Bunyamwera orthobunyavirus S-segment untranslated regions mediate poly(A) tail-independent translation

SARS-CoV-2 nsp1 mediates broad inhibition of translation in mammals

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) non-structural protein 1 (nsp1) promotes innate immune evasion by inhibiting host translation in human cells. However, the role of nsp1 in other host species remains elusive, especially in bats-natural reservoirs of sarbecoviruses with a markedly different innate immune system than humans. We reveal that nsp1 potently inhibits translation in Rhinolophus lepidus bat cells, which belong to the same genus as known sarbecovirus reservoir hosts. We determined a cryoelectron microscopy structure of nsp1 bound to the R. lepidus 40S ribosomal subunit, showing that it blocks the mRNA entry channel by targeting a highly conserved site among mammals. Accordingly, we found that nsp1 blocked protein translation in mammalian cells from several species, underscoring its broadly inhibitory activity and conserved role in numerous SARS-CoV-2 hosts. Our findings illuminate the arms race between coronaviruses and mammalian host immunity, providing a foundation for understanding the determinants of viral maintenance in bat hosts and spillover.

SARS-CoV-2 nsp1 mediates broad inhibition of translation in mammals

SARS-CoV-2 nonstructural protein 1 (nsp1) promotes innate immune evasion by inhibiting host translation in human cells. However, the role of nsp1 in other host species remains elusive, especially in bats which are natural reservoirs of sarbecoviruses and possess a markedly different innate immune system than humans. Here, we reveal that SARS-CoV-2 nsp1 potently inhibits translation in bat cells from Rhinolophus lepidus, belonging to the same genus as known sarbecovirus reservoirs hosts. We determined a cryo-electron microscopy structure of SARS-CoV-2 nsp1 bound to the Rhinolophus lepidus 40S ribosome and show that it blocks the mRNA entry channel via targeting a highly conserved site among mammals. Accordingly, we found that nsp1 blocked protein translation in mammalian cell lines from several species, underscoring its broadly inhibitory activity and conserved role in numerous SARS-CoV-2 hosts. Our findings illuminate the arms race between coronaviruses and mammalian host immunity (including bats), providing a foundation for understanding the determinants of viral maintenance in bat hosts and spillovers.

Innate Immune Response Against Batai Virus, Bunyamwera Virus, and Their Reassortants

Orthobunyaviruses (OBVs) represent a diverse group of RNA viruses, encompassing a progressively increasing number of arboviruses that cause disease in both humans and livestock. Yet, studies investigating these viruses remain scarce despite the critical importance of such knowledge for assessing their zoonotic potential. In this study, we conducted an evaluation of the early immune response against the understudied Batai virus (BATV), as well as the influence of reassortment with the Bunyamwera virus (BUNV) on this response. Using RNA sequencing of infected murine bone marrow-derived dendritic cells, complemented by qPCR assays, we assessed the innate immune response at the transcriptome level. Additionally, we extended the qPCR analysis by including human THP-1-derived dendritic cells and ovine SFT-R cells to identify differences across species. Our results provide the first evidence that BATV elicits a strong innate immune response compared to BUNV, which largely evades early detection. Reassortants exhibited intermediate phenotypes, although unique changes in the early immune response were found as well. These findings provide a starting point for a better understanding of the immune response to BATV. Furthermore, they raise the question of whether reassortment induces changes in the innate immune response that might contribute to the differences in pathogenicity between reassortant OBVs and their parental generations.

Rational design of an artificial tethered enzyme for non-templated post-transcriptional mRNA polyadenylation by the second generation of the C3P3 system

We have previously introduced the first generation of C3P3, an artificial system that allows the autonomous in-vivo production of mRNA with m7GpppN-cap. While C3P3-G1 synthesized much larger amounts of capped mRNA in human cells than conventional nuclear expression systems, it produced a proportionately much smaller amount of the corresponding proteins, indicating a clear defect of mRNA translatability. A possible mechanism for this poor translatability could be the rudimentary polyadenylation of the mRNA produced by the C3P3-G1 system. We therefore sought to develop the C3P3-G2 system using an artificial enzyme to post-transcriptionally lengthen the poly(A) tail. This system is based on the mutant mouse poly(A) polymerase alpha fused at its N terminus with an N peptide from the λ virus, which binds to BoxBr sequences placed in the 3'UTR region of the mRNA of interest. The resulting system selectively brings mPAPαm7 to the target mRNA to elongate its poly(A)-tail to a length of few hundred adenosine. Such elongation of the poly(A) tail leads to an increase in protein expression levels of about 2.5-3 times in cultured human cells compared to the C3P3-G1 system. Finally, the coding sequence of the tethered mutant poly(A) polymerase can be efficiently fused to that of the C3P3-G1 enzyme via an F2A sequence, thus constituting the single-ORF C3P3-G2 enzyme. These technical developments constitute an important milestone in improving the performance of the C3P3 system, paving the way for its applications in bioproduction and non-viral human gene therapy.

Poly(A) tale: From A to A; RNA polyadenylation in prokaryotes and eukaryotes

Most eukaryotic mRNAs and different non-coding RNAs undergo a form of 3' end processing known as polyadenylation. Polyadenylation machinery is present in almost all organisms except few species. In bacteria, the machinery has evolved from PNPase, which adds heteropolymeric tails, to a poly(A)-specific polymerase. Differently, a complex machinery for accurate polyadenylation and several non-canonical poly(A) polymerases are developed in eukaryotes. The role of poly(A) tail has also evolved from serving as a degradative signal to a stabilizing modification that also regulates translation. In this review, we discuss poly(A) tail emergence in prokaryotes and its development into a stable, yet dynamic feature at the 3' end of mRNAs in eukaryotes. We also describe how appearance of novel poly(A) polymerases gives cells flexibility to shape poly(A) tail. We explain how poly(A) tail dynamics help regulate cognate RNA metabolism in a context-dependent manner, such as during oocyte maturation. Finally, we describe specific mRNAs in metazoans that bear stem-loops instead of poly(A) tails. We conclude with how recent discoveries about poly(A) tail can be applied to mRNA technology. This article is categorized under: RNA Evolution and Genomics > RNA and Ribonucleoprotein Evolution RNA Processing > 3' End Processing RNA Turnover and Surveillance > Regulation of RNA Stability.

Mechanisms and consequences of mRNA destabilization during viral infections

During viral infection there is dynamic interplay between the virus and the host to regulate gene expression. In many cases, the host induces the expression of antiviral genes to combat infection, while the virus uses "host shut-off" systems to better compete for cellular resources and to limit the induction of the host antiviral response. Viral mechanisms for host shut-off involve targeting translation, altering host RNA processing, and/or inducing the degradation of host mRNAs. In this review, we discuss the diverse mechanisms viruses use to degrade host mRNAs. In addition, the widespread degradation of host mRNAs can have common consequences including the accumulation of RNA binding proteins in the nucleus, which leads to altered RNA processing, mRNA export, and changes to transcription.

Translation-A tug of war during viral infection

Viral reproduction is contingent on viral protein synthesis that relies on the host ribosomes. As such, viruses have evolved remarkable strategies to hijack the host translational apparatus in order to favor viral protein production and to interfere with cellular innate defenses. Here, we describe the approaches viruses use to exploit the translation machinery, focusing on commonalities across diverse viral families, and discuss the functional relevance of this process. We illustrate the complementary strategies host cells utilize to block viral protein production and consider how cells ensure an efficient antiviral response that relies on translation during this tug of war over the ribosome. Finally, we highlight potential roles mRNA modifications and ribosome quality control play in translational regulation and innate immunity. We address these topics in the context of the COVID-19 pandemic and focus on the gaps in our current knowledge of these mechanisms, specifically in viruses with pandemic potential.

A little less aggregation a little more replication: Viral manipulation of stress granules

Recent exciting studies have uncovered how membrane-less organelles, also known as biocondensates, are providing cells with rapid response pathways, allowing them to re-organize their cellular contents and adapt to stressful conditions. Their assembly is driven by the phase separation of their RNAs and intrinsically disordered protein components into condensed foci. Among these, stress granules (SGs) are dynamic cytoplasmic biocondensates that form in response to many stresses, including activation of the integrated stress response or viral infections. SGs sit at the crossroads between antiviral signaling and translation because they concentrate signaling proteins and components of the innate immune response, in addition to translation machinery and stalled mRNAs. Consequently, they have been proposed to contribute to antiviral activities, and therefore are targeted by viral countermeasures. Equally, SGs components can be commandeered by viruses for their own efficient replication. Phase separation processes are an important component of the viral life cycle, for example, driving the assembly of replication factories or inclusion bodies. Therefore, in this review, we will outline the recent understanding of this complex interplay and tug of war between viruses, SGs, and their components. This article is categorized under: RNA in Disease and Development > RNA in Disease Translation > Regulation RNA Interactions with Proteins and Other Molecules > RNA-Protein Complexes.

The SARS-CoV-2 targeted human RNA binding proteins network biology to investigate COVID-19 associated manifestations

The global coronavirus disease 2019 (COVID-19) pandemic caused by the SARS-CoV-2 virus has had unprecedented social and economic ramifications. Identifying targets for drug repurposing could be an effective means to present new and fast treatments. Furthermore, the risk of morbidity and mortality from COVID-19 goes up when there are coexisting medical conditions, however, the underlying mechanisms remain unclear. In the current study, we have adopted a network-based systems biology approach to investigate the RNA binding proteins (RBPs)-based molecular interplay between COVID-19, various human cancers, and neurological disorders. The network based on RBPs commonly involved in the three disease conditions consisted of nine RBPs connecting 10 different cancer types, 22 brain disorders, and COVID-19 infection, ultimately hinting at the comorbidities and complexity of COVID-19. Further, we underscored five miRNAs with reported antiviral properties that target all of the nine shared RBPs and are thus therapeutically valuable. As a strategy to improve the clinical conditions in comorbidities associated with COVID-19, we propose perturbing the shared RBPs by drug repurposing. The network-based analysis presented hereby contributes to a better knowledge of the molecular underpinnings of the comorbidities associated with COVID-19.

A small stem-loop-forming region within the 3'-UTR of a non-polyadenylated LCMV mRNA promotes translation

Mammalian arenavirus (mammarenavirus) mRNAs are characterized by 5′-capped and 3′-nonpolyadenylated untranslated regions (UTRs). We previously reported that the nonpolyadenylated 3′-UTR of viral mRNA (vmRNA), which is derived from the noncoding intergenic region (IGR), regulates viral protein levels at the posttranscriptional level. This finding provided the basis for the development of novel live-attenuated vaccines (LAVs) against human pathogenic mammarenaviruses. Detailed information about the roles of specific vmRNA 3′-UTR sequences in controlling translation efficiency will help in understanding the mechanism underlying attenuation by IGR manipulations. Here, we characterize the roles of cis-acting mRNA regulatory sequences of a prototypic mammarenavirus, lymphocytic choriomeningitis virus (LCMV), in modulating translational efficiency. Using in vitro transcribed RNA mimics encoding a reporter gene, we demonstrate that the 3′-UTR of nucleoprotein (NP) mRNA without a poly(A) tail promotes translation in a poly(A)-binding protein-independent manner. Comparison with the 3′-UTR of glycoprotein precursor mRNA, which is translated less efficiently, revealed that a 10-nucleotide sequence proximal to the NP open reading frame is essential for promoting translation. Modification of this 10-nucleotide sequence also impacted reporter gene expression in recombinant LCMV. Our findings will enable rational design of the 10-nucleotide sequence to further improve our mammarenavirus LAV candidates and to develop a novel LCMV vector capable of controlling foreign gene expression.

Localization and Functional Roles of Components of the Translation Apparatus in the Eukaryotic Cell Nucleus

Components of the translation apparatus, including ribosomal proteins, have been found in cell nuclei in various organisms. Components of the translation apparatus are involved in various nuclear processes, particularly those associated with genome integrity control and the nuclear stages of gene expression, such as transcription, mRNA processing, and mRNA export. Components of the translation apparatus control intranuclear trafficking; the nuclear import and export of RNA and proteins; and regulate the activity, stability, and functional recruitment of nuclear proteins. The nuclear translocation of these components is often involved in the cell response to stimulation and stress, in addition to playing critical roles in oncogenesis and viral infection. Many components of the translation apparatus are moonlighting proteins, involved in integral cell stress response and coupling of gene expression subprocesses. Thus, this phenomenon represents a significant interest for both basic and applied molecular biology. Here, we provide an overview of the current data regarding the molecular functions of translation factors and ribosomal proteins in the cell nucleus.

Nervous Necrosis Virus Coat Protein Mediates Host Translation Shutoff through Nuclear Translocalization and Degradation of Polyadenylate Binding Protein

Nervous necrosis virus (NNV) belongs to the Betanodavirus genus of the Nodaviridae family and is the main cause of viral nervous necrosis disease in marine fish larvae and juveniles worldwide. The NNV virion contains two positive-sense, single-stranded RNA genomes, which encode RNA-dependent RNA polymerase, coat protein, and B2 protein. Interestingly, NNV infection can shut off host translation in orange-spotted grouper (Epinephelus coioides) brain cells; however, the detailed mechanisms of this action remain unknown. In this study, we discovered that the host translation factor, polyadenylate binding protein (PABP), is a key target during NNV takeover of host translation machinery. Additionally, ectopic expression of NNV coat protein is sufficient to trigger nuclear translocalization and degradation of PABP, followed by translation shutoff. A direct interaction between NNV coat protein and PABP was demonstrated, and this binding requires the NNV coat protein N-terminal shell domain and PABP proline-rich linker region. Notably, we also showed that degradation of PABP during later stages of infection is mediated by the ubiquitin-proteasome pathway. Thus, our study reveals that the NNV coat protein hijacks host PABP, causing its relocalization to the nucleus and promoting its degradation to stimulate host translation shutoff. IMPORTANCE Globally, more than 200 species of aquacultured and wild marine fish are susceptible to NNV infection. Devastating outbreaks of this virus have been responsible for massive economic damage in the aquaculture industry, but the molecular mechanisms by which NNV affects its host remain largely unclear. In this study, we show that NNV hijacks translation in host brain cells, with the viral coat protein binding to host PABP to promote its nuclear translocalization and degradation. This previously unknown mechanism of NNV-induced host translation shutoff greatly enhances the understanding of NNV pathogenesis and provides useful insights and novel tools for development of NNV treatments, such as the use of orange-spotted grouper brain cells as an in vitro model system.

Mutagenic Analysis of Hazara Nairovirus Nontranslated Regions during Single- and Multistep Growth Identifies both Attenuating and Functionally Critical Sequences for Virus Replication

Hazara nairovirus (HAZV) is a member of the family Nairoviridae in the order Bunyavirales and closely related to Crimean-Congo hemorrhagic fever virus, which is responsible for severe and fatal human disease. The HAZV genome comprises three segments of negative-sense RNA, named S, M, and L, with nontranslated regions (NTRs) flanking a single open reading frame. NTR sequences regulate RNA synthesis and, by analogy with other segmented negative-sense RNA viruses, may direct activities such as virus assembly and innate immune modulation. The terminal-proximal nucleotides of 3' and 5' NTRs exhibit extensive terminal complementarity; the first 11 nucleotides are strictly conserved and form promoter element 1 (PE1), with adjacent segment-specific nucleotides forming PE2. To explore the functionality of NTR nucleotides within the context of the nairovirus multiplication cycle, we designed infectious HAZV mutants bearing successive deletions throughout both S segment NTRs. Fitness of rescued viruses was assessed in single-step and multistep growth, which revealed that the 3' NTR was highly tolerant to change, whereas several deletions of centrally located nucleotides in the 5' NTR led to significantly reduced growth, indicative of functional disruption. Deletions that encroached upon PE1 and PE2 ablated virus growth and identified additional adjacent nucleotides critical for viability. Mutational analysis of PE2 suggest that its signaling ability relies solely on interterminal base pairing and is an independent cis-acting signaling module. This study represents the first mutagenic analysis of nairoviral NTRs in the context of the infectious cycle, and the mechanistic implications of our findings for nairovirus RNA synthesis are discussed.IMPORTANCE Nairoviruses are a group of RNA viruses that include many serious pathogens of humans and animals, including one of the most serious human pathogens in existence, Crimean-Congo hemorrhagic fever virus. The ability of nairoviruses to multiply and cause disease is controlled in major part by nucleotides that flank the 3' and 5' ends of nairoviral genes, called nontranslated regions (NTRs). NTR nucleotides interact with other virus components to perform critical steps of the virus multiplication cycle, such as mRNA transcription and RNA replication, with other roles being likely. To better understand how NTRs work, we performed the first comprehensive investigation of the importance of NTR nucleotides in the context of the entire nairovirus replication cycle. We identified both dispensable and critical NTR nucleotides, as well as highlighting the importance of 3' and 5' NTR interactions in virus growth, thus providing the first functional map of the nairovirus NTRs.

Tick-borne encephalitis virus inhibits rRNA synthesis and host protein production in human cells of neural origin

Tick-borne encephalitis virus (TBEV), a member of the genus Flavivirus (Flaviviridae), is a causative agent of a severe neuroinfection. Recently, several flaviviruses have been shown to interact with host protein synthesis. In order to determine whether TBEV interacts with this host process in its natural target cells, we analysed de novo protein synthesis in a human cell line derived from cerebellar medulloblastoma (DAOY HTB-186). We observed a significant decrease in the rate of host protein synthesis, including the housekeeping genes HPRT1 and GAPDH and the known interferon-stimulated gene viperin. In addition, TBEV infection resulted in a specific decrease of RNA polymerase I (POLR1) transcripts, 18S and 28S rRNAs and their precursor, 45-47S pre-rRNA, but had no effect on the POLR3 transcribed 5S rRNA levels. To our knowledge, this is the first report of flavivirus-induced decrease of specifically POLR1 rRNA transcripts accompanied by host translational shut-off.

Tick-borne encephalitis virus (TBEV) is a causative agent of a severe human neuroinfection that threatens Europe and Asia. Little is known about the interaction of this neurotropic virus with neural cells, even though this may be important to better understand why or how TBEV can cause high pathogenicity in humans, especially following neural cell infection. Here, we showed that TBEV induced host translational shut-off in cells of neural origin. In addition, TBEV interfered also with the expression of host ribosomal RNAs. Interestingly, the transcriptional shut-off was documented for rRNA species transcribed by RNA polymerase I (18S rRNA, 28S rRNA and their precursor 45-47S pre-rRNA), but not for RNA polymerase III rRNA transcripts (5S rRNA). Artificial inhibition of host translation using cycloheximide resulted in the decrease of all rRNA species. Based on these data, TBEV seems to specifically target transcription of RNA polymerase I. These new findings further increase our understanding of TBEV interactions with a key target cell type.

Rotavirus Infection Alters Splicing of the Stress-Related Transcription Factor XBP1

XBP1 is a stress-regulated transcription factor also involved in mammalian host defenses and innate immune response. Our investigation of XBP1 RNA splicing during rotavirus infection revealed that an additional XBP1 RNA (XBP1es) that corresponded to exon skipping in the XBP1 pre-RNA is induced depending on the rotavirus strain used. We show that the translation product of XBP1es (XBP1es) has trans-activation properties similar to those of XBP1 on ER stress response element (ERSE) containing promoters. Using monoreassortant between ES⁺ ("skipping") and ES^- ("nonskipping") strains of rotavirus, we show that gene 7 encoding the viral translation enhancer NSP3 is involved in this phenomenon and that exon skipping parallels the nuclear relocalization of cytoplasmic PABP. We further show, using recombinant rotaviruses carrying chimeric gene 7, that the ES⁺ phenotype is linked to the eIF4G-binding domain of NSP3. Because the XBP1 transcription factor is involved in stress and immunological responses, our results suggest an alternative way to activate XBP1 upon viral infection or nuclear localization of PABP.IMPORTANCE Rotavirus is one of the most important pathogens causing severe gastroenteritis in young children worldwide. Here we show that infection with several rotavirus strains induces an alternative splicing of the RNA encoding the stressed-induced transcription factor XBP1. The genetic determinant of XBP1 splicing is the viral RNA translation enhancer NSP3. Since XBP1 is involved in cellular stress and immune responses and since the XBP1 protein made from the alternatively spliced RNA is an active transcription factor, our observations raise the question of whether alternative splicing is a cellular response to rotavirus infection.

Cytoplasmic Relocalization and Colocalization with Viroplasms of Host Cell Proteins, and Their Role in Rotavirus Infection

Rotavirus replicates in the cytoplasm of infected cells in unique virus-induced cytoplasmic inclusion bodies called viroplasms (VMs), which are nucleated by two essential viral nonstructural proteins, NSP2 and NSP5. However, the precise composition of the VM, the intracellular localization of host proteins during virus infection, and their association with VMs or role in rotavirus growth remained largely unexplored. Mass spectrometry analyses revealed the presence of several host heterogeneous nuclear ribonucleoproteins (hnRNPs), AU-rich element-binding proteins (ARE-BPs), and cytoplasmic proteins from uninfected MA104 cell extracts in the pulldown (PD) complexes of the purified viroplasmic proteins NSP2 and NSP5. Immunoblot analyses of PD complexes from RNase-treated and untreated cell extracts, analyses of coimmunoprecipitation complexes using RNase-treated infected cell lysates, and direct binding assays using purified recombinant proteins further demonstrated that the interactions of the majority of the hnRNPs and ARE-BPs with viroplasmic proteins are RNA independent. Time course immunoblot analysis of the nuclear and cytoplasmic fractions from rotavirus-infected and mock-infected cells and immunofluorescence confocal microscopy analyses of virus-infected cells revealed a surprising sequestration of the majority of the relocalized host proteins in viroplasms. Analyses of ectopic overexpression and small interfering RNA (siRNA)-mediated downregulation of expression revealed that host proteins either promote or inhibit viral protein expression and progeny virus production in virus-infected cells. This study demonstrates that rotavirus induces the cytoplasmic relocalization and sequestration of a large number of nuclear and cytoplasmic proteins in viroplasms, subverting essential cellular processes in both compartments to promote rapid virus growth, and reveals that the composition of rotavirus viroplasms is much more complex than is currently understood.IMPORTANCE Rotavirus replicates exclusively in the cytoplasm. Knowledge on the relocalization of nuclear proteins to the cytoplasm or the role(s) of host proteins in rotavirus infection is very limited. In this study, it is demonstrated that rotavirus infection induces the cytoplasmic relocalization of a large number of nuclear RNA-binding proteins (hnRNPs and AU-rich element-binding proteins). Except for a few, most nuclear hnRNPs and ARE-BPs, nuclear transport proteins, and some cytoplasmic proteins directly interact with the viroplasmic proteins NSP2 and NSP5 in an RNA-independent manner and become sequestered in the viroplasms of infected cells. The host proteins differentially affected viral gene expression and virus growth. This study demonstrates that rotavirus induces the relocalization and sequestration of a large number of host proteins in viroplasms, affecting host processes in both compartments and generating conditions conducive for virus growth in the cytoplasm of infected cells.

Bioinformatics and Translation Elongation

Codon usage depends on mutation bias, tRNA-mediated selection, and the need for high efficiency and accuracy in translation. One codon in a synonymous codon family is often strongly over-used, especially in highly expressed genes, which often leads to a high dN/dS ratio because dS is very small. Many different codon usage indices have been proposed to measure codon usage and codon adaptation. Sense codon could be misread by release factors and stop codons misread by tRNAs, which also contribute to codon usage in rare cases. This chapter outlines the conceptual framework on codon evolution, illustrates codon-specific and gene-specific codon usage indices, and presents their applications. A new index for codon adaptation that accounts for background mutation bias (Index of Translation Elongation) is presented and contrasted with codon adaptation index (CAI) which does not consider background mutation bias. They are used to re-analyze data from a recent paper claiming that translation elongation efficiency matters little in protein production. The reanalysis disproves the claim.

Bunyaviridae RdRps: structure, motifs, and RNA synthesis machinery

Bunyaviridae family is the largest and most diverse family of RNA viruses. It has more than 350 members divided into five genera: Orthobunyavirus, Phlebovirus, Nairovirus, Hantavirus, and Tospovirus. They are present in the five continents, causing recurrent epidemics, epizootics, and considerable agricultural loss. The genome of bunyaviruses is divided into three segments of negative single-stranded RNA according to their relative size: L (Large), M (Medium) and S (Small) segment. Bunyaviridae RNA-dependent RNA polymerase (RdRp) is encoded by the L segment, and is in charge of the replication and transcription of the viral RNA in the cytoplasm of the infected cell. Viral RdRps share a characteristic right hand-like structure with three subdomains: finger, palm, and thumb subdomains that define the formation of the catalytic cavity. In addition to the N-terminal endonuclease domain, eight conserved motifs (A-H) have been identified in the RdRp of Bunyaviridae. In this review, we have summarized the recent insights from the structural and functional studies of RdRp to understand the roles of different motifs shared by RdRps, the mechanism of viral RNA replication, genome segment packaging by the nucleoprotein, cap-snatching, mRNA transcription, and other RNA mechanisms of bunyaviruses.

Poly(A)-binding proteins and mRNA localization: who rules the roost?

RNA-binding proteins are often multifunctional, interact with a variety of protein partners and display complex localizations within cells. Mammalian cytoplasmic poly(A)-binding proteins (PABPs) are multifunctional RNA-binding proteins that regulate multiple aspects of mRNA translation and stability. Although predominantly diffusely cytoplasmic at steady state, they shuttle through the nucleus and can be localized to a variety of cytoplasmic foci, including those associated with mRNA storage and localized translation. Intriguingly, PABP sub-cellular distribution can alter dramatically in response to cellular stress or viral infection, becoming predominantly nuclear and/or being enriched in induced cytoplasmic foci. However, relatively little is known about the mechanisms that govern this distribution/relocalization and in many cases PABP functions within specific sites remain unclear. Here we discuss the emerging evidence with respect to these questions in mammals.

A Cap-to-Tail Guide to mRNA Translation Strategies in Virus-Infected Cells

Although viruses require cellular functions to replicate, their absolute dependence upon the host translation machinery to produce polypeptides indispensable for their reproduction is most conspicuous. Despite their incredible diversity, the mRNAs produced by all viruses must engage cellular ribosomes. This has proven to be anything but a passive process and has revealed a remarkable array of tactics for rapidly subverting control over and dominating cellular regulatory pathways that influence translation initiation, elongation, and termination. Besides enforcing viral mRNA translation, these processes profoundly impact host cell-intrinsic immune defenses at the ready to deny foreign mRNA access to ribosomes and block protein synthesis. Finally, genome size constraints have driven the evolution of resourceful strategies for maximizing viral coding capacity. Here, we review the amazing strategies that work to regulate translation in virus-infected cells, highlighting both virus-specific tactics and the tremendous insight they provide into fundamental translational control mechanisms in health and disease.

Rift Valley fever virus NSs protein functions and the similarity to other bunyavirus NSs proteins

Rift Valley fever is a mosquito-borne zoonotic disease that affects both ruminants and humans. The nonstructural (NS) protein, which is a major virulence factor for Rift Valley fever virus (RVFV), is encoded on the S-segment. Through the cullin 1-Skp1-Fbox E3 ligase complex, the NSs protein promotes the degradation of at least two host proteins, the TFIIH p62 and the PKR proteins. NSs protein bridges the Fbox protein with subsequent substrates, and facilitates the transfer of ubiquitin. The SAP30-YY1 complex also bridges the NSs protein with chromatin DNA, affecting cohesion and segregation of chromatin DNA as well as the activation of interferon-β promoter. The presence of NSs filaments in the nucleus induces DNA damage responses and causes cell-cycle arrest, p53 activation, and apoptosis. Despite the fact that NSs proteins have poor amino acid similarity among bunyaviruses, the strategy utilized to hijack host cells are similar. This review will provide and summarize an update of recent findings pertaining to the biological functions of the NSs protein of RVFV as well as the differences from those of other bunyaviruses.

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).

Elucidation of RNA binding regions of gonadotropin-regulated testicular RNA helicase (GRTH/DDX25) to transcripts of a chromatin remodeling protein essential for spermatogenesis

BACKGROUND

Gonadotropin-regulated testicular RNA helicase (GRTH) is a testis-specific member of the DEAD-box family of RNA helicases present in Leydig and germ cells. It is a transport protein of mRNAs from nucleus to cytoplasmic sites and is essential for posttranscriptional regulation and completion of spermatogenesis. Transition protein 2 (Tp2), which associates with GRTH and is required for spermatid elongation, failed to express in GRTH null mice with impaired mRNA nuclear export. The present study determines GRTH binding motifs/regions that associate with Tp2 mRNA transcripts.

MATERIALS AND METHODS

RNA-protein interaction was analyzed using biotin-labeled electrophoretic mobility gel shift assays (EMSA). 3'-biotin-labeled RNA (Tp2) was incubated with mGRTH protein (full length/sequential deletion of specific and conserved RNA helicase motifs of GRTH) expressed from in vitro TNT coupled reticulocyte lysate system. Binding specificity was further elucidated by mutagenesis and antibody supershift analysis.

RESULTS

RNA-EMSA revealed that the 3' UTR of Tp2 RNA (127 nt from TGA) was retarded in presence of full length GRTH. Nucleotide sequences downstream of TGA of the Tp2 transcript (1-47 and 78-127 nt) are important for binding to GRTH. Sequential deletions/point mutations in GRTH revealed region(s) of conserved binding motifs of RNA helicases (Ia and V) essential for GRTH binding to Tp2 mRNA.

CONCLUSIONS

Our studies provide insights into the association of Tp2 expression via binding to the conserved RNA binding motifs of GRTH protein and the basis for understanding GRTH in the regulation of the genes essential for germ cell elongation and completion of spermatogenesis.

General Molecular Strategy for Development of Arenavirus Live-Attenuated Vaccines

Hemorrhagic fever arenaviruses (HFA) pose important public health problems in regions where they are endemic. Thus, Lassa virus (LASV) infects several hundred thousand individuals yearly in West Africa, causing a large number of Lassa fever cases associated with high morbidity and mortality. Concerns about human-pathogenic arenaviruses are exacerbated because of the lack of FDA-licensed arenavirus vaccines and because current antiarenaviral therapy is limited to an off-label use of ribavirin that is only partially effective. The Mopeia virus (MOPV)/LASV reassortant (ML29) is a LASV candidate live-attenuated vaccine (LAV) that has shown promising results in animal models. Nevertheless, the mechanism of ML29 attenuation remains unknown, which raises concerns about the phenotypic stability of ML29 in response to additional mutations. Development of LAVs based on well-defined molecular mechanisms of attenuation will represent a major step in combatting HFA. We used the prototypic arenavirus lymphocytic choriomeningitis virus (LCMV) to develop a general molecular strategy for arenavirus attenuation. Our approach involved replacement of the noncoding intergenic region (IGR) of the L genome segment with the IGR of the S genome segment to generate a recombinant LCMV, rLCMV(IGR/S-S), that was highly attenuated in vivo but induced protection against a lethal challenge with wild-type LCMV. Attenuation of rLCMV(IGR/S-S) was associated with a stable reorganization of the control of viral gene expression. This strategy can facilitate the rapid development of LAVs with the antigenic composition of the parental HFA and a mechanism of attenuation that minimizes concerns about increased virulence that could be caused by genetic changes in the LAV.

IMPORTANCE

Hemorrhagic fever arenaviruses (HFA) cause high morbidity and mortality, and pose important public health problems in the regions where they are endemic. Implementation of live-attenuated vaccines (LAV) will represent a major step in combatting HFA. Here we have used the prototypic arenavirus LCMV to document a general molecular strategy for arenavirus attenuation that can facilitate the rapid development of safe and effective, as well as stable, LAV to combat HFA.

Orthobunyaviruses and innate immunity induction: alieNSs vs. PredatoRRs

The taxonomic group of Orthobunyaviruses is gaining increased attention, as several emerging members are causing devastating illnesses among humans and livestock. These viruses are transmitted to mammals by arthropods (mostly mosquitoes) during the blood meal. The nature of their genomic RNA predisposes orthobunyaviruses for eliciting a strong innate immune response mediated by pathogen recognition receptors (PRRs), especially the cytoplasmic RIG-I. However, the PRR responses are in fact disabled by the viral non-structural protein NSs. NSs imposes a strong block of cellular gene expression by inhibiting elongating RNA polymerase II. In this review, we will give an overview on the current state of knowledge regarding the interactions between orthobunyaviruses, the PRR axis, and NSs.

Evolution of the Bunyamwera virus polymerase to accommodate deletions within genomic untranslated region sequences

The untranslated regions (UTR) present at the ends of bunyavirus genome segments are required for essential steps in the virus life cycle and provide signals for encapsidation by nucleocapsid protein and the promoters for RNA transcription and replication as well as for mRNA transcription termination. For the prototype bunyavirus, Bunyamwera virus (BUNV), only the terminal 11 nucleotides (nt) of the segments are identical. Thereafter, the UTRs are highly variable both in length and in sequence. Furthermore, apart from the conserved termini, the UTRs of different viruses are highly variable. We previously generated recombinant BUNV carrying the minimal UTRs on all three segments that were attenuated for growth in cell culture. Following serial passage of these viruses, the viruses acquired increased fitness, and amino acid changes were observed to accumulate in the viral polymerase (L protein) of most mutant viruses, with the vast majority of the amino acid changes occurring in the C-terminal region. The function of this domain within L remains unknown, but by using a minigenome assay we showed that it might be involved in UTR recognition. Moreover, we identified an amino acid mutation within the polymerase that, when introduced into an otherwise wild-type BUNV, resulted in a virus with a temperature-sensitive phenotype. Viruses carrying temperature-sensitive mutations are good candidates for the design of live attenuated vaccines. We suggest that a combination of stable deletions of the UTRs together with the introduction of temperature-sensitive mutations in both the nucleocapsid and the polymerase could be used to design live attenuated vaccines against serious pathogens within the family Bunyaviridae.

IMPORTANCE

Virus growth in tissue culture can be attenuated by introduction of mutations in both coding and noncoding sequences. We generated attenuated Bunyamwera viruses by deleting sequences within both the 3' and 5' untranslated regions (UTR) on each genome segment and showed that the viruses regained fitness following serial passage in cell culture. The fitter viruses had acquired amino acid changes predominantly in the C-terminal domain of the viral polymerase (L protein), and by using minigenome assays we showed that the mutant polymerases were better adapted to recognizing the mutant UTRs. We suggest that deletions within the UTRs should be incorporated along with other specific mutations, including deletion of the major virulence gene encoding the NSs protein and introduction of temperature-sensitive mutations, in the design of attenuated bunyaviruses that could have potential as vaccines.

Poly(A) binding protein 1 enhances cap-independent translation initiation of neurovirulence factor from avian herpesvirus

Poly(A) binding protein 1 (PABP1) plays a central role in mRNA translation and stability and is a target by many viruses in diverse manners. We report a novel viral translational control strategy involving the recruitment of PABP1 to the 5' leader internal ribosome entry site (5L IRES) of an immediate-early (IE) bicistronic mRNA that encodes the neurovirulence protein (pp14) from the avian herpesvirus Marek's disease virus serotype 1 (MDV1). We provide evidence for the interaction between an internal poly(A) sequence within the 5L IRES and PABP1 which may occur concomitantly with the recruitment of PABP1 to the poly(A) tail. RNA interference and reverse genetic mutagenesis results show that a subset of virally encoded-microRNAs (miRNAs) targets the inhibitor of PABP1, known as paip2, and therefore plays an indirect role in PABP1 recruitment strategy by increasing the available pool of active PABP1. We propose a model that may offer a mechanistic explanation for the cap-independent enhancement of the activity of the 5L IRES by recruitment of a bona fide initiation protein to the 5' end of the message and that is, from the affinity binding data, still compatible with the formation of 'closed loop' structure of mRNA.

Mapping overlapping functional elements embedded within the protein-coding regions of RNA viruses

Identification of the full complement of genes and other functional elements in any virus is crucial to fully understand its molecular biology and guide the development of effective control strategies. RNA viruses have compact multifunctional genomes that frequently contain overlapping genes and non-coding functional elements embedded within protein-coding sequences. Overlapping features often escape detection because it can be difficult to disentangle the multiple roles of the constituent nucleotides via mutational analyses, while high-throughput experimental techniques are often unable to distinguish functional elements from incidental features. However, RNA viruses evolve very rapidly so that, even within a single species, substitutions rapidly accumulate at neutral or near-neutral sites providing great potential for comparative genomics to distinguish the signature of purifying selection. Computationally identified features can then be efficiently targeted for experimental analysis. Here we analyze alignments of protein-coding virus sequences to identify regions where there is a statistically significant reduction in the degree of variability at synonymous sites, a characteristic signature of overlapping functional elements. Having previously tested this technique by experimental verification of discoveries in selected viruses, we now analyze sequence alignments for ∼700 RNA virus species to identify hundreds of such regions, many of which have not been previously described.

Orthobunyaviruses: recent genetic and structural insights

Orthobunyaviruses, which have small, tripartite, negative-sense RNA genomes and structurally simple virions composed of just four proteins, can have devastating effects on human health and well-being, either by causing disease in humans or by causing disease in livestock and crops. In this Review, I describe the recent genetic and structural advances that have revealed important insights into the composition of orthobunyavirus virions, viral transcription and replication and viral interactions with the host innate immune response. Lastly, I highlight outstanding questions and areas of future research.

Influenza a virus host shutoff disables antiviral stress-induced translation arrest

Influenza A virus (IAV) polymerase complexes function in the nucleus of infected cells, generating mRNAs that bear 5′ caps and poly(A) tails, and which are exported to the cytoplasm and translated by host machinery. Host antiviral defences include mechanisms that detect the stress of virus infection and arrest cap-dependent mRNA translation, which normally results in the formation of cytoplasmic aggregates of translationally stalled mRNA-protein complexes known as stress granules (SGs). It remains unclear how IAV ensures preferential translation of viral gene products while evading stress-induced translation arrest. Here, we demonstrate that at early stages of infection both viral and host mRNAs are sensitive to drug-induced translation arrest and SG formation. By contrast, at later stages of infection, IAV becomes partially resistant to stress-induced translation arrest, thereby maintaining ongoing translation of viral gene products. To this end, the virus deploys multiple proteins that block stress-induced SG formation: 1) non-structural protein 1 (NS1) inactivates the antiviral double-stranded RNA (dsRNA)-activated kinase PKR, thereby preventing eIF2α phosphorylation and SG formation; 2) nucleoprotein (NP) inhibits SG formation without affecting eIF2α phosphorylation; 3) host-shutoff protein polymerase-acidic protein-X (PA-X) strongly inhibits SG formation concomitant with dramatic depletion of cytoplasmic poly(A) RNA and nuclear accumulation of poly(A)-binding protein. Recombinant viruses with disrupted PA-X host shutoff function fail to effectively inhibit stress-induced SG formation. The existence of three distinct mechanisms of IAV-mediated SG blockade reveals the magnitude of the threat of stress-induced translation arrest during viral replication.

Like all viruses, Influenza A virus (IAV) is absolutely dependent on host-cell protein synthesis machinery. This dependence makes the virus vulnerable to the innate ability of cells to inhibit protein synthesis in response to various types of stress. This inhibition, termed translation arrest, helps cells survive adverse conditions by re-dedicating their energy to stress responses. When cells arrest translation, they form stress granules: depots of untranslated mRNAs and associated proteins. Translation arrest and formation of stress granules can be induced pharmacologically, and in this work we sought to determine whether stress granule induction would be effective in blocking IAV replication. Here we demonstrate that treatment of cells with inducers of stress granules at early times after infection resulted in blockade of viral protein synthesis and stopped viral replication. At later times post-infection, by contrast, IAV proteins prevented pharmacological induction of stress granules. We identified three viral proteins – more than in any virus to date – that work in concert to prevent stress granule formation. Taken together, our studies reveal a multipronged approach for viral suppression of translation arrest, and identify a window of opportunity early in infection when pharmacological induction of stress granules has a strong antiviral effect.

The mammalian orthoreovirus bicistronic M3 mRNA initiates translation using a 5' end-dependent, scanning mechanism that does not require interaction of 5'-3' untranslated regions

Mammalian orthoreovirus mRNAs possess short 5' UTR, lack 3' poly(A) tails, and may lack 5' cap structures at late times post-infection. As such, the mechanisms by which these viral mRNAs recruit ribosomes remain completely unknown. Toward addressing this question, we used bicistronic MRV M3 mRNA to analyze the role of 5' and 3' UTRs during MRV protein synthesis. The 5' UTR was found to be dispensable for translation initiation; however, reducing its length promoted increased downstream initiation. Modifying start site Kozak context altered the ratio of upstream to downstream initiation, whereas mutations in the 3' UTR did not. Moreover, an M3 mRNA lacking a 3' UTR was able to rescue MRV infection to WT levels in an siRNA trans-complementation assay. Together, these data allow us to propose a model in which the MRV M3 mRNA initiates translation using a 5' end-dependent, scanning mechanism that does not require the viral mRNA 3' UTR or 5'-3' UTRs interaction.

Nuclear relocalization of polyadenylate binding protein during rift valley fever virus infection involves expression of the NSs gene

Rift Valley fever virus (RVFV), an ambisense member of the family Bunyaviridae, genus Phlebovirus, is the causative agent of Rift Valley fever, an important zoonotic infection in Africa and the Middle East. Phlebovirus proteins are translated from virally transcribed mRNAs that, like host mRNA, are capped but, unlike host mRNAs, are not polyadenylated. Here, we investigated the role of PABP1 during RVFV infection of HeLa cells. Immunofluorescence studies of infected cells demonstrated a gross relocalization of PABP1 to the nucleus late in infection. Immunofluorescence microscopy studies of nuclear proteins revealed costaining between PABP1 and markers of nuclear speckles. PABP1 relocalization was sharply decreased in cells infected with a strain of RVFV lacking the gene encoding the RVFV nonstructural protein S (NSs). To determine whether PABP1 was required for RVFV infection, we measured the production of nucleocapsid protein (N) in cells transfected with small interfering RNAs (siRNAs) targeting PABP1. We found that the overall percentage of RVFV N-positive cells was not changed by siRNA treatment, indicating that PABP1 was not required for RVFV infection. However, when we analyzed populations of cells producing high versus low levels of PABP1, we found that the percentage of RVFV N-positive cells was decreased in cell populations producing physiologic levels of PABP1 and increased in cells with reduced levels of PABP1. Together, these results suggest that production of the NSs protein during RVFV infection leads to sequestration of PABP1 in the nuclear speckles, creating a state within the cell that favors viral protein production.

Characterization of messenger RNA termini in Schmallenberg virus and related Simbuviruses

Schmallenberg virus (SBV) is an emerging arbovirus infecting ruminants in Europe. SBV belongs to the Bunyaviridae family within the Simbu serogroup. Its genome comprises three segments, small (S), medium (M) and large (L), that together encode six proteins and contain NTRs. NTRs are involved in initiation and termination of transcription and in genome packaging. This study explored the 3' mRNA termini of SBV and related Simbuviruses. In addition, the 5' termini of SBV messenger RNA (mRNA) were characterized. For the three SBV segments, cap-snatching was found to initiate mRNA transcription both in vivo and in vitro. The presence of extraneous nucleotides between host RNA leaders and the viral termini fits with the previously described prime-and-realign theory. At the 3' termini, common features were identified for SBV and related Simbuviruses. However, different patterns were observed for the termini of the three segments from the same virus type.

Rotavirus prevents the expression of host responses by blocking the nucleocytoplasmic transport of polyadenylated mRNAs

Rotaviruses are the most important agent of severe gastroenteritis in young children. Early in infection, these viruses take over the host translation machinery, causing a severe shutoff of cell protein synthesis while viral proteins are efficiently synthesized. In infected cells, there is an accumulation of the cytoplasmic poly(A)-binding protein in the nucleus, induced by the viral protein NSP3. Here we found that poly(A)-containing mRNAs also accumulate and become hyperadenylated in the nuclei of infected cells. Using reporter genes bearing the untranslated regions (UTRs) of cellular or viral genes, we found that the viral UTRs do not determine the efficiency of translation of mRNAs in rotavirus-infected cells. Furthermore, we showed that while a polyadenylated reporter mRNA directly delivered into the cytoplasm of infected cells was efficiently translated, the same reporter introduced as a plasmid that needs to be transcribed and exported to the cytoplasm was poorly translated. Altogether, these results suggest that nuclear retention of poly(A)-containing mRNAs is one of the main strategies of rotavirus to control cell translation and therefore the host antiviral and stress responses.

Tinkering with translation: protein synthesis in virus-infected cells

Viruses are obligate intracellular parasites, and their replication requires host cell functions. Although the size, composition, complexity, and functions encoded by their genomes are remarkably diverse, all viruses rely absolutely on the protein synthesis machinery of their host cells. Lacking their own translational apparatus, they must recruit cellular ribosomes in order to translate viral mRNAs and produce the protein products required for their replication. In addition, there are other constraints on viral protein production. Crucially, host innate defenses and stress responses capable of inactivating the translation machinery must be effectively neutralized. Furthermore, the limited coding capacity of the viral genome needs to be used optimally. These demands have resulted in complex interactions between virus and host that exploit ostensibly virus-specific mechanisms and, at the same time, illuminate the functioning of the cellular protein synthesis apparatus.

Interplay between polyadenylate-binding protein 1 and Kaposi's sarcoma-associated herpesvirus ORF57 in accumulation of polyadenylated nuclear RNA, a viral long noncoding RNA

Polyadenylate-binding protein cytoplasmic 1 (PABPC1) is a cytoplasmic-nuclear shuttling protein important for protein translation initiation and both RNA processing and stability. We report that PABPC1 forms a complex with the Kaposi's sarcoma-associated herpesvirus (KSHV) ORF57 protein, which allows ORF57 to interact with a 9-nucleotide (nt) core element of KSHV polyadenylated nuclear (PAN) RNA, a viral long noncoding RNA (lncRNA), and increase PAN stability. The N-terminal RNA recognition motifs (RRMs) of PABPC1 are necessary for the direct interaction with ORF57. During KSHV lytic infection, the expression of viral ORF57 leads to a substantial decrease in overall PABPC1 expression, along with a shift in the cellular distribution of the remaining PABPC1 to the nucleus. Interestingly, PABPC1 and ORF57 have opposing functions in modulating PAN steady-state accumulation. The suppressive effect of PABPC1 specific to PAN expression is alleviated by small interfering RNA knockdown of PABPC1 or by overexpression of ORF57. Conversely, ectopic PABPC1 reduces ORF57 steady-state protein levels and induces aberrant polyadenylation of PAN and thereby indirectly inhibits ORF57-mediated PAN accumulation. However, E1B-AP5 (heterogeneous nuclear ribonucleoprotein U-like 1), which interacts with a region outside the 9-nt core to stimulate PAN expression, does not interact or even colocalize with ORF57. Unlike PABPC1, the nuclear distribution of E1B-AP5 remains unchanged by viral lytic infection or overexpression of ORF57. Together, these data indicate that PABPC1 is an important cellular target of viral ORF57 to directly upregulate PAN accumulation during viral lytic infection, and the ability of host PABPC1 to disrupt ORF57 expression is a strategic host counterbalancing mechanism.

Attenuation of bunyamwera orthobunyavirus replication by targeted mutagenesis of genomic untranslated regions and creation of viable viruses with minimal genome segments

Bunyamwera virus (BUNV) is the prototype virus for both the genus Orthobunyavirus and the family Bunyaviridae. BUNV has a tripartite, negative-sense RNA genome. The coding region of each segment is flanked by untranslated regions (UTRs) that are partially complementary. The UTRs play an important role in the virus life cycle by promoting transcription, replication, and encapsidation of the viral genome. Using reverse genetics, we generated recombinant viruses that contained deletions within the 3' and/or 5' UTRs of the L or M segments to determine the minimal UTRs competent for virus viability. We then generated viruses carrying deleted UTRs in all three segments. These viruses were grossly attenuated in tissue culture, being significantly impaired in their ability to produce plaques in BHK cells, and had a reduced capacity to cause host cell protein shutoff. After serial passage in tissue culture, some viruses partially recovered fitness, generating higher titers and producing larger plaques. We determined the complete nucleotide sequence for each virus. The deleted UTR sequences were maintained, and no amino acid changes were observed in the nonstructural proteins (NSs and NSm), the nucleocapsid protein (N), or the Gn glycoprotein. One virus had a single amino acid substitution in Gc. Three viruses contained amino acid changes in the viral polymerase that mostly occurred in the C-terminal domain of the L protein. Although the role of this domain remains unknown, we suggest that those changes might be involved in the evolution of the polymerase to recognize the deleted UTRs more efficiently.

Viperin, MTAP44, and protein kinase R contribute to the interferon-induced inhibition of Bunyamwera Orthobunyavirus replication

The first line of defense against viral infection is the interferon (IFN) response, which culminates in the expression of hundreds of proteins with presumed antiviral activity, and must be overcome by a virus for successful replication. The nonstructural NSs protein is the primary IFN antagonist encoded by Bunyamwera virus (BUNV), the prototype of the Orthobunyavirus genus and the family Bunyaviridae. The NSs protein interferes with RNA polymerase II-mediated transcription, thereby inhibiting cellular mRNA production, including IFN mRNAs. A recombinant virus, rBUNdelNSs, that is unable to express the NSs protein does not inhibit cellular transcription and is a strong IFN inducer. We report here that cells stimulated into the antiviral state by IFN-β treatment were protected against wild-type BUNV and rBUNdelNSs infection but addition of IFN-β after infection had little effect on the replication cycle of either virus. By screening a panel of cell lines that overexpressed individual IFN-stimulated genes, we found that protein kinase R (PKR), MTAP44, and particularly viperin appreciably restricted BUNV replication. The enzymatic activities of PKR and viperin were required for their inhibitory activities. Taken together, our data show that the restriction of BUNV replication mediated by IFN is an accumulated effect of at least three IFN-stimulated genes that probably act on different stages of the viral replication cycle.

The small genome segment of Bunyamwera orthobunyavirus harbours a single transcription-termination signal

Transcription termination of the mRNA produced from the small (S) genome segment of Bunyamwera orthobunyavirus (BUNV) has previously been mapped to two cis-acting sequences located within the 5' UTR using a virus-free replication assay. The ability of these sequences to terminate transcription was attributed to the shared pentanucleotide motif 3'-UGUCG-5'. Taking advantage of our plasmid-based rescue system to generate recombinant viruses, we re-evaluated the importance of both pentanucleotide motifs as well as that of two other conserved sequences in transcription termination in vivo. Analysis of the 3' ends of positive-stranded viral RNAs derived from the S segment revealed that only the region around the upstream pentanucleotide motif mediated transcription termination in cells infected with wild-type BUNV, leading to mRNAs that were about 100 nt shorter than antigenome RNA. Furthermore, the downstream motif was not recognized in recombinant viruses in which the upstream signal has been disrupted. Our results suggest that in the context of virus infection transcription termination of the BUNV S genome segment mRNA is exclusively directed by the upstream-termination signal. Interestingly, within this region we identified a motif similar to a transcription-termination sequence used by Rift Valley fever phlebovirus.

Analysis of the Tomato spotted wilt virus ambisense S RNA-encoded hairpin structure in translation

Background

The intergenic region (IR) of ambisense RNA segments from animal- and plant-infecting (-)RNA viruses functions as a bidirectional transcription terminator. The IR sequence of the Tomato spotted wilt virus (TSWV) ambisense S RNA contains stretches that are highly rich in A-residues and U-residues and is predicted to fold into a stable hairpin structure. The presence of this hairpin structure sequence in the 3′ untranslated region (UTR) of TSWV mRNAs implies a possible role in translation.

Methodology/Principal Findings

To analyse the role of the predicted hairpin structure in translation, various Renilla luciferase constructs containing modified 3′ and/or 5′ UTR sequences of the TSWV S RNA encoded nucleocapsid (N) gene were analyzed for expression. While good luciferase expression levels were obtained from constructs containing the 5′ UTR and the 3′ UTR, luciferase expression was lost when the hairpin structure sequence was removed from the 3′ UTR. Constructs that only lacked the 5′ UTR, still rendered good expression levels. When in addition the entire 3′ UTR was exchanged for that of the S RNA encoded non-structural (NSs) gene transcript, containing the complementary hairpin folding sequence, the loss of luciferase expression could only be recovered by providing the 5′ UTR sequence of the NSs transcript. Luciferase activity remained unaltered when the hairpin structure sequence was swapped for the analogous one from Tomato yellow ring virus, another distinct tospovirus. The addition of N and NSs proteins further increased luciferase expression levels from hairpin structure containing constructs.

Conclusions/Significance

The results suggest a role for the predicted hairpin structure in translation in concert with the viral N and NSs proteins. The presence of stretches highly rich in A-residues does not rule out a concerted action with a poly(A)-tail-binding protein. A common transcription termination and translation strategy for plant- and animal-infecting ambisense RNA viruses is being discussed.

Using SILAC and quantitative proteomics to investigate the interactions between viral and host proteomes

Viruses continue to pose some of the greatest threats to human and animal health, and food security worldwide. Therefore, new approaches are required to increase our understanding of virus-host cell interactions and subsequently design more effective therapeutic countermeasures. Quantitative proteomics based on stable isotope labeling by amino acids in cell culture (SILAC), coupled to LC-MS/MS and bioinformatic analysis, is providing an excellent resource for studying host cell proteomes and can readily be applied for the study of virus infection. Here, we review this approach and discuss how virus-host cell interactions can best be studied, what is realistically feasible, and the potential limitations. For example, sub-cellular fractionation can reduce sample complexity for LC-MS/MS, increase data return and provide information regarding protein trafficking between different cellular compartments. The key to successful quantitative proteomics combines good experimental design and appropriate sample preparation with statistical analysis and validation of the MS data through the use of independent techniques and functional analysis. The annotation of the human genome and the increasing availability of biological reagents such as antibodies, provide the optimum parameters for studying viruses that infect humans, in human cell lines. SILAC-based quantitative proteomics can also be used to study the interactome of viral proteins with the host cell. Coupling proteomic studies with global transcriptomic and RNA depletion experiments will provide great insights into the complexity of the infection process, and potentially reveal new antiviral targets.

Nuclear relocalisation of cytoplasmic poly(A)-binding proteins PABP1 and PABP4 in response to UV irradiation reveals mRNA-dependent export of metazoan PABPs

Poly(A)-binding protein 1 (PABP1) has a fundamental role in the regulation of mRNA translation and stability, both of which are crucial for a wide variety of cellular processes. Although generally a diffuse cytoplasmic protein, it can be found in discrete foci such as stress and neuronal granules. Mammals encode several additional cytoplasmic PABPs that remain poorly characterised, and with the exception of PABP4, appear to be restricted in their expression to a small number of cell types. We have found that PABP4, similarly to PABP1, is a diffusely cytoplasmic protein that can be localised to stress granules. However, UV exposure unexpectedly relocalised both proteins to the nucleus. Nuclear relocalisation of PABPs was accompanied by a reduction in protein synthesis but was not linked to apoptosis. In examining the mechanism of PABP relocalisation, we found that it was related to a change in the distribution of poly(A) RNA within cells. Further investigation revealed that this change in RNA distribution was not affected by PABP knockdown but that perturbations that block mRNA export recapitulate PABP relocalisation. Our results support a model in which nuclear export of PABPs is dependent on ongoing mRNA export, and that a block in this process following UV exposure leads to accumulation of cytoplasmic PABPs in the nucleus. These data also provide mechanistic insight into reports that transcriptional inhibitors and expression of certain viral proteins cause relocation of PABP to the nucleus.

Bunyavirus: structure and replication

The Bunyaviridae family is comprised of a large number of negative-sense, single-stranded RNA viruses that infect animals, insects, and plants. The tripartite genome of bunyaviruses, encapsidated in the form of individual ribonucleoprotein complexes, encodes four structural proteins, the glycoproteins Gc and Gn, the nucleoprotein N, and the viral polymerase L. Some bunyaviruses also use an ambi-sense strategy to encode the nonstructural proteins NSs and NSm. While some bunyaviruses have a T = 12 icosahedral symmetry, others only have locally ordered capsids, or capsids with no detectable symmetry. Bunyaviruses enter cells through clathrin-mediated endocytosis or phagocytosis. In endosome, viral glycoproteins facilitate membrane fusion at acidic pH, thus allowing bunyaviruses to uncoat and deliver their genomic RNA into host cytoplasm. Bunyaviruses replicate in cytoplasm where the viral polymerase L catalyzes both transcription and replication of the viral genome. While transcription requires a cap primer for initiation and ends at specific termination signals before the 3' end of the template is reached, replication copies the entire template and does not depend on any primer for initiation. This review will discuss some of the most interesting aspects of bunyavirus replication, including L protein/N protein-mediated cap snatching, prime-and-realign for transcription and replication initiation, translation-coupled transcription, sequence/secondary structure-dependent transcription termination, ribonucleoprotein encapsidation, and N protein-mediated initiation of viral protein translation. Recent developments on the structure and functional characterization of the bunyavirus capsid and the RNA synthesis machineries (including both protein L and N) will also be discussed.

Viral subversion of the host protein synthesis machinery

Although viruses encode many of the functions that are required for viral replication, they are completely reliant on the protein synthesis machinery that is present in their host cells.

Recruiting cellular ribosomes to translate viral mRNAs represents a crucial step in the replication of all viruses.

To ensure translation of their mRNAs, viruses use a diverse collection of strategies (probably pirated from their cellular hosts) to commandeer key translation factors that are required for the initiation, elongation and termination steps of translation.

Viruses also neutralize host defences that seek to incapacitate the translation machinery in infected cells.

Viruses rely on the translation machinery of the host cell to produce the proteins that are essential for their replication. Here, Walsh and Mohr discuss the diverse strategies by which viruses subvert the host protein synthesis machinery and regulate the translation of viral mRNAs.

Viruses are fully reliant on the translation machinery of their host cells to produce the polypeptides that are essential for viral replication. Consequently, viruses recruit host ribosomes to translate viral mRNAs, typically using virally encoded functions to seize control of cellular translation factors and the host signalling pathways that regulate their activity. This not only ensures that viral proteins will be produced, but also stifles innate host defences that are aimed at inhibiting the capacity of infected cells for protein synthesis. Remarkably, nearly every step of the translation process can be targeted by virally encoded functions. This Review discusses the diverse strategies that viruses use to subvert host protein synthesis functions and regulate mRNA translation in infected cells.

A viral nuclear noncoding RNA binds re-localized poly(A) binding protein and is required for late KSHV gene expression

During the lytic phase of infection, the gamma herpesvirus Kaposi's Sarcoma-Associated Herpesvirus (KSHV) expresses a highly abundant, 1.1 kb nuclear noncoding RNA of unknown function. We observe that this polyadenylated nuclear (PAN) RNA avidly binds host poly(A)-binding protein C1 (PABPC1), which normally functions in the cytoplasm to bind the poly(A) tails of mRNAs, regulating mRNA stability and translation efficiency. During the lytic phase of KSHV infection, PABPC1 is re-localized to the nucleus as a consequence of expression of the viral shutoff exonuclease (SOX) protein; SOX also mediates the host shutoff effect in which host mRNAs are downregulated while viral mRNAs are selectively expressed. We show that whereas PAN RNA is not required for the host shutoff effect or for PABPC1 re-localization, SOX strongly upregulates the levels of PAN RNA in transient transfection experiments. This upregulation is destroyed by the same SOX mutation that ablates the host shutoff effect and PABPC1 nuclear re-localization or by removal of the poly(A) tail of PAN. In cells induced into the KSHV lytic phase, depletion of PAN RNA using RNase H-targeting antisense oligonucleotides reveals that it is necessary for the production of late viral proteins from mRNAs that are themselves polyadenylated. Our results add to the repertoire of functions ascribed to long noncoding RNAs and suggest a mechanism of action for nuclear noncoding RNAs in gamma herpesvirus infection.

Almost all eukaryotic messenger RNAs (mRNAs) have a string of 150–200 adenylates at the 3′ end. This poly(A) tail has been implicated as important for regulating mRNA translation, stability and export. During the lytic phase of infection of Kaposi's Sarcoma-Associated Herpesvirus (KSHV), a noncoding viral RNA is synthesized that resembles an mRNA in that it is transcribed by RNA polymerase II, is methyl-G capped at the 5′ end, and is polyadenylated at the 3′ end; yet this RNA is never exported to the cytoplasm for translation. Rather, it builds up in the nucleus to exceedingly high levels. We present evidence that the function of this abundant, polyadenylated nuclear (PAN) RNA is to bind poly(A) binding protein, which normally binds poly(A) tails of mRNAs in the cytoplasm but is re-localized into the nucleus during lytic KSHV infection. The interaction between PAN RNA and re-localized poly(A) binding protein is important for formation of new virus, in particular for the synthesis of proteins made late in infection. Our study provides new insight into the function of this noncoding RNA during KSHV infection and expands recent discoveries regarding re-localization of poly(A) binding protein during many viral infections.

Recent advances in the molecular and cellular biology of bunyaviruses

The family Bunyaviridae of segmented, negative-stranded RNA viruses includes over 350 members that infect a bewildering variety of animals and plants. Many of these bunyaviruses are the causative agents of serious disease in their respective hosts, and are classified as emerging viruses because of their increased incidence in new populations and geographical locations throughout the world. Emerging bunyaviruses, such as Crimean-Congo hemorrhagic fever virus, tomato spotted wilt virus and Rift Valley fever virus, are currently attracting great interest due to migration of their arthropod vectors, a situation possibly linked to climate change. These and other examples of continued emergence suggest that bunyaviruses will probably continue to pose a sustained global threat to agricultural productivity, animal welfare and human health. The threat of emergence is particularly acute in light of the lack of effective preventative or therapeutic treatments for any of these viruses, making their study an important priority. This review presents recent advances in the understanding of the bunyavirus life cycle, including aspects of their molecular, cellular and structural biology. Whilst special emphasis is placed upon the emerging bunyaviruses, we also describe the extensive body of work involving model bunyaviruses, which have been the subject of major contributions to our overall understanding of this important group of viruses.

Translation initiation: a regulatory role for poly(A) tracts in front of the AUG codon in Saccharomyces cerevisiae

The 5'-UTR serves as the loading dock for ribosomes during translation initiation and is the key site for translation regulation. Many genes in the yeast Saccharomyces cerevisiae contain poly(A) tracts in their 5'-UTRs. We studied these pre-AUG poly(A) tracts in a set of 3274 recently identified 5'-UTRs in the yeast to characterize their effect on in vivo protein abundance, ribosomal density, and protein synthesis rate in the yeast. The protein abundance and the protein synthesis rate increase with the length of the poly(A), but exhibit a dramatic decrease when the poly(A) length is ≥12. The ribosomal density also reaches the lowest level when the poly(A) length is ≥12. This supports the hypothesis that a pre-AUG poly(A) tract can bind to translation initiation factors to enhance translation initiation, but a long (≥12) pre-AUG poly(A) tract will bind to Pab1p, whose binding size is 12 consecutive A residues in yeast, resulting in repression of translation. The hypothesis explains why a long pre-AUG poly(A) leads to more efficient translation initiation than a short one when PABP is absent, and why pre-AUG poly(A) is short in the early genes but long in the late genes of vaccinia virus.

Importin alpha-mediated nuclear import of cytoplasmic poly(A) binding protein occurs as a direct consequence of cytoplasmic mRNA depletion

Recent studies have found the cytoplasmic poly(A) binding protein (PABPC) to have opposing effects on gene expression when concentrated in the cytoplasm versus in the nucleus. PABPC is predominantly cytoplasmic at steady state, where it enhances protein synthesis through simultaneous interactions with mRNA and translation factors. However, it accumulates dramatically within the nucleus in response to various pathogenic and nonpathogenic stresses, leading to an inhibition of mRNA export. The molecular events that trigger relocalization of PABPC and the mechanisms by which it translocates into the nucleus to block gene expression are not understood. Here, we reveal an RNA-based mechanism of retaining PABPC in the cytoplasm. Expression either of viral proteins that promote mRNA turnover or of a cytoplasmic deadenylase drives nuclear relocalization of PABPC in a manner dependent on the PABPC RNA recognition motifs (RRMs). Using multiple independent binding sites within its RRMs, PABPC interacts with importin α, a component of the classical import pathway. Finally, we demonstrate that the direct association of PABPC with importin α is antagonized by the presence of poly(A) RNA, supporting a model in which RNA binding masks nuclear import signals within the PABPC RRMs, thereby ensuring efficient cytoplasmic retention of this protein in normal cells. These findings further suggest that cells must carefully calibrate the ratio of PABPC to mRNA, as events that offset this balance can dramatically influence gene expression.

Getting the message direct manipulation of host mRNA accumulation during gammaherpesvirus lytic infection

The Gammaherpesvirinae subfamily of herpesviruses comprises lymphotropic viruses, including the oncogenic human pathogens Epstein-Barr virus and Kaposi's sarcoma-associated herpesvirus. During lytic infection, gammaherpesviruses manipulate host gene expression to optimize the cellular environment for viral replication and to evade the immune response. Additionally, although a lytically infected cell will itself be killed in the process of viral replication, lytic infection can contribute to pathogenesis by inducing the secretion of paracrine factors with functions in cell survival and proliferation, and angiogenesis. The mechanisms by which these viruses manipulate host gene expression are varied and target the accumulation of cellular mRNAs and their translation, signaling pathways, and protein stability. Here, we discuss how gammaherpesviral proteins directly influence host mRNA biogenesis and stability, either selectively or globally, in order to fine-tune the cellular environment to the advantage of the virus. Appreciation of the mechanisms by which these viruses interface with and adapt normal cellular processes continues to inform our understanding of gammaherpesviral biology and the regulation of mRNA accumulation and turnover in our own cells.