Japanese encephalitis virus core protein inhibits stress granule formation through an interaction with Caprin-1 and facilitates viral propagation

Two Birds With One Stone: RNA Virus Strategies to Manipulate G3BP1 and Other Stress Granule Components

Stress granules (SGs) are membrane-less organelles forming in the cytoplasm in response to various types of stress, including viral infection. SGs and SG-associated proteins can play either a proviral role, by facilitating viral replication, or an antiviral role, by limiting the translation capacity, sequestering viral RNA, or contributing to the innate immune response of the cell. Consequently, viruses frequently target stress granules while counteracting cellular translation shut-off and the antiviral response. One strategy is to sequester SG components, not only to impair their assembly but also to repurpose and incorporate them into viral replication sites. G3BP1 is a key SG protein, driving its nucleation through protein-protein and protein-RNA interactions. Many cellular proteins, including other SG components, interact with G3BP1 via their ΦxFG motifs. Notably, SARS-CoV N proteins and alphaviral nsP3 proteins contain similar motifs, allowing them to compete for G3BP1. Several SG proteins have been shown to interact with the flaviviral capsid protein, which is primarily responsible for anchoring the viral genome inside the virion. There are also numerous examples of structured elements within coronaviral and flaviviral RNAs recruiting or sponging SG proteins. Despite these insights, the structural and biochemical details of SG-virus interactions remain largely unexplored and are known only for a handful of cases. Exploring their molecular relevance for infection and discovering new examples of direct SG-virus contacts is highly important, as advances in this area will open new possibilities for the design of targeted therapies and potentially broad-spectrum antivirals.

Structural insights into the Caprin-2 HR1 domain in canonical Wnt signaling

The canonical Wnt signaling pathway plays crucial roles in cell fate decisions as well as in pathogenesis of various diseases. Previously, we reported Caprin-2 as a new regulator of canonical Wnt signaling through a mechanism of facilitating LRP5/6 phosphorylation. Here, we resolved the crystal structure of the N-terminal homologous region 1 (HR1) domain of human Caprin-2. HR1 domain is so far only observed in Caprin-2 and its homologous protein Caprin-1, and the function of this domain remains largely mysterious. Here, the structure showed that HR1 domain of human Caprin-2 forms a homo-dimer and exhibits an overall structure roughly resembling the appearance of a pair of scissors. Moreover, we found that residues R200 and R201, which located at a basic cluster within the N-terminal "blades" region, are critical for Caprin-2's localization to the plasma membrane. In line with this, mutations targeting these two residues decrease Caprin-2's activity in the canonical Wnt signaling. Overall, we characterized a previously unknown "scissors"-like structure of the full-length HR1 domain and revealed its function in mediating Caprin-2's localization to the plasma membrane.

SARS-CoV-2 nucleocapsid protein promotes self-deacetylation by inducing HDAC6 to facilitate viral replication

BACKGROUND

The global outbreak of COVID-19 caused by the SARS-CoV-2 has led to millions of deaths. This unanticipated emergency has prompted virologists across the globe to delve deeper into the intricate dynamicity of the host-virus interface with an aim to identify antiviral targets and elucidate host and viral determinants of severe disease.

AIM

The present study was undertaken to analyse the role of histone deacetylase 6 (HDAC6) in regulating SARS-CoV-2 infection.

RESULTS

Gradual increase in HDAC6 expression was observed in different SARS-CoV-2-permissive cell lines following SARS-CoV-2 infection. The SARS-CoV-2 nucleocapsid protein (N protein) was identified as the primary viral factor responsible for upregulating HDAC6 expression. Downregulation of HDAC6 using shRNA or a specific inhibitor tubacin resulted in reduced viral replication suggesting proviral role of its deacetylase activity. Further investigations uncovered the interaction of HDAC6 with stress granule protein G3BP1 and N protein during infection. HDAC6-mediated deacetylation of SARS-CoV-2 N protein was found to be crucial for its association with G3BP1.

CONCLUSION

This study provides valuable insights into the molecular mechanisms underlying the disruption of cytoplasmic stress granules during SARS-CoV-2 infection and highlights the significance of HDAC6 in the process.

The NS2B-PP1α-eIF2α axis: Inhibiting stress granule formation and Boosting Zika virus replication

Stress granules (SGs), formed by untranslated messenger ribonucleoproteins (mRNPs) during cellular stress in eukaryotes, have been linked to flavivirus interference without clear understanding. This study reveals the role of Zika virus (ZIKV) NS2B as a scaffold protein mediating interaction between protein phosphatase 1α (PP1α) and eukaryotic initiation factor 2α (eIF2α). This interaction promotes eIF2α dephosphorylation by PP1α, inhibiting SG formation. The NS2B-PP1α complex exhibits remarkable stability, resisting ubiquitin-induced degradation and amplifying eIF2α dephosphorylation, thus promoting ZIKV replication. In contrast, the NS2BV35A mutant, interacting exclusively with eIF2α, fails to inhibit SG formation, resulting in reduced viral replication and diminished impact on brain organoid growth. These findings reveal PP1α's dual role in ZIKV infection, inducing interferon production as an antiviral factor and suppressing SG formation as a viral promoter. Moreover, we found that NS2B also serves as a versatile mechanism employed by flaviviruses to counter host antiviral defenses, primarily by broadly inhibiting SG formation. This research advances our comprehension of the complex interplay in flavivirus-host interactions, offering potential for innovative therapeutic strategies against flavivirus infections.

Phosphorylation of G3BP1 is involved in the regulation of PDCoV-induced inflammatory response

Stress granules (SGs), the main component is GTPase-activating protein-binding protein 1 (G3BP1), which are assembled during viral infection and function to sequester host and viral mRNAs and proteins, are part of the antiviral responses. In this study, we found that porcine deltacoronavirus (PDCoV) infection induced stable formation of robust SGs in cells through a PERK (protein kinase R-like endoplasmic reticulum kinase)-dependent mechanism. Overexpression of SGs marker proteins G3BP1 significantly reduced PDCoV replication in vitro, while inhibition of endogenous G3BP1 enhanced PDCoV replication. Moreover, PDCoV infected LLC-PK1 cells raise the phosphorylation level of G3BP1. By overexpression of the G3BP1 phosphorylated protein or the G3BP1 dephosphorylated protein, we found that phosphorylation of G3BP1 is involved in the regulation of PDCoV-induced inflammatory response. Taken together, our study presents a vital aspect of the host innate response to invading pathogens and reveals attractive host targets for antiviral target.

Dengue virus pathogenesis and host molecular machineries

Dengue viruses (DENV) are positive-stranded RNA viruses belonging to the Flaviviridae family. DENV is the causative agent of dengue, the most rapidly spreading viral disease transmitted by mosquitoes. Each year, millions of people contract the virus through bites from infected female mosquitoes of the Aedes species. In the majority of individuals, the infection is asymptomatic, and the immune system successfully manages to control virus replication within a few days. Symptomatic individuals may present with a mild fever (Dengue fever or DF) that may or may not progress to a more critical disease termed Dengue hemorrhagic fever (DHF) or the fatal Dengue shock syndrome (DSS). In the absence of a universally accepted prophylactic vaccine or therapeutic drug, treatment is mostly restricted to supportive measures. Similar to many other viruses that induce acute illness, DENV has developed several ways to modulate host metabolism to create an environment conducive to genome replication and the dissemination of viral progeny. To search for new therapeutic options, understanding the underlying host-virus regulatory system involved in various biological processes of the viral life cycle is essential. This review aims to summarize the complex interaction between DENV and the host cellular machinery, comprising regulatory mechanisms at various molecular levels such as epigenetic modulation of the host genome, transcription of host genes, translation of viral and host mRNAs, post-transcriptional regulation of the host transcriptome, post-translational regulation of viral proteins, and pathways involved in protein degradation.

A Balancing Act: The Viral-Host Battle over RNA Binding Proteins

A defining feature of a productive viral infection is the co-opting of host cell resources for viral replication. Despite the host repertoire of molecular functions and biological counter measures, viruses still subvert host defenses to take control of cellular factors such as RNA binding proteins (RBPs). RBPs are involved in virtually all steps of mRNA life, forming ribonucleoprotein complexes (mRNPs) in a highly ordered and regulated process to control RNA fate and stability in the cell. As such, the hallmark of the viral takeover of a cell is the reshaping of RNA fate to modulate host gene expression and evade immune responses by altering RBP interactions. Here, we provide an extensive review of work in this area, particularly on the duality of the formation of RNP complexes that can be either pro- or antiviral. Overall, in this review, we highlight the various ways viruses co-opt RBPs to regulate RNA stability and modulate the outcome of infection by gathering novel insights gained from research studies in this field.

Intrinsic factors behind long COVID: IV. Hypothetical roles of the SARS-CoV-2 nucleocapsid protein and its liquid-liquid phase separation

When the SARS-CoV-2 virus infects humans, it leads to a condition called COVID-19 that has a wide spectrum of clinical manifestations, from no symptoms to acute respiratory distress syndrome. The virus initiates damage by attaching to the ACE-2 protein on the surface of endothelial cells that line the blood vessels and using these cells as hosts for replication. Reactive oxygen species levels are increased during viral replication, which leads to oxidative stress. About three-fifths (~60%) of the people who get infected with the virus eradicate it from their body after 28 days and recover their normal activity. However, a large fraction (~40%) of the people who are infected with the virus suffer from various symptoms (anosmia and/or ageusia, fatigue, cough, myalgia, cognitive impairment, insomnia, dyspnea, and tachycardia) beyond 12 weeks and are diagnosed with a syndrome called long COVID. Long-term clinical studies in a group of people who contracted SARS-CoV-2 have been contrasted with a noninfected matched group of people. A subset of infected people can be distinguished by a set of cytokine markers to have persistent, low-grade inflammation and often self-report two or more bothersome symptoms. No medication can alleviate their symptoms efficiently. Coronavirus nucleocapsid proteins have been investigated extensively as potential drug targets due to their key roles in virus replication, among which is their ability to bind their respective genomic RNAs for incorporation into emerging virions. This review highlights basic studies of the nucleocapsid protein and its ability to undergo liquid-liquid phase separation. We hypothesize that this ability of the nucleocapsid protein for phase separation may contribute to long COVID. This hypothesis unlocks new investigation angles and could potentially open novel avenues for a better understanding of long COVID and treating this condition.

G3BP1-dependent condensation of translationally inactive viral RNAs antagonizes infection

G3BP1 is an RNA binding protein that condenses untranslating messenger RNAs into stress granules (SGs). G3BP1 is inactivated by multiple viruses and is thought to antagonize viral replication by SG-enhanced antiviral signaling. Here, we show that neither G3BP1 nor SGs generally alter the activation of innate immune pathways. Instead, we show that the RNAs encoded by West Nile virus, Zika virus, and severe acute respiratory syndrome coronavirus 2 are prone to G3BP1-dependent RNA condensation, which is enhanced by limiting translation initiation and correlates with the disruption of viral replication organelles and viral RNA replication. We show that these viruses counteract condensation of their RNA genomes by inhibiting the RNA condensing function of G3BP proteins, hijacking the RNA decondensing activity of eIF4A, and/or maintaining efficient translation. These findings argue that RNA condensation can function as an intrinsic antiviral mechanism, which explains why many viruses inactivate G3BP proteins and suggests that SGs may have arisen as a vestige of this antiviral mechanism.

Gadd45β is critical for regulation of type I interferon signaling by facilitating G3BP-mediated stress granule formation

Stress granules (SGs) constitute a signaling hub that plays a critical role in type I interferon responses. Here, we report that growth arrest and DNA damage-inducible beta (Gadd45β) act as a positive regulator of SG-mediated interferon signaling by targeting G3BP upon RNA virus infection. Gadd45β deficiency markedly impairs SG formation and SG-mediated activation of interferon signaling in vitro. Gadd45β knockout mice are highly susceptible to RNA virus infection, and their ability to produce interferon and cytokines is severely impaired. Specifically, Gadd45β interacts with the RNA-binding domain of G3BP, leading to conformational expansion of G3BP1 via dissolution of its autoinhibitory electrostatic intramolecular interaction. The acidic loop 1- and RNA-binding properties of Gadd45β markedly increase the conformational expansion and RNA-binding affinity of the G3BP1-Gadd45β complex, thereby promoting assembly of SGs. These findings suggest a role for Gadd45β as a component and critical regulator of G3BP1-mediated SG formation, which facilitates RLR-mediated interferon signaling.

Biomolecular phase separation in stress granule assembly and virus infection

Liquid-liquid phase separation (LLPS) has emerged as a crucial mechanism for cellular compartmentalization. One prominent example of this is the stress granule. Found in various types of cells, stress granule is a biomolecular condensate formed through phase separation. It comprises numerous RNA and RNA-binding proteins. Over the past decades, substantial knowledge has been gained about the composition and dynamics of stress granules. SGs can regulate various signaling pathways and have been associated with numerous human diseases, such as neurodegenerative diseases, cancer, and infectious diseases. The threat of viral infections continues to loom over society. Both DNA and RNA viruses depend on host cells for replication. Intriguingly, many stages of the viral life cycle are closely tied to RNA metabolism in human cells. The field of biomolecular condensates has rapidly advanced in recent times. In this context, we aim to summarize research on stress granules and their link to viral infections. Notably, stress granules triggered by viral infections behave differently from the canonical stress granules triggered by sodium arsenite (SA) and heat shock. Studying stress granules in the context of viral infections could offer a valuable platform to link viral replication processes and host anti-viral responses. A deeper understanding of these biological processes could pave the way for innovative interventions and treatments for viral infectious diseases. They could potentially bridge the gap between basic biological processes and interactions between viruses and their hosts.

CAPRIN1 Is Required for Control of Viral Replication Complexes by Interferon Gamma

Replication complexes (RCs), formed by positive-strand (+) RNA viruses through rearrangements of host endomembranes, protect their replicating RNA from host innate immune defenses. We have shown that two evolutionarily conserved defense systems, autophagy and interferon (IFN), target viral RCs and inhibit viral replication collaboratively. However, the mechanism by which autophagy proteins target viral RCs and the role of IFN-inducible GTPases in the disruption of RCs remains poorly understood. Here, using murine norovirus (MNV) as a model (+) RNA virus, we show that the guanylate binding protein 1 (GBP1) is the human GTPase responsible for inhibiting RCs. Furthermore, we found that ATG16L1 mediates the LC3 targeting of MNV RC by binding to WIPI2B and CAPRIN1, and that IFN gamma-mediated control of MNV replication was dependent on CAPRIN1. Collectively, this study identifies a novel mechanism for the autophagy machinery-mediated recognition and inhibition of viral RCs, a hallmark of (+) RNA virus replication. IMPORTANCE Replication complexes provide a microenvironment important for (+) RNA virus replication and shield it from host immune response. Previously we have shown that interferon gamma (IFNG) disrupts the RC of MNV via evolutionarily conserved autophagy proteins and IFN-inducible GTPases. Elucidating the mechanism of targeting of viral RC by ATG16L1 and IFN-induced GTPase will pave the way for development of therapeutics targeting the viral replication complexes. Here, we have identified GBP1 as the sole GBP targeting viral RC and uncovered the novel role of CAPRIN1 in recruiting ATG16L1 to the viral RC.

Multiple functions of stress granules in viral infection at a glance

Stress granules (SGs) are distinct RNA granules induced by various stresses, which are evolutionarily conserved across species. In general, SGs act as a conservative and essential self-protection mechanism during stress responses. Viruses have a long evolutionary history and viral infections can trigger a series of cellular stress responses, which may interact with SG formation. Targeting SGs is believed as one of the critical and conservative measures for viruses to tackle the inhibition of host cells. In this systematic review, we have summarized the role of SGs in viral infection and categorized their relationships into three tables, with a particular focus on Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) infection. Moreover, we have outlined several kinds of drugs targeting SGs according to different pathways, most of which are potentially effective against SARS-CoV-2. We believe this review would offer a new view for the researchers and clinicians to attempt to develop more efficacious treatments for virus infection, particularly for the treatment of SARS-CoV-2 infection.

Peste Des Petits Ruminants Virus Nucleocapsid Protein Interacts with Protein Kinase R-Activating Protein and Induces Stress Granules To Promote Viral Replication

The pathogenic mechanisms of peste des petits ruminants virus (PPRV) infection remain poorly understood, leaving peste des petits ruminants (PPR) control and eradication especially difficult. Here, we determined that PPRV nucleocapsid (N) protein triggers formation of stress granules (SGs) to benefit viral replication. A mass spectrometry-based profiling of the interactome of PPRV N protein revealed that PPRV N protein interacted with protein kinase R (PKR)-activating protein (PACT), and this interaction was confirmed in the context of PPRV infection. PACT was essential for PPRV replication. Besides, the ectopic expression of N activated the PKR/eIF2α (α subunit of eukaryotic initiation factor 2) pathway through induction of PKR phosphorylation, but it did not induce PKR phosphorylation in PACT-deficient (PACT^-/-) cells. PPRV N interacted with PACT, impairing the interaction between PACT and a PKR inhibitor, transactivation response RNA-binding protein (TRBP), which subsequently enhanced the interaction between PACT and PKR and thus promoted the activation of PKR and eIF2α phosphorylation, resulting in formation of stress granules (SGs). Consistently, PPRV infection induced SG formation through activation of the PKR/eIF2α pathway, and knockdown of N impaired PPRV-induced SG formation. PPRV-induced SG formation significantly decreased in PACT^-/- cells as well. The role of SG formation in PPRV replication was subsequently investigated, which showed that SG formation plays a positive role in PPRV replication. By using an RNA fluorescence in situ hybridization assay, we found that PPRV-induced SGs hid cellular mRNA rather than viral mRNA. Altogether, our data provide the first evidence that PPRV N protein plays a role in modulating the PKR/eIF2α/SG axis and promotes virus replication through targeting PACT. IMPORTANCE Stress granule (SG) formation is a conserved cellular strategy to reduce stress-related damage regulating cell survival. A mass spectrometry-based profiling of the interactome of PPRV N protein revealed that PPRV N interacted with PACT to regulate the assembly of SGs. N protein inhibited the interaction between PACT and a PKR inhibitor, TRBP, through binding to the M1 domain of PACT, which enhanced the interaction between PACT and PKR and thus promoted PKR activation and subsequent eIF2α phosphorylation as well as SG formation. The regulatory function of N protein was strikingly abrogated in PACT^-/- cells. SGs induced by PPRV infection through the PKR/eIF2α pathway are PACT dependent. The loss-of-function assay indicated that PPRV-induced SGs were critical for PPRV replication. We concluded that the PPRV N protein manipulates the host PKR/eIF2α/SG axis to favor virus replication.

Simultaneous membrane and RNA binding by tick-borne encephalitis virus capsid protein

Tick-borne encephalitis virus is an enveloped, pathogenic, RNA virus in the family Flaviviridae, genus Flavivirus. Viral particles are formed when the nucleocapsid, consisting of an RNA genome and multiple copies of the capsid protein, buds through the endoplasmic reticulum membrane and acquires the viral envelope and the associated proteins. The coordination of the nucleocapsid components to the sites of assembly and budding are poorly understood. Here, we investigate the interactions of the wild-type and truncated capsid proteins with membranes with biophysical methods and model membrane systems. We show that capsid protein initially binds membranes via electrostatic interactions with negatively-charged lipids, which is followed by membrane insertion. Additionally, we show that membrane-bound capsid protein can recruit viral genomic RNA. We confirm the biological relevance of the biophysical findings by using mass spectrometry to show that purified virions contain negatively-charged lipids. Our results suggest that nucleocapsid assembly is coordinated by negatively-charged membrane patches on the endoplasmic reticulum and that the capsid protein mediates direct contacts between the nucleocapsid and the membrane.

Stress granules: potential therapeutic targets for infectious and inflammatory diseases

Eukaryotic cells are stimulated by external pressure such as that derived from heat shock, oxidative stress, nutrient deficiencies, or infections, which induce the formation of stress granules (SGs) that facilitates cellular adaptation to environmental pressures. As aggregated products of the translation initiation complex in the cytoplasm, SGs play important roles in cell gene expression and homeostasis. Infection induces SGs formation. Specifically, a pathogen that invades a host cell leverages the host cell translation machinery to complete the pathogen life cycle. In response, the host cell suspends translation, which leads to SGs formation, to resist pathogen invasion. This article reviews the production and function of SGs, the interaction between SGs and pathogens, and the relationship between SGs and pathogen-induced innate immunity to provide directions for further research into anti-infection and anti-inflammatory disease strategies.

Japanese Encephalitis Virus-Infected Cells

RNA virus infections have been a leading cause of pandemics. Aided by global warming and increased connectivity, their threat is likely to increase over time. The flaviviruses are one such RNA virus family, and its prototypes such as the Japanese encephalitis virus (JEV), Dengue virus, Zika virus, West Nile virus, etc., pose a significant health burden on several endemic countries. All viruses start off their life cycle with an infected cell, wherein a series of events are set in motion as the virus and host battle for autonomy. With their remarkable capacity to hijack cellular systems and, subvert/escape defence pathways, viruses are able to establish infection and disseminate in the body, causing disease. Using this strategy, JEV replicates and spreads through several cell types such as epithelial cells, fibroblasts, monocytes and macrophages, and ultimately breaches the blood-brain barrier to infect neurons and microglia. The neurotropic nature of JEV, its high burden on the paediatric population, and its lack of any specific antivirals/treatment strategies emphasise the need for biomedical research-driven solutions. Here, we highlight the latest research developments on Japanese encephalitis virus-infected cells and discuss how these can aid in the development of future therapies.

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.

African Swine Fever Virus Exhibits Distinct Replication Defects in Different Cell Types

African swine fever virus (ASFV) causes one of the most devastating diseases affecting pigs and wild suids, a worldwide epizootic situation exacerbated in recent years due to the lack of vaccine or effective treatment. ASFV has a restricted cell tropism, and is prone to replicate in porcine monocytes and alveolar macrophages with high efficiency. Here, the replication capabilities of ASFV were examined in swine pulmonary alveolar macrophages (PAMs) and compared with 3D4/21, PK-15, MA-104 and Marc-145 cell lines using PCR, qPCR and Western blot with monoclonal antibodies against the viral p30 and p72 proteins. The results showed that ASFV has a variety of infection characteristics in PAMs and showed four cell lines with distinct defects during virus early transcription-translation, genome replication and late protein synthesis. Furthermore, an antiviral role of the stress granule pathway was revealed against ASFV, and ASFV infection inhibited stress granule formation in PAMs but not 3D4/21. These results will help to deepen our knowledge on ASFV infection and to develop ASFV susceptible cell lines.

Nucleolar Protein Treacle Is Important for the Efficient Growth of Mumps Virus

The nucleolus is the largest structure in the nucleus, and it plays roles in mediating cellular stress responses and regulating cell proliferation, as well as in ribosome biosynthesis. The nucleolus is composed of a variety of nucleolar factors that interact with each other in a complex manner to enable its function. Many viral proteins interact with nucleolar factors as well, affecting cellular morphology and function. Here, to investigate the association between mumps virus (MuV) infection and the nucleolus, we evaluated the necessity of nucleolar factors for MuV proliferation by performing a knockdown of these factors with small interfering (si)RNAs. Our results reveal that suppressing the expression of Treacle, which is required for ribosome biosynthesis, reduced the proliferative potential of MuV. Additionally, the one-step growth kinetics results indicate that Treacle knockdown did not affect the viral RNA and protein synthesis of MuV, but it did impair the production of infectious virus particles. Viral matrix protein (M) was considered a candidate Treacle interaction partner because it functions in the process of particle formation in the viral life cycle and is partially localized in the nucleolus. Our data confirm that MuV M can interact with Treacle and colocalize with it in the nucleolus. Furthermore, we found that viral infection induces relocalization of Treacle in the nucleus. Together, these findings suggest that interaction with Treacle in the nucleolus is important for the M protein to exert its functions late in the MuV life cycle. IMPORTANCE The nucleolus, which is the site of ribosome biosynthesis, is a target organelle for many viruses. It is increasingly evident that viruses can favor their own replication and multiplication by interacting with various nucleolar factors. In this study, we found that the nucleolar protein Treacle, known to function in the transcription and processing of pre-rRNA, is required for the efficient propagation of mumps virus (MuV). Specifically, our data indicate that Treacle is not involved in viral RNA or protein synthesis but is important in the processes leading to viral particle production in MuV infection. Additionally, we determined that MuV matrix protein (M), which functions mainly in viral particle assembly and budding, colocalized and interacted with Treacle. Furthermore, we found that Treacle is distributed throughout the nucleus in MuV-infected cells. Our research shows that the interaction between M and Treacle supports efficient viral growth in the late stage of MuV infection.

Molecular pathogenesis of Japanese encephalitis and possible therapeutic strategies

Japanese encephalitis virus (JEV), a single-stranded, enveloped RNA virus, is a health concern across Asian countries, associated with severe neurological disorders, especially in children. Primarily, pigs, bats, and birds are the natural hosts for JEV, but humans are infected incidentally. JEV requires a few host proteins for its entry and replication inside the mammalian host cell. The endoplasmic reticulum (ER) plays a significant role in JEV genome replication and assembly. During this process, the ER undergoes stress due to its remodelling and accumulation of viral particles and unfolded proteins, leading to an unfolded protein response (UPR). Here, we review the overall strategy used by JEV to infect the host cell and various cytopathic effects caused by JEV infection. We also highlight the role of JEV structural proteins (SPs) and non-structural proteins (NSPs) at various stages of the JEV life cycle that are involved in up- and downregulation of different host proteins and are potentially relevant for developing efficient therapeutic drugs.

Melatonin: Regulation of Viral Phase Separation and Epitranscriptomics in Post-Acute Sequelae of COVID-19

The relentless, protracted evolution of the SARS-CoV-2 virus imposes tremendous pressure on herd immunity and demands versatile adaptations by the human host genome to counter transcriptomic and epitranscriptomic alterations associated with a wide range of short- and long-term manifestations during acute infection and post-acute recovery, respectively. To promote viral replication during active infection and viral persistence, the SARS-CoV-2 envelope protein regulates host cell microenvironment including pH and ion concentrations to maintain a high oxidative environment that supports template switching, causing extensive mitochondrial damage and activation of pro-inflammatory cytokine signaling cascades. Oxidative stress and mitochondrial distress induce dynamic changes to both the host and viral RNA m6A methylome, and can trigger the derepression of long interspersed nuclear element 1 (LINE1), resulting in global hypomethylation, epigenetic changes, and genomic instability. The timely application of melatonin during early infection enhances host innate antiviral immune responses by preventing the formation of "viral factories" by nucleocapsid liquid-liquid phase separation that effectively blockades viral genome transcription and packaging, the disassembly of stress granules, and the sequestration of DEAD-box RNA helicases, including DDX3X, vital to immune signaling. Melatonin prevents membrane depolarization and protects cristae morphology to suppress glycolysis via antioxidant-dependent and -independent mechanisms. By restraining the derepression of LINE1 via multifaceted strategies, and maintaining the balance in m6A RNA modifications, melatonin could be the quintessential ancient molecule that significantly influences the outcome of the constant struggle between virus and host to gain transcriptomic and epitranscriptomic dominance over the host genome during acute infection and PASC.

Stressful steps: Progress and challenges in understanding stress-induced mRNA condensation and accumulation in stress granules

Stress-induced condensation of mRNA and protein into massive cytosolic clusters is conserved across eukaryotes. Known as stress granules when visible by imaging, these structures remarkably have no broadly accepted biological function, mechanism of formation or dispersal, or even molecular composition. As part of a larger surge of interest in biomolecular condensation, studies of stress granules and related RNA/protein condensates have increasingly probed the biochemical underpinnings of condensation. Here, we review open questions and recent advances, including the stages from initial condensate formation to accumulation in mature stress granules, mechanisms by which stress-induced condensates form and dissolve, and surprising twists in understanding the RNA components of stress granules and their role in condensation. We outline grand challenges in understanding stress-induced RNA condensation, centering on the unique and substantial barriers in the molecular study of cellular structures, such as stress granules, for which no biological function has been firmly established.

Flavivirus recruits the valosin-containing protein-NPL4 complex to induce stress granule disassembly for efficient viral genome replication

Flaviviruses are human pathogens that can cause severe diseases, such as dengue fever and Japanese encephalitis, which can lead to death. Valosin-containing protein (VCP)/p97, a cellular ATPase associated with diverse cellular activities (AAA-ATPase), is reported to have multiple roles in flavivirus replication. Nevertheless, the importance of each role still has not been addressed. In this study, the functions of 17 VCP mutants that are reportedly unable to interact with the VCP cofactors were validated using the short-interfering RNA rescue experiments. Our findings of this study suggested that VCP exerts its functions in replication of the Japanese encephalitis virus by interacting with the VCP cofactor nuclear protein localization 4 (NPL4). We show that the depletion of NPL4 impaired the early stage of viral genome replication. In addition, we demonstrate that the direct interaction between NPL4 and viral nonstructural protein (NS4B) is critical for the translocation of NS4B to the sites of viral replication. Finally, we found that Japanese encephalitis virus and dengue virus promoted stress granule formation only in VCP inhibitor-treated cells and the expression of NS4B or VCP attenuated stress granule formation mediated by protein kinase R, which is generally known to be activated by type I interferon and viral genome RNA. These results suggest that the NS4B-mediated recruitment of VCP to the virus replication site inhibits cellular stress responses and consequently facilitates viral protein synthesis in the flavivirus-infected cells.

[Advances in the reverse genetics system for RNA viruses]

RNA viruses are responsible for several infectious diseases, including dengue fever, Zika fever, and COVID-19. Reverse genetics is a powerful tool to elucidate which domain or mutations in RNA viruses determine their pathogenicity and ability to evade antiviral drugs and host immune response. Previous reverse genetics systems for flaviviruses and coronaviruses have been technically challenging and time-consuming, thereby hampering the further understanding of events during viral evolution. A novel reverse genetics system-circular polymerase extension reaction (CPER)-has been developed to overcome this limitation. CPER is based on PCR-mediated assembly of DNA fragments that encode the whole genome of these viruses. CPER requires a relatively short time to introduce specific mutations into the viral genome of flaviviruses and SARS-CoV-2. In this review article, we explain the mode of action of this system and discuss the future direction of reverse genetics for RNA viruses.

Research Progress on the Structure and Function of G3BP

Ras-GTPase-activating protein (SH3 domain)-binding protein (G3BP) is an RNA binding protein. G3BP is a key component of stress granules (SGs) and can interact with many host proteins to regulate the expression of SGs. As an antiviral factor, G3BP can interact with viral proteins to regulate the assembly of SGs and thus exert antiviral effects. However, many viruses can also use G3BP as a proximal factor and recruit translation initiation factors to promote viral proliferation. G3BP regulates mRNA translation and attenuation to regulate gene expression; therefore, it is closely related to diseases, such as cancer, embryonic death, arteriosclerosis, and neurodevelopmental disorders. This review discusses the important discoveries and developments related G3BP in the biological field over the past 20 years, which includes the formation of SGs, interaction with viruses, stability of RNA, and disease progression.

How Do Flaviviruses Hijack Host Cell Functions by Phase Separation?

Viral proteins interact with different sets of host cell components throughout the viral life cycle and are known to localize to the intracellular membraneless organelles (MLOs) of the host cell, where formation/dissolution is regulated by phase separation of intrinsically disordered proteins and regions (IDPs/IDRs). Viral proteins are rich in IDRs, implying that viruses utilize IDRs to regulate phase separation of the host cell organelles and augment replication by commandeering the functions of the organelles and/or sneaking into the organelles to evade the host immune response. This review aims to integrate current knowledge of the structural properties and intracellular localizations of viral IDPs to understand viral strategies in the host cell. First, the properties of viral IDRs are reviewed and similarities and differences with those of eukaryotes are described. The higher IDR content in viruses with smaller genomes suggests that IDRs are essential characteristics of viral proteins. Then, the interactions of the IDRs of flaviviruses with the MLOs of the host cell are investigated with emphasis on the viral proteins localized in the nucleoli and stress granules. Finally, the possible roles of viral IDRs in regulation of the phase separation of organelles and future possibilities for antiviral drug development are discussed.

Inhibition of eIF2α Phosphorylation by Peste des Petits Ruminant Virus Phosphoprotein Facilitates Viral Replication

Peste des petits ruminant virus (PPRV) causes a highly contagious disease in small ruminants. The molecular mechanism of PPRV replication and its interactions with hosts are poorly studied. In other paramyxoviruses, the viral phosphoprotein (P) has been associated with multiple functions for key biological processes such as the regulation of transcription, translation, and the control of cell cycle. Phosphorylation of the α subunit of eukaryotic initiation factor 2 (eIF2α) is an important process for gene regulation in host cells under stress, including viral infection. In the present study, molecular mechanisms associated with PPRV replication and viral interaction with host cells were investigated. We describe the ability of PPRV to dephosphorylate eIF2α and the potential of PPRV P protein to induce the host cellular growth arrest DNA damage protein (GADD34), which is known to be associated with eIF2α dephosphorylation. Furthermore, we observed that PPRV P protein alone could block PERK/eIF2α phosphorylation. We speculate that PPRV exploits eIF2α dephosphorylation to facilitate viral replication and that PPRV P protein is involved in this molecular mechanism. This work provides new insights into further understanding PPRV pathobiology and its viral/host interactions.

Structure and function of capsid protein in flavivirus infection and its applications in the development of vaccines and therapeutics

Flaviviruses are enveloped single positive-stranded RNA viruses. The capsid (C), a structural protein of flavivirus, is dimeric and alpha-helical, with several special structural and functional features. The functions of the C protein go far beyond a structural role in virions. It is not only responsible for encapsidation to protect the viral RNA but also able to interact with various host proteins to promote virus proliferation. Therefore, the C protein plays an important role in infected host cells and the viral life cycle. Flaviviruses have been shown to affect the health of humans and animals. Thus, there is an urgent need to effectively control flavivirus infections. The structure of the flavivirus virion has been determined, but there is relatively little information about the function of the C protein. Hence, a greater understanding of the role of the C protein in viral infections will help to discover novel antiviral strategies and provide a promising starting point for the further development of flavivirus vaccines or therapeutics.

Stress granules: regulators or by-products?

Cells have to deal with conditions that can cause damage to biomolecules and eventually cell death. To protect against these adverse conditions and promote recovery, cells undergo dramatic changes upon exposure to stress. This involves activation of signaling pathways, cell cycle arrest, translational reprogramming, and reorganization of the cytoplasm. Notably, many stress conditions cause a global inhibition of mRNA translation accompanied by the formation of cytoplasmic condensates called stress granules (SGs), which sequester mRNA together with RNA-binding proteins, translation initiation factors, and other components. SGs are highly conserved in eukaryotes, suggesting that they perform an important function during the stress response. Over the years, many different roles have been assigned to SGs, including translational control, mRNA storage, regulation of mRNA decay, antiviral innate immune response, and modulation of signaling pathways. Most of our understanding, however, has been deduced from correlative data based upon the composition of SGs and only recently have technological innovations allowed hypotheses for SG function to be directly tested. Here, we discuss these challenges and explore the evidence related to the function of SGs.

LncRNA GIRGL drives CAPRIN1-mediated phase separation to suppress glutaminase-1 translation under glutamine deprivation

Glutamine constitutes an essential source of both carbon and nitrogen for numerous biosynthetic processes. The first and rate-limiting step of glutaminolysis involves the generation of glutamate from glutamine, catalyzed by glutaminase-1 (GLS1). Shortages of glutamine result in reductions in GLS1, but the underlying mechanisms are not fully known. Here, we characterize a long noncoding RNA, GIRGL (glutamine insufficiency regulator of glutaminase lncRNA), that is induced upon glutamine starvation. Manipulating GIRGL revealed a relationship between its expression and the translational suppression of GLS1. Cellular GIRGL levels are balanced by a combination of transactivation by c-JUN together with negative stability regulation via HuR/Ago2. Increased levels of GIRGL in the absence of glutamine drive formation of a complex between dimers of CAPRIN1 and GLS1 mRNA, serving to promote liquid-liquid phase separation of CAPRIN1 and inducing stress granule formation. Suppressing GLS1 mRNA translation enables cancer cells to survive under prolonged glutamine deprivation stress.

Arthropods Under Pressure: Stress Responses and Immunity at the Pathogen-Vector Interface

Understanding what influences the ability of some arthropods to harbor and transmit pathogens may be key for controlling the spread of vector-borne diseases. Arthropod immunity has a central role in dictating vector competence for pathogen acquisition and transmission. Microbial infection elicits immune responses and imparts stress on the host by causing physical damage and nutrient deprivation, which triggers evolutionarily conserved stress response pathways aimed at restoring cellular homeostasis. Recent studies increasingly recognize that eukaryotic stress responses and innate immunity are closely intertwined. Herein, we describe two well-characterized and evolutionarily conserved mechanisms, the Unfolded Protein Response (UPR) and the Integrated Stress Response (ISR), and examine evidence that these stress responses impact immune signaling. We then describe how multiple pathogens, including vector-borne microbes, interface with stress responses in mammals. Owing to the well-conserved nature of the UPR and ISR, we speculate that similar mechanisms may be occurring in arthropod vectors and ultimately impacting vector competence. We conclude this Perspective by positing that novel insights into vector competence will emerge when considering that stress-signaling pathways may be influencing the arthropod immune network.

Seneca Valley Virus 3C Protease Inhibits Stress Granule Formation by Disrupting eIF4GI-G3BP1 Interaction

Stress granules (SGs) are the sites of mRNA storage and related to the regulation of mRNA translation, which are dynamic structures in response to various environmental stresses and viral infections. Seneca Valley virus (SVV), an oncolytic RNA virus belonging to Picornaviridae family, can cause vesicular disease (VD) indistinguished from foot-and-mouth disease (FMD) and other pig VDs. In this study, we found that SVV induced SG formation in the early stage of infection in a PKR-eIF2α dependent manner, as demonstrated by the recruitment of marker proteins of G3BP1 and eIF4GI. Surprisingly, we found that downregulating SG marker proteins TIA1 or G3BP1, or expressing an eIF2α non-phosphorylatable mutant inhibited SG formation, but this inhibition of transient SG formation had no significant effect on SVV propagation. Depletion of G3BP1 significantly attenuated the activation of NF-κB signaling pathway. In addition, we found that SVV inhibited SG formation at the late stage of infection and 3C protease was essential for the inhibition depending on its enzyme activity. Furthermore, we also found that 3C protease blocked the SG formation by disrupting eIF4GI-G3BP1 interaction. Overall, our results demonstrate that SVV induces transient SG formation in an eIF2α phosphorylation and PKR-dependent manner, and that 3C protease inhibits SG formation by interfering eIF4GI-G3BP1 interaction.

Dance with the Devil: Stress Granules and Signaling in Antiviral Responses

Cells have evolved highly specialized sentinels that detect viral infection and elicit an antiviral response. Among these, the stress-sensing protein kinase R, which is activated by double-stranded RNA, mediates suppression of the host translation machinery as a strategy to limit viral replication. Non-translating mRNAs rapidly condensate by phase separation into cytosolic stress granules, together with numerous RNA-binding proteins and components of signal transduction pathways. Growing evidence suggests that the integrated stress response, and stress granules in particular, contribute to antiviral defense. This review summarizes the current understanding of how stress and innate immune signaling act in concert to mount an effective response against virus infection, with a particular focus on the potential role of stress granules in the coordination of antiviral signaling cascades.

Crystal structure of a 123 amino acids dimerization domain of Drosophila Caprin

Cytoplasmic activation/proliferation-associated protein (Caprin) proteins assume diverse functions in many important biological processes, including synaptic plasticity, stress response, innate immune response, and cellular proliferation. The Caprin family members are characterized by the presence of a highly conserved homologous region (HR1) at the N-terminus and arginine-glycine-rich (RGG) boxes at the C-terminus. We had previously determined the crystal structures of human Caprin-1 and Caprin-2 fragments corresponding to the C-terminal 2/3 of HR1. Both fragments adopt homodimeric structures. Based on sequence conservation, we speculated that all Caprin proteins should have similar homodimeric structures. Here we report the crystal structure of a fragment (residues 187-309) of Drosophila melanogaster Caprin (dCaprin). The dCaprin fragment adopts an all α-helical fold which self-associates to form a homodimer. The overall dCaprin homodimeric structure is similar to the Caprin-1 and Caprin-2 homodimeric structures. Most of the amino acids residues mediating homodimerization in the three structures are conserved among all Caprin family members. These structural and sequence data suggest that homodimerization through a conserved dimerization domain is a common structural feature of the Caprin protein family. The dimeric structures may also be involved in interaction with Caprin partners. Dimer formation creates a V-shape concave surface that may serve as a protein binding groove. The concave surfaces in Caprin-1, Caprin-2, and dCaprin should have different and specific binding partners due to the large difference in electrostatic potentials. We propose the existence of a multi-functional domain in Caprin proteins, which not only mediate homodimerization but also involve in interaction with specific Caprin partners.

Role of RNA-binding proteins during the late stages of Flavivirus replication cycle

The genus Flavivirus encompasses several worldwide-distributed arthropod-borne viruses including, dengue virus, Japanese encephalitis virus, West Nile virus, yellow fever virus, Zika virus, and tick-borne encephalitis virus. Infection with these viruses manifest with symptoms ranging from febrile illness to life- threatening hypotensive shock and encephalitis. Therefore, flaviviruses pose a great risk to public health. Currently, preventive measures are falling short to control epidemics and there are no antivirals against any Flavivirus.Flaviviruses carry a single stranded positive-sense RNA genome that plays multiple roles in infected cells: it is translated into viral proteins, used as template for genome replication, it is the precursor of the subgenomic flaviviral RNA and it is assembled into new virions. Furthermore, viral RNA genomes are also packaged into extracellular vesicles, e.g. exosomes, which represent an alternate mode of virus dissemination.Because RNA molecules are at the center of Flavivirus replication cycle, viral and host RNA-binding proteins (RBPs) are critical determinants of infection. Numerous studies have revealed the function of RBPs during Flavivirus infection, particularly at the level of RNA translation and replication. These proteins, however, are also critical participants at the late stages of the replication cycle. Here we revise the function of host RBPs and the viral proteins capsid, NS2A and NS3, during the packaging of viral RNA and the assembly of new virus particles. Furthermore, we go through the evidence pointing towards the importance of host RBPs in mediating cellular RNA export with the idea that the biogenesis of exosomes harboring Flavivirus RNA would follow an analogous pathway.

Crosstalk between RNA Metabolism and Cellular Stress Responses during Zika Virus Replication

Zika virus (ZIKV) is a mosquito-borne virus associated with neurological disorders such as Guillain-Barré syndrome and microcephaly. In humans, ZIKV is able to replicate in cell types from different tissues including placental cells, neurons, and microglia. This intricate virus-cell interaction is accompanied by virally induced changes in the infected cell aimed to promote viral replication as well as cellular responses aimed to counteract or tolerate the virus. Early in the infection, the 11-kb positive-sense RNA genome recruit ribosomes in the cytoplasm and the complex is translocated to the endoplasmic reticulum (ER) for viral protein synthesis. In this process, ZIKV replication is known to induce cellular stress, which triggers both the expression of innate immune genes and the phosphorylation of eukaryotic translation initiation factor 2 (eIF2α), shutting-off host protein synthesis. Remodeling of the ER during ZIKV replication also triggers the unfolded protein response (UPR), which induces changes in the cellular transcriptional landscapes aimed to tolerate infection or trigger apoptosis. Alternatively, ZIKV replication induces changes in the adenosine methylation patterns of specific host mRNAs, which have different consequences in viral replication and cellular fate. In addition, the ZIKV RNA genome undergoes adenosine methylation by the host machinery, which results in the inhibition of viral replication. However, despite these relevant findings, the full scope of these processes to the outcome of infection remains poorly elucidated. This review summarizes relevant aspects of the complex crosstalk between RNA metabolism and cellular stress responses against ZIKV and discusses their possible impact on viral pathogenesis.

Proximity RNA Labeling by APEX-Seq Reveals the Organization of Translation Initiation Complexes and Repressive RNA Granules

Diverse ribonucleoprotein complexes control mRNA processing, translation, and decay. Transcripts in these complexes localize to specific regions of the cell and can condense into non-membrane-bound structures such as stress granules. It has proven challenging to map the RNA composition of these large and dynamic structures, however. We therefore developed an RNA proximity labeling technique, APEX-seq, which uses the ascorbate peroxidase APEX2 to probe the spatial organization of the transcriptome. We show that APEX-seq can resolve the localization of RNAs within the cell and determine their enrichment or depletion near key RNA-binding proteins. Matching the spatial transcriptome, as revealed by APEX-seq, with the spatial proteome determined by APEX-mass spectrometry (APEX-MS), obtained precisely in parallel, provides new insights into the organization of translation initiation complexes on active mRNAs and unanticipated complexity in stress granule composition. Our novel technique allows a powerful and general approach to explore the spatial environment of macromolecules.

Inducible degradation of lncRNA Sros1 promotes IFN-γ-mediated activation of innate immune responses by stabilizing Stat1 mRNA

Interferon-γ (IFN-γ) is essential for the innate immune response to intracellular bacteria. Noncoding RNAs and RNA-binding proteins (RBPs) need to be further considered in studies of regulation of the IFN-γ-activated signaling pathway in macrophages. In the present study, we found that the microRNA miR-1 promoted IFN-γ-mediated clearance of Listeria monocytogenes in macrophages by indirectly stabilizing the Stat1 messenger RNA through the degradation of the cytoplasmic long noncoding RNA Sros1. Inducible degradation or genetic loss of Sros1 led to enhanced IFN-γ-dependent activation of the innate immune response. Mechanistically, Sros1 blocked the binding of Stat1 mRNA to the RBP CAPRIN1, which stabilized the Stat1 mRNA and, consequently, promoted IFN-γ-STAT1-mediated innate immunity. These observations shed light on the complex RNA-RNA regulatory networks involved in cytokine-initiated innate responses in host-pathogen interactions.

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.

Functional Correlation between Subcellular Localizations of Japanese Encephalitis Virus Capsid Protein and Virus Production

The flavivirus capsid protein is considered to be essential for the formation of nucleocapsid complexes with viral genomic RNA at the viral replication organelle that appears on the endoplasmic reticulum (ER), as well as for incorporation into virus particles. However, this protein is also detected at the lipid droplet (LD) and nucleolus, and physiological roles of these off-site localizations are still unclear. In this study, we made a series of alanine substitution mutants of Japanese encephalitis virus (JEV) capsid protein that cover all polar and hydrophobic amino acid residues to identify the molecular surfaces required for virus particle formation and for localization at the LD and nucleolus. Five mutants exhibited a defect in the formation of infectious particles, and two of these mutants failed to be incorporated into the subviral particles (SVP). Three mutants lost the ability to localize to the nucleolus, and only a single mutant, with mutations at α2, was unable to localize to the LD. Unlike the cytoplasmic capsid protein, the nucleolar capsid protein was resistant to detergent treatment, and the α2 mutant was hypersensitive to detergent treatment. To scrutinize the relationship between these localizations and viral particle formation, we made eight additional alanine substitution mutants and found that all the mutants that did not localize at the LD or nucleolus failed to form normal viral particles. These results support the functional correlation between LD or nucleolus localization of the flaviviral capsid protein and the formation of infectious viral particles.IMPORTANCE This study is the first to report the comprehensive mutagenesis of a flavivirus capsid protein. We assessed the requirement of each molecular surface for infectious viral particle formation as well as for LD and nucleolar localization and found functional relationships between the subcellular localization of the virus capsid protein and infectious virus particle formation. We developed a system to independently assess the packaging of viral RNA and that of the capsid protein and found a molecular surface of the capsid protein that is crucial for packaging of viral RNA but not for packaging of the capsid protein itself. We also characterized the biochemical properties of capsid protein mutants and found that the capsid protein localizes at the nucleolus in a different manner than for its localization to the LD. Our comprehensive alanine-scanning mutagenesis study will aid in the development of antiflavivirus small molecules that can target the flavivirus capsid protein.

Japanese Encephalitis Virus Induces Apoptosis and Encephalitis by Activating the PERK Pathway

Accumulated evidence demonstrates that Japanese encephalitis virus (JEV) infection triggers endoplasmic reticulum (ER) stress and neuron apoptosis. ER stress sensor protein kinase R-like endoplasmic reticulum kinase (PERK) has been reported to induce apoptosis under acute or prolonged ER stress. However, the precise role of PERK in JEV-induced apoptosis and encephalitis remains unknown. Here, we report that JEV infection activates the PERK-ATF4-CHOP apoptosis pathway both in vitro and in vivo PERK activation also promotes the formation of stress granule, which in turn represses JEV-induced apoptosis. However, PERK inhibitor reduces apoptosis, indicating that JEV-activated PERK predominantly induces apoptosis via the PERK-ATF4-CHOP apoptosis pathway. Among JEV proteins that have been reported to induce ER stress, only JEV NS4B can induce PERK activation. PERK has been reported to form an active molecule by dimerization. The coimmunoprecipitation assay shows that NS4B interacts with PERK. Moreover, glycerol gradient centrifugation shows that NS4B induces PERK dimerization. Both the LIG-FHA and the LIG-WD40 domains within NS4B are required to induce PERK dimerization, suggesting that JEV NS4B pulls two PERK molecules together by simultaneously interacting with them via different motifs. PERK deactivation reduces brain cell damage and encephalitis during JEV infection. Furthermore, expression of JEV NS4B is sufficient to induce encephalitis via PERK in mice, indicating that JEV activates PERK primarily via its NS4B to cause encephalitis. Taken together, our findings provide a novel insight into JEV-caused encephalitis.IMPORTANCE Japanese encephalitis virus (JEV) infection triggers endoplasmic reticulum (ER) stress and neuron apoptosis. ER stress sensor protein kinase R-like endoplasmic reticulum kinase (PERK) has been reported to induce apoptosis under acute or prolonged ER stress. However, whether the PERK pathway of ER stress response plays important roles in JEV-induced apoptosis and encephalitis remains unknown. Here, we found that JEV infection activates ER stress sensor PERK in neuronal cells and mouse brains. PERK activation induces apoptosis via the PERK-ATF4-CHOP apoptosis pathway upon JEV infection. Among the JEV proteins prM, E, NS1, NS2A, NS2B, and NS4B, only NS4B activates PERK. Moreover, activated PERK participates in apoptosis and encephalitis induced by JEV and NS4B. These findings provide a novel therapeutic approach for JEV-caused encephalitis.

Zika Virus Subverts Stress Granules To Promote and Restrict Viral Gene Expression

Many viruses inhibit SGs. In this study, we observed that ZIKV restricts SG assembly, likely by relocalizing and subverting specific SG proteins to modulate ZIKV replication. This ZIKV-SG protein interaction is interesting, as many SG proteins are also known to function in neuronal granules, which are critical in neural development and function. Moreover, dysregulation of different SG proteins in neurons has been shown to play a role in the progression of neurodegenerative diseases. The likely consequences of ZIKV modulating SG assembly and subverting specific SG proteins are alterations to cellular mRNA transcription, splicing, stability, and translation. Such changes in cellular ribostasis could profoundly affect neural development and contribute to the devastating developmental and neurological anomalies observed following intrauterine ZIKV infection. Our study provides new insights into virus-host interactions and the identification of the SG proteins that may contribute to the unusual pathogenesis associated with this reemerging arbovirus.

Flaviviruses limit the cell stress response by preventing the formation of stress granules (SGs) and modulate viral gene expression by subverting different proteins involved in the stress granule pathway. In this study, we investigated the formation of stress granules during Zika virus (ZIKV) infection and the role stress granule proteins play during the viral life cycle. Using immunofluorescence and confocal microscopy, we determined that ZIKV disrupted the formation of arsenite-induced stress granules and changed the subcellular distribution, but not the abundance or integrity, of stress granule proteins. We also investigated the role of different stress granule proteins in ZIKV infection by using target-specific short interfering RNAs to deplete Ataxin2, G3BP1, HuR, TIA-1, TIAR, and YB1. Knockdown of TIA-1 and TIAR affected ZIKV protein and RNA levels but not viral titers. Conversely, depletion of Ataxin2 and YB1 decreased virion production despite having only a small effect on ZIKV protein expression. Notably, however, depletion of G3BP1 and HuR decreased and increased ZIKV gene expression and virion production, respectively. Using an MR766 Gaussia Luciferase reporter genome together with knockdown and overexpression assays, G3BP1 and HuR were found to modulate ZIKV replication. These data indicate that ZIKV disrupts the formation of stress granules by sequestering stress granule proteins required for replication, where G3BP1 functions to promote ZIKV infection while HuR exhibits an antiviral effect. The results of ZIKV relocalizing and subverting select stress granule proteins might have broader consequences on cellular RNA homeostasis and contribute to cellular gene dysregulation and ZIKV pathogenesis.

IMPORTANCE Many viruses inhibit SGs. In this study, we observed that ZIKV restricts SG assembly, likely by relocalizing and subverting specific SG proteins to modulate ZIKV replication. This ZIKV-SG protein interaction is interesting, as many SG proteins are also known to function in neuronal granules, which are critical in neural development and function. Moreover, dysregulation of different SG proteins in neurons has been shown to play a role in the progression of neurodegenerative diseases. The likely consequences of ZIKV modulating SG assembly and subverting specific SG proteins are alterations to cellular mRNA transcription, splicing, stability, and translation. Such changes in cellular ribostasis could profoundly affect neural development and contribute to the devastating developmental and neurological anomalies observed following intrauterine ZIKV infection. Our study provides new insights into virus-host interactions and the identification of the SG proteins that may contribute to the unusual pathogenesis associated with this reemerging arbovirus.

The R2TP complex regulates paramyxovirus RNA synthesis

The regulation of paramyxovirus RNA synthesis by host proteins is poorly understood. Here, we identified a novel regulation mechanism of paramyxovirus RNA synthesis by the Hsp90 co-chaperone R2TP complex. We showed that the R2TP complex interacted with the paramyxovirus polymerase L protein and that silencing of the R2TP complex led to uncontrolled upregulation of mumps virus (MuV) gene transcription but not genome replication. Regulation by the R2TP complex was critical for MuV replication and evasion of host innate immune responses. The R2TP complex also regulated measles virus (MeV) RNA synthesis, but its function was inhibitory and not beneficial to MeV, as MeV evaded host innate immune responses in the absence of the R2TP complex. The identification of the R2TP complex as a critical host factor sheds new light on the regulation of paramyxovirus RNA synthesis.

The family Paramyxoviridae includes several important human and animal pathogens such as mumps virus (MuV) and measles virus (MeV). Paramyxovirus RNA synthesis is strictly regulated by both viral and host proteins. In this study, we identified the R2TP complex as a novel host factor regulating paramyxovirus RNA synthesis. The R2TP complex is a Hsp90 co-chaperone and is involved in Hsp90-mediated assembly of large protein complexes. We showed that the R2TP complex precisely regulated MuV transcription by interacting with the polymerase L protein. This regulation was critical for MuV evasion of host innate immune responses and for viral replication. We also showed that the R2TP complex regulated MeV RNA synthesis, but that its function was inhibitory and not beneficial to MeV. Our findings support a novel regulation mechanism of paramyxovirus RNA synthesis that is directly relevant to its biology and life cycle, and provide the first evidence linking the R2TP complex to defense against viral infection.

Role of caprin-1 in carcinogenesis

RNA-binding proteins serve an essential role in post-transcriptional gene regulation. Cytoplasmic activation/proliferation-associated protein-1 (caprin-1) is an RNA-binding protein that participates in the regulation of cell cycle control-associated genes. Caprin-1 acts alone or in combination with other RNA-binding proteins, such as RasGAP SH3-domain-binding protein 1 and fragile X mental retardation protein. In the tumorigenesis process, caprin-1 primarily functions by activating cell proliferation and upregulating the expression of immune checkpoint proteins. Through the formation of stress granules, caprin-1 is also involved in the process by which tumor cells adapt to adverse conditions, which contributes to radiation and chemotherapy resistance. Given its role in various clinical malignancies, caprin-1 holds the potential to be used as a biomarker and a target for the development of novel therapeutics. The present review describes this newly identified putative oncogenic protein and its possible impact on tumorigenesis.

SG formation relies on eIF4GI-G3BP interaction which is targeted by picornavirus stress antagonists

Typical stress granules (tSGs) are stalled translation pre-initiation complex aggregations in the cytoplasm, and their formation is a common consequence of translation initiation inhibition under stress. We previously found that 2A protease of picornaviruses blocks tSG formation and induces atypical SG formation, but the molecular mechanism by which 2A inhibits tSG formation remains unclear. Here, we found that eukaryotic translation initiation factor 4 gamma1 (eIF4GI) is critical for tSG formation by interacting with Ras-GTPase-activating protein SH3-domain-binding protein (G3BP), and this interaction is mediated by aa 182-203 of eIF4GI and the RNA-binding domain of G3BP. Upon eIF4GI-G3BP interaction, eIF4GI can assemble into tSGs and rescue tSG formation. Finally, we found that 2A or L protein of picornaviruses blocks tSG formation by disrupting eIF4GI-G3BP interaction. Our findings provide the first evidence that eIF4GI-G3BP interaction is indispensable for tSG formation, and 2A or L protein of picornaviruses interferes eIF4GI-G3BP interaction, thereby blocking tSG formation.

Crystal structure of a dimerization domain of human Caprin-2: similar overall dimeric fold but different molecular surface properties to that of human Caprin-1

Human Caprin-1 and Caprin-2 are prototypic members of the caprin (cytoplasmic activation/proliferation-associated protein) protein family. Vertebrate caprin proteins contain two highly conserved homologous regions (HR1 and HR2) and C-terminal RGG motifs. Drosophila caprin (dCaprin) shares HR1 and RGG motifs but lacks HR2. Caprin-1 and Caprin-2 have important and non-redundant functions. The detailed molecular mechanisms of their actions remain largely unknown. Previously, we determined the crystal structure of a ∼120-residue fragment of Caprin-1 within the HR1 region. The structure has a novel all α-helical fold that self-associates to form a homodimer. In this study, the crystal structure of a corresponding fragment from Caprin-2 is reported. The Caprin-2 fragment has similar protein fold and dimeric structure as that of the Caprin-1 fragment. Structural comparison reveals that the molecular interactions mediating homodimerization of Caprin-1 and Caprin-2 are largely conserved in the two systems. Structural-modelling study of the corresponding dCaprin fragment indicates that dCaprin may also adopt a similar dimeric structure. The presence of a dimerization domain within HR1 may represent an evolutionarily conserved feature of the caprin protein family. Interestingly, while Caprin-1 and Caprin-2 adopt similar overall dimeric structures, the two structures have quite different molecular surface properties. In the Caprin-1 dimeric structure, some of the surface areas are known or suspected to function as binding sites for Carpin-1-interacting proteins. The different surface properties of the caprin dimeric structures may dictate their intermolecular interaction with specific protein partners. Communicated by Ramaswamy H. Sarma.

Importin α5 negatively regulates importin β1-mediated nuclear import of Newcastle disease virus matrix protein and viral replication and pathogenicity in chicken fibroblasts

The matrix (M) protein of Newcastle disease virus (NDV) is demonstrated to localize in the nucleus via intrinsic nuclear localization signal (NLS), but cellular proteins involved in the nuclear import of NDV M protein and the role of M's nuclear localization in the replication and pathogenicity of NDV remain unclear. In this study, importin β1 was screened to interact with NDV M protein by yeast two-hybrid screening. This interaction was subsequently confirmed by co-immunoprecipitation and pull-down assays. In vitro binding studies indicated that the NLS region of M protein and the amino acids 336–433 of importin β1 that belonged to the RanGTP binding region were important for binding. Importantly, a recombinant virus with M/NLS mutation resulted in a pathotype change of NDV and attenuated viral replication and pathogenicity in chicken fibroblasts and SPF chickens. In agreement with the binding data, nuclear import of NDV M protein in digitonin-permeabilized HeLa cells required both importin β1 and RanGTP. Interestingly, importin α5 was verified to interact with M protein through binding importin β1. However, importin β1 or importin α5 depletion by siRNA resulted in different results, which showed the obviously cytoplasmic or nuclear accumulation of M protein and the remarkably decreased or increased replication ability and pathogenicity of NDV in chicken fibroblasts, respectively. Our findings therefore demonstrate for the first time the nuclear import mechanism of NDV M protein and the negative regulation role of importin α5 in importin β1-mediated nuclear import of M protein and the replication and pathogenicity of a paramyxovirus.

Tellurium/Bovine Serum Albumin Nanocomposites Inducing the Formation of Stress Granules in a Protein Kinase R-Dependent Manner

The effect of nanoparticles (NPs) on cellular stress responses is important to the understanding of nanotoxicities and developing safe therapies. Although the relationship between NPs and cellular stress responses has been preliminarily investigated, stress responses to NPs remain unclear. Here, tellurium/bovine serum albumin (Te/BSA) nanocomposites were prepared using sodium tellurite, BSA, and glutathione as precursors. The as-prepared Te/BSA nanocomposites, with particle size similar to that of many viruses, are found to induce the formation of stress granules (SGs), a kind of cytoplasmic RNA granule formed under various stresses. The SGs in Te/BSA nanocomposite-treated cells are composed of T-cell internal antigen 1 (TIA1), TIA1-related protein, and eukaryotic initiation factor 3η. Using chemical inhibitors and small interfering RNA-mediated silencing, protein kinase R (PKR) is identified as the α-subunit of eukaryotic initiation factor 2 (eIF2α)-kinase activated upon Te/BSA nanocomposite incubation, which is also the dominant kinase responsible for eIF2α activation under virus infection. Mechanistically, PKR is activated in a heparin-dependent manner. This study reveals a biological effect of Te/BSA nanocomposites on stress responses, providing a preliminary basis for further research on viruslike particles and the application of NPs in biology.

DDX5 RNA Helicases: Emerging Roles in Viral Infection

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