A Cool Look at Positive-Strand RNA Virus Replication Organelles: New Insights from Cryo-Electron Microscopy.

Positive-strand RNA viruses encompass a variety of established and emerging eukaryotic pathogens. Their genome replication is confined to specialized cytoplasmic membrane compartments known as replication organelles (ROs). These ROs derive from host membranes, transformed into distinct structures such as invaginated spherules or intricate membrane networks including single- and/or double-membrane vesicles. ROs play a vital role in orchestrating viral RNA synthesis and evading detection by innate immune sensors of the host. In recent years, groundbreaking cryo-electron microscopy studies conducted with several prototypic viruses have significantly advanced our understanding of RO structure and function. Notably, these studies unveiled the presence of crown-shaped multimeric viral protein complexes that seem to actively participate in viral RNA synthesis and regulate the release of newly synthesized RNA into the cytosol for translation and packaging. These findings have shed light on novel viral functions and fascinating macromolecular complexes that delineate promising new avenues for future research.

RNA genome packaging and capsid assembly of bluetongue virus visualized in host cells.

Unlike those of double-stranded DNA (dsDNA), single-stranded DNA (ssDNA), and ssRNA viruses, the mechanism of genome packaging of dsRNA viruses is poorly understood. Here, we combined the techniques of high-resolution cryoelectron microscopy (cryo-EM), cellular cryoelectron tomography (cryo-ET), and structure-guided mutagenesis to investigate genome packaging and capsid assembly of bluetongue virus (BTV), a member of the Reoviridae family of dsRNA viruses. A total of eleven assembly states of BTV capsid were captured, with resolutions up to 2.8 Å, with most visualized in the host cytoplasm. ATPase VP6 was found underneath the vertices of capsid shell protein VP3 as an RNA-harboring pentamer, facilitating RNA packaging. RNA packaging expands the VP3 shell, which then engages middle- and outer-layer proteins to generate infectious virions. These revealed "duality" characteristics of the BTV assembly mechanism reconcile previous contradictory co-assembly and core-filling models and provide insights into the mysterious RNA packaging and capsid assembly of Reoviridae members and beyond.

Molecular mechanisms of stress-induced reactivation in mumps virus condensates.

Negative-stranded RNA viruses can establish long-term persistent infection in the form of large intracellular inclusions in the human host and cause chronic diseases. Here, we uncover how cellular stress disrupts the metastable host-virus equilibrium in persistent infection and induces viral replication in a culture model of mumps virus. Using a combination of cell biology, whole-cell proteomics, and cryo-electron tomography, we show that persistent viral replication factories are dynamic condensates and identify the largely disordered viral phosphoprotein as a driver of their assembly. Upon stress, increased phosphorylation of the phosphoprotein at its interaction interface with the viral polymerase coincides with the formation of a stable replication complex. By obtaining atomic models for the authentic mumps virus nucleocapsid, we elucidate a concomitant conformational change that exposes the viral genome to its replication machinery. These events constitute a stress-mediated switch within viral condensates that provide an environment to support upregulation of viral replication.

Translation and Replication Dynamics of Single RNA Viruses.

RNA viruses are among the most prevalent pathogens and are a major burden on society. Although RNA viruses have been studied extensively, little is known about the processes that occur during the first several hours of infection because of a lack of sensitive assays. Here we develop a single-molecule imaging assay, virus infection real-time imaging (VIRIM), to study translation and replication of individual RNA viruses in live cells. VIRIM uncovered a striking heterogeneity in replication dynamics between cells and revealed extensive coordination between translation and replication of single viral RNAs. Furthermore, using VIRIM, we identify the replication step of the incoming viral RNA as a major bottleneck of successful infection and identify host genes that are responsible for inhibition of early virus replication. Single-molecule imaging of virus infection is a powerful tool to study virus replication and virus-host interactions that may be broadly applicable to RNA viruses.

Global analysis of protein-RNA interactions in SARS-CoV-2-infected cells reveals key regulators of infection.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) causes coronavirus disease 2019 (COVID-19). SARS-CoV-2 relies on cellular RNA-binding proteins (RBPs) to replicate and spread, although which RBPs control its life cycle remains largely unknown. Here, we employ a multi-omic approach to identify systematically and comprehensively the cellular and viral RBPs that are involved in SARS-CoV-2 infection. We reveal that SARS-CoV-2 infection profoundly remodels the cellular RNA-bound proteome, which includes wide-ranging effects on RNA metabolic pathways, non-canonical RBPs, and antiviral factors. Moreover, we apply a new method to identify the proteins that directly interact with viral RNA, uncovering dozens of cellular RBPs and six viral proteins. Among them are several components of the tRNA ligase complex, which we show regulate SARS-CoV-2 infection. Furthermore, we discover that available drugs targeting host RBPs that interact with SARS-CoV-2 RNA inhibit infection. Collectively, our results uncover a new universe of host-virus interactions with potential for new antiviral therapies against COVID-19.

Growth Factor Receptor Signaling Inhibition Prevents SARS-CoV-2 Replication.

SARS-CoV-2 infections are rapidly spreading around the globe. The rapid development of therapies is of major importance. However, our lack of understanding of the molecular processes and host cell signaling events underlying SARS-CoV-2 infection hinders therapy development. We use a SARS-CoV-2 infection system in permissible human cells to study signaling changes by phosphoproteomics. We identify viral protein phosphorylation and define phosphorylation-driven host cell signaling changes upon infection. Growth factor receptor (GFR) signaling and downstream pathways are activated. Drug-protein network analyses revealed GFR signaling as key pathways targetable by approved drugs. The inhibition of GFR downstream signaling by five compounds prevents SARS-CoV-2 replication in cells, assessed by cytopathic effect, viral dsRNA production, and viral RNA release into the supernatant. This study describes host cell signaling events upon SARS-CoV-2 infection and reveals GFR signaling as a central pathway essential for SARS-CoV-2 replication. It provides novel strategies for COVID-19 treatment.

Viral cis-regulatory elements as sensors of cellular states and environmental cues.

To withstand a hostile cellular environment and replicate, viruses must sense, interpret, and respond to many internal and external cues. Retroviruses and DNA viruses can intercept these cues impinging on host transcription factors via cis-regulatory elements (CREs) in viral genomes, allowing them to sense and coordinate context-specific responses to varied signals. Here, we explore the characteristics of viral CREs, the classes of signals and host transcription factors that regulate them, and how this informs outcomes of viral replication, immune evasion, and latency. We propose that viral CREs constitute central hubs for signal integration from multiple pathways and that sequence variation between viral isolates can rapidly rewire sensing mechanisms, contributing to the variability observed in patient outcomes.

Host caveolin-1 facilitates Zika virus infection by promoting viral RNA replication.

Zika virus (ZIKV) has gained notoriety in recent years because there are no targeted therapies or vaccines available so far. Caveolin-1 (Cav-1) in host cells plays crucial functions in the invasion of many viruses. However, its specific involvement in ZIKV infection has remained unclear. Here, we reveal that depleting Cav-1 leads to a substantial reduction in ZIKV RNA levels, protein expression and viral particle production, indicating that ZIKV exploits Cav-1 for its infection. By dissecting each stage of the viral life cycle, we unveil that, unlike its invasion role in many other viruses, Cav-1 depletion selectively impairs ZIKV replication, resulting in altered replication dynamics and reduced strand-specific RNA levels, but does not affect viral entry, maturation and release. These results reveal an unforeseen function of Cav-1 in facilitating ZIKV replication, which provides new insights into the intricate interaction between Cav-1 and ZIKV and underscores Cav-1 as a potential candidate for anti-ZIKV approaches.

Nucleolin binds to and regulates transcription of hepatitis B virus covalently closed circular DNA minichromosome.

Hepatitis B virus (HBV) remains a major public health threat with nearly 300 million people chronically infected worldwide who are at a high risk of developing hepatocellular carcinoma. Current therapies are effective in suppressing HBV replication but rarely lead to cure. Current therapies do not affect the HBV covalently closed circular DNA (cccDNA), which serves as the template for viral transcription and replication and is highly stable in infected cells to ensure viral persistence. In this study, we aim to identify and elucidate the functional role of cccDNA-associated host factors using affinity purification and protein mass spectrometry in HBV-infected cells. Nucleolin was identified as a key cccDNA-binding protein and shown to play an important role in HBV cccDNA transcription, likely via epigenetic regulation. Targeting nucleolin to silence cccDNA transcription in infected hepatocytes may be a promising therapeutic strategy for a functional cure of HBV.

Nonstructural protein 4 of human norovirus self-assembles into various membrane-bridging multimers.

Single-stranded, positive-sense RNA ((+)RNA) viruses replicate their genomes in virus-induced intracellular membrane compartments. (+)RNA viruses dedicate a significant part of their small genomes (a few thousands to a few tens of thousands of bases) to the generation of these compartments by encoding membrane-interacting proteins and/or protein domains. Noroviruses are a very diverse genus of (+)RNA viruses including human and animal pathogens. Human noroviruses are the major cause of acute gastroenteritis worldwide, with genogroup II genotype 4 (GII.4) noroviruses accounting for the vast majority of infections. Three viral proteins encoded in the N terminus of the viral replication polyprotein direct intracellular membrane rearrangements associated with norovirus replication. Of these three, nonstructural protein 4 (NS4) seems to be the most important, although its exact functions in replication organelle formation are unknown. Here, we produce, purify, and characterize GII.4 NS4. AlphaFold modeling combined with experimental data refines and corrects our previous crude structural model of NS4. Using simple artificial liposomes, we report an extensive characterization of the membrane properties of NS4. We find that NS4 self-assembles and thereby bridges liposomes together. Cryo-EM, NMR, and membrane flotation show formation of several distinct NS4 assemblies, at least two of them bridging pairs of membranes together in different fashions. Noroviruses belong to (+)RNA viruses whose replication compartment is extruded from the target endomembrane and generates double-membrane vesicles. Our data establish that the 21-kDa GII.4 human norovirus NS4 can, in the absence of any other factor, recapitulate in tubo several features, including membrane apposition, that occur in such processes.

Electron microscopy: The key to resolve RNA viruses replication organelles.

Positive-sense single-stranded RNA viruses significantly reshape intracellular membranes to generate viral replication organelles that form a controlled niche in which nucleic acids, enzymes, and cofactors accumulate to assure an efficient replication of the viral genome. In recent years, advancements in electron microscopy (EM) techniques have enabled imaging of these viral factories in a near-native state providing significantly higher molecular details that have led to progress in our general understanding of virus biology. In this review, we describe the contribution of the cutting-edge EM approaches to the current knowledge of replication organelles biogenesis, structure, and functions.

Non-canonical proline-tyrosine interactions with multiple host proteins regulate Ebola virus infection.

The Ebola virus VP30 protein interacts with the viral nucleoprotein and with host protein RBBP6 via PPxPxY motifs that adopt non-canonical orientations, as compared to other proline-rich motifs. An affinity tag-purification mass spectrometry approach identified additional PPxPxY-containing host proteins hnRNP L, hnRNPUL1, and PEG10, as VP30 interactors. hnRNP L and PEG10, like RBBP6, inhibit viral RNA synthesis and EBOV infection, whereas hnRNPUL1 enhances. RBBP6 and hnRNP L modulate VP30 phosphorylation, increase viral transcription, and exert additive effects on viral RNA synthesis. PEG10 has more modest inhibitory effects on EBOV replication. hnRNPUL1 positively affects viral RNA synthesis but in a VP30-independent manner. Binding studies demonstrate variable capacity of the PPxPxY motifs from these proteins to bind VP30, define PxPPPPxY as an optimal binding motif, and identify the fifth proline and the tyrosine as most critical for interaction. Competition binding and hydrogen-deuterium exchange mass spectrometry studies demonstrate that each protein binds a similar interface on VP30. VP30 therefore presents a novel proline recognition domain that is targeted by multiple host proteins to modulate viral transcription.

Barley stripe mosaic virus γb protein disrupts chloroplast antioxidant defenses to optimize viral replication.

The plant antioxidant system plays important roles in response to diverse abiotic and biotic stresses. However, the effects of virus infection on host redox homeostasis and how antioxidant defense pathway is manipulated by viruses remain poorly understood. We previously demonstrated that the Barley stripe mosaic virus (BSMV) γb protein is recruited to the chloroplast by the viral αa replicase to enhance viral replication. Here, we show that BSMV infection induces chloroplast oxidative stress. The versatile γb protein interacts directly with NADPH-dependent thioredoxin reductase C (NTRC), a core component of chloroplast antioxidant systems. Overexpression of NbNTRC significantly impairs BSMV replication in Nicotiana benthamiana plants, whereas disruption of NbNTRC expression leads to increased viral accumulation and infection severity. To counter NTRC-mediated defenses, BSMV employs the γb protein to competitively interfere with NbNTRC binding to 2-Cys Prx. Altogether, this study indicates that beyond acting as a helicase enhancer, γb also subverts NTRC-mediated chloroplast antioxidant defenses to create an oxidative microenvironment conducive to viral replication.

Lipidomics reveals the significance and mechanism of the cellular ceramide metabolism for rotavirus replication.

As one of the most important causative agents of severe gastroenteritis in children, piglets, and other young animals, species A rotaviruses have adversely impacted both human health and the global swine industry. Vaccines against rotaviruses (RVs) are insufficiently effective, and no specific treatment is available. To understand the relationships between porcine RV (PoRV) infection and enterocytes in terms of the cellular lipid metabolism, we performed an untargeted liquid chromatography mass spectrometry (LC-MS) lipidomics analysis of PoRV-infected IPEC-J2 cells. Herein, a total of 451 lipids (263 upregulated lipids and 188 downregulated lipids), spanning sphingolipid, glycerolipid, and glycerophospholipids, were significantly altered compared with the mock-infected group. Interestingly, almost all the ceramides among these lipids were upregulated during PoRV infection. LC-MS analysis was used to validated the lipidomics data and demonstrated that PoRV replication increased the levels of long-chain ceramides (C16-ceramide, C18-ceramide, and C24-ceramide) in cells. Furthermore, we found that these long-chain ceramides markedly inhibited PoRV infection and that their antiviral actions were exerted in the replication stage of PoRV infection. Moreover, downregulation of endogenous ceramides with the ceramide metabolic inhibitors enhanced PoRV propagation. Increasing the levels of ceramides by the addition of C6-ceramide strikingly suppressed the replication of diverse RV strains. We further found that the treatment with an apoptotic inhibitor could reverse the antiviral activity of ceramide against PoRV replication, demonstrating that ceramide restricted RV infection by inducing apoptosis. Altogether, this study revealed that ceramides played an antiviral role against RV infection, providing potential approaches for the development of antiviral therapies.IMPORTANCERotaviruses (RVs) are among the most important zoonosis viruses, which mainly infected enterocytes of the intestinal epithelium causing diarrhea in children and the young of many mammalian and avian species. Lipids play an essential role in viral infection. A comprehensive understanding of the interaction between RV and lipid metabolism in the enterocytes will be helpful to control RV infection. Here, we mapped changes in enterocyte lipids following porcine RV (PoRV) infection using an untargeted lipidomics approach. We found that PoRV infection altered the metabolism of various lipid species, especially ceramides (derivatives of the sphingosine). We further demonstrated that PoRV infection increased the accumulation of ceramides and that ceramides exerted antiviral effects on RV replication by inducing apoptosis. Our findings fill a gap in understanding the alterations of lipid metabolism in RV-infected enterocytes and highlight the antiviral effects of ceramides on RV infection, suggesting potential approaches to control RV infection.

A dual fluorescent herpes simplex virus type 1 recombinant reveals divergent outcomes of neuronal infection.

Critical stages of lytic herpes simplex virus type 1 (HSV-1) replication are marked by the sequential expression of immediate early (IE) to early (E), then late (L) viral genes. HSV-1 can also persist in neuronal cells via a non-replicative, transcriptionally repressed infection called latency. The regulation of lytic and latent transcriptional profiles is critical to HSV-1 pathogenesis and persistence. We sought a fluorescence-based approach to observe the outcome of neuronal HSV-1 infection at the single-cell level. To achieve this goal, we constructed and characterized a novel HSV-1 recombinant that enables discrimination between lytic and latent infection. The dual reporter HSV-1 encodes a human cytomegalovirus-immediate early (hCMV-IE) promoter-driven enhanced yellow fluorescent protein (eYFP) to visualize the establishment of infection and an endogenous mCherry-VP26 fusion to report lytic replication. We confirmed that viral gene expression, replication, and spread of infection are not altered by the incorporation of the fluorescent reporters, and fluorescent protein (FP) detection virtuously reports the progression of lytic replication. We demonstrate that the outcome of HSV-1 infection of compartmentalized primary neurons is determined by viral inoculating dose: high-dose axonal inoculation proceeds to lytic replication, whereas low-dose axonal inoculation establishes a latent HSV-1 infection. Interfering with low-dose axonal inoculation via small molecule drugs reports divergent phenotypes of eYFP and mCherry reporter detection, correlating with altered states of viral gene expression. We report that the transcriptional state of neuronal HSV-1 infection is variable in response to changes in the intracellular neuronal environment.IMPORTANCEHerpes simplex virus type 1 (HSV-1) is a prevalent human pathogen that infects approximately 67% of the global human population. HSV-1 invades the peripheral nervous system, where latent HSV-1 infection persists within the host for life. Immunological evasion, viral persistence, and herpetic pathologies are determined by the regulation of HSV-1 gene expression. Studying HSV-1 gene expression during neuronal infection is challenging but essential for the development of antiviral therapeutics and interventions. We used a recombinant HSV-1 to evaluate viral gene expression during infection of primary neurons. Manipulation of cell signaling pathways impacts the establishment and transcriptional state of HSV-1 latency in neurons. The work here provides critical insight into the cellular and viral factors contributing to the establishment of latent HSV-1 infection.

Equine herpesvirus type 1 (EHV-1) replication at the upper respiratory entry site is inhibited by neutralizing EHV-1-specific IgG1 and IgG4/7 mucosal antibodies.

Equine herpesvirus type 1 (EHV-1) is a contagious respiratory pathogen that infects the mucosa of the upper respiratory tract (URT). Mucosal immune responses at the URT provide the first line of defense against EHV-1 and are crucial for orchestrating immunity. To define host-pathogen interactions, we characterized B-cell responses, antibody isotype functions, and EHV-1 replication of susceptible (non-immune) and clinically protected (immune) horses after experimental EHV-1 infection. Nasal secretion and nasal wash samples were collected and used for the isolation of DNA, RNA, and mucosal antibodies. Shedding of infectious virus, EHV-1 copy numbers, viral RNA expression, and host B-cell activation in the URT were compared based on host immune status. Mucosal EHV-1-specific antibody responses were associated with EHV-1 shedding and viral RNA transcription. Finally, mucosal immunoglobulin G (IgG) and IgA isotypes were purified and tested for neutralizing capabilities. IgG1 and IgG4/7 neutralized EHV-1, while IgG3/5, IgG6, and IgA did not. Immune horses secreted high amounts of mucosal EHV-1-specific IgG4/7 antibodies and quickly upregulated B-cell pathway genes, while EHV-1 was undetected by virus isolation and PCR. RNA transcription analysis reinforced incomplete viral replication in immune horses. In contrast, complete viral replication with high viral copy numbers and shedding of infectious viruses was characteristic for non-immune horses, together with low or absent EHV-1-specific neutralizing antibodies during viral replication. These data confirm that pre-existing mucosal IgG1 and IgG4/7 and rapid B-cell activation upon EHV-1 infection are essential for virus neutralization, regulation of viral replication, and mucosal immunity against EHV-1.IMPORTANCEEquine herpesvirus type 1 (EHV-1) causes respiratory disease, abortion storms, and neurologic outbreaks known as equine herpes myeloencephalopathy (EHM). EHV-1 is transmitted with respiratory secretions by nose-to-nose contact or via fomites. The virus initially infects the epithelium of the upper respiratory tract (URT). Host-pathogen interactions and mucosal immunity at the viral entry site provide the first line of defense against the EHV-1. Robust mucosal immunity can be essential in protecting against EHV-1 and to reduce EHM outbreaks. It has previously been shown that immune horses do not establish cell-associated viremia, the prerequisite for EHM. Here, we demonstrate how mucosal antibodies can prevent the replication of EHV-1 at the epithelium of the URT and, thereby, the progression of the virus to the peripheral blood. The findings improve the mechanistic understanding of mucosal immunity against EHV-1 and can support the development of enhanced diagnostic tools, vaccines against EHM, and the management of EHV-1 outbreaks.

A nanobody inhibiting porcine reproductive and respiratory syndrome virus replication via blocking self-interaction of viral nucleocapsid protein.

Porcine reproductive and respiratory syndrome (PRRS) is a serious global pig industry disease. Understanding the mechanism of viral replication and developing efficient antiviral strategies are necessary for combating with PRRS virus (PRRSV) infection. Recently, nanobody is considered to be a promising antiviral drug, especially for respiratory viruses. The present study evaluated two nanobodies against PRRSV nucleocapsid (N) protein (PRRSV-N-Nb1 and -Nb2) for their anti-PRRSV activity in vitro and in vivo. The results showed that intracellularly expressed PRRSV-N-Nb1 significantly inhibited PRRSV-2 replication in MARC-145 cells (approximately 100%). Then, the PRRSV-N-Nb1 fused with porcine IgG Fc (Nb1-pFc) as a delivering tag was produced and used to determine its effect on PRRSV-2 replication in porcine alveolar macrophages (PAMs) and pigs. The inhibition rate of Nb1-pFc against PRRSV-2 in PAMs could reach >90%, and it can also inhibit viral replication in vivo. Epitope mapping showed that the motif Serine 105 (S105) in PRRSV-2 N protein was the key amino acid binding to PRRSV-N-Nb1, which is also pivotal for the self-interaction of N protein via binding to Arginine 97. Moreover, viral particles were not successfully rescued when the S105 motif was mutated to Alanine (S105A). Attachment, entry, genome replication, release, docking model analysis, and blocking enzyme-linked immunosorbent assay (ELISA) indicated that the binding of PRRSV-N-Nb1 to N protein could block its self-binding, which prevents the viral replication of PRRSV. PRRSV-N-Nb1 may be a promising drug to counter PRRSV-2 infection. We also provided some new insights into the molecular basis of PRRSV N protein self-binding and assembly of viral particles.IMPORTANCEPorcine reproductive and respiratory syndrome virus (PRRSV) causes serious economic losses to the swine industry worldwide, and there are no highly effective strategies for prevention. Nanobodies are considered a promising novel approach for treating diseases because of their ease of production and low costing. Here, we showed that PRRSV-N-Nb1 against PRRSV-N protein significantly inhibited PRRSV-2 replication in vitro and in vivo. Furthermore, we demonstrated that the motif Serine 105 (S105) in PRRSV-N protein was the key amino acid to interact with PRRSV-N-Nb1 and bond to its motif R97, which is important for the self-binding of N protein. The PRRSV-N-Nb1 could block the self-interaction of N protein following viral assembly. These findings not only provide insights into the molecular basis of PRRSV N protein self-binding as a key factor for viral replication for the first time but also highlight a novel target for the development of anti-PRRSV replication drugs.

SQSTM1 downregulates avian metapneumovirus subgroup C replication via mediating selective autophagic degradation of viral M2-2 protein.

Avian metapneumovirus subgroup C (aMPV/C), an important pathogen causing acute respiratory infection in chickens and turkeys, contributes to substantial economic losses in the poultry industry worldwide. aMPV/C has been reported to induce autophagy, which is beneficial to virus replication. Sequestosome 1 (SQSTM1/P62), a selective autophagic receptor, plays a crucial role in viral replication by clearing ubiquitinated proteins. However, the relationship between SQSTM1-mediated selective autophagy and aMPV/C replication is unclear. In this study, we found that the expression of SQSTM1 negatively regulates aMPV/C replication by reducing viral protein expression and viral titers. Further studies revealed that the interaction between SQSTM1 and aMPV/C M2-2 protein is mediated via the Phox and Bem1 (PB1) domain of the former, which recognizes a ubiquitinated lysine at position 67 of the M2-2 protein, and finally degrades M2-2 via SQSTM1-mediated selective autophagy. Collectively, our results reveal that SQSTM1 degrades M2-2 via a process of selective autophagy to suppress aMPV/C replication, thereby providing novel insights for the prevention and control of aMPV/C infection.IMPORTANCEThe selective autophagy plays an important role in virus replication. As an emerging pathogen of avian respiratory virus, clarification of the effect of SQSTM1, a selective autophagic receptor, on aMPV/C replication in host cells enables us to better understand the viral pathogenesis. Previous study showed that aMPV/C infection reduced the SQSTM1 expression accompanied by virus proliferation, but the specific regulatory mechanism between them was still unclear. In this study, we demonstrated for the first time that SQSTM1 recognizes the 67th amino acid of M2-2 protein by the interaction between them, followed by M2-2 degradation via the SQSTM1-mediated selective autophagy, and finally inhibits aMPV/C replication. This information supplies the mechanism by which SQSTM1 negatively regulates viral replication, and provides new insights for preventing and controlling aMPV/C infection.

AAV2 can replicate its DNA by a rolling hairpin or rolling circle mechanism, depending on the helper virus.

Adeno-associated virus type 2 (AAV2) is a small, non-pathogenic, helper virus-dependent parvovirus with a single-stranded (ss) DNA genome of approximately 4.7 kb. AAV2 DNA replication requires the presence of a helper virus such as adenovirus type 5 (AdV5) or herpes simplex virus type 1 (HSV-1) and is generally assumed to occur as a strand-displacement rolling hairpin (RHR) mechanism initiated at the AAV2 3' inverted terminal repeat (ITR). We have recently shown that AAV2 replication supported by HSV-1 leads to the formation of double-stranded head-to-tail concatemers, which provides evidence for a rolling circle replication (RCR) mechanism. We have revisited AAV2 DNA replication and specifically compared the formation of AAV2 replication intermediates in the presence of either HSV-1 or AdV5 as the helper virus. The results confirmed that the AAV2 DNA replication mechanism is helper virus-dependent and follows a strand-displacement RHR mechanism when AdV5 is the helper virus and primarily an RCR mechanism when HSV-1 is the helper virus. We also demonstrate that recombination plays a negligible role in AAV2 genome replication. Interestingly, the formation of high-molecular-weight AAV2 DNA concatemers in the presence of HSV-1 as the helper virus was dependent on an intact HSV-1 DNA polymerase. IMPORTANCE: AAV is a small helper virus-dependent, non-pathogenic parvovirus. The AAV genome replication mechanism was extensively studied in the presence of AdV as the helper virus and described to proceed using RHR. Surprisingly, HSV-1 co-infection facilitates RCR of the AAV2 DNA. We directly compared AdV5 and HSV-1 supported AAV2 DNA replication and showed that AAV2 can adapt its replication mechanism to the helper virus. A detailed understanding of the AAV replication mechanism expands our knowledge of virus biology and can contribute to increase gene therapy vector production.

African swine fever virus pH240R enhances viral replication via inhibition of the type I IFN signaling pathway.

African swine fever (ASF) is an acute, hemorrhagic, and severe infectious disease caused by ASF virus (ASFV) infection. At present, there are still no safe and effective drugs and vaccines to prevent ASF. Mining the important proteins encoded by ASFV that affect the virulence and replication of ASFV is the key to developing effective vaccines and drugs. In this study, ASFV pH240R, a capsid protein of ASFV, was found to inhibit the type I interferon (IFN) signaling pathway. Mechanistically, pH240R interacted with IFNAR1 and IFNAR2 to disrupt the interaction of IFNAR1-TYK2 and IFNAR2-JAK1. Additionally, pH240R inhibited the phosphorylation of IFNAR1, TYK2, and JAK1 induced by IFN-α, resulting in the suppression of the nuclear import of STAT1 and STAT2 and the expression of IFN-stimulated genes (ISGs). Consistent with these results, H240R-deficient ASFV (ASFV-∆H240R) infection induced more ISGs in porcine alveolar macrophages compared with its parental ASFV HLJ/18. We also found that pH240R enhanced viral replication via inhibition of ISGs expression. Taken together, our results clarify that pH240R enhances ASFV replication by inhibiting the JAK-STAT signaling pathway, which highlights the possibility of pH240R as a potential drug target.IMPORTANCEThe innate immune response is the host's first line of defense against pathogen infection, which has been reported to affect the replication and virulence of African swine fever virus (ASFV) isolates. Identification of ASFV-encoded proteins that affect the virulence and replication of ASFV is the key step in developing more effective vaccines and drugs. In this study, we found that pH240R interacted with IFNAR1 and IFNAR2 by disrupting the interaction of IFNAR1-TYK2 and IFNAR2-JAK1, resulting in the suppression of the expression of interferon (IFN)-stimulated genes (ISGs). Consistent with these results, H240R-deficient ASFV (ASFV-∆H240R) infection induces more ISGs' expression compared with its parental ASFV HLJ/18. We also found that pH240R enhanced viral replication via inhibition of ISGs' expression. Taken together, our findings showed that pH240R enhances ASFV replication by inhibiting the IFN-JAK-STAT axis, which highlights the possibility of pH240R as a potential drug target.