Endoplasmic Reticulum Protein SCAP Inhibits Dengue Virus NS2B3 Protease by Suppressing Its K27-Linked Polyubiquitylation

A fast-growing dengue virus mutant reveals a dual role of STING in response to infection

The four dengue viruses (DENVs) have evolved multiple mechanisms to ensure its survival. Among these mechanisms is the ability to regulate its replication rate, which may contribute to avoiding premature immune activation that limit infection dissemination: DENVs associated with dengue epidemics have shown slower replication rate than pre-epidemic strains. Correspondingly, wild-type DENVs replicate more slowly than their clinically attenuated derivatives. To understand how DENVs 'make haste slowly', we generated and screened for DENV2 mutants with accelerated replication that also induced high type-I interferon (IFN) expression in infected cells. We chanced upon a single NS2B-I114T amino acid substitution, in an otherwise highly conserved amino acid residue. Accelerated DENV2 replication damaged host DNA as mutant infection was dependent on host DNA damage repair factors, namely RAD21, EID3 and NEK5. DNA damage induced cGAS/STING signalling and activated early type-I IFN response that inhibited infection dissemination. Unexpectedly, STING activation also supported mutant DENV replication in infected cells through STING-induced autophagy. Our findings thus show that DENV NS2B has multi-faceted role in controlling DENV replication rate and immune evasion and suggest that the dual role of STING in supporting virus replication within infected cells but inhibiting infection dissemination could be particularly advantageous for live attenuated vaccine development.

Functions and mechanisms of the Ubc13-UEV complex and lysine 63-linked polyubiquitination in plants

Ubiquitination is one of the best-known post-translational modifications in eukaryotes, in which different linkage types of polyubiquitination result in different outputs of the target proteins. Distinct from the well-characterized K48-linked polyubiquitination that usually serves as a signal for degradation of the target protein, K63-linked polyubiquitination often requires a unique E2 heterodimer Ubc13-UEV and alters the target protein activity instead of marking it for degradation. This review focuses on recent advances on the roles of Ubc13-UEV-mediated K63-linked polyubiquitination in plant growth, development, and response to environmental stresses.

Flaviviridae Nonstructural Proteins: The Role in Molecular Mechanisms of Triggering Inflammation

Members of the Flaviviridae family are posing a significant threat to human health worldwide. Many flaviviruses are capable of inducing severe inflammation in humans. Flaviviridae nonstructural proteins, apart from their canonical roles in viral replication, have noncanonical functions strongly affecting antiviral innate immunity. Among these functions, antagonism of type I IFN is the most investigated; meanwhile, more data are accumulated on their role in the other pathways of innate response. This review systematizes the last known data on the role of Flaviviridae nonstructural proteins in molecular mechanisms of triggering inflammation, with an emphasis on their interactions with TLRs and RLRs, interference with NF-κB and cGAS-STING signaling, and activation of inflammasomes.

Dual-Role Ubiquitination Regulation Shuttling the Entire Life Cycle of the Flaviviridae

Ubiquitination is a reversible protein post-translational modification that regulates various pivotal physiological and pathological processes in all eukaryotes. Recently, the antiviral immune response is enhanced by the regulation of ubiquitination. Intriguingly, Flaviviridae viruses can ingeniously hijack the ubiquitination system to help them survive, which has become a hot topic among worldwide researchers. The Flaviviridae family members, such as HCV and CSFV, can cause serious diseases of humans and animals around the world. The multiple roles of ubiquitination involved in the life cycle of Flaviviridae family would open new sight for future development of antiviral tactic. Here, we discuss recent advances with regard to functional roles of ubiquitination and some ubiquitin-like modifications in the life cycle of Flaviviridae infection, shedding new light on the antiviral mechanism research and therapeutic drug development.

Biogenesis and Breakdown of Lipid Droplets in Pathological Conditions

Lipid droplets (LD) have long been considered as mere fat drops; however, LD have lately been revealed to be ubiquitous, dynamic and to be present in diverse organelles in which they have a wide range of key functions. Although incompletely understood, the biogenesis of eukaryotic LD initiates with the synthesis of neutral lipids (NL) by enzymes located in the endoplasmic reticulum (ER). The accumulation of NL leads to their segregation into nanometric nuclei which then grow into lenses between the ER leaflets as they are further filled with NL. The lipid composition and interfacial tensions of both ER and the lenses modulate their shape which, together with specific ER proteins, determine the proneness of LD to bud from the ER toward the cytoplasm. The most important function of LD is the buffering of energy. But far beyond this, LD are actively integrated into physiological processes, such as lipid metabolism, control of protein homeostasis, sequestration of toxic lipid metabolic intermediates, protection from stress, and proliferation of tumours. Besides, LD may serve as platforms for pathogen replication and defense. To accomplish these functions, from biogenesis to breakdown, eukaryotic LD have developed mechanisms to travel within the cytoplasm and to establish contact with other organelles. When nutrient deprivation occurs, LD undergo breakdown (lipolysis), which begins with the LD-associated members of the perilipins family PLIN2 and PLIN3 chaperone-mediated autophagy degradation (CMA), a specific type of autophagy that selectively degrades a subset of cytosolic proteins in lysosomes. Indeed, PLINs CMA degradation is a prerequisite for further true lipolysis, which occurs via cytosolic lipases or by lysosome luminal lipases when autophagosomes engulf portions of LD and target them to lysosomes. LD play a crucial role in several pathophysiological processes. Increased accumulation of LD in non-adipose cells is commonly observed in numerous infectious diseases caused by intracellular pathogens including viral, bacterial, and parasite infections, and is gradually recognized as a prominent characteristic in a variety of cancers. This review discusses current evidence related to the modulation of LD biogenesis and breakdown caused by intracellular pathogens and cancer.

Regulation of innate immune signaling pathways by autophagy in dengue virus infection

Autophagy is not only an intracellular recycling degradation system that maintains cellular homeostasis but is also a component of innate immunity that contributes to host defense against viral infection. The viral components as well as viral particles trapped in autophagosomes can be delivered to lysosomes for degradation. Abundant evidence indicates that dengue virus (DENV) has evolved the potent ability to hijack or subvert autophagy process for escaping host immunity and promoting viral replication. Moreover, autophagy is often required to deliver viral components to pattern recognition receptors signaling for interferon (IFN)-mediated viral elimination. Hence, this review summarizes DENV-induced autophagy, which exhibits dual effects on proviral activity of promoting replication and antiviral activity to eliminating viral particles.

Japanese Encephalitis Virus NS2B-3 Protein Complex Promotes Cell Apoptosis and Viral Particle Release by Down-Regulating the Expression of AXL

Japanese encephalitis virus (JEV) is a flavivirus transmitted by mosquitoes that causes severe encephalitis in humans and animals. It has been suggested that AXL, a transmembrane protein, can promote the replication of various flaviviruses, such as dengue (DENV), Zika (ZIKV), and West Nile (WNV) viruses. However, the effect of AXL on JEV infection has not yet been determined. In the present study, we demonstrate that AXL is down-regulated after JEV infection in the late stage. JEV NS2B-3 protein specifically interacted with AXL, and promoted AXL degradation through the ubiquitin-proteasome pathway. AXL-degradation increased cell apoptosis by disrupting phosphatidylinositol 3-kinase (PI3K)/Akt signal transduction. In addition, the degradation of AXL promoted JEV release to supernatant, whereas the virus in the cell lysates decreased. The supplementation of AXL ligand Gas6 inhibited the JEV-mediated degradation of AXL. Altogether, we discover a new function of NS2B-3 during the process of JEV replication, and provide a new insight into the interactions between JEV and cell hosts.

Contributions of Ubiquitin and Ubiquitination to Flaviviral Antagonism of Type I IFN

Flaviviruses implement a broad range of antagonism strategies against the host antiviral response. A pivotal component of the early host response is production and signaling of type I interferon (IFN-I). Ubiquitin, a prevalent cellular protein-modifying molecule, is heavily involved in the cellular regulation of this and other immune response pathways. Viruses use ubiquitin and ubiquitin machinery to antagonize various steps of these pathways through diverse mechanisms. Here, we highlight ways in which flaviviruses use or inhibit ubiquitin to antagonize the antiviral IFN-I response.

K27-linked noncanonic ubiquitination in immune regulation

Ubiquitination is a common form of posttranslational modification that has been implicated in regulating considerable immune signaling pathways. The functions of canonic K48- and K63-linked ubiquitination have been well studied. However, the roles of noncanonic ubiquitination remain largely unexplored and require further investigations. There is increasing evidence suggesting that K27-linked noncanonic ubiquitination turns out to be indispensable to both innate immune signaling and T cell signaling. In this review, we provide an overview of the latest findings related to K27-linked ubiquitination, and highlight the crucial roles of K27-linked ubiquitination in regulating antimicrobial response, cytokine signaling and response, as well as T cell activation and differentiation. We also propose interesting areas for better understanding how K27-linked ubiquitination regulates immunity.

Beyond K48 and K63: non-canonical protein ubiquitination

Protein ubiquitination has become one of the most extensively studied post-translational modifications. Originally discovered as a critical element in highly regulated proteolysis, ubiquitination is now regarded as essential for many other cellular processes. This results from the unique features of ubiquitin (Ub) and its ability to form various homo- and heterotypic linkage types involving one of the seven different lysine residues or the free amino group located at its N-terminus. While K48- and K63-linked chains are broadly covered in the literature, the other types of chains assembled through K6, K11, K27, K29, and K33 residues deserve equal attention in the light of the latest discoveries. Here, we provide a concise summary of recent advances in the field of these poorly understood Ub linkages and their possible roles in vivo.

Hijacking of Lipid Droplets by Hepatitis C, Dengue and Zika Viruses-From Viral Protein Moonlighting to Extracellular Release

Hijacking and manipulation of host cell biosynthetic pathways by human enveloped viruses are essential for the viral lifecycle. Flaviviridae members, including hepatitis C, dengue and Zika viruses, extensively manipulate host lipid metabolism, underlining the importance of lipid droplets (LDs) in viral infection. LDs are dynamic cytoplasmic organelles that can act as sequestration platforms for a unique subset of host and viral proteins. Transient recruitment and mobilization of proteins to LDs during viral infection impacts host-cell biological properties, LD functionality and canonical protein functions. Notably, recent studies identified LDs in the nucleus and also identified that LDs are transported extracellularly via an autophagy-mediated mechanism, indicating a novel role for autophagy in Flaviviridae infections. These developments underline an unsuspected diversity and localization of LDs and potential moonlighting functions of LD-associated proteins during infection. This review summarizes recent breakthroughs concerning the LD hijacking activities of hepatitis C, dengue and Zika viruses and potential roles of cytoplasmic, nuclear and extracellular LD-associated viral proteins during infection.

Innate Immune DNA Sensing of Flaviviruses

Flaviviruses are arthropod-borne RNA viruses that have been used extensively to study host antiviral responses. Often selected just to represent standard single-stranded positive-sense RNA viruses in early studies, the Flavivirus genus over time has taught us how truly unique it is in its remarkable ability to target not just the RNA sensory pathways but also the cytosolic DNA sensing system for its successful replication inside the host cell. This review summarizes the main developments on the unexpected antagonistic strategies utilized by different flaviviruses, with RNA genomes, against the host cyclic GAMP synthase (cGAS)/stimulator of interferon genes (STING) cytosolic DNA sensing pathway in mammalian systems. On the basis of the recent advancements on this topic, we hypothesize that the mechanisms of viral sensing and innate immunity are much more fluid than what we had anticipated, and both viral and host factors will continue to be found as important factors contributing to the host innate immune system in the future.

DNA-induced 2'3'-cGAMP enhances haplotype-specific human STING cleavage by dengue protease

Dengue virus (DENV) antagonizes the DNA sensing cGAS-STING pathway to subvert innate immunity, but how DENV protease-mediated human STING cleavage contributes to DENV pathogenesis remains obscure. Here, we found that STING haplotype frequency varies among different subhuman populations, and different haplotypes respond differently to DENV protease. The cleavage of a DENV protease-sensitive STING can be further enhanced by coculture with neighboring cells producing 2′3′-cGAMP, either by DNA transfection of cGAS or by reactivating Epstein–Barr virus from latent infection. Thus, DENV infection trims down human STING-mediated innate immunity in a haplotype-specific manner. The genetic background of host STING and bystander coinfection of pathogens triggering 2′3′-cGAMP production may be the missing link between STING cleavage and DENV pathogenesis.

The cytosolic DNA sensor cGMP-AMP synthase (cGAS) synthesizes the noncanonical cyclic dinucleotide 2′3′-cGAMP to activate the adaptor protein stimulator of IFN genes (STING), thus awakening host immunity in response to DNA pathogen infection. However, dengue virus (DENV), an RNA virus without a DNA stage in its life cycle, also manipulates cGAS-STING–mediated innate immunity by proteolytic degradation of STING. Here, we found that the sensitivity of STING to DENV protease varied with different human STING haplotypes. Exogenous DNA further enhanced DENV protease’s ability to interact and cleave protease-sensitive STING. DNA-enhanced STING cleavage was reduced in cGAS-knockdown cells and triggered by the cGAS product 2′3′-cGAMP. The source of DNA may not be endogenous mitochondrial DNA but rather exogenous reactivated viral DNA. Cells producing 2′3′-cGAMP by overexpressing cGAS or with DNA virus reactivation enhanced STING cleavage in neighboring cells harboring DENV protease. DENV infection reduced host innate immunity in cells with the protease-sensitive STING haplotype, whose homozygote genotype frequency was found significantly reduced in Taiwanese people with dengue fever. Therefore, the human STING genetic background and DNA pathogen coinfection may be the missing links contributing to DENV pathogenesis.

Dengue proteins with their role in pathogenesis, and strategies for developing an effective anti-dengue treatment: A review

Dengue virus is an arbovirus belonging to class Flaviviridae Its clinical manifestation ranges from asymptomatic to extreme conditions (dengue hemorrhagic fever/dengue shock syndrome). A lot of research has been done on this ailment, yet there is no effective treatment available for the disease. This review provides the systematic understanding of all dengue proteins, role of its structural proteins (C-protein, E-protein, prM) in virus entry, assembly, and secretion in host cell, and nonstructural proteins (NS1, NS2a, NS2b, NS3, NS4a, NS4b, and NS5) in viral assembly, replication, and immune evasion during dengue progression and pathogenesis. Furthermore, the review has highlighted the controversies related to the only commercially available dengue vaccine, that is, Dengvaxia, and the risk associated with it. Lastly, it provides an insight regarding various approaches for developing an effective anti-dengue treatment.

Porcine RING Finger Protein 114 Inhibits Classical Swine Fever Virus Replication via K27-Linked Polyubiquitination of Viral NS4B

Porcine RING finger protein 114 (pRNF114) is a member of the RING domain E3 ligases. In this study, it was shown that pRNF114 is a potential anti-CSFV factor and the anti-CSFV effect of pRNF114 depends on its E3 ligase activity. Notably, pRNF114 targets and catalyzes the K27-linked polyubiquitination of the NS4B protein and then promotes proteasome-dependent degradation of NS4B, inhibiting the replication of CSFV. To our knowledge, pRNF114 is the first E3 ligase to be identified as being involved in anti-CSFV activity, and targeting NS4B could be a crucial route for antiviral development.

In the host, many RING domain E3 ligases have been reported to inhibit viral replication through various mechanisms. In a previous screen, we found that porcine RING finger protein 114 (pRNF114), a RING domain E3 ubiquitin ligase, inhibits classical swine fever virus (CSFV) replication. This study aimed to clarify the underlying antiviral mechanism of pRNF114 against CSFV. Upon CSFV infection, pRNF114 mRNA was upregulated both in vitro and in vivo. CSFV replication was significantly suppressed in PK-pRNF114 cells stably expressing pRNF114 by the lentivirus-delivered system, whereas CSFV growth was enhanced in PK-15 cells with RNF114 knockout by the CRISPR/Cas9 system. The RING domain of pRNF114, which has E3 ubiquitin ligase activity, is crucial for its antiviral activity. Mechanistically, pRNF114 interacted with the CSFV NS4B protein through their C-terminal domains, which led to the K27-linked polyubiquitination and degradation of NS4B through a proteasome-dependent pathway. Collectively, these findings indicate that pRNF114 as a critical regulator of CSFV replication and uncover a mechanism by which pRNF114 employs its E3 ubiquitin ligase activity to inhibit CSFV replication.

IMPORTANCE Porcine RING finger protein 114 (pRNF114) is a member of the RING domain E3 ligases. In this study, it was shown that pRNF114 is a potential anti-CSFV factor and the anti-CSFV effect of pRNF114 depends on its E3 ligase activity. Notably, pRNF114 targets and catalyzes the K27-linked polyubiquitination of the NS4B protein and then promotes proteasome-dependent degradation of NS4B, inhibiting the replication of CSFV. To our knowledge, pRNF114 is the first E3 ligase to be identified as being involved in anti-CSFV activity, and targeting NS4B could be a crucial route for antiviral development.

TRAF6 Plays a Proviral Role in Tick-Borne Flavivirus Infection through Interaction with the NS3 Protease

Tick-borne flaviviruses (TBFVs) can cause life-threatening encephalitis and hemorrhagic fever. To identify virus-host interactions that may be exploited as therapeutic targets, we analyzed the TBFV polyprotein in silico for antiviral protein-binding motifs. We obtained two putative tumor necrosis factor receptor-associated factor 6 (TRAF6)-binding motifs (TBMs) within the protease domain of the viral nonstructural 3 (NS3) protein. Here, we show that TBFV NS3 interacted with TRAF6 during infection and that TRAF6 supports TBFV replication. The proviral role of TRAF6 was not seen with mosquito-borne flaviviruses, consistent with the lack of conserved TBMs. Mutation of the second TBM within NS3 disrupted TRAF6 binding, coincident with reduced abundance of mature, autocatalytically derived form of the NS3 protease and significant virus attenuation in vitro. Our studies reveal insights into how flaviviruses exploit innate immunity for the purpose of viral replication and identify a potential target for therapeutic design.

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Langat virus (LGTV) NS3 protease interacts with TRAF6 during infection

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Tick-borne, unlike mosquito-borne, flaviviruses use TRAF6 for optimal replication

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E117A mutation of LGTV NS3 reduces TRAF6 binding and mature protease abundance

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LGTV with a mutated TRAF6-binding motif is attenuated in vitro

Interferon-stimulated TRIM69 interrupts dengue virus replication by ubiquitinating viral nonstructural protein 3

In order to eliminate viral infections, hundreds of interferon-stimulated genes (ISGs) are induced via type I interferons (IFNs). However, the functions and mechanisms of most ISGs are largely unclear. A tripartite motif (TRIM) protein encoding gene TRIM69 is induced by dengue virus (DENV) infection as an ISG. TRIM69 restricts DENV replication, and its RING domain, which has the E3 ubiquitin ligase activity, is critical for its antiviral activity. An in vivo study further confirmed that TRIM69 contributes to the control of DENV infection in immunocompetent mice. Unlike many other TRIM family members, TRIM69 is not involved in modulation of IFN signaling. Instead, TRIM69 interacts with DENV Nonstructural Protein 3 (NS3) directly and mediates its polyubiquitination and degradation. Finally, Lys104 of NS3 is identified as the target of TRIM69-mediated ubiquitination. Our study demonstrates that TRIM69 restricts DENV replication by specifically ubiquitinating a viral nonstructural protein.

TRIM21 Promotes Innate Immune Response to RNA Viral Infection through Lys27-Linked Polyubiquitination of MAVS

Human innate immunity responds to viral infection by activating the production of interferons (IFNs) and proinflammatory cytokines. The mitochondrial adaptor molecule MAVS plays a critical role in innate immune response to viral infection. In this study, we show that TRIM21 (tripartite motif-containing protein 21) interacts with MAVS to positively regulate innate immunity. Under viral infection, TRIM21 is upregulated through the IFN/JAK/STAT signaling pathway. Knockdown of TRIM21 dramatically impairs innate immune response to viral infection. Moreover, TRIM21 interacts with MAVS and catalyzes its K27-linked polyubiquitination, thereby promoting the recruitment of TBK1 to MAVS. Specifically, the PRY-SPRY domain of TRIM21 is the key domain for its interaction with MAVS, while the RING domain of TRIM21 facilitates the polyubiquitination chains of MAVS. In addition, the MAVS-mediated innate immune response is enhanced by both the PRY-SPRY and RING domains of TRIM21. Mutation analyses of all the lysine residues of MAVS further revealed that Lys325 of MAVS is catalyzed by TRIM21 for the K27-linked polyubiquitination. Overall, this study reveals a novel mechanism by which TRIM21 promotes the K27-linked polyubiquitination of MAVS to positively regulate innate immune response, thereby inhibiting viral infection.IMPORTANCE Activation of innate immunity is essential for host cells to restrict the spread of invading viruses and other pathogens. MAVS plays a critical role in innate immune response to RNA viral infection. In this study, we demonstrated that TRIM21 targets MAVS to positively regulate innate immunity. Notably, TRIM21 targets and catalyzes K27-linked polyubiquitination of MAVS and then promotes the recruitment of TBK1 to MAVS, leading to upregulation of innate immunity. Our study outlines a novel mechanism by which the IFN signaling pathway blocks RNA virus to escape immune elimination.

Species-specific disruption of STING-dependent antiviral cellular defenses by the Zika virus NS2B3 protease

The limited host tropism of numerous viruses causing disease in humans remains incompletely understood. One example is Zika virus (ZIKV), an RNA virus that has reemerged in recent years. Here, we demonstrate that ZIKV efficiently infects fibroblasts from humans, great apes, New and Old World monkeys, but not rodents. ZIKV infection in human-but not murine-cells impairs responses to agonists of the cGMP-AMP synthase/stimulator of IFN genes (cGAS/STING) signaling pathway, suggesting that viral mechanisms to evade antiviral defenses are less effective in rodent cells. Indeed, human, but not mouse, STING is subject to cleavage by proteases encoded by ZIKV, dengue virus, West Nile virus, and Japanese encephalitis virus, but not that of yellow fever virus. The protease cleavage site, located between positions 78/79 of human STING, is only partially conserved in nonhuman primates and rodents, rendering these orthologs resistant to degradation. Genetic disruption of STING increases the susceptibility of mouse-but not human-cells to ZIKV. Accordingly, expression of only mouse, not human, STING in murine STING knockout cells rescues the ZIKV suppression phenotype. STING-deficient mice, however, did not exhibit increased susceptibility, suggesting that other redundant antiviral pathways control ZIKV infection in vivo. Collectively, our data demonstrate that numerous RNA viruses evade cGAS/STING-dependent signaling and affirm the importance of this pathway in shaping the host range of ZIKV. Furthermore, our results explain-at least in part-the decreased permissivity of rodent cells to ZIKV, which could aid in the development of mice model with inheritable susceptibility to ZIKV and other flaviviruses.

Biochemistry and Molecular Biology of Flaviviruses

Flaviviruses, such as dengue, Japanese encephalitis, tick-borne encephalitis, West Nile, yellow fever, and Zika viruses, are critically important human pathogens that sicken a staggeringly high number of humans every year. Most of these pathogens are transmitted by mosquitos, and not surprisingly, as the earth warms and human populations grow and move, their geographic reach is increasing. Flaviviruses are simple RNA-protein machines that carry out protein synthesis, genome replication, and virion packaging in close association with cellular lipid membranes. In this review, we examine the molecular biology of flaviviruses touching on the structure and function of viral components and how these interact with host factors. The latter are functionally divided into pro-viral and antiviral factors, both of which, not surprisingly, include many RNA binding proteins. In the interface between the virus and the hosts we highlight the role of a noncoding RNA produced by flaviviruses to impair antiviral host immune responses. Throughout the review, we highlight areas of intense investigation, or a need for it, and potential targets and tools to consider in the important battle against pathogenic flaviviruses.

Immune Response to Dengue and Zika

Flaviviruses such as dengue (DENV), yellow fever (YFV), West Nile (WNV), and Zika (ZIKV) are human pathogens of global significance. In particular, DENV causes the most prevalent mosquito-borne viral diseases in humans, and ZIKV emerged from obscurity into the spotlight in 2016 as the etiologic agent of congenital Zika syndrome. Owing to the recent emergence of ZIKV as a global pandemic threat, the roles of the immune system during ZIKV infections are as yet unclear. In contrast, decades of DENV research implicate a dual role for the immune system in protection against and pathogenesis of DENV infection. As DENV and ZIKV are closely related, knowledge based on DENV studies has been used to prioritize investigation of ZIKV immunity and pathogenesis, and to accelerate ZIKV diagnostic, therapeutic, and vaccine design. This review discusses the following topics related to innate and adaptive immune responses to DENV and ZIKV: the interferon system as the key mechanism of host defense and viral target for immune evasion, antibody-mediated protection versus antibody-dependent enhancement, and T cell-mediated protection versus original T cell antigenic sin. Understanding the mechanisms that regulate the balance between immune-mediated protection and pathogenesis during DENV and ZIKV infections is critical toward development of safe and effective DENV and ZIKV therapeutics and vaccines.

K48-linked polyubiquitination of dengue virus NS1 protein inhibits its interaction with the viral partner NS4B

Dengue virus (DENV) is a member of the Flaviviridae family, which is transmitted to mammalian species through arthropods, and causes dengue fever or severe dengue fever in humans. The DENV genome encodes for multiple nonstructural (NS) proteins including NS1. NS1 plays an essential role in replication by interacting with other viral proteins including NS4B, however how these interactions are regulated during virus infection is not known. By using bioinformatics, mass spectrometry analysis, and co-immunoprecipitation assays, here we show that DENV-NS1 is ubiquitinated on multiples lysine residues during DENV infection, including K189, a lysine residue previously shown to be important for efficient DENV replication. Data from in vitro and cell culture experiments indicate that dengue NS1 undergoes modification with K48-linked polyubiquitin chains, which usually target proteins to the proteasome for degradation. Furthermore, ubiquitinated NS1 was detected in lysates as well as in supernatants of human and mosquito infected cells. Ubiquitin deconjugation of NS1 using the deubiquitinase OTU resulted in increased interaction with the viral protein NS4B suggesting that ubiquitinated NS1 has reduced affinity for NS4B. In support of these data, a K189R mutation on NS1, which abrogates ubiquitination on amino acid residue 189 of NS1, also increased NS1-NS4B interactions. Our work describes a new mechanism of regulation of NS1-NS4B interactions and suggests that ubiquitination of NS1 may affect DENV replication.

RNA-virus proteases counteracting host innate immunity

Virus invasion triggers host immune responses, in particular, innate immune responses. Pathogen‐associated molecular patterns of viruses (such as dsRNA, ssRNA, or viral proteins) released during virus replication are detected by the corresponding pattern‐recognition receptors of the host, and innate immune responses are induced. Through production of type‐I and type‐III interferons as well as various other cytokines, the host innate immune system forms the frontline to protect host cells and inhibit virus infection. Not surprisingly, viruses have evolved diverse strategies to counter this antiviral system. In this review, we discuss the multiple strategies used by proteases of positive‐sense single‐stranded RNA viruses of the families Picornaviridae, Coronaviridae, and Flaviviridae, when counteracting host innate immune responses.