HIV-2 genomic RNA accumulates in stress granules in the absence of active translation

RNA binding protein TIAR modulates HBV replication by tipping the balance of pgRNA translation

The pregenomic RNA (pgRNA) of hepatitis B virus (HBV) serves not only as a bicistronic message RNA to translate core protein (Cp) and DNA polymerase (Pol), but also as the template for reverse transcriptional replication of viral DNA upon packaging into nucleocapsid. Although it is well known that pgRNA translates much more Cp than Pol, the molecular mechanism underlying the regulation of Cp and Pol translation efficiency from pgRNA remains elusive. In this study, we systematically profiled HBV nucleocapsid- and pgRNA-associated cellular proteins by proteomic analysis and identified TIA-1-related protein (TIAR) as a novel cellular protein that binds pgRNA and promotes HBV DNA replication. Interestingly, loss- and gain-of-function genetic analyses showed that manipulation of TIAR expression did not alter the levels of HBV transcripts nor the secretion of HBsAg and HBeAg in human hepatoma cells supporting HBV replication. However, Ribo-seq and PRM-based mass spectrometry analyses demonstrated that TIAR increased the translation of Pol but decreased the translation of Cp from pgRNA. RNA immunoprecipitation (RIP) and pulldown assays further revealed that TIAR directly binds pgRNA at the 5' stem-loop (ε). Moreover, HBV replication or Cp expression induced the increased expression and redistribution of TIAR from the nucleus to the cytoplasm of hepatocytes. Our results thus imply that TIAR is a novel cellular factor that regulates HBV replication by binding to the 5' ε structure of pgRNA to tip the balance of Cp and Pol translation. Through induction of TIAR translocation from the nucleus to the cytoplasm, Cp indirectly regulates the Pol translation and balances Cp and Pol expression levels in infected hepatocytes to ensure efficient viral replication.

T-Cell Intracellular Antigen 1-Like Protein in Physiology and Pathology

T-cell intracellular antigen 1 (TIA1)-related/like (TIAR/TIAL1) protein is a multifunctional RNA-binding protein (RBP) involved in regulating many aspects of gene expression, independently or in combination with its paralog TIA1. TIAR was first described in 1992 by Paul Anderson’s lab in relation to the development of a cell death phenotype in immune system cells, as it possesses nucleolytic activity against cytotoxic lymphocyte target cells. Similar to TIA1, it is characterized by a subcellular nucleo-cytoplasmic localization and ubiquitous expression in the cells of different tissues of higher organisms. In this paper, we review the relevant structural and functional information available about TIAR from a triple perspective (molecular, cellular and pathophysiological), paying special attention to its expression and regulation in cellular events and processes linked to human pathophysiology.

Human Retrovirus Genomic RNA Packaging

Two non-covalently linked copies of the retrovirus genome are specifically recruited to the site of virus particle assembly and packaged into released particles. Retroviral RNA packaging requires RNA export of the unspliced genomic RNA from the nucleus, translocation of the genome to virus assembly sites, and specific interaction with Gag, the main viral structural protein. While some aspects of the RNA packaging process are understood, many others remain poorly understood. In this review, we provide an update on recent advancements in understanding the mechanism of RNA packaging for retroviruses that cause disease in humans, i.e., HIV-1, HIV-2, and HTLV-1, as well as advances in the understanding of the details of genomic RNA nuclear export, genome translocation to virus assembly sites, and genomic RNA dimerization.

Strategies for Success. Viral Infections and Membraneless Organelles

Regulation of RNA homeostasis or “RNAstasis” is a central step in eukaryotic gene expression. From transcription to decay, cellular messenger RNAs (mRNAs) associate with specific proteins in order to regulate their entire cycle, including mRNA localization, translation and degradation, among others. The best characterized of such RNA-protein complexes, today named membraneless organelles, are Stress Granules (SGs) and Processing Bodies (PBs) which are involved in RNA storage and RNA decay/storage, respectively. Given that SGs and PBs are generally associated with repression of gene expression, viruses have evolved different mechanisms to counteract their assembly or to use them in their favor to successfully replicate within the host environment. In this review we summarize the current knowledge about the viral regulation of SGs and PBs, which could be a potential novel target for the development of broad-spectrum antiviral therapies.

A Rev-CBP80-eIF4AI complex drives Gag synthesis from the HIV-1 unspliced mRNA

Gag synthesis from the full-length unspliced mRNA is critical for the production of the viral progeny during human immunodeficiency virus type-1 (HIV-1) replication. While most spliced mRNAs follow the canonical gene expression pathway in which the recruitment of the nuclear cap-binding complex (CBC) and the exon junction complex (EJC) largely stimulates the rates of nuclear export and translation, the unspliced mRNA relies on the viral protein Rev to reach the cytoplasm and recruit the host translational machinery. Here, we confirm that Rev ensures high levels of Gag synthesis by driving nuclear export and translation of the unspliced mRNA. These functions of Rev are supported by the CBC subunit CBP80, which binds Rev and the unspliced mRNA in the nucleus and the cytoplasm. We also demonstrate that Rev interacts with the DEAD-box RNA helicase eIF4AI, which translocates to the nucleus and cooperates with the viral protein to promote Gag synthesis. Finally, we show that the Rev/RRE axis is important for the assembly of a CBP80-eIF4AI complex onto the unspliced mRNA. Together, our results provide further evidence towards the understanding of the molecular mechanisms by which Rev drives Gag synthesis from the unspliced mRNA during HIV-1 replication.

HIV-1 requires Staufen1 to dissociate stress granules and to produce infectious viral particles

The human immunodeficiency virus type 1 (HIV-1) genomic RNA (vRNA) has two major fates during viral replication: to serve as the template for the major structural and enzymatic proteins, or to be encapsidated and packaged into assembling virions to serve as the genomic vRNA in budding viruses. The dynamic balance between vRNA translation and encapsidation is mediated by numerous host proteins, including Staufen1. During HIV-1 infection, HIV-1 recruits Staufen1 to assemble a distinct ribonucleoprotein complex promoting vRNA encapsidation and viral assembly. Staufen1 also rescues vRNA translation and gene expression during conditions of cellular stress. In this work, we utilized novel Staufen1^-/- gene-edited cells to further characterize the contribution of Staufen1 in HIV-1 replication. We observed a marked deficiency in the ability of HIV-1 to dissociate stress granules (SGs) in Staufen1-deficient cells and remarkably, the vRNA repositioned to SGs. These phenotypes were rescued by Staufen1 expression in trans or in cis, but not by a dsRBD-binding mutant, Staufen1F135A. The mistrafficking of the vRNA in these Staufen1^-/- cells was also accompanied by a dramatic decrease in viral production and infectivity. This work provides novel insight into the mechanisms by which HIV-1 uses Staufen1 to ensure optimal vRNA translation and trafficking, supporting an integral role for Staufen1 in the HIV-1 life cycle, positioning it as an attractive target for next-generation antiretroviral agents.

Battling for Ribosomes: Translational Control at the Forefront of the Antiviral Response

In the early stages of infection, gaining control of the cellular protein synthesis machinery including its ribosomes is the ultimate combat objective for a virus. To successfully replicate, viruses unequivocally need to usurp and redeploy this machinery for translation of their own mRNA. In response, the host triggers global shutdown of translation while paradoxically allowing swift synthesis of antiviral proteins as a strategy to limit collateral damage. This fundamental conflict at the level of translational control defines the outcome of infection. As part of this special issue on molecular mechanisms of early virus-host cell interactions, we review the current state of knowledge regarding translational control during viral infection with specific emphasis on protein kinase RNA-activated and mammalian target of rapamycin-mediated mechanisms. We also describe recent technological advances that will allow unprecedented insight into how viruses and host cells battle for ribosomes.

Cytosine methylation by DNMT2 facilitates stability and survival of HIV-1 RNA in the host cell during infection

The enigmatic methyltransferase, DNMT2 (DNA methyltransferase 2), structurally resembles a DNA methyltransferase, but has been shown to be a tRNA methyltransferase targeting cytosine within a specific CpG in different tRNA molecules. We had previously shown that, during environmental stress conditions, DNMT2 is re-localized from the nucleus to the cytoplasmic stress granules (SGs) and is associated with RNA-processing proteins. In the present study, we show that DNMT2 binds and methylates various mRNA species in a sequence-independent manner and gets re-localized to SGs in a phosphorylation-dependent manner. Importantly, our results indicate that HIV-1 enhances its survivability in the host cell by utilizing this RNA methylation capability of DNMT2 to increase the stability of its own genome. Upon infection, DNMT2 re-localizes from the nucleus to the SGs and methylates HIV-1 RNA. This DNMT2-dependent methylation provided post-transcriptional stability to the HIV-1 RNA. Furthermore, DNMT2 overexpression increased the HIV-1 viral titre. This would suggest that HIV hijacks the RNA-processing machinery within the SGs to ensure its own survival in the host cell. Thus, our findings provide for a novel mechanism by which virus tries to modulate the host cell machinery to its own advantage.

Characterization of the interaction between the HIV-1 Gag structural polyprotein and the cellular ribosomal protein L7 and its implication in viral nucleic acid remodeling

Background

In HIV-1 infected cells, the integrated viral DNA is transcribed by the host cell machinery to generate the full length HIV-1 RNA (FL RNA) that serves as mRNA encoding for the Gag and GagPol precursors. Virion formation is orchestrated by Gag, and the current view is that a specific interaction between newly made Gag molecules and FL RNA initiates the process. This in turn would cause FL RNA dimerization by the NC domain of Gag (GagNC). However the RNA chaperoning activity of unprocessed Gag is low as compared to the mature NC protein. This prompted us to search for GagNC co-factors.

Results

Here we report that RPL7, a major ribosomal protein involved in translation regulation, is a partner of Gag via its interaction with the NC domain. This interaction is mediated by the NC zinc fingers and the N- and C-termini of RPL7, respectively, but seems independent of RNA binding, Gag oligomerization and its interaction with the plasma membrane. Interestingly, RPL7 is shown for the first time to exhibit a potent DNA/RNA chaperone activity higher than that of Gag. In addition, Gag and RPL7 can function in concert to drive rapid nucleic acid hybridization.

Conclusions

Our results show that GagNC interacts with the ribosomal protein RPL7 endowed with nucleic acid chaperone activity, favoring the notion that RPL7 could be a Gag helper chaperoning factor possibly contributing to the start of Gag assembly.

Electronic supplementary material

The online version of this article (doi:10.1186/s12977-016-0287-4) contains supplementary material, which is available to authorized users.

Who Regulates Whom? An Overview of RNA Granules and Viral Infections

After viral infection, host cells respond by mounting an anti-viral stress response in order to create a hostile atmosphere for viral replication, leading to the shut-off of mRNA translation (protein synthesis) and the assembly of RNA granules. Two of these RNA granules have been well characterized in yeast and mammalian cells, stress granules (SGs), which are translationally silent sites of RNA triage and processing bodies (PBs), which are involved in mRNA degradation. This review discusses the role of these RNA granules in the evasion of anti-viral stress responses through virus-induced remodeling of cellular ribonucleoproteins (RNPs).

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.