{"gene":"UBE2N","run_date":"2026-06-10T10:51:56","timeline":{"discoveries":[{"year":2001,"finding":"Crystal structure of the Mms2/Ubc13 heterodimer reveals that the active site of Ubc13 sits at the intersection of two channels that are potential binding sites for the two substrate ubiquitins; mutations that destabilize the heterodimer interface confer marked UV sensitivity, demonstrating that the intact heterodimer is necessary for DNA repair.","method":"X-ray crystallography + mutagenesis + UV sensitivity assay","journal":"Cell","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structure with mutagenesis and functional validation, foundational study replicated by multiple labs","pmids":["11440714"],"is_preprint":false},{"year":2001,"finding":"Human Mms2 and Ubc13 form a stable heterodimer; the activated heterodimer transfers ubiquitin via the Ubc13 active-site thioester exclusively to Lys63 of an untethered acceptor ubiquitin; NMR mapping identifies a surface on acceptor ubiquitin that interacts with Mms2, indicating Mms2 orients the acceptor ubiquitin to place Lys63 near the Ubc13 active site.","method":"In vitro ubiquitin thioester/chain-assembly assay + 1H-15N HSQC NMR mapping","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — reconstituted in vitro activity with structural NMR mapping; multiple orthogonal methods in one study; independently replicated","pmids":["11504715"],"is_preprint":false},{"year":2003,"finding":"NMR-based structural model of the human Mms2·Ubc13 heterodimer bound to both acceptor and donor ubiquitins defines the molecular basis for Lys63-linked chain synthesis; thermodynamic/kinetic measurements show Mms2 and Ubc13 interact with a Kd ~49 nM and the heterodimer binds acceptor ubiquitin with a Kd ~28 µM, markedly tighter than Mms2 alone (~98 µM).","method":"1H-15N NMR spectroscopy + isothermal titration calorimetry","journal":"The Journal of biological chemistry / Biochemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — NMR structural model with quantitative ITC thermodynamics; two orthogonal methods; consistent with crystal structures","pmids":["12569095","12834344"],"is_preprint":false},{"year":2005,"finding":"Mms2 and Ubc13 interact through a single key Mms2 residue that inserts into a pocket on Ubc13; structure-guided mutations at the heterodimer interface abolish Lys63-linked polyubiquitination and DNA repair complementation, demonstrating that specific interface residues determine E2-variant selectivity.","method":"Yeast two-hybrid, GST pull-down, surface plasmon resonance, in vitro ubiquitin conjugation, functional complementation","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — multiple orthogonal methods (structural, biochemical, genetic) in one study with mutagenesis validation","pmids":["15749714"],"is_preprint":false},{"year":2005,"finding":"Mammalian Ubc13 pairs with two distinct UEV proteins (Mms2 and Uev1A) that direct it to different cellular processes: Ubc13-Mms2 is required for DNA damage repair but not NF-κB activation, whereas Ubc13-Uev1A is required for NF-κB activation but not DNA repair; the two UEVs also differentially modulate the length of Lys63-linked polyubiquitin chains.","method":"Functional complementation in yeast, NF-κB reporter assays, siRNA knockdown, in vitro ubiquitin chain-length analysis","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — epistasis/functional separation with multiple orthogonal methods, independently corroborated by structural studies","pmids":["16129784"],"is_preprint":false},{"year":2006,"finding":"Crystal structure of the Mms2-Ubc13-ubiquitin covalent intermediate (donor ubiquitin linked to Ubc13 active-site Cys) reveals at atomic resolution how Mms2 positions the acceptor ubiquitin Lys63 into the Ubc13 active site for selective chain elongation.","method":"X-ray crystallography of covalent UEV-E2-Ub intermediate","journal":"Nature structural & molecular biology","confidence":"High","confidence_rationale":"Tier 1 / Strong — atomic crystal structure of the catalytic intermediate; direct mechanistic visualization","pmids":["16980971"],"is_preprint":false},{"year":2006,"finding":"Conditional ablation of Ubc13 in B cells and macrophages causes defective B cell development and impaired activation; in response to all tested stimuli except TNF, Ubc13-deficient cells show near-normal NF-κB activation but markedly impaired MAP kinase activation; Ubc13-induced MAP kinase activation requires ubiquitination of the adaptor IKKγ (NEMO).","method":"Conditional knockout mice, NF-κB and MAPK signaling assays, ubiquitination assays","journal":"Nature immunology","confidence":"High","confidence_rationale":"Tier 2 / Strong — clean conditional KO with defined cellular phenotypes and pathway placement; replicated in multiple cell types","pmids":["16862162"],"is_preprint":false},{"year":2006,"finding":"RNF8, an FHA-RING ubiquitin ligase, physically interacts with Ubc13 and is required, together with Ubc13, to recruit the Rap80/Abraxas/BRCA1/BRCC36 A complex to DNA damage foci; this constitutes a sequential E3 ubiquitin ligase cascade generating K63-linked polyubiquitin chains at damage sites.","method":"siRNA knockdown, Co-IP, immunofluorescence focus formation assay","journal":"Proceedings of the National Academy of Sciences","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal Co-IP, genetic knockdown with defined pathway placement; replicated across multiple subsequent studies","pmids":["18077395"],"is_preprint":false},{"year":2006,"finding":"SHPRH, the human Rad5 homolog, physically interacts with both Rad6-Rad18 and Mms2-Ubc13 complexes and acts as the E3 ubiquitin ligase indispensable for Mms2-Ubc13-dependent polyubiquitination of PCNA.","method":"Co-IP, in vitro ubiquitination assay, siRNA knockdown","journal":"Proceedings of the National Academy of Sciences","confidence":"High","confidence_rationale":"Tier 1–2 / Moderate — reconstituted in vitro ubiquitination plus Co-IP; multiple orthogonal methods","pmids":["17108083"],"is_preprint":false},{"year":2006,"finding":"The RING finger protein RNF8 binds UBC13 through its RING domain and stimulates K63-linked (and K48-linked) self-polyubiquitylation; RNF8 co-localizes with UBC13 in the nucleus.","method":"Co-IP/pull-down, in vitro ubiquitination assay, immunofluorescence co-localization","journal":"Journal of cellular biochemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — Co-IP plus in vitro assay; single lab, two orthogonal methods","pmids":["16215985"],"is_preprint":false},{"year":2006,"finding":"Ubc13 elicits K63-linked ubiquitination of p53, which attenuates Hdm2-induced polyubiquitination; Ubc13 increases p53 stability while decreasing its transcriptional activity and promoting its cytoplasmic localization; these effects require Ubc13 catalytic activity and involve increasing monomeric (non-tetrameric) p53.","method":"Co-IP, K63-specific ubiquitination assay, subcellular fractionation, reporter assay, mutagenesis","journal":"Molecular and cellular biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal methods (Co-IP, fractionation, reporter) in one lab; Ubc13 catalytic mutant controls included","pmids":["17000756"],"is_preprint":false},{"year":2007,"finding":"Disruption or siRNA depletion of UBC13 in DT40 or human cells causes chromosome instability, hypersensitivity to UV and ionizing radiation, and impaired DNA double-strand break repair by homologous recombination; specifically, BRCA1 E3 ligase function activation, Rad51 nucleoprotein filament formation, and ssDNA/RPA complex generation at DSBs are abolished in Ubc13-deficient cells.","method":"Gene disruption (DT40), siRNA knockdown, HR assay, immunofluorescence (BRCA1/Rad51/RPA foci)","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic disruption and siRNA with multiple defined phenotypic readouts in two cell systems; pathway placement by epistasis","pmids":["17349954"],"is_preprint":false},{"year":2007,"finding":"Ubc13 is an essential component of TRAF-mediated inflammatory signaling; heterozygous Ubc13 knockout mice show reduced TRAF6 ubiquitination in vivo, reduced cytokine secretion, and impaired NF-κB, JNK, and p38 MAPK activation in macrophages/splenocytes; homozygous knockout is embryonic lethal.","method":"Gene ablation in mice, in vivo ubiquitination assay, cytokine ELISA, signaling pathway analysis","journal":"Proceedings of the National Academy of Sciences","confidence":"High","confidence_rationale":"Tier 2 / Strong — clean knockout with multiple defined signaling and phenotypic readouts; in vivo ubiquitination confirmed","pmids":["17404240"],"is_preprint":false},{"year":2003,"finding":"TNF-induced GCKR and SAPK/JNK activation depends on TRAF2 and the Ubc13-Uev1A complex; Ubc13 interference inhibits TNF- and TRAF2-mediated GCKR and SAPK activation; TNF signaling leads to TRAF2 K63-linked polyubiquitination and oligomerization, and GCKR ubiquitination and activation, all sensitive to Ubc13 disruption.","method":"Dominant-negative interference, siRNA knockdown, co-IP, ubiquitination assay, kinase activity assay","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Strong — dominant-negative plus siRNA knockdown, multiple orthogonal readouts; pathway positioning established","pmids":["12591926"],"is_preprint":false},{"year":2007,"finding":"The HTLV-1 Tax oncoprotein is K63-polyubiquitinated in a Ubc13-dependent manner; Tax interacts with Ubc13; Ubc13 knockdown abrogates Tax ubiquitination and NF-κB activation; Tax ubiquitination is required for its interaction with NEMO.","method":"Co-IP, siRNA knockdown, K63-specific ubiquitination assay, NF-κB reporter assay","journal":"Journal of virology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — Co-IP plus siRNA knockdown and ubiquitination assay; single lab, multiple orthogonal methods","pmids":["17942533"],"is_preprint":false},{"year":2005,"finding":"ISG15 covalently modifies Ubc13 at Lys92, and this ISGylation suppresses Ubc13's ability to form a thioester intermediate with ubiquitin, thereby inhibiting its ubiquitin-conjugating activity.","method":"In vitro ISGylation assay, ubiquitin thioester formation assay, biochemical purification of ISGylated Ubc13","journal":"Biochemical and biophysical research communications (two independent papers)","confidence":"High","confidence_rationale":"Tier 1 / Strong — direct biochemical assay of thioester formation, modification site identified by mass spectrometry; independently replicated by two labs in same year","pmids":["16112642","16122702"],"is_preprint":false},{"year":2004,"finding":"The TRAF6 RING finger domain directly binds Ubc13; either a single Cys-to-Ser substitution in the TRAF6 RING or a surface mutation on Ubc13 predicted to contact RING finger proteins abolishes the interaction; TRAF6 also self-interacts through its N-terminal RING-containing domain.","method":"Yeast two-hybrid, mutagenesis","journal":"FEBS letters","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — yeast two-hybrid interaction with mutagenesis; single lab, relatively limited in vitro validation","pmids":["15147900"],"is_preprint":false},{"year":2008,"finding":"The RNF8 RING domain anchors UBC13 at DNA damage sites independently of E2 variant proteins (Mms2/Uev1A); RNF8-UBC13 without E2 variants is sufficient to catalyze ubiquitin conjugation and promote 53BP1 accumulation at DSBs; only RING domains from UBC13-binding E3s enable this activity.","method":"Mutagenesis, immunofluorescence foci assay, in vitro ubiquitination assay, siRNA knockdown","journal":"Molecular and cellular biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple in vitro and cellular assays; single lab; challenges prior model of obligate E2V requirement","pmids":["18678647"],"is_preprint":false},{"year":2009,"finding":"The Rap80-BRCC36 deubiquitinating enzyme complex antagonizes RNF8-Ubc13-dependent ubiquitination at DSBs; BRCC36 knockdown or catalytic mutant restores 53BP1 recruitment and γH2AX ubiquitination following RNF8 depletion, revealing that opposing RNF8-Ubc13 ligase and Rap80-BRCC36 DUB activities determine steady-state ubiquitin levels at DSBs.","method":"siRNA knockdown, expression of catalytic mutants, immunofluorescence, epistasis analysis","journal":"Proceedings of the National Academy of Sciences","confidence":"High","confidence_rationale":"Tier 2 / Strong — epistatic analysis with catalytic mutants and multiple readouts; pathway antagonism directly demonstrated","pmids":["19202061"],"is_preprint":false},{"year":2009,"finding":"JNK phosphorylates p53 at Thr81 on polysomes, which is required for dissociation of Ubc13 from p53; without JNK activity or with a non-phosphorylatable p53 T81 mutant, the Ubc13-p53 complex is maintained, inhibiting p53 multimerization and transcriptional activation; thus JNK and Ubc13 cooperate to regulate p53 multimerization on polysomes.","method":"Co-IP, polysome fractionation, site-directed mutagenesis of p53 T81, JNK inhibition","journal":"Proceedings of the National Academy of Sciences","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal methods (Co-IP, fractionation, mutagenesis) in one lab","pmids":["19651615"],"is_preprint":false},{"year":2009,"finding":"Conditional knockout of the Ubc13 gene causes severe loss of hematopoietic stem cells and immune cell lineages, thymus/bone marrow atrophy, and mouse lethality; loss of Ubc13 results in accumulation of β-catenin and hyperexpression of Wnt target genes, placing Ubc13 upstream of Wnt signaling in hematopoietic stem cell maintenance.","method":"Conditional knockout mice, flow cytometry, Western blot for β-catenin, Wnt target gene expression","journal":"Proceedings of the National Academy of Sciences","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — clean conditional KO with defined phenotype and pathway (Wnt/β-catenin) placement; single lab","pmids":["19926860"],"is_preprint":false},{"year":2012,"finding":"The Shigella effector OspI is a glutamine deamidase that specifically deamidates Gln100 of UBC13 to glutamic acid (Q100E), abolishing its E2 ubiquitin-conjugating activity required for TRAF6 activation, thereby dampening NF-κB inflammatory signaling; crystal structure of OspI reveals a Cys-His-Asp catalytic triad required for deamidation.","method":"Crystal structure of OspI (2.0 Å), mass spectrometry identification of Q100E, in vitro E2 activity assay, catalytic triad mutagenesis, NF-κB signaling assay","journal":"Nature","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structure, identification of modification site by MS, biochemical activity assay, mutagenesis validation; multiple orthogonal methods","pmids":["22407319"],"is_preprint":false},{"year":2012,"finding":"Crystal structure of RNF8 RING domain in complex with Ubc13/Mms2 (ternary complex) reveals that RNF8 dimerizes via a coiled-coil and binds Ubc13/Mms2 to stimulate K63 ubiquitin chain formation; RNF168, in contrast, is a RING monomer and does not catalyze K63 polyubiquitylation with Ubc13/Mms2.","method":"X-ray crystallography, in vitro ubiquitination assay, mutagenesis disrupting RNF8-Ubc13 interface or RNF8 coiled-coil","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structure with mutagenesis and in vitro reconstitution; multiple orthogonal methods","pmids":["22589545"],"is_preprint":false},{"year":2012,"finding":"Crystal structure of human OTUB1 in complex with UBC13 and MMS2 (3.15 Å) shows OTUB1 inhibits UBC13 E2 activity non-catalytically; OTUB1 strongly suppresses K63-linked tri-ubiquitin but allows di-ubiquitin production by capping the di-ubiquitin on the UBC13-MMS2 complex; structure-guided OTUB1 mutants that disrupt the UBC13 interface fail to inhibit K63 ubiquitination in vitro and in vivo.","method":"X-ray crystallography (3.15 Å), surface plasmon resonance, mutagenesis, in vitro ubiquitination assay, cellular DSB response assay","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structure, SPR, mutagenesis, in vitro and in vivo functional validation; multiple orthogonal methods","pmids":["22679021"],"is_preprint":false},{"year":2012,"finding":"Ubc13-specific ablation in regulatory T cells impairs their suppressive function in vivo and renders them susceptible to acquiring TH1 and TH17 effector phenotypes; this function involves downstream IKK activation; the Ubc13-IKK axis controls IL-10 and SOCS1 expression in Treg cells.","method":"Treg cell-specific conditional knockout mice, in vivo suppression assay, cytokine profiling, kinase activity assay","journal":"Nature immunology","confidence":"High","confidence_rationale":"Tier 2 / Strong — clean cell-type-specific conditional KO with defined cellular phenotype and pathway placement; in vivo","pmids":["22484734"],"is_preprint":false},{"year":2012,"finding":"The small molecule NSC697923 inhibits Ubc13-Uev1A by blocking formation of the Ubc13-ubiquitin thioester conjugate, suppresses constitutive NF-κB activity in DLBCL cells, and inhibits DLBCL cell proliferation and survival.","method":"Biochemical thioester formation assay, NF-κB reporter, cell proliferation/viability assay, siRNA knockdown","journal":"Blood","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct biochemical mechanism (thioester inhibition) and cellular effects; single lab","pmids":["22791293"],"is_preprint":false},{"year":2013,"finding":"Crystal structures of OspI-Ubc13 complexes (2.96 Å and 2.3 Å) show OspI uses hydrophobic and charged surfaces to engage the α1 helix, L1 and L2 loops of Ubc13, with Gln100 positioned in the OspI catalytic pocket; Ubc13 binding induces structural rearrangement of the OspI catalytic pocket; the OspI-binding surface on Ubc13 largely overlaps with E3 ligase and DUB binding surfaces.","method":"X-ray crystallography (2.96 Å and 2.3 Å), mutagenesis, binding assays","journal":"Journal of molecular biology / PLoS pathogens","confidence":"High","confidence_rationale":"Tier 1 / Strong — two independent crystal structures from two labs at high resolution with mutagenesis validation","pmids":["23542009","23633953"],"is_preprint":false},{"year":2013,"finding":"UBC13 mediates K63-linked ubiquitination of TAK1 at Lys158 during H. pylori CagA-induced NF-κB activation; mutation of TAK1 K158R prevents its ubiquitination and impairs NF-κB activation; dominant-negative Ubc13 or siRNA knockdown abolishes CagA-facilitated TAK1 and NF-κB activation.","method":"Site-directed mutagenesis (TAK1 K158R), siRNA knockdown, dominant-negative Ubc13, ubiquitination assay, NF-κB reporter","journal":"Journal of cellular biochemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — mutagenesis identifies specific ubiquitination site; dominant-negative and siRNA convergent; single lab","pmids":["23606331"],"is_preprint":false},{"year":2013,"finding":"MDC1 is ubiquitylated on Lys1977 of its tandem BRCT domain in a UBC13-dependent manner, and this ubiquitylation is required for direct MDC1 binding to RAP80 through RAP80's ubiquitin-interacting motifs.","method":"Co-IP, UBC13 knockdown, mutagenesis (K1977 identification), immunofluorescence","journal":"DNA repair","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — Co-IP with UBC13 knockdown and site identification; single lab","pmids":["21622030"],"is_preprint":false},{"year":2014,"finding":"UBE2N knockdown specifically prevents K63-linked ubiquitylation at mitochondrial sites during PINK1/Parkin-dependent mitophagy; combined knockdown of UBE2N, UBE2L3, and UBE2D2/3 substantially reduces mitochondrial polyubiquitylation, p62 recruitment, and autophagic clearance of depolarized mitochondria.","method":"siRNA knockdown, linkage-specific ubiquitin antibodies, immunofluorescence, autophagic flux assay","journal":"Journal of cell science","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — siRNA knockdown with linkage-specific antibodies; single lab; combined knockdown approach","pmids":["24906799"],"is_preprint":false},{"year":2014,"finding":"Ubc13 is required for breast cancer metastasis but is largely dispensable for primary tumor growth; Ubc13 is required for TGFβ-induced TAK1 and p38 MAPK activation (non-SMAD signaling) to control metastasis-promoting gene expression; pharmacological p38 inhibition attenuates BCa metastasis in mice.","method":"Conditional knockout in mouse model, lung colonization assay, TGFβ signaling analysis (SMAD vs. non-SMAD), p38 inhibitor in vivo","journal":"Proceedings of the National Academy of Sciences","confidence":"High","confidence_rationale":"Tier 2 / Strong — clean conditional KO with in vivo metastasis assay, pathway placement via SMAD/non-SMAD dissection; in vivo pharmacological rescue","pmids":["25189770"],"is_preprint":false},{"year":2014,"finding":"STAT3 acts as a transcriptional repressor of the Ube2n (Ubc13) gene; in RANKL-activated macrophages, STAT3 is stimulated by autocrine IL-6 and inhibits Ets-1/Set1 methyltransferase/H3K4me3 accumulation at the Ube2n promoter; depletion of Ubc13 in Stat3-deficient macrophages suppresses excessive TRAF6-mediated K63 ubiquitination and NF-κB responses.","method":"ChIP (Ets-1, Set1, H3K4me3 at Ube2n promoter), siRNA knockdown, NF-κB reporter, macrophage activation assay","journal":"Nature communications","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — ChIP with functional knockdown epistasis; single lab; multiple readouts","pmids":["25503582"],"is_preprint":false},{"year":2015,"finding":"NSC697923 and BAY 11-7082 both inhibit Ubc13 by covalent Michael addition at the Ubc13 active-site cysteine; crystal structures of both Ubc13-inhibitor adducts reveal that both exploit a binding groove unique to Ubc13; a Ubc13 mutant resistant to NSC697923 shows that NF-κB and DNA damage signaling inhibition by NSC697923 is largely due to specific Ubc13 inhibition.","method":"X-ray crystallography of inhibitor-Ubc13 adducts, resistance mutant, cellular NF-κB and DNA damage signaling assays","journal":"ACS chemical biology","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structures plus resistance mutant demonstrating on-target specificity; multiple orthogonal methods","pmids":["25909880"],"is_preprint":false},{"year":2016,"finding":"RNF8 stimulates Ubc13 polyubiquitination activity by modulating the conformation of ubiquitin covalently linked to the Ubc13 active site; crystal structure of the activated RNF8-Ubc13~ubiquitin complex shows this allosteric activation; structure-guided separation-of-function mutations that impair E2 stimulation abolish DSB signaling, 53BP1 recruitment, and specifically BRCA1 (but not 53BP1) recruitment by chromatin-targeted RNF168.","method":"X-ray crystallography + SAXS solution conformation, separation-of-function mutagenesis, cellular immunofluorescence","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structure, SAXS, and separation-of-function mutations with cellular validation; multiple orthogonal methods","pmids":["26903517"],"is_preprint":false},{"year":2016,"finding":"GPS2 (G-protein Pathway Suppressor 2) directly inhibits Ubc13 enzymatic activity in B cells; GPS2 B cell-specific deletion causes developmental defects at multiple stages of B cell differentiation by derepressing TLR, BCR, and AKT/FOXO1 signaling through elevated Ubc13-mediated K63 ubiquitination.","method":"In vitro Ubc13 activity assay with GPS2, conditional knockout mice, B cell developmental analysis","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct in vitro enzymatic inhibition demonstrated plus in vivo KO phenotype; single lab","pmids":["28039360"],"is_preprint":false},{"year":2017,"finding":"In cerebellar granule neurons, RNF8 and UBC13 form a cytoplasmic (not nuclear) complex that suppresses synapse differentiation in vivo; knockdown or conditional KO increases parallel fiber presynaptic boutons and functional synapses; RNF8 interacts with HERC2 and NEURL4, and knockdown of these phenocopies RNF8/UBC13 loss.","method":"In vivo knockdown, conditional knockout, electrophysiology (PF/PC synapse), proteomics, immunofluorescence","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 2 / Strong — in vivo KO with electrophysiological and morphological validation, proteomics for complex identification; multiple orthogonal methods","pmids":["29097665"],"is_preprint":false},{"year":2017,"finding":"TRAF6 coiled-coil (CC) domain mediates TRAF6 oligomerization, which primes interaction with the Ubc13~Ub conjugate (not unloaded Ubc13); this interaction is required for processive assembly of long K63-linked polyubiquitin chains and TAK1 activation in IL-1R/TLR pathways; fusion of the CC domain to CHIP/STUB1 confers NF-κB activation capacity.","method":"Mutagenesis of CC domain, in vitro ubiquitination processivity assay, co-IP, NF-κB reporter, domain fusion experiments","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — reconstituted in vitro processivity assay with mutagenesis, domain fusion gain-of-function, multiple orthogonal methods","pmids":["28993672"],"is_preprint":false},{"year":2017,"finding":"Ubc13 K63-linked ubiquitination of RHBDF2 (iRhom2) is promoted by the Uev1A-Ubc13 complex together with CHIP, facilitating TACE maturation and subsequent shedding of the TNFα receptor, thereby acting as a negative regulator of TNFα-induced NF-κB signaling.","method":"Co-IP, in vitro ubiquitination assay, TACE maturation assay, NF-κB reporter","journal":"Cellular signalling","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — Co-IP and in vitro ubiquitination; single lab, multiple readouts","pmids":["29069608"],"is_preprint":false},{"year":2017,"finding":"Ube2N preferentially facilitates production of unanchored K63-linked polyubiquitin chains (while Ube2D3 promotes covalent conjugation) downstream of RIG-I/Riplet; both types of polyubiquitin chains are required for RIG-I to induce MAVS aggregation on mitochondria, triggering innate immune signaling.","method":"Chromatographic purification, in vitro ubiquitination assay, MAVS aggregation assay, siRNA knockdown","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — biochemical purification, reconstituted in vitro assay, mechanistic dissection of chain type; multiple orthogonal methods","pmids":["28469175"],"is_preprint":false},{"year":2018,"finding":"Legionella pneumophila effector MavC is a transglutaminase that catalyzes monoubiquitination of UBE2N by covalent crosslinking of ubiquitin Gln40 to UBE2N Lys92 (and Lys94) via a γ-glutamyl-ε-Lys isopeptide bond; the catalytic residue is Cys74; this modification abolishes UBE2N's ability to form K63-type polyubiquitin chains and dampens NF-κB signaling.","method":"Biochemical transglutaminase assay, mass spectrometry (modification site), mutagenesis of Cys74, K63-chain formation assay, NF-κB reporter","journal":"Nature microbiology","confidence":"High","confidence_rationale":"Tier 1 / Strong — novel enzymatic mechanism defined, modification sites identified by MS, catalytic residue mutated; multiple orthogonal methods","pmids":["30420781"],"is_preprint":false},{"year":2019,"finding":"MavC targets the thioester-linked Ube2N~ubiquitin conjugate (not free Ube2N) for intramolecular transglutamination; ubiquitin shows increased affinity for MavC when tethered to Ube2N; crystal structures of MavC with substrate mimic and crosslinked product reveal the insertion domain is crucial for substrate recognition and that transamidation is favored over deamidation.","method":"Crystal structures of MavC-Ube2N~Ub complexes, biochemical transglutamination/deamidation assay, binding affinity measurements","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structures of substrate and product complexes with reconstituted biochemical assays; two labs converged on mechanism","pmids":["32398758"],"is_preprint":false},{"year":2019,"finding":"A tri-ionic motif in TRIM21 (and TRIM5) RING domain provides anchor points that wrap the Ube2N~Ub complex around the RING, locking the closed conformation required for ubiquitin discharge; mutation of these anchor points specifically inhibits ubiquitination with Ube2N/Ube2V2 but not Ube2D1, establishing an E2-specific catalytic mechanism for this class of RING E3s.","method":"NMR, mutagenesis, in vitro ubiquitination assay, viral neutralization assay, immune signaling assay","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1 / Strong — NMR structural data with mutagenesis and multiple functional assays; E2-specificity mechanism defined","pmids":["31582740"],"is_preprint":false},{"year":2019,"finding":"L. pneumophila effector MvcA (50% identity to MavC) uses its deamidase catalytic triad to remove the MavC-installed ubiquitin from UBE2N (deubiquitination), restoring UBE2N activity; structural analysis of MvcA-UBE2N-Ub reveals the insertion domain is critical for substrate recognition; this temporal regulation of UBE2N activity is required for efficient intracellular bacterial replication.","method":"Biochemical deubiquitination assay, crystal/structural analysis, mutagenesis of catalytic triad, bacterial replication assay","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 1 / Strong — reconstituted enzymatic reversal of modification, structural analysis, mutagenesis, functional validation","pmids":["31825121"],"is_preprint":false},{"year":2019,"finding":"Deamidation of UBC13 at Gln100 (Q100E by OspI) inhibits interaction with TRAF6 RING domain by forming a new intramolecular salt bridge in UBC13 that competes with a critical intermolecular salt bridge at the UBC13/TRAF6 RING interface; this additionally prevents transient interactions needed for the closed E2-RING complex.","method":"NMR chemical shift perturbation, mutagenesis, binding affinity measurement, in vitro ubiquitination assay","journal":"eLife","confidence":"High","confidence_rationale":"Tier 1 / Strong — NMR structural analysis with mutagenesis revealing salt-bridge competition mechanism; multiple orthogonal methods","pmids":["31638574"],"is_preprint":false},{"year":2020,"finding":"OTUB1 deubiquitinase is stabilized by K48-linked deubiquitination in dendritic cells, leading to increased K63-linked ubiquitination of IRAK1 and TRAF6 via UBC13, thereby augmenting NF-κB activation and inflammatory cytokine production; DC-specific OTUB1 deletion impairs IL-12 production and immune defense against T. gondii.","method":"Conditional knockout mice, co-IP, linkage-specific ubiquitination assay, cytokine measurement, infection model","journal":"Cellular & molecular immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — Co-IP plus conditional KO with multiple in vivo readouts; single lab","pmids":["32024978"],"is_preprint":false},{"year":2020,"finding":"UBC13-mediated K63-linked polyubiquitination promotes MRE11 recruitment to TOP2-DNA adduct-blocked DSBs via RAP80/BRCA1 localization and BRCA1-MRE11 complex formation, facilitating nucleolytic removal of blocking adducts before NHEJ; UBC13 and MRE11 are dispensable for repair of clean DSBs but responsible for >50% and >70% of NHEJ-dependent repair of radiation-induced dirty DSBs, respectively.","method":"Auxin-inducible degron knockdown, NHEJ assay, immunofluorescence (RAP80, BRCA1, MRE11 foci), epistasis analysis","journal":"iScience","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — clean degron depletion with multiple cellular assays; pathway placement by epistasis; single lab","pmids":["32283528"],"is_preprint":false},{"year":2021,"finding":"Ubc13 associates with NLRP3 and promotes K63-linked polyubiquitination at Lys565 and Lys687 of NLRP3; Ubc13 knockdown/knockout or catalytic inhibition dramatically impairs NLRP3 inflammasome activation in macrophages, indicating K63 ubiquitination of NLRP3 is required for its activation.","method":"Co-IP, siRNA/CRISPR knockout, mass spectrometry (ubiquitination site identification), inflammasome activation assay (IL-1β, ASC speck formation)","journal":"Journal of immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — Co-IP, MS site identification, KO with functional readout; single lab","pmids":["33893171"],"is_preprint":false},{"year":2021,"finding":"LGP2 helicase inhibits K63-linked polyubiquitination by directly associating with and sequestering Ubc13/UBE2N via its Hel2i subdomain, thereby inactivating multiple K63-Ub ligases (TRAF6, TRIM25, RNF125) and broadly suppressing innate immune signaling.","method":"Co-IP/pull-down of LGP2-Ubc13, in vitro ubiquitination assay, K63-Ub immunoblot, signaling reporter assay","journal":"Cell reports","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct interaction demonstrated and in vitro ubiquitination inhibition; single lab, multiple E3 substrates tested","pmids":["34965427"],"is_preprint":false},{"year":2021,"finding":"RNF213 is a K63-linked E3 ubiquitin ligase that interacts with UBC13 (identified by yeast two-hybrid with RNF213 RING domain as bait); RNF213 undergoes K63-linked autoubiquitination in a UBC13-dependent manner; this axis is required for angiogenic cell motility and invasion in HUVEC cells.","method":"Yeast two-hybrid, Co-IP, K63-specific ubiquitination assay, UBC13 knockdown, HUVEC cell migration/invasion assay","journal":"FASEB BioAdvances","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — yeast two-hybrid plus Co-IP plus ubiquitination assay; functional knockdown; single lab","pmids":["33842849"],"is_preprint":false},{"year":2022,"finding":"In AML, TRIM21 is identified as the E3 ligase that partners with activated UBE2N to modulate UBE2N-dependent proteostasis; UBE2N inhibition reduces levels of K63-ubiquitinated target proteins, leading to their increased K48-linked ubiquitination and degradation through the immunoproteasome; this is selective for immunoproteasome-positive AML cells.","method":"Interactome screen, proteomic analysis, whole-genome CRISPR-activation screen, K48/K63 ubiquitination assays, enzymatically defective mouse model","journal":"Science translational medicine / Journal of clinical investigation","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — interactome screen plus proteomics and CRISPR screen; enzymatically defective mouse model; single group","pmids":["35263148","40371639"],"is_preprint":false},{"year":2023,"finding":"SET7 methyltransferase methylates OTUB1 at Lys122; this methylation does not affect OTUB1 DUB activity but impairs its non-canonical binding to UBC13, relieving OTUB1-mediated suppression of K63-linked ubiquitination and promoting ferroptosis.","method":"In vitro methylation assay, Co-IP (OTUB1-UBC13 interaction), methylation-mimic mutant, cell viability and ROS assay","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct in vitro methylation and binding assay with mutant; functional consequence in cells; single lab","pmids":["36822329"],"is_preprint":false},{"year":2018,"finding":"FAM177A1 competitively binds TRAF6 and impairs its interaction with Ubc13, thereby inhibiting TRAF6-mediated K63 polyubiquitination, downstream recruitment of signaling molecules, and NF-κB activation in response to IL-1β.","method":"Co-IP, competition binding assay (TRAF6-Ubc13 displacement), NF-κB reporter, siRNA knockdown","journal":"Journal of immunology","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — Co-IP competition assay and siRNA knockdown; single lab; indirect inhibition mechanism","pmids":["34799425"],"is_preprint":false}],"current_model":"UBE2N (Ubc13) is the sole E2 ubiquitin-conjugating enzyme capable of synthesizing Lys63-linked polyubiquitin chains; it does so by forming a heterodimer with a catalytically inactive UEV protein (Mms2/UEV1A), wherein the UEV positions the acceptor ubiquitin Lys63 into the Ubc13 active site, as established by crystal structures including the covalent E2~Ub intermediate; different UEV partners (Mms2 vs. Uev1A) direct Ubc13 to distinct pathways (DNA damage repair vs. NF-κB/MAP kinase signaling, respectively); Ubc13 partners with multiple RING E3 ligases (RNF8, TRAF6, TRIM21, RNF213, SHPRH, and others) that recruit and allosterically activate it at specific cellular locations; its activity is negatively regulated by ISGylation at Lys92 (blocking thioester formation), by OTUB1 non-canonical binding (capping di-ubiquitin on the E2-UEV complex), by the Shigella effector OspI (deamidation of Gln100 disrupting E3 binding), by the Legionella effector MavC (transglutaminase-mediated monoubiquitination targeting the charged E2~Ub conjugate, reversed by MvcA), by GPS2 and LGP2 (direct sequestration), and at the transcriptional level by STAT3; Ubc13-generated K63 chains serve as non-proteolytic signals required for DNA DSB signaling (RNF8-dependent recruitment of BRCA1/RAP80), homologous recombination initiation, postreplication DNA repair (PCNA polyubiquitination), NF-κB and MAP kinase activation in multiple immune cell types, MAVS aggregation in antiviral signaling, NLRP3 inflammasome activation, p53 monomerization and cytoplasmic localization, mitophagy, and synapse suppression in neurons."},"narrative":{"mechanistic_narrative":"UBE2N (Ubc13) is the dedicated E2 ubiquitin-conjugating enzyme for Lys63-linked polyubiquitin chain synthesis, a non-proteolytic signal that drives DNA double-strand break responses, innate and adaptive immune signaling, and additional cellular programs [PMID:11504715, PMID:17349954, PMID:17404240]. Catalysis requires heterodimerization with a catalytically inactive ubiquitin-conjugating enzyme variant (UEV): Mms2 or Uev1A binds a pocket on Ubc13 and orients an untethered acceptor ubiquitin so that its Lys63 is positioned into the Ubc13 active site for selective chain elongation, as defined by NMR models and the crystal structure of the covalent UEV–Ubc13–ubiquitin intermediate [PMID:11440714, PMID:11504715, PMID:16980971]. The choice of UEV partner partitions Ubc13 between pathways — Ubc13-Mms2 supports DNA repair while Ubc13-Uev1A supports NF-κB activation — and tunes chain length [PMID:16129784]. Ubc13 is recruited and allosterically activated by multiple RING E3 ligases that wrap around the Ubc13~ubiquitin conjugate to lock the closed, discharge-competent conformation, including RNF8 in the DNA damage response, TRAF6 in inflammatory signaling, and TRIM21 [PMID:22589545, PMID:26903517, PMID:28993672, PMID:31582740]. In the nucleus, RNF8-Ubc13 builds K63 chains at break sites that recruit the RAP80/Abraxas/BRCA1/BRCC36 complex and 53BP1, license homologous recombination through BRCA1 activation and Rad51 filament formation, and is opposed by the Rap80-BRCC36 deubiquitinase to set steady-state ubiquitin levels [PMID:18077395, PMID:17349954, PMID:19202061]; Ubc13-Mms2 with the E3 SHPRH also polyubiquitinates PCNA in postreplication repair [PMID:17108083]. In immune cells, Ubc13 mediates TRAF6/TRAF2-dependent K63 ubiquitination to activate MAP kinases and NF-κB, controlling B cell, macrophage, and regulatory T cell function, and is essential for hematopoietic stem cell maintenance and embryonic viability [PMID:16862162, PMID:17404240, PMID:12591926, PMID:22484734, PMID:19926860]. Beyond canonical chains, Ube2N generates unanchored K63 polyubiquitin required for RIG-I-driven MAVS aggregation in antiviral signaling and promotes NLRP3 inflammasome activation [PMID:28469175, PMID:33893171]. Ubc13 activity is restrained by diverse mechanisms: ISGylation at Lys92 blocks thioester formation, OTUB1 non-catalytically caps di-ubiquitin on the E2-UEV complex, and the direct binders GPS2 and LGP2 sequester the enzyme [PMID:16112642, PMID:16122702, PMID:22679021, PMID:28039360, PMID:34965427]. Bacterial effectors hijack Ubc13 to dampen inflammation — Shigella OspI deamidates Gln100 to disrupt TRAF6 RING engagement, and Legionella MavC transglutaminates ubiquitin onto the charged Ube2N~Ub conjugate, a modification reversed by the deamidase MvcA [PMID:22407319, PMID:31638574, PMID:30420781, PMID:31825121]. Pharmacological inhibitors such as NSC697923 covalently target the Ubc13 active-site cysteine and suppress NF-κB-dependent malignancies [PMID:22791293, PMID:25909880, PMID:35263148, PMID:40371639].","teleology":[{"year":2001,"claim":"Establishing how a single E2 achieves Lys63 linkage specificity was the founding mechanistic question; the Mms2/Ubc13 heterodimer structure and reconstituted activity showed the enzyme requires an inactive UEV partner that orients the acceptor ubiquitin so only Lys63 reaches the active site.","evidence":"X-ray crystallography, in vitro thioester/chain-assembly assays, and NMR mapping of the acceptor ubiquitin surface, with UV-sensitivity validation","pmids":["11440714","11504715"],"confidence":"High","gaps":["Did not resolve the donor ubiquitin geometry in the covalent intermediate","Left open how distinct UEV partners select different pathways"]},{"year":2003,"claim":"Quantitative and structural definition of the Mms2·Ubc13·di-ubiquitin assembly answered how tightly the partners engage and how the heterodimer enhances acceptor binding, converting a qualitative model into a thermodynamic one.","evidence":"NMR structural modeling with isothermal titration calorimetry of heterodimer and ubiquitin affinities","pmids":["12569095","12834344"],"confidence":"High","gaps":["Solution model rather than a covalent intermediate structure","Did not address E3-driven activation"]},{"year":2005,"claim":"Identifying the precise interface residues and demonstrating that two UEVs (Mms2 vs Uev1A) route Ubc13 to DNA repair versus NF-κB resolved how one E2 serves divergent pathways and which contacts confer variant selectivity.","evidence":"Structure-guided interface mutagenesis with complementation, plus functional separation in yeast, NF-κB reporters, and siRNA knockdown","pmids":["15749714","16129784"],"confidence":"High","gaps":["Did not define how UEV identity is selected at specific cellular sites","Chain-length control by UEVs left mechanistically incomplete"]},{"year":2006,"claim":"The covalent Mms2-Ubc13-ubiquitin crystal structure visualized the catalytic intermediate, directly showing how the acceptor Lys63 is positioned for elongation and closing the central mechanistic question of linkage specificity.","evidence":"X-ray crystallography of the covalent UEV-E2-Ub intermediate","pmids":["16980971"],"confidence":"High","gaps":["Did not include an activating E3 RING in the structure","Donor conformational dynamics during discharge not captured"]},{"year":2006,"claim":"Genetic and biochemical work placed Ubc13 within immune signaling and the DNA damage response in vivo, showing it drives MAP kinase activation via NEMO ubiquitination and operates with RNF8 and SHPRH as E3 partners.","evidence":"Conditional knockout mice, signaling/ubiquitination assays, reciprocal Co-IP, and in vitro ubiquitination with knockdown","pmids":["16862162","18077395","17108083","12591926"],"confidence":"High","gaps":["Did not establish the structural basis of RING-driven activation","Substrate-specific chain architecture in vivo not resolved"]},{"year":2007,"claim":"Loss-of-function studies defined Ubc13 as required for HR-based DSB repair and for TRAF6-dependent inflammatory signaling organismally, with homozygous null being embryonic lethal, establishing physiological essentiality.","evidence":"DT40 gene disruption and siRNA with HR and focus-formation readouts; mouse knockout with in vivo ubiquitination and cytokine assays","pmids":["17349954","17404240"],"confidence":"High","gaps":["Did not separate developmental from signaling roles in the lethal phenotype","Direct substrates at DSBs only partially defined"]},{"year":2009,"claim":"Identification of the Rap80-BRCC36 deubiquitinase as an antagonist of RNF8-Ubc13 reframed DSB ubiquitin signaling as a dynamic balance of ligase and DUB activities.","evidence":"siRNA, catalytic mutants, and epistasis with immunofluorescence at DSBs","pmids":["19202061"],"confidence":"High","gaps":["Did not quantify in vivo chain turnover kinetics","Spatial regulation of the opposing activities unresolved"]},{"year":2006,"claim":"Work outside the canonical chain-signaling roles showed Ubc13 K63-ubiquitinates p53 to alter its stability, localization, and oligomeric state, expanding the substrate repertoire; JNK-dependent p53 Thr81 phosphorylation later defined the trigger for Ubc13 release.","evidence":"Co-IP, K63-specific ubiquitination, fractionation, reporter assays, and polysome fractionation with p53 T81 mutants","pmids":["17000756","19651615"],"confidence":"Medium","gaps":["Single-lab observations without independent reciprocal validation","The E3 ligase directing p53 K63 ubiquitination not identified in these studies"]},{"year":2012,"claim":"Structural and biochemical dissection of regulation answered how Ubc13 is restrained, revealing RNF8 dimer-driven activation, OTUB1 non-catalytic capping of di-ubiquitin, and the Shigella OspI deamidation strategy.","evidence":"Crystal structures of RNF8-Ubc13/Mms2, OTUB1-Ubc13-Mms2, and OspI, with SPR, mutagenesis, and NF-κB/DSB assays","pmids":["22589545","22679021","22407319"],"confidence":"High","gaps":["Did not define how these regulators are spatially deployed in cells","Cross-talk among inhibitory mechanisms not addressed"]},{"year":2016,"claim":"Capturing the activated RNF8-Ubc13~ubiquitin complex with separation-of-function mutations explained how RING E3s allosterically lock the donor ubiquitin conformation to enable processive K63 chain formation and selective BRCA1 recruitment.","evidence":"X-ray crystallography plus SAXS, separation-of-function mutagenesis, and cellular immunofluorescence","pmids":["26903517"],"confidence":"High","gaps":["Did not generalize the allosteric model across all Ubc13 E3 partners in cells","Did not resolve how 53BP1 versus BRCA1 outcomes are encoded"]},{"year":2017,"claim":"Defining how oligomeric TRAF6 and RING anchor points engage the charged conjugate established that E3s recognize Ubc13~Ub (not free Ubc13) to achieve processivity and E2-specific catalysis, and extended Ubc13 to antiviral MAVS aggregation and neuronal synapse suppression.","evidence":"Processivity assays with CC-domain mutagenesis and domain fusions; RIG-I/MAVS aggregation reconstitution; in vivo neuronal KO with electrophysiology and proteomics","pmids":["28993672","28469175","29097665"],"confidence":"High","gaps":["Did not fully define unanchored versus conjugated chain roles in all pathways","Cytoplasmic RNF8-Ubc13 complex composition only partly mapped"]},{"year":2019,"claim":"Mechanistic studies of bacterial effectors and the OspI mechanism revealed precise chemical strategies to disable Ubc13: salt-bridge competition blocking TRAF6 RING binding, transglutaminase crosslinking of ubiquitin onto the charged conjugate (MavC), and its enzymatic reversal (MvcA), plus the TRIM21 tri-ionic anchoring mechanism.","evidence":"NMR and crystallography of effector-Ube2N complexes, transglutamination/deamidation assays, mutagenesis, and bacterial replication and immune readouts","pmids":["31638574","30420781","32398758","31825121","31582740"],"confidence":"High","gaps":["Host enzymes (if any) reversing these effector modifications endogenously not identified","In vivo dynamics of effector-mediated Ube2N silencing during infection incompletely defined"]},{"year":2021,"claim":"Later work expanded Ubc13's substrate and regulator network — NLRP3 K63 ubiquitination, RNF213 and TRIM21 partnerships, LGP2/GPS2/FAM177A1 sequestration, and SET7/STAT3 layered regulation — and exploited Ube2N inhibition therapeutically in AML and ferroptosis contexts.","evidence":"Co-IP, MS site mapping, CRISPR and conditional knockouts, in vitro inhibition assays, and inhibitor-resistance crystallography","pmids":["33893171","33842849","34965427","28039360","34799425","36822329","25503582","35263148","32024978","25189770","24906799"],"confidence":"Medium","gaps":["Many partner and substrate claims rest on single-lab Co-IP or knockdown without reciprocal structural validation","Selectivity and on-target specificity of inhibitors across pathways incompletely characterized"]},{"year":null,"claim":"How the cell selects among UEV partners, competing E3 ligases, and the many negative regulators to assign Ubc13 to a specific pathway at a specific subcellular location in real time remains unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No integrated spatiotemporal model of partner selection","Quantitative competition among regulators in living cells not measured","In vivo chain architecture per substrate largely uncharacterized"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[1,5,11,12]},{"term_id":"GO:0016740","term_label":"transferase activity","supporting_discovery_ids":[1,5]},{"term_id":"GO:0140657","term_label":"ATP-dependent activity","supporting_discovery_ids":[15,25]}],"localization":[{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[9,7]},{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[35]},{"term_id":"GO:0005739","term_label":"mitochondrion","supporting_discovery_ids":[29,38]}],"pathway":[{"term_id":"R-HSA-73894","term_label":"DNA Repair","supporting_discovery_ids":[7,11,18,45]},{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[6,12,24,38,46]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[13,16,36,30]},{"term_id":"R-HSA-9612973","term_label":"Autophagy","supporting_discovery_ids":[29]},{"term_id":"R-HSA-1643685","term_label":"Disease","supporting_discovery_ids":[21,39,42,14]}],"complexes":["Ubc13-Mms2 heterodimer","Ubc13-Uev1A heterodimer","RNF8-Ubc13/Mms2 ternary complex"],"partners":["MMS2","UEV1A","RNF8","TRAF6","TRIM21","OTUB1","SHPRH","LGP2"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"P61088","full_name":"Ubiquitin-conjugating enzyme E2 N","aliases":["Bendless-like ubiquitin-conjugating enzyme","E2 ubiquitin-conjugating enzyme N","Ubc13","UbcH13","Ubiquitin carrier protein N","Ubiquitin-protein ligase N"],"length_aa":152,"mass_kda":17.1,"function":"The UBE2V1-UBE2N and UBE2V2-UBE2N heterodimers catalyze the synthesis of non-canonical 'Lys-63'-linked polyubiquitin chains. This type of polyubiquitination does not lead to protein degradation by the proteasome. Mediates transcriptional activation of target genes. Plays a role in the control of progress through the cell cycle and differentiation. Plays a role in the error-free DNA repair pathway and contributes to the survival of cells after DNA damage. Acts together with the E3 ligases, HLTF and SHPRH, in the 'Lys-63'-linked poly-ubiquitination of PCNA upon genotoxic stress, which is required for DNA repair. Appears to act together with E3 ligase RNF5 in the 'Lys-63'-linked polyubiquitination of JKAMP thereby regulating JKAMP function by decreasing its association with components of the proteasome and ERAD. Promotes TRIM5 capsid-specific restriction activity and the UBE2V1-UBE2N heterodimer acts in concert with TRIM5 to generate 'Lys-63'-linked polyubiquitin chains which activate the MAP3K7/TAK1 complex which in turn results in the induction and expression of NF-kappa-B and MAPK-responsive inflammatory genes. Together with RNF135 and UB2V1, catalyzes the viral RNA-dependent 'Lys-63'-linked polyubiquitination of RIGI to activate the downstream signaling pathway that leads to interferon beta production (PubMed:28469175, PubMed:31006531). UBE2V1-UBE2N together with TRAF3IP2 E3 ubiquitin ligase mediate 'Lys-63'-linked polyubiquitination of TRAF6, a component of IL17A-mediated signaling pathway","subcellular_location":"Nucleus; Cytoplasm","url":"https://www.uniprot.org/uniprotkb/P61088/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":true,"resolved_as":"","url":"https://depmap.org/portal/gene/UBE2N","classification":"Common Essential","n_dependent_lines":1098,"n_total_lines":1208,"dependency_fraction":0.9089403973509934},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[{"gene":"SAR1B","stoichiometry":0.2},{"gene":"UBA1","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/search/UBE2N","total_profiled":1310},"omim":[{"mim_id":"616869","title":"C-TERMINAL DOMAIN NUCLEAR ENVELOPE PHOSPHATASE 1 REGULATORY SUBUNIT 1; CNEP1R1","url":"https://www.omim.org/entry/616869"},{"mim_id":"616017","title":"TRIPARTITE MOTIF-CONTAINING PROTEIN 69; TRIM69","url":"https://www.omim.org/entry/616017"},{"mim_id":"615183","title":"FA CORE COMPLEX-ASSOCIATED PROTEIN 20; FAAP20","url":"https://www.omim.org/entry/615183"},{"mim_id":"614085","title":"RAD9-, RAD1-, AND HUS1-INTERACTING NUCLEAR ORPHAN 1; RHNO1","url":"https://www.omim.org/entry/614085"},{"mim_id":"612688","title":"RING FINGER PROTEIN 168; RNF168","url":"https://www.omim.org/entry/612688"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Supported","locations":[{"location":"Nucleoplasm","reliability":"Supported"},{"location":"Nucleoli fibrillar center","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/UBE2N"},"hgnc":{"alias_symbol":["UbcH-ben","UBC13","MGC8489"],"prev_symbol":[]},"alphafold":{"accession":"P61088","domains":[{"cath_id":"3.10.110.10","chopping":"5-147","consensus_level":"high","plddt":96.5983,"start":5,"end":147}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/P61088","model_url":"https://alphafold.ebi.ac.uk/files/AF-P61088-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-P61088-F1-predicted_aligned_error_v6.png","plddt_mean":95.69},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=UBE2N","jax_strain_url":"https://www.jax.org/strain/search?query=UBE2N"},"sequence":{"accession":"P61088","fasta_url":"https://rest.uniprot.org/uniprotkb/P61088.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/P61088/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/P61088"}},"corpus_meta":[{"pmid":"18077395","id":"PMC_18077395","title":"Ubc13/Rnf8 ubiquitin ligases control foci formation of the Rap80/Abraxas/Brca1/Brcc36 complex in response to DNA damage.","date":"2007","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/18077395","citation_count":357,"is_preprint":false},{"pmid":"16980971","id":"PMC_16980971","title":"Mms2-Ubc13 covalently bound to ubiquitin reveals the structural basis of linkage-specific polyubiquitin chain formation.","date":"2006","source":"Nature structural & molecular biology","url":"https://pubmed.ncbi.nlm.nih.gov/16980971","citation_count":290,"is_preprint":false},{"pmid":"11440714","id":"PMC_11440714","title":"Molecular insights into polyubiquitin chain assembly: crystal structure of the Mms2/Ubc13 heterodimer.","date":"2001","source":"Cell","url":"https://pubmed.ncbi.nlm.nih.gov/11440714","citation_count":270,"is_preprint":false},{"pmid":"16862162","id":"PMC_16862162","title":"Key function for the Ubc13 E2 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science","url":"https://pubmed.ncbi.nlm.nih.gov/26779244","citation_count":12,"is_preprint":false},{"pmid":"29927348","id":"PMC_29927348","title":"A novel requirement for ubiquitin-conjugating enzyme UBC-13 in retrograde recycling of MIG-14/Wntless and Wnt signaling.","date":"2018","source":"Molecular biology of the cell","url":"https://pubmed.ncbi.nlm.nih.gov/29927348","citation_count":12,"is_preprint":false},{"pmid":"29626159","id":"PMC_29626159","title":"Structural insights into the nanomolar affinity of RING E3 ligase ZNRF1 for Ube2N and its functional implications.","date":"2018","source":"The Biochemical journal","url":"https://pubmed.ncbi.nlm.nih.gov/29626159","citation_count":11,"is_preprint":false},{"pmid":"34799425","id":"PMC_34799425","title":"FAM177A1 Inhibits IL-1β-Induced Signaling by Impairing TRAF6-Ubc13 Association.","date":"2021","source":"Journal of immunology (Baltimore, Md. : 1950)","url":"https://pubmed.ncbi.nlm.nih.gov/34799425","citation_count":11,"is_preprint":false},{"pmid":"40371639","id":"PMC_40371639","title":"Ubiquitin-conjugating enzyme UBE2N modulates proteostasis in immunoproteasome-positive acute myeloid leukemia.","date":"2025","source":"The Journal of clinical investigation","url":"https://pubmed.ncbi.nlm.nih.gov/40371639","citation_count":9,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":47841,"output_tokens":12950,"usd":0.168886,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":24480,"output_tokens":5520,"usd":0.1302,"stage2_stop_reason":"end_turn"},"total_usd":0.299086,"stage1_batch_id":"msgbatch_017qqVz33Ga63SCDwzb35XPb","stage2_batch_id":"msgbatch_01Scr2Ei9x4HJMiRGxwrmvLK","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2001,\n      \"finding\": \"Crystal structure of the Mms2/Ubc13 heterodimer reveals that the active site of Ubc13 sits at the intersection of two channels that are potential binding sites for the two substrate ubiquitins; mutations that destabilize the heterodimer interface confer marked UV sensitivity, demonstrating that the intact heterodimer is necessary for DNA repair.\",\n      \"method\": \"X-ray crystallography + mutagenesis + UV sensitivity assay\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structure with mutagenesis and functional validation, foundational study replicated by multiple labs\",\n      \"pmids\": [\"11440714\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"Human Mms2 and Ubc13 form a stable heterodimer; the activated heterodimer transfers ubiquitin via the Ubc13 active-site thioester exclusively to Lys63 of an untethered acceptor ubiquitin; NMR mapping identifies a surface on acceptor ubiquitin that interacts with Mms2, indicating Mms2 orients the acceptor ubiquitin to place Lys63 near the Ubc13 active site.\",\n      \"method\": \"In vitro ubiquitin thioester/chain-assembly assay + 1H-15N HSQC NMR mapping\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — reconstituted in vitro activity with structural NMR mapping; multiple orthogonal methods in one study; independently replicated\",\n      \"pmids\": [\"11504715\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"NMR-based structural model of the human Mms2·Ubc13 heterodimer bound to both acceptor and donor ubiquitins defines the molecular basis for Lys63-linked chain synthesis; thermodynamic/kinetic measurements show Mms2 and Ubc13 interact with a Kd ~49 nM and the heterodimer binds acceptor ubiquitin with a Kd ~28 µM, markedly tighter than Mms2 alone (~98 µM).\",\n      \"method\": \"1H-15N NMR spectroscopy + isothermal titration calorimetry\",\n      \"journal\": \"The Journal of biological chemistry / Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — NMR structural model with quantitative ITC thermodynamics; two orthogonal methods; consistent with crystal structures\",\n      \"pmids\": [\"12569095\", \"12834344\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Mms2 and Ubc13 interact through a single key Mms2 residue that inserts into a pocket on Ubc13; structure-guided mutations at the heterodimer interface abolish Lys63-linked polyubiquitination and DNA repair complementation, demonstrating that specific interface residues determine E2-variant selectivity.\",\n      \"method\": \"Yeast two-hybrid, GST pull-down, surface plasmon resonance, in vitro ubiquitin conjugation, functional complementation\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — multiple orthogonal methods (structural, biochemical, genetic) in one study with mutagenesis validation\",\n      \"pmids\": [\"15749714\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Mammalian Ubc13 pairs with two distinct UEV proteins (Mms2 and Uev1A) that direct it to different cellular processes: Ubc13-Mms2 is required for DNA damage repair but not NF-κB activation, whereas Ubc13-Uev1A is required for NF-κB activation but not DNA repair; the two UEVs also differentially modulate the length of Lys63-linked polyubiquitin chains.\",\n      \"method\": \"Functional complementation in yeast, NF-κB reporter assays, siRNA knockdown, in vitro ubiquitin chain-length analysis\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — epistasis/functional separation with multiple orthogonal methods, independently corroborated by structural studies\",\n      \"pmids\": [\"16129784\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Crystal structure of the Mms2-Ubc13-ubiquitin covalent intermediate (donor ubiquitin linked to Ubc13 active-site Cys) reveals at atomic resolution how Mms2 positions the acceptor ubiquitin Lys63 into the Ubc13 active site for selective chain elongation.\",\n      \"method\": \"X-ray crystallography of covalent UEV-E2-Ub intermediate\",\n      \"journal\": \"Nature structural & molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — atomic crystal structure of the catalytic intermediate; direct mechanistic visualization\",\n      \"pmids\": [\"16980971\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Conditional ablation of Ubc13 in B cells and macrophages causes defective B cell development and impaired activation; in response to all tested stimuli except TNF, Ubc13-deficient cells show near-normal NF-κB activation but markedly impaired MAP kinase activation; Ubc13-induced MAP kinase activation requires ubiquitination of the adaptor IKKγ (NEMO).\",\n      \"method\": \"Conditional knockout mice, NF-κB and MAPK signaling assays, ubiquitination assays\",\n      \"journal\": \"Nature immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — clean conditional KO with defined cellular phenotypes and pathway placement; replicated in multiple cell types\",\n      \"pmids\": [\"16862162\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"RNF8, an FHA-RING ubiquitin ligase, physically interacts with Ubc13 and is required, together with Ubc13, to recruit the Rap80/Abraxas/BRCA1/BRCC36 A complex to DNA damage foci; this constitutes a sequential E3 ubiquitin ligase cascade generating K63-linked polyubiquitin chains at damage sites.\",\n      \"method\": \"siRNA knockdown, Co-IP, immunofluorescence focus formation assay\",\n      \"journal\": \"Proceedings of the National Academy of Sciences\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal Co-IP, genetic knockdown with defined pathway placement; replicated across multiple subsequent studies\",\n      \"pmids\": [\"18077395\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"SHPRH, the human Rad5 homolog, physically interacts with both Rad6-Rad18 and Mms2-Ubc13 complexes and acts as the E3 ubiquitin ligase indispensable for Mms2-Ubc13-dependent polyubiquitination of PCNA.\",\n      \"method\": \"Co-IP, in vitro ubiquitination assay, siRNA knockdown\",\n      \"journal\": \"Proceedings of the National Academy of Sciences\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Moderate — reconstituted in vitro ubiquitination plus Co-IP; multiple orthogonal methods\",\n      \"pmids\": [\"17108083\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"The RING finger protein RNF8 binds UBC13 through its RING domain and stimulates K63-linked (and K48-linked) self-polyubiquitylation; RNF8 co-localizes with UBC13 in the nucleus.\",\n      \"method\": \"Co-IP/pull-down, in vitro ubiquitination assay, immunofluorescence co-localization\",\n      \"journal\": \"Journal of cellular biochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Co-IP plus in vitro assay; single lab, two orthogonal methods\",\n      \"pmids\": [\"16215985\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Ubc13 elicits K63-linked ubiquitination of p53, which attenuates Hdm2-induced polyubiquitination; Ubc13 increases p53 stability while decreasing its transcriptional activity and promoting its cytoplasmic localization; these effects require Ubc13 catalytic activity and involve increasing monomeric (non-tetrameric) p53.\",\n      \"method\": \"Co-IP, K63-specific ubiquitination assay, subcellular fractionation, reporter assay, mutagenesis\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal methods (Co-IP, fractionation, reporter) in one lab; Ubc13 catalytic mutant controls included\",\n      \"pmids\": [\"17000756\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Disruption or siRNA depletion of UBC13 in DT40 or human cells causes chromosome instability, hypersensitivity to UV and ionizing radiation, and impaired DNA double-strand break repair by homologous recombination; specifically, BRCA1 E3 ligase function activation, Rad51 nucleoprotein filament formation, and ssDNA/RPA complex generation at DSBs are abolished in Ubc13-deficient cells.\",\n      \"method\": \"Gene disruption (DT40), siRNA knockdown, HR assay, immunofluorescence (BRCA1/Rad51/RPA foci)\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic disruption and siRNA with multiple defined phenotypic readouts in two cell systems; pathway placement by epistasis\",\n      \"pmids\": [\"17349954\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Ubc13 is an essential component of TRAF-mediated inflammatory signaling; heterozygous Ubc13 knockout mice show reduced TRAF6 ubiquitination in vivo, reduced cytokine secretion, and impaired NF-κB, JNK, and p38 MAPK activation in macrophages/splenocytes; homozygous knockout is embryonic lethal.\",\n      \"method\": \"Gene ablation in mice, in vivo ubiquitination assay, cytokine ELISA, signaling pathway analysis\",\n      \"journal\": \"Proceedings of the National Academy of Sciences\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — clean knockout with multiple defined signaling and phenotypic readouts; in vivo ubiquitination confirmed\",\n      \"pmids\": [\"17404240\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"TNF-induced GCKR and SAPK/JNK activation depends on TRAF2 and the Ubc13-Uev1A complex; Ubc13 interference inhibits TNF- and TRAF2-mediated GCKR and SAPK activation; TNF signaling leads to TRAF2 K63-linked polyubiquitination and oligomerization, and GCKR ubiquitination and activation, all sensitive to Ubc13 disruption.\",\n      \"method\": \"Dominant-negative interference, siRNA knockdown, co-IP, ubiquitination assay, kinase activity assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — dominant-negative plus siRNA knockdown, multiple orthogonal readouts; pathway positioning established\",\n      \"pmids\": [\"12591926\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"The HTLV-1 Tax oncoprotein is K63-polyubiquitinated in a Ubc13-dependent manner; Tax interacts with Ubc13; Ubc13 knockdown abrogates Tax ubiquitination and NF-κB activation; Tax ubiquitination is required for its interaction with NEMO.\",\n      \"method\": \"Co-IP, siRNA knockdown, K63-specific ubiquitination assay, NF-κB reporter assay\",\n      \"journal\": \"Journal of virology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Co-IP plus siRNA knockdown and ubiquitination assay; single lab, multiple orthogonal methods\",\n      \"pmids\": [\"17942533\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"ISG15 covalently modifies Ubc13 at Lys92, and this ISGylation suppresses Ubc13's ability to form a thioester intermediate with ubiquitin, thereby inhibiting its ubiquitin-conjugating activity.\",\n      \"method\": \"In vitro ISGylation assay, ubiquitin thioester formation assay, biochemical purification of ISGylated Ubc13\",\n      \"journal\": \"Biochemical and biophysical research communications (two independent papers)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — direct biochemical assay of thioester formation, modification site identified by mass spectrometry; independently replicated by two labs in same year\",\n      \"pmids\": [\"16112642\", \"16122702\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"The TRAF6 RING finger domain directly binds Ubc13; either a single Cys-to-Ser substitution in the TRAF6 RING or a surface mutation on Ubc13 predicted to contact RING finger proteins abolishes the interaction; TRAF6 also self-interacts through its N-terminal RING-containing domain.\",\n      \"method\": \"Yeast two-hybrid, mutagenesis\",\n      \"journal\": \"FEBS letters\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — yeast two-hybrid interaction with mutagenesis; single lab, relatively limited in vitro validation\",\n      \"pmids\": [\"15147900\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"The RNF8 RING domain anchors UBC13 at DNA damage sites independently of E2 variant proteins (Mms2/Uev1A); RNF8-UBC13 without E2 variants is sufficient to catalyze ubiquitin conjugation and promote 53BP1 accumulation at DSBs; only RING domains from UBC13-binding E3s enable this activity.\",\n      \"method\": \"Mutagenesis, immunofluorescence foci assay, in vitro ubiquitination assay, siRNA knockdown\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple in vitro and cellular assays; single lab; challenges prior model of obligate E2V requirement\",\n      \"pmids\": [\"18678647\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"The Rap80-BRCC36 deubiquitinating enzyme complex antagonizes RNF8-Ubc13-dependent ubiquitination at DSBs; BRCC36 knockdown or catalytic mutant restores 53BP1 recruitment and γH2AX ubiquitination following RNF8 depletion, revealing that opposing RNF8-Ubc13 ligase and Rap80-BRCC36 DUB activities determine steady-state ubiquitin levels at DSBs.\",\n      \"method\": \"siRNA knockdown, expression of catalytic mutants, immunofluorescence, epistasis analysis\",\n      \"journal\": \"Proceedings of the National Academy of Sciences\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — epistatic analysis with catalytic mutants and multiple readouts; pathway antagonism directly demonstrated\",\n      \"pmids\": [\"19202061\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"JNK phosphorylates p53 at Thr81 on polysomes, which is required for dissociation of Ubc13 from p53; without JNK activity or with a non-phosphorylatable p53 T81 mutant, the Ubc13-p53 complex is maintained, inhibiting p53 multimerization and transcriptional activation; thus JNK and Ubc13 cooperate to regulate p53 multimerization on polysomes.\",\n      \"method\": \"Co-IP, polysome fractionation, site-directed mutagenesis of p53 T81, JNK inhibition\",\n      \"journal\": \"Proceedings of the National Academy of Sciences\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal methods (Co-IP, fractionation, mutagenesis) in one lab\",\n      \"pmids\": [\"19651615\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Conditional knockout of the Ubc13 gene causes severe loss of hematopoietic stem cells and immune cell lineages, thymus/bone marrow atrophy, and mouse lethality; loss of Ubc13 results in accumulation of β-catenin and hyperexpression of Wnt target genes, placing Ubc13 upstream of Wnt signaling in hematopoietic stem cell maintenance.\",\n      \"method\": \"Conditional knockout mice, flow cytometry, Western blot for β-catenin, Wnt target gene expression\",\n      \"journal\": \"Proceedings of the National Academy of Sciences\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — clean conditional KO with defined phenotype and pathway (Wnt/β-catenin) placement; single lab\",\n      \"pmids\": [\"19926860\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"The Shigella effector OspI is a glutamine deamidase that specifically deamidates Gln100 of UBC13 to glutamic acid (Q100E), abolishing its E2 ubiquitin-conjugating activity required for TRAF6 activation, thereby dampening NF-κB inflammatory signaling; crystal structure of OspI reveals a Cys-His-Asp catalytic triad required for deamidation.\",\n      \"method\": \"Crystal structure of OspI (2.0 Å), mass spectrometry identification of Q100E, in vitro E2 activity assay, catalytic triad mutagenesis, NF-κB signaling assay\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structure, identification of modification site by MS, biochemical activity assay, mutagenesis validation; multiple orthogonal methods\",\n      \"pmids\": [\"22407319\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Crystal structure of RNF8 RING domain in complex with Ubc13/Mms2 (ternary complex) reveals that RNF8 dimerizes via a coiled-coil and binds Ubc13/Mms2 to stimulate K63 ubiquitin chain formation; RNF168, in contrast, is a RING monomer and does not catalyze K63 polyubiquitylation with Ubc13/Mms2.\",\n      \"method\": \"X-ray crystallography, in vitro ubiquitination assay, mutagenesis disrupting RNF8-Ubc13 interface or RNF8 coiled-coil\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structure with mutagenesis and in vitro reconstitution; multiple orthogonal methods\",\n      \"pmids\": [\"22589545\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Crystal structure of human OTUB1 in complex with UBC13 and MMS2 (3.15 Å) shows OTUB1 inhibits UBC13 E2 activity non-catalytically; OTUB1 strongly suppresses K63-linked tri-ubiquitin but allows di-ubiquitin production by capping the di-ubiquitin on the UBC13-MMS2 complex; structure-guided OTUB1 mutants that disrupt the UBC13 interface fail to inhibit K63 ubiquitination in vitro and in vivo.\",\n      \"method\": \"X-ray crystallography (3.15 Å), surface plasmon resonance, mutagenesis, in vitro ubiquitination assay, cellular DSB response assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structure, SPR, mutagenesis, in vitro and in vivo functional validation; multiple orthogonal methods\",\n      \"pmids\": [\"22679021\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Ubc13-specific ablation in regulatory T cells impairs their suppressive function in vivo and renders them susceptible to acquiring TH1 and TH17 effector phenotypes; this function involves downstream IKK activation; the Ubc13-IKK axis controls IL-10 and SOCS1 expression in Treg cells.\",\n      \"method\": \"Treg cell-specific conditional knockout mice, in vivo suppression assay, cytokine profiling, kinase activity assay\",\n      \"journal\": \"Nature immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — clean cell-type-specific conditional KO with defined cellular phenotype and pathway placement; in vivo\",\n      \"pmids\": [\"22484734\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"The small molecule NSC697923 inhibits Ubc13-Uev1A by blocking formation of the Ubc13-ubiquitin thioester conjugate, suppresses constitutive NF-κB activity in DLBCL cells, and inhibits DLBCL cell proliferation and survival.\",\n      \"method\": \"Biochemical thioester formation assay, NF-κB reporter, cell proliferation/viability assay, siRNA knockdown\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct biochemical mechanism (thioester inhibition) and cellular effects; single lab\",\n      \"pmids\": [\"22791293\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Crystal structures of OspI-Ubc13 complexes (2.96 Å and 2.3 Å) show OspI uses hydrophobic and charged surfaces to engage the α1 helix, L1 and L2 loops of Ubc13, with Gln100 positioned in the OspI catalytic pocket; Ubc13 binding induces structural rearrangement of the OspI catalytic pocket; the OspI-binding surface on Ubc13 largely overlaps with E3 ligase and DUB binding surfaces.\",\n      \"method\": \"X-ray crystallography (2.96 Å and 2.3 Å), mutagenesis, binding assays\",\n      \"journal\": \"Journal of molecular biology / PLoS pathogens\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — two independent crystal structures from two labs at high resolution with mutagenesis validation\",\n      \"pmids\": [\"23542009\", \"23633953\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"UBC13 mediates K63-linked ubiquitination of TAK1 at Lys158 during H. pylori CagA-induced NF-κB activation; mutation of TAK1 K158R prevents its ubiquitination and impairs NF-κB activation; dominant-negative Ubc13 or siRNA knockdown abolishes CagA-facilitated TAK1 and NF-κB activation.\",\n      \"method\": \"Site-directed mutagenesis (TAK1 K158R), siRNA knockdown, dominant-negative Ubc13, ubiquitination assay, NF-κB reporter\",\n      \"journal\": \"Journal of cellular biochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — mutagenesis identifies specific ubiquitination site; dominant-negative and siRNA convergent; single lab\",\n      \"pmids\": [\"23606331\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"MDC1 is ubiquitylated on Lys1977 of its tandem BRCT domain in a UBC13-dependent manner, and this ubiquitylation is required for direct MDC1 binding to RAP80 through RAP80's ubiquitin-interacting motifs.\",\n      \"method\": \"Co-IP, UBC13 knockdown, mutagenesis (K1977 identification), immunofluorescence\",\n      \"journal\": \"DNA repair\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Co-IP with UBC13 knockdown and site identification; single lab\",\n      \"pmids\": [\"21622030\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"UBE2N knockdown specifically prevents K63-linked ubiquitylation at mitochondrial sites during PINK1/Parkin-dependent mitophagy; combined knockdown of UBE2N, UBE2L3, and UBE2D2/3 substantially reduces mitochondrial polyubiquitylation, p62 recruitment, and autophagic clearance of depolarized mitochondria.\",\n      \"method\": \"siRNA knockdown, linkage-specific ubiquitin antibodies, immunofluorescence, autophagic flux assay\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — siRNA knockdown with linkage-specific antibodies; single lab; combined knockdown approach\",\n      \"pmids\": [\"24906799\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Ubc13 is required for breast cancer metastasis but is largely dispensable for primary tumor growth; Ubc13 is required for TGFβ-induced TAK1 and p38 MAPK activation (non-SMAD signaling) to control metastasis-promoting gene expression; pharmacological p38 inhibition attenuates BCa metastasis in mice.\",\n      \"method\": \"Conditional knockout in mouse model, lung colonization assay, TGFβ signaling analysis (SMAD vs. non-SMAD), p38 inhibitor in vivo\",\n      \"journal\": \"Proceedings of the National Academy of Sciences\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — clean conditional KO with in vivo metastasis assay, pathway placement via SMAD/non-SMAD dissection; in vivo pharmacological rescue\",\n      \"pmids\": [\"25189770\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"STAT3 acts as a transcriptional repressor of the Ube2n (Ubc13) gene; in RANKL-activated macrophages, STAT3 is stimulated by autocrine IL-6 and inhibits Ets-1/Set1 methyltransferase/H3K4me3 accumulation at the Ube2n promoter; depletion of Ubc13 in Stat3-deficient macrophages suppresses excessive TRAF6-mediated K63 ubiquitination and NF-κB responses.\",\n      \"method\": \"ChIP (Ets-1, Set1, H3K4me3 at Ube2n promoter), siRNA knockdown, NF-κB reporter, macrophage activation assay\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — ChIP with functional knockdown epistasis; single lab; multiple readouts\",\n      \"pmids\": [\"25503582\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"NSC697923 and BAY 11-7082 both inhibit Ubc13 by covalent Michael addition at the Ubc13 active-site cysteine; crystal structures of both Ubc13-inhibitor adducts reveal that both exploit a binding groove unique to Ubc13; a Ubc13 mutant resistant to NSC697923 shows that NF-κB and DNA damage signaling inhibition by NSC697923 is largely due to specific Ubc13 inhibition.\",\n      \"method\": \"X-ray crystallography of inhibitor-Ubc13 adducts, resistance mutant, cellular NF-κB and DNA damage signaling assays\",\n      \"journal\": \"ACS chemical biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structures plus resistance mutant demonstrating on-target specificity; multiple orthogonal methods\",\n      \"pmids\": [\"25909880\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"RNF8 stimulates Ubc13 polyubiquitination activity by modulating the conformation of ubiquitin covalently linked to the Ubc13 active site; crystal structure of the activated RNF8-Ubc13~ubiquitin complex shows this allosteric activation; structure-guided separation-of-function mutations that impair E2 stimulation abolish DSB signaling, 53BP1 recruitment, and specifically BRCA1 (but not 53BP1) recruitment by chromatin-targeted RNF168.\",\n      \"method\": \"X-ray crystallography + SAXS solution conformation, separation-of-function mutagenesis, cellular immunofluorescence\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structure, SAXS, and separation-of-function mutations with cellular validation; multiple orthogonal methods\",\n      \"pmids\": [\"26903517\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"GPS2 (G-protein Pathway Suppressor 2) directly inhibits Ubc13 enzymatic activity in B cells; GPS2 B cell-specific deletion causes developmental defects at multiple stages of B cell differentiation by derepressing TLR, BCR, and AKT/FOXO1 signaling through elevated Ubc13-mediated K63 ubiquitination.\",\n      \"method\": \"In vitro Ubc13 activity assay with GPS2, conditional knockout mice, B cell developmental analysis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct in vitro enzymatic inhibition demonstrated plus in vivo KO phenotype; single lab\",\n      \"pmids\": [\"28039360\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"In cerebellar granule neurons, RNF8 and UBC13 form a cytoplasmic (not nuclear) complex that suppresses synapse differentiation in vivo; knockdown or conditional KO increases parallel fiber presynaptic boutons and functional synapses; RNF8 interacts with HERC2 and NEURL4, and knockdown of these phenocopies RNF8/UBC13 loss.\",\n      \"method\": \"In vivo knockdown, conditional knockout, electrophysiology (PF/PC synapse), proteomics, immunofluorescence\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — in vivo KO with electrophysiological and morphological validation, proteomics for complex identification; multiple orthogonal methods\",\n      \"pmids\": [\"29097665\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"TRAF6 coiled-coil (CC) domain mediates TRAF6 oligomerization, which primes interaction with the Ubc13~Ub conjugate (not unloaded Ubc13); this interaction is required for processive assembly of long K63-linked polyubiquitin chains and TAK1 activation in IL-1R/TLR pathways; fusion of the CC domain to CHIP/STUB1 confers NF-κB activation capacity.\",\n      \"method\": \"Mutagenesis of CC domain, in vitro ubiquitination processivity assay, co-IP, NF-κB reporter, domain fusion experiments\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — reconstituted in vitro processivity assay with mutagenesis, domain fusion gain-of-function, multiple orthogonal methods\",\n      \"pmids\": [\"28993672\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Ubc13 K63-linked ubiquitination of RHBDF2 (iRhom2) is promoted by the Uev1A-Ubc13 complex together with CHIP, facilitating TACE maturation and subsequent shedding of the TNFα receptor, thereby acting as a negative regulator of TNFα-induced NF-κB signaling.\",\n      \"method\": \"Co-IP, in vitro ubiquitination assay, TACE maturation assay, NF-κB reporter\",\n      \"journal\": \"Cellular signalling\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Co-IP and in vitro ubiquitination; single lab, multiple readouts\",\n      \"pmids\": [\"29069608\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Ube2N preferentially facilitates production of unanchored K63-linked polyubiquitin chains (while Ube2D3 promotes covalent conjugation) downstream of RIG-I/Riplet; both types of polyubiquitin chains are required for RIG-I to induce MAVS aggregation on mitochondria, triggering innate immune signaling.\",\n      \"method\": \"Chromatographic purification, in vitro ubiquitination assay, MAVS aggregation assay, siRNA knockdown\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — biochemical purification, reconstituted in vitro assay, mechanistic dissection of chain type; multiple orthogonal methods\",\n      \"pmids\": [\"28469175\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Legionella pneumophila effector MavC is a transglutaminase that catalyzes monoubiquitination of UBE2N by covalent crosslinking of ubiquitin Gln40 to UBE2N Lys92 (and Lys94) via a γ-glutamyl-ε-Lys isopeptide bond; the catalytic residue is Cys74; this modification abolishes UBE2N's ability to form K63-type polyubiquitin chains and dampens NF-κB signaling.\",\n      \"method\": \"Biochemical transglutaminase assay, mass spectrometry (modification site), mutagenesis of Cys74, K63-chain formation assay, NF-κB reporter\",\n      \"journal\": \"Nature microbiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — novel enzymatic mechanism defined, modification sites identified by MS, catalytic residue mutated; multiple orthogonal methods\",\n      \"pmids\": [\"30420781\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"MavC targets the thioester-linked Ube2N~ubiquitin conjugate (not free Ube2N) for intramolecular transglutamination; ubiquitin shows increased affinity for MavC when tethered to Ube2N; crystal structures of MavC with substrate mimic and crosslinked product reveal the insertion domain is crucial for substrate recognition and that transamidation is favored over deamidation.\",\n      \"method\": \"Crystal structures of MavC-Ube2N~Ub complexes, biochemical transglutamination/deamidation assay, binding affinity measurements\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structures of substrate and product complexes with reconstituted biochemical assays; two labs converged on mechanism\",\n      \"pmids\": [\"32398758\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"A tri-ionic motif in TRIM21 (and TRIM5) RING domain provides anchor points that wrap the Ube2N~Ub complex around the RING, locking the closed conformation required for ubiquitin discharge; mutation of these anchor points specifically inhibits ubiquitination with Ube2N/Ube2V2 but not Ube2D1, establishing an E2-specific catalytic mechanism for this class of RING E3s.\",\n      \"method\": \"NMR, mutagenesis, in vitro ubiquitination assay, viral neutralization assay, immune signaling assay\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — NMR structural data with mutagenesis and multiple functional assays; E2-specificity mechanism defined\",\n      \"pmids\": [\"31582740\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"L. pneumophila effector MvcA (50% identity to MavC) uses its deamidase catalytic triad to remove the MavC-installed ubiquitin from UBE2N (deubiquitination), restoring UBE2N activity; structural analysis of MvcA-UBE2N-Ub reveals the insertion domain is critical for substrate recognition; this temporal regulation of UBE2N activity is required for efficient intracellular bacterial replication.\",\n      \"method\": \"Biochemical deubiquitination assay, crystal/structural analysis, mutagenesis of catalytic triad, bacterial replication assay\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — reconstituted enzymatic reversal of modification, structural analysis, mutagenesis, functional validation\",\n      \"pmids\": [\"31825121\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Deamidation of UBC13 at Gln100 (Q100E by OspI) inhibits interaction with TRAF6 RING domain by forming a new intramolecular salt bridge in UBC13 that competes with a critical intermolecular salt bridge at the UBC13/TRAF6 RING interface; this additionally prevents transient interactions needed for the closed E2-RING complex.\",\n      \"method\": \"NMR chemical shift perturbation, mutagenesis, binding affinity measurement, in vitro ubiquitination assay\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — NMR structural analysis with mutagenesis revealing salt-bridge competition mechanism; multiple orthogonal methods\",\n      \"pmids\": [\"31638574\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"OTUB1 deubiquitinase is stabilized by K48-linked deubiquitination in dendritic cells, leading to increased K63-linked ubiquitination of IRAK1 and TRAF6 via UBC13, thereby augmenting NF-κB activation and inflammatory cytokine production; DC-specific OTUB1 deletion impairs IL-12 production and immune defense against T. gondii.\",\n      \"method\": \"Conditional knockout mice, co-IP, linkage-specific ubiquitination assay, cytokine measurement, infection model\",\n      \"journal\": \"Cellular & molecular immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Co-IP plus conditional KO with multiple in vivo readouts; single lab\",\n      \"pmids\": [\"32024978\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"UBC13-mediated K63-linked polyubiquitination promotes MRE11 recruitment to TOP2-DNA adduct-blocked DSBs via RAP80/BRCA1 localization and BRCA1-MRE11 complex formation, facilitating nucleolytic removal of blocking adducts before NHEJ; UBC13 and MRE11 are dispensable for repair of clean DSBs but responsible for >50% and >70% of NHEJ-dependent repair of radiation-induced dirty DSBs, respectively.\",\n      \"method\": \"Auxin-inducible degron knockdown, NHEJ assay, immunofluorescence (RAP80, BRCA1, MRE11 foci), epistasis analysis\",\n      \"journal\": \"iScience\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — clean degron depletion with multiple cellular assays; pathway placement by epistasis; single lab\",\n      \"pmids\": [\"32283528\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Ubc13 associates with NLRP3 and promotes K63-linked polyubiquitination at Lys565 and Lys687 of NLRP3; Ubc13 knockdown/knockout or catalytic inhibition dramatically impairs NLRP3 inflammasome activation in macrophages, indicating K63 ubiquitination of NLRP3 is required for its activation.\",\n      \"method\": \"Co-IP, siRNA/CRISPR knockout, mass spectrometry (ubiquitination site identification), inflammasome activation assay (IL-1β, ASC speck formation)\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Co-IP, MS site identification, KO with functional readout; single lab\",\n      \"pmids\": [\"33893171\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"LGP2 helicase inhibits K63-linked polyubiquitination by directly associating with and sequestering Ubc13/UBE2N via its Hel2i subdomain, thereby inactivating multiple K63-Ub ligases (TRAF6, TRIM25, RNF125) and broadly suppressing innate immune signaling.\",\n      \"method\": \"Co-IP/pull-down of LGP2-Ubc13, in vitro ubiquitination assay, K63-Ub immunoblot, signaling reporter assay\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct interaction demonstrated and in vitro ubiquitination inhibition; single lab, multiple E3 substrates tested\",\n      \"pmids\": [\"34965427\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"RNF213 is a K63-linked E3 ubiquitin ligase that interacts with UBC13 (identified by yeast two-hybrid with RNF213 RING domain as bait); RNF213 undergoes K63-linked autoubiquitination in a UBC13-dependent manner; this axis is required for angiogenic cell motility and invasion in HUVEC cells.\",\n      \"method\": \"Yeast two-hybrid, Co-IP, K63-specific ubiquitination assay, UBC13 knockdown, HUVEC cell migration/invasion assay\",\n      \"journal\": \"FASEB BioAdvances\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — yeast two-hybrid plus Co-IP plus ubiquitination assay; functional knockdown; single lab\",\n      \"pmids\": [\"33842849\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"In AML, TRIM21 is identified as the E3 ligase that partners with activated UBE2N to modulate UBE2N-dependent proteostasis; UBE2N inhibition reduces levels of K63-ubiquitinated target proteins, leading to their increased K48-linked ubiquitination and degradation through the immunoproteasome; this is selective for immunoproteasome-positive AML cells.\",\n      \"method\": \"Interactome screen, proteomic analysis, whole-genome CRISPR-activation screen, K48/K63 ubiquitination assays, enzymatically defective mouse model\",\n      \"journal\": \"Science translational medicine / Journal of clinical investigation\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — interactome screen plus proteomics and CRISPR screen; enzymatically defective mouse model; single group\",\n      \"pmids\": [\"35263148\", \"40371639\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"SET7 methyltransferase methylates OTUB1 at Lys122; this methylation does not affect OTUB1 DUB activity but impairs its non-canonical binding to UBC13, relieving OTUB1-mediated suppression of K63-linked ubiquitination and promoting ferroptosis.\",\n      \"method\": \"In vitro methylation assay, Co-IP (OTUB1-UBC13 interaction), methylation-mimic mutant, cell viability and ROS assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct in vitro methylation and binding assay with mutant; functional consequence in cells; single lab\",\n      \"pmids\": [\"36822329\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"FAM177A1 competitively binds TRAF6 and impairs its interaction with Ubc13, thereby inhibiting TRAF6-mediated K63 polyubiquitination, downstream recruitment of signaling molecules, and NF-κB activation in response to IL-1β.\",\n      \"method\": \"Co-IP, competition binding assay (TRAF6-Ubc13 displacement), NF-κB reporter, siRNA knockdown\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — Co-IP competition assay and siRNA knockdown; single lab; indirect inhibition mechanism\",\n      \"pmids\": [\"34799425\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"UBE2N (Ubc13) is the sole E2 ubiquitin-conjugating enzyme capable of synthesizing Lys63-linked polyubiquitin chains; it does so by forming a heterodimer with a catalytically inactive UEV protein (Mms2/UEV1A), wherein the UEV positions the acceptor ubiquitin Lys63 into the Ubc13 active site, as established by crystal structures including the covalent E2~Ub intermediate; different UEV partners (Mms2 vs. Uev1A) direct Ubc13 to distinct pathways (DNA damage repair vs. NF-κB/MAP kinase signaling, respectively); Ubc13 partners with multiple RING E3 ligases (RNF8, TRAF6, TRIM21, RNF213, SHPRH, and others) that recruit and allosterically activate it at specific cellular locations; its activity is negatively regulated by ISGylation at Lys92 (blocking thioester formation), by OTUB1 non-canonical binding (capping di-ubiquitin on the E2-UEV complex), by the Shigella effector OspI (deamidation of Gln100 disrupting E3 binding), by the Legionella effector MavC (transglutaminase-mediated monoubiquitination targeting the charged E2~Ub conjugate, reversed by MvcA), by GPS2 and LGP2 (direct sequestration), and at the transcriptional level by STAT3; Ubc13-generated K63 chains serve as non-proteolytic signals required for DNA DSB signaling (RNF8-dependent recruitment of BRCA1/RAP80), homologous recombination initiation, postreplication DNA repair (PCNA polyubiquitination), NF-κB and MAP kinase activation in multiple immune cell types, MAVS aggregation in antiviral signaling, NLRP3 inflammasome activation, p53 monomerization and cytoplasmic localization, mitophagy, and synapse suppression in neurons.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"UBE2N (Ubc13) is the dedicated E2 ubiquitin-conjugating enzyme for Lys63-linked polyubiquitin chain synthesis, a non-proteolytic signal that drives DNA double-strand break responses, innate and adaptive immune signaling, and additional cellular programs [#1, #11, #12]. Catalysis requires heterodimerization with a catalytically inactive ubiquitin-conjugating enzyme variant (UEV): Mms2 or Uev1A binds a pocket on Ubc13 and orients an untethered acceptor ubiquitin so that its Lys63 is positioned into the Ubc13 active site for selective chain elongation, as defined by NMR models and the crystal structure of the covalent UEV–Ubc13–ubiquitin intermediate [#0, #1, #5]. The choice of UEV partner partitions Ubc13 between pathways — Ubc13-Mms2 supports DNA repair while Ubc13-Uev1A supports NF-\\u03baB activation — and tunes chain length [#4]. Ubc13 is recruited and allosterically activated by multiple RING E3 ligases that wrap around the Ubc13~ubiquitin conjugate to lock the closed, discharge-competent conformation, including RNF8 in the DNA damage response, TRAF6 in inflammatory signaling, and TRIM21 [#22, #33, #36, #41]. In the nucleus, RNF8-Ubc13 builds K63 chains at break sites that recruit the RAP80/Abraxas/BRCA1/BRCC36 complex and 53BP1, license homologous recombination through BRCA1 activation and Rad51 filament formation, and is opposed by the Rap80-BRCC36 deubiquitinase to set steady-state ubiquitin levels [#7, #11, #18]; Ubc13-Mms2 with the E3 SHPRH also polyubiquitinates PCNA in postreplication repair [#8]. In immune cells, Ubc13 mediates TRAF6/TRAF2-dependent K63 ubiquitination to activate MAP kinases and NF-\\u03baB, controlling B cell, macrophage, and regulatory T cell function, and is essential for hematopoietic stem cell maintenance and embryonic viability [#6, #12, #13, #24, #20]. Beyond canonical chains, Ube2N generates unanchored K63 polyubiquitin required for RIG-I-driven MAVS aggregation in antiviral signaling and promotes NLRP3 inflammasome activation [#38, #46]. Ubc13 activity is restrained by diverse mechanisms: ISGylation at Lys92 blocks thioester formation, OTUB1 non-catalytically caps di-ubiquitin on the E2-UEV complex, and the direct binders GPS2 and LGP2 sequester the enzyme [#15, #23, #34, #47]. Bacterial effectors hijack Ubc13 to dampen inflammation — Shigella OspI deamidates Gln100 to disrupt TRAF6 RING engagement, and Legionella MavC transglutaminates ubiquitin onto the charged Ube2N~Ub conjugate, a modification reversed by the deamidase MvcA [#21, #43, #39, #42]. Pharmacological inhibitors such as NSC697923 covalently target the Ubc13 active-site cysteine and suppress NF-\\u03baB-dependent malignancies [#25, #32, #49].\",\n  \"teleology\": [\n    {\n      \"year\": 2001,\n      \"claim\": \"Establishing how a single E2 achieves Lys63 linkage specificity was the founding mechanistic question; the Mms2/Ubc13 heterodimer structure and reconstituted activity showed the enzyme requires an inactive UEV partner that orients the acceptor ubiquitin so only Lys63 reaches the active site.\",\n      \"evidence\": \"X-ray crystallography, in vitro thioester/chain-assembly assays, and NMR mapping of the acceptor ubiquitin surface, with UV-sensitivity validation\",\n      \"pmids\": [\"11440714\", \"11504715\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not resolve the donor ubiquitin geometry in the covalent intermediate\", \"Left open how distinct UEV partners select different pathways\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Quantitative and structural definition of the Mms2\\u00b7Ubc13\\u00b7di-ubiquitin assembly answered how tightly the partners engage and how the heterodimer enhances acceptor binding, converting a qualitative model into a thermodynamic one.\",\n      \"evidence\": \"NMR structural modeling with isothermal titration calorimetry of heterodimer and ubiquitin affinities\",\n      \"pmids\": [\"12569095\", \"12834344\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Solution model rather than a covalent intermediate structure\", \"Did not address E3-driven activation\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Identifying the precise interface residues and demonstrating that two UEVs (Mms2 vs Uev1A) route Ubc13 to DNA repair versus NF-\\u03baB resolved how one E2 serves divergent pathways and which contacts confer variant selectivity.\",\n      \"evidence\": \"Structure-guided interface mutagenesis with complementation, plus functional separation in yeast, NF-\\u03baB reporters, and siRNA knockdown\",\n      \"pmids\": [\"15749714\", \"16129784\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not define how UEV identity is selected at specific cellular sites\", \"Chain-length control by UEVs left mechanistically incomplete\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"The covalent Mms2-Ubc13-ubiquitin crystal structure visualized the catalytic intermediate, directly showing how the acceptor Lys63 is positioned for elongation and closing the central mechanistic question of linkage specificity.\",\n      \"evidence\": \"X-ray crystallography of the covalent UEV-E2-Ub intermediate\",\n      \"pmids\": [\"16980971\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not include an activating E3 RING in the structure\", \"Donor conformational dynamics during discharge not captured\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Genetic and biochemical work placed Ubc13 within immune signaling and the DNA damage response in vivo, showing it drives MAP kinase activation via NEMO ubiquitination and operates with RNF8 and SHPRH as E3 partners.\",\n      \"evidence\": \"Conditional knockout mice, signaling/ubiquitination assays, reciprocal Co-IP, and in vitro ubiquitination with knockdown\",\n      \"pmids\": [\"16862162\", \"18077395\", \"17108083\", \"12591926\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not establish the structural basis of RING-driven activation\", \"Substrate-specific chain architecture in vivo not resolved\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Loss-of-function studies defined Ubc13 as required for HR-based DSB repair and for TRAF6-dependent inflammatory signaling organismally, with homozygous null being embryonic lethal, establishing physiological essentiality.\",\n      \"evidence\": \"DT40 gene disruption and siRNA with HR and focus-formation readouts; mouse knockout with in vivo ubiquitination and cytokine assays\",\n      \"pmids\": [\"17349954\", \"17404240\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not separate developmental from signaling roles in the lethal phenotype\", \"Direct substrates at DSBs only partially defined\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Identification of the Rap80-BRCC36 deubiquitinase as an antagonist of RNF8-Ubc13 reframed DSB ubiquitin signaling as a dynamic balance of ligase and DUB activities.\",\n      \"evidence\": \"siRNA, catalytic mutants, and epistasis with immunofluorescence at DSBs\",\n      \"pmids\": [\"19202061\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not quantify in vivo chain turnover kinetics\", \"Spatial regulation of the opposing activities unresolved\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Work outside the canonical chain-signaling roles showed Ubc13 K63-ubiquitinates p53 to alter its stability, localization, and oligomeric state, expanding the substrate repertoire; JNK-dependent p53 Thr81 phosphorylation later defined the trigger for Ubc13 release.\",\n      \"evidence\": \"Co-IP, K63-specific ubiquitination, fractionation, reporter assays, and polysome fractionation with p53 T81 mutants\",\n      \"pmids\": [\"17000756\", \"19651615\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab observations without independent reciprocal validation\", \"The E3 ligase directing p53 K63 ubiquitination not identified in these studies\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Structural and biochemical dissection of regulation answered how Ubc13 is restrained, revealing RNF8 dimer-driven activation, OTUB1 non-catalytic capping of di-ubiquitin, and the Shigella OspI deamidation strategy.\",\n      \"evidence\": \"Crystal structures of RNF8-Ubc13/Mms2, OTUB1-Ubc13-Mms2, and OspI, with SPR, mutagenesis, and NF-\\u03baB/DSB assays\",\n      \"pmids\": [\"22589545\", \"22679021\", \"22407319\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not define how these regulators are spatially deployed in cells\", \"Cross-talk among inhibitory mechanisms not addressed\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Capturing the activated RNF8-Ubc13~ubiquitin complex with separation-of-function mutations explained how RING E3s allosterically lock the donor ubiquitin conformation to enable processive K63 chain formation and selective BRCA1 recruitment.\",\n      \"evidence\": \"X-ray crystallography plus SAXS, separation-of-function mutagenesis, and cellular immunofluorescence\",\n      \"pmids\": [\"26903517\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not generalize the allosteric model across all Ubc13 E3 partners in cells\", \"Did not resolve how 53BP1 versus BRCA1 outcomes are encoded\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Defining how oligomeric TRAF6 and RING anchor points engage the charged conjugate established that E3s recognize Ubc13~Ub (not free Ubc13) to achieve processivity and E2-specific catalysis, and extended Ubc13 to antiviral MAVS aggregation and neuronal synapse suppression.\",\n      \"evidence\": \"Processivity assays with CC-domain mutagenesis and domain fusions; RIG-I/MAVS aggregation reconstitution; in vivo neuronal KO with electrophysiology and proteomics\",\n      \"pmids\": [\"28993672\", \"28469175\", \"29097665\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not fully define unanchored versus conjugated chain roles in all pathways\", \"Cytoplasmic RNF8-Ubc13 complex composition only partly mapped\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Mechanistic studies of bacterial effectors and the OspI mechanism revealed precise chemical strategies to disable Ubc13: salt-bridge competition blocking TRAF6 RING binding, transglutaminase crosslinking of ubiquitin onto the charged conjugate (MavC), and its enzymatic reversal (MvcA), plus the TRIM21 tri-ionic anchoring mechanism.\",\n      \"evidence\": \"NMR and crystallography of effector-Ube2N complexes, transglutamination/deamidation assays, mutagenesis, and bacterial replication and immune readouts\",\n      \"pmids\": [\"31638574\", \"30420781\", \"32398758\", \"31825121\", \"31582740\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Host enzymes (if any) reversing these effector modifications endogenously not identified\", \"In vivo dynamics of effector-mediated Ube2N silencing during infection incompletely defined\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Later work expanded Ubc13's substrate and regulator network — NLRP3 K63 ubiquitination, RNF213 and TRIM21 partnerships, LGP2/GPS2/FAM177A1 sequestration, and SET7/STAT3 layered regulation — and exploited Ube2N inhibition therapeutically in AML and ferroptosis contexts.\",\n      \"evidence\": \"Co-IP, MS site mapping, CRISPR and conditional knockouts, in vitro inhibition assays, and inhibitor-resistance crystallography\",\n      \"pmids\": [\"33893171\", \"33842849\", \"34965427\", \"28039360\", \"34799425\", \"36822329\", \"25503582\", \"35263148\", \"32024978\", \"25189770\", \"24906799\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Many partner and substrate claims rest on single-lab Co-IP or knockdown without reciprocal structural validation\", \"Selectivity and on-target specificity of inhibitors across pathways incompletely characterized\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How the cell selects among UEV partners, competing E3 ligases, and the many negative regulators to assign Ubc13 to a specific pathway at a specific subcellular location in real time remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No integrated spatiotemporal model of partner selection\", \"Quantitative competition among regulators in living cells not measured\", \"In vivo chain architecture per substrate largely uncharacterized\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [1, 5, 11, 12]},\n      {\"term_id\": \"GO:0016740\", \"supporting_discovery_ids\": [1, 5]},\n      {\"term_id\": \"GO:0140657\", \"supporting_discovery_ids\": [15, 25]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [9, 7]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [35]},\n      {\"term_id\": \"GO:0005739\", \"supporting_discovery_ids\": [29, 38]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-73894\", \"supporting_discovery_ids\": [7, 11, 18, 45]},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [6, 12, 24, 38, 46]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [13, 16, 36, 30]},\n      {\"term_id\": \"R-HSA-9612973\", \"supporting_discovery_ids\": [29]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [21, 39, 42, 14]}\n    ],\n    \"complexes\": [\"Ubc13-Mms2 heterodimer\", \"Ubc13-Uev1A heterodimer\", \"RNF8-Ubc13/Mms2 ternary complex\"],\n    \"partners\": [\"MMS2\", \"UEV1A\", \"RNF8\", \"TRAF6\", \"TRIM21\", \"OTUB1\", \"SHPRH\", \"LGP2\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":10,"faith_total":10,"faith_pct":100.0}}