Listerin promotes cGAS protein degradation through the ESCRT pathway to negatively regulate cGAS-mediated immune response.

The enzyme cyclic GMP-AMP synthase (cGAS) is a key sensor for detecting misplaced double-stranded DNA (dsDNA) of genomic, mitochondrial, and microbial origin. It synthesizes 2'3'-cGAMP, which in turn activates the stimulator of interferon genes pathway, leading to the initiation of innate immune responses. Here, we identified Listerin as a negative regulator of cGAS-mediated innate immune response. We found that Listerin interacts with cGAS on endosomes and promotes its K63-linked ubiquitination through recruitment of the E3 ligase TRIM27. The polyubiquitinated cGAS is then recognized by the endosomal sorting complexes required for transport machinery and sorted into endosomes for degradation. Listerin deficiency enhances the innate antiviral response to herpes simplex virus 1 infection. Genetic deletion of Listerin also deteriorates the neuroinflammation and the ALS disease progress in an ALS mice model; overexpression of Listerin can robustly ameliorate disease progression in ALS mice. Thus, our work uncovers a mechanism for cGAS regulation and suggests that Listerin may be a promising therapeutic target for ALS disease.

Interplay between TRIM7 and antiviral immunity.

TRIM7 has been demonstrated to have significant roles in promoting host defense against viral infections and regulating immune signaling pathways. As an E3 ubiquitin ligase, it catalyzes the ubiquitination of various substrates, including adaptor proteins (MAVS and STING) and transcription factors (NF-κB and IRF3), thereby exerting positive or negative regulation on immune signal pathways. However, viruses have developed immune evasion mechanisms to counteract TRIM7. Some viruses can inhibit TRIM7 function by targeting it for degradation or sequestering it away from its targets. Moreover, TRIM7 may even facilitate viral infection by ubiquitinating viral proteins, including envelope proteins that are critical for tissue and species tropism. A comprehensive understanding of the interaction between TRIM7 and antiviral immunity is crucial for the development of innovative treatments for viral diseases.

RNF123 Mediates Ubiquitination and Degradation of SOCS1 To Regulate Type I Interferon Production during Duck Tembusu Virus Infection.

Many RING domain E3 ubiquitin ligases play critical roles in fine-tuning the innate immune response, yet little is known about their regulatory role in flavivirus-induced innate immunity. In previous studies, we found that the suppressor of cytokine signaling 1 (SOCS1) protein mainly undergoes lysine 48 (K48)-linked ubiquitination. However, the E3 ubiquitin ligase that promotes the K48-linked ubiquitination of SOCS1 is unknown. In the present study, we found that RING finger protein 123 (RNF123) binds to the SH2 domain of SOCS1 through its RING domain and facilitates the K48-linked ubiquitination of the K114 and K137 residues of SOCS1. Further studies found that RNF123 promoted the proteasomal degradation of SOCS1 and promoted Toll-like receptor 3 (TLR3)- and interferon (IFN) regulatory factor 7 (IRF7)-mediated type I IFN production during duck Tembusu virus (DTMUV) infection through SOCS1, ultimately inhibiting DTMUV replication. Overall, these findings demonstrate a novel mechanism by which RNF123 regulates type I IFN signaling during DTMUV infection by targeting SOCS1 degradation. IMPORTANCE In recent years, posttranslational modification (PTM) has gradually become a research hot spot in the field of innate immunity regulation, and ubiquitination is one of the critical PTMs. DTMUV has seriously endangered the development of the waterfowl industry in Southeast Asian countries since its outbreak in 2009. Previous studies have shown that SOCS1 is modified by K48-linked ubiquitination during DTMUV infection, but E3 ubiquitin ligase catalyzing the ubiquitination of SOCS1 has not been reported. Here, we identify for the first time that RNF123 acts as an E3 ubiquitin ligase that regulates TLR3- and IRF7-induced type I IFN signaling during DTMUV infection by targeting the K48-linked ubiquitination of the K114 and K137 residues of SOCS1 and the proteasomal degradation of SOCS1.

RNF5: inhibiting antiviral immunity and shaping virus life cycle.

RNF5 is an E3 ubiquitin ligase involved in various physiological processes such as protein localization and cancer progression. Recent studies have shown that RNF5 significantly inhibits antiviral innate immunity by promoting the ubiquitination and degradation of STING and MAVS, which are essential adaptor proteins, as well as their downstream signal IRF3. The abundance of RNF5 is delicately regulated by both host factors and viruses. Host factors have been found to restrict RNF5-mediated ubiquitination, maintaining the stability of STING or MAVS through distinct mechanisms. Meanwhile, viruses have developed ingenious strategies to hijack RNF5 to ubiquitinate and degrade immune proteins. Moreover, recent studies have revealed the multifaceted roles of RNF5 in the life cycle of various viruses, including SARS-CoV-2 and KSHV. Based on these emerging discoveries, RNF5 represents a novel means of modulating antiviral immunity. In this review, we summarize the latest research on the roles of RNF5 in antiviral immunity and virus life cycle. This comprehensive understanding could offer valuable insights into exploring potential therapeutic applications focused on targeting RNF5 during viral infections.

Antibody targeting of E3 ubiquitin ligases for receptor degradation.

Most current therapies that target plasma membrane receptors function by antagonizing ligand binding or enzymatic activities. However, typical mammalian proteins comprise multiple domains that execute discrete but coordinated activities. Thus, inhibition of one domain often incompletely suppresses the function of a protein. Indeed, targeted protein degradation technologies, including proteolysis-targeting chimeras1 (PROTACs), have highlighted clinically important advantages of target degradation over inhibition2. However, the generation of heterobifunctional compounds binding to two targets with high affinity is complex, particularly when oral bioavailability is required3. Here we describe the development of proteolysis-targeting antibodies (PROTABs) that tether cell-surface E3 ubiquitin ligases to transmembrane proteins, resulting in target degradation both in vitro and in vivo. Focusing on zinc- and ring finger 3 (ZNRF3), a Wnt-responsive ligase, we show that this approach can enable colorectal cancer-specific degradation. Notably, by examining a matrix of additional cell-surface E3 ubiquitin ligases and transmembrane receptors, we demonstrate that this technology is amendable for 'on-demand' degradation. Furthermore, we offer insights on the ground rules governing target degradation by engineering optimized antibody formats. In summary, this work describes a strategy for the rapid development of potent, bioavailable and tissue-selective degraders of cell-surface proteins.

Pseudorabies virus tegument protein UL13 recruits RNF5 to inhibit STING-mediated antiviral immunity.

Pseudorabies virus (PRV) has evolved various immune evasion mechanisms that target host antiviral immune responses. However, it is unclear whether and how PRV encoded proteins modulate the cGAS-STING axis for immune evasion. Here, we show that PRV tegument protein UL13 inhibits STING-mediated antiviral signaling via regulation of STING stability. Mechanistically, UL13 interacts with the CDN domain of STING and recruits the E3 ligase RING-finger protein 5 (RNF5) to promote K27-/K29-linked ubiquitination and degradation of STING. Consequently, deficiency of RNF5 enhances host antiviral immune responses triggered by PRV infection. In addition, mutant PRV lacking UL13 impaired in antagonism of STING-mediated production of type I IFNs and shows attenuated pathogenicity in mice. Our findings suggest that PRV UL13 functions as an antagonist of IFN signaling via a novel mechanism by targeting STING to persistently evade host antiviral responses.

E3 Ubiquitin Ligase Riplet Is Expressed in T Cells and Suppresses T Cell-Mediated Antitumor Immune Responses.

The E3 ubiquitin ligase Riplet mediates retinoic acid-inducible gene-I polyubiquitination and is essential for viral-induced expression of type I IFNs in dendritic cells and macrophages. The function of Riplet in innate immunity has been well demonstrated; however, its role in adaptive immunity during the antitumor immune response is unclear. In this study, we examined the role of Riplet in the T cell-mediated antitumor immune response. Riplet was expressed in T cells and upregulated in CD8+ T cells in response to TCR-mediated stimulation. Furthermore, PR domain containing 1, eomesodermin, and killer cell lectin-like receptor G1 expression was increased in effector CD8+ T cells by Riplet knockout in vitro, which suggests that Riplet is involved in the effector function of CD8+ T cells. Our results indicated that Riplet deficiency augmented the antitumor response of MO4 (OVA-expressing melanoma)-bearing mice treated with OVA peptide-pulsed dendritic cells. Moreover, both CD4+ and CD8+ T cells played important roles in Riplet-mediated augmentation of the antitumor immune response. In tumor-draining lymph nodes, the Th1 response was promoted, and the induction of OVA-specific CD8+ T cells and IFN-γ production were enhanced by Riplet deficiency. Furthermore, the IFN-γ response and OVA-specific cytotoxicity of CD8+ T cells in tumor tissue were augmented by Riplet deficiency. The expression of Cxcl9fluorescence-minus-one and Cxcl10 mRNA was also enhanced in the tumor microenvironment by Riplet knockout, consistent with the augmented recruitment of CTLs. Overall, we clarified a function of Riplet in T cells, which is to suppress the antitumor immune response through modulating Th1 and CTLs.

Decreased Expression of Programmed Death Ligand-L1 by Seven in Absentia Homolog 2 in Cholangiocarcinoma Enhances T-Cell-Mediated Antitumor Activity.

N6-methyladenosine (m6A) has been reported as an important mechanism of post-transcriptional regulation. Programmed death ligand 1 (PD-L1) is a primary immune inhibitory molecule expressed on tumor cells that promotes immune evasion. In addition, seven in absentia homolog 2 (Siah2), a RING E3 ubiquitin ligase, has been involved in tumorigenesis and cancer progression. However, the role of m6A-METTL14-Siah2-PD-L1 axis in immunotherapy remains to be elucidated. In this study, we showed that METTL14, a component of the m6A methyltransferase complex, induced Siah2 expression in cholangiocarcinoma (CCA). METTL14 was shown to enrich m6A modifications in the 3'UTR region of the Siah2 mRNA, thereby promoting its degradation in an YTHDF2-dependent manner. Furthermore, co-immunoprecipitation experiments demonstrated that Siah2 interacted with PD-L1 by promoting its K63-linked ubiquitination. We also observed that in vitro and in vivo Siah2 knockdown inhibited T cells expansion and cytotoxicity by sustaining tumor cell PD-L1 expression. The METTL14-Siah2-PD-L1-regulating axis was further confirmed in human CCA specimens. Analysis of specimens from patients receiving anti-PD1 immunotherapy suggested that tumors with low Siah2 levels were more sensitive to anti-PD1 immunotherapy. Taken together, our results evidenced a new regulatory mechanism of Siah2 by METTL14-induced mRNA epigenetic modification and the potential role of Siah2 in cancer immunotherapy.

Rare germline heterozygous missense variants in BRCA1-associated protein 1, BAP1, cause a syndromic neurodevelopmental disorder.

Nuclear deubiquitinase BAP1 (BRCA1-associated protein 1) is a core component of multiprotein complexes that promote transcription by reversing the ubiquitination of histone 2A (H2A). BAP1 is a tumor suppressor whose germline loss-of-function variants predispose to cancer. To our knowledge, there are very rare examples of different germline variants in the same gene causing either a neurodevelopmental disorder (NDD) or a tumor predisposition syndrome. Here, we report a series of 11 de novo germline heterozygous missense BAP1 variants associated with a rare syndromic NDD. Functional analysis showed that most of the variants cannot rescue the consequences of BAP1 inactivation, suggesting a loss-of-function mechanism. In T cells isolated from two affected children, H2A deubiquitination was impaired. In matching peripheral blood mononuclear cells, histone H3 K27 acetylation ChIP-seq indicated that these BAP1 variants induced genome-wide chromatin state alterations, with enrichment for regulatory regions surrounding genes of the ubiquitin-proteasome system (UPS). Altogether, these results define a clinical syndrome caused by rare germline missense BAP1 variants that alter chromatin remodeling through abnormal histone ubiquitination and lead to transcriptional dysregulation of developmental genes.

Arabidopsis PUB2 and PUB4 connect signaling components of pattern-triggered immunity.

Plants use pattern recognition receptors (PRRs) to detect pathogen-associated molecular patterns (PAMPs) and activate pattern-triggered immunity (PTI). Precise regulation of information from PRRs to downstream signaling components is vital to mounting an appropriate immune response and requires dynamic interactions of these PTI components. We used transcriptome profiling, phenotypic analysis, molecular genetics, and protein-protein interaction analysis to understand the roles of the Arabidopsis plant U-box (PUB) proteins PUB2 and PUB4 in disease resistance and PTI signaling. Loss of function of both PUB2 and PUB4 diminishes the PAMP-triggered oxidative bursts and dampens mitogen-activated protein kinase signaling, resulting in a severe compromise in resistance to not only pathogenic but also nonpathogenic strains of Pseudomonas syringae. Within PUB4, the E3 ligase activity is dispensable, but the armadillo repeat region is essential and sufficient for its function in immunity. PUB2 and PUB4 interact with PTI signaling components, including FLS2, BIK1, PBL27, and RbohD, and enhance FLS2-BIK1 and BIK1-RbohD interactions. Our study reveals that PUB2 and PUB4 are critical components of plant immunity and connect PTI components to positively regulate defense responses.

Progress on Poxvirus E3 Ubiquitin Ligases and Adaptor Proteins.

Poxviruses have evolved a variety of innate immunity evasion mechanisms, some of which involve poxvirus-encoded E3 ubiquitin ligases and adaptor proteins. Based on their functional domains and ubiquitin transfer mechanisms, these poxvirus-encoded E3 ubiquitin ligases and adaptor proteins can be divided into five categories: PRANC, ANK/BC, BBK, P28/RING, and MARCH proteins. Although the substrates of many poxvirus E3 ubiquitin ligases remain to be discovered, most of the identified substrates are components of the innate immune system. In this review, we discuss the current research progress on poxvirus-encoded E3 ubiquitin ligases and adaptor proteins to provide mechanistic insights into the interplay between these viruses and their hosts.

The Role of E3 Ubiquitin Ligases and Deubiquitinases in Inflammatory Bowel Disease: Friend or Foe?

Inflammatory bowel disease (IBD), which include Crohn's disease (CD) and ulcerative colitis (UC), exhibits a complex multifactorial pathogenesis involving genetic susceptibility, imbalance of gut microbiota, mucosal immune disorder and environmental factors. Recent studies reported associations between ubiquitination and deubiquitination and the occurrence and development of inflammatory bowel disease. Ubiquitination modification, one of the most important types of post-translational modifications, is a multi-step enzymatic process involved in the regulation of various physiological processes of cells, including cell cycle progression, cell differentiation, apoptosis, and innate and adaptive immune responses. Alterations in ubiquitination and deubiquitination can lead to various diseases, including IBD. Here, we review the role of E3 ubiquitin ligases and deubiquitinases (DUBs) and their mediated ubiquitination and deubiquitination modifications in the pathogenesis of IBD. We highlight the importance of this type of posttranslational modification in the development of inflammation, and provide guidance for the future development of targeted therapeutics in IBD.

MALT1-Dependent Cleavage of HOIL1 Modulates Canonical NF-κB Signaling and Inflammatory Responsiveness.

Nuclear factor kappa B (NF-κB) is a critical transcription factor involved in regulating cell activation, inflammation, and survival. The linear ubiquitin chain assembly complex (LUBAC) which consists of HOIL1, HOIP, and SHARPIN, catalyzes the linear ubiquitination of target proteins-a post-translational modification that is essential for NF-κB activation. Human germline pathogenic variants that dysregulate linear ubiquitination and NF-κB signaling are associated with immunodeficiency and/or autoinflammation including dermatitis, recurrent fevers, systemic inflammation and enteropathy. We previously identified MALT1 paracaspase as a novel negative regulator of LUBAC by proteolytic cleavage of HOIL1. To directly investigate the impact of HOIL1 cleavage activity on the inflammatory response, we employed a stable transduction system to express and directly compare non-cleavable HOIL1 with wild-type HOIL1 in primary HOIL1-deficient patient skin fibroblasts. We discovered that non-cleavable HOIL1 resulted in enhanced NF-κB signaling in response to innate stimuli. Transcriptomics revealed enrichment of inflammation and proinflammatory cytokine-related pathways after stimulation. Multiplexed cytokine assays confirmed a 'hyperinflammatory' phenotype in these cells. This work highlights the physiological importance of MALT1-dependent cleavage and modulation of HOIL1 on NF-κB signaling and inflammation, provides a mechanism for the autoinflammation observed in MALT1-deficient patients, and will inform the development of therapeutics that target MALT1 paracaspase and LUBAC function in treating autoinflammatory skin diseases.

African Swine Fever Virus pI215L Negatively Regulates cGAS-STING Signaling Pathway through Recruiting RNF138 to Inhibit K63-Linked Ubiquitination of TBK1.

African swine fever is a severe animal infectious disease caused by African swine fever virus (ASFV), and the morbidity and mortality associated with virulent ASFV isolates are as high as 100%. Previous studies showed that the ability of ASFV to antagonize IFN production is closely related to its pathogenicity. Here, we report that ASFV HLJ/18 infection induced low levels of type I IFN and inhibited cGMP-AMP-induced type I IFN production in porcine alveolar macrophages that were isolated from specific pathogen-free Landrace piglets. Subsequently, an unbiased screen was performed to screen the ASFV genes with inhibitory effects on the type I IFN production. ASFV pI215L, a viral E2 ubiquitin-conjugating enzyme, was identified as one of the strongest inhibitory effectors on the production of type I IFN. Knockdown of pI215L expression inhibited ASFV replication and enhanced IFN-ß production. However, inhibition of type I IFN production by pI215L was independent of its E2 enzyme activity. Furthermore, we found that pI215L inhibited type I IFN production and K63-linked polyubiquitination of TANK-binding kinase 1 through pI215L-binding RING finger protein 138 (RNF138). ASFV pI215L enhanced the interaction between RNF138 and RNF128 and promoted RNF138 to degrade RNF128, which resulted in reduced K63-linked polyubiquitination of TANK-binding kinase 1 and type І IFN production. Taken together, our findings reveal a novel immune escape mechanism of ASFV, which provides a clue to the design and development of an immune-sensitive attenuated live vaccine.

Self-guarding of MORC3 enables virulence factor-triggered immunity.

Pathogens use virulence factors to inhibit the immune system1. The guard hypothesis2,3 postulates that hosts monitor (or 'guard') critical innate immune pathways such that their disruption by virulence factors provokes a secondary immune response1. Here we describe a 'self-guarded' immune pathway in human monocytes, in which guarding and guarded functions are combined in one protein. We find that this pathway is triggered by ICP0, a key virulence factor of herpes simplex virus type 1, resulting in robust induction of anti-viral type I interferon (IFN). Notably, induction of IFN by ICP0 is independent of canonical immune pathways and the IRF3 and IRF7 transcription factors. A CRISPR screen identified the ICP0 target MORC34 as an essential negative regulator of IFN. Loss of MORC3 recapitulates the IRF3- and IRF7-independent IFN response induced by ICP0. Mechanistically, ICP0 degrades MORC3, which leads to de-repression of a MORC3-regulated DNA element (MRE) adjacent to the IFNB1 locus. The MRE is required in cis for IFNB1 induction by the MORC3 pathway, but is not required for canonical IFN-inducing pathways. As well as repressing the MRE to regulate IFNB1, MORC3 is also a direct restriction factor of HSV-15. Our results thus suggest a model in which the primary anti-viral function of MORC3 is self-guarded by its secondary IFN-repressing function-thus, a virus that degrades MORC3 to avoid its primary anti-viral function will unleash the secondary anti-viral IFN response.

Kaposi's sarcoma-associated herpesvirus ubiquitin ligases downregulate cell surface expression of l-selectin.

Kaposi's sarcoma-associated herpesvirus (KSHV) is an oncogenic etiological factor for Kaposi's sarcoma and primary effusion lymphoma in immunocompromised patients. KSHV utilizes two immune evasion E3 ubiquitin ligases, namely K3 and K5, to downregulate the expression of antigen-presenting molecules and ligands of natural killer (NK) cells in the host cells through an ubiquitin-dependent endocytic mechanism. This allows the infected cells to evade surveillance and elimination by cytotoxic lymphocytes and NK cells. The number of host cell molecular substrates reported for these ubiquitin ligases is limited. The identification of novel substrates for these ligases will aid in elucidating the mechanism underlying immune evasion of KSHV. This study demonstrated that K5 downregulated the cell surface expression of l-selectin, a C-type lectin-like adhesion receptor expressed in the lymphocytes. Tryptophan residue located at the centre of the E2-binding site in the K5 RINGv domain was essential to downregulate l-selectin expression. Additionally, the lysine residues located at the cytoplasmic tail of l-selectin were required for the K5-mediated downregulation of l-selectin. K5 promoted the degradation of l-selectin through polyubiquitination. These results suggest that K5 downregulates l-selectin expression on the cell surface by promoting polyubiquitination and ubiquitin-dependent endocytosis, which indicated that l-selectin is a novel substrate for K5. Additionally, K3 downregulated l-selectin expression. The findings of this study will aid in the elucidation of a novel immune evasion mechanism in KSHV.

Lymph node-resident dendritic cells drive TH2 cell development involving MARCH1.

Type 2 T helper (TH2) cells are protective against parasitic worm infections but also aggravate allergic inflammation. Although the role of dendritic cells (DCs) in TH2 cell differentiation is well established, the underlying mechanisms are largely unknown. Here, we show that DC induction of TH2 cells depends on membrane-associated RING-CH-1 (MARCH1) ubiquitin ligase. The pro-TH2 effect of MARCH1 relied on lymph node (LN)­resident DCs, which triggered T cell receptor (TCR) signaling and induced GATA-3 expression from naïve CD4+ T cells independent of tissue-driven migratory DCs. Mice with mutations in the ubiquitin acceptor sites of MHCII and CD86, the two substrates of MARCH1, failed to develop TH2 cells. These findings suggest that TH2 cell development depends on ubiquitin-mediated clearance of antigen-presenting and costimulatory molecules by LN-resident DCs and consequent control of TCR signaling.

Negative Regulation of RNF90 on RNA Virus-Triggered Antiviral Immune Responses Targeting MAVS.

The antiviral innate immunity is the first line of host defense against viral infection. Mitochondrial antiviral signaling protein (MAVS, also named Cardif/IPS-1/VISA) is a critical protein in RNA virus-induced antiviral signaling pathways. Our previous research suggested that E3 ubiquitin-protein ligases RING-finger protein (RNF90) negatively regulate cellular antiviral responses by targeting STING for degradation, though its role in RNA virus infection remains unknown. This study demonstrated that RNF90 negatively regulated RNA virus-triggered antiviral innate immune responses in RNF90-silenced PMA-THP1 cells, RNF90-deficient cells (including HaCaTs, MEFs, and BMDMs), and RNF90-deficient mice. However, RNF90 regulated RNA virus-triggered antiviral innate immune responses independent of STING. RNF90 promoted K48-linked ubiquitination of MAVS and its proteasome-dependent degradation, leading to the inhibition of innate immune responses. Altogether, our findings suggested a novel function and mechanism of RNF90 in antiviral innate immunity.

MNSFß Regulates TNFα Production by Interacting with RC3H1 in Human Macrophages, and Dysfunction of MNSFß in Decidual Macrophages Is Associated With Recurrent Pregnancy Loss.

Decidual macrophages (dMϕ) are the second largest population of leukocytes at the maternal-fetal interface and play critical roles in maintaining pregnancy. Our previous studies demonstrated the active involvement of monoclonal nonspecific suppressor factor-ß (MNSFß) in embryonic implantation and pregnancy success. MNSFß is a ubiquitously expressed ubiquitin-like protein that also exhibits immune regulatory potential, but its function in human dMϕ remains unknown. Here, we observed that the proportion of CD11chigh (CD11cHI) dMϕ was significantly increased in dMϕ derived from patients with recurrent pregnancy loss (RPL dMϕ) compared to those derived from normal pregnant women (Control dMϕ). The production of MNSFß and TNFα by RPL dMϕ was also significantly increased compared to that by Control dMϕ. Conditioned medium from RPL dMϕ exerted an inhibitory effect on the invasiveness of human trophoblastic HTR8/SVneo cells, and this effect could be partially reversed by a neutralizing antibody against TNFα. Bioinformatics analysis indicated a potential interaction between MNSFß and RC3H1, a suppressor of TNFα transcription. Immunoprecipitation experiments with human Mϕ differentiated from the human monocyte cell line Thp1 (Thp1-derived Mϕ) proved the binding of MNSFß to RC3H1. Specific knockdown of MNSFß in Thp1-derived Mϕ led to a marked decrease in TNFα production, which could be reversed by inhibiting RC3H1 expression. Interestingly, a significant decrease in the protein level of RC3H1 was observed in RPL dMϕ. Together, our findings indicate that aberrantly increased MNSFß expression in dMϕ may promote TNFα production via its interaction with RC3H1, and these phenomena could result in the disruption of the immune balance at the maternal-fetal interface and thus pregnancy loss.

The RING-type protein BOI negatively regulates the protein level of a CC-NBS-LRR in Arabidopsis.

Nucleotide-binding site and leucine-rich repeat receptors (NLRs) play pivotal roles in plant immunity. The regulation of NLR stability is essential to ensure effective immunity, whereas the exact mechanism is largely unclear. The Arabidopsis CC-NBS-LRR protein L5 (At1g12290) can induce cell death in Nicotiana benthamiana, but not in Arabidopsis thaliana. We screened the interactors of L5 by yeast two-hybrid, and found that the BOI can interact with the CC domain of L5. Transiently expressed BOI reduced the protein level of L5, and suppressed the auoactivity of L5 in N. benthamiana. BOI can interact and ubiquitinate L5 in vitro, and mediate the proteasomal degradation of L5 in N. benthamiana and Arabidopsis. The Lys425 in the NBS domain of L5 is the critical unbiquitin site for the degradation. In conclusion, our results reveal a mechanism for the control of the stability of L5 protein and for the suppressed of L5-triggered cell death by a RING-type E3 ligase through the ubiquitin proteasome system.