX1, although DDX3 does not. These data indicate that DDX1 and DDX21 have similar biochemical activities with regard to the Rev/RRE system, while DDX3 differs.

Venezuelan equine encephalitis virus non-structural protein 3 (nsP3) interacts with RNA helicases DDX1 and DDX3 in infected cells

The mosquito-borne New World alphavirus, Venezuelan equine encephalitis virus (VEEV) is a Category B select agent with no approved vaccines or therapies to treat infected humans. Therefore it is imperative to identify novel targets that can be targeted for effective therapeutic intervention. We aimed to identify and validate interactions of VEEV nonstructural protein 3 (nsP3) with host proteins and determine the consequences of these interactions to viral multiplication. We used a HA tagged nsP3 infectious clone (rTC-83-nsP3-HA) to identify and validate two RNA helicases: DDX1 and DDX3 that interacted with VEEV-nsP3. In addition, DDX1 and DDX3 knockdown resulted in a decrease in infectious viral titers. Furthermore, we propose a functional model where the nsP3:DDX3 complex interacts with the host translational machinery and is essential in the viral life cycle. This study will lead to future investigations in understanding the importance of VEEV-nsP3 to viral multiplication and apply the information for the discovery of novel host targets as therapeutic options.

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VEEV nsP3 interacted with the host helicases DDX1 and DDX3 in infected cells.

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Depletion of DDX1 or DDX3 negatively impacted viral multiplication and decreased infectious viral titers.

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nsP3 may interact with the host translational machinery through DDX3.

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The small molecule DDX3 inhibitor RK33 negatively impacted VEEV multiplication.

Cytoplasmic RNA Granules and Viral Infection

RNA granules are dynamic cellular structures essential for proper gene expression and homeostasis. The two principal types of cytoplasmic RNA granules are stress granules, which contain stalled translation initiation complexes, and processing bodies (P bodies), which concentrate factors involved in mRNA degradation. RNA granules are associated with gene silencing of transcripts; thus, viruses repress RNA granule functions to favor replication. This article discusses the breadth of viral interactions with cytoplasmic RNA granules, focusing on mechanisms that modulate the functions of RNA granules and that typically promote viral replication. Currently, mechanisms for virus manipulation of RNA granules can be loosely grouped into three nonexclusive categories: (a) cleavage of key RNA granule factors, (b) regulation of PKR activation, and (c) co-opting of RNA granule factors for new roles in viral replication. Viral modulation of RNA granules supports productive infection by inhibiting their gene-silencing functions and counteracting their role in linking stress sensing with innate immune activation.

HIV-1 Nef-associated Factor 1 Enhances Viral Production by Interacting with CRM1 to Promote Nuclear Export of Unspliced HIV-1 gag mRNA

HIV-1 depends on host-cell-encoded factors to complete its life cycle. A comprehensive understanding of how HIV-1 manipulates host machineries during viral infection can facilitate the identification of host targets for antiviral drugs or gene therapy. The cellular protein Naf1 (HIV-1 Nef-associated factor 1) is a CRM1-dependent nucleo-cytoplasmic shuttling protein, and has been identified to regulate multiple receptor-mediated signal pathways in inflammation. The cytoplasm-located Naf1 can inhibit NF-κB activation through binding to A20, and the loss of Naf1 controlled NF-κB activation is associated with multiple autoimmune diseases. However, the effect of Naf1 on HIV-1 mRNA expression has not been characterized. In this study we found that the nucleus-located Naf1 could promote nuclear export of unspliced HIV-1 gag mRNA. We demonstrated that the association between Naf1 and CRM1 was required for this function as the inhibition or knockdown of CRM1 expression significantly impaired Naf1-promoted HIV-1 production. The mutation of Naf1 nuclear export signals (NESs) that account for CRM1 recruitment for nuclear export decreased Naf1 function. Additionally, the mutation of the nuclear localization signal (NLS) of Naf1 diminished its ability to promote HIV-1 production, demonstrating that the shuttling property of Naf1 is required for this function. Our results reveal a novel role of Naf1 in enhancing HIV-1 production, and provide a potential therapeutic target for controlling HIV-1 infection.

Tudor staphylococcal nuclease links formation of stress granules and processing bodies with mRNA catabolism in Arabidopsis

Tudor Staphylococcal Nuclease (TSN or Tudor-SN; also known as SND1) is an evolutionarily conserved protein involved in the transcriptional and posttranscriptional regulation of gene expression in animals. Although TSN was found to be indispensable for normal plant development and stress tolerance, the molecular mechanisms underlying these functions remain elusive. Here, we show that Arabidopsis thaliana TSN is essential for the integrity and function of cytoplasmic messenger ribonucleoprotein (mRNP) complexes called stress granules (SGs) and processing bodies (PBs), sites of posttranscriptional gene regulation during stress. TSN associates with SGs following their microtubule-dependent assembly and plays a scaffolding role in both SGs and PBs. The enzymatically active tandem repeat of four SN domains is crucial for targeting TSN to the cytoplasmic mRNA complexes and is sufficient for the cytoprotective function of TSN during stress. Furthermore, our work connects the cytoprotective function of TSN with its positive role in stress-induced mRNA decapping. While stress led to a pronounced increase in the accumulation of uncapped mRNAs in wild-type plants, this increase was abrogated in TSN knockout plants. Taken together, our results establish TSN as a key enzymatic component of the catabolic machinery responsible for the processing of mRNAs in the cytoplasmic mRNP complexes during stress.

The Ded1/DDX3 subfamily of DEAD-box RNA helicases

In eukaryotic organisms, the orthologs of the DEAD-box RNA helicase Ded1p from yeast and DDX3 from human form a well-defined subfamily that is characterized by high sequence conservation in their helicase core and their N- and C- termini. Individual members of this Ded1/DDX3 subfamily perform multiple functions in RNA metabolism in both nucleus and cytoplasm. Ded1/DDX3 subfamily members have also been implicated in cellular signaling pathways and are targeted by diverse viruses. In this review, we discuss the considerable body of work on the biochemistry and biology of these proteins, including the recently discovered link of human DDX3 to tumorigenesis.

A HIV-1 Tat mutant protein disrupts HIV-1 Rev function by targeting the DEAD-box RNA helicase DDX1

BACKGROUND

Previously we described a transdominant negative mutant of the HIV-1 Tat protein, termed Nullbasic, that downregulated the steady state levels of unspliced and singly spliced viral mRNA, an activity caused by inhibition of HIV-1 Rev activity. Nullbasic also altered the subcellular localizations of Rev and other cellular proteins, including CRM1, B23 and C23 in a Rev-dependent manner, suggesting that Nullbasic may disrupt Rev function and trafficking by intervening with an unidentified component of the Rev nucleocytoplasmic transport complex.

RESULTS

To seek a possible mechanism that could explain how Nullbasic inhibits Rev activity, we used a proteomics approach to identify host cellular proteins that interact with Nullbasic. Forty-six Nullbasic-binding proteins were identified by mass spectrometry including the DEAD-box RNA helicase, DDX1. To determine the effect of DDX1 on Nullbasic-mediated Rev activity, we performed cell-based immunoprecipitation assays, Rev reporter assays and bio-layer interferometry (BLI) assays. Interaction between DDX1 and Nullbasic was observed by co-immunoprecipitation of Nullbasic with endogenous DDX1 from cell lysates. BLI assays showed a direct interaction between Nullbasic and DDX1. Nullbasic affected DDX1 subcellular distribution in a Rev-independent manner. Interestingly overexpression of DDX1 in cells not only restored Rev-dependent mRNA export and gene expression in a Rev reporter assay but also partly reversed Nullbasic-induced Rev subcellular mislocalization. Moreover, HIV-1 wild type Tat co-immunoprecipitated with DDX1 and overexpression of Tat could rescue the unspliced viral mRNA levels inhibited by Nullbasic in HIV-1 expressing cells.

CONCLUSIONS

Nullbasic was used to further define the complex mechanisms involved in the Rev-dependent nuclear export of the 9 kb and 4 kb viral RNAs. All together, these data indicate that DDX1 can be sequestered by Nullbasic leading to destabilization of the Rev nucleocytoplasmic transport complex and decreased levels of Rev-dependent viral transcripts. The outcomes support a role for DDX1 in maintenance of a Rev nuclear complex that transports viral RRE-containing mRNA to the cytoplasm. To our knowledge Nullbasic is the first anti-HIV protein that specifically targets the cellular protein DDX1 to block Rev's activity. Furthermore, our research raises the possibility that wild type Tat may play a previously unrecognized but very important role in Rev function.

Multiple functions of DDX3 RNA helicase in gene regulation, tumorigenesis, and viral infection

The DEAD-box RNA helicase DDX3 is a multifunctional protein involved in all aspects of RNA metabolism, including transcription, splicing, mRNA nuclear export, translation, RNA decay and ribosome biogenesis. In addition, DDX3 is also implicated in cell cycle regulation, apoptosis, Wnt-β-catenin signaling, tumorigenesis, and viral infection. Notably, recent studies suggest that DDX3 is a component of anti-viral innate immune signaling pathways. Indeed, DDX3 contributes to enhance the induction of anti-viral mediators, interferon (IFN) regulatory factor 3 and type I IFN. However, DDX3 seems to be an important target for several viruses, such as human immunodeficiency virus type 1 (HIV-1), hepatitis C virus (HCV), hepatitis B virus (HBV), and poxvirus. DDX3 interacts with HIV-1 Rev or HCV Core protein and modulates its function. At least, DDX3 is required for both HIV-1 and HCV replication. Therefore, DDX3 could be a novel therapeutic target for the development of drug against HIV-1 and HCV.

The DEAD-box RNA helicase 5 positively regulates the replication of porcine reproductive and respiratory syndrome virus by interacting with viral Nsp9 in vitro

The nonstructural protein 9 (Nsp9) of porcine reproductive and respiratory syndrome virus (PRRSV) has been recognized to play important roles in viral replication. The present study first screened that the DEAD-box RNA helicase 5 (DDX5) was a cellular protein interacting with the Nsp9 of PRRSV by a yeast two-hybrid method in a pulmonary alveolar macrophages (PAMs) cDNA library. Next, DDX5 was shown to interact with viral Nsp9 in the co-transfected HEK293 cells with the DDX5- and Nsp9-expressing plasmids, and the interaction between endogenous DDX5 and Nsp9 was also confirmed in MARC-145 cells infected with the Nsp9-expressing lentiviruses. Then, the interacting domains between DDX5 and Nsp9 were determined to be the DEXDc and HELICc domains in DDX5 and the RdRp domain in Nsp9, respectively. Moreover, in the HEK293 cells, MARC-145 cells and PAM cell lines co-transfected with the DDX5- and Nsp9-expressing plasmids, Nsp9 was shown to co-localize with DDX5 in the cytoplasm with a perinuclear pattern, and meanwhile in PRRSV-infected MARC-145 cells and PAMs, endogenous DDX5 was also found to co-localize with Nsp9. Finally, silencing the DDX5 gene in MARC-145 cells significantly impacted the replication of PRRSV, and while the over-expression of DDX5 could slightly enhance viral replication. These findings indicate that DDX5 positively regulates the replication of PRRSV via its interaction with viral Nsp9 in vitro.

Translation initiation of the HIV-1 mRNA

Translation initiation of the full-length mRNA of the human immunodeficiency virus can occur via several different mechanisms to maintain production of viral structural proteins throughout the replication cycle. HIV-1 viral protein synthesis can occur by the use of both a cap-dependant and IRES-driven mechanism depending on the physiological conditions of the cell and the status of the ongoing infection. For both of these mechanisms there is a need for several viral and cellular co-factors for optimal translation of the viral mRNA. In this review we will describe the mechanism used by the full-length mRNA to initiate translation highlighting the role of co-factors within this process. A particular emphasis will be given to the role of the DDX3 RNA helicase in HIV-1 mRNA translation initiation.