{"gene":"UBE2D2","run_date":"2026-06-10T10:51:56","timeline":{"discoveries":[{"year":1990,"finding":"Yeast UBC4 (ortholog of UBE2D2) mediates selective degradation of short-lived and abnormal proteins by generating high molecular weight ubiquitin-protein conjugates in vivo; loss of UBC4/UBC5 markedly reduces turnover of short-lived proteins and canavanyl-peptides but not long-lived proteins.","method":"Yeast genetics (ubc4ubc5 deletion mutants), in vivo ubiquitin conjugate detection, heat stress phenotype analysis","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 2 / Strong — foundational study with genetic deletion, in vivo conjugation assay, and multiple phenotypic readouts; independently replicated across subsequent studies","pmids":["2154373"],"is_preprint":false},{"year":1993,"finding":"Crystal structure of yeast Ubc4 (UBE2D2 ortholog) determined at 2.7 Å; the enzyme is an α/β protein with the ubiquitin-accepting cysteine located in a cleft between two loops, and the conserved surface adjacent to the active-site cysteine is proposed to function in protein–protein binding during ubiquitin thiol ester formation.","method":"X-ray crystallography (molecular replacement, 2.7 Å resolution)","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structure with functional interpretation of active-site architecture; foundational structural paper","pmids":["8268156"],"is_preprint":false},{"year":1995,"finding":"Human UBC4 (UBE2D2) is required for E6AP (E6-associated protein)-mediated ubiquitinylation of p53 in the HPV E6 pathway; reconstitution of p53 ubiquitinylation from purified components showed UBC4 specifically ubiquitinylates E6AP, and in vivo inhibition of UBC4 blocks E6-stimulated p53 degradation.","method":"In vitro ubiquitinylation reconstitution with purified components, in vivo inhibition assay","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 / Moderate — biochemical reconstitution from purified components plus in vivo inhibition, single lab","pmids":["7724550"],"is_preprint":false},{"year":1995,"finding":"Yeast UBC4 monoubiquitinates itself in vivo at K144 in an intermolecular (E2–E2) reaction; a second lysine (K64) is required for ubiquitination at K144; cross-linking demonstrates direct UBC4–UBC4 homointeraction in vitro.","method":"In vivo epitope-tagged ubiquitin coexpression, site-directed mutagenesis, chemical mapping, in vitro cross-linking","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1-2 / Moderate — mutagenesis plus in vitro cross-linking plus in vivo detection; multiple orthogonal methods in one study","pmids":["7756256"],"is_preprint":false},{"year":2000,"finding":"The RING-H2 protein APC11 and UBC4 (UBE2D2 ortholog) are sufficient to ubiquitinate APC substrates securin and cyclin B in vitro; APC11 alone supports multi-ubiquitin chain synthesis with E1 and UBC4, and the RING-H2 finger integrity is required for this activity.","method":"In vitro ubiquitination assay with recombinant proteins expressed in E. coli, RING domain mutagenesis","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 / Moderate — in vitro reconstitution with purified recombinant proteins plus mutagenesis, single lab","pmids":["10922056"],"is_preprint":false},{"year":2000,"finding":"UBC4 (UBE2D2) and Ubc3 catalyze SCF(β-TRCP)-dependent, phosphorylation-dependent ubiquitination of IκBα; both E2s associate with the SCF(β-TRCP) complex isolated from human cells, and UBC4 is 19-fold more efficient than Ubc3 in catalyzing this reaction in vitro.","method":"In vitro ubiquitination reconstitution with recombinant components, co-immunoprecipitation with human cell SCF complex","journal":"Oncogene","confidence":"High","confidence_rationale":"Tier 1 / Moderate — in vitro reconstitution, quantitative comparison, Co-IP with endogenous complex; single lab","pmids":["10918611"],"is_preprint":false},{"year":2004,"finding":"NMR solution structure of human UbcH5B (UBE2D2) determined; residues of UbcH5B important for binding to the CNOT4 RING domain were identified by NMR chemical shift perturbation, and a structural model of the UbcH5B/CNOT4 RING complex was generated by HADDOCK docking, revealing differences from the c-Cbl/UbcH7 complex at specific residues relevant to E2/E3 specificity.","method":"NMR chemical shift perturbation, homology modeling, HADDOCK docking, mutagenesis","journal":"Structure (London, England : 1993)","confidence":"High","confidence_rationale":"Tier 1 / Moderate — NMR structure plus functional mapping plus docking; multiple orthogonal methods in single study","pmids":["15062086"],"is_preprint":false},{"year":2004,"finding":"UbcH5B/C (UBE2D2/3) are the physiological E2 enzymes for Mdm2-mediated ubiquitination and degradation of p53 in vivo; siRNA knockdown of UbcH5B/C in cells causes accumulation of both Mdm2 and p53, inhibits p53 ubiquitination and degradation, while in vitro screening showed only UbcH5A/B/C and E2-25K support Mdm2-mediated p53 ubiquitination.","method":"In vitro E2 panel screen, siRNA knockdown in mammalian cells, p53/Mdm2 ubiquitination and stability assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Strong — in vitro screen combined with siRNA knockdown and in-cell ubiquitination assay; two orthogonal approaches","pmids":["15280377"],"is_preprint":false},{"year":2004,"finding":"NMR solution structure of UbcH5B (UBE2D2) solved; the N-terminal helix involved in E3 binding displays a different orientation compared to crystal structures; Asn77 adopts multiple side-chain conformations in solution (contrasting single conformation in crystals), with implications for catalytic function.","method":"NMR spectroscopy (relaxation data, automated NOE assignments, homology modeling)","journal":"Journal of molecular biology","confidence":"Medium","confidence_rationale":"Tier 1 / Weak — NMR structure in solution, single lab, structural interpretation without full mutagenesis validation","pmids":["15522302"],"is_preprint":false},{"year":2008,"finding":"Ubc4/5 (UBE2D2 ortholog) cooperates with the E3 c-Cbl to ubiquitinate EGFR; upon EGF stimulation Ubc4/5 and c-Cbl co-relocalize from plasma membrane to Hrs-positive endosomes, indicating EGFR continues to be ubiquitinated after internalization to facilitate polyubiquitination and subsequent lysosomal sorting.","method":"Localization (fluorescence microscopy), siRNA knockdown, in vitro ubiquitination assay, dominant-negative ubiquitin mutant experiments, EGFR degradation assay","journal":"Molecular biology of the cell","confidence":"High","confidence_rationale":"Tier 2 / Moderate — knockdown, localization, in vitro activity, and dominant-negative experiments provide multiple orthogonal lines of evidence","pmids":["18508924"],"is_preprint":false},{"year":2008,"finding":"UBE2D2 is the E2 ubiquitin-conjugating enzyme responsible for SCF(FBXW2)-mediated ubiquitination and proteasomal degradation of the placental transcription factor GCM1; UBE2D2 enzyme activity is required for GCM1 ubiquitination and association with the SCF(FBXW2) complex, and UBE2D2 knockdown prolongs GCM1 half-life in vivo.","method":"In vitro ubiquitination assay, Co-IP with SCF complex, RNA interference knockdown, pulse-chase half-life measurement","journal":"Biology of reproduction","confidence":"High","confidence_rationale":"Tier 2 / Moderate — in vitro assay, Co-IP, siRNA knockdown, and half-life assay; multiple orthogonal methods","pmids":["18703417"],"is_preprint":false},{"year":2010,"finding":"Crystal structure of UbcH5b~ubiquitin intermediate (oxyester-linked) determined at 2.2 Å; the conjugate self-assembles into an infinite spiral through backside interaction, providing multiple E2 active sites; biochemical assays show the self-assembled UbcH5b~Ub bridges the gap between substrate lysine and the E2 catalytic cysteine to enable efficient ubiquitination.","method":"X-ray crystallography (2.2 Å), in vitro ubiquitination biochemical assays","journal":"Structure (London, England : 1993)","confidence":"High","confidence_rationale":"Tier 1 / Moderate — crystal structure of catalytic intermediate combined with biochemical validation; single lab but two orthogonal methods","pmids":["20152160"],"is_preprint":false},{"year":2010,"finding":"Crystal structures of E4B U box domain free and bound to UbcH5c and Ubc4 (UBE2D2 ortholog) determined by X-ray crystallography; the E4B U box is monomeric (unlike other U box domains), stabilized by a hydrogen-bond network; structural and calorimetric/NMR binding data suggest allosteric regulation of UbcH5c/Ubc4 by E4B U box.","method":"X-ray crystallography, NMR spectroscopy, isothermal titration calorimetry","journal":"Structure (London, England : 1993)","confidence":"High","confidence_rationale":"Tier 1 / Moderate — crystal structures plus NMR plus ITC; multiple orthogonal methods in single study","pmids":["20696396"],"is_preprint":false},{"year":2011,"finding":"The essential function of Ubc4/Ubc5 (UBE2D2 orthologs) in yeast is HECT E3-dependent (likely Rsp5); mutation N78S that specifically abolishes RING E3-catalyzed isopeptide bond formation but not HECT E3 transthiolation rescues lethality of ubc4/ubc5 deletion, establishing that Ubc4 acts as a monoubiquitinating E2 in RING E3 pathways while performing a critical function with a HECT E3.","method":"Yeast genetics (deletion and rescue with point mutants), in vitro ubiquitination assay, active-site and E3-binding mutagenesis","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1-2 / Moderate — epistasis genetics plus in vitro mutagenesis with mechanistic distinction between RING and HECT pathways","pmids":["21357418"],"is_preprint":false},{"year":2014,"finding":"UBE2D2 (along with UBE2D3, UBE2N, UBE2L3) is essential for Parkin-dependent mitophagy; UBE2D2/3 knockdown reduces autophagic clearance of depolarized mitochondria without affecting PINK1 stabilization or Parkin translocation; combined knockdown of all these E2s reduces mitochondrial polyubiquitylation and p62 recruitment, and UBE2D2/3 contribute to ubiquitination of mitofusins, TOM20, TOM70, VDAC1, and Parkin.","method":"siRNA knockdown in mammalian cells, mitophagy assay (autophagic clearance), immunofluorescence, ubiquitylation assays for specific substrates","journal":"Journal of cell science","confidence":"High","confidence_rationale":"Tier 2 / Moderate — systematic siRNA knockdown with multiple substrate and functional readouts; single lab with several orthogonal assays","pmids":["24906799"],"is_preprint":false},{"year":2015,"finding":"RING E3 AO7 (RNF25) binds UbcH5B (UBE2D2) with unusually high affinity via a unique UbcH5B-binding region (U5BR) connected by a linker to its RING domain, forming a clamp around the E2; the U5BR contacts the backside of UbcH5B distinct from both the active site and RING-interacting region; high-affinity clamp binding blocks stimulatory non-covalent ubiquitin binding to the UbcH5B backside, thereby decreasing ubiquitination rate.","method":"Co-crystallization, X-ray crystallography, ubiquitination rate assays, RING domain mutagenesis","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — crystal structure plus functional ubiquitination assays plus mutagenesis; multiple orthogonal methods","pmids":["26475854"],"is_preprint":false},{"year":2016,"finding":"UBE2D2 does not interact with the muscle E3 ligase MuRF1 and is not the cognate E2 for MuRF1-dependent muscle wasting; yeast two-hybrid, surface plasmon resonance, and cell-based assays all showed no functional interaction between UBE2D2 and MuRF1, and UBE2D2 was unable to promote MuRF1-dependent α-actin degradation.","method":"Yeast two-hybrid, surface plasmon resonance (SPR), HEK293T cell-based ubiquitination/degradation assay, mRNA quantification during atrophy","journal":"The international journal of biochemistry & cell biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — three independent methods all yielding negative result; well-controlled negative finding","pmids":["27378730"],"is_preprint":false},{"year":2018,"finding":"Crystal structure of cIAP1 RING dimer bound to UbcH5B covalently linked to ubiquitin (UbcH5B-Ub) and a non-covalent ubiquitin at 1.7 Å; cIAP1 RING promotes a closed UbcH5B-Ub conformation priming the thioester for transfer; non-covalent ubiquitin binding to the UbcH5B backside abuts the α1β1-loop and further stabilizes the closed active conformation.","method":"X-ray crystallography (1.7 Å), biochemical ubiquitin transfer assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — high-resolution crystal structure of ternary complex plus biochemical functional analyses; single lab","pmids":["30523153"],"is_preprint":false},{"year":2021,"finding":"UbcH5b (UBE2D2) supports HECTD3 auto-ubiquitination in vitro; the triterpenoid PC3-15 directly binds UbcH5b and inhibits UbcH5b-mediated p62 ubiquitination; the UbcH5b–p62 axis confers TNBC cell resistance to lapatinib by promoting autophagy.","method":"In vitro ubiquitination assay, FRET-based inhibitor screen, direct binding assay, siRNA knockdown, in vivo mouse xenograft","journal":"Cancer letters","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vitro assays plus direct binding plus cellular and in vivo studies; multiple orthogonal methods, single lab","pmids":["33607208"],"is_preprint":false},{"year":2021,"finding":"miR-30b-5p upregulation by ox-LDL reduces UBE2D2 ubiquitination activity, stabilizing KAT2B; elevated KAT2B then acetylates HMGB1, causing its nuclear exit and secretion, which promotes M1 macrophage polarization and recruitment.","method":"Luciferase reporter assay, co-immunoprecipitation, ubiquitination assay, qRT-PCR, flow cytometry, transwell assay","journal":"Atherosclerosis","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — multiple assays (Co-IP, ubiquitination, luciferase) supporting the pathway; single lab","pmids":["33812169"],"is_preprint":false},{"year":2022,"finding":"MUL1 RING domain recruits both UBE2D2 and the substrate p53-TAD simultaneously; RING(MUL1) binding induces the closed conformation of UBE2D2~Ub and accelerates its hydrolysis (suppressed by N77A mutation); TADp53 binding affinity to MUL1-RING is enhanced in complex with UBE2D2~Ub, indicating multivalent substrate recognition underlies ubiquitination of this intrinsically disordered protein.","method":"NMR, crystal structure determination of RING(MUL1):UBE2D2 complex, oxyester mimetic UBE2D2~Ub assays, hydrolysis kinetics, binding affinity measurements","journal":"The FEBS journal","confidence":"High","confidence_rationale":"Tier 1 / Moderate — structural characterization combined with biochemical assays and mutagenesis; multiple orthogonal methods in single study","pmids":["35048531"],"is_preprint":false},{"year":2023,"finding":"Depletion of UBE2D2 (or UBE2D1) in endothelial cells increases steady-state VEGFR2 levels at the plasma membrane, enhances VEGFR2 recycling, amplifies VEGF-A-stimulated MAPK/PLCγ1/Akt signaling, and stimulates endothelial tubulogenesis; establishing UBE2D2 as a regulator of VEGFR2 ubiquitination, trafficking, and angiogenic signaling.","method":"siRNA knockdown (reverse genetics screen), flow cytometry, cell-surface biotinylation, recycling assay, Western blot signaling analysis, tubulogenesis assay","journal":"Journal of cell science","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — systematic knockdown with multiple cellular readouts; single lab","pmids":["37226882"],"is_preprint":false},{"year":2024,"finding":"Ubiquitin variants (UbVs) that inhibit Ube2d2 (UbcH5b) were identified; two characterized UbVs bind Ube2d2 with low micromolar affinity at a site overlapping with E1 binding, and the more inhibitory UbV additionally blocks the non-covalent ubiquitin-binding backside site, inhibiting ubiquitin chain building.","method":"Structural characterization (crystallography/biophysics implied), binding affinity measurements, in vitro ubiquitin chain building assay, specificity profiling within Ube2d family","journal":"The FEBS journal","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — structural/biophysical characterization plus functional chain-building assay; single lab, abstract-level description","pmids":["39473070"],"is_preprint":false}],"current_model":"UBE2D2 (UbcH5B/UBC4) is a ubiquitin-conjugating E2 enzyme that, after loading ubiquitin onto its catalytic cysteine via E1, cooperates with a broad range of both RING (e.g., APC11, c-Cbl, Mdm2, MUL1, cIAP1, FBXW2-SCF, SCF-β-TRCP, AO7/RNF25) and HECT (e.g., E6AP, HECTD3, Rsp5) E3 ligases to ubiquitinate substrates including p53, IκBα, cyclin B, securin, EGFR, GCM1, and mitochondrial proteins; its closed catalytic conformation is stabilized by RING domain binding and by non-covalent ubiquitin engagement at its backside surface, while its self-assembled UbcH5b~Ub spiral intermediate facilitates processive substrate ubiquitination, and it plays essential cellular roles in proteasomal degradation of short-lived/abnormal proteins, PINK1-Parkin-dependent mitophagy, VEGFR2 trafficking/angiogenesis, and cell cycle progression."},"narrative":{"mechanistic_narrative":"UBE2D2 (UbcH5B/UBC4) is a promiscuous ubiquitin-conjugating E2 enzyme central to proteasomal turnover of short-lived and abnormal proteins, a role first established genetically in yeast where loss of UBC4/UBC5 selectively impairs degradation of unstable substrates [PMID:2154373]. After charging with ubiquitin via E1, it transfers ubiquitin to substrate lysines in cooperation with a broad array of RING and HECT E3 ligases, including E6AP for p53 [PMID:7724550], Mdm2 for p53 [PMID:15280377], the APC11 RING for securin and cyclin B [PMID:10922056], SCF complexes for IκBα and the transcription factor GCM1 [PMID:10918611, PMID:18703417], and c-Cbl for EGFR [PMID:18508924]. Its catalytic mechanism is governed by conformational control: RING-domain binding (e.g., cIAP1, MUL1) and non-covalent ubiquitin engagement at the E2 backside surface stabilize a closed UbcH5B~Ub conformation that primes the thioester for transfer [PMID:30523153, PMID:35048531], and self-assembly of the UbcH5b~Ub conjugate into a spiral via backside interactions provides multiple active sites for processive ubiquitination [PMID:20152160]. UBE2D2 supports diverse cellular processes, contributing to PINK1-Parkin-dependent mitophagy through ubiquitination of mitochondrial proteins [PMID:24906799], to VEGFR2 trafficking and angiogenic signaling in endothelial cells [PMID:37226882], and—via genetic analysis in yeast—it performs an essential HECT-E3-dependent function while acting as a monoubiquitinating E2 in RING-E3 pathways [PMID:21357418].","teleology":[{"year":1990,"claim":"Established the core physiological role of this E2 in selective protein turnover, answering whether UBC4/UBC5 activity is required for degradation of unstable proteins.","evidence":"Yeast deletion genetics with in vivo ubiquitin conjugate detection and stress phenotyping","pmids":["2154373"],"confidence":"High","gaps":["Did not identify specific E3 partners or substrates","Mammalian ortholog function not directly tested"]},{"year":1993,"claim":"Defined the catalytic architecture, locating the ubiquitin-accepting cysteine in a cleft and identifying a conserved surface for E3 binding.","evidence":"X-ray crystallography of yeast Ubc4 at 2.7 Å","pmids":["8268156"],"confidence":"High","gaps":["No substrate or E3-bound complex","Solution dynamics not addressed"]},{"year":1995,"claim":"Connected the E2 to specific E3-substrate pathways (E6AP/p53) and revealed intermolecular self-ubiquitination, showing how the enzyme engages HECT ligases and itself.","evidence":"In vitro reconstitution with purified components, in vivo inhibition, mutagenesis, and cross-linking","pmids":["7724550","7756256"],"confidence":"High","gaps":["Physiological significance of K144 self-ubiquitination unclear","p53 work limited to the HPV E6 context"]},{"year":2000,"claim":"Demonstrated broad E3 partnership by reconstituting APC11- and SCF(β-TRCP)-dependent ubiquitination, establishing UBC4 as the preferred E2 for cell-cycle and IκBα substrates.","evidence":"In vitro ubiquitination with recombinant proteins, RING mutagenesis, and Co-IP with human SCF complex","pmids":["10922056","10918611"],"confidence":"High","gaps":["In-cell requirement for these specific reactions not fully established","Chain-type selectivity not resolved"]},{"year":2004,"claim":"Mapped E2/E3 interaction surfaces and identified UbcH5B/C as the physiological E2 for Mdm2-mediated p53 degradation, linking structure to in-cell function.","evidence":"NMR structures with chemical-shift mapping and docking; in vitro E2 panel screen plus siRNA knockdown with stability assays","pmids":["15062086","15280377","15522302"],"confidence":"High","gaps":["Redundancy among UbcH5 isoforms not fully dissected","Solution vs crystal conformational differences functionally unvalidated (#8 Medium)"]},{"year":2008,"claim":"Extended the substrate repertoire to receptor trafficking (EGFR via c-Cbl) and placental transcription factor turnover (GCM1 via SCF(FBXW2)), with spatial regulation at endosomes.","evidence":"siRNA knockdown, fluorescence localization, in vitro ubiquitination, Co-IP, and pulse-chase half-life measurement","pmids":["18508924","18703417"],"confidence":"High","gaps":["Direct vs indirect contribution to endosomal sorting not separated","Chain linkage on these substrates not determined"]},{"year":2010,"claim":"Revealed the mechanistic basis of processivity and allosteric control through the self-assembled UbcH5b~Ub spiral and E4B U-box binding.","evidence":"Crystal structure of UbcH5b~Ub oxyester intermediate with biochemical assays; E4B U-box crystal structures with NMR and ITC","pmids":["20152160","20696396"],"confidence":"High","gaps":["Physiological prevalence of the spiral intermediate uncertain","Functional consequence of E4B allostery in cells not shown"]},{"year":2011,"claim":"Resolved the division of labor between RING and HECT pathways, showing the essential cellular function depends on HECT E3 (Rsp5) transthiolation while RING E3s drive isopeptide bond formation.","evidence":"Yeast deletion/rescue with the N78S mutant, in vitro ubiquitination, and active-site mutagenesis","pmids":["21357418"],"confidence":"High","gaps":["Identity of the essential HECT substrates not defined","Mammalian equivalence of this RING/HECT split untested"]},{"year":2014,"claim":"Defined a role in PINK1-Parkin mitophagy, showing the E2 is required for mitochondrial substrate ubiquitination downstream of Parkin recruitment.","evidence":"siRNA knockdown of multiple E2s, mitophagy clearance assays, immunofluorescence, and substrate ubiquitylation assays","pmids":["24906799"],"confidence":"High","gaps":["Functional redundancy with UBE2D3/UBE2N/UBE2L3 obscures individual contribution","Direct vs indirect substrate ubiquitination not distinguished"]},{"year":2015,"claim":"Showed that E3 binding can be inhibitory, with the AO7/RNF25 U5BR clamp blocking the stimulatory backside ubiquitin site to slow ubiquitination.","evidence":"Co-crystallization, X-ray crystallography, ubiquitination rate assays, and RING mutagenesis","pmids":["26475854"],"confidence":"High","gaps":["Cellular relevance of AO7-mediated inhibition not established","Generality across other E3s unknown"]},{"year":2016,"claim":"Delimited E2/E3 specificity by ruling out MuRF1 as a cognate partner, refining which ligases recruit this E2.","evidence":"Yeast two-hybrid, SPR, and cell-based degradation assays (negative result)","pmids":["27378730"],"confidence":"Medium","gaps":["Negative finding for one E3 does not generalize","Conditions or modifications enabling weak interaction not exhaustively tested"]},{"year":2018,"claim":"Provided high-resolution mechanistic detail of how a RING dimer (cIAP1) and non-covalent backside ubiquitin together stabilize the catalytically primed closed conformation.","evidence":"X-ray crystallography at 1.7 Å of the ternary complex with ubiquitin transfer assays","pmids":["30523153"],"confidence":"High","gaps":["Snapshot does not capture transfer dynamics","Substrate-bound geometry not included"]},{"year":2021,"claim":"Extended substrate and disease links to HECTD3 auto-ubiquitination, p62/autophagy-mediated drug resistance, and KAT2B/HMGB1-driven macrophage polarization, while identifying a small-molecule inhibitor binding the E2.","evidence":"In vitro ubiquitination, FRET inhibitor screen and direct binding, siRNA knockdown, xenograft; plus luciferase, Co-IP, ubiquitination, and flow cytometry","pmids":["33607208","33812169"],"confidence":"Medium","gaps":["Pathway-level rather than direct mechanistic dissection (#19)","Inhibitor selectivity within the UbcH5 family not fully defined (#18)"]},{"year":2022,"claim":"Demonstrated multivalent substrate recognition, showing MUL1 RING simultaneously recruits E2 and the disordered p53-TAD and enhances substrate affinity in the conjugate complex.","evidence":"NMR, crystal structure of RING(MUL1):UBE2D2, oxyester-mimetic conjugate assays, and hydrolysis kinetics with N77A mutant","pmids":["35048531"],"confidence":"High","gaps":["Cellular evidence for MUL1-mediated p53 ubiquitination not provided","Applicability to other disordered substrates untested"]},{"year":2023,"claim":"Established a cellular role in VEGFR2 trafficking and angiogenic signaling, showing depletion enhances receptor recycling and downstream signaling.","evidence":"siRNA knockdown, cell-surface biotinylation, recycling assay, signaling Western blots, and tubulogenesis assay in endothelial cells","pmids":["37226882"],"confidence":"Medium","gaps":["Cognate E3 for VEGFR2 ubiquitination not identified","Direct ubiquitination of VEGFR2 by UBE2D2 not shown"]},{"year":2024,"claim":"Developed ubiquitin-variant inhibitors that block E1 and backside-binding surfaces, providing tools to probe and inhibit chain-building activity.","evidence":"Structural/biophysical characterization, binding affinity, in vitro chain-building assay, and UBE2D-family specificity profiling","pmids":["39473070"],"confidence":"Medium","gaps":["Cellular activity of UbVs not demonstrated","Selectivity over UBE2D1/3 only partially resolved"]},{"year":null,"claim":"How the broad E3 promiscuity of UBE2D2 is restricted to specific substrates and chain types in distinct cellular contexts remains unresolved.","evidence":"","pmids":[],"confidence":"High","gaps":["Cognate E3s for VEGFR2 and essential yeast HECT substrates undefined","Regulation of isoform redundancy (UBE2D1/2/3) in vivo unclear","Determinants of chain linkage specificity not established"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[2,4,7,9,10,20]},{"term_id":"GO:0016740","term_label":"transferase activity","supporting_discovery_ids":[0,11,17]},{"term_id":"GO:0031386","term_label":"protein tag activity","supporting_discovery_ids":[3,11]}],"localization":[{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[9]},{"term_id":"GO:0005768","term_label":"endosome","supporting_discovery_ids":[9]}],"pathway":[{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[0,2,7,10]},{"term_id":"R-HSA-1640170","term_label":"Cell Cycle","supporting_discovery_ids":[4]},{"term_id":"R-HSA-9612973","term_label":"Autophagy","supporting_discovery_ids":[14,18]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[21]}],"complexes":["SCF(β-TRCP)","SCF(FBXW2)","APC/C"],"partners":["MDM2","E6AP","APC11","C-CBL","CIAP1","MUL1","RNF25","E4B"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"P62837","full_name":"Ubiquitin-conjugating enzyme E2 D2","aliases":["(E3-independent) E2 ubiquitin-conjugating enzyme D2","E2 ubiquitin-conjugating enzyme D2","Ubiquitin carrier protein D2","Ubiquitin-conjugating enzyme E2(17)KB 2","Ubiquitin-conjugating enzyme E2-17 kDa 2","Ubiquitin-protein ligase D2","p53-regulated ubiquitin-conjugating enzyme 1"],"length_aa":147,"mass_kda":16.7,"function":"Accepts ubiquitin from the E1 complex and catalyzes its covalent attachment to other proteins (PubMed:10329681, PubMed:18042044, PubMed:18703417, PubMed:20061386, PubMed:20403326, PubMed:20525694, PubMed:26475854, PubMed:28322253). Catalyzes 'Lys-48'-linked polyubiquitination (PubMed:10329681, PubMed:18042044, PubMed:18359941, PubMed:18703417, PubMed:20061386, PubMed:20403326, PubMed:20525694, PubMed:26475854). Mediates the selective degradation of short-lived and abnormal proteins (PubMed:10329681, PubMed:18042044, PubMed:18359941, PubMed:18703417, PubMed:20061386, PubMed:20403326, PubMed:20525694, PubMed:26475854). Functions in the E6/E6-AP-induced ubiquitination of p53/TP53 (PubMed:15280377). Mediates ubiquitination of PEX5 and SQSTM1 and autoubiquitination of STUB1 and TRAF6 (PubMed:18359941, PubMed:28322253). Involved in the signal-induced conjugation and subsequent degradation of NFKBIA, FBXW2-mediated GCM1 ubiquitination and degradation, MDM2-dependent degradation of p53/TP53 and the activation of MAVS in the mitochondria by RIGI in response to viral infection (PubMed:18703417, PubMed:20403326). Essential for viral activation of IRF3 (PubMed:19854139)","subcellular_location":"","url":"https://www.uniprot.org/uniprotkb/P62837/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/UBE2D2","classification":"Not Classified","n_dependent_lines":2,"n_total_lines":1208,"dependency_fraction":0.0016556291390728477},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/UBE2D2","total_profiled":1310},"omim":[{"mim_id":"621515","title":"UBIQUITIN-CONJUGATING ENZYME E2 D4; UBE2D4","url":"https://www.omim.org/entry/621515"},{"mim_id":"616386","title":"POTASSIUM CHANNEL TETRAMERIZATION DOMAIN-CONTAINING PROTEIN 17; KCTD17","url":"https://www.omim.org/entry/616386"},{"mim_id":"616014","title":"RING FINGER PROTEIN 25; RNF25","url":"https://www.omim.org/entry/616014"},{"mim_id":"614534","title":"ANAPHASE-PROMOTING COMPLEX SUBUNIT 11; ANAPC11","url":"https://www.omim.org/entry/614534"},{"mim_id":"614472","title":"RING FINGER PROTEIN 123; RNF123","url":"https://www.omim.org/entry/614472"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Plasma membrane","reliability":"Approved"},{"location":"Cytosol","reliability":"Approved"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in 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loss of UBC4/UBC5 markedly reduces turnover of short-lived proteins and canavanyl-peptides but not long-lived proteins.\",\n      \"method\": \"Yeast genetics (ubc4ubc5 deletion mutants), in vivo ubiquitin conjugate detection, heat stress phenotype analysis\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — foundational study with genetic deletion, in vivo conjugation assay, and multiple phenotypic readouts; independently replicated across subsequent studies\",\n      \"pmids\": [\"2154373\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1993,\n      \"finding\": \"Crystal structure of yeast Ubc4 (UBE2D2 ortholog) determined at 2.7 Å; the enzyme is an α/β protein with the ubiquitin-accepting cysteine located in a cleft between two loops, and the conserved surface adjacent to the active-site cysteine is proposed to function in protein–protein binding during ubiquitin thiol ester formation.\",\n      \"method\": \"X-ray crystallography (molecular replacement, 2.7 Å resolution)\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structure with functional interpretation of active-site architecture; foundational structural paper\",\n      \"pmids\": [\"8268156\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"Human UBC4 (UBE2D2) is required for E6AP (E6-associated protein)-mediated ubiquitinylation of p53 in the HPV E6 pathway; reconstitution of p53 ubiquitinylation from purified components showed UBC4 specifically ubiquitinylates E6AP, and in vivo inhibition of UBC4 blocks E6-stimulated p53 degradation.\",\n      \"method\": \"In vitro ubiquitinylation reconstitution with purified components, in vivo inhibition assay\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — biochemical reconstitution from purified components plus in vivo inhibition, single lab\",\n      \"pmids\": [\"7724550\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"Yeast UBC4 monoubiquitinates itself in vivo at K144 in an intermolecular (E2–E2) reaction; a second lysine (K64) is required for ubiquitination at K144; cross-linking demonstrates direct UBC4–UBC4 homointeraction in vitro.\",\n      \"method\": \"In vivo epitope-tagged ubiquitin coexpression, site-directed mutagenesis, chemical mapping, in vitro cross-linking\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Moderate — mutagenesis plus in vitro cross-linking plus in vivo detection; multiple orthogonal methods in one study\",\n      \"pmids\": [\"7756256\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"The RING-H2 protein APC11 and UBC4 (UBE2D2 ortholog) are sufficient to ubiquitinate APC substrates securin and cyclin B in vitro; APC11 alone supports multi-ubiquitin chain synthesis with E1 and UBC4, and the RING-H2 finger integrity is required for this activity.\",\n      \"method\": \"In vitro ubiquitination assay with recombinant proteins expressed in E. coli, RING domain mutagenesis\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro reconstitution with purified recombinant proteins plus mutagenesis, single lab\",\n      \"pmids\": [\"10922056\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"UBC4 (UBE2D2) and Ubc3 catalyze SCF(β-TRCP)-dependent, phosphorylation-dependent ubiquitination of IκBα; both E2s associate with the SCF(β-TRCP) complex isolated from human cells, and UBC4 is 19-fold more efficient than Ubc3 in catalyzing this reaction in vitro.\",\n      \"method\": \"In vitro ubiquitination reconstitution with recombinant components, co-immunoprecipitation with human cell SCF complex\",\n      \"journal\": \"Oncogene\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro reconstitution, quantitative comparison, Co-IP with endogenous complex; single lab\",\n      \"pmids\": [\"10918611\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"NMR solution structure of human UbcH5B (UBE2D2) determined; residues of UbcH5B important for binding to the CNOT4 RING domain were identified by NMR chemical shift perturbation, and a structural model of the UbcH5B/CNOT4 RING complex was generated by HADDOCK docking, revealing differences from the c-Cbl/UbcH7 complex at specific residues relevant to E2/E3 specificity.\",\n      \"method\": \"NMR chemical shift perturbation, homology modeling, HADDOCK docking, mutagenesis\",\n      \"journal\": \"Structure (London, England : 1993)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — NMR structure plus functional mapping plus docking; multiple orthogonal methods in single study\",\n      \"pmids\": [\"15062086\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"UbcH5B/C (UBE2D2/3) are the physiological E2 enzymes for Mdm2-mediated ubiquitination and degradation of p53 in vivo; siRNA knockdown of UbcH5B/C in cells causes accumulation of both Mdm2 and p53, inhibits p53 ubiquitination and degradation, while in vitro screening showed only UbcH5A/B/C and E2-25K support Mdm2-mediated p53 ubiquitination.\",\n      \"method\": \"In vitro E2 panel screen, siRNA knockdown in mammalian cells, p53/Mdm2 ubiquitination and stability assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — in vitro screen combined with siRNA knockdown and in-cell ubiquitination assay; two orthogonal approaches\",\n      \"pmids\": [\"15280377\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"NMR solution structure of UbcH5B (UBE2D2) solved; the N-terminal helix involved in E3 binding displays a different orientation compared to crystal structures; Asn77 adopts multiple side-chain conformations in solution (contrasting single conformation in crystals), with implications for catalytic function.\",\n      \"method\": \"NMR spectroscopy (relaxation data, automated NOE assignments, homology modeling)\",\n      \"journal\": \"Journal of molecular biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Weak — NMR structure in solution, single lab, structural interpretation without full mutagenesis validation\",\n      \"pmids\": [\"15522302\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Ubc4/5 (UBE2D2 ortholog) cooperates with the E3 c-Cbl to ubiquitinate EGFR; upon EGF stimulation Ubc4/5 and c-Cbl co-relocalize from plasma membrane to Hrs-positive endosomes, indicating EGFR continues to be ubiquitinated after internalization to facilitate polyubiquitination and subsequent lysosomal sorting.\",\n      \"method\": \"Localization (fluorescence microscopy), siRNA knockdown, in vitro ubiquitination assay, dominant-negative ubiquitin mutant experiments, EGFR degradation assay\",\n      \"journal\": \"Molecular biology of the cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — knockdown, localization, in vitro activity, and dominant-negative experiments provide multiple orthogonal lines of evidence\",\n      \"pmids\": [\"18508924\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"UBE2D2 is the E2 ubiquitin-conjugating enzyme responsible for SCF(FBXW2)-mediated ubiquitination and proteasomal degradation of the placental transcription factor GCM1; UBE2D2 enzyme activity is required for GCM1 ubiquitination and association with the SCF(FBXW2) complex, and UBE2D2 knockdown prolongs GCM1 half-life in vivo.\",\n      \"method\": \"In vitro ubiquitination assay, Co-IP with SCF complex, RNA interference knockdown, pulse-chase half-life measurement\",\n      \"journal\": \"Biology of reproduction\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vitro assay, Co-IP, siRNA knockdown, and half-life assay; multiple orthogonal methods\",\n      \"pmids\": [\"18703417\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Crystal structure of UbcH5b~ubiquitin intermediate (oxyester-linked) determined at 2.2 Å; the conjugate self-assembles into an infinite spiral through backside interaction, providing multiple E2 active sites; biochemical assays show the self-assembled UbcH5b~Ub bridges the gap between substrate lysine and the E2 catalytic cysteine to enable efficient ubiquitination.\",\n      \"method\": \"X-ray crystallography (2.2 Å), in vitro ubiquitination biochemical assays\",\n      \"journal\": \"Structure (London, England : 1993)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — crystal structure of catalytic intermediate combined with biochemical validation; single lab but two orthogonal methods\",\n      \"pmids\": [\"20152160\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Crystal structures of E4B U box domain free and bound to UbcH5c and Ubc4 (UBE2D2 ortholog) determined by X-ray crystallography; the E4B U box is monomeric (unlike other U box domains), stabilized by a hydrogen-bond network; structural and calorimetric/NMR binding data suggest allosteric regulation of UbcH5c/Ubc4 by E4B U box.\",\n      \"method\": \"X-ray crystallography, NMR spectroscopy, isothermal titration calorimetry\",\n      \"journal\": \"Structure (London, England : 1993)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — crystal structures plus NMR plus ITC; multiple orthogonal methods in single study\",\n      \"pmids\": [\"20696396\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"The essential function of Ubc4/Ubc5 (UBE2D2 orthologs) in yeast is HECT E3-dependent (likely Rsp5); mutation N78S that specifically abolishes RING E3-catalyzed isopeptide bond formation but not HECT E3 transthiolation rescues lethality of ubc4/ubc5 deletion, establishing that Ubc4 acts as a monoubiquitinating E2 in RING E3 pathways while performing a critical function with a HECT E3.\",\n      \"method\": \"Yeast genetics (deletion and rescue with point mutants), in vitro ubiquitination assay, active-site and E3-binding mutagenesis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Moderate — epistasis genetics plus in vitro mutagenesis with mechanistic distinction between RING and HECT pathways\",\n      \"pmids\": [\"21357418\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"UBE2D2 (along with UBE2D3, UBE2N, UBE2L3) is essential for Parkin-dependent mitophagy; UBE2D2/3 knockdown reduces autophagic clearance of depolarized mitochondria without affecting PINK1 stabilization or Parkin translocation; combined knockdown of all these E2s reduces mitochondrial polyubiquitylation and p62 recruitment, and UBE2D2/3 contribute to ubiquitination of mitofusins, TOM20, TOM70, VDAC1, and Parkin.\",\n      \"method\": \"siRNA knockdown in mammalian cells, mitophagy assay (autophagic clearance), immunofluorescence, ubiquitylation assays for specific substrates\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — systematic siRNA knockdown with multiple substrate and functional readouts; single lab with several orthogonal assays\",\n      \"pmids\": [\"24906799\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"RING E3 AO7 (RNF25) binds UbcH5B (UBE2D2) with unusually high affinity via a unique UbcH5B-binding region (U5BR) connected by a linker to its RING domain, forming a clamp around the E2; the U5BR contacts the backside of UbcH5B distinct from both the active site and RING-interacting region; high-affinity clamp binding blocks stimulatory non-covalent ubiquitin binding to the UbcH5B backside, thereby decreasing ubiquitination rate.\",\n      \"method\": \"Co-crystallization, X-ray crystallography, ubiquitination rate assays, RING domain mutagenesis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — crystal structure plus functional ubiquitination assays plus mutagenesis; multiple orthogonal methods\",\n      \"pmids\": [\"26475854\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"UBE2D2 does not interact with the muscle E3 ligase MuRF1 and is not the cognate E2 for MuRF1-dependent muscle wasting; yeast two-hybrid, surface plasmon resonance, and cell-based assays all showed no functional interaction between UBE2D2 and MuRF1, and UBE2D2 was unable to promote MuRF1-dependent α-actin degradation.\",\n      \"method\": \"Yeast two-hybrid, surface plasmon resonance (SPR), HEK293T cell-based ubiquitination/degradation assay, mRNA quantification during atrophy\",\n      \"journal\": \"The international journal of biochemistry & cell biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — three independent methods all yielding negative result; well-controlled negative finding\",\n      \"pmids\": [\"27378730\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Crystal structure of cIAP1 RING dimer bound to UbcH5B covalently linked to ubiquitin (UbcH5B-Ub) and a non-covalent ubiquitin at 1.7 Å; cIAP1 RING promotes a closed UbcH5B-Ub conformation priming the thioester for transfer; non-covalent ubiquitin binding to the UbcH5B backside abuts the α1β1-loop and further stabilizes the closed active conformation.\",\n      \"method\": \"X-ray crystallography (1.7 Å), biochemical ubiquitin transfer assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — high-resolution crystal structure of ternary complex plus biochemical functional analyses; single lab\",\n      \"pmids\": [\"30523153\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"UbcH5b (UBE2D2) supports HECTD3 auto-ubiquitination in vitro; the triterpenoid PC3-15 directly binds UbcH5b and inhibits UbcH5b-mediated p62 ubiquitination; the UbcH5b–p62 axis confers TNBC cell resistance to lapatinib by promoting autophagy.\",\n      \"method\": \"In vitro ubiquitination assay, FRET-based inhibitor screen, direct binding assay, siRNA knockdown, in vivo mouse xenograft\",\n      \"journal\": \"Cancer letters\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vitro assays plus direct binding plus cellular and in vivo studies; multiple orthogonal methods, single lab\",\n      \"pmids\": [\"33607208\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"miR-30b-5p upregulation by ox-LDL reduces UBE2D2 ubiquitination activity, stabilizing KAT2B; elevated KAT2B then acetylates HMGB1, causing its nuclear exit and secretion, which promotes M1 macrophage polarization and recruitment.\",\n      \"method\": \"Luciferase reporter assay, co-immunoprecipitation, ubiquitination assay, qRT-PCR, flow cytometry, transwell assay\",\n      \"journal\": \"Atherosclerosis\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — multiple assays (Co-IP, ubiquitination, luciferase) supporting the pathway; single lab\",\n      \"pmids\": [\"33812169\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"MUL1 RING domain recruits both UBE2D2 and the substrate p53-TAD simultaneously; RING(MUL1) binding induces the closed conformation of UBE2D2~Ub and accelerates its hydrolysis (suppressed by N77A mutation); TADp53 binding affinity to MUL1-RING is enhanced in complex with UBE2D2~Ub, indicating multivalent substrate recognition underlies ubiquitination of this intrinsically disordered protein.\",\n      \"method\": \"NMR, crystal structure determination of RING(MUL1):UBE2D2 complex, oxyester mimetic UBE2D2~Ub assays, hydrolysis kinetics, binding affinity measurements\",\n      \"journal\": \"The FEBS journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — structural characterization combined with biochemical assays and mutagenesis; multiple orthogonal methods in single study\",\n      \"pmids\": [\"35048531\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Depletion of UBE2D2 (or UBE2D1) in endothelial cells increases steady-state VEGFR2 levels at the plasma membrane, enhances VEGFR2 recycling, amplifies VEGF-A-stimulated MAPK/PLCγ1/Akt signaling, and stimulates endothelial tubulogenesis; establishing UBE2D2 as a regulator of VEGFR2 ubiquitination, trafficking, and angiogenic signaling.\",\n      \"method\": \"siRNA knockdown (reverse genetics screen), flow cytometry, cell-surface biotinylation, recycling assay, Western blot signaling analysis, tubulogenesis assay\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — systematic knockdown with multiple cellular readouts; single lab\",\n      \"pmids\": [\"37226882\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Ubiquitin variants (UbVs) that inhibit Ube2d2 (UbcH5b) were identified; two characterized UbVs bind Ube2d2 with low micromolar affinity at a site overlapping with E1 binding, and the more inhibitory UbV additionally blocks the non-covalent ubiquitin-binding backside site, inhibiting ubiquitin chain building.\",\n      \"method\": \"Structural characterization (crystallography/biophysics implied), binding affinity measurements, in vitro ubiquitin chain building assay, specificity profiling within Ube2d family\",\n      \"journal\": \"The FEBS journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — structural/biophysical characterization plus functional chain-building assay; single lab, abstract-level description\",\n      \"pmids\": [\"39473070\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"UBE2D2 (UbcH5B/UBC4) is a ubiquitin-conjugating E2 enzyme that, after loading ubiquitin onto its catalytic cysteine via E1, cooperates with a broad range of both RING (e.g., APC11, c-Cbl, Mdm2, MUL1, cIAP1, FBXW2-SCF, SCF-β-TRCP, AO7/RNF25) and HECT (e.g., E6AP, HECTD3, Rsp5) E3 ligases to ubiquitinate substrates including p53, IκBα, cyclin B, securin, EGFR, GCM1, and mitochondrial proteins; its closed catalytic conformation is stabilized by RING domain binding and by non-covalent ubiquitin engagement at its backside surface, while its self-assembled UbcH5b~Ub spiral intermediate facilitates processive substrate ubiquitination, and it plays essential cellular roles in proteasomal degradation of short-lived/abnormal proteins, PINK1-Parkin-dependent mitophagy, VEGFR2 trafficking/angiogenesis, and cell cycle progression.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"UBE2D2 (UbcH5B/UBC4) is a promiscuous ubiquitin-conjugating E2 enzyme central to proteasomal turnover of short-lived and abnormal proteins, a role first established genetically in yeast where loss of UBC4/UBC5 selectively impairs degradation of unstable substrates [#0]. After charging with ubiquitin via E1, it transfers ubiquitin to substrate lysines in cooperation with a broad array of RING and HECT E3 ligases, including E6AP for p53 [#2], Mdm2 for p53 [#7], the APC11 RING for securin and cyclin B [#4], SCF complexes for IκBα and the transcription factor GCM1 [#5, #10], and c-Cbl for EGFR [#9]. Its catalytic mechanism is governed by conformational control: RING-domain binding (e.g., cIAP1, MUL1) and non-covalent ubiquitin engagement at the E2 backside surface stabilize a closed UbcH5B~Ub conformation that primes the thioester for transfer [#17, #20], and self-assembly of the UbcH5b~Ub conjugate into a spiral via backside interactions provides multiple active sites for processive ubiquitination [#11]. UBE2D2 supports diverse cellular processes, contributing to PINK1-Parkin-dependent mitophagy through ubiquitination of mitochondrial proteins [#14], to VEGFR2 trafficking and angiogenic signaling in endothelial cells [#21], and—via genetic analysis in yeast—it performs an essential HECT-E3-dependent function while acting as a monoubiquitinating E2 in RING-E3 pathways [#13].\",\n  \"teleology\": [\n    {\n      \"year\": 1990,\n      \"claim\": \"Established the core physiological role of this E2 in selective protein turnover, answering whether UBC4/UBC5 activity is required for degradation of unstable proteins.\",\n      \"evidence\": \"Yeast deletion genetics with in vivo ubiquitin conjugate detection and stress phenotyping\",\n      \"pmids\": [\"2154373\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not identify specific E3 partners or substrates\", \"Mammalian ortholog function not directly tested\"]\n    },\n    {\n      \"year\": 1993,\n      \"claim\": \"Defined the catalytic architecture, locating the ubiquitin-accepting cysteine in a cleft and identifying a conserved surface for E3 binding.\",\n      \"evidence\": \"X-ray crystallography of yeast Ubc4 at 2.7 Å\",\n      \"pmids\": [\"8268156\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No substrate or E3-bound complex\", \"Solution dynamics not addressed\"]\n    },\n    {\n      \"year\": 1995,\n      \"claim\": \"Connected the E2 to specific E3-substrate pathways (E6AP/p53) and revealed intermolecular self-ubiquitination, showing how the enzyme engages HECT ligases and itself.\",\n      \"evidence\": \"In vitro reconstitution with purified components, in vivo inhibition, mutagenesis, and cross-linking\",\n      \"pmids\": [\"7724550\", \"7756256\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Physiological significance of K144 self-ubiquitination unclear\", \"p53 work limited to the HPV E6 context\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"Demonstrated broad E3 partnership by reconstituting APC11- and SCF(β-TRCP)-dependent ubiquitination, establishing UBC4 as the preferred E2 for cell-cycle and IκBα substrates.\",\n      \"evidence\": \"In vitro ubiquitination with recombinant proteins, RING mutagenesis, and Co-IP with human SCF complex\",\n      \"pmids\": [\"10922056\", \"10918611\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In-cell requirement for these specific reactions not fully established\", \"Chain-type selectivity not resolved\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Mapped E2/E3 interaction surfaces and identified UbcH5B/C as the physiological E2 for Mdm2-mediated p53 degradation, linking structure to in-cell function.\",\n      \"evidence\": \"NMR structures with chemical-shift mapping and docking; in vitro E2 panel screen plus siRNA knockdown with stability assays\",\n      \"pmids\": [\"15062086\", \"15280377\", \"15522302\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Redundancy among UbcH5 isoforms not fully dissected\", \"Solution vs crystal conformational differences functionally unvalidated (#8 Medium)\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Extended the substrate repertoire to receptor trafficking (EGFR via c-Cbl) and placental transcription factor turnover (GCM1 via SCF(FBXW2)), with spatial regulation at endosomes.\",\n      \"evidence\": \"siRNA knockdown, fluorescence localization, in vitro ubiquitination, Co-IP, and pulse-chase half-life measurement\",\n      \"pmids\": [\"18508924\", \"18703417\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct vs indirect contribution to endosomal sorting not separated\", \"Chain linkage on these substrates not determined\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Revealed the mechanistic basis of processivity and allosteric control through the self-assembled UbcH5b~Ub spiral and E4B U-box binding.\",\n      \"evidence\": \"Crystal structure of UbcH5b~Ub oxyester intermediate with biochemical assays; E4B U-box crystal structures with NMR and ITC\",\n      \"pmids\": [\"20152160\", \"20696396\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Physiological prevalence of the spiral intermediate uncertain\", \"Functional consequence of E4B allostery in cells not shown\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Resolved the division of labor between RING and HECT pathways, showing the essential cellular function depends on HECT E3 (Rsp5) transthiolation while RING E3s drive isopeptide bond formation.\",\n      \"evidence\": \"Yeast deletion/rescue with the N78S mutant, in vitro ubiquitination, and active-site mutagenesis\",\n      \"pmids\": [\"21357418\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Identity of the essential HECT substrates not defined\", \"Mammalian equivalence of this RING/HECT split untested\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Defined a role in PINK1-Parkin mitophagy, showing the E2 is required for mitochondrial substrate ubiquitination downstream of Parkin recruitment.\",\n      \"evidence\": \"siRNA knockdown of multiple E2s, mitophagy clearance assays, immunofluorescence, and substrate ubiquitylation assays\",\n      \"pmids\": [\"24906799\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Functional redundancy with UBE2D3/UBE2N/UBE2L3 obscures individual contribution\", \"Direct vs indirect substrate ubiquitination not distinguished\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Showed that E3 binding can be inhibitory, with the AO7/RNF25 U5BR clamp blocking the stimulatory backside ubiquitin site to slow ubiquitination.\",\n      \"evidence\": \"Co-crystallization, X-ray crystallography, ubiquitination rate assays, and RING mutagenesis\",\n      \"pmids\": [\"26475854\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Cellular relevance of AO7-mediated inhibition not established\", \"Generality across other E3s unknown\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Delimited E2/E3 specificity by ruling out MuRF1 as a cognate partner, refining which ligases recruit this E2.\",\n      \"evidence\": \"Yeast two-hybrid, SPR, and cell-based degradation assays (negative result)\",\n      \"pmids\": [\"27378730\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Negative finding for one E3 does not generalize\", \"Conditions or modifications enabling weak interaction not exhaustively tested\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Provided high-resolution mechanistic detail of how a RING dimer (cIAP1) and non-covalent backside ubiquitin together stabilize the catalytically primed closed conformation.\",\n      \"evidence\": \"X-ray crystallography at 1.7 Å of the ternary complex with ubiquitin transfer assays\",\n      \"pmids\": [\"30523153\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Snapshot does not capture transfer dynamics\", \"Substrate-bound geometry not included\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Extended substrate and disease links to HECTD3 auto-ubiquitination, p62/autophagy-mediated drug resistance, and KAT2B/HMGB1-driven macrophage polarization, while identifying a small-molecule inhibitor binding the E2.\",\n      \"evidence\": \"In vitro ubiquitination, FRET inhibitor screen and direct binding, siRNA knockdown, xenograft; plus luciferase, Co-IP, ubiquitination, and flow cytometry\",\n      \"pmids\": [\"33607208\", \"33812169\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Pathway-level rather than direct mechanistic dissection (#19)\", \"Inhibitor selectivity within the UbcH5 family not fully defined (#18)\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Demonstrated multivalent substrate recognition, showing MUL1 RING simultaneously recruits E2 and the disordered p53-TAD and enhances substrate affinity in the conjugate complex.\",\n      \"evidence\": \"NMR, crystal structure of RING(MUL1):UBE2D2, oxyester-mimetic conjugate assays, and hydrolysis kinetics with N77A mutant\",\n      \"pmids\": [\"35048531\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Cellular evidence for MUL1-mediated p53 ubiquitination not provided\", \"Applicability to other disordered substrates untested\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Established a cellular role in VEGFR2 trafficking and angiogenic signaling, showing depletion enhances receptor recycling and downstream signaling.\",\n      \"evidence\": \"siRNA knockdown, cell-surface biotinylation, recycling assay, signaling Western blots, and tubulogenesis assay in endothelial cells\",\n      \"pmids\": [\"37226882\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Cognate E3 for VEGFR2 ubiquitination not identified\", \"Direct ubiquitination of VEGFR2 by UBE2D2 not shown\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Developed ubiquitin-variant inhibitors that block E1 and backside-binding surfaces, providing tools to probe and inhibit chain-building activity.\",\n      \"evidence\": \"Structural/biophysical characterization, binding affinity, in vitro chain-building assay, and UBE2D-family specificity profiling\",\n      \"pmids\": [\"39473070\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Cellular activity of UbVs not demonstrated\", \"Selectivity over UBE2D1/3 only partially resolved\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How the broad E3 promiscuity of UBE2D2 is restricted to specific substrates and chain types in distinct cellular contexts remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Cognate E3s for VEGFR2 and essential yeast HECT substrates undefined\", \"Regulation of isoform redundancy (UBE2D1/2/3) in vivo unclear\", \"Determinants of chain linkage specificity not established\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [2, 4, 7, 9, 10, 20]},\n      {\"term_id\": \"GO:0016740\", \"supporting_discovery_ids\": [0, 11, 17]},\n      {\"term_id\": \"GO:0031386\", \"supporting_discovery_ids\": [3, 11]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [9]},\n      {\"term_id\": \"GO:0005768\", \"supporting_discovery_ids\": [9]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [0, 2, 7, 10]},\n      {\"term_id\": \"R-HSA-1640170\", \"supporting_discovery_ids\": [4]},\n      {\"term_id\": \"R-HSA-9612973\", \"supporting_discovery_ids\": [14, 18]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [21]}\n    ],\n    \"complexes\": [\"SCF(β-TRCP)\", \"SCF(FBXW2)\", \"APC/C\"],\n    \"partners\": [\"MDM2\", \"E6AP\", \"APC11\", \"c-Cbl\", \"cIAP1\", \"MUL1\", \"RNF25\", \"E4B\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":4,"faith_total":4,"faith_pct":100.0}}