Trimethylguanosine capping selectively promotes expression of Rev-dependent HIV-1 RNAs

Translational Control of the HIV Unspliced Genomic RNA

Post-transcriptional control in both HIV-1 and HIV-2 is a highly regulated process that commences in the nucleus of the host infected cell and finishes by the expression of viral proteins in the cytoplasm. Expression of the unspliced genomic RNA is particularly controlled at the level of RNA splicing, export, and translation. It appears increasingly obvious that all these steps are interconnected and they result in the building of a viral ribonucleoprotein complex (RNP) that must be efficiently translated in the cytosolic compartment. This review summarizes our knowledge about the genesis, localization, and expression of this viral RNP.

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.

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.

Functional analyses reveal extensive RRE plasticity in primary HIV-1 sequences selected under selective pressure

BACKGROUND

HIV-1 Rev response element (RRE) is a functional region of viral RNA lying immediately downstream to the junction of gp120 and gp41 in the env coding sequence. The RRE is essential for HIV replication and binds with the Rev protein to facilitate the export of viral mRNA from nucleus to cytoplasm. It has been suggested that changes in the predicted secondary structure of primary RRE sequences impact the function of the RREs; however, functional assays have not yet been performed. The aim of this study was to characterize the genetic, structural and functional variation in the RRE primary sequences selected in vivo by Enfuvirtide pressure.

RESULTS

Multiple RRE variants were obtained from viruses isolated from patients who failed an Enfuvirtide-containing regimen. Different alterations were observed in the predicted RRE secondary structures, with the abrogation of the primary Rev binding site in one of the variants. In spite of this, most of the RRE variants were able to bind Rev and promote the cytoplasmic export of the viral mRNAs with equivalent efficiency in a cell-based assay. Only RRE45 and RRE40-45 showed an impaired ability to bind Rev in a gel-shift binding assay. Unexpectedly, this impairment was not reflected in functional capacity when RNA export was evaluated using a reporter assay, or during virus replication in lymphoid cells, suggesting that in vivo the RRE would be highly malleable.

CONCLUSIONS

The Rev-RRE functionality is unaffected in RRE variants selected in patients failing an ENF-containing regimen. Our data show that the current understanding of the Rev-RRE complex structure does not suffice and fails to rationally predict the function of naturally occurring RRE mutants. Therefore, this data should be taken into account in the development of antiviral agents that target the RRE-Rev complex.

Hypermethylated-capped selenoprotein mRNAs in mammals

Mammalian mRNAs are generated by complex and coordinated biogenesis pathways and acquire 5′-end m⁷G caps that play fundamental roles in processing and translation. Here we show that several selenoprotein mRNAs are not recognized efficiently by translation initiation factor eIF4E because they bear a hypermethylated cap. This cap modification is acquired via a 5′-end maturation pathway similar to that of the small nucle(ol)ar RNAs (sn- and snoRNAs). Our findings also establish that the trimethylguanosine synthase 1 (Tgs1) interacts with selenoprotein mRNAs for cap hypermethylation and that assembly chaperones and core proteins devoted to sn- and snoRNP maturation contribute to recruiting Tgs1 to selenoprotein mRNPs. We further demonstrate that the hypermethylated-capped selenoprotein mRNAs localize to the cytoplasm, are associated with polysomes and thus translated. Moreover, we found that the activity of Tgs1, but not of eIF4E, is required for the synthesis of the GPx1 selenoprotein in vivo.

Life of psi: how full-length HIV-1 RNAs become packaged genomes in the viral particles

As a member of the retrovirus family, HIV-1 packages its RNA genome into particles and replicates through a DNA intermediate that integrates into the host cellular genome. The multiple genes encoded by HIV-1 are expressed from the same promoter and their expression is regulated by splicing and ribosomal frameshift. The full-length HIV-1 RNA plays a central role in viral replication as it serves as the genome in the progeny virus and is used as the template for Gag and GagPol translation. In this review, we summarize findings that contribute to our current understanding of how full-length RNA is expressed and transported, cis- and trans-acting elements important for RNA packaging, the locations and timing of RNA:RNA and RNA:Gag interactions, and the processes required for this RNA to be packaged into viral particles.

The myxobacterial metabolite ratjadone A inhibits HIV infection by blocking the Rev/CRM1-mediated nuclear export pathway

BACKGROUND

The nuclear export of unspliced and partially spliced HIV-1 mRNA is mediated by the recognition of a leucine-rich nuclear export signal (NES) in the HIV Rev protein by the host protein CRM1/Exportin1. This makes the CRM1-Rev complex an attractive target for the development of new antiviral drugs. Here we tested the anti-HIV efficacy of ratjadone A, a CRM1 inhibitor derived from myxobacteria.

RESULTS

Ratjadone A inhibits HIV infection in vitro in a dose-dependent manner with EC₅₀ values at the nanomolar range. The inhibitory effect of ratjadone A occurs around 12 hours post-infection and is specific for the Rev/CRM1-mediated nuclear export pathway. By using a drug affinity responsive target stability (DARTS) assay we could demonstrate that ratjadone A interferes with the formation of the CRM1-Rev-NES complex by binding to CRM1 but not to Rev.

CONCLUSION

Ratjadone A exhibits strong anti-HIV activity but low selectivity due to toxic effects. Although this limits its potential use as a therapeutic drug, further studies with derivatives of ratjadones might help to overcome these difficulties in the future.

ERK2-mediated phosphorylation of transcriptional coactivator binding protein PIMT/NCoA6IP at Ser298 augments hepatic gluconeogenesis

PRIP-Interacting protein with methyl transferase domain (PIMT) serves as a molecular bridge between CREB-binding protein (CBP)/ E1A binding protein p300 (Ep300) -anchored histone acetyl transferase and the Mediator complex sub-unit1 (Med1) and modulates nuclear receptor transcription. Here, we report that ERK2 phosphorylates PIMT at Ser(298) and enhances its ability to activate PEPCK promoter. We observed that PIMT is recruited to PEPCK promoter and adenoviral-mediated over-expression of PIMT in rat primary hepatocytes up-regulated expression of gluconeogenic genes including PEPCK. Reporter experiments with phosphomimetic PIMT mutant (PIMT(S298D)) suggested that conformational change may play an important role in PIMT-dependent PEPCK promoter activity. Overexpression of PIMT and Med1 together augmented hepatic glucose output in an additive manner. Importantly, expression of gluconeogenic genes and hepatic glucose output were suppressed in isolated liver specific PIMT knockout mouse hepatocytes. Furthermore, consistent with reporter experiments, PIMT(S298D) but not PIMT(S298A) augmented hepatic glucose output via up-regulating the expression of gluconeogenic genes. Pharmacological blockade of MAPK/ERK pathway using U0126, abolished PIMT/Med1-dependent gluconeogenic program leading to reduced hepatic glucose output. Further, systemic administration of T4 hormone to rats activated ERK1/2 resulting in enhanced PIMT ser(298) phosphorylation. Phosphorylation of PIMT led to its increased binding to the PEPCK promoter, increased PEPCK expression and induction of gluconeogenesis in liver. Thus, ERK2-mediated phosphorylation of PIMT at Ser(298) is essential in hepatic gluconeogenesis, demonstrating an important role of PIMT in the pathogenesis of hyperglycemia.

Putting an 'End' to HIV mRNAs: capping and polyadenylation as potential therapeutic targets

Like most cellular mRNAs, the 5′ end of HIV mRNAs is capped and the 3′ end matured by the process of polyadenylation. There are, however, several rather unique and interesting aspects of these post-transcriptional processes on HIV transcripts. Capping of the highly structured 5′ end of HIV mRNAs is influenced by the viral TAT protein and a population of HIV mRNAs contains a trimethyl-G cap reminiscent of U snRNAs involved in splicing. HIV polyadenylation involves active repression of a promoter-proximal polyadenylation signal, auxiliary upstream regulatory elements and moonlighting polyadenylation factors that have additional impacts on HIV biology outside of the constraints of classical mRNA 3’ end formation. This review describes these post-transcriptional novelties of HIV gene expression as well as their implications in viral biology and as possible targets for therapeutic intervention.

Potential therapeutic applications of RNA cap analogs

Cap analogs are chemically modified derivatives of the unique cap structure present at the 5´ end of all eukaryotic mRNAs and several non-coding RNAs. Until recently, cap analogs have served primarily as tools in the study of RNA metabolism. Continuing advances in our understanding of cap biological functions (including RNA stabilization, pre-mRNA splicing, initiation of mRNA translation, as well as cellular transport of mRNAs and snRNAs) and the consequences of the disruption of these processes - resulting in serious medical disorders - have opened new possibilities for pharmaceutical applications of these compounds. In this review, the medicinal potential of cap analogs in areas, such as cancer treatment (including eIF4E targeting and mRNA-based immunotherapy), spinal muscular atrophy treatment, antiviral therapy and the improvement of the localization of nucleus-targeting drugs, are highlighted. Advances achieved to date, challenges, plausible solutions and prospects for the future development of cap analog-based drug design are described.

Posttranscriptional regulation of retroviral gene expression: primary RNA transcripts play three roles as pre-mRNA, mRNA, and genomic RNA

After reverse transcription of the retroviral RNA genome and integration of the DNA provirus into the host genome, host machinery is used for viral gene expression along with viral proteins and RNA regulatory elements. Here, we discuss co-transcriptional and posttranscriptional regulation of retroviral gene expression, comparing simple and complex retroviruses. Cellular RNA polymerase II synthesizes full-length viral primary RNA transcripts that are capped and polyadenylated. All retroviruses generate a singly spliced env mRNA from this primary transcript, which encodes the viral glycoproteins. In addition, complex viral RNAs are alternatively spliced to generate accessory proteins, such as Rev, which is involved in posttranscriptional regulation of HIV-1 RNA. Importantly, the splicing of all retroviruses is incomplete; they must maintain and export a fraction of their primary RNA transcripts. This unspliced RNA functions both as the major mRNA for Gag and Pol proteins and as the packaged genomic RNA. Different retroviruses export their unspliced viral RNA from the nucleus to the cytoplasm by either Tap-dependent or Rev/CRM1-dependent routes. Translation of the unspliced mRNA involves frame-shifting or termination codon suppression so that the Gag proteins, which make up the capsid, are expressed more abundantly than the Pol proteins, which are the viral enzymes. After the viral polyproteins assemble into viral particles and bud from the cell membrane, a viral encoded protease cleaves them. Some retroviruses have evolved mechanisms to protect their unspliced RNA from decay by nonsense-mediated RNA decay and to prevent genome editing by the cellular APOBEC deaminases.

The DEAD-box helicase DDX3 substitutes for the cap-binding protein eIF4E to promote compartmentalized translation initiation of the HIV-1 genomic RNA

Here, we show a novel molecular mechanism promoted by the DEAD-box RNA helicase DDX3 for translation of the HIV-1 genomic RNA. This occurs through the adenosine triphosphate-dependent formation of a translation initiation complex that is assembled at the 5′ m⁷GTP cap of the HIV-1 mRNA. This is due to the property of DDX3 to substitute for the initiation factor eIF4E in the binding of the HIV-1 m⁷GTP 5′ cap structure where it nucleates the formation of a core DDX3/PABP/eIF4G trimeric complex on the HIV-1 genomic RNA. By using RNA fluorescence in situ hybridization coupled to indirect immunofluorescence, we further show that this viral ribonucleoprotein complex is addressed to compartmentalized cytoplasmic foci where the translation initiation complex is assembled.

NEAT1 long noncoding RNA and paraspeckle bodies modulate HIV-1 posttranscriptional expression

Most of the human genome is transcribed into protein-noncoding RNAs (ncRNAs), including small ncRNAs and long ncRNAs (lncRNAs). Over the past decade, rapidly emerging evidence has increasingly supported the view that lncRNAs serve key regulatory and functional roles in mammal cells. HIV-1 replication relies on various cell functions. To date, while the involvement of host protein factors and microRNAs (miRNAs) in the HIV-1 life cycle has been extensively studied, the relationship between lncRNAs and HIV-1 remains uncharacterized. Here, we have profiled 83 disease-related lncRNAs in HIV-1-infected T cells. We found NEAT1 to be one of several lncRNAs whose expression is changed by HIV-1 infection, and we have characterized its role in HIV-1 replication. We In the abstract, added definition of INS OK, or should "cis-acting" be added?report here that the knockdown of NEAT1 enhances virus production through increased nucleus-to-cytoplasm export of Rev-dependent instability element (INS)-containing HIV-1 mRNAs. IMPORTANCE Long protein-noncoding RNAs (lncRNAs) play roles in regulating gene expression and modulating protein activities. There is emerging evidence that lncRNAs are involved in the replication of viruses. To our knowledge, this report is the first to characterize a role contributed by an lncRNA, NEAT1, to HIV-1 replication. NEAT1 is essential for the integrity of the nuclear paraspeckle substructure. Based on our findings from NEAT1 knockdown, we have identified the nuclear paraspeckle body as another important subcellular organelle for HIV-1 replication.

NEAT1 long noncoding RNA and paraspeckle bodies modulate HIV-1 posttranscriptional expression

Most of the human genome is transcribed into protein-noncoding RNAs (ncRNAs), including small ncRNAs and long ncRNAs (lncRNAs). Over the past decade, rapidly emerging evidence has increasingly supported the view that lncRNAs serve key regulatory and functional roles in mammal cells. HIV-1 replication relies on various cell functions. To date, while the involvement of host protein factors and microRNAs (miRNAs) in the HIV-1 life cycle has been extensively studied, the relationship between lncRNAs and HIV-1 remains uncharacterized. Here, we have profiled 83 disease-related lncRNAs in HIV-1-infected T cells. We found NEAT1 to be one of several lncRNAs whose expression is changed by HIV-1 infection, and we have characterized its role in HIV-1 replication. We In the abstract, added definition of INS OK, or should "cis-acting" be added?report here that the knockdown of NEAT1 enhances virus production through increased nucleus-to-cytoplasm export of Rev-dependent instability element (INS)-containing HIV-1 mRNAs. IMPORTANCE Long protein-noncoding RNAs (lncRNAs) play roles in regulating gene expression and modulating protein activities. There is emerging evidence that lncRNAs are involved in the replication of viruses. To our knowledge, this report is the first to characterize a role contributed by an lncRNA, NEAT1, to HIV-1 replication. NEAT1 is essential for the integrity of the nuclear paraspeckle substructure. Based on our findings from NEAT1 knockdown, we have identified the nuclear paraspeckle body as another important subcellular organelle for HIV-1 replication.

Human CRM1 augments production of infectious human and feline immunodeficiency viruses from murine cells

Productive replication of human immunodeficiency virus type 1 (HIV-1) occurs efficiently only in humans. The posttranscriptional stages of the HIV-1 life cycle proceed poorly in mouse cells, with a resulting defect in viral assembly and release. Previous work has shown that the presence of human chromosome 2 increases HIV-1 production in mouse cells. Recent studies have shown that human chromosome region maintenance 1 (hCRM1) stimulates Gag release from rodent cells. Here we report that expressions of hCRM1 in murine cells resulted in marked increases in the production of infectious HIV-1 and feline immunodeficiency virus (FIV). HIV-1 production was also increased by hSRp40, and a combination of hCRM1 and hSRp40 resulted in a more-than-additive effect on HIV-1 release. In contrast, the overexpression of mouse CRM1 (mCRM1) minimally affected HIV-1 and FIV production and did not antagonize hCRM1. In the presence of hCRM1 there were large increases in the amounts of released capsid, which paralleled the increases in the infectious titers. Consistent with this finding, the ratios of unspliced to spliced HIV-1 mRNAs in mouse cells expressing hCRM1 and SRp40 became similar to those of human cells. Furthermore, imaging of intron-containing FIV RNA showed that hCRM1 increased RNA export to the cytoplasm.By testing chimeras between mCRM1 and hCRM1 and comparing those sequences to feline CRM1, we mapped the functional domain to HEAT (Huntingtin, elongation factor 3, protein phosphatase 2A, and the yeast kinase TOR1) repeats 4A to 9A and a triple point mutant in repeat 9A, which showed a loss of function. Structural analysis suggested that this region of hCRM1 may serve as a binding site for viral or cellular factors to facilitate lentiviral RNA nuclear export.

Translation initiation is driven by different mechanisms on the HIV-1 and HIV-2 genomic RNAs

The human immunodeficiency virus (HIV) unspliced full length genomic RNA possesses features of an eukaryotic cellular mRNA as it is capped at its 5' end and polyadenylated at its 3' extremity. This genomic RNA is used both for the production of the viral structural and enzymatic proteins (Gag and Pol, respectively) and as genome for encapsidation in the newly formed viral particle. Although both of these processes are critical for viral replication, they should be controlled in a timely manner for a coherent progression into the viral cycle. Some of this regulation is exerted at the level of translational control and takes place on the viral 5' untranslated region and the beginning of the gag coding region. In this review, we have focused on the different initiation mechanisms (cap- and internal ribosome entry site (IRES)-dependent) that are used by the HIV-1 and HIV-2 genomic RNAs and the cellular and viral factors that can modulate their expression. Interestingly, although HIV-1 and HIV-2 share many similarities in the overall clinical syndrome they produce, in some aspects of their replication cycle, and in the structure of their respective genome, they exhibit some differences in the way that ribosomes are recruited on the gag mRNA to initiate translation and produce the viral proteins; this will be discussed in the light of the literature.

From promoting to inhibiting: diverse roles of helicases in HIV-1 Replication

Helicases hydrolyze nucleotide triphosphates (NTPs) and use the energy to modify the structures of nucleic acids. They are key players in every cellular process involving RNA or DNA. Human immunodeficiency virus type 1 (HIV-1) does not encode a helicase, thus it has to exploit cellular helicases in order to efficiently replicate its RNA genome. Indeed, several helicases have been found to specifically associate with HIV-1 and promote viral replication. However, studies have also revealed a couple of helicases that inhibit HIV-1 replication; these findings suggest that HIV-1 can either benefit from the function of cellular helicases or become curtailed by these enzymes. In this review, we focus on what is known about how a specific helicase associates with HIV-1 and how a distinct step of HIV-1 replication is affected. Despite many helicases having demonstrated roles in HIV-1 replication and dozens of other helicase candidates awaiting to be tested, a deeper appreciation of their involvement in the HIV-1 life cycle is hindered by our limited knowledge at the enzymatic and molecular levels regarding how helicases shape the conformation and structure of viral RNA-protein complexes and how these conformational changes are translated into functional outcomes in the context of viral replication.

Early embryonic lethality of mice with disrupted transcription cofactor PIMT/NCOA6IP/Tgs1 gene

PIMT (also known as PIPMT/NCOA6IP/Tgs1), first isolated as a transcription coactivator PRIP (NCOA6)-interacting 96-kDa protein with RNA-binding property, possesses RNA methyltransferase activity. As a transcription coactivator binding protein, PIMT enhances the nuclear receptor transcriptional activity and its methyltransferase property is involved in the formation of the 2,2,7-trimethylguanosine cap of non-coding small RNAs, but the in vivo functions of this gene have not been fully explored. To elucidate the biological functions, we used gene targeting to generate mice with a disrupted PIMT/Tgs1 gene. Disruption of PIMT gene results in early embryonic lethality due to impairment of development around the blastocyst and uterine implantation stages. We show that PIMT is expressed in all cells of the E3.5day blastocyst in the mouse. PIMT null mutation abolished PIMT expression in all cells of the blastocyst and caused a reduction in the expression of Oct4 and Nanog transcription factor proteins in the E3.5 blastocyst resulting in the near failure to form inner cell mass (ICM). With conditional deletion of PIMT gene, mouse embryonic fibroblasts (MEFs) exhibit defective wound healing in the scratch assay and a reduction in cell proliferation due to decreased G₀/G₁ transition and G₂/M phase cell cycle arrest. We conclude that PIMT/NCOA6IP, which is expressed in all cells of the 3.5 day stage blastocyst, is indispensable for early embryonic development.

Multi-Faceted Post-Transcriptional Functions of HIV-1 Rev

Post-transcriptional regulation of HIV-1 gene expression is largely governed by the activities of the viral Rev protein. In this minireview, the multiple post-transcriptional activities of Rev in the export of partially spliced and unspliced HIV-1 RNAs from the nucleus to the cytoplasm, in the translation of HIV-1 transcripts, and in the packaging of viral genomic RNAs are reviewed in brief.

Dynamic Post-Transcriptional Regulation of HIV-1 Gene Expression

Gene expression of the human immunodeficiency virus type 1 (HIV-1) is a highly regulated process. Basal transcription of the integrated provirus generates early transcripts that encode for the viral products Tat and Rev. Tat promotes the elongation of RNA polymerase while Rev mediates the nuclear export of viral RNAs that contain the Rev-responsive RNA element (RRE). These RNAs are exported from the nucleus to allow expression of Gag-Pol and Env proteins and for the production of full-length genomic RNAs. A balance exists between completely processed mRNAs and RRE-containing RNAs. Rev functions as an adaptor that recruits cellular factors to re-direct singly spliced and unspliced viral RNAs to nuclear export. The aim of this review is to address the dynamic regulation of this post-transcriptional pathway in light of recent findings that implicate several novel cellular cofactors of Rev function.

Initiation of HIV-1 reverse transcription and functional role of nucleocapsid-mediated tRNA/viral genome interactions

HIV-1 reverse transcription is initiated from a tRNA(Lys)(3) molecule annealed to the viral RNA at the primer binding site (PBS). The annealing of tRNA(Lys)(3) requires the opening of its three-dimensional structure and RNA rearrangements to form an efficient initiation complex recognized by the reverse transcriptase. This annealing is mediated by the nucleocapsid protein (NC). In this paper, we first review the actual knowledge about HIV-1 viral RNA and tRNA(Lys)(3) structures. Then, we summarize the studies explaining how NC chaperones the formation of the tRNA(Lys)(3)/PBS binary complex. Additional NMR data that investigated the NC interaction with tRNA(Lys)(3) D-loop are presented. Lastly, we focused on the additional interactions occurring between tRNA(Lys)(3) and the viral RNA and showed that they are dependent on HIV-1 isolates, i.e. the sequence and the structure of the viral RNA.

Functional and structural analysis of the internal ribosome entry site present in the mRNA of natural variants of the HIV-1

The 5′untranslated regions (UTR) of the full length mRNA of the HIV-1 proviral clones pNL4.3 and pLAI, harbor an internal ribosomal entry site (IRES). In this study we extend this finding by demonstrating that the mRNA 5′UTRs of natural variants of HIV-1 also exhibit IRES-activity. Cap-independent translational activity was demonstrated using bicistronic mRNAs in HeLa cells and in Xenopus laevis oocytes. The possibility that expression of the downstream cistron in these constructs was due to alternative splicing or to cryptic promoter activity was ruled out. The HIV-1 variants exhibited significant 5′UTR nucleotide diversity with respect to the control sequence recovered from pNL4.3. Interestingly, translational activity from the 5′UTR of some of the HIV-1 variants was enhanced relative to that observed for the 5′UTR of pNL4.3. In an attempt to explain these findings we probed the secondary structure of the variant HIV-1 5′UTRs using enzymatic and chemical approaches. Yet subsequent structural analyses did not reveal significant variations when compared to the pNL4.3-5′UTR. Thus, the increased IRES-activity observed for some of the HIV-1 variants cannot be ascribed to a specific structural modification. A model to explain these findings is proposed.

TRIM5 structure, HIV-1 capsid recognition, and innate immune signaling

TRIM5 is a restriction factor that blocks retrovirus infection soon after the virion core enters the cell cytoplasm. Restriction activity is targeted to the virion core via recognition of the capsid protein lattice that encases the viral genomic RNA. In common with all of the many TRIM family members, TRIM5 has RING, B-box, and coiled-coil domains. As an E3 ubiquitin ligase TRIM5 cooperates with the heterodimeric E2, UBC13/UEV1A, to activate the TAK1 (MAP3K7) kinase, NF-κB and AP-1 signaling, and the transcription of inflammatory cytokines and chemokines. TAK1, UBC13, and UEV1A all contribute to TRIM5-mediated retrovirus restriction activity. Interaction of the carboxy-terminal PRYSPRY or cyclophilin domains of TRIM5 with the retroviral capsid lattice stimulates the formation of a complementary lattice by TRIM5, with greatly increased TRIM5 E3 activity, and host cell signal transduction. Structural and biochemical studies on TRIM5 have opened a much needed window on how the innate immune system detects the distinct molecular features of HIV-1 and other retroviruses.

Thriving under stress: selective translation of HIV-1 structural protein mRNA during Vpr-mediated impairment of eIF4E translation activity

Translation is a regulated process and is pivotal to proper cell growth and homeostasis. All retroviruses rely on the host translational machinery for viral protein synthesis and thus may be susceptible to its perturbation in response to stress, co-infection, and/or cell cycle arrest. HIV-1 infection arrests the cell cycle in the G2/M phase, potentially disrupting the regulation of host cell translation. In this study, we present evidence that HIV-1 infection downregulates translation in lymphocytes, attributable to the cell cycle arrest induced by the HIV-1 accessory protein Vpr. The molecular basis of the translation suppression is reduced accumulation of the active form of the translation initiation factor 4E (eIF4E). However, synthesis of viral structural proteins is sustained despite the general suppression of protein production. HIV-1 mRNA translation is sustained due to the distinct composition of the HIV-1 ribonucleoprotein complexes. RNA-coimmunoprecipitation assays determined that the HIV-1 unspliced and singly spliced transcripts are predominantly associated with nuclear cap binding protein 80 (CBP80) in contrast to completely-spliced viral and cellular mRNAs that are associated with eIF4E. The active translation of the nuclear cap binding complex (CBC)-bound viral mRNAs is demonstrated by ribosomal RNA profile analyses. Thus, our findings have uncovered that the maintenance of CBC association is a novel mechanism used by HIV-1 to bypass downregulation of eIF4E activity and sustain viral protein synthesis. We speculate that a subset of CBP80-bound cellular mRNAs contribute to recovery from significant cellular stress, including human retrovirus infection.

Retroviruses are intracellular parasites that utilize the host translation machinery to catalyze viral protein synthesis. The activity of the translation machinery fluctuates during cell cycle progression and is reduced in the G2/M phase. HIV-1 infection causes the cells to arrest in the G2/M phase, which has the potential to alter the activity of the translation machinery. Herein several lines of evidence demonstrated that lymphocyte mRNA translation is suppressed by the action of HIV-1 accessory protein Vpr. The molecular basis of translation suppression is reduced activity of the rate-limiting translation intitation factor, eIF4E. However, synthesis of the viral structural proteins is sustained and is due to the difference in composition of the viral and cellular mRNA-ribonucleoprotein complexes. Both cellular and completely spliced viral mRNAs are predominantly associated with the cytoplasmic cap binding protein, eIF4E. In contrast, unspliced HIV-1 mRNAs are predominantly associated with the components of the nuclear cap binding complex (CBC). The retention of CBC on the viral mRNAs provides a mechanism to sustain viral protein synthesis. This newly characterized interface of the virus-host-protein synthesis machinery is likely a cellular adaptation used to enable synthesis of proteins that reengage the cell cycle and facilitate recovery from stress.

Characterization of the HIV-1 RNA associated proteome identifies Matrin 3 as a nuclear cofactor of Rev function

Background

Central to the fully competent replication cycle of the human immunodeficiency virus type 1 (HIV-1) is the nuclear export of unspliced and partially spliced RNAs mediated by the Rev posttranscriptional activator and the Rev response element (RRE).

Results

Here, we introduce a novel method to explore the proteome associated with the nuclear HIV-1 RNAs. At the core of the method is the generation of cell lines harboring an integrated provirus carrying RNA binding sites for the MS2 bacteriophage protein. Flag-tagged MS2 is then used for affinity purification of the viral RNA. By this approach we found that the viral RNA is associated with the host nuclear matrix component MATR3 (Matrin 3) and that its modulation affected Rev activity. Knockdown of MATR3 suppressed Rev/RRE function in the export of unspliced HIV-1 RNAs. However, MATR3 was able to associate with Rev only through the presence of RRE-containing viral RNA.

Conclusions

In this work, we exploited a novel proteomic method to identify MATR3 as a cellular cofactor of Rev activity. MATR3 binds viral RNA and is required for the Rev/RRE mediated nuclear export of unspliced HIV-1 RNAs.

Matrin 3 is a co-factor for HIV-1 Rev in regulating post-transcriptional viral gene expression

Post-transcriptional regulation of HIV-1 gene expression is mediated by interactions between viral transcripts and viral/cellular proteins. For HIV-1, post-transcriptional nuclear control allows for the export of intron-containing RNAs which are normally retained in the nucleus. Specific signals on the viral RNAs, such as instability sequences (INS) and Rev responsive element (RRE), are binding sites for viral and cellular factors that serve to regulate RNA-export. The HIV-1 encoded viral Rev protein binds to the RRE found on unspliced and incompletely spliced viral RNAs. Binding by Rev directs the export of these RNAs from the nucleus to the cytoplasm. Previously, Rev co-factors have been found to include cellular factors such as CRM1, DDX3, PIMT and others. In this work, the nuclear matrix protein Matrin 3 is shown to bind Rev/RRE-containing viral RNA. This binding interaction stabilizes unspliced and partially spliced HIV-1 transcripts leading to increased cytoplasmic expression of these viral RNAs.

An essential role for trimethylguanosine RNA caps in Saccharomyces cerevisiae meiosis and their requirement for splicing of SAE3 and PCH2 meiotic pre-mRNAs

Tgs1 is the enzyme that converts m⁷G RNA caps to the 2,2,7-trimethylguanosine (TMG) caps characteristic of spliceosomal snRNAs. Fungi grow vegetatively without TMG caps, thereby raising the question of what cellular transactions, if any, are TMG cap-dependent. Here, we report that Saccharomyces cerevisiae Tgs1 methyltransferase activity is essential for meiosis. tgs1Δ cells are specifically defective in splicing PCH2 and SAE3 meiotic pre-mRNAs. The TMG requirement for SAE3 splicing is alleviated by two intron mutations: a UAUUAAC to UACUAAC change that restores a consensus branchpoint and disruption of a stem–loop encompassing the branchpoint. The TMG requirement for PCH2 splicing is alleviated by a CACUAAC to UACUAAC change restoring a consensus branchpoint and by shortening the PCH2 5′ exon. Placing the SAE3 and PCH2 introns within a HIS3 reporter confers Tgs1-dependent histidine prototrophy, signifying that the respective introns are portable determinants of TMG-dependent gene expression. Analysis of in vitro splicing in extracts of TGS1 versus tgs1Δ cells showed that SAE3 intron removal was enfeebled without TMG caps, whereas splicing of ACT1 was unaffected. Our findings illuminate a new mode of tunable splicing, a reliance on TMG caps for an essential developmental RNA transaction, and three genetically distinct meiotic splicing regulons in budding yeast.

Rev-ing up post-transcriptional HIV-1 RNA expression

The post-transcriptional export of spliced and unspliced HIV-1 (human immunodeficiency virus type 1) RNAs from the nucleus to the cytoplasm is a complex process. Part of the complexity arises from the fact that eukaryotic cells normally retain unspliced RNAs in the nucleus preventing their exit into the cytoplasm. HIV-1 has evolved a protein, Rev, that participates in the export of unspliced / partially spliced viral RNAs from the nucleus. It has been documented that several cellular factors cooperate in trans with Rev, and certain cis-RNA motifs / features are important for transcripts to be recognized by Rev and its co-factors. Here, the post-transcriptional activities of Rev are discussed in the context of a recent finding that an RNA cap methyltransferase contributes to the expression of unspliced / partially spliced HIV-1 transcripts.

Insights into cellular factors that regulate HIV-1 replication in human cells

Retroviruses integrate into the host cell's chromosome. Accordingly, many aspects of the life cycle of retroviruses like HIV-1 are intimately linked to the functions of cellular proteins and RNAs. In this review, we discuss in brief recent genomewide screens for the identification of cellular proteins that assist HIV-1 replication in human cells. We also review findings for other cellular moieties that help or restrict the viral life cycle.