Tuning BRCA1 and BARD1 activity to investigate RING ubiquitin ligase mechanisms

The tumor‐suppressor protein BRCA1 works with BARD1 to catalyze the transfer of ubiquitin onto protein substrates. The N‐terminal regions of BRCA1 and BARD1 that contain their RING domains are responsible for dimerization and ubiquitin ligase activity. This activity is a common feature among hundreds of human RING domain‐containing proteins. RING domains bind and activate E2 ubiquitin‐conjugating enzymes to promote ubiquitin transfer to substrates. We show that the identity of residues at specific positions in the RING domain can tune activity levels up or down. We report substitutions that create a structurally intact BRCA1/BARD1 heterodimer that is inactive in vitro with all E2 enzymes. Other substitutions in BRCA1 or BARD1 RING domains result in hyperactivity, revealing that both proteins have evolved attenuated activity. Loss of attenuation results in decreased product specificity, providing a rationale for why nature has tuned BRCA1 activity. The ability to tune BRCA1 provides powerful tools for understanding its biological functions and provides a basis to assess mechanisms for rescuing the activity of cancer‐associated variations. Beyond the applicability to BRCA1, we show the identity of residues at tuning positions that can be used to predict and modulate the activity of an unrelated RING E3 ligase. These findings provide valuable insights into understanding the mechanism and function of RING E3 ligases like BRCA1.     

牵牛胁迫应答基因PnLOG2的克隆及功能验证

RING型E3泛素连接酶在植物应答非生物胁迫过程中发挥着重要功能。该研究从圆叶牵牛中克隆出RING型E3泛素连接酶基因PnLOG2,该基因序列号为XM_019321049.1。利用ORF Finder预测PnLOG2基因编码开放阅读框长度为912 bp (51~992 bp),编码313个氨基酸,蛋白分子质量34.38 kD,理论等电点为5.14。系统发育分析表明,PnLOG2基因与番茄亲缘关系最近。组织特异性分析表明,PnLOG2基因在牵牛不同组织均有表达,在老茎和新叶中表达量较高。qRT PCR分析结果表明,PnLOG2基因在圆叶牵牛根和叶中受干旱、盐碱胁迫诱导显著上调表达。通过异源表达PnLOG2基因于酵母细胞中,发现干旱、盐碱胁迫下PnLOG2基因提高了重组酵母的耐盐和耐旱能力,但降低了对碱的耐受性。该研究初步阐明了PnLOG2基因在干旱、盐碱胁迫下的功能,为进一步研究RING型E3泛素连接酶在非生物胁迫中的机理提供了理论依据。     

Arabidopsis C3HC4‐RING finger E3 ubiquitin ligase AtAIRP4 positively regulates stress‐responsive abscisic acid signaling

Degradation of proteins via the ubiquitin system is an important step in many stress signaling pathways in plants. E3 ligases recognize ligand proteins and dictate the high specificity of protein degradation, and thus, play a pivotal role in ubiquitination. Here, we identified a gene, named Arabidopsis thaliana abscisic acid (ABA)‐insensitive RING protein 4 (AtAIRP4), which is induced by ABA and other stress treatments. AtAIRP4 encodes a cellular protein with a C3HC4‐RING finger domain in its C‐terminal side, which has in vitro E3 ligase activity. Loss of AtAIRP4 leads to a decrease in sensitivity of root elongation and stomatal closure to ABA, whereas overexpression of this gene in the T‐DNA insertion mutant atairp4 effectively recovered the ABA‐associated phenotypes. AtAIRP4 overexpression plants were hypersensitive to salt and osmotic stresses during seed germination, and showed drought avoidance compared with the wild‐type and atairp4 mutant plants. In addition, the expression levels of ABA‐ and drought‐induced marker genes in AtAIRP4 overexpression plants were markedly higher than those in the wild‐type and atairp4 mutant plants. Hence, these results indicate that AtAIRP4 may act as a positive regulator of ABA‐mediated drought avoidance and a negative regulator of salt tolerance in Arabidopsis.     

Identification of a Protein that Interacts with LeATL6 Ubiquitin‐protein Ligase E3 Upregulated in Tomato Treated with Elicitin‐Like Cell Wall Proteins of Pythium oligandrum

The expression of LeATL6, which encodes RING‐H2 zinc finger ubiquitin‐protein ligase E3, is highly induced in tomato roots treated with the elicitin‐like cell wall protein fraction (CWP) from the non‐pathogenic oomycete Pythium oligandrum, which enhances resistance to pathogens through a jasmonic acid (JA)‐dependent signalling pathway. In this study, the role of LeATL6 for CWP‐induced defence response was further analysed. To screen the putative target protein of LeATL6 for the CWP‐induced defence mechanism in tomato, we used a yeast two‐hybrid system to screen five clones encoding a protein that interacts with LeATL6. Four clones had a function associated with the ubiquitin‐proteasome system. Another positive clone encoded a protein sharing homology with S‐adenosylmethionine decarboxylase (SAMDC). In CWP‐treated tomato roots, SAMDC activity was clearly suppressed. Thus, the interaction of SAMDC with LeATL6 and the decreased SAMDC activity may be associated with JA‐dependent induced resistance in tomato treated with P. oligandrum.     

Structure and catalytic activation of the TRIM23 RING E3 ubiquitin ligase

Tripartite motif (TRIM) proteins comprise a large family of RING‐type ubiquitin E3 ligases that regulate important biological processes. An emerging general model is that TRIMs form elongated antiparallel coiled‐coil dimers that prevent interaction of the two attendant RING domains. The RING domains themselves bind E2 conjugating enzymes as dimers, implying that an active TRIM ligase requires higher‐order oligomerization of the basal coiled‐coil dimers. Here, we report crystal structures of the TRIM23 RING domain in isolation and in complex with an E2–ubiquitin conjugate. Our results indicate that TRIM23 enzymatic activity requires RING dimerization, consistent with the general model of TRIM activation.     

Solution structure of the E3 ligase HOIL-1 Ubl domain

The E3 ligases HOIL‐1 and parkin are each comprised of an N‐terminal ubiquitin‐like (Ubl) domain followed by a zinc‐binding region and C‐terminal RING–In‐between‐RING–RING domains. These two proteins, involved in the ubiquitin‐mediated degradation pathway, are the only two known E3 ligases to share this type of multidomain architecture. Further, the Ubl domain of both HOIL‐1 and parkin has been shown to interact with the S5a subunit of the 26S proteasome. The solution structure of the HOIL‐1 Ubl domain was solved using NMR spectroscopy to compare it with that of parkin to determine the structural elements responsible for S5a intermolecular interactions. The final ensemble of 20 structures had a β‐grasp Ubl‐fold with an overall backbone RMSD of 0.59 ± 0.10 Å in the structured regions between I55 and L131. HOIL‐1 had a unique extension of both β1 and β2 sheets compared to parkin and other Ubl domains, a result of a four‐residue insertion in this region. A similar 15‐residue hydrophobic core in the HOIL‐1 Ubl domain resulted in a comparable stability to the parkin Ubl, but significantly lower than that observed for ubiquitin. A comparison with parkin and other Ubl domains indicates that HOIL‐1 likely uses a conserved hydrophobic patch (W58, V102, Y127, Y129) found on the β1 face, the β3–β4 loop and β5, as well as a C‐terminal basic residue (R134) to recruit the S5a subunit as part of the ubiquitin‐mediated proteolysis pathway.     

Disruption of gene SPL35, encoding a novel CUE domain‐containing protein,leads to cell death and enhanced disease response in rice

Lesion mimic mutants that exhibit spontaneous hypersensitive response (HR)‐like necrotic lesions are ideal experimental systems for elucidating molecular mechanisms involved in plant cell death and defence responses. Here we report identification of a rice lesion mimic mutant, spotted leaf 35 (spl35), and cloning of the causal gene by TAIL‐PCR strategy. spl35 exhibited decreased chlorophyll content, higher accumulation of H₂O₂, up‐regulated expression of defence‐related marker genes, and enhanced resistance to both fungal and bacterial pathogens of rice. The SPL35 gene encodes a novel CUE (coupling of ubiquitin conjugation to ER degradation) domain‐containing protein that is predominantly localized in cytosol, ER and unknown punctate compartment(s). SPL35 is constitutively expressed in all organs, and both overexpression and knockdown of SPL35 cause the lesion mimic phenotype. SPL35 directly interacts with the E2 protein OsUBC5a and the coatomer subunit delta proteins Delta‐COP1 and Delta‐COP2 through the CUE domain, and down‐regulation of these interacting proteins also cause development of HR‐like lesions resembling those in spl35 and activation of defence responses, indicating that SPL35 may be involved in the ubiquitination and vesicular trafficking pathways. Our findings provide insight into a role of SPL35 in regulating cell death and defence response in plants.     

Molecular characterization of rice arsenic‐induced RING finger E3 ligase 2 (OsAIR2) and its heterogeneous overexpression in Arabidopsis thaliana

Arsenic (As) accumulation adversely affects the growth and productivity of plants and poses a serious threat to human health and food security. In this study, we identified one As‐responsive R eally I nteresting N ew G ene (RING) E3 ubiquitin ligase gene from rice root tissues during As stress. We named it Oryza sativa As‐Induced RING E3 ligase 2 (OsAIR2). Expression of OsAIR2 was induced under various abiotic stress conditions, including heat, salt, drought and As exposure. Results of an in vitro ubiquitination assay showed that OsAIR2 possesses an E3 ligase activity. Within the cell, OsAIR2 was found to be localized to the Golgi apparatus. Using yeast two‐hybrid (Y2H) assay, the 3‐ketoacyl‐CoA thiolase (KAT) protein was identified as an interaction partner. We found that the O. sativa KAT1 (OsKAT1) is localized to the cytosol and peroxisomes. Moreover, in vitro pull‐down assay verified the physical interaction between OsAIR2 and OsKAT1. Interestingly, in vitro ubiquitination assay and in vivo proteasomal degradation assay revealed that OsAIR2 ubiquitinates OsKAT1 and promotes the degradation of OsKAT1 via the 26S proteasome degradation pathway. Heterogeneous overexpression of OsAIR2 in Arabidopsis improved the seed germination and increased the root length under arsenate stress conditions. Therefore, these results suggest that OsAIR2 may be associated with the plant response to As stress and acts as a positive regulator of As stress tolerance.     

The pepper E3 ubiquitin ligase RING1 gene, CaRING1, is required for cell death and the salicylic acid-dependent defense response

Ubiquitination is essential for ubiquitin/proteasome-mediated protein degradation in plant development and defense. Here, we identified a novel E3 ubiquitin ligase RING1 gene, CaRING1, from pepper (Capsicum annuum). In pepper, CaRING1 expression is induced by avirulent Xanthomonas campestris pv vesicatoria infection. CaRING1 contains an amino-terminal transmembrane domain and a carboxyl-terminal RING domain. In addition, it displays in vitro E3 ubiquitin ligase activity, and the RING domain is essential for E3 ubiquitin ligase activity in CaRING1. CaRING1 also localizes to the plasma membrane. In pepper plants, virus-induced gene silencing of CaRING1 confers enhanced susceptibility to avirulent X. campestris pv vesicatoria infection, which is accompanied by compromised hypersensitive cell death, reduced expression of PATHOGENESIS-RELATED1, and lowered salicylic acid levels in leaves. Transient expression of CaRING1 in pepper leaves induces cell death and the defense response that requires the E3 ubiquitin ligase activity of CaRING1. By contrast, overexpression of CaRING1 in Arabidopsis (Arabidopsis thaliana) confers enhanced resistance to hemibiotrophic Pseudomonas syringae pv tomato and biotrophic Hyaloperonospora arabidopsidis infections. Taken together, these results suggest that CaRING1 is involved in the induction of cell death and the regulation of ubiquitination during the defense response to microbial pathogens.     

Functional role of TRIM E3 ligase oligomerization and regulation of catalytic activity

TRIM E3 ubiquitin ligases regulate a wide variety of cellular processes and are particularly important during innate immune signalling events. They are characterized by a conserved tripartite motif in their N‐terminal portion which comprises a canonical RING domain, one or two B‐box domains and a coiled‐coil region that mediates ligase dimerization. Self‐association via the coiled‐coil has been suggested to be crucial for catalytic activity of TRIMs; however, the precise molecular mechanism underlying this observation remains elusive. Here, we provide a detailed characterization of the TRIM ligases TRIM25 and TRIM32 and show how their oligomeric state is linked to catalytic activity. The crystal structure of a complex between the TRIM25 RING domain and an ubiquitin‐loaded E2 identifies the structural and mechanistic features that promote a closed E2~Ub conformation to activate the thioester for ubiquitin transfer allowing us to propose a model for the regulation of activity in the full‐length protein. Our data reveal an unexpected diversity in the self‐association mechanism of TRIMs that might be crucial for their biological function.     

Identification of a novel E3 ubiquitin ligase that is required for suppression of premature senescence in Arabidopsis

During leaf senescence, resources are recycled by redistribution to younger leaves and reproductive organs. Candidate pathways for the regulation of onset and progression of leaf senescence include ubiquitin‐dependent turnover of key proteins. Here, we identified a novel plant U‐box E3 ubiquitin ligase that prevents premature senescence in Arabidopsis plants, and named it SENESCENCE‐ASSOCIATED E3 UBIQUITIN LIGASE 1 (SAUL1). Using in vitro ubiquitination assays, we show that SAUL1 has E3 ubiquitin ligase activity. We isolated two alleles of saul1 mutants that show premature senescence under low light conditions. The visible yellowing of leaves is accompanied by reduced chlorophyll content, decreased photochemical efficiency of photosystem II and increased expression of senescence genes. In addition, saul1 mutants exhibit enhanced abscisic acid (ABA) biosynthesis. We show that application of ABA to Arabidopsis is sufficient to trigger leaf senescence, and that this response is abolished in the ABA‐insensitive mutants abi1‐1 and abi2‐1, but enhanced in the ABA‐hypersensitive mutant era1‐3. We found that increased ABA levels coincide with enhanced activity of Arabidopsis aldehyde oxidase 3 (AAO3) and accumulation of AAO3 protein in saul1 mutants. Using label transfer experiments, we showed that interactions between SAUL1 and AAO3 occur. This suggests that SAUL1 participates in targeting AAO3 for ubiquitin‐dependent degradation via the 26S proteasome to prevent premature senescence.     

Ubiquitin‐Activated Interaction Traps (UBAITs) identify E3 ligase binding partners

We describe a new class of reagents for identifying substrates, adaptors, and regulators of HECT and RING E3s. UBAITs (Ub iquitin‐A ctivated I nteraction T raps) are E3‐ubiquitin fusion proteins and, in an E1‐ and E2‐dependent manner, the C‐terminal ubiquitin moiety forms an amide linkage to proteins that interact with the E3, enabling covalent co‐purification of the E3 with partner proteins. We designed UBAITs for both HECT (Rsp5, Itch) and RING (Psh1, RNF126, RNF168) E3s. For HECT E3s, trapping of interacting proteins occurred in vitro either through an E3 thioester‐linked lariat intermediate or through an E2 thioester intermediate, and both WT and active‐site mutant UBAITs trapped known interacting proteins in yeast and human cells. Yeast Psh1 and human RNF126 and RNF168 UBAITs also trapped known interacting proteins when expressed in cells. Human RNF168 is a key mediator of ubiquitin signaling that promotes DNA double‐strand break repair. Using the RNF168 UBAIT, we identify H2AZ—a histone protein involved in DNA repair—as a new target of this E3 ligase. These results demonstrate that UBAITs represent powerful tools for profiling a wide range of ubiquitin ligases.     

A plant‐specific in vitro ubiquitination analysis system

Protein ubiquitination requires the concerted action of three enzymes: ubiquitin‐activating enzyme (E1), ubiquitin‐conjugating enzyme (E2) and ubiquitin ligase (E3). These ubiquitination enzymes belong to an abundant protein family that is encoded in all eukaryotic genomes. Describing their biochemical characteristics is an important part of their functional analysis. It has been recognized that various E2/E3 specificities exist, and that detection of E3 ubiquitination activity in vitro may depend on the recruitment of E2s. Here, we describe the development of an in vitro ubiquitination system based on proteins encoded by genes from Arabidopsis. It includes most varieties of Arabidopsis E2 proteins, which are tested with several RING‐finger type E3 ligases. This system permits determination of E3 activity in combination with most of the E2 sub‐groups that have been identified in the Arabidopsis genome. At the same time, E2/E3 specificities have also been explored. The components used in this system are all from plants, particularly Arabidopsis, making it very suitable for ubiquitination assays of plant proteins. Some E2 proteins that are not easily expressed in Escherichia coli were transiently expressed and purified from plants before use in ubiquitination assays. This system is also adaptable to proteins of species other than plants. In this system, we also analyzed two mutated forms of ubiquitin, K48R and K63R, to detect various types of ubiquitin conjugation.     

Characterization of three Arabidopsis homologs of human RING membrane anchor E3 ubiquitin ligase

Ubiquitination affects diverse physiological processes in eukaryotic cells. AtRMA1 was previously identified as an Arabidopsis homolog of human RING membrane-anchor E3 ubiquitin (Ub) ligase. Here, we identified two additional AtRMA homologs, AtRMA2 and AtRMA3. The predicted AtRMA proteins contain a RING motif and a trans-membrane domain in their N-terminal and extreme C-terminal regions, respectively. Bacterially expressed AtRMAs exhibited E3 ligase activity in vitro, which was abrogated by mutation of the conserved cysteine residue in their RING domains. In vivo targeting experiments using an Arabidopsis protoplast-transfection system showed that all three AtRMAs are localized to the ER. Although RT-PCR analysis indicated that AtRMA mRNAs were expressed constitutively in all tissues examined, their promoter activities were differentially detected in a tissue-specific fashion in AtRMA-promoter::GUS transgenic Arabidopsis plants. The AtRMA1 and AtRMA3 genes are predominantly expressed in major tissues, such as cotyledons, leaves, shoot–root junction, roots, and anthers, while AtRMA2 expression is restricted to the root tips and leaf hydathodes. We suggest that a ubiquitnation pathway involving these AtRMA E3 Ub ligases may play a role in the growth and development of Arabidopsis.     

An E3 ubiquitin ligase from Nicotiana benthamiana targets the replicase of Bamboo mosaic virus and restricts its replication

One up-regulated host gene identified previously was found involved in the infection process of Bamboo mosaic virus (BaMV), a single-stranded positive-sense RNA virus. The full length cDNA of this gene was cloned by 5′ and 3′-rapid amplification of cDNA ends and found to encode a polypeptide containing a conserved really interesting new gene (RING) domain and a transmembrane domain. The gene might function as an ubiquitin E3 ligase. We designated this protein in Nicotiana benthamiana as ubiquitin E3 ligase containing RING domain 1 (NbUbE3R1). Further characterization by using Tobacco rattle virus-based virus-induced gene silencing (loss-of-function) revealed that increased BaMV accumulation was in both knockdown plants and protoplasts. The gene might have a defensive role in the replication step of BaMV infection. To further inspect the functional role of NbUbE3R1 in BaMV accumulation, NbUbE3R1 was expressed in N. benthamiana plants. The wild-type NbUbE3R1-orange fluorescent protein (NbUbE3R1-OFP), NbUbE3R1/△TM-OFP (removal of the transmembrane domain) and NbUbE3R1/mRING-OFP (mutation at the RING domain, the E2 interaction site) were transiently expressed in plants. NbUbE3R1 and its derivatives all functioned in restricting the accumulation of BaMV. The common feature of these constructs was the intact substrate-interacting domain. Yeast two-hybrid and co-immunoprecipitation experiments used to determine the possible viral-encoded substrate of NbUbE3R1 revealed the replicase of BaMV as the possible substrate. In conclusion, we identified an up-regulated gene, NbUbE3R1 that plays a role in BaMV replication.