Virus-induced Signaling Adaptor Research Articles

BAG6 negatively regulates the RLR signaling pathway by targeting VISA/MAVS.

The virus-induced signaling adaptor protein VISA (also known as MAVS, ISP-1, Cardif) is a critical adaptor protein in the innate immune response to RNA virus infection. Upon viral infection, VISA self-aggregates to form a sizeable prion-like complex and recruits downstream signal components for signal transduction. Here, we discover that BAG6 (BCL2-associated athanogene 6, formerly BAT3 or Scythe) is an essential negative regulator in the RIG-I-like receptor signaling pathway. BAG6 inhibits the aggregation of VISA by promoting the K48-linked ubiquitination and specifically attenuates the recruitment of TRAF2 by VISA to inhibit RLR signaling. The aggregation of VISA and the interaction of VISA and TRAF2 are enhanced in BAG6-deficient cell lines after viral infection, resulting in the enhanced transcription level of downstream antiviral genes. Our research shows that BAG6 is a critical regulating factor in RIG-I/VISA-mediated innate immune response by targeting VISA.

Thioredoxin 2 Negatively Regulates Innate Immunity to RNA Viruses by Disrupting the Assembly of the Virus-Induced Signaling Adaptor Complex.

The virus-induced signaling adaptor (VISA) complex plays a critical role in the innate immune response to RNA viruses. However, the mechanism of VISA complex formation remains unclear. Here, we demonstrate that thioredoxin 2 (TRX2) interacts with VISA at mitochondria both in vivo and in vitro Knockdown and knockout of TRX2 enhanced the formation of the VISA-associated complex, as well as virus-triggered activation of interferon regulatory factor 3 (IRF3) and transcription of the interferon beta 1 (IFNB1) gene. TRX2 inhibits the formation of VISA aggregates by repressing reactive oxygen species (ROS) production, thereby disrupting the assembly of the VISA complex. Furthermore, our data suggest that the C93 residue of TRX2 is essential for inhibition of VISA aggregation, whereas the C283 residue of VISA is required for VISA aggregation. Collectively, these findings uncover a novel mechanism of TRX2 that negatively regulates VISA complex formation.IMPORTANCE The VISA-associated complex plays pivotal roles in inducing type I interferons (IFNs) and eliciting the innate antiviral response. Many host proteins are identified as VISA-associated-complex proteins, but how VISA complex formation is regulated by host proteins remains enigmatic. We identified the TRX2 protein as an important regulator of VISA complex formation. Knockout of TRX2 increases virus- or poly(I·C)-triggered induction of type I IFNs at the VISA level. Mechanistically, TRX2 inhibits the production of ROS at its C93 site, which impairs VISA aggregates at its C283 site, and subsequently impedes the assembly of the VISA complex. Our findings suggest that TRX2 plays an important role in the regulation of VISA complex assembly.

SNX8 modulates the innate immune response to RNA viruses by regulating the aggregation of VISA.

The mitochondrial virus-induced signaling adaptor (VISA, also called mitochondrial antiviral signaling, MAVS) protein is a central adaptor in the innate immune response to cytosolic viral RNA. Viral infection causes the aggregation of VISA, which is important for its recruitment of downstream signaling components. How VISA aggregation is regulated remains unknown. Here, we found that sorting nexin 8 (SNX8) is a positive regulator of the RNA virus-triggered induction of downstream effector genes and innate immune response. The brains and lungs of Snx8-/- mice infected with RNA viruses exhibited lower serum cytokine levels and higher viral titers than those of wild-type mice, resulting in higher lethality. Mechanistically, viral infection induced the translocation of SNX8 from the cytosol to mitochondria and its increased association with VISA, leading to VISA aggregation, its recruitment of downstream signaling components and the induction of downstream antiviral genes. Our findings suggest that SNX8 is a critical component of the RIG-I-like receptor (RLR)-mediated innate immune response by modulating VISA aggregation and activation.

RACK1 attenuates RLR antiviral signaling by targeting VISA-TRAF complexes

Virus-induced signaling adaptor (VISA), which mediates the production of type I interferon, is crucial for the retinoic acid-inducible gene I (RIG-I)-like receptor (RLR) signaling pathway. Upon viral infection, RIG-I recognizes double-stranded viral RNA and interacts with VISA to mediate antiviral innate immunity. However, the mechanisms underlying RIG/VISA-mediated antiviral regulation remain unclear. In this study, we confirmed that receptor for activated C kinase 1 (RACK1) interacts with VISA and attenuates the RIG/VISA-mediated antiviral innate immune signaling pathway. Overexpression of RACK1 inhibited the interferon-β (IFN-β) promoter; interferon-stimulated response element (ISRE); nuclear factor kappa B (NF-κB) activation; and dimerization of interferon regulatory factor 3 (IRF3) mediated by RIG-I, VISA, and TANK-binding kinase 1 (TBK1). A reduction in RACK1 expression level upon small interfering RNA knockdown increased RIG/VISA-mediated antiviral transduction. Additionally, RACK1 disrupted formation of the VISA-tumor necrosis factor receptor-associated factor 2 (TRAF2), VISA-TRAF3, and VISA-TRAF6 complexes during RIG-I/VISA-mediated signal transduction. Additionally, RACK1 enhanced K48-linked ubiquitination of VISA, attenuated its K63-linked ubiquitination, and decreased VISA-mediated antiviral signal transduction. Together, these results indicate that RACK1 interacts with VISA to repress downstream signaling and downregulates virus-induced IFN-β production in the RIG-I/VISA signaling pathway.

FKBP8 inhibits virus-induced RLR-VISA signaling.

The mitochondrial antiviral signal protein mitochondrial antiviral signaling protein, also known as virus-induced signaling adaptor (VISA), plays a key role in regulating host innate immune signaling pathways. This study identifies FK506 binding protein 8 (FKBP8) as a candidate interacting protein of VISA through the yeast two-hybrid technique. The interaction of FKBP8 with VISA, retinoic acid inducible protein 1 (RIG-I), and IFN regulatory factor 3 (IRF3) was confirmed during viral infection in mammalian cells by coimmunoprecipitation. Overexpression of FKBP8 using a eukaryotic expression plasmid significantly attenuated Sendai virus-induced activation of the promoter interferons β (IFN-β), and transcription factors nuclear factor κ-light chain enhancer of activated B cells (NF-κB) and IFN-stimulated response element (ISRE). Overexpression of FKBP8 also decreased dimer-IRF3 activity, but enhanced virus replication. Conversely, knockdown of FKBP8 expression by RNA interference showed opposite effects. Further studies indicated that FKBP8 acts as a negative interacting partner to regulate RLR-VISA signaling by acting on VISA and TANK binding kinase 1 (TBK1). Additionally, FKBP8 played a negative role on virus-induced signaling by inhibiting the formation of TBK1-IRF3 and VISA-TRAF3 complexes. Notably, FKBP8 also promoted the degradation of TBK1, RIG-I, and TRAF3 resulting from FKBP8 reinforced Sendai virus-induced endogenous polyubiquitination of RIG-I, TBK1, and TNF receptor-associated factor 3 (TRAF3). Therefore, a novel function of FKBP8 in innate immunity antiviral signaling regulation was revealed in this study.

TTLL12 Inhibits the Activation of Cellular Antiviral Signaling through Interaction with VISA/MAVS.

Upon virus infection, host cells use retinoic-acid-inducible geneI I (RIG-I)-like receptors to recognize viral RNA and activate type I IFN expression. To investigate the role of protein methylation in the antiviral signaling pathway, we screened all the SET domain-containing proteins and identified TTLL12 as a negative regulator of RIG-I signaling. TTLL12 contains SET and TTL domains, which are predicted to have lysine methyltransferase and tubulin tyrosine ligase activities, respectively. Exogenous expression of TTLL12 represses IFN-β expression induced by Sendai virus. TTLL12 deficiency by RNA interference and CRISPR-gRNA techniques increases the induced IFN-β expression and inhibits virus replication in the cell. The global gene expression profiling indicated that TTLL12 specifically inhibits the expression of the downstream genes of innate immunity pathways. Cell fractionation and fluorescent staining indicated that TTLL12 is localized in the cytosol. The mutagenesis study suggested that TTLL12's ability to repress the RIG-I pathway is probably not dependent on protein modifications. Instead, TTLL12 directly interacts with virus-induced signaling adaptor (VISA), TBK1, and IKKε, and inhibits the interactions of VISA with other signaling molecules. Taken together, our findings demonstrate TTLL12 as a negative regulator of RNA-virus-induced type I IFN expression by inhibiting the interaction of VISA with other proteins.

Duck Tembusu Virus Nonstructural Protein 1 Antagonizes IFN-β Signaling Pathways by Targeting VISA.

Duck Tembusu virus (DTMUV) is an emergent infectious pathogen that has caused severe disease in ducks and huge economic losses to the poultry industry in China since 2009. Previously, we showed that DTMUV inhibits IFN-β induction early in infection; however, the mechanisms of the inhibition of innate immune responses remain poorly understood. In this study, we screened DTMUV-encoded structural and nonstructural proteins using reporter assays and found that DTMUV NS1 markedly suppressed virus-triggered IFN-β expression by inhibiting retinoic acid-inducible gene I-like receptor signaling. Moreover, we found that DTMUV NS1 specifically interacted with the C-terminal domain of virus-induced signaling adaptor and impaired the association of retinoic acid-inducible gene I or melanoma differentiation-associated gene 5 and virus-induced signaling adaptor, thereby downregulating the retinoic acid-inducible gene I-like receptor-mediated signal transduction and cellular antiviral responses, leading to evasion of the innate immune response. Together, our findings reveal a novel mechanism manipulated by DTMUV to circumvent the host antiviral immune response.

Esterase D enhances type I interferon signal transduction to suppress foot-and-mouth disease virus replication.

The enzymatic activities of esterase D (ESD) are involved in many human diseases. However, no antiviral property of ESD has been described to date. Foot-and-mouth disease virus (FMDV) is the etiological agent of foot-and-mouth disease. In this study, we showed that FMDV infection triggered ESD expression. Overexpression of ESD significantly suppressed FMDV replication and knockdown of ESD expression enhanced virus replication, showing an essential antiviral role of ESD. Furthermore, we found that Sendai-virus-induced interferon (IFN) signaling was enhanced by upregulation of ESD, and ESD promoted activation of the IFN-β promoter simulated by IFN regulatory factor (IRF)3 or its upstream molecules (retinoic acid-inducible gene-I, melanoma differentiation-associated protein 5, virus-induced signaling adaptor and TANK binding kinase 1). Detailed analysis revealed that ESD protein enhanced IRF3 phosphorylation during FMDV infection. Overexpression of ESD also promoted the expression of various antiviral interferon-stimulated genes (ISGs) and knockdown of ESD impaired the expression of these antiviral genes during FMDV infection. Our findings demonstrate a new mechanism evolved by ESD to enhance type I IFN signal transduction and suppress viral replication during FMDV infection.

Highly Pathogenic Porcine Reproductive and Respiratory Syndrome Virus Nsp4 Cleaves VISA to Impair Antiviral Responses Mediated by RIG-I-like Receptors.

Porcine reproductive and respiratory syndrome virus (PRRSV) is one of the most significant etiological agents in the swine industry worldwide. It has been reported that PRRSV infection can modulate host immune responses, and innate immune evasion is thought to play a vital role in PRRSV pathogenesis. In this study, we demonstrated that highly pathogenic PRRSV (HP-PRRSV) infection specifically down-regulated virus-induced signaling adaptor (VISA), a unique adaptor molecule that is essential for retinoic acid induced gene-I (RIG-I) and melanoma differentiation associated gene 5 (MDA5) signal transduction. Moreover, we verified that nsp4 inhibited IRF3 activation induced by signaling molecules, including RIG-I, MDA5, VISA, and TBK1, but not IRF3. Subsequently, we demonstrated that HP-PRRSV nsp4 down-regulated VISA and suppressed type I IFN induction. Importantly, VISA was cleaved by nsp4 and released from mitochondrial membrane, which interrupted the downstream signaling of VISA. However, catalytically inactive mutant of nsp4 abolished its ability to cleave VISA. Interestingly, nsp4 of typical PRRSV strain CH-1a had no effect on VISA. Taken together, these findings reveal a strategy evolved by HP-PRRSV to counteract anti-viral innate immune signaling, which complements the known PRRSV-mediated immune-evasion mechanisms.

Baculovirus vectors expressing F proteins in combination with virus-induced signaling adaptor (VISA) molecules confer protection against respiratory syncytial virus infection

Baculovirus has been exploited for use as a novel vaccine vector. To investigate the feasibility and efficacy of recombinant baculoviruses (rBVs) expressing respiratory syncytial virus (RSV) fusion (F) proteins, four constructs (Bac-tF/64, Bac-CF, Bac-CF/tF64 and Bac-CF/tF64-VISA) were generated. Bac-tF64 displays the F ectodomain (tF) on the envelope of rBVs, whereas Bac-CF expresses full-length F protein in transduced mammalian cells. Bac-CF/tF64 not only displays tF on the envelope but also expresses F in cells. Bac-CF/tF64-VISA comprises Bac-CF/tF64 harboring the virus-induced signaling adaptor (VISA) gene. After administration to BALB/c mice, all four vectors elicited RSV neutralizing antibody (Ab), systemic Ab (IgG, IgG1, and IgG2a), and cytokine responses. Compared with Bac-tF64, mice inoculated with Bac-CF and Bac-CF/tF64 exhibited an increased mixed Th1/Th2 cytokine response, increased ratios of IgG2a/IgG1 antibody responses, and reduced immunopathology upon RSV challenge. Intriguingly, co-expression of VISA reduced Th2 cytokine (IL-4, IL-5, and IL-10) production induced by Bac-CF/tF64, thus relieving lung pathology upon a subsequent RSV challenge. Our results indicated that the Bac-CF/tF64 vector incorporated with the VISA molecule may provide an effective vaccine strategy for protection against RSV.

Role of RIG-1 signaling in the Pathogenesis of Alzheimer’s Disease (172.1)

Abstract Recent evidence has accumulated that Alzheimer’s disease (AD) is associated with a central nervous system (CNS) inflammatory response. Neuroinflammation is a complex response against harmful effects of diverse stimuli and plays a critical role in the pathogenesis of AD. An important arm of the inflammatory response is the innate immune response. The innate immune response is regulated by pattern recognition receptors (PRRs) and their signaling pathways. Here we report that the expression of the innate immune response proteins, retinoic acid-inducible gene-I (RIG-1), virus-induced signaling adaptor (VISA), and the NACHT leucine-rich domain and pyrin-containing protein, NLRP12 are significantly elevated in neurons of the temporal cortex, the cerebrospinal fluid and plasma in presymptomatic AD patients, post-mortem diagnosis. Increased expression of RIG-1 and VISA were not found in pathologically confirmed cases of amyotrophic lateral sclerosis, Diffuse Lewy Body disease or Huntington’s disease, suggesting that activation of RIG-1 signaling may be a specific marker for early AD pathogenesis. Moreover, in vitro results show that stimulation of RIG-1 signaling induces expression of amyloid precursor protein and β-amyloid, supporting the idea that activation of the RIG-1 signaling system occurs in presymptomatic AD.

Ankrd17 positively regulates RIG‐I‐like receptor (RLR)‐mediated immune signaling

Retinoic acid-inducible gene-I (RIG-I)-like receptors (RLRs), such as RIG-I, melanoma differentiation-associated gene 5 (MDA5), and virus-induced signaling adaptor (VISA), are intracellular molecules that sense diverse viral RNAs and trigger immune responses. In this study, we demonstrate that the ankyrin repeat protein ankrd17 interacts with RIG-I, MDA5, and VISA and upregulates RLR-mediated immune signaling. Overexpression of ankrd17 enhances RLR-mediated activation of IRF-3 and NF-κB and upregulates the transcription of IFN-β. It also promotes RLR signaling in response to poly (I:C), influenza virus RNA, and Sendai virus. Consistently, knockdown of ankrd17 impairs RLR signaling. Furthermore, we demonstrate that ankrd17 enhances the interaction of RIG-I and MDA5 with VISA; the ankyrin repeat domain of ankrd17 is required for its interaction with RIG-I as well as for its function in regulating the RLR pathway. Taken together, our results indicate that ankrd17 is a positive regulator of the RLR signaling pathway.

The complement C1qA enhances retinoic acid‐inducible gene‐I‐mediated immune signalling

The cellular innate immune response is essential for recognizing and defending against viral infection. Retinoic acid-inducible gene-I (RIG-I) and virus-induced signaling adaptor (VISA) mediated immune signalling is critically involved in RNA-virus-induced innate immune responses. Here we demonstrate that the complement C1qA interacts with different RIG-I pathway components and enhances RIG-I-VISA-mediated signalling pathway as well as TBK1-mediated activation of interferon-β (IFN-β) promoter. Our data show that over-expression of C1qA up-regulates RIG-I-mediated activation of IFN-stimulated responsive element (ISRE) and nuclear factor-κB reporters and IFN-β transcription, but not IFN regulatory factor-3-mediated and inhibitor of κB kinase-mediated activation of ISRE and nuclear factor-κB promoter. In addition, C1qA can counteract the function of the C1q receptor gC1qR in RIG-I-mediated signalling. Our results reveal the important role of complement C1qA in the innate immune response.

TRAF6 and IRF7 Control HIV Replication in Macrophages

The innate immune system recognizes virus infection and evokes antiviral responses which include producing type I interferons (IFNs). The induction of IFN provides a crucial mechanism of antiviral defense by upregulating interferon-stimulated genes (ISGs) that restrict viral replication. ISGs inhibit the replication of many viruses by acting at different steps of their viral cycle. Specifically, IFN treatment prior to in vitro human immunodeficiency virus (HIV) infection stops or significantly delays HIV-1 production indicating that potent inhibitory factors are generated. We report that HIV-1 infection of primary human macrophages decreases tumor necrosis factor receptor-associated factor 6 (TRAF6) and virus-induced signaling adaptor (VISA) expression, which are both components of the IFN signaling pathway controlling viral replication. Knocking down the expression of TRAF6 in macrophages increased HIV-1 replication and augmented the expression of IRF7 but not IRF3. Suppressing VISA had no impact on viral replication. Overexpression of IRF7 resulted in enhanced viral replication while knocking down IRF7 expression in macrophages significantly reduced viral output. These findings are the first demonstration that TRAF6 can regulate HIV-1 production and furthermore that expression of IRF7 promotes HIV-1 replication.

The virus-induced signaling adaptor molecule enhances DNA-raised immune protection against H5N1 influenza virus infection in mice

As an adaptor molecule in the retinoic acid-inducible gene-I (RIG-I) signaling pathway, the virus-induced signaling adaptor (VISA) molecule activates NF-κB and IRF3 and thereby leads to the production of type I interferons (IFNs). To explore the potential of VISA as a genetic adjuvant for DNA vaccines, a eukaryotic expression plasmid, pVISA, was generated by cloning the VISA gene into the pVAX1vector. For comparison, the pTRIF plasmid was similarly constructed, encoding the known genetic adjuvant TRIF (TIR-domain-containing adapter-inducing interferon-β), an adapter in the Toll-like receptor (TLR) signaling pathway. Mice were immunized with the chimeric DNA vaccine pHA/NP 147–155, which encodes the HA (hemagglutinin) fused with NP (nucleoprotein) CTL epitope (NP 147–155) of H5N1 influenza virus, either alone or in combination with pVISA or pTRIF. Antigen-specific immune responses were examined in immunized mice. Our results demonstrate that co-immunization of the pHA/NP 147–155 plasmid with the VISA adjuvant augmented DNA-raised cellular immune responses and provided protection against H5N1 influenza virus challenge in mice. In addition, our data suggest that VISA acts as a stronger adjuvant for DNA immunization than TRIF. We conclude that co-inoculation with a vector expressing the adaptor molecule VISA enhanced the protective immunity against H5N1 infection induced by pHA/NP 147–155 and that VISA could be developed as a novel genetic adjuvant for DNA vaccines.

Hepatitis B virus X protein suppresses virus-triggered IRF3 activation and IFN-β induction by disrupting the VISA-associated complex

Viral RNAs produced during viral infection are recognized by the cytoplasmic RNA helicases retinoic acid-inducible gene-I (RIG-I) and melanoma differentiation-associated gene 5 (MDA5). A central adapter protein downstream of RIG-I and MDA5 is the mitochondrial membrane protein virus-induced signaling adaptor (VISA), which mediates the induction of type I interferons (IFNs) through the activation of transcription factors such as nuclear factor-kappaB (NF-kappaB) and IFN-regulatory factor-3 (IRF3). Here we found that hepatitis B virus (HBV)-encoded X protein (HBx) acts as an inhibitor of virus-triggered IRF3 activation and IFN-beta induction. Reporter and plaque assays indicate that HBx inhibits signaling by components upstream but not downstream of VISA. Immunoprecipitation experiments indicate that HBx interacts with VISA and disrupts the association of VISA with its upstream and downstream components. These findings suggest that HBx acts as a suppressor of virus-triggered induction of type I IFNs, which explains the observation that HBV causes transient and chronic infection in hepatocytes but fails to activate the pattern recognition receptor-mediated IFN induction pathways.

The E3 Ubiquitin Ligase RNF5 Targets Virus-Induced Signaling Adaptor for Ubiquitination and Degradation

Viral infection activates transcription factors, such as NF-kappaB and IFN regulatory factor 3, which collaborate to induce type I IFNs and elicit innate antiviral response. Virus-induced signaling adaptor (VISA) has been identified as a critical adaptor required for virus-triggered induction of type I IFNs. In this study, we showed that the E3 ubiquitin ligase RING-finger protein 5 (RNF5) interacted with VISA at mitochondria in a viral infection-dependent manner. Domain mapping experiments indicated that the C-terminal transmembrane domain of VISA was required for its interaction with RNF5. RNF5 targeted VISA at K362 and K461 for K48-linked ubiquitination and degradation after viral infection, whereas knockdown of RNF5 reversed virus-induced downregulation of VISA at the early phase. These findings suggest that RNF5-mediated ubiquitination and degradation of VISA is one of the mechanisms of the regulation of virus-triggered induction of type I IFNs and cellular antiviral response.

DEAD/H BOX 3 (DDX3) helicase binds the RIG‐I adaptor IPS‐1 to up‐regulate IFN‐β‐inducing potential

Retinoic acid-inducible gene-I (RIG-I)-like receptors (RLR) are members of the DEAD box helicases, and recognize viral RNA in the cytoplasm, leading to IFN-beta induction through the adaptor IFN-beta promoter stimulator-1 (IPS-1) (also known as Cardif, mitochondrial antiviral signaling protein or virus-induced signaling adaptor). Since uninfected cells usually harbor a trace of RIG-I, other RNA-binding proteins may participate in assembling viral RNA into the IPS-1 pathway during the initial response to infection. We searched for proteins coupling with human IPS-1 by yeast two-hybrid and identified another DEAD (Asp-Glu-Ala-Asp) box helicase, DDX3 (DEAD/H BOX 3). DDX3 can bind viral RNA to join it in the IPS-1 complex. Unlike RIG-I, DDX3 was constitutively expressed in cells, and some fraction of DDX3 is colocalized with IPS-1 around mitochondria. The 622-662 a.a DDX3 C-terminal region (DDX3-C) directly bound to the IPS-1 CARD-like domain, and the whole DDX3 protein also associated with RLR. By reporter assay, DDX3 helped IPS-1 up-regulate IFN-beta promoter activation and knockdown of DDX3 by siRNA resulted in reduced IFN-beta induction. This activity was conserved on the DDX3-C fragment. DDX3 only marginally enhanced IFN-beta promoter activation induced by transfected TANK-binding kinase 1 (TBK1) or I-kappa-B kinase-epsilon (IKKepsilon). Forced expression of DDX3 augmented virus-mediated IFN-beta induction and host cell protection against virus infection. Hence, DDX3 is an antiviral IPS-1 enhancer.

Polo-like Kinase 1 (PLK1) Regulates Interferon (IFN) Induction by MAVS

The mitochondria-bound adapter MAVS participates in IFN induction by recruitment of downstream partners such as members of the TRAF family, leading to activation of NF-kappaB, and the IRF3 pathways. A yeast two-hybrid search for MAVS-interacting proteins yielded the Polo-box domain (PBD) of the mitotic Polo-like kinase PLK1. We showed that PBD associates with two different domains of MAVS in both dependent and independent phosphorylation events. The phosphodependent association requires the phosphopeptide binding ability of PBD. It takes place downstream of the proline-rich domain of MAVS, within an STP motif, characteristic of the binding of PLK1 to its targets, where the central Thr234 residue is phosphorylated. Its phosphoindependent association takes place at the C terminus of MAVS. PLK1 strongly inhibits the ability of MAVS to activate the IRF3 and NF-kappaB pathways and to induce IFN. Reciprocally, depletion of PLK1 can increase IFN induction in response to RIG-I/SeV or RIG-I/poly(I)-poly(C) treatments. This inhibition is dependent on the phosphoindependent association of PBD at the C terminus of MAVS where it disrupts the association of MAVS with its downstream partner TRAF3. IFN induction was strongly inhibited in cells arrested in G2/M by nocodazole, which provokes increased expression of endogenous PLK1. Interestingly, depletion of PLK1 from these nocodazole-treated cells could restore, at least partially, IFN induction. Altogether, these data demonstrate a new function for PLK1 as a regulator of IFN induction and provide the basis for the development of inhibitors preventing the PLK1/MAVS association to sustain innate immunity.

Severe Acute Respiratory Syndrome Coronavirus M Protein Inhibits Type I Interferon Production by Impeding the Formation of TRAF3·TANK·TBK1/IKKϵ Complex

Severe acute respiratory syndrome (SARS) coronavirus is highly pathogenic in humans and evades innate immunity at multiple levels. It has evolved various strategies to counteract the production and action of type I interferons, which mobilize the front-line defense against viral infection. In this study we demonstrate that SARS coronavirus M protein inhibits gene transcription of type I interferons. M protein potently antagonizes the activation of interferon-stimulated response element-dependent transcription by double-stranded RNA, RIG-I, MDA5, TBK1, IKKϵ, and virus-induced signaling adaptor (VISA) but has no influence on the transcriptional activity of this element when IRF3 or IRF7 is overexpressed. M protein physically associates with RIG-I, TBK1, IKKϵ, and TRAF3 and likely sequesters some of them in membrane-associated cytoplasmic compartments. Consequently, the expression of M protein prevents the formation of TRAF3·TANK·TBK1/IKKϵ complex and thereby inhibits TBK1/IKKϵ-dependent activation of IRF3/IRF7 transcription factors. Taken together, our findings reveal a new mechanism by which SARS coronavirus circumvents the production of type I interferons.