{"gene":"UBE2B","run_date":"2026-04-28T21:43:00","timeline":{"discoveries":[{"year":1991,"finding":"UBC2/RAD6 (yeast ortholog of UBE2B) is essential for multiubiquitination and degradation of N-end rule substrates, and physically associates with UBR1 (N-recognin), the recognition component of the N-end rule pathway.","method":"Genetic epistasis, in vivo ubiquitination assays, physical co-purification","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1-2 — foundational study with multiple orthogonal methods, replicated by subsequent work","pmids":["1651502"],"is_preprint":false},{"year":1993,"finding":"The acidic C-terminal region of Ubc2 is required for both physical stability and functional activity of the N-recognin (UBR1)/Ubc2 complex; the active-site Cys-88 is required for catalytic activity but not for N-recognin binding; a ~170-residue C-terminal fragment of N-recognin (UBR1) was identified as the Ubc2-interacting domain by two-hybrid assay.","method":"Mutagenesis (C88A active site mutant), two-hybrid interaction cloning, dominant-negative inhibition assay","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1-2 — active-site and domain mutagenesis with in vivo functional readout, replicated across labs","pmids":["8505328"],"is_preprint":false},{"year":1992,"finding":"The catalytic domain of RAD6/UBC2 is essential for its DNA repair and growth functions, and these functions are distinct from those conferred by the CDC34 tail; a chimeric E2 combining the RAD6 catalytic domain with the CDC34 tail retains both RAD6 and CDC34 activities, demonstrating domain modularity.","method":"Chimeric protein construction, complementation assays in yeast","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 1 — domain-swapping with functional reconstitution, defining catalytic vs regulatory domain contributions","pmids":["1639076"],"is_preprint":false},{"year":1997,"finding":"Ubiquitin-conjugating activity of RAD6/UBC2 (active-site C88) is essential for telomeric silencing; neither Rad18 nor Ubr1 (N-end rule pathway component) is required, indicating silencing is a distinct branch of the RAD6 pathway.","method":"Null mutant and point mutant (C88A, C88S) complementation, genetic epistasis","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 2 — genetic epistasis with catalytic-dead mutants defining pathway branch","pmids":["9343433"],"is_preprint":false},{"year":1999,"finding":"Human Rad6B (UBE2B) mediates ubiquitin-dependent proteolysis of cAMP-induced transcriptional repressors hICERIIγ and hATF5 in mammalian cells; degradation requires active ubiquitin-conjugating enzyme activity and results in abrogation of repressor function; endogenous ICER protein is elevated in mHR6B-/- cells.","method":"Transfection ubiquitination assays, dominant-negative/antisense constructs, mHR6B knockout cells, yeast two-hybrid substrate identification","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods including knockout cells and in vivo ubiquitination assays","pmids":["10373550"],"is_preprint":false},{"year":2000,"finding":"Human RAD18 protein interacts with both HHR6A and HHR6B (UBE2A and UBE2B), forming stable protein complexes when co-expressed in yeast cells, purified to near homogeneity.","method":"Co-expression and co-purification, complex isolation","journal":"Nucleic acids research","confidence":"High","confidence_rationale":"Tier 2 — reciprocal purification of stable complex, confirmed biochemically","pmids":["10908344"],"is_preprint":false},{"year":2004,"finding":"RAD6B (UBE2B) is cell cycle-regulated with maximal protein levels in late S/G2 phase; upon DNA damage (adriamycin, cisplatin), RAD6B is recruited from nucleus to chromatin together with RAD18, PCNA, and phosphohistone H3; RAD6B overexpression confers chemoresistance via enhanced post-replication repair (PRR), while antisense depletion causes PRR deficiency and chemosensitivity.","method":"Cell cycle synchronization/flow cytometry, in vivo chromatin crosslinking/fractionation, stable transfection KO/OE, PRR assay (DNA molecular weight analysis)","journal":"Oncogene","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods including chromatin fractionation, PRR assay, and loss/gain of function with defined molecular readouts","pmids":["14981545"],"is_preprint":false},{"year":2005,"finding":"Fission yeast Rhp6/Ubc2 (RAD6 ortholog) and Ubr1 E3 ligase ubiquitinate the nuclear envelope protein Cut8, which physically interacts with and tethers the proteasome; non-ubiquitinatable Cut8 (K-all-R) fails to enrich nuclear proteasome; rhp6 and ubr1 null mutants also fail to enrich nuclear proteasome, linking RAD6/UBC2 activity to nuclear proteasome localization.","method":"Genetic knockouts, ubiquitination-site mutagenesis (K-all-R), subcellular fractionation, co-immunoprecipitation, DNA damage sensitivity assays","journal":"Cell","confidence":"High","confidence_rationale":"Tier 1-2 — substrate identification with mutagenesis and functional consequence (nuclear proteasome enrichment), published in Cell","pmids":["16096059"],"is_preprint":false},{"year":2008,"finding":"Human RAD18 complexed with RAD6B (UBE2B) preferentially binds forked and single-stranded DNA structures via the SAP domain of RAD18 (residues 248-282); SAP domain mutation abolishes RAD18 accumulation at DNA damage sites, prevents guidance of DNA Pol η to stalled replication forks, and impairs PCNA monoubiquitination.","method":"In vitro DNA binding assays, SAP domain mutagenesis, in vivo localization (fluorescence microscopy), UV sensitivity complementation","journal":"Genes to cells : devoted to molecular & cellular mechanisms","confidence":"High","confidence_rationale":"Tier 1-2 — domain mutagenesis with both in vitro biochemical and in vivo functional validation","pmids":["18363965"],"is_preprint":false},{"year":2008,"finding":"Rad6B (UBE2B) overexpression in breast cells induces β-catenin accumulation via K63-linked polyubiquitination that renders β-catenin insensitive to 26S proteasome degradation; in vitro ubiquitination assays confirm Rad6B directly mediates β-catenin polyubiquitination; Rad6B silencing suppresses β-catenin mono- and polyubiquitination and transcriptional activity.","method":"Stable overexpression/siRNA knockdown, in vitro ubiquitination assay, chromatin immunoprecipitation (ubiquitinated β-catenin on chromatin), cycloheximide chase, TOP/Flash reporter assay","journal":"Cancer research","confidence":"High","confidence_rationale":"Tier 1-2 — in vitro ubiquitination assay combined with multiple cellular readouts and loss/gain of function","pmids":["18339854"],"is_preprint":false},{"year":2010,"finding":"CDK1/2-dependent phosphorylation of UBE2B/Ubc2 at Ser120 downregulates N-end rule-dependent degradation: S120D (phosphomimetic) mutant shows 20-fold reduced activity with E3α/Ubr1 (8-fold increase in Km, 2.5-fold decrease in Vmax) while S120A mutant shows 8-fold reduction in Vmax for polyubiquitin chain elongation; phosphorylation does not affect E1-catalyzed E2 transthiolation.","method":"In vitro kinetics of E3α-catalyzed ubiquitination, site-directed mutagenesis (S120D/S120A), in vivo N-end rule reporter assays in T47D cells","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — in vitro kinetic analysis with mutagenesis and in vivo functional validation","pmids":["21041297"],"is_preprint":false},{"year":2012,"finding":"Rad6B (UBE2B) ubiquitinates β-catenin at lysine 394 (K394) as the major site; Rad6B-interacting region maps to amino acids 131-181 of β-catenin and 50-116 of Rad6B; K394R mutation abolishes ~50% of Rad6B-induced β-catenin ubiquitination and reduces β-catenin transcriptional activity and stability (not rescued by proteasome inhibitor MG132).","method":"GST pulldown with deletion mutants, in vitro and in vivo ubiquitination assays, site-directed mutagenesis (K394R), TOP/Flash luciferase reporter, co-immunoprecipitation","journal":"Biochimica et biophysica acta","confidence":"High","confidence_rationale":"Tier 1-2 — site-specific mutagenesis with in vitro and in vivo ubiquitination confirmation and functional readout","pmids":["22705350"],"is_preprint":false},{"year":2013,"finding":"UBE2B (Ube2b) is a direct transcriptional target of androgen receptor (AR) in mouse Sertoli cells; AR binds androgen-responsive elements in the Ube2b promoter; testosterone-induced UBE2B upregulates H2A ubiquitylation, and UBE2B knockdown blocks testosterone-induced H2A ubiquitylation.","method":"Luciferase reporter assay, EMSA, ChIP assay, Western blot, qRT-PCR, siRNA knockdown, AR knockout mouse model","journal":"Biology of reproduction","confidence":"High","confidence_rationale":"Tier 2 — ChIP, EMSA and luciferase with orthogonal functional ubiquitination readout","pmids":["23863405"],"is_preprint":false},{"year":2015,"finding":"UBE2B is upregulated in catabolic myotubes (dexamethasone-treated C2C12); UBE2B knockdown causes a sharp decrease in total (-18%) and K48-linked (-28%) ubiquitin conjugates, implicating UBE2B in proteasome-mediated myofibrillar protein degradation in muscle catabolism.","method":"siRNA knockdown, ubiquitin conjugate immunoblotting (total and K48-linked), E2 expression screen","journal":"Journal of cachexia, sarcopenia and muscle","confidence":"Medium","confidence_rationale":"Tier 2 — KD with specific molecular readout (K48-Ub conjugates), single lab","pmids":["27239408"],"is_preprint":false},{"year":2019,"finding":"RAD6B (UBE2B) mediates PCNA monoubiquitination (translesion synthesis marker) and FANCD2 activation (Fanconi anemia pathway); RAD6B also monoubiquitinates H2AX to modulate γH2AX levels; RAD6B silencing or inhibition impairs homologous recombination and restart of cisplatin-stalled replication forks.","method":"RAD6-selective inhibitor (SMI#9), RAD6B siRNA knockdown, DR-GFP homologous recombination assay, DNA fiber assay, immunofluorescence foci (γH2AX, RAD51, FANCD2, POL η), Western blot","journal":"Biochimica et biophysica acta. Molecular basis of disease","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods (inhibitor + siRNA, functional HR assay, fiber assay) with specific molecular readouts","pmids":["31639439"],"is_preprint":false},{"year":2019,"finding":"Ube2b (UBE2B) mediates ubiquitination and degradation of DNMT3a in dorsal hippocampus neurons during repeated opiate self-administration; DNMT3a degradation leads to demethylation of the CaMKK1 gene promoter, facilitating CaMKK1/CaMKIα/βPIX/Rac1 cascade activation and actin cytoskeleton remodeling underlying behavioral plasticity.","method":"In vivo rat model (heroin self-administration), co-immunoprecipitation, ubiquitination assay, promoter methylation analysis, viral vector-mediated Ube2b overexpression/knockdown, behavioral tests","journal":"Molecular psychiatry","confidence":"High","confidence_rationale":"Tier 2 — substrate identification with pathway epistasis, in vivo model with multiple molecular and behavioral readouts","pmids":["31576007"],"is_preprint":false},{"year":2019,"finding":"RAD6B (UBE2B) mediates polyubiquitination of histones H2A and H2B; RAD6B deficiency in neurons results in increased genomic instability (γH2AX), elevated p53/p21 levels, neuronal senescence, and neurodegeneration with learning/memory deficits in mice.","method":"RAD6B knockout mouse, X-ray irradiation, immunofluorescence, histone ubiquitination assays, behavioral experiments","journal":"Frontiers in cellular neuroscience","confidence":"Medium","confidence_rationale":"Tier 2 — KO mouse with defined molecular phenotype, single study","pmids":["31507381"],"is_preprint":false},{"year":2019,"finding":"RAD6B (UBE2B) splice variants (RAD6BΔexon4 and RAD6Bintron5ins) retain intact catalytic domains and exhibit functional in vivo ubiquitin-conjugating activity, expressed as 14 and 15 kDa proteins specifically in melanomas but not normal melanocytes.","method":"RT-PCR splice variant characterization, in vivo ubiquitination activity assay, Western blot, whole exome sequencing","journal":"Cells","confidence":"Medium","confidence_rationale":"Tier 2 — functional ubiquitination assay on novel isoforms, single lab","pmids":["31683936"],"is_preprint":false},{"year":2022,"finding":"UBE2B forms a heterodimer complex with E3 ligase RAD18; the UBE2B/RAD18 complex monoubiquitinates ZMYM2, increasing its protein stability; RAD18 knockdown impairs UBE2B-induced ZMYM2 monoubiquitination; UBE2B overexpression shifts ZMYM2 from polyubiquitinated (degraded) to monoubiquitinated (stabilized) form.","method":"Co-immunoprecipitation, immunofluorescence colocalization, cycloheximide chase assay, monoubiquitination assay, in vivo xenograft tumor model, RAD18 siRNA knockdown","journal":"Bioengineered","confidence":"Medium","confidence_rationale":"Tier 2 — co-IP with functional ubiquitination and stability readout, single lab","pmids":["35313791"],"is_preprint":false},{"year":2024,"finding":"UBR4 contains a distinct E3 module with a 'hemiRING' zinc finger, a UBR zinc-finger interacting (UZI) subdomain, and an N-terminal affinity region that specifically recruits UBE2A and UBE2B as cognate E2s; crystal structure of the E2-E3 complex reveals atomic-level specificity determinants; the UZI subdomain allosterically and modestly activates the Ub-loaded UBE2A/UBE2B.","method":"X-ray crystallography (E2-E3 complex structure), mutagenesis, in vitro ubiquitination reconstitution, biochemical binding assays","journal":"Nature structural & molecular biology","confidence":"High","confidence_rationale":"Tier 1 — crystal structure with functional reconstitution and mutagenesis validation","pmids":["38182926"],"is_preprint":false},{"year":2006,"finding":"The human RAD6B (UBE2B) gene is a direct transcriptional target of TCF-4/β-catenin/p300; β-catenin assembled on TCF binding elements in the Rad6B promoter drives its transcription; this creates a positive feedback loop where Rad6B stabilizes β-catenin and β-catenin drives Rad6B expression.","method":"EMSA, Western blot of EMSA, UV cross-linking, chromatin immunoprecipitation, luciferase reporter assay, co-transfection","journal":"Molecular cancer research : MCR","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods (ChIP, EMSA, luciferase) from single lab demonstrating direct transcriptional regulation","pmids":["17050667"],"is_preprint":false},{"year":2026,"finding":"UBE2B interacts with E3 ligase BIRC2 to catalyze K63-linked ubiquitination of TRAF1, amplifying NF-κB signaling; NF-κB subunit P65 directly binds the UBE2B promoter to enhance its transcription, creating a feedforward loop; UBE2B promotes gastric cancer cell proliferation in vitro and in vivo.","method":"Co-immunoprecipitation, K63-linked ubiquitination assay, ChIP assay, luciferase reporter assay, in vivo xenograft, bioinformatics/clinical validation","journal":"Molecular cancer research : MCR","confidence":"Medium","confidence_rationale":"Tier 2 — multiple mechanistic assays (K63-Ub assay, ChIP, luciferase), single lab","pmids":["41661096"],"is_preprint":false},{"year":2003,"finding":"Mouse Ube2b (UBE2B) null mutant spermatids show nuclear and sperm periaxonemal structural anomalies; transition proteins, protamines, and actin distribute normally, but periaxonemal structures are abnormally distributed, demonstrating UBE2B is required for periaxonemal assembly during spermatogenesis.","method":"Ube2b knockout mouse, immuno-electron microscopy, in situ hybridization, electron microscopy","journal":"Molecular reproduction and development","confidence":"High","confidence_rationale":"Tier 2 — KO mouse with high-resolution ultrastructural analysis defining specific spermatogenic defect","pmids":["12784252"],"is_preprint":false},{"year":2018,"finding":"RAD6B (UBE2B) polyubiquitinates histones H2A and H2B in mouse spermatogenesis; RNF8 monoubiquitinates H2A and H2B; loss of RAD6B leads to male sterility associated with absent histone polyubiquitination; RAD6B-deficient spermatocytes show senescence contributing to germ cell loss.","method":"RNF8 and RAD6B knockout mice, histone ubiquitination assays, fertility/offspring count assays, senescence markers","journal":"Cell cycle (Georgetown, Tex.)","confidence":"Medium","confidence_rationale":"Tier 2 — KO mice with direct histone ubiquitination assay, single lab","pmids":["28825854"],"is_preprint":false},{"year":2025,"finding":"UBE2B promotes ubiquitination and degradation of U2AF1 (splicing factor), thereby modulating the p53/p21 signaling pathway to induce endothelial apoptosis during renal ischemia-reperfusion injury.","method":"UBE2B overexpression/knockdown in endothelial cells, co-immunoprecipitation, ubiquitination assay, apoptosis/proliferation assays","journal":"Molecular biology reports","confidence":"Low","confidence_rationale":"Tier 3 — single pulldown/ubiquitination with functional readout, single lab, limited validation","pmids":["41186784"],"is_preprint":false}],"current_model":"UBE2B (RAD6B/HHR6B) is a ubiquitin-conjugating E2 enzyme that works with multiple E3 ligases—including UBR1/E3α (N-end rule degradation), RAD18 (PCNA monoubiquitination and translesion synthesis), UBR4 (N-degron pathway, via a hemiRING module), RNF8, and BIRC2—to ubiquitinate diverse substrates (N-end rule proteins, PCNA, H2A, H2B, β-catenin at K394 via K63 linkage, DNMT3a, TRAF1, ZMYM2); its activity is regulated by CDK1/2-dependent phosphorylation at Ser120 (which differentially modulates its cognate E3 interactions), it is transcriptionally driven by the β-catenin/TCF-4 axis (creating a positive feedback loop that stabilizes β-catenin and activates Wnt signaling), and it is essential for post-replication DNA repair/translesion synthesis, N-end rule proteolysis, spermatogenesis (histone replacement and periaxonemal assembly), nuclear proteasome enrichment, and neuronal DNA damage response."},"narrative":{"teleology":[{"year":1991,"claim":"Establishing that UBC2/RAD6 is the obligate E2 for N-end rule degradation and physically associates with UBR1 answered the fundamental question of which ubiquitin-conjugating enzyme mediates this proteolytic pathway.","evidence":"Genetic epistasis and co-purification of yeast Ubc2 with N-recognin/UBR1","pmids":["1651502"],"confidence":"High","gaps":["Mammalian UBE2B–UBR1 interaction not yet demonstrated","Substrate scope of the complex unknown"]},{"year":1992,"claim":"Domain-swap experiments showed that the RAD6 catalytic core is modular and independently sufficient for DNA repair function, separating its repair role from its C-terminal regulatory tail.","evidence":"RAD6/CDC34 chimeric proteins tested for complementation of rad6Δ and cdc34 phenotypes in yeast","pmids":["1639076"],"confidence":"High","gaps":["Which E3 partners require the acidic tail versus the catalytic domain","Structural basis of domain modularity unresolved"]},{"year":1993,"claim":"Mapping the UBR1-binding region to a C-terminal fragment and showing that the acidic tail of Ubc2 stabilizes the E2–E3 complex distinguished the catalytic and E3-interaction surfaces of the enzyme.","evidence":"Two-hybrid mapping, C88A mutagenesis, dominant-negative assays in yeast","pmids":["8505328"],"confidence":"High","gaps":["Atomic-resolution structure of the UBE2B–UBR1 interface not determined","How acidic tail modulates activity kinetically not quantified"]},{"year":1997,"claim":"Demonstrating that RAD6 catalytic activity is required for telomeric silencing but neither RAD18 nor UBR1 is involved revealed a third, genetically distinct effector branch of the RAD6 pathway.","evidence":"Catalytic-dead mutants (C88A/C88S) and rad18Δ/ubr1Δ epistasis in yeast silencing assays","pmids":["9343433"],"confidence":"High","gaps":["E3 ligase for silencing branch unidentified","Histone substrate specificity in silencing not shown"]},{"year":1999,"claim":"Identifying human RAD6B as the E2 that ubiquitinates and degrades cAMP-responsive transcriptional repressors (ICERII γ, ATF5) extended its substrates beyond DNA repair into mammalian transcriptional regulation.","evidence":"Ubiquitination assays, dominant-negative constructs, and elevated ICER in mHR6B−/− mouse cells","pmids":["10373550"],"confidence":"High","gaps":["E3 partner for ICER/ATF5 degradation not identified","Physiological consequence of ICER stabilization beyond cAMP signaling unclear"]},{"year":2000,"claim":"Biochemical isolation of a stable human RAD18–UBE2B complex established the conserved E2–E3 partnership that channels RAD6 activity toward DNA damage tolerance in human cells.","evidence":"Co-expression in yeast and purification of the RAD18–HHR6B heterodimer","pmids":["10908344"],"confidence":"High","gaps":["PCNA monoubiquitination by this complex not yet shown directly","Stoichiometry and structural details of the complex unresolved"]},{"year":2003,"claim":"Ube2b knockout mice revealed that UBE2B is required for periaxonemal structural assembly during spermiogenesis, defining its non-redundant role with UBE2A in male fertility.","evidence":"Immuno-electron microscopy and ultrastructural analysis of Ube2b−/− spermatids","pmids":["12784252"],"confidence":"High","gaps":["Specific ubiquitination substrates mediating periaxonemal assembly not identified","Whether histone replacement defects contribute to the structural phenotype unclear"]},{"year":2004,"claim":"Showing that RAD6B peaks in late S/G2, is recruited to chromatin with RAD18 and PCNA after DNA damage, and confers chemoresistance via post-replication repair linked its cell-cycle regulation to a specific repair function.","evidence":"Chromatin fractionation, PRR assay, and gain/loss-of-function in synchronized cells treated with cisplatin/adriamycin","pmids":["14981545"],"confidence":"High","gaps":["Whether PCNA monoubiquitination is the sole PRR-relevant substrate not resolved","Mechanism of cell-cycle-dependent RAD6B accumulation unknown"]},{"year":2005,"claim":"Discovery that RAD6/UBR1-mediated ubiquitination of Cut8 is required to tether proteasomes to the nuclear envelope expanded UBE2B's role to proteasome localization.","evidence":"Cut8 K-to-R mutagenesis, rhp6Δ/ubr1Δ mutants, subcellular fractionation of proteasome in fission yeast","pmids":["16096059"],"confidence":"High","gaps":["Whether a mammalian Cut8 ortholog or analogous mechanism exists is unknown","Whether proteasome mis-localization explains any RAD6-null phenotypes untested"]},{"year":2006,"claim":"Identifying UBE2B as a direct transcriptional target of TCF-4/β-catenin established a positive-feedback loop coupling Wnt pathway activation to E2 enzyme abundance.","evidence":"ChIP, EMSA, and luciferase reporter assays on the RAD6B promoter with β-catenin/TCF-4","pmids":["17050667"],"confidence":"High","gaps":["Whether this loop operates in vivo in Wnt-driven tumors not shown","Quantitative contribution of this loop versus other β-catenin stabilization mechanisms unknown"]},{"year":2008,"claim":"Demonstrating that the RAD18 SAP domain directs the RAD18–UBE2B complex to fork structures and is required for PCNA monoubiquitination and Pol η recruitment resolved how substrate targeting occurs in translesion synthesis.","evidence":"In vitro DNA binding with SAP domain mutants, UV sensitivity complementation, PCNA ubiquitination and Pol η foci assays","pmids":["18363965"],"confidence":"High","gaps":["Whether UBE2B contributes to DNA-binding specificity unknown","Role of other RAD18 domains in complex activation uncharacterized"]},{"year":2008,"claim":"Revealing that UBE2B directly K63-polyubiquitinates β-catenin, stabilizing it against proteasomal degradation, identified a non-canonical ubiquitin signal that activates rather than degrades a key oncoprotein.","evidence":"In vitro ubiquitination, K63-linkage-specific antibodies, siRNA knockdown, TOP/Flash reporter in breast cancer cells","pmids":["18339854"],"confidence":"High","gaps":["E3 ligase cooperating with UBE2B for β-catenin ubiquitination not identified","How K63-Ub renders β-catenin proteasome-resistant mechanistically unclear"]},{"year":2010,"claim":"Quantitative kinetic analysis of CDK1/2-mediated Ser120 phosphorylation showed it selectively impairs UBE2B's activity with UBR1/E3α (20-fold) without affecting E1 charging, revealing cell-cycle-dependent tuning of E2–E3 partnerships.","evidence":"In vitro kinetics (Km/Vmax) with S120D/S120A mutants, in vivo N-end rule reporter assay","pmids":["21041297"],"confidence":"High","gaps":["Impact of Ser120 phosphorylation on RAD18 or other E3 partnerships not measured","In vivo stoichiometry of phosphorylated UBE2B across cell cycle unknown"]},{"year":2012,"claim":"Mapping the UBE2B-β-catenin interface and identifying K394 as the major ubiquitination site provided substrate-level resolution for the non-degradative ubiquitin signal that stabilizes β-catenin.","evidence":"Deletion mapping, K394R mutagenesis, in vitro/in vivo ubiquitination, TOP/Flash and stability assays","pmids":["22705350"],"confidence":"High","gaps":["Other ubiquitination sites on β-catenin by UBE2B likely exist (K394R only reduces ~50%)","Structural basis of the UBE2B–β-catenin interaction unresolved"]},{"year":2013,"claim":"Showing that androgen receptor directly drives UBE2B transcription in Sertoli cells and that UBE2B mediates testosterone-induced H2A ubiquitylation connected hormonal regulation of spermatogenesis to chromatin ubiquitination.","evidence":"AR ChIP on Ube2b promoter, EMSA, luciferase reporter, siRNA-mediated UBE2B knockdown abolishing H2A-Ub","pmids":["23863405"],"confidence":"High","gaps":["Specific H2A sites ubiquitinated by UBE2B in Sertoli cells not mapped","Whether AR-driven UBE2B is sufficient to explain spermatogenic histone replacement unknown"]},{"year":2018,"claim":"Genetic dissection using RNF8 and RAD6B knockout mice showed that RNF8 monoubiquitinates and RAD6B polyubiquitinates H2A/H2B during spermatogenesis, and that RAD6B loss causes spermatocyte senescence and male sterility.","evidence":"Double and single KO mice, histone ubiquitination assays, senescence markers, fertility assays","pmids":["28825854"],"confidence":"Medium","gaps":["Whether RNF8 is the E3 that directs RAD6B polyubiquitination of histones or acts independently not fully resolved","Chain-type specificity of histone polyubiquitination in spermatogenesis not determined"]},{"year":2019,"claim":"Multiple studies converged to show UBE2B is central to the neuronal DNA damage response: it monoubiquitinates PCNA and H2AX, activates FANCD2, and supports homologous recombination; its loss causes genomic instability, neuronal senescence, and memory deficits; and it ubiquitinates DNMT3a to drive epigenetic remodeling underlying addiction-related plasticity.","evidence":"RAD6B KO mice (neurodegeneration phenotype), RAD6 inhibitor + siRNA with HR/fiber assays, in vivo heroin self-administration model with co-IP/ubiquitination/methylation analysis","pmids":["31639439","31507381","31576007"],"confidence":"High","gaps":["Whether UBE2B-mediated H2AX monoubiquitination is direct or requires a specific E3 not established","Relative contributions of UBE2A versus UBE2B in neurons unclear"]},{"year":2022,"claim":"Identification of ZMYM2 as a substrate whose stability is increased by UBE2B/RAD18-dependent monoubiquitination (shifting it from poly- to mono-Ub) revealed that UBE2B can switch substrate fate from degradation to stabilization.","evidence":"Co-IP, cycloheximide chase, monoubiquitination assay, RAD18 knockdown, xenograft model","pmids":["35313791"],"confidence":"Medium","gaps":["ZMYM2 ubiquitination site not mapped","Whether ZMYM2 stabilization mediates the oncogenic effects not mechanistically resolved"]},{"year":2024,"claim":"Crystal structure of UBR4's hemiRING–UBE2B complex revealed the atomic determinants of E2 selectivity and showed that the UZI subdomain allosterically activates ubiquitin-loaded UBE2B, defining a new E3 architecture for the N-degron pathway.","evidence":"X-ray crystallography, structure-guided mutagenesis, in vitro ubiquitination reconstitution","pmids":["38182926"],"confidence":"High","gaps":["Physiological substrates of the UBR4–UBE2B complex in mammalian cells not identified","Whether UBR4 competes with UBR1 for UBE2B binding in vivo unknown"]},{"year":2025,"claim":"UBE2B was reported to cooperate with BIRC2 to K63-polyubiquitinate TRAF1, amplifying NF-κB signaling in a feedforward loop with P65-driven UBE2B transcription, connecting UBE2B to innate immune and inflammatory signaling.","evidence":"Co-IP, K63-Ub assay, ChIP on UBE2B promoter, luciferase reporter, xenograft in gastric cancer cells","pmids":["41661096"],"confidence":"Medium","gaps":["Endogenous validation of BIRC2–UBE2B interaction in non-cancer contexts missing","Whether K63-Ub on TRAF1 is direct or scaffold-mediated not resolved"]},{"year":null,"claim":"How UBE2B is selectively allocated among its numerous E3 partners in vivo—and whether Ser120 phosphorylation differentially modulates non-UBR1 partnerships (RAD18, UBR4, BIRC2)—remains the central unresolved question for understanding pathway-specific regulation.","evidence":"","pmids":[],"confidence":"Low","gaps":["Quantitative in vivo measurements of UBE2B partitioning among E3s lacking","Structural basis for phospho-Ser120 selectivity beyond UBR1 not determined","Whether UBE2A fully compensates for UBE2B loss in specific tissues remains untested"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[0,1,4,9,10,11,14,15,16,23,24]},{"term_id":"GO:0016874","term_label":"ligase activity","supporting_discovery_ids":[0,9,19]}],"localization":[{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[6,7,14]},{"term_id":"GO:0005694","term_label":"chromosome","supporting_discovery_ids":[6,12,16,23]}],"pathway":[{"term_id":"R-HSA-73894","term_label":"DNA Repair","supporting_discovery_ids":[2,6,8,14]},{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[0,4,10,15,19]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[9,11,20,21]},{"term_id":"R-HSA-4839726","term_label":"Chromatin organization","supporting_discovery_ids":[12,16,23]},{"term_id":"R-HSA-1474165","term_label":"Reproduction","supporting_discovery_ids":[22,23]}],"complexes":["RAD18–UBE2B heterodimer","UBR1–UBE2B complex","UBR4–UBE2B complex"],"partners":["RAD18","UBR1","UBR4","BIRC2","RNF8","CTNNB1","DNMT3A","ZMYM2"],"other_free_text":[]},"mechanistic_narrative":"UBE2B (RAD6B/HHR6B) is a ubiquitin-conjugating (E2) enzyme that partners with multiple E3 ligases to regulate DNA damage tolerance, chromatin remodeling, protein quality control, and signal transduction. In complex with RAD18, UBE2B monoubiquitinates PCNA to activate translesion synthesis and post-replication repair, and polyubiquitinates histones H2A and H2B—functions essential for spermatogenesis and neuronal genome maintenance [PMID:18363965, PMID:31639439, PMID:25854, PMID:31507381]. Through the N-end rule E3 ligase UBR1 and the structurally distinct hemiRING E3 UBR4, UBE2B mediates N-degron-dependent proteolysis, an activity down-regulated by CDK1/2 phosphorylation at Ser120 [PMID:1651502, PMID:21041297, PMID:38182926]. UBE2B also stabilizes β-catenin via K63-linked polyubiquitination at K394—creating a positive-feedback loop with TCF-4/β-catenin-driven UBE2B transcription—and ubiquitinates substrates including DNMT3a, TRAF1 (with BIRC2, K63-linked), and ZMYM2 (with RAD18) to modulate Wnt, NF-κB, and epigenetic pathways [PMID:18339854, PMID:22705350, PMID:17050667, PMID:31576007, PMID:41661096, PMID:35313791]."},"prefetch_data":{"uniprot":{"accession":"P63146","full_name":"Ubiquitin-conjugating enzyme E2 B","aliases":["E2 ubiquitin-conjugating enzyme B","RAD6 homolog B","HR6B","hHR6B","Ubiquitin carrier protein B","Ubiquitin-conjugating enzyme E2-17 kDa","Ubiquitin-protein ligase B"],"length_aa":152,"mass_kda":17.3,"function":"E2 ubiquitin-conjugating enzyme that accepts ubiquitin from the ubiquitin-activating enzyme E1 and transfers it to a E3 ubiquitin-protein ligase (PubMed:16337599, PubMed:17108083, PubMed:17130289, PubMed:1717990, PubMed:20061386). In vitro catalyzes 'Lys-11'-, as well as 'Lys-48'- and 'Lys-63'-linked polyubiquitination (PubMed:20061386). Together with the E3 enzyme BRE1 (RNF20 and/or RNF40), plays a role in transcription regulation by catalyzing the monoubiquitination of histone H2B at 'Lys-120' to form H2BK120ub1 (PubMed:16337599). H2BK120ub1 gives a specific tag for epigenetic transcriptional activation, elongation by RNA polymerase II, telomeric silencing, and is also a prerequisite for H3K4me and H3K79me formation (PubMed:16337599). May play a role in DNA repair (PubMed:8062904). Associates to the E3 ligase RAD18 to form the UBE2B-RAD18 ubiquitin ligase complex involved in mono-ubiquitination of DNA-associated PCNA on 'Lys-164' (PubMed:17108083, PubMed:17130289). In association with the E3 enzyme UBR4, is involved in N-end rule-dependent protein degradation (PubMed:38182926). May be involved in neurite outgrowth (By similarity)","subcellular_location":"Cell membrane; Nucleus","url":"https://www.uniprot.org/uniprotkb/P63146/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/UBE2B","classification":"Not Classified","n_dependent_lines":0,"n_total_lines":1208,"dependency_fraction":0.0},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/UBE2B","total_profiled":1310},"omim":[{"mim_id":"613831","title":"UBIQUITIN PROTEIN LIGASE E3 COMPONENT N-RECOGNIN 3; UBR3","url":"https://www.omim.org/entry/613831"},{"mim_id":"609134","title":"UBIQUITIN-PROTEIN LIGASE E3 COMPONENT N-RECOGNIN 2; UBR2","url":"https://www.omim.org/entry/609134"},{"mim_id":"606398","title":"ACTIVATING TRANSCRIPTION FACTOR 5; ATF5","url":"https://www.omim.org/entry/606398"},{"mim_id":"179095","title":"UBIQUITIN-CONJUGATING ENZYME E2 B; UBE2B","url":"https://www.omim.org/entry/179095"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Nucleoplasm","reliability":"Approved"},{"location":"Plasma membrane","reliability":"Approved"},{"location":"Cytosol","reliability":"Approved"},{"location":"Vesicles","reliability":"Additional"},{"location":"Flagellar centriole","reliability":"Additional"},{"location":"Mid piece","reliability":"Additional"},{"location":"Principal piece","reliability":"Additional"}],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in all","driving_tissues":[{"tissue":"skeletal muscle","ntpm":202.4}],"url":"https://www.proteinatlas.org/search/UBE2B"},"hgnc":{"alias_symbol":["UBC2","HHR6B","RAD6B"],"prev_symbol":[]},"alphafold":{"accession":"P63146","domains":[{"cath_id":"3.10.110.10","chopping":"3-148","consensus_level":"high","plddt":95.995,"start":3,"end":148}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/P63146","model_url":"https://alphafold.ebi.ac.uk/files/AF-P63146-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-P63146-F1-predicted_aligned_error_v6.png","plddt_mean":95.69},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=UBE2B","jax_strain_url":"https://www.jax.org/strain/search?query=UBE2B"},"sequence":{"accession":"P63146","fasta_url":"https://rest.uniprot.org/uniprotkb/P63146.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/P63146/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/P63146"}},"corpus_meta":[{"pmid":"1651502","id":"PMC_1651502","title":"The N-end rule is mediated by 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of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — foundational study with multiple orthogonal methods, replicated by subsequent work\",\n      \"pmids\": [\"1651502\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1993,\n      \"finding\": \"The acidic C-terminal region of Ubc2 is required for both physical stability and functional activity of the N-recognin (UBR1)/Ubc2 complex; the active-site Cys-88 is required for catalytic activity but not for N-recognin binding; a ~170-residue C-terminal fragment of N-recognin (UBR1) was identified as the Ubc2-interacting domain by two-hybrid assay.\",\n      \"method\": \"Mutagenesis (C88A active site mutant), two-hybrid interaction cloning, dominant-negative inhibition assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — active-site and domain mutagenesis with in vivo functional readout, replicated across labs\",\n      \"pmids\": [\"8505328\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1992,\n      \"finding\": \"The catalytic domain of RAD6/UBC2 is essential for its DNA repair and growth functions, and these functions are distinct from those conferred by the CDC34 tail; a chimeric E2 combining the RAD6 catalytic domain with the CDC34 tail retains both RAD6 and CDC34 activities, demonstrating domain modularity.\",\n      \"method\": \"Chimeric protein construction, complementation assays in yeast\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — domain-swapping with functional reconstitution, defining catalytic vs regulatory domain contributions\",\n      \"pmids\": [\"1639076\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"Ubiquitin-conjugating activity of RAD6/UBC2 (active-site C88) is essential for telomeric silencing; neither Rad18 nor Ubr1 (N-end rule pathway component) is required, indicating silencing is a distinct branch of the RAD6 pathway.\",\n      \"method\": \"Null mutant and point mutant (C88A, C88S) complementation, genetic epistasis\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis with catalytic-dead mutants defining pathway branch\",\n      \"pmids\": [\"9343433\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"Human Rad6B (UBE2B) mediates ubiquitin-dependent proteolysis of cAMP-induced transcriptional repressors hICERIIγ and hATF5 in mammalian cells; degradation requires active ubiquitin-conjugating enzyme activity and results in abrogation of repressor function; endogenous ICER protein is elevated in mHR6B-/- cells.\",\n      \"method\": \"Transfection ubiquitination assays, dominant-negative/antisense constructs, mHR6B knockout cells, yeast two-hybrid substrate identification\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods including knockout cells and in vivo ubiquitination assays\",\n      \"pmids\": [\"10373550\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"Human RAD18 protein interacts with both HHR6A and HHR6B (UBE2A and UBE2B), forming stable protein complexes when co-expressed in yeast cells, purified to near homogeneity.\",\n      \"method\": \"Co-expression and co-purification, complex isolation\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal purification of stable complex, confirmed biochemically\",\n      \"pmids\": [\"10908344\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"RAD6B (UBE2B) is cell cycle-regulated with maximal protein levels in late S/G2 phase; upon DNA damage (adriamycin, cisplatin), RAD6B is recruited from nucleus to chromatin together with RAD18, PCNA, and phosphohistone H3; RAD6B overexpression confers chemoresistance via enhanced post-replication repair (PRR), while antisense depletion causes PRR deficiency and chemosensitivity.\",\n      \"method\": \"Cell cycle synchronization/flow cytometry, in vivo chromatin crosslinking/fractionation, stable transfection KO/OE, PRR assay (DNA molecular weight analysis)\",\n      \"journal\": \"Oncogene\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods including chromatin fractionation, PRR assay, and loss/gain of function with defined molecular readouts\",\n      \"pmids\": [\"14981545\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Fission yeast Rhp6/Ubc2 (RAD6 ortholog) and Ubr1 E3 ligase ubiquitinate the nuclear envelope protein Cut8, which physically interacts with and tethers the proteasome; non-ubiquitinatable Cut8 (K-all-R) fails to enrich nuclear proteasome; rhp6 and ubr1 null mutants also fail to enrich nuclear proteasome, linking RAD6/UBC2 activity to nuclear proteasome localization.\",\n      \"method\": \"Genetic knockouts, ubiquitination-site mutagenesis (K-all-R), subcellular fractionation, co-immunoprecipitation, DNA damage sensitivity assays\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — substrate identification with mutagenesis and functional consequence (nuclear proteasome enrichment), published in Cell\",\n      \"pmids\": [\"16096059\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Human RAD18 complexed with RAD6B (UBE2B) preferentially binds forked and single-stranded DNA structures via the SAP domain of RAD18 (residues 248-282); SAP domain mutation abolishes RAD18 accumulation at DNA damage sites, prevents guidance of DNA Pol η to stalled replication forks, and impairs PCNA monoubiquitination.\",\n      \"method\": \"In vitro DNA binding assays, SAP domain mutagenesis, in vivo localization (fluorescence microscopy), UV sensitivity complementation\",\n      \"journal\": \"Genes to cells : devoted to molecular & cellular mechanisms\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — domain mutagenesis with both in vitro biochemical and in vivo functional validation\",\n      \"pmids\": [\"18363965\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Rad6B (UBE2B) overexpression in breast cells induces β-catenin accumulation via K63-linked polyubiquitination that renders β-catenin insensitive to 26S proteasome degradation; in vitro ubiquitination assays confirm Rad6B directly mediates β-catenin polyubiquitination; Rad6B silencing suppresses β-catenin mono- and polyubiquitination and transcriptional activity.\",\n      \"method\": \"Stable overexpression/siRNA knockdown, in vitro ubiquitination assay, chromatin immunoprecipitation (ubiquitinated β-catenin on chromatin), cycloheximide chase, TOP/Flash reporter assay\",\n      \"journal\": \"Cancer research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — in vitro ubiquitination assay combined with multiple cellular readouts and loss/gain of function\",\n      \"pmids\": [\"18339854\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"CDK1/2-dependent phosphorylation of UBE2B/Ubc2 at Ser120 downregulates N-end rule-dependent degradation: S120D (phosphomimetic) mutant shows 20-fold reduced activity with E3α/Ubr1 (8-fold increase in Km, 2.5-fold decrease in Vmax) while S120A mutant shows 8-fold reduction in Vmax for polyubiquitin chain elongation; phosphorylation does not affect E1-catalyzed E2 transthiolation.\",\n      \"method\": \"In vitro kinetics of E3α-catalyzed ubiquitination, site-directed mutagenesis (S120D/S120A), in vivo N-end rule reporter assays in T47D cells\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro kinetic analysis with mutagenesis and in vivo functional validation\",\n      \"pmids\": [\"21041297\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Rad6B (UBE2B) ubiquitinates β-catenin at lysine 394 (K394) as the major site; Rad6B-interacting region maps to amino acids 131-181 of β-catenin and 50-116 of Rad6B; K394R mutation abolishes ~50% of Rad6B-induced β-catenin ubiquitination and reduces β-catenin transcriptional activity and stability (not rescued by proteasome inhibitor MG132).\",\n      \"method\": \"GST pulldown with deletion mutants, in vitro and in vivo ubiquitination assays, site-directed mutagenesis (K394R), TOP/Flash luciferase reporter, co-immunoprecipitation\",\n      \"journal\": \"Biochimica et biophysica acta\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — site-specific mutagenesis with in vitro and in vivo ubiquitination confirmation and functional readout\",\n      \"pmids\": [\"22705350\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"UBE2B (Ube2b) is a direct transcriptional target of androgen receptor (AR) in mouse Sertoli cells; AR binds androgen-responsive elements in the Ube2b promoter; testosterone-induced UBE2B upregulates H2A ubiquitylation, and UBE2B knockdown blocks testosterone-induced H2A ubiquitylation.\",\n      \"method\": \"Luciferase reporter assay, EMSA, ChIP assay, Western blot, qRT-PCR, siRNA knockdown, AR knockout mouse model\",\n      \"journal\": \"Biology of reproduction\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — ChIP, EMSA and luciferase with orthogonal functional ubiquitination readout\",\n      \"pmids\": [\"23863405\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"UBE2B is upregulated in catabolic myotubes (dexamethasone-treated C2C12); UBE2B knockdown causes a sharp decrease in total (-18%) and K48-linked (-28%) ubiquitin conjugates, implicating UBE2B in proteasome-mediated myofibrillar protein degradation in muscle catabolism.\",\n      \"method\": \"siRNA knockdown, ubiquitin conjugate immunoblotting (total and K48-linked), E2 expression screen\",\n      \"journal\": \"Journal of cachexia, sarcopenia and muscle\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — KD with specific molecular readout (K48-Ub conjugates), single lab\",\n      \"pmids\": [\"27239408\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"RAD6B (UBE2B) mediates PCNA monoubiquitination (translesion synthesis marker) and FANCD2 activation (Fanconi anemia pathway); RAD6B also monoubiquitinates H2AX to modulate γH2AX levels; RAD6B silencing or inhibition impairs homologous recombination and restart of cisplatin-stalled replication forks.\",\n      \"method\": \"RAD6-selective inhibitor (SMI#9), RAD6B siRNA knockdown, DR-GFP homologous recombination assay, DNA fiber assay, immunofluorescence foci (γH2AX, RAD51, FANCD2, POL η), Western blot\",\n      \"journal\": \"Biochimica et biophysica acta. Molecular basis of disease\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (inhibitor + siRNA, functional HR assay, fiber assay) with specific molecular readouts\",\n      \"pmids\": [\"31639439\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Ube2b (UBE2B) mediates ubiquitination and degradation of DNMT3a in dorsal hippocampus neurons during repeated opiate self-administration; DNMT3a degradation leads to demethylation of the CaMKK1 gene promoter, facilitating CaMKK1/CaMKIα/βPIX/Rac1 cascade activation and actin cytoskeleton remodeling underlying behavioral plasticity.\",\n      \"method\": \"In vivo rat model (heroin self-administration), co-immunoprecipitation, ubiquitination assay, promoter methylation analysis, viral vector-mediated Ube2b overexpression/knockdown, behavioral tests\",\n      \"journal\": \"Molecular psychiatry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — substrate identification with pathway epistasis, in vivo model with multiple molecular and behavioral readouts\",\n      \"pmids\": [\"31576007\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"RAD6B (UBE2B) mediates polyubiquitination of histones H2A and H2B; RAD6B deficiency in neurons results in increased genomic instability (γH2AX), elevated p53/p21 levels, neuronal senescence, and neurodegeneration with learning/memory deficits in mice.\",\n      \"method\": \"RAD6B knockout mouse, X-ray irradiation, immunofluorescence, histone ubiquitination assays, behavioral experiments\",\n      \"journal\": \"Frontiers in cellular neuroscience\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — KO mouse with defined molecular phenotype, single study\",\n      \"pmids\": [\"31507381\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"RAD6B (UBE2B) splice variants (RAD6BΔexon4 and RAD6Bintron5ins) retain intact catalytic domains and exhibit functional in vivo ubiquitin-conjugating activity, expressed as 14 and 15 kDa proteins specifically in melanomas but not normal melanocytes.\",\n      \"method\": \"RT-PCR splice variant characterization, in vivo ubiquitination activity assay, Western blot, whole exome sequencing\",\n      \"journal\": \"Cells\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — functional ubiquitination assay on novel isoforms, single lab\",\n      \"pmids\": [\"31683936\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"UBE2B forms a heterodimer complex with E3 ligase RAD18; the UBE2B/RAD18 complex monoubiquitinates ZMYM2, increasing its protein stability; RAD18 knockdown impairs UBE2B-induced ZMYM2 monoubiquitination; UBE2B overexpression shifts ZMYM2 from polyubiquitinated (degraded) to monoubiquitinated (stabilized) form.\",\n      \"method\": \"Co-immunoprecipitation, immunofluorescence colocalization, cycloheximide chase assay, monoubiquitination assay, in vivo xenograft tumor model, RAD18 siRNA knockdown\",\n      \"journal\": \"Bioengineered\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — co-IP with functional ubiquitination and stability readout, single lab\",\n      \"pmids\": [\"35313791\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"UBR4 contains a distinct E3 module with a 'hemiRING' zinc finger, a UBR zinc-finger interacting (UZI) subdomain, and an N-terminal affinity region that specifically recruits UBE2A and UBE2B as cognate E2s; crystal structure of the E2-E3 complex reveals atomic-level specificity determinants; the UZI subdomain allosterically and modestly activates the Ub-loaded UBE2A/UBE2B.\",\n      \"method\": \"X-ray crystallography (E2-E3 complex structure), mutagenesis, in vitro ubiquitination reconstitution, biochemical binding assays\",\n      \"journal\": \"Nature structural & molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — crystal structure with functional reconstitution and mutagenesis validation\",\n      \"pmids\": [\"38182926\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"The human RAD6B (UBE2B) gene is a direct transcriptional target of TCF-4/β-catenin/p300; β-catenin assembled on TCF binding elements in the Rad6B promoter drives its transcription; this creates a positive feedback loop where Rad6B stabilizes β-catenin and β-catenin drives Rad6B expression.\",\n      \"method\": \"EMSA, Western blot of EMSA, UV cross-linking, chromatin immunoprecipitation, luciferase reporter assay, co-transfection\",\n      \"journal\": \"Molecular cancer research : MCR\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (ChIP, EMSA, luciferase) from single lab demonstrating direct transcriptional regulation\",\n      \"pmids\": [\"17050667\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"UBE2B interacts with E3 ligase BIRC2 to catalyze K63-linked ubiquitination of TRAF1, amplifying NF-κB signaling; NF-κB subunit P65 directly binds the UBE2B promoter to enhance its transcription, creating a feedforward loop; UBE2B promotes gastric cancer cell proliferation in vitro and in vivo.\",\n      \"method\": \"Co-immunoprecipitation, K63-linked ubiquitination assay, ChIP assay, luciferase reporter assay, in vivo xenograft, bioinformatics/clinical validation\",\n      \"journal\": \"Molecular cancer research : MCR\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple mechanistic assays (K63-Ub assay, ChIP, luciferase), single lab\",\n      \"pmids\": [\"41661096\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Mouse Ube2b (UBE2B) null mutant spermatids show nuclear and sperm periaxonemal structural anomalies; transition proteins, protamines, and actin distribute normally, but periaxonemal structures are abnormally distributed, demonstrating UBE2B is required for periaxonemal assembly during spermatogenesis.\",\n      \"method\": \"Ube2b knockout mouse, immuno-electron microscopy, in situ hybridization, electron microscopy\",\n      \"journal\": \"Molecular reproduction and development\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — KO mouse with high-resolution ultrastructural analysis defining specific spermatogenic defect\",\n      \"pmids\": [\"12784252\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"RAD6B (UBE2B) polyubiquitinates histones H2A and H2B in mouse spermatogenesis; RNF8 monoubiquitinates H2A and H2B; loss of RAD6B leads to male sterility associated with absent histone polyubiquitination; RAD6B-deficient spermatocytes show senescence contributing to germ cell loss.\",\n      \"method\": \"RNF8 and RAD6B knockout mice, histone ubiquitination assays, fertility/offspring count assays, senescence markers\",\n      \"journal\": \"Cell cycle (Georgetown, Tex.)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — KO mice with direct histone ubiquitination assay, single lab\",\n      \"pmids\": [\"28825854\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"UBE2B promotes ubiquitination and degradation of U2AF1 (splicing factor), thereby modulating the p53/p21 signaling pathway to induce endothelial apoptosis during renal ischemia-reperfusion injury.\",\n      \"method\": \"UBE2B overexpression/knockdown in endothelial cells, co-immunoprecipitation, ubiquitination assay, apoptosis/proliferation assays\",\n      \"journal\": \"Molecular biology reports\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — single pulldown/ubiquitination with functional readout, single lab, limited validation\",\n      \"pmids\": [\"41186784\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"UBE2B (RAD6B/HHR6B) is a ubiquitin-conjugating E2 enzyme that works with multiple E3 ligases—including UBR1/E3α (N-end rule degradation), RAD18 (PCNA monoubiquitination and translesion synthesis), UBR4 (N-degron pathway, via a hemiRING module), RNF8, and BIRC2—to ubiquitinate diverse substrates (N-end rule proteins, PCNA, H2A, H2B, β-catenin at K394 via K63 linkage, DNMT3a, TRAF1, ZMYM2); its activity is regulated by CDK1/2-dependent phosphorylation at Ser120 (which differentially modulates its cognate E3 interactions), it is transcriptionally driven by the β-catenin/TCF-4 axis (creating a positive feedback loop that stabilizes β-catenin and activates Wnt signaling), and it is essential for post-replication DNA repair/translesion synthesis, N-end rule proteolysis, spermatogenesis (histone replacement and periaxonemal assembly), nuclear proteasome enrichment, and neuronal DNA damage response.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"UBE2B (RAD6B/HHR6B) is a ubiquitin-conjugating (E2) enzyme that partners with multiple E3 ligases to regulate DNA damage tolerance, chromatin remodeling, protein quality control, and signal transduction. In complex with RAD18, UBE2B monoubiquitinates PCNA to activate translesion synthesis and post-replication repair, and polyubiquitinates histones H2A and H2B—functions essential for spermatogenesis and neuronal genome maintenance [PMID:18363965, PMID:31639439, PMID:25854, PMID:31507381]. Through the N-end rule E3 ligase UBR1 and the structurally distinct hemiRING E3 UBR4, UBE2B mediates N-degron-dependent proteolysis, an activity down-regulated by CDK1/2 phosphorylation at Ser120 [PMID:1651502, PMID:21041297, PMID:38182926]. UBE2B also stabilizes β-catenin via K63-linked polyubiquitination at K394—creating a positive-feedback loop with TCF-4/β-catenin-driven UBE2B transcription—and ubiquitinates substrates including DNMT3a, TRAF1 (with BIRC2, K63-linked), and ZMYM2 (with RAD18) to modulate Wnt, NF-κB, and epigenetic pathways [PMID:18339854, PMID:22705350, PMID:17050667, PMID:31576007, PMID:41661096, PMID:35313791].\",\n  \"teleology\": [\n    {\n      \"year\": 1991,\n      \"claim\": \"Establishing that UBC2/RAD6 is the obligate E2 for N-end rule degradation and physically associates with UBR1 answered the fundamental question of which ubiquitin-conjugating enzyme mediates this proteolytic pathway.\",\n      \"evidence\": \"Genetic epistasis and co-purification of yeast Ubc2 with N-recognin/UBR1\",\n      \"pmids\": [\"1651502\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mammalian UBE2B–UBR1 interaction not yet demonstrated\", \"Substrate scope of the complex unknown\"]\n    },\n    {\n      \"year\": 1992,\n      \"claim\": \"Domain-swap experiments showed that the RAD6 catalytic core is modular and independently sufficient for DNA repair function, separating its repair role from its C-terminal regulatory tail.\",\n      \"evidence\": \"RAD6/CDC34 chimeric proteins tested for complementation of rad6Δ and cdc34 phenotypes in yeast\",\n      \"pmids\": [\"1639076\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Which E3 partners require the acidic tail versus the catalytic domain\", \"Structural basis of domain modularity unresolved\"]\n    },\n    {\n      \"year\": 1993,\n      \"claim\": \"Mapping the UBR1-binding region to a C-terminal fragment and showing that the acidic tail of Ubc2 stabilizes the E2–E3 complex distinguished the catalytic and E3-interaction surfaces of the enzyme.\",\n      \"evidence\": \"Two-hybrid mapping, C88A mutagenesis, dominant-negative assays in yeast\",\n      \"pmids\": [\"8505328\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Atomic-resolution structure of the UBE2B–UBR1 interface not determined\", \"How acidic tail modulates activity kinetically not quantified\"]\n    },\n    {\n      \"year\": 1997,\n      \"claim\": \"Demonstrating that RAD6 catalytic activity is required for telomeric silencing but neither RAD18 nor UBR1 is involved revealed a third, genetically distinct effector branch of the RAD6 pathway.\",\n      \"evidence\": \"Catalytic-dead mutants (C88A/C88S) and rad18Δ/ubr1Δ epistasis in yeast silencing assays\",\n      \"pmids\": [\"9343433\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"E3 ligase for silencing branch unidentified\", \"Histone substrate specificity in silencing not shown\"]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Identifying human RAD6B as the E2 that ubiquitinates and degrades cAMP-responsive transcriptional repressors (ICERII γ, ATF5) extended its substrates beyond DNA repair into mammalian transcriptional regulation.\",\n      \"evidence\": \"Ubiquitination assays, dominant-negative constructs, and elevated ICER in mHR6B−/− mouse cells\",\n      \"pmids\": [\"10373550\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"E3 partner for ICER/ATF5 degradation not identified\", \"Physiological consequence of ICER stabilization beyond cAMP signaling unclear\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"Biochemical isolation of a stable human RAD18–UBE2B complex established the conserved E2–E3 partnership that channels RAD6 activity toward DNA damage tolerance in human cells.\",\n      \"evidence\": \"Co-expression in yeast and purification of the RAD18–HHR6B heterodimer\",\n      \"pmids\": [\"10908344\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"PCNA monoubiquitination by this complex not yet shown directly\", \"Stoichiometry and structural details of the complex unresolved\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Ube2b knockout mice revealed that UBE2B is required for periaxonemal structural assembly during spermiogenesis, defining its non-redundant role with UBE2A in male fertility.\",\n      \"evidence\": \"Immuno-electron microscopy and ultrastructural analysis of Ube2b−/− spermatids\",\n      \"pmids\": [\"12784252\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Specific ubiquitination substrates mediating periaxonemal assembly not identified\", \"Whether histone replacement defects contribute to the structural phenotype unclear\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Showing that RAD6B peaks in late S/G2, is recruited to chromatin with RAD18 and PCNA after DNA damage, and confers chemoresistance via post-replication repair linked its cell-cycle regulation to a specific repair function.\",\n      \"evidence\": \"Chromatin fractionation, PRR assay, and gain/loss-of-function in synchronized cells treated with cisplatin/adriamycin\",\n      \"pmids\": [\"14981545\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether PCNA monoubiquitination is the sole PRR-relevant substrate not resolved\", \"Mechanism of cell-cycle-dependent RAD6B accumulation unknown\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Discovery that RAD6/UBR1-mediated ubiquitination of Cut8 is required to tether proteasomes to the nuclear envelope expanded UBE2B's role to proteasome localization.\",\n      \"evidence\": \"Cut8 K-to-R mutagenesis, rhp6Δ/ubr1Δ mutants, subcellular fractionation of proteasome in fission yeast\",\n      \"pmids\": [\"16096059\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether a mammalian Cut8 ortholog or analogous mechanism exists is unknown\", \"Whether proteasome mis-localization explains any RAD6-null phenotypes untested\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Identifying UBE2B as a direct transcriptional target of TCF-4/β-catenin established a positive-feedback loop coupling Wnt pathway activation to E2 enzyme abundance.\",\n      \"evidence\": \"ChIP, EMSA, and luciferase reporter assays on the RAD6B promoter with β-catenin/TCF-4\",\n      \"pmids\": [\"17050667\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether this loop operates in vivo in Wnt-driven tumors not shown\", \"Quantitative contribution of this loop versus other β-catenin stabilization mechanisms unknown\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Demonstrating that the RAD18 SAP domain directs the RAD18–UBE2B complex to fork structures and is required for PCNA monoubiquitination and Pol η recruitment resolved how substrate targeting occurs in translesion synthesis.\",\n      \"evidence\": \"In vitro DNA binding with SAP domain mutants, UV sensitivity complementation, PCNA ubiquitination and Pol η foci assays\",\n      \"pmids\": [\"18363965\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether UBE2B contributes to DNA-binding specificity unknown\", \"Role of other RAD18 domains in complex activation uncharacterized\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Revealing that UBE2B directly K63-polyubiquitinates β-catenin, stabilizing it against proteasomal degradation, identified a non-canonical ubiquitin signal that activates rather than degrades a key oncoprotein.\",\n      \"evidence\": \"In vitro ubiquitination, K63-linkage-specific antibodies, siRNA knockdown, TOP/Flash reporter in breast cancer cells\",\n      \"pmids\": [\"18339854\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"E3 ligase cooperating with UBE2B for β-catenin ubiquitination not identified\", \"How K63-Ub renders β-catenin proteasome-resistant mechanistically unclear\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Quantitative kinetic analysis of CDK1/2-mediated Ser120 phosphorylation showed it selectively impairs UBE2B's activity with UBR1/E3α (20-fold) without affecting E1 charging, revealing cell-cycle-dependent tuning of E2–E3 partnerships.\",\n      \"evidence\": \"In vitro kinetics (Km/Vmax) with S120D/S120A mutants, in vivo N-end rule reporter assay\",\n      \"pmids\": [\"21041297\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Impact of Ser120 phosphorylation on RAD18 or other E3 partnerships not measured\", \"In vivo stoichiometry of phosphorylated UBE2B across cell cycle unknown\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Mapping the UBE2B-β-catenin interface and identifying K394 as the major ubiquitination site provided substrate-level resolution for the non-degradative ubiquitin signal that stabilizes β-catenin.\",\n      \"evidence\": \"Deletion mapping, K394R mutagenesis, in vitro/in vivo ubiquitination, TOP/Flash and stability assays\",\n      \"pmids\": [\"22705350\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Other ubiquitination sites on β-catenin by UBE2B likely exist (K394R only reduces ~50%)\", \"Structural basis of the UBE2B–β-catenin interaction unresolved\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Showing that androgen receptor directly drives UBE2B transcription in Sertoli cells and that UBE2B mediates testosterone-induced H2A ubiquitylation connected hormonal regulation of spermatogenesis to chromatin ubiquitination.\",\n      \"evidence\": \"AR ChIP on Ube2b promoter, EMSA, luciferase reporter, siRNA-mediated UBE2B knockdown abolishing H2A-Ub\",\n      \"pmids\": [\"23863405\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Specific H2A sites ubiquitinated by UBE2B in Sertoli cells not mapped\", \"Whether AR-driven UBE2B is sufficient to explain spermatogenic histone replacement unknown\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Genetic dissection using RNF8 and RAD6B knockout mice showed that RNF8 monoubiquitinates and RAD6B polyubiquitinates H2A/H2B during spermatogenesis, and that RAD6B loss causes spermatocyte senescence and male sterility.\",\n      \"evidence\": \"Double and single KO mice, histone ubiquitination assays, senescence markers, fertility assays\",\n      \"pmids\": [\"28825854\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether RNF8 is the E3 that directs RAD6B polyubiquitination of histones or acts independently not fully resolved\", \"Chain-type specificity of histone polyubiquitination in spermatogenesis not determined\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Multiple studies converged to show UBE2B is central to the neuronal DNA damage response: it monoubiquitinates PCNA and H2AX, activates FANCD2, and supports homologous recombination; its loss causes genomic instability, neuronal senescence, and memory deficits; and it ubiquitinates DNMT3a to drive epigenetic remodeling underlying addiction-related plasticity.\",\n      \"evidence\": \"RAD6B KO mice (neurodegeneration phenotype), RAD6 inhibitor + siRNA with HR/fiber assays, in vivo heroin self-administration model with co-IP/ubiquitination/methylation analysis\",\n      \"pmids\": [\"31639439\", \"31507381\", \"31576007\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether UBE2B-mediated H2AX monoubiquitination is direct or requires a specific E3 not established\", \"Relative contributions of UBE2A versus UBE2B in neurons unclear\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Identification of ZMYM2 as a substrate whose stability is increased by UBE2B/RAD18-dependent monoubiquitination (shifting it from poly- to mono-Ub) revealed that UBE2B can switch substrate fate from degradation to stabilization.\",\n      \"evidence\": \"Co-IP, cycloheximide chase, monoubiquitination assay, RAD18 knockdown, xenograft model\",\n      \"pmids\": [\"35313791\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"ZMYM2 ubiquitination site not mapped\", \"Whether ZMYM2 stabilization mediates the oncogenic effects not mechanistically resolved\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Crystal structure of UBR4's hemiRING–UBE2B complex revealed the atomic determinants of E2 selectivity and showed that the UZI subdomain allosterically activates ubiquitin-loaded UBE2B, defining a new E3 architecture for the N-degron pathway.\",\n      \"evidence\": \"X-ray crystallography, structure-guided mutagenesis, in vitro ubiquitination reconstitution\",\n      \"pmids\": [\"38182926\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Physiological substrates of the UBR4–UBE2B complex in mammalian cells not identified\", \"Whether UBR4 competes with UBR1 for UBE2B binding in vivo unknown\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"UBE2B was reported to cooperate with BIRC2 to K63-polyubiquitinate TRAF1, amplifying NF-κB signaling in a feedforward loop with P65-driven UBE2B transcription, connecting UBE2B to innate immune and inflammatory signaling.\",\n      \"evidence\": \"Co-IP, K63-Ub assay, ChIP on UBE2B promoter, luciferase reporter, xenograft in gastric cancer cells\",\n      \"pmids\": [\"41661096\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Endogenous validation of BIRC2–UBE2B interaction in non-cancer contexts missing\", \"Whether K63-Ub on TRAF1 is direct or scaffold-mediated not resolved\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How UBE2B is selectively allocated among its numerous E3 partners in vivo—and whether Ser120 phosphorylation differentially modulates non-UBR1 partnerships (RAD18, UBR4, BIRC2)—remains the central unresolved question for understanding pathway-specific regulation.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"Quantitative in vivo measurements of UBE2B partitioning among E3s lacking\", \"Structural basis for phospho-Ser120 selectivity beyond UBR1 not determined\", \"Whether UBE2A fully compensates for UBE2B loss in specific tissues remains untested\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [0, 1, 4, 9, 10, 11, 14, 15, 16, 23, 24]},\n      {\"term_id\": \"GO:0016874\", \"supporting_discovery_ids\": [0, 9, 19]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [6, 7, 14]},\n      {\"term_id\": \"GO:0005694\", \"supporting_discovery_ids\": [6, 12, 16, 23]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-73894\", \"supporting_discovery_ids\": [2, 6, 8, 14]},\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [0, 4, 10, 15, 19]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [9, 11, 20, 21]},\n      {\"term_id\": \"R-HSA-4839726\", \"supporting_discovery_ids\": [12, 16, 23]},\n      {\"term_id\": \"R-HSA-1474165\", \"supporting_discovery_ids\": [22, 23]}\n    ],\n    \"complexes\": [\n      \"RAD18–UBE2B heterodimer\",\n      \"UBR1–UBE2B complex\",\n      \"UBR4–UBE2B complex\"\n    ],\n    \"partners\": [\n      \"RAD18\",\n      \"UBR1\",\n      \"UBR4\",\n      \"BIRC2\",\n      \"RNF8\",\n      \"CTNNB1\",\n      \"DNMT3A\",\n      \"ZMYM2\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}