Retinoic Acid-inducible Gene I Ubiquitination Research Articles

Housekeeping U1 snRNA facilitates antiviral innate immunity by promoting TRIM25-mediated RIG-I activation

U1 small nuclear RNA (snRNA) is an abundant and evolutionarily conserved 164-nucleotide RNA species that functions in pre-mRNA splicing, and it is considered to be a housekeeping non-coding RNA. However, the role of U1 snRNA in regulating host antiviral immunity remains largely unexplored. Here, we find that RNVU1-18, a U1 pseudogene, is significantly upregulated in the host infected with RNA viruses, including influenza and respiratory syncytial virus. Overexpression of U1 snRNA protects cells against RNA viruses, while knockdown of U1 snRNA leads to more viral burden invitro and invivo. Knockout of RNVU1-18 is sufficient to impair the type I interferon-dependent antiviral innate immunity. U1 snRNA is required to fully activate the retinoic acid-inducible gene I (RIG-I)-dependent antiviral signaling, since it interacts with tripartite motif 25 (TRIM25) and enhances the RIG-I-TRIM25 interaction to trigger K63-linked ubiquitination of RIG-I. Our study reveals the important role of housekeeping U1 snRNA in regulating host antiviral innate immunity and restricting RNA virus infection.

Hazara Orthonairovirus Nucleoprotein Antagonizes Type I Interferon Production by Inhibition of RIG-I Ubiquitination.

Viruses have evolved various strategies to evade the host innate immune system. The relationship between nairoviruses and the interferon (IFN) system is poorly understood. We investigated whether and how nairoviruses antagonize host innate immunity using Hazara orthonairovirus (HAZV) as a surrogate model for Crimean-Congo hemorrhagic fever virus. HAZV nucleoprotein (N) was found to interact with the tripartite motif-containing protein 25 (TRIM25). The N-terminal region of N protein and the C-terminal region of TRIM25 are important for their interaction. Overexpression of N protein results in weakened interaction of TRIM25 with retinoic acid-inducible gene I (RIG-I). Furthermore, K63-linked polyubiquitination of RIG-I is inhibited in the presence of N protein. Our data collectively suggest that HAZV N protein interferes with the binding of TRIM25 to RIG-I and subsequent K63-linked polyubiquitination of RIG-I, which leads to inhibition of type I IFN production.

TRIM22 orchestrates the proliferation of GBMs and the benefits of TMZ by coordinating the modification and degradation of RIG-I

Tripartite motif 22 (TRIM22) is an agonist of nuclear factor κB (NF-κB) that plays an important role in the proliferation and drug sensitivity of glioblastoma (GBM). However, the molecular mechanism underlying the protein network between TRIM22 and nuclear factor κB (NF-κB) in GBM remains unclear. Here, we found that knockout of TRIM22 effectively inhibited tumor proliferation and increased the sensitivity of GBM cells to temozolomide (TMZ) in vivo and in vitro. Moreover, TRIM22 forms a complex with cytosolic purine 5-nucleotidase (NT5C2) in GBM and regulates the ubiquitination of retinoic acid-inducible gene-I (RIG-I). TRIM22 promotes the K63-linked ubiquitination of RIG-I, while NT5C2 is responsible for K48-linked ubiquitination. This regulation directly affects the RIG-I/NF-κB/cell division cycle and apoptosis regulator protein 1 (CCAR1) signaling axis. Ubiquitin modification inhibitor of RIG-I restores the inhibition of tumor growth induced by TRIM22 knockout. The follow-up results showed that compared with patients with high TRIM22 expression, patients with low TRIM22 expression had a longer survival time and were more sensitive to treatment with TMZ. Our results revealed that the TRIM22-NT5C2 complex orchestrates the proliferation of GBM and benefits of TMZ through post-translational modification of RIG-I and the regulation of the RIG-I/NF-κB/CCAR1 pathway and is a promising target for single-pathway multi-target therapy.

Human metapneumovirus M2-2 protein inhibits RIG-I signaling by preventing TRIM25-mediated RIG-I ubiquitination.

Retinoic acid-inducible gene I (RIG-I) is a receptor that senses viral RNA and interacts with mitochondrial antiviral signaling (MAVS) protein, leading to the production of type I interferons and inflammatory cytokines to establish an antiviral state. This signaling axis is initiated by the K63-linked RIG-I ubiquitination, mediated by E3 ubiquitin ligases such as TRIM25. However, many viruses, including several members of the family Paramyxoviridae and human respiratory syncytial virus (HRSV), a member of the family Pneumoviridae, escape the immune system by targeting RIG-I/TRIM25 signaling. In this study, we screened human metapneumovirus (HMPV) open reading frames (ORFs) for their ability to block RIG-I signaling reconstituted in HEK293T cells by transfection with TRIM25 and RIG-I CARD (an N-terminal CARD domain that is constitutively active in RIG-I signaling). HMPV M2-2 was the most potent inhibitor of RIG-I/TRIM25-mediated interferon (IFN)-β activation. M2-2 silencing induced the activation of transcription factors (IRF and NF-kB) downstream of RIG-I signaling in A549 cells. Moreover, M2-2 inhibited RIG-I ubiquitination and CARD-dependent interactions with MAVS. Immunoprecipitation revealed that M2-2 forms a stable complex with RIG-I CARD/TRIM25 via direct interaction with the SPRY domain of TRIM25. Similarly, HRSV NS1 also formed a stable complex with RIG-I CARD/TRIM25 and inhibited RIG-I ubiquitination. Notably, the inhibitory actions of HMPV M2-2 and HRSV NS1 are similar to those of V proteins of several members of the Paramyxoviridae family. In this study, we have identified a novel mechanism of immune escape by HMPV, similar to that of Pneumoviridae and Paramyxoviridae family members.

LncNSPL facilitates influenza A viral immune escape by restricting TRIM25-mediated K63-linked RIG-I ubiquitination

SummaryLong noncoding RNAs (lncRNAs) participate in host antiviral responses; however, how viruses exploit host lncRNAs for immune evasion remains largely unexplored. Functional screening of differentially expressed lncRNA profile in patients infected with influenza A virus (IAV) revealed that lncNSPL (Gene Symbol: LOC105370355) was highly expressed in monocytes. Deregulated lncNSPL expression in infected monocytes significantly increased type I interferon (IFN-I) production and inhibited IAV replication. Moreover, lncNSPL overexpression in mice increased the susceptibility to IAV infection and impaired IFN-I production. LncNSPL directly bound to retinoic acid-inducible gene I (RIG-I) and blocked the interaction between RIG-I and E3 ligase tripartite interaction motif 25 (TRIM25), reducing TRIM25-mediated lysine 63 (K63)-linked RIG-I ubiquitination and limiting the downstream production of antiviral mediators during the late stage of IAV infection. Our findings provide mechanistic insights into the means by which lncNSPL promotes IAV replication and immune escape via restricting the TRIM25-mediated RIG-I K63-linked ubiquitination. Thus, lncNSPL may represent a promising pharmaceutical target for anti-IAV therapy.

Conserved function of crucian carp cGAS in the MITA-mediated interferon signaling

Mammalian cyclic GMP-AMP synthase (cGAS) is pivotal for cytosolic DNA-triggered interferon (IFN) response. However, the function of cGAS in fish IFN response remains unclear. Our recent study has reported that cGAS from crucian and grass carps downregulates the IFN response by attenuating the K63-linked ubiquitination of retinoic acid-inducible gene-I (RIG-I) and its interaction with mitochondrial antiviral signaling protein (MAVS). Here, the function of crucian carp cGAS was further investigated. We found that crucian carp cGAS directly binds to poly deoxyadenylic-deoxythymidylic acid (poly (dA:dT)) and exhibits mediator of IFN regulatory factor 3 (IRF3) activation (MITA)-dependent activation of the IFN response, indicating a conserved function of crucian carp cGAS in the MITA-mediated IFN signaling. However, crucian carp cGAS could suppress the IFN activation stimulated by polyinosinic: polycytidylic acid (poly (I:C)) in time- and dose-dependent manners. These data collectively suggest complicated functions of crucian carp cGAS in the IFN antiviral response.

Grass Carp Reovirus VP56 Allies VP4, Recruits, Blocks, and Degrades RIG-I to More Effectively Attenuate IFN Responses and Facilitate Viral Evasion.

ABSTRACTGrass carp reovirus (GCRV), the most virulent aquareovirus, causes epidemic hemorrhagic disease and tremendous economic loss in freshwater aquaculture industry. VP56, a putative fibrin inlaying the outer surface of GCRV-II and GCRV-III, is involved in cell attachment. In the present study, we found that VP56 localizes at the early endosome, lysosome, and endoplasmic reticulum, recruits the cytoplasmic viral RNA sensor retinoic acid-inducible gene I (RIG-I) and binds to it. The interaction between VP56 and RIG-I was detected by endogenous coimmunoprecipitation (co-IP), glutathione S-transferase (GST) pulldown, and subsequent liquid chromatography-tandem mass spectrometry (LC-MS/MS) and was then confirmed by traditional co-IPs and a novel far-red mNeptune-based bimolecular fluorescence complementation system. VP56 binds to the helicase domain of RIG-I. VP56 enhances K48-linked ubiquitination of RIG-I to degrade it by the proteasomal pathway. Thus, VP56 impedes the initial immune function of RIG-I by dual mechanisms (blockade and degradation) and attenuates signaling from RIG-I recognizing viral RNA, subsequently weakening downstream signaling transduction and interferon (IFN) responses. Accordingly, host antiviral effectors are reduced, and cytopathic effects are increased. These findings were corroborated by RNA sequencing (RNA-seq) and VP56 knockdown. Finally, we found that VP56 and the major outer capsid protein VP4 bind together in the cytosol to enhance the degradation of RIG-I and more efficiently facilitate viral replication. Collectively, the results indicated that VP56 allies VP4, recruits, blocks, and degrades RIG-I, thereby attenuating IFNs and antiviral effectors to facilitate viral evasion more effectively. This study reveals a virus attacking target and an escaping strategy from host antiviral immunity for GCRV and will help understand mechanisms of infection of reoviruses.IMPORTANCE Grass carp reovirus (GCRV) fibrin VP56 and major outer capsid protein VP4 inlay and locate on the outer surface of GCRV-II and GCRV-III, which causes tremendous loss in grass carp and black carp industries. Fibrin is involved in cell attachment and plays an important role in reovirus infection. The present study identified the interaction proteins of VP56 and found that VP56 and VP4 bind to the different domains of the viral RNA sensor retinoic acid-inducible gene I (RIG-I) in grass carp to block RIG-I sensing of viral RNA and induce RIG-I degradation by the proteasomal pathway to attenuate signaling transduction, thereby suppressing interferons (IFNs) and antiviral effectors, facilitating viral replication. VP56 and VP4 bind together in the cytosol to more efficiently facilitate viral evasion. This study reveals a virus attacking a target and an escaping strategy from host antiviral immunity for GCRV and will be helpful in understanding the mechanisms of infection of reoviruses.

The E3 Ubiquitin Ligase TRIM25 Inhibits Tembusu Virus Replication in vitro.

Duck Tembusu virus (DTMUV) is a newly emerging pathogenic flavivirus that has caused significant economic losses to the duck industry in China since 2010 due to egg production losses and neurological dysfunction. DTMUV is a public health concern because the infection spreads rapidly among birds. Retinoic acid-inducible gene-I (RIG-I)serves as an innate immune sensor and plays a key role in host antiviral defenses. Tripartite motif-containing protein 25 (TRIM25), an E3 ubiquitin ligase, is pivotal for RIG-I ubiquitination and activation. In addition, TRIM25 acts as an interferon-stimulated gene and mediates the antiviral activity. However, the effect of duck TRIM25 on DTMUV has not been assessed. Herein, we reportthe antiviral function of TRIM25 against DTMUV. First, we constructed the pcDNA3.1-c-myc-duTRIM25 plasmid. TRIM25 has a 2052 bp open reading frame that encodes a predicted 684 amino acid protein consisting of a RING finger domain, a B-box domain, a coiled-coil domain, and a PRY/SPRY domain. The protein sequence identity with chicken, mouse, and human TRIM25 is 69.7, 47.8, and 48.3%, respectively. TRIM25 was upregulated in BHK-21 cells, duck embryo fibroblasts, and 293T cellsupon DTMUV infection. The expression of viral RNA and proteins was significantly lower in cells over expressing TRIM25 than in control cells. Furthermore, siRNA-mediated silencing of TRIM25 increased the production of viral progeny. These results help elucidate the molecular mechanisms underlying the host response to DTMUV infection and suggest potential control measures for DTMUV outbreaks.

Reciprocal regulation of RIG-I and XRCC4 connects DNA repair with RIG-I immune signaling

The RNA-sensing pathway contributes to type I interferon (IFN) production induced by DNA damaging agents. However, the potential involvement of RNA sensors in DNA repair is unknown. Here, we found that retinoic acid-inducible gene I (RIG-I), a key cytosolic RNA sensor that recognizes RNA virus and initiates the MAVS-IRF3-type I IFN signaling cascade, is recruited to double-stranded breaks (DSBs) and suppresses non-homologous end joining (NHEJ). Mechanistically, RIG-I interacts with XRCC4, and the RIG-I/XRCC4 interaction impedes the formation of XRCC4/LIG4/XLF complex at DSBs. High expression of RIG-I compromises DNA repair and sensitizes cancer cells to irradiation treatment. In contrast, depletion of RIG-I renders cells resistant to irradiation in vitro and in vivo. In addition, this mechanism suggests a protective role of RIG-I in hindering retrovirus integration into the host genome by suppressing the NHEJ pathway. Reciprocally, XRCC4, while suppressed for its DNA repair function, has a critical role in RIG-I immune signaling through RIG-I interaction. XRCC4 promotes RIG-I signaling by enhancing oligomerization and ubiquitination of RIG-I, thereby suppressing RNA virus replication in host cells. In vivo, silencing XRCC4 in mouse lung promotes influenza virus replication in mice and these mice display faster body weight loss, poorer survival, and a greater degree of lung injury caused by influenza virus infection. This reciprocal regulation of RIG-I and XRCC4 reveals a new function of RIG-I in suppressing DNA repair and virus integration into the host genome, and meanwhile endues XRCC4 with a crucial role in potentiating innate immune response, thereby helping host to prevail in the battle against virus.

HSPBP1 facilitates cellular RLR-mediated antiviral response by inhibiting the K48-linked ubiquitination of RIG-I

Retinoic acid-inducible gene I (RIG-I) plays a critical role in the recognition of intracytoplasmic viral RNA. Upon binding to the RNA of invading viruses, the activated RIG-I translocates to mitochondria, where it recruits adapter protein MAVS, causing a series of signaling cascades.In this study, we demonstrated that Hsp70 binding protein 1 (HSPBP1) promotes RIG-I-mediated signal transduction. The overexpression of HSPBP1 can increase the stability of RIG-I protein by inhibiting its K48-linked ubiquitination, and promote the activation of IRF3 and the production of IFN-β induced by Sendai virus. Knockdown and knockout of HSPBP1 leads to down-regulation of virus-induced RIG-I expression, inhibits IRF3 activation, and reduces the production of IFNB1.These results indicate that HSPBP1 positively regulates the antiviral signal pathway induced by inhibiting the K48-linked ubiquitination of RIG-I.

Antagonism of Type I Interferon by Severe Acute Respiratory Syndrome Coronavirus 2.

The coronavirus disease 2019 (COVID-19) pandemic is caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), warranting urgent study of the molecular mechanisms of SARS-CoV-2 infection and host immune response. Type I interferon (IFN-I) is a key component of host innate immune system responsible for eliminating the virus at the early stage of infection. In contrast, SARS-CoV-2 has evolved multiple strategies to evade innate immune response to facilitate viral replication, transmission, and pathogenesis. This review summarizes the recent progresses on SARS-CoV-2 proteins that antagonize host IFN-I production and/or signaling. These progresses have provided knowledge for new vaccine and antiviral development to prevent and control COVID-19.

NS5 Conservative Site Is Required for Zika Virus to Restrict the RIG-I Signaling.

During host–virus co-evolution, cells develop innate immune systems to inhibit virus invasion, while viruses employ strategies to suppress immune responses and maintain infection. Here, we reveal that Zika virus (ZIKV), a re-emerging arbovirus causing public concerns and devastating complications, restricts host immune responses through a distinct mechanism. ZIKV nonstructural protein 5 (NS5) interacts with the host retinoic acid-inducible gene I (RIG-I), an essential signaling molecule for defending pathogen infections. NS5 subsequently represses K63-linked polyubiquitination of RIG-I, attenuates the phosphorylation and nuclear translocation of interferon regulatory factor 3 (IRF3), and inhibits the expression and production of interferon-β (IFN-β), thereby restricting the RIG-I signaling pathway. Interestingly, we demonstrate that the methyltransferase (MTase) domain of NS5 is required for the repression of RIG-I ubiquitination, IRF3 activation, and IFN-β production. Detailed studies further reveal that the conservative active site D146 of NS5 is critical for the suppression of the RIG-I signaling. Therefore, we uncover an essential role of NS5 conservative site D146 in ZIKV-mediated repression of innate immune system, illustrate a distinct mechanism by which ZIKV evades host immune responses, and discover a potential target for anti-viral infection.

The influenza NS1 protein modulates RIG-I activation via a strain-specific direct interaction with the second CARD of RIG-I

The influenza NS1 protein modulates RIG-I activation via a strain-specific direct interaction with the second CARD of RIG-I

Recovery of TRIM25-Mediated RIG-I Ubiquitination through Suppression of NS1 by RNA Aptamers.

Non-structural protein 1 (NS1) of influenza virus has been shown to inhibit the innate immune response by blocking the induction of interferon (IFN). In this study, we isolated two single-stranded RNA aptamers specific to NS1 with Kd values of 1.62 ± 0.30 nM and 1.97 ± 0.27 nM, respectively, using a systematic evolution of ligand by exponential enrichment (SELEX) procedure. The selected aptamers were able to inhibit the interaction of NS1 with tripartite motif-containing protein 25 (TRIM25), and suppression of NS1 enabled retinoic acid inducible gene I (RIG-I) to be ubiquitinated regularly by TRIM25. Additional luciferase reporter assay and quantitative real-time PCR (RT-PCR) experiments demonstrated that suppression of NS1 by the selected aptamers induced IFN production. It is noted that viral replication was also inhibited through IFN induction in the presence of the selected aptamers. These results suggest that the isolated aptamers are strongly expected to be new therapeutic agents against influenza infection.

RIPLET, and not TRIM25, is required for endogenous RIG-I-dependent antiviral responses.

The innate immune system is our first line of defense against viral pathogens. Host cell pattern recognition receptors sense viral components and initiate immune signaling cascades that result in the production of an array of cytokines to combat infection. Retinoic acid-inducible gene-I (RIG-I) is a pattern recognition receptor that recognizes viral RNA and, when activated, results in the production of type I and III interferons (IFNs) and the upregulation of IFN-stimulated genes. Ubiquitination of RIG-I by the E3 ligases tripartite motif-containing 25 (TRIM25) and Riplet is thought to be requisite for RIG-I activation; however, recent studies have questioned the relative importance of these two enzymes for RIG-I signaling. In this study, we show that deletion of Trim25 does not affect the IFN response to either influenza A virus (IAV), influenza B virus, Sendai virus or several RIG-I agonists. This is in contrast to deletion of either Rig-i or Riplet, which completely abrogated RIG-I-dependent IFN responses. This was consistent in both mouse and human cell lines, as well as in normal human bronchial cells. With most of the current TRIM25 literature based on exogenous expression, these findings provide critical evidence that Riplet, and not TRIM25, is required endogenously for the ubiquitination of RIG-I. Despite this, loss of TRIM25 results in greater susceptibility to IAV infection invivo, suggesting that it may have an alternative role in host antiviral defense. This study refines our understanding of RIG-I signaling in viral infections and will inform future studies in the field.

Human Respiratory Syncytial Virus NS 1 Targets TRIM25 to Suppress RIG-I Ubiquitination and Subsequent RIG-I-Mediated Antiviral Signaling.

Respiratory syncytial virus (RSV) causes severe acute lower respiratory tract disease. Retinoic acid-inducible gene-I (RIG-I) serves as an innate immune sensor and triggers antiviral responses upon recognizing viral infections including RSV. Since tripartite motif-containing protein 25 (TRIM25)-mediated K63-polyubiquitination is crucial for RIG-I activation, several viruses target initial RIG-I activation through ubiquitination. RSV NS1 and NS2 have been shown to interfere with RIG-I-mediated antiviral signaling. In this study, we explored the possibility that NS1 suppresses RIG-I-mediated antiviral signaling by targeting TRIM25. Ubiquitination of ectopically expressed RIG-I-2Cards domain was decreased by RSV infection, indicating that RSV possesses ability to inhibit TRIM25-mediated RIG-I ubiquitination. Similarly, ectopic expression of NS1 sufficiently suppressed TRIM25-mediated RIG-I ubiquitination. Furthermore, interaction between NS1 and TRIM25 was detected by a co-immunoprecipitation assay. Further biochemical assays showed that the SPRY domain of TRIM25, which is responsible for interaction with RIG-I, interacted sufficiently with NS1. Suppression of RIG-I ubiquitination by NS1 resulted in decreased interaction between RIG-I and its downstream molecule, MAVS. The suppressive effect of NS1 on RIG-I signaling could be abrogated by overexpression of TRIM25. Collectively, this study suggests that RSV NS1 interacts with TRIM25 and interferes with RIG-I ubiquitination to suppress type-I interferon signaling.

Positive regulatory role of c-Src-mediated TRIM25 tyrosine phosphorylation on RIG-I ubiquitination and RIG-I-mediated antiviral signaling pathway

Retinoic acid-inducible gene I (RIG-I) detects viral RNAs and induces antiviral responses. During viral RNA recognition by RIG-I, tripartite motif protein 25 (TRIM25) plays a critical regulatory role by inducing K63-linked RIG-I polyubiquitination. Previous proteomics analysis revealed several phosphorylation sites on TRIM25, including tyrosine 278 (Y278), yet the roles of these modifications remain elusive. Here, we demonstrated that TRIM25 interacted with c-Src and underwent tyrosine phosphorylation by c-Src kinase upon viral infection and the phosphorylation is required for the complete activation of RIG-I signaling. Analysis using a c-Src inhibitor and TRIM25 mutant, in which tyrosine 278 is substituted by phenylalanine (Y278F), suggested that the phosphorylation positively regulates K63-linked polyubiquitination of RIG-I and subsequent antiviral signaling. The TRIM25 Y278F mutant displayed decreased E3-ubiquitin ligase activity in vitro, suggesting that this phosphorylation event affects the E3-ligase activity of TRIM25. Thus, we provide a molecular mechanism of c-Src-mediated positive regulation of RIG-I signaling.

Stratified ubiquitination of RIG-I creates robust immune response and induces selective gene expression.

The activation of retinoic acid-inducible gene I (RIG-I), an indispensable viral RNA sensor in mammals, is subtly regulated by ubiquitination. Although multiple ubiquitination sites at the amino terminus of RIG-I have been identified, their functional allocations in RIG-I activation remain elusive. We identified a stratified model for RIG-I amino-terminal ubiquitination, in which initiation at either Lys164 or Lys172 allows subsequent ubiquitination at other lysines, to trigger and amplify RIG-I activation. Experimental and mathematical modeling showed that multisite ubiquitination provides robustness in RIG-I-mediated type I interferon (IFN) signaling. Furthermore, the flexibly controlled ultrasensitivity and IFN activation intensity determine the specificity of the IFN-stimulated gene transcription and manipulate cell fate in antiviral immune response. Our work demonstrates that tunable type I IFN signaling can be regulated through multisite RIG-I ubiquitination and elucidates a new paradigm for dynamic regulation in RIG-I-mediated antiviral signaling.

ZBP1/DAI ubiquitination and sensing of influenza vRNPs activate programmed cell death

Innate sensing of influenza virus infection induces activation of programmed cell death pathways. We have recently identified Z-DNA-binding protein 1 (ZBP1) as an innate sensor of influenza A virus (IAV). ZBP1-mediated IAV sensing is critical for triggering programmed cell death in the infected lungs. Surprisingly, little is known about the mechanisms regulating ZBP1 activation to induce programmed cell death. Here, we report that the sensing of IAV RNA by retinoic acid inducible gene I (RIG-I) initiates ZBP1-mediated cell death via the RIG-I-MAVS-IFN-β signaling axis. IAV infection induces ubiquitination of ZBP1, suggesting potential regulation of ZBP1 function through posttranslational modifications. We further demonstrate that ZBP1 senses viral ribonucleoprotein (vRNP) complexes of IAV to trigger cell death. These findings collectively indicate that ZBP1 activation requires RIG-I signaling, ubiquitination, and vRNP sensing to trigger activation of programmed cell death pathways during IAV infection. The mechanism of ZBP1 activation described here may have broader implications in the context of virus-induced cell death.

The Severe Acute Respiratory Syndrome Coronavirus Nucleocapsid Inhibits Type I Interferon Production by Interfering with TRIM25-Mediated RIG-I Ubiquitination.

Severe acute respiratory syndrome (SARS) is a respiratory disease, caused by a coronavirus (SARS-CoV), that is characterized by atypical pneumonia. The nucleocapsid protein (N protein) of SARS-CoV plays an important role in inhibition of type I interferon (IFN) production via an unknown mechanism. In this study, the SARS-CoV N protein was found to bind to the SPRY domain of the tripartite motif protein 25 (TRIM25) E3 ubiquitin ligase, thereby interfering with the association between TRIM25 and retinoic acid-inducible gene I (RIG-I) and inhibiting TRIM25-mediated RIG-I ubiquitination and activation. Type I IFN production induced by poly I·C or Sendai virus (SeV) was suppressed by the SARS-CoV N protein. SARS-CoV replication was increased by overexpression of the full-length N protein but not N amino acids 1 to 361, which could not interact with TRIM25. These findings provide an insightful interpretation of the SARS-CoV-mediated host innate immune suppression caused by the N protein.IMPORTANCE The SARS-CoV N protein is essential for the viral life cycle and plays a key role in the virus-host interaction. We demonstrated that the interaction between the C terminus of the N protein and the SPRY domain of TRIM25 inhibited TRIM25-mediated RIG-I ubiquitination, which resulted in the inhibition of IFN production. We also found that the Middle East respiratory syndrome CoV (MERS-CoV) N protein interacted with TRIM25 and inhibited RIG-I signaling. The outcomes of these findings indicate the function of the coronavirus N protein in modulating the host's initial innate immune response.