MAVS recruits multiple ubiquitin E3 ligases to activate antiviral signaling cascades

Multifaceted roles of TRIM38 in innate immune and inflammatory responses

The tripartite motif-containing (TRIM) proteins represent the largest E3 ubiquitin ligase family. The multifaceted roles of TRIM38 in innate immunity and inflammation have been intensively investigated in recent years. TRIM38 is essential for cytosolic RNA or DNA sensor-mediated innate immune responses to both RNA and DNA viruses, while negatively regulating TLR3/4- and TNF/IL-1β-triggered inflammatory responses. In these processes, TRIM38 acts as an E3 ubiquitin or SUMO ligase, which targets key cellular signaling components, or as an enzymatic activity-independent regulator. This review summarizes recent advances that highlight the critical roles of TRIM38 in the regulation of proper innate immune and inflammatory responses.

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.

TAX1BP1 Restrains Virus-Induced Apoptosis by Facilitating Itch-Mediated Degradation of the Mitochondrial Adaptor MAVS

The host response to RNA virus infection consists of an intrinsic innate immune response and the induction of apoptosis as mechanisms to restrict viral replication. The mitochondrial adaptor molecule MAVS plays critical roles in coordinating both virus-induced type I interferon production and apoptosis; however, the regulation of MAVS-mediated apoptosis is poorly understood. Here, we show that the adaptor protein TAX1BP1 functions as a negative regulator of virus-induced apoptosis. TAX1BP1-deficient cells are highly sensitive to apoptosis in response to infection with the RNA viruses vesicular stomatitis virus and Sendai virus and to transfection with poly(I·C). TAX1BP1 undergoes degradation during RNA virus infection, and loss of TAX1BP1 is associated with apoptotic cell death. TAX1BP1 deficiency augments virus-induced activation of proapoptotic c-Jun N-terminal kinase (JNK) signaling. Virus infection promotes the mitochondrial localization of TAX1BP1 and concomitant interaction with the mitochondrial adaptor MAVS. TAX1BP1 recruits the E3 ligase Itch to MAVS to trigger its ubiquitination and degradation, and loss of TAX1BP1 or Itch results in increased MAVS protein expression. Together, these results indicate that TAX1BP1 functions as an adaptor molecule for Itch to target MAVS during RNA virus infection and thus restrict virus-induced apoptosis.

Protect this house: cytosolic sensing of viruses

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Cells are equipped with pattern recognition receptors to sense invading viruses.

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Nucleic acids of RNA viruses are sensed by RIG-I like receptors in the cytosol.

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Foreign DNA is sensed by cGAS and other DNA sensors in the cytosol.

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These pattern recognition receptors activate adaptor proteins to initiate antiviral innate immune responses.

The ability to recognize invading viral pathogens and to distinguish their components from those of the host cell is critical to initiate the innate immune response. The efficiency of this detection is an important factor in determining the susceptibility of the cell to viral infection. Innate sensing of viruses is, therefore, an indispensable step in the line of defense for cells and organisms. Recent discoveries have uncovered novel sensors of viral components and hallmarks of infection, as well as mechanisms by which cells discriminate between self and non-self. This review highlights the mechanisms used by cells to detect viral pathogens in the cytosol, and recent advances in the field of cytosolic sensing of viruses.

Molecular cloning and functional characterization of feline MAVS

The mitochondrial anti-viral signaling protein (MAVS) plays an important role in the type I IFN response. In this study, two feline MAVS transcripts were cloned. Both transcripts have the same open reading frame encoding 523 amino acids. The putative protein shares 76.6 % similarity with canine and exhibits similarity to human, mouse, rat, bovine and porcine, ranging from 46.1 to 65.8 %. Deletion mutant analysis indicated that the transmembrane (TM) domain is necessary for localization in the mitochondrial membrane, and both the caspase activation and recruitment domain and TM domain are indispensible for activating the IFN-β response. Additionally, Sendai virus-induced IFN-β promoter activation was significantly inhibited by siRNA targeting MAVS. Finally, miniMAVS, a second protein encoded by MAVS mRNA, was identified, which interfered with the IFN-β response via the inhibition of NF-κB activation. The identification of MAVS will promote the understanding and control of feline infectious diseases.

Functional motifs responsible for human metapneumovirus M2-2-mediated innate immune evasion

Human metapneumovirus (hMPV) is a major cause of lower respiratory infection in young children. Repeated infections occur throughout life, but its immune evasion mechanisms are largely unknown. We recently found that hMPV M2-2 protein elicits immune evasion by targeting mitochondrial antiviral-signaling protein (MAVS), an antiviral signaling molecule. However, the molecular mechanisms underlying such inhibition are not known. Our mutagenesis studies revealed that PDZ-binding motifs, 29-DEMI-32 and 39-KEALSDGI-46, located in an immune inhibitory region of M2-2, are responsible for M2-2-mediated immune evasion. We also found both motifs prevent TRAF5 and TRAF6, the MAVS downstream adaptors, to be recruited to MAVS, while the motif 39-KEALSDGI-46 also blocks TRAF3 migrating to MAVS. In parallel, these TRAFs are important in activating transcription factors NF-kB and/or IRF-3 by hMPV. Our findings collectively demonstrate that M2-2 uses its PDZ motifs to launch the hMPV immune evasion through blocking the interaction of MAVS and its downstream TRAFs.

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This manuscript describes a molecular mechanism underlying the immune evasion of hMPV.

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Results create the design basis for safer and more effective hMPV vaccines/therapeutic molecules.

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We demonstrate the contribution of TRAFs in antiviral responses to hMPV infection.

JMJD8 is a positive regulator of TNF-induced NF-κB signaling

TNF-induced signaling mediates pleiotropic biological consequences including inflammation, immunity, cell proliferation and apoptosis. Misregulation of TNF signaling has been attributed as a major cause of chronic inflammatory diseases and cancer. Jumonji domain-containing protein 8 (JMJD8) belongs to the JmjC family. However, only part of the family members has been described as hydroxylase enzymes that function as histone demethylases. Here, we report that JMJD8 positively regulates TNF-induced NF-κB signaling. Silencing the expression of JMJD8 using RNA interference (RNAi) greatly suppresses TNF-induced expression of several NF-κB-dependent genes. Furthermore, knockdown of JMJD8 expression reduces RIP ubiquitination, IKK kinase activity, delays IκBα degradation and subsequently blocks nuclear translocation of p65. In addition, JMJD8 deficiency enhances TNF-induced apoptosis. Taken together, these findings indicate that JMJD8 functions as a positive regulator of TNF-induced NF-κB signaling.

Interaction of SARS and MERS Coronaviruses with the Antiviral Interferon Response

Severe Acute Respiratory Syndrome (SARS) and Middle East Respiratory Syndrome (MERS) are the most severe coronavirus (CoV)-associated diseases in humans. The causative agents, SARS-CoV and MERS-CoV, are of zoonotic origin but may be transmitted to humans, causing severe and often fatal respiratory disease in their new host. The two coronaviruses are thought to encode an unusually large number of factors that allow them to thrive and replicate in the presence of efficient host defense mechanisms, especially the antiviral interferon system. Here, we review the recent progress in our understanding of the strategies that highly pathogenic coronaviruses employ to escape, dampen, or block the antiviral interferon response in human cells.

MAVS maintains mitochondrial homeostasis via autophagy

Mitochondrial antiviral signalling protein (MAVS) acts as a critical adaptor protein to transduce antiviral signalling by physically interacting with activated RIG-I and MDA5 receptors. MAVS executes its functions at the outer membrane of mitochondria to regulate downstream antiviral signalling, indicating that the mitochondria provides a functional platform for innate antiviral signalling transduction. However, little is known about whether and how MAVS-mediated antiviral signalling contributes to mitochondrial homeostasis. Here we show that the activation of MAVS is sufficient to induce autophagic signalling, which may mediate the turnover of the damaged mitochondria. Importantly, we find MAVS directly interacts with LC3 through its LC3-binding motif ‘YxxI’, suggesting that MAVS might act as an autophagy receptor to mediate mitochondrial turnover upon excessive activation of RLR signalling. Furthermore, we provide evidence that both MAVS self-aggregation and its interaction with TRAF2/6 proteins are important for MAVS-mediated mitochondrial turnover. Collectively, our findings suggest that MAVS acts as a potential receptor for mitochondria-associated autophagic signalling to maintain mitochondrial homeostasis.

Soluble IL-6 Receptor and IL-27 Subunit p28 Protein Complex Mediate the Antiviral Response through the Type III IFN Pathway

Previously, we demonstrated that the soluble IL-6R (sIL-6R) plays an important role in the host antiviral response through induction of type I IFN and sIL-6R-mediated antiviral action via the IL-27 subunit p28; however, the mechanism that underlies sIL-6R and p28 antiviral action and whether type III IFN is involved remain unknown. In this study, we constructed a sIL-6R and p28 fusion protein (sIL-6R/p28 FP) and demonstrated that the fusion protein has stronger antiviral activity than sIL-6R alone. Consequently, knockout of sIL-6R inhibited virus-triggered IFN-λ1 expression. In addition, sIL-6R/p28 FP associated with mitochondrial antiviral signaling protein and TNFR-associated factor 6, the retinoic acid-inducible gene I adapter complex, and the antiviral activity mediated by sIL-6R/p28 FP was dependent on mitochondrial antiviral signaling protein. Furthermore, significantly reduced binding of p50/p65 and IFN regulatory factor 3 to the IFN-λ1 promoter was observed in sIL-6R knockout cells compared with the control cells. Interestingly, a novel heterodimer of c-Fos and activating transcription factor 1 was identified as a crucial transcriptional activator of IFN-λ1 The sIL-6R/p28 FP upregulated IFN-λ1 expression by increasing the binding abilities of c-Fos and activating transcription factor 1 to the IFN-λ1 promoter via the p38 MAPK signaling pathway. In conclusion, these results demonstrate the important role of sIL-6R/p28 FP in mediating virus-induced type III IFN production.

Mechanism of TRIM25 Catalytic Activation in the Antiviral RIG-I Pathway

Antiviral response pathways induce interferon by higher-order assembly of signaling complexes called signalosomes. Assembly of the RIG-I signalosome is regulated by K63-linked polyubiquitin chains, which are synthesized by the E3 ubiquitin ligase, TRIM25. We have previously shown that the TRIM25 coiled-coil domain is a stable, antiparallel dimer that positions two catalytic RING domains on opposite ends of an elongated rod. We now show that the RING domain is a separate self-association motif that engages ubiquitin-conjugated E2 enzymes as a dimer. RING dimerization is required for catalysis, TRIM25-mediated RIG-I ubiquitination, interferon induction, and antiviral activity. We also provide evidence that RING dimerization and E3 ligase activity are promoted by binding of the TRIM25 SPRY domain to the RIG-I effector domain. These results indicate that TRIM25 actively participates in higher-order assembly of the RIG-I signalosome and helps to fine-tune the efficiency of the RIG-I-mediated antiviral response.

Mycotoxin Patulin Suppresses Innate Immune Responses by Mitochondrial Dysfunction and p62/Sequestosome-1-dependent Mitophagy

Innate immune responses are important for pathogen elimination and adaptive immune response activation. However, excess inflammation may contribute to immunopathology and disease progression (e.g. inflammation-associated hepatocellular carcinoma). Immune modulation resulting from pattern recognition receptor-induced responses is a potential strategy for controlling immunopathology and related diseases. This study demonstrates that the mycotoxin patulin suppresses Toll-like receptor- and RIG-I/MAVS-dependent cytokine production through GSH depletion, mitochondrial dysfunction, the activation of p62-associated mitophagy, and p62-TRAF6 interaction. Blockade of autophagy restored the immunosuppressive activity of patulin, and pharmacological activation of p62-dependent mitophagy directly reduced RIG-I-like receptor-dependent inflammatory cytokine production. These results demonstrated that p62-dependent mitophagy has an immunosuppressive role to innate immune response and might serve as a potential immunomodulatory target for inflammation-associated diseases.

PPM1A silences cytosolic RNA sensing and antiviral defense through direct dephosphorylation of MAVS and TBK1

Cytosolic RNA sensing is a prerequisite for initiation of innate immune response against RNA viral pathogens. Signaling through RIG-I (retinoic acid-inducible gene I)-like receptors (RLRs) to TBK1 (Tank-binding kinase 1)/IKKε (IκB kinase ε) kinases is transduced by mitochondria-associated MAVS (mitochondrial antiviral signaling protein). However, the precise mechanism of how MAVS-mediated TBK1/IKKε activation is strictly controlled still remains obscure. We reported that protein phosphatase magnesium-dependent 1A (PPM1A; also known as PP2Cα), depending on its catalytic ability, dampened the RLR-IRF3 (interferon regulatory factor 3) axis to silence cytosolic RNA sensing signaling. We demonstrated that PPM1A was an inherent partner of the TBK1/IKKε complex, targeted both MAVS and TBK1/IKKε for dephosphorylation, and thus disrupted MAVS-driven formation of signaling complex. Conversely, a high level of MAVS can dissociate the TBK1/PPM1A complex to override PPM1A-mediated inhibition. Loss of PPM1A through gene ablation in human embryonic kidney 293 cells and mouse primary macrophages enabled robustly enhanced antiviral responses. Consequently, Ppm1a(-/-) mice resisted to RNA virus attack, and transgenic zebrafish expressing PPM1A displayed profoundly increased RNA virus vulnerability. These findings identify PPM1A as the first known phosphatase of MAVS and elucidate the physiological function of PPM1A in antiviral immunity on whole animals.

Hepatitis B Virus-Induced Parkin-Dependent Recruitment of Linear Ubiquitin Assembly Complex (LUBAC) to Mitochondria and Attenuation of Innate Immunity

Hepatitis B virus (HBV) suppresses innate immune signaling to establish persistent infection. Although HBV is a DNA virus, its pre-genomic RNA (pgRNA) can be sensed by RIG-I and activates MAVS to mediate interferon (IFN) λ synthesis. Despite of the activation of RIG-I-MAVS axis by pgRNA, the underlying mechanism explaining how HBV infection fails to induce interferon-αβ (IFN) synthesis remained uncharacterized. We demonstrate that HBV induced parkin is able to recruit the linear ubiquitin assembly complex (LUBAC) to mitochondria and abrogates IFN β synthesis. Parkin interacts with MAVS, accumulates unanchored linear polyubiquitin chains on MAVS via LUBAC, to disrupt MAVS signalosome and attenuate IRF3 activation. This study highlights the novel role of parkin in antiviral signaling which involves LUBAC being recruited to the mitochondria. These results provide avenues of investigations on the role of mitochondrial dynamics in innate immunity.

Hepatitis B virus (HBV) chronic infection is one of the major causes of hepatocellular carcinoma. HBV infection is associated with mitochondrial dysfunction. We previously showed that persistent infection of HBV requires rapid clearance of impaired mitochondria by mitophagy, a cellular quality control process that insures survival of HBV infected cells. During the process, Parkin, an RBR E3 ligase, is recruited to mitochondria to induce mitophagy. In this study, we show that the Parkin, plays a critical role in the modulation of innate immune signaling. Using HBV expressing cells, we show that the Parkin recruits linear ubiquitin assembly complex (LUBAC) to the mitochondria and subsequently inhibits downstream signaling of mitochondrial antiviral signaling protein (MAVS). Mitochondrial LUBAC then catalyzes linear ubiquitin chains on MAVS, which abrogates its downstream events such as MAVS-TRAFs interaction and abolishes IRF3 phosphorylation. The results of this study highlight the molecular details explaining how HBV can suppress interferon synthesis implicating a mitophagy-independent role of Parkin. HBV-induced mitochondrial damage serves as the platform for recruitment of Parkin and LUBAC, which together modify MAVS by ubiquitination and cripples its downstream signaling.

Phleboviruses and the Type I Interferon Response

The genus Phlebovirus of the family Bunyaviridae contains a number of emerging virus species which pose a threat to both human and animal health. Most prominent members include Rift Valley fever virus (RVFV), sandfly fever Naples virus (SFNV), sandfly fever Sicilian virus (SFSV), Toscana virus (TOSV), Punta Toro virus (PTV), and the two new members severe fever with thrombocytopenia syndrome virus (SFTSV) and Heartland virus (HRTV). The nonstructural protein NSs is well established as the main phleboviral virulence factor in the mammalian host. NSs acts as antagonist of the antiviral type I interferon (IFN) system. Recent progress in the elucidation of the molecular functions of a growing list of NSs proteins highlights the astonishing variety of strategies employed by phleboviruses to evade the IFN system.

Tumor necrosis factor receptor- associated factor 6 (TRAF6) regulation of development, function, and homeostasis of the immune system

Tumor necrosis factor receptor (TNFR)-associated factor 6 (TRAF6) is an adapter protein that mediates a wide array of protein-protein interactions via its TRAF domain and a RING finger domain that possesses non-conventional E3 ubiquitin ligase activity. First identified nearly two decades ago as a mediator of interleukin-1 receptor (IL-1R)-mediated activation of NFκB, TRAF6 has since been identified as an actor downstream of multiple receptor families with immunoregulatory functions, including members of the TNFR superfamily, the Toll-like receptor (TLR) family, tumor growth factor-β receptors (TGFβR), and T-cell receptor (TCR). In addition to NFκB, TRAF6 may also direct activation of mitogen-activated protein kinase (MAPK), phosphoinositide 3-kinase (PI3K), and interferon regulatory factor pathways. In the context of the immune system, TRAF6-mediated signals have proven critical for the development, homeostasis, and/or activation of B cells, T cells, and myeloid cells, including macrophages, dendritic cells, and osteoclasts, as well as for organogenesis of thymic and secondary lymphoid tissues. In multiple cellular contexts, TRAF6 function is essential not only for proper activation of the immune system but also for maintaining immune tolerance, and more recent work has begun to identify mechanisms of contextual specificity for TRAF6, involving both regulatory protein interactions, and messenger RNA regulation by microRNAs.

Linear ubiquitination in immunity

Linear ubiquitination is a post‐translational protein modification recently discovered to be crucial for innate and adaptive immune signaling. The function of linear ubiquitin chains is regulated at multiple levels: generation, recognition, and removal. These chains are generated by the linear ubiquitin chain assembly complex (LUBAC), the only known ubiquitin E3 capable of forming the linear ubiquitin linkage de novo. LUBAC is not only relevant for activation of nuclear factor‐κB (NF‐κB) and mitogen‐activated protein kinases (MAPKs) in various signaling pathways, but importantly, it also regulates cell death downstream of immune receptors capable of inducing this response. Recognition of the linear ubiquitin linkage is specifically mediated by certain ubiquitin receptors, which is crucial for translation into the intended signaling outputs. LUBAC deficiency results in attenuated gene activation and increased cell death, causing pathologic conditions in both, mice, and humans. Removal of ubiquitin chains is mediated by deubiquitinases (DUBs). Two of them, OTULIN and CYLD, are constitutively associated with LUBAC. Here, we review the current knowledge on linear ubiquitination in immune signaling pathways and the biochemical mechanisms as to how linear polyubiquitin exerts its functions distinctly from those of other ubiquitin linkage types.

Ubiquitination of the Transcription Factor IRF-3 Activates RIPA, the Apoptotic Pathway that Protects Mice from Viral Pathogenesis

The transcription factor IRF-3 mediates cellular antiviral response by inducing the expression of interferon and other antiviral proteins. In RNA-virus infected cells, IRF-3's transcriptional activation is triggered primarily by RIG-I-like receptors (RLR), which can also activate the RLR-induced IRF-3-mediated pathway of apoptosis (RIPA). Here, we have reported that the pathway of IRF-3 activation in RIPA was independent of and distinct from the known pathway of transcriptional activation of IRF-3. It required linear polyubiquitination of two specific lysine residues of IRF-3 by LUBAC, the linear polyubiquitinating enzyme complex, which bound IRF-3 in signal-dependent fashion. To evaluate the role of RIPA in viral pathogenesis, we engineered a genetically targeted mouse, which expressed a mutant IRF-3 that was RIPA-competent but transcriptionally inert; this single-action IRF-3 could protect mice from lethal viral infection. Our observations indicated that IRF-3-mediated apoptosis of virus-infected cells could be an effective antiviral mechanism, without expression of the interferon-stimulated genes.

Mitochondria and Antiviral Immunity

Mitochondria are unique dynamic organelles that evolved from free-living bacteria into endosymbionts of mammalian hosts (Sagan 1967; Hatefi 1985). They have a distinct ~16.6 kb closed circular DNA genome coding for 13 polypeptides (Taanman 1999). In addition, a majority of the ~1500 mitochondrial proteins are encoded in the nucleus and transported to the mitochondria (Bonawitz et al. 2006). Mitochondria have two membranes: an outer smooth membrane and a highly folded inner membrane called cristae, which encompasses the matrix that houses the enzymes of the tricarboxylic acid (TCA) cycle and lipid metabolism. The inner mitochondrial membrane houses the protein complexes comprising the electron transport chain (ETC) (Hatefi 1985).

Ubiquitin signaling in immune responses

Ubiquitination has emerged as a crucial mechanism that regulates signal transduction in diverse biological processes, including different aspects of immune functions. Ubiquitination regulates pattern-recognition receptor signaling that mediates both innate immune responses and dendritic cell maturation required for initiation of adaptive immune responses. Ubiquitination also regulates the development, activation, and differentiation of T cells, thereby maintaining efficient adaptive immune responses to pathogens and immunological tolerance to self-tissues. Like phosphorylation, ubiquitination is a reversible reaction tightly controlled by the opposing actions of ubiquitin ligases and deubiquitinases. Deregulated ubiquitination events are associated with immunological disorders, including autoimmune and inflammatory diseases.

CARMA3 Is a Host Factor Regulating the Balance of Inflammatory and Antiviral Responses against Viral Infection

Host response to RNA virus infection is sensed by RNA sensors such as RIG-I, which induces MAVS-mediated NF-κB and IRF3 activation to promote inflammatory and antiviral responses, respectively. Here, we have found that CARMA3, a scaffold protein previously shown to mediate NF-κB activation induced by GPCR and EGFR, positively regulates MAVS-induced NF-κB activation. However, our data suggest that CARMA3 sequesters MAVS from forming high-molecular-weight aggregates, thereby suppressing TBK1/IRF3 activation. Interestingly, following NF-κB activation upon virus infection, CARMA3 is targeted for proteasome-dependent degradation, which releases MAVS to activate IRF3. When challenged with vesicular stomatitis virus or influenza A virus, CARMA3-deficient mice showed reduced disease symptoms compared to those of wild-type mice as a result of less inflammation and a stronger ability to clear infected virus. Altogether, our results reveal the role of CARMA3 in regulating the balance of host antiviral and pro-inflammatory responses against RNA virus infection.

Regulation of antiviral innate immune signaling by stress-induced RNA granules

Activation of antiviral innate immunity is triggered by cellular pattern recognition receptors. Retinoic acid inducible gene-I (RIG-I)-like receptors (RLRs) detect viral non-self RNA in cytoplasm of virus-infected cells and play a critical role in the clearance of the invaded viruses through production of antiviral cytokines. Among the three known RLRs, RIG-I and melanoma differentiation-associated gene 5 recognize distinct non-self signatures of viral RNA and activate antiviral signaling. Recent reports have clearly described the molecular machinery underlying the activation of RLRs and interactions with the downstream adaptor, mitochondrial antiviral signaling protein (MAVS). RLRs and MAVS are thought to form large multimeric filaments around cytoplasmic organelles depending on the presence of Lys63-linked ubiquitin chains. Furthermore, RLRs have been shown to localize to stress-induced ribonucleoprotein aggregate known as stress granules and utilize them as a platform for recognition/activation of signaling. In this review, we will focus on the current understanding of RLR-mediated signal activation and the interactions with stress-induced RNA granules.

Filament assemblies in foreign nucleic acid sensors

Helical filamentous assembly is ubiquitous in biology, but was only recently realized to be broadly employed in the innate immune system of vertebrates. Accumulating evidence suggests that the filamentous assemblies and helical oligomerization play important roles in detection of foreign nucleic acids and activation of the signaling pathways to produce antiviral and inflammatory mediators. In this review, we focus on the helical assemblies observed in the signaling pathways of RIG-I-like receptors (RLRs) and AIM2-like receptors (ALRs). We describe ligand-dependent oligomerization of receptor, receptor-dependent oligomerization of signaling adaptor molecules, and their functional implications and regulations.

Molecular characterization and expression analysis of mitochondrial antiviral signaling protein gene in sea perch, Lateolabrax japonicus

The mitochondrial antiviral signaling protein (MAVS) is vital for host defenses against viral infection by inducing expression of type I interferon. Here, the MAVS of sea perch (Lateolabrax japonicus) (LjMAVS) was cloned and analyzed. The complete cDNA sequence of LjMAVS was 3207 bp and encoded a polypeptide of 601 amino acids. LjMAVS contains an N-terminal CARD-like domain, a central proline-rich domain and a C-terminal transmembrane domain. Phylogenetic analysis indicated that LjMAVS exhibited the closest relationship to O. fasciatus MAVS. LjMAVS was ubiquitously expressed in all tested tissues of healthy fish. The expression of LjMAVS was significantly increased post nervous necrosis virus (NNV) infection in vivo in all the selected tissues. Furthermore, time course analysis showed that LjMAVS transcripts significantly increased in the brain, spleen and kidney tissues after NNV infection. LjMAVS mRNA expression was significantly up-regulated in vitro after poly I:C stimulation. The viral gene transcription of RGNNV was significantly decreased in LjMAVS over-expressing LJB cells. These findings provide useful information for further elucidating the function ofLjMAVS in antiviral innate immune against NNV in sea perch.

Molecular characterization and functional analysis of tumor necrosis factor receptor-associated factor 2 in the Pacific oyster

Tumor necrosis factor receptor (TNFR)-associated factors (TRAFs) are a family of crucial adaptors, playing vital roles in mediating signal transduction in immune signaling pathways, including RIG-I-like receptor (RLR) signaling pathway. In the present study, a new TRAF family member (CgTRAF2) was identified in the Pacific oyster, Crassostrea gigas. Comparison and phylogenetic analysis revealed that CgTRAF2 could be a new member of the invertebrate TRAF2 family. Quantitative real-time PCR revealed that CgTRAF2 mRNA was highly expressed in the digestive gland, gills, and hemocytes, and it was significantly up-regulated after Vibrio alginolyticus and ostreid herpesvirus 1 (OsHV-1) challenge. The CgTRAF2 mRNA expression profile in different developmental stages of oyster larvae suggested that CgTRAF2 could function in early larval development. CgTRAF2 mRNA expression pattern, after the silence of CgMAVS (Mitochondrial Antiviral Signaling) -like, indicated that CgTRAF2 might function downstream of CgMAVS-like. Moreover, the subcellular localization analysis revealed that CgTRAF2 was localized in cytoplasm, and it may play predominately important roles in signal transduction. Collectively, these results demonstrated that CgTRAF2 might play important roles in the innate immunity and larval development of the Pacific oyster.

WDR82 Negatively Regulates Cellular Antiviral Response by Mediating TRAF3 Polyubiquitination in Multiple Cell Lines

Upon virus infection, retinoic acid-inducible gene I-like receptors in host cells recognize viral RNA and activate type I IFN expression. Previously, we identified WD repeat domain (WDR) 5 as one positive regulator for pathway activation. In this study, we report that WDR82, a homolog protein of WDR5, acts opposite to WDR5 and inhibits the activation of the retinoic acid-inducible gene I signaling pathway. WDR82 overexpression inhibits virus-triggered pathway activation, whereas its knockdown enhances induced IFN-β expression. WDR82 is localized on the mitochondria, and its first N-terminal WD40 domain is critical for localization. WDR82 interacts with TNFR-associated factor (TRAF) 3, and its overexpression promotes K48-linked, but not K63-linked, polyubiquitination on TRAF3. Furthermore, WDR82 knockdown inhibits viral replication in the cell, whereas its overexpression has the opposite effect. Interestingly, WDR82 regulates Sendai virus-induced IFNB1 expression in a cell type-specific manner. Taken together, our findings demonstrate that WDR82 is a negative regulator of virus-triggered type I IFNs pathway through mediating TRAF3 polyubiquitination status and stability on mitochondria.

Global quantitative proteomic analysis of human glioma cells profiled host protein expression in response to enterovirus type 71 infection

Enterovirus 71 (EV71) is one of the leading causes of hand, foot and mouth disease with neurological complications in some cases. To study the pathogenesis of EV71 infection, large-scale analyses of EV71 infected cells have been performed. However, most of these studies employed rhabdomyosarcoma (RD) cells or used transcriptomic strategy. Here, we performed SILAC-based quantitative proteomic analysis of EV71-infected U251 cells, a human glioma cell line. A total of 3125 host proteins were quantified, in which 451 were differentially regulated as a result of EV71 infection at 8 or 20 hpi or both. Gene Ontology analysis indicates the regulated proteins were enriched in "metabolic process", "biological regulation" and "cellular process", implying that these biological processes were affected by EV71 infection. Furthermore, functional study indicated that TRAF2 and TRAF6 among the up-regulated proteins could inhibit the replication of EV71 at the early phase post infection, and the anti-EV71 function of both proteins was independent of interferon β. Our study not only provided an overview of cellular response to EV71 infection in a human glioma cell line, but also found that TRAF2 and TRAF6 might be potential targets to inhibit the replication of EV71. All MS data have been deposited in the ProteomeXchange with identifier PXD002454 (http://proteomecentral.proteomexchange.org/dataset/PXD002454).

Ring finger protein 166 potentiates RNA virus-induced interferon-β production via enhancing the ubiquitination of TRAF3 and TRAF6

Host cells orchestrate the production of IFN-β upon detecting invading viral pathogens. Here, we report that Ring finger protein 166 (RNF166) potentiates RNA virus-triggered IFN-β production. Overexpression of RNF166 rather than its homologous proteins RNF114, RNF125, and RNF138, enhanced Sendai virus (SeV)-induced activation of the IFN-β promoter. Knockdown of endogenous RNF166, but not other RNFs, inhibited the IFN-β production induced by SeV and encephalomyocarditis virus. RNF166 interacted with TRAF3 and TRAF6. SeV-induced ubiquitination of TRAF3 and TRAF6 was suppressed when endogenous RNF166 rather than RNF114/138 was knocked down. These findings suggest that RNF166 positively regulates RNA virus-triggered IFN-β production by enhancing the ubiquitination of TRAF3 and TRAF6.

Structural Insights into mitochondrial antiviral signaling protein (MAVS)-tumor necrosis factor receptor-associated factor 6 (TRAF6) signaling

In response to viral infection, cytosolic retinoic acid-inducible gene I-like receptors sense viral RNA and promote oligomerization of mitochondrial antiviral signaling protein (MAVS), which then recruits tumor necrosis factor receptor-associated factor (TRAF) family proteins, including TRAF6, to activate an antiviral response. Currently, the interaction between MAVS and TRAF6 is only partially understood, and atomic details are lacking. Here, we demonstrated that MAVS directly interacts with TRAF6 through its potential TRAF6-binding motif 2 (T6BM2; amino acids 455-460). Further, we solved the crystal structure of MAVS T6BM2 in complex with the TRAF6 TRAF_C domain at 2.95 Å resolution. T6BM2 of MAVS binds to the canonical adaptor-binding groove of the TRAF_C domain. Structure-directed mutational analyses in vitro and in cells revealed that MAVS binding to TRAF6 via T6BM2 instead of T6BM1 is essential but not sufficient for an optimal antiviral response. Particularly, a MAVS mutant Y460E retained its TRAF6-binding ability as predicted but showed significantly impaired signaling activity, highlighting the functional importance of this tyrosine. Moreover, these observations were further confirmed in MAVS(-/-) mouse embryonic fibroblast cells. Collectively, our work provides a structural basis for understanding the MAVS-TRAF6 antiviral response.

IRTKS negatively regulates antiviral immunity through PCBP2 sumoylation-mediated MAVS degradation

RNA virus infection is recognized by the RIG-I family of receptors that activate the mitochondrial adaptor MAVS, leading to the clearance of viruses. Antiviral signalling activation requires strict modulation to avoid damage to the host from exacerbated inflammation. Insulin receptor tyrosine kinase substrate (IRTKS) participates in actin bundling and insulin signalling and its deficiency causes insulin resistance. However, whether IRTKS is involved in the regulation of innate immunity remains elusive. Here we show that IRTKS deficiency causes enhanced innate immune responses against RNA viruses. IRTKS-mediated suppression of antiviral responses depends on the RIG-I-MAVS signalling pathway. IRTKS recruits the E2 ligase Ubc9 to sumoylate PCBP2 in the nucleus, which causes its cytoplasmic translocation during viral infection. The sumoylated PCBP2 associates with MAVS to initiate its degradation, leading to downregulation of antiviral responses. Thus, IRTKS functions as a negative modulator of excessive inflammation.

Modulation of Innate Immune Signalling by Lipid-Mediated MAVS Transmembrane Domain Oligomerization

RIG-I-like receptors detect viral RNA in infected cells and promote oligomerization of the outer mitochondrial membrane protein MAVS to induce innate immunity to viral infection through type I interferon production. Mitochondrial reactive oxygen species (mROS) have been shown to enhance anti-viral MAVS signalling, but the mechanisms have remained obscure. Using a biochemical oligomerization-reporter fused to the transmembrane domain of MAVS, we found that mROS inducers promoted lipid-dependent MAVS transmembrane domain oligomerization in the plane of the outer mitochondrial membrane. These events were mirrored by Sendai virus infection, which similarly induced lipid peroxidation and promoted lipid-dependent MAVS transmembrane domain oligomerization. Our observations point to a role for mROS-induced changes in lipid bilayer properties in modulating antiviral innate signalling by favouring the oligomerization of MAVS transmembrane domain in the outer-mitochondrial membrane.

The mitochondrial ubiquitin ligase MARCH5 resolves MAVS aggregates during antiviral signalling

Mitochondria serve as platforms for innate immunity. The mitochondrial antiviral signalling (MAVS) protein forms aggregates that elicit robust type-I interferon induction on viral infection, but persistent MAVS signalling leads to host immunopathology; it remains unknown how these signalling aggregates are resolved. Here we identify the mitochondria-resident E3 ligase, MARCH5, as a negative regulator of MAVS aggregates. March5^+/− mice and MARCH5-deficient immune cells exhibit low viral replication and elevated type-I interferon responses to RNA viruses. MARCH5 binds MAVS only during viral stimulation when MAVS forms aggregates, and these interactions require the RING domain of MARCH5 and the CARD domain of MAVS. MARCH5, but not its RING mutant (MARCH5^H43W), reduces the level of MAVS aggregates. MARCH5 transfers ubiquitin to Lys7 and Lys500 of MAVS and promotes its proteasome-mediated degradation. Our results indicate that MARCH5 modulates MAVS-mediated antiviral signalling, preventing excessive immune reactions.

RNA viral infections trigger an immune response mediated by the formation of aggregates of the MAVS protein. Here the authors show that the mitochondrial protein MARCH5 modulates this response by transferring ubiquitin to MAVS aggregates, thus promoting their proteasomal degradation.

LGP2 synergy with MDA5 in RLR-mediated RNA recognition and antiviral signaling

Mammalian cells have the ability to recognize virus infection and mount a powerful antiviral response. Pattern recognition receptor proteins detect molecular signatures of virus infection and activate antiviral signaling. The RIG-I-like receptor (RLR) proteins are expressed in the cytoplasm of nearly all cells and specifically recognize virus-derived RNA species as a molecular feature discriminating the pathogen from the host. The RLR family is composed of three homologous proteins, RIG-I, MDA5, and LGP2. All RLRs have the ability to detect virus-derived dsRNA and hydrolyze ATP, but display individual differences in enzymatic activity, intrinsic ability to recognize RNA, and mechanisms of activation. Emerging evidence suggests that MDA5 and RIG-I utilize distinct mechanisms to form oligomeric complexes along dsRNA. Aligning of their signaling domains creates a platform capable of propagating and amplifying antiviral signaling responses. LGP2 with intact ATP hydrolysis is critical for the MDA5-mediated antiviral response, but LGP2 lacks the domains essential for activation of antiviral signaling, leaving the role of LGP2 in antiviral signaling unclear. Recent studies revealed a mechanistic basis of synergy between LGP2 and MDA5 leading to enhanced antiviral signaling. This review briefly summarizes the RLR system, and focuses on the relationship between LGP2 and MDA5, describing in detail how these two proteins work together to detect foreign RNA and generate a fully functional antiviral response.

An Internally Translated MAVS Variant Exposes Its Amino-terminal TRAF-Binding Motifs to Deregulate Interferon Induction

Activation of pattern recognition receptors and proper regulation of downstream signaling are crucial for host innate immune response. Upon infection, the NF-κB and interferon regulatory factors (IRF) are often simultaneously activated to defeat invading pathogens. Mechanisms concerning differential activation of NF-κB and IRF are not well understood. Here we report that a MAVS variant inhibits interferon (IFN) induction, while enabling NF-κB activation. Employing herpesviral proteins that selectively activate NF-κB signaling, we discovered that a MAVS variant of ~50 kDa, thus designated MAVS50, was produced from internal translation initiation. MAVS50 preferentially interacts with TRAF2 and TRAF6, and activates NF-κB. By contrast, MAVS50 inhibits the IRF activation and suppresses IFN induction. Biochemical analysis showed that MAVS50, exposing a degenerate TRAF-binding motif within its N-terminus, effectively competed with full-length MAVS for recruiting TRAF2 and TRAF6. Ablation of the TRAF-binding motif of MAVS50 impaired its inhibitory effect on IRF activation and IFN induction. These results collectively identify a new means by which signaling events is differentially regulated via exposing key internally embedded interaction motifs, implying a more ubiquitous regulatory role of truncated proteins arose from internal translation and other related mechanisms.

Host innate immune signaling plays critical roles in defeating pathogen infection. In response to viral infection, cellular signaling events cumulate in the activation of NF-κB and interferon regulatory factors. How these two signaling ramifications are differentially regulated remains an open question. Here we report an internally translated MAVS variant deregulates IRF activation via exposing N-terminal TRAF-binding motifs. As such, the short form of MAVS efficiently competes for binding to TRAF2 and TRAF6 against full-length MAVS, thereby sequestering key adaptors from the signaling cascades mediated by full-length MAVS. Our study uncovers a delicate regulatory mechanism of truncated proteins bearing key protein-interacting motifs that is enabled by internal translation initiation and potentially other relevant means.

Activation and regulation of DNA-driven immune responses

The innate immune system provides early defense against infections and also plays a key role in monitoring alterations of homeostasis in the body. DNA is highly immunostimulatory, and recent advances in this field have led to the identification of the innate immune sensors responsible for the recognition of DNA as well as the downstream pathways that are activated. Moreover, information on how cells regulate DNA-driven immune responses to avoid excessive inflammation is now emerging. Finally, several reports have demonstrated how defects in DNA sensing, signaling, and regulation are associated with susceptibility to infections or inflammatory diseases in humans and model organisms. In this review, the current literature on DNA-stimulated innate immune activation is discussed, and important new questions facing this field are proposed.

An autoinhibitory mechanism modulates MAVS activity in antiviral innate immune response

In response to virus infection, RIG-I senses viral RNA and activates the adaptor protein MAVS, which then forms prion-like filaments and stimulates a specific signalling pathway leading to type I interferon production to restrict virus proliferation. However, the mechanisms by which MAVS activity is regulated remain elusive. Here we identify distinct regions of MAVS responsible for activation of transcription factors interferon regulatory factor 3 (IRF3) and nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB). These IRF3- and NF-κB-stimulating regions recruit preferential TNF receptor-associated factors (TRAFs) for downstream signalling. Strikingly, these regions' activities are inhibited by their respective adjacent regions in quiescent MAVS. Our data thus show that an autoinhibitory mechanism modulates MAVS activity in unstimulated cells and, on viral infection, individual regions of MAVS are released following MAVS filament formation to activate antiviral signalling cascades.

Targeting signaling factors for degradation, an emerging mechanism for TRAF functions

Tumor necrosis factor receptor (TNFR)-associated factors (TRAFs) form a family of proteins that are best known as signaling adapters of TNFRs. However, emerging evidence suggests that TRAF proteins, particularly TRAF2 and TRAF3, also regulate signal transduction by controlling the fate of intracellular signaling factors. A well-recognized function of TRAF2 and TRAF3 in this aspect is to mediate ubiquitin-dependent degradation of nuclear factor-κB (NF-κB)-inducing kinase (NIK), an action required for the control of NIK-regulated non-canonical NF-κB signaling pathway. TRAF2 and TRAF3 form a complex with the E3 ubiquitin ligase cIAP (cIAP1 or cIAP2), in which TRAF3 serves as the NIK-binding adapter. Recent evidence suggests that the cIAP-TRAF2-TRAF3 E3 complex also targets additional signaling factors for ubiquitin-dependent degradation, thereby regulating important aspects of immune and inflammatory responses. This review provides both historical aspects and new insights into the signaling functions of this ubiquitination system.

How RIG-I like receptors activate MAVS

RIG-I and MDA5 are well-conserved cytoplasmic pattern recognition receptors that detect viral RNAs during infection and activate the type I interferon (IFN)-mediated antiviral immune response. While much is known about how these receptors recognize viral RNAs, how they interact with their common signaling adaptor molecule MAVS and activate the downstream signaling pathway had been less clear. Previous studies have shown that the signaling domains (tandem CARDs or 2CARDs) of RIG-I and MDA5 must form homo-oligomers in order to interact with MAVS, and that their interactions lead to filament formation of MAVS, a pre-requisite for downstream signal activation. More recent data suggest that multiple mechanisms synergistically promote tetramer formation of RIG-I 2CARD, and that this tetramer resembles a lock-washer, which serves as a helical template to nucleate the MAVS filament. We here summarize these recent findings and discuss the current understanding of the signal activation mechanisms of RIG-I and MDA5.

Perception of self: distinguishing autoimmunity from autoinflammation

Rheumatic diseases can be divided in two groups, autoinflammatory and autoimmune disorders. The clinical presentation of both types of diseases overlap, but the pathological pathways underlying rheumatic autoinflammation and autoimmunity are distinct and are the subject of ongoing research. There are a number of ways in which these groups of diseases differ in terms of disease mechanisms and therapeutic responses. First, autoinflammatory diseases are driven by endogenous danger signals, metabolic mediators and cytokines, whereas autoimmunity involves the activation of T and B cells, the latter requiring V-(D)-J recombination of receptor-chain gene segments for maturation. Second, the efficacy of biologic agents directed against proinflammatory cytokines (for example IL-1β and TNF) also highlights differences between autoinflammatory and autoimmune processes. Finally, whereas autoinflammatory diseases are mostly driven by inflammasome-induced IL-1β and IL-18 production, autoimmune diseases are associated with type I interferon (IFN) signatures in blood. In this Review, we provide an overview of the monocyte intracellular pathways that drive autoinflammation and autoimmunity. We convey recent findings on how the type I IFN pathway can modulate IL-1β signalling (and vice versa), and discuss why IL-1β-mediated autoinflammatory diseases do not perpetuate into autoimmunity. The origins of intracellular autoantigens in autoimmune disorders are also discussed. Finally, we suggest how new mechanistic knowledge of autoinflammatory and autoimmune diseases might help improve treatment strategies to benefit patient care.

Mitochondrial antiviral-signalling protein plays an essential role in host immunity against human metapneumovirus

Human metapneumovirus (hMPV) is a common cause of respiratory tract infection in the paediatrics population. Recently, we and others have shown that retinoic acid-inducible gene 1 (RIG-I)-like receptors (RLRs) are essential for hMPV-induced cellular antiviral signalling. However, the contribution of those receptors to host immunity against pulmonary hMPV infection is largely unexplored. In this study, mice deficient in mitochondrial antiviral-signalling protein (MAVS), an adaptor of RLRs, were used to investigate the role(s) of these receptors in pulmonary immune responses to hMPV infection. MAVS deletion significantly impaired the induction of antiviral and pro-inflammatory cytokines and the recruitment of immune cells to the bronchoalveolar lavage fluid by hMPV. Compared with WT mice, mice lacking MAVS demonstrated decreased abilities to activate pulmonary dendritic cells (DCs) and abnormal primary T-cell responses to hMPV infection. In addition, mice deficient in MAVS had a higher peak of viral load at day 5 post-infection (p.i.) than WT mice, but were able to clear hMPV by day 7 p.i. similarly to WT mice. Taken together, our data indicate a role of MAVS-mediated pathways in the pulmonary immune responses to hMPV infection and the early control of hMPV replication.

Aryl Hydrocarbon Receptor Interacting Protein Targets IRF7 to Suppress Antiviral Signaling and the Induction of Type I Interferon

The transcription factor IRF7 (interferon regulatory factor 7) is a key regulator of type I interferon and plays essential roles in restricting virus infection and spread. IRF7 activation is tightly regulated to prevent excessive inflammation and autoimmunity; however, how IRF7 is suppressed by negative regulators remains poorly understood. Here, we have identified AIP (aryl hydrocarbon receptor interacting protein) as a new binding partner of IRF7. The interaction between AIP and IRF7 is enhanced upon virus infection, and AIP potently inhibits IRF7-induced type I IFN (IFNα/β) production. Overexpression of AIP blocks virus-induced activation of IFN, whereas knockdown of AIP by siRNA potentiates virally activated IFN production. Consistently, AIP-deficient murine embryonic fibroblasts are highly resistant to virus infection because of increased production of IFNα/β. AIP inhibits IRF7 function by antagonizing the nuclear localization of IRF7. Together, our study identifies AIP as a novel inhibitor of IRF7 and a negative regulator of innate antiviral signaling.

MAVS Promotes Inflammasome Activation by Targeting ASC for K63-Linked Ubiquitination via the E3 Ligase TRAF3

Stringent control of inflammasome signaling pathway is important for maintaining immunological balance, yet the molecular mechanisms responsible for its tight regulation are still poorly understood. In this study, we found that the signaling pathway dependent on mitochondrial antiviral signaling protein (MAVS) was required for the optimal activation of apoptosis-associated specklike protein (ASC)-dependent inflammasome. In particular, TNFR-associated factor 3 was found to be a direct E3 ligase for ASC. Ubiquitination of ASC at Lys(174) was critical for speck formation and inflammasome activation. Deficiency in MAVS or TNFR-associated factor 3 impaired ASC ubiquitination and cytosolic aggregates formation, resulting in reduced inflammasome response upon RNA virus infection. This study has identified a previously unrecognized role of MAVS in the regulation of inflammasome signaling and provided molecular insight into the mechanisms by which ubiquitination of ASC controls inflammasome activity through the formation of ASC specks.

Sensing and responding to cytosolic viruses invasions: An orchestra of kaleidoscopic ubiquitinations

Ubiquitin is a versatile molecular signature that modulates diverse cellular processes via proteasome-dependent and proteasome-independent mechanisms. The covalent and/or non-covalent binding of mono-ubiquitin and/or poly-ubiquitin chains to a target protein broadens the dynamic and functional spectra for signal integration. Different linkages of poly-ubiquitin chains determine specific physiological or pathological functions of target proteins. Accumulating evidences has revealed the essential roles of ubiquitination in orchestrating the host defenses against cytosolic RNA or DNA from viral infections. In this review, we summarize the current progress regarding the understanding of ubiquitin-mediated regulation of the RIG-I and STING antiviral signaling pathways and discuss certain critical issues that remain to be resolved in future studies.

Regulation of cellular innate antiviral signaling by ubiquitin modification

Host pattern-recognition receptors (PRRs) recognize pathogen-associated molecular patterns generated by invading viruses and initiate a series of signaling cascades that lead to the activation of interferon-regulatory factor 3 (IRF3) and nuclear factor-κB (NF-κB) and subsequent induction of type I interferons (IFNs). Posttranslational modification of proteins by ubiquitin plays an essential role in mediating or regulating the virus-triggered PRRs-mediated signaling. Deubiquitination is the reversible process of ubiquitination and its role in regulating PRRs-mediated signaling has recently been explored. In this review, we first summarize the ubiquitination events in PRRs-mediated signaling that is triggered by viral nucleic acid and then focus on host and viral deubiquitinating enzymes-mediated regulation of virus-triggered signaling that modulates the activation of IRF3 and NF-κB and subsequent induction of type I IFNs.

Phosphorylation of innate immune adaptor proteins MAVS, STING, and TRIF induces IRF3 activation

During virus infection, the adaptor proteins MAVS and STING transduce signals from the cytosolic nucleic acid sensors RIG-I and cGAS, respectively, to induce type I interferons (IFNs) and other antiviral molecules. Here we show that MAVS and STING harbor two conserved serine and threonine clusters that are phosphorylated by the kinases IKK and/or TBK1 in response to stimulation. Phosphorylated MAVS and STING then bind to a positively charged surface of interferon regulatory factor 3 (IRF3) and thereby recruit IRF3 for its phosphorylation and activation by TBK1. We further show that TRIF, an adaptor protein in Toll-like receptor signaling, activates IRF3 through a similar phosphorylation-dependent mechanism. These results reveal that phosphorylation of innate adaptor proteins is an essential and conserved mechanism that selectively recruits IRF3 to activate the type I IFN pathway.