{"gene":"UBR4","run_date":"2026-06-10T10:51:56","timeline":{"discoveries":[{"year":2024,"finding":"UBR4 contains a distinct catalytic E3 module comprising a 'hemiRING' zinc finger, a helical-rich UBR zinc-finger interacting (UZI) subdomain, and an N-terminal affinity region that recruits cognate E2 conjugating enzymes UBE2A and UBE2B. The UZI subdomain allosterically activates the Ub-loaded E2 (E2~Ub). The hemiRING specifically recognizes UBE2A/UBE2B through determinants revealed by crystal structure of the E2-E3 complex.","method":"Crystal/cryo-EM structure of E2-E3 complex, in vitro ubiquitination assays, mutagenesis","journal":"Nature structural & molecular biology","confidence":"High","confidence_rationale":"Tier 1 / Strong — atomic-resolution structure combined with in vitro functional assays and mutagenesis in a single rigorous study","pmids":["38182926"],"is_preprint":false},{"year":2025,"finding":"Cryo-EM analysis of UBR4 in complex with cofactors KCMF1 and CALM1 reveals a 1.3-megadalton ring structure with a central substrate-binding arena and flexibly attached catalytic units. UBR4 acts as an E4 ubiquitin chain-elongating ligase, extending K48-linked ubiquitin chains on pre-ubiquitinated substrates. Efficient substrate targeting requires both preubiquitination and specific N-degrons, with KCMF1 acting as a substrate filter.","method":"Cryo-electron microscopy, in vitro ubiquitination reconstitution, epistasis analysis","journal":"Science (New York, N.Y.)","confidence":"High","confidence_rationale":"Tier 1 / Strong — cryo-EM structure plus reconstitution in vitro with epistasis in a single rigorous study","pmids":["40875847"],"is_preprint":false},{"year":2024,"finding":"The UBR4-KCMF1 ubiquitin ligase complex acts downstream of a priming E3 that mono-ubiquitinates orphan subunits; UBR4 then recognizes both the orphan protein and its mono-ubiquitin mark and builds K48-linked poly-ubiquitin chains for proteasomal degradation. This convergence point is required for efficient degradation of orphan subunits from the chaperonin, proteasome cap, proteasome core, and a protein targeting complex.","method":"Epistasis analysis in cells, in vitro reconstitution assays","journal":"bioRxiv (preprint)","confidence":"High","confidence_rationale":"Tier 1 / Moderate — in vitro reconstitution plus cellular epistasis, preprint single lab but two orthogonal methods","pmids":["bio_10.1101_2024.08.07.607117"],"is_preprint":true},{"year":2013,"finding":"UBR4 (p600) recognizes N-degrons through its UBR box as an N-recognin of the N-end rule pathway, mediating ubiquitylation-dependent proteolysis. UBR4-deficient mice die during embryogenesis with impaired yolk sac vascular development. In the yolk sac endoderm, UBR4 regulates autophagic flux including LC3 lipidation/activation and autophagic double-membrane structure formation.","method":"Genetic knockout mouse model, autophagic flux assays, LC3 lipidation assays","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 / Strong — clean KO with defined developmental and cellular phenotypes, multiple orthogonal readouts","pmids":["23431188"],"is_preprint":false},{"year":2013,"finding":"Dengue virus NS5 protein bridges STAT2 and UBR4, recruiting UBR4 to promote proteasome-dependent STAT2 degradation. Processed (but not unprocessed) NS5 preferentially binds UBR4. UBR4 knockdown reduces DENV-mediated STAT2 degradation and impairs viral replication in IFN-I competent cells.","method":"Co-immunoprecipitation, siRNA knockdown, proteasome inhibitor assays, viral replication assays","journal":"PLoS pathogens","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal Co-IP, siRNA KD with functional viral and STAT2 degradation readouts, multiple orthogonal methods","pmids":["23555265"],"is_preprint":false},{"year":2023,"finding":"Structural analysis of the UBR box of UBR4 reveals it recognizes type-2 N-terminal amino acids (containing aromatic rings) and type-1 N-terminal arginine through two phenylalanines on a hydrophobic surface, a distinct mechanism from other UBR box-containing proteins. The second ligand residue binding mechanism was also characterized.","method":"X-ray crystallography / structural analysis, binding assays","journal":"Communications biology","confidence":"High","confidence_rationale":"Tier 1 / Moderate — crystal structure with functional binding characterization, single lab but atomic-resolution structural data","pmids":["38030679"],"is_preprint":false},{"year":2019,"finding":"Loss of UBR4 in Drosophila and mouse myofibers promotes hypertrophy via decreased ubiquitination and degradation of the HAT1/RBBP4/RBBP7 histone-binding complex. RNAi screening in Drosophila identified UBR4 as an N-end rule ubiquitin ligase that limits myofiber size.","method":"RNAi screening in Drosophila, mouse KO, ubiquitination assays, proteomics (mass spectrometry)","journal":"Cell reports","confidence":"High","confidence_rationale":"Tier 2 / Strong — loss-of-function in two organisms with defined substrate identification by mass spectrometry and ubiquitination assays","pmids":["31365869"],"is_preprint":false},{"year":2021,"finding":"UBR4 levels increase in skeletal muscle with aging and UBR4 increases proteasome proteolytic activity. Muscle-specific UBR4 loss rescues age-associated myofiber atrophy in mice but reduces specific force and accelerates protein quality control decline, demonstrating that UBR4 antithetically regulates myofiber size and muscle protein quality control.","method":"Muscle-specific KO mouse model, proteasome activity assays, in vivo muscle function measurements","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 2 / Strong — muscle-specific KO with multiple orthogonal functional phenotypic readouts in mice and Drosophila","pmids":["33658508"],"is_preprint":false},{"year":2015,"finding":"UBR4 associates with influenza A virus M2 protein (identified by affinity purification/mass spectrometry and Co-IP) and promotes apical transport of viral proteins; UBR4 knockdown impairs viral budding.","method":"Affinity purification-mass spectrometry, Co-immunoprecipitation, RNAi knockdown, viral protein localization assays","journal":"Cell host & microbe","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — Co-IP plus functional knockdown with viral phenotypic readout, single study","pmids":["26651948"],"is_preprint":false},{"year":2015,"finding":"UBR4 forms a complex with KCMF1 and the E2 enzyme RAD6. NMR and in vitro interaction mapping showed KCMF1 C-terminus binds directly to RAD6, while KCMF1 N-terminal domains interact with UBR4. KCMF1 and RAD6 colocalize at late endosomes and lysosomes. RAD6A mutations (R7W, R11Q) found in X-linked intellectual disability specifically disrupt interaction with KCMF1 and UBR4.","method":"Affinity purification-mass spectrometry, NMR, in vivo and in vitro interaction mapping, co-localization microscopy","journal":"Molecular & cellular proteomics : MCP","confidence":"High","confidence_rationale":"Tier 1 / Moderate — NMR structure plus in vitro interaction mapping plus cellular colocalization, single lab with multiple orthogonal methods","pmids":["25582440"],"is_preprint":false},{"year":2016,"finding":"In Drosophila, POE/UBR4 acts as an E3 ubiquitin ligase together with E2 UBC6 and co-factor UFD4 to target MAPK for proteasomal degradation, opposing the deubiquitinase USP47. KCMF1 was identified as another key component modulating MAPK levels through this complex. Genetic interaction screening (RNAi-based epistasis) established the pathway order.","method":"RNAi-based genetic interaction screen (epistasis), Drosophila genetics, MAPK half-life measurements","journal":"PLoS biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — epistasis by RNAi screen with defined pathway order, single lab","pmids":["27552662"],"is_preprint":false},{"year":2018,"finding":"UBR4 is recruited to maturing endosomes through interaction with Ca2+-bound calmodulin, and this endosomal recruitment is essential for biogenesis of early endosomes (EEs) and trafficking of endocytosed cargo. Loss of UBR4 disrupts endosome-lysosome pathway function in mouse embryos.","method":"Co-immunoprecipitation (calmodulin interaction), cellular fractionation, live-cell imaging, mouse embryo analysis, KO phenotyping","journal":"Journal of cell science","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — Co-IP interaction, KO with defined endosomal phenotype, single lab","pmids":["30111582"],"is_preprint":false},{"year":2018,"finding":"UBR4-deficient mice die around E9.5-10.5 with defects in neurogenesis and cardiovascular development correlated with depletion of plasma membrane proteins and impaired multivesicular body (MVB) biogenesis, suggesting UBR4 governs proteome-wide turnover of cell surface proteins through the endosomal pathway.","method":"Conditional KO mouse model (UBR box deletion), proteomics of plasma membrane fractions, MVB biogenesis assays","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — KO mouse with proteomics and MVB phenotype, single lab","pmids":["30157281"],"is_preprint":false},{"year":2016,"finding":"The ubiquitin ligase UBR4 is a key component of the podocin interactome in podocytes and glomeruli (identified by purification and MS). UBR4 co-localizes with podocin and controls its ubiquitylation at K301 and a second lysine, regulating stability and disassembly of podocin/MEC-2 multimeric complexes. In C. elegans, Ubr4 degrades mislocalized MEC-2 multimers.","method":"Affinity purification-mass spectrometry, co-immunoprecipitation, ubiquitylomics, C. elegans genetics, molecular dynamics simulations","journal":"Human molecular genetics","confidence":"High","confidence_rationale":"Tier 2 / Strong — AP-MS identification, site-specific ubiquitylomics, C. elegans in vivo validation, multiple orthogonal methods","pmids":["26792178"],"is_preprint":false},{"year":2022,"finding":"UBR4/POE ubiquitin ligase promotes neuropeptide trafficking in clock neurons by maintaining expression of Coronin 7, which is required for budding of Golgi-derived transport vesicles. Ubr4-deficient mice show resistance to jetlag; poe knockdown flies are prone to arrhythmicity due to reduced axonal trafficking of circadian neuropeptides.","method":"Mouse KO, Drosophila RNAi, live imaging, neuropeptide trafficking assays, Golgi export assays","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 2 / Strong — KO/KD in two organisms with defined molecular mechanism (Coronin 7 regulation), multiple orthogonal readouts","pmids":["35332162"],"is_preprint":false},{"year":2023,"finding":"UBR4 ubiquitinates YAP at its substrate to promote YAP degradation and nuclear-cytoplasmic translocation, thereby reducing endometrial fibrosis. P65 (NF-κB) binds the UBR4 promoter to transcriptionally upregulate UBR4 expression in menstrual blood stromal cells.","method":"siRNA knockdown, ubiquitination assays, nuclear/cytoplasmic fractionation, chromatin immunoprecipitation (P65 on UBR4 promoter), co-immunoprecipitation","journal":"Journal of nanobiotechnology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — ubiquitination assay plus ChIP plus fractionation, single lab","pmids":["37644565"],"is_preprint":false},{"year":2023,"finding":"UBR4 (E3 ligase) cooperates with UBE2L6 (E2) to ubiquitinate EZH2 at K381, promoting its degradation. UHRF1-mediated CpG methylation downregulates UBE2L6 in low-pigmented melanoma cells, stabilizing EZH2. This UHRF1/UBE2L6/UBR4 axis controls EZH2 abundance and melanocytic differentiation states.","method":"Co-immunoprecipitation, mass spectrometry, ubiquitination assays, siRNA knockdown, animal studies","journal":"Oncogene","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — Co-IP plus site-specific ubiquitination assays plus animal xenograft validation, single lab","pmids":["36906655"],"is_preprint":false},{"year":2024,"finding":"UBR4 promotes proteasomal degradation of MLH1 through its ubiquitin ligase activity; the region of MLH1 encompassing amino acids 516-650 is crucial for UBR4-dependent degradation. PMS2 binding to this segment shields MLH1 from UBR4-mediated degradation. The deubiquitylase USP5 opposes UBR4 to stabilize MLH1. UBR4 or USP5 deficiency affects cellular response to 6-TG, linking them to mismatch repair.","method":"Co-immunoprecipitation, siRNA knockdown, ubiquitination assays, domain-mapping (deletion constructs), 6-TG sensitivity assay","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — Co-IP, domain mapping, functional drug response assay, single lab","pmids":["39032648"],"is_preprint":false},{"year":2024,"finding":"NatB (N-terminal acetyltransferase B)-mediated N-terminal acetylation protects procaspase-8 from UBR4 (and UBR1) Arg/N-recognin-mediated degradation. In Naa20-/- MEFs lacking NatB catalytic activity, procaspase-8 is degraded via UBR4, reducing responsiveness to extrinsic apoptotic stimuli. UBR4 silencing in Naa20-/- cells partially restores procaspase-8 levels and apoptotic response.","method":"MEF KO (Naa20-/-), quantitative N-terminomics, label-free quantification, targeted proteomics, siRNA knockdown of UBR4, apoptosis assays","journal":"Molecular and cellular biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic KO combined with siRNA epistasis and proteomics, single lab with multiple methods","pmids":["39099191"],"is_preprint":false},{"year":2024,"finding":"UBR4 exclusively (not UBR1 or UBR2) targets N-degrons bearing position-3 arginine or lysine residues after MetAP2-mediated co-translational cleavage of N-terminal methionine preceding second-position threonine or valines. This MetAP2-dependent Arg/N-degron pathway is a distinct substrate-recognition mechanism for UBR4.","method":"Reporter assays, CRISPR-Cas9 knockout of N-recognins (UBR1, UBR2, UBR4), MetAP2 inhibition","journal":"bioRxiv (preprint)","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — CRISPR KO epistasis with reporter assays demonstrating UBR4 specificity, preprint single lab","pmids":["bio_10.1101_2024.10.03.616566"],"is_preprint":true},{"year":2024,"finding":"NAP1L1 protects BIRC2 from UBR4-mediated ubiquitin-proteasome degradation. NAP1L1 knockdown promotes UBR4-dependent ubiquitination and degradation of BIRC2, inhibiting hepatocellular carcinoma cell proliferation and apoptotic escape. UBR4 was identified as the intermediate molecule linking NAP1L1 and BIRC2 by Co-IP and mass spectrometry.","method":"Co-immunoprecipitation, mass spectrometry, siRNA knockdown, ubiquitination assays","journal":"Cell death discovery","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — Co-IP plus mass spectrometry plus functional KD assays, single lab","pmids":["38538582"],"is_preprint":false},{"year":2026,"finding":"UBR4 promotes ubiquitination and proteasomal degradation of the kinase HRI, thereby constraining integrated stress response (ISR) overactivation and alleviating ISR-mediated inhibition of mitophagy in proximal tubular epithelial cells. Tubule-specific Ubr4 deficiency exacerbates cisplatin-induced kidney injury by allowing HRI-mediated ISR overactivation.","method":"Tubule-specific KO mouse model, ubiquitination assays, HRI protein stability assays, ISR pathway readouts, mitophagy assays","journal":"Free radical biology & medicine","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — tissue-specific KO with defined substrate (HRI) and pathway rescue, single lab","pmids":["41548766"],"is_preprint":false},{"year":2026,"finding":"UBR4 mediates clathrin-dependent EGFR endocytosis and degradation in colorectal cancer cells. UBR4 knockdown promotes EGFR recycling rather than degradation, enhancing cell proliferation and reducing apoptosis in response to cetuximab.","method":"siRNA knockdown, co-immunoprecipitation, tandem mass spectrometry, immunofluorescence, cell viability and apoptosis assays","journal":"Journal of gastroenterology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — Co-IP/MS identification plus functional siRNA KD with multiple cellular readouts, single lab","pmids":["41491123"],"is_preprint":false},{"year":2025,"finding":"UBR4 knockout in A549 lung cancer cells induces cellular senescence with defective mitochondria caused by impaired mitophagy. Restoration of UBR4 or antioxidant treatment reverses the ΔUBR4 phenotypes, suggesting UBR4 regulates mitochondrial quality control to prevent senescence-associated tumor suppression.","method":"CRISPR KO, senescence assays, mitophagy flux assays, xenograft mouse models, antioxidant rescue experiments","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — CRISPR KO with multiple phenotypic and rescue readouts in vitro and in vivo, single lab","pmids":["40531870"],"is_preprint":false},{"year":2016,"finding":"In the murine SCN, UBR4 protein expression is time-of-day-dependent and light-inducible. UBR4 is localized exclusively to arginine vasopressin (AVP)-expressing neurons of the SCN shell, as determined by immunofluorescence microscopy. Photic stimulation in the early subjective night increases the number of UBR4-expressing SCN cells.","method":"Proteome-wide SCN screen, immunofluorescence microscopy, temporal expression profiling","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — direct localization by immunofluorescence with temporal functional correlation, replicated across time points but single lab","pmids":["25084275"],"is_preprint":false},{"year":2022,"finding":"UBR4 depletion reduces RVFV titers and viral RNA production, identifying UBR4 as a host factor required for Rift Valley fever virus replication. UBR4 was identified as a Gn glycoprotein-interacting protein by proteomics-based approach using V5-epitope tagged virus.","method":"Proteomics (affinity purification-mass spectrometry), siRNA knockdown, viral titer assays, viral RNA quantification","journal":"Virology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — AP-MS identification plus functional KD with viral replication readout, single lab","pmids":["35032865"],"is_preprint":false},{"year":2026,"finding":"CaMKK2 co-immunoprecipitates with UBR4 and the 19S proteasome regulatory particle (RP) in chondrocytes; CaMKK2 scaffolds UBR4 and 19S RP around polyubiquitinated proteins such as Sox9 for proteasomal degradation, independently of its kinase activity, while CaMKK2 kinase activity phosphorylates Psmc5 (19S ATPase) to enhance proteasome activity.","method":"Co-immunoprecipitation-mass spectrometry, kinase assays, phosphosite identification by mass spectrometry, proteasome activity assays, kinase-dead mutants","journal":"bioRxiv (preprint)","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — Co-IP/MS with kinase assay and proteasome activity readout, preprint single lab with multiple orthogonal methods","pmids":["42182340"],"is_preprint":true},{"year":2024,"finding":"In IgG4-RD, a UBR4 variant prevents lysosomal degradation of the phosphatase CD45 in T cells, contributing to T cell hyperresponsiveness. This was established in patient-derived T cells carrying the specific UBR4 variant.","method":"Patient cell-based assays, CD45 protein stability measurements, lysosomal degradation assays","journal":"The Journal of clinical investigation","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single variant analysis in patient cells, no in vitro reconstitution or biochemical mechanism established","pmids":["38885295"],"is_preprint":false}],"current_model":"UBR4 is a large (~574 kDa) E3/E4 ubiquitin ligase of the N-degron pathway that assembles into a ~1.3 MDa ring complex with KCMF1 and calmodulin; its hemiRING catalytic module specifically recruits E2s UBE2A/UBE2B to extend K48-linked ubiquitin chains on substrates bearing N-degrons or preexisting mono-ubiquitin marks, thereby targeting diverse substrates—including orphan protein complex subunits, STAT2, podocin, MAPK, EZH2, MLH1, BIRC2, HRI, and the HAT1/RBBP4/RBBP7 histone-binding complex—for proteasomal degradation, while also regulating endosomal/lysosomal trafficking, autophagy, neuropeptide secretion, and muscle proteostasis."},"narrative":{"mechanistic_narrative":"UBR4 is a large E3/E4 ubiquitin ligase of the N-degron (N-end rule) pathway that directs ubiquitin-proteasome and endolysosomal turnover of a broad set of substrates across development, proteostasis, and quality control [PMID:23431188, PMID:31365869]. Its catalytic core comprises a hemiRING zinc finger and a UZI subdomain that specifically recruit and allosterically activate the cognate E2 enzymes UBE2A/UBE2B (RAD6), determining E2 selectivity [PMID:38182926, PMID:25582440]. Substrate selection operates through a UBR box that recognizes type-1 (N-terminal Arg) and type-2 (aromatic) N-degrons via a distinct two-phenylalanine surface, with specialized recognition of MetAP2-generated position-3 Arg/Lys degrons [PMID:38030679, PMID:bio_10.1101_2024.10.03.616566]. UBR4 assembles with KCMF1 and calmodulin into a ~1.3 MDa ring with a central substrate arena; it functions predominantly as an E4 chain-elongating ligase that extends K48-linked chains on substrates already bearing priming mono-ubiquitin and an N-degron, with KCMF1 acting as a substrate filter — a logic that drives clearance of orphan subunits from the chaperonin, proteasome cap/core, and targeting complexes [PMID:40875847, PMID:bio_10.1101_2024.08.07.607117]. Through these activities UBR4 degrades diverse targets including the HAT1/RBBP4/RBBP7 complex to limit myofiber size [PMID:31365869], podocin multimers [PMID:26792178], MAPK [PMID:27552662], EZH2 [PMID:36906655], MLH1 [PMID:39032648], YAP [PMID:37644565], BIRC2 [PMID:38538582], and the stress kinase HRI [PMID:41548766], and is exploited by Dengue NS5 to degrade STAT2 [PMID:23555265]. Independently of degradation, UBR4 governs endosomal/lysosomal trafficking via calmodulin-dependent recruitment to maturing endosomes, multivesicular body biogenesis, autophagic flux, mitophagy-linked mitochondrial quality control, and Golgi-derived neuropeptide secretion [PMID:23431188, PMID:30111582, PMID:30157281, PMID:35332162, PMID:40531870]. UBR4 loss is embryonic lethal in mice with vascular, neural, and cardiovascular defects tied to surface-proteome turnover [PMID:23431188, PMID:30157281].","teleology":[{"year":2013,"claim":"Established UBR4 as a bona fide N-recognin of the N-end rule pathway and tied it to autophagy and embryonic development, defining its core biological role.","evidence":"UBR knockout mouse with developmental phenotyping and LC3 lipidation/autophagic flux assays","pmids":["23431188"],"confidence":"High","gaps":["Did not identify direct degradation substrates","Link between N-degron recognition and autophagy regulation left mechanistically unresolved"]},{"year":2013,"claim":"Showed UBR4 can be hijacked by a viral adaptor to degrade a host substrate, demonstrating substrate recruitment can occur via bridging proteins rather than intrinsic N-degron recognition alone.","evidence":"Reciprocal Co-IP, siRNA knockdown, proteasome inhibition, and viral replication assays with Dengue NS5/STAT2","pmids":["23555265"],"confidence":"High","gaps":["Whether STAT2 carries an N-degron or is recruited solely via NS5 bridging not resolved","Direct ubiquitination of STAT2 by UBR4 not reconstituted"]},{"year":2015,"claim":"Defined the UBR4-KCMF1-RAD6 module architecture and connected it to endolysosomal compartments and X-linked intellectual disability through RAD6A mutations.","evidence":"AP-MS, NMR, in vitro interaction mapping, and colocalization microscopy","pmids":["25582440"],"confidence":"High","gaps":["Substrates targeted by this complex at endosomes/lysosomes not identified","Catalytic logic of the assembly not yet defined"]},{"year":2016,"claim":"Identified specific physiological substrates and trafficking roles — podocin multimers and a MAPK degradation pathway — broadening UBR4 from a developmental factor to a substrate-specific ligase.","evidence":"AP-MS/ubiquitylomics with C. elegans validation (podocin); RNAi epistasis in Drosophila (MAPK/UBC6/UFD4/KCMF1)","pmids":["26792178","27552662"],"confidence":"High","gaps":["Site-specific ubiquitination logic generalizability across substrates unknown","Mammalian conservation of the MAPK pathway not established"]},{"year":2018,"claim":"Mechanistically linked UBR4 to endosome biogenesis and surface-proteome turnover via calmodulin-dependent recruitment, explaining embryonic lethality through trafficking failure.","evidence":"Co-IP (calmodulin), fractionation, live imaging, KO mouse embryo proteomics and MVB assays","pmids":["30111582","30157281"],"confidence":"Medium","gaps":["Whether endosomal roles require ligase activity or are scaffolding functions unclear","Direct surface-protein substrates not individually validated"]},{"year":2019,"claim":"Demonstrated that UBR4-mediated degradation of the HAT1/RBBP4/RBBP7 complex limits myofiber size, establishing a conserved proteostatic role in muscle.","evidence":"Drosophila RNAi screen, mouse KO, ubiquitination assays, mass spectrometry","pmids":["31365869"],"confidence":"High","gaps":["How histone-binding complex turnover controls hypertrophy mechanistically unresolved"]},{"year":2021,"claim":"Revealed an antithetical role in which UBR4 boosts proteasome activity and protein quality control even as it restrains myofiber size, complicating its role in age-associated atrophy.","evidence":"Muscle-specific KO mouse, proteasome activity and in vivo force measurements","pmids":["33658508"],"confidence":"High","gaps":["Molecular basis for UBR4 enhancement of proteasome activity not defined"]},{"year":2022,"claim":"Showed a degradation-independent trafficking function: UBR4 maintains Coronin 7 to drive Golgi-derived neuropeptide vesicle budding and circadian neuropeptide transport.","evidence":"Mouse KO, Drosophila RNAi, live imaging, Golgi export and neuropeptide trafficking assays","pmids":["35332162"],"confidence":"High","gaps":["Whether Coronin 7 is a UBR4 ubiquitination substrate or stabilized by another route not resolved"]},{"year":2023,"claim":"Resolved the structural basis of UBR box N-degron recognition, showing a distinct two-phenylalanine mechanism for type-1 and type-2 residues.","evidence":"X-ray crystallography of the UBR box with binding assays","pmids":["38030679"],"confidence":"High","gaps":["Structure does not connect N-degron binding to catalytic chain assembly","Second-ligand binding contribution to substrate selection in vivo unverified"]},{"year":2023,"claim":"Expanded the substrate repertoire to disease-relevant targets — EZH2 (with E2 UBE2L6) and YAP — and showed UBR4 itself is transcriptionally controlled by NF-kB.","evidence":"Co-IP, site-specific ubiquitination assays, fractionation, ChIP, animal studies","pmids":["36906655","37644565"],"confidence":"Medium","gaps":["Whether these substrates are recognized via N-degron or other determinants unclear","Single-lab substrate assignments without structural validation"]},{"year":2024,"claim":"Defined the dedicated catalytic E3 module (hemiRING + UZI) and its E2 selectivity, providing the structural mechanism of UBE2A/UBE2B recruitment and activation.","evidence":"Crystal/cryo-EM of E2-E3 complex, in vitro ubiquitination, mutagenesis","pmids":["38182926"],"confidence":"High","gaps":["How the catalytic module is positioned relative to substrates in the full assembly not resolved in this study"]},{"year":2024,"claim":"Established UBR4 as a convergence point acting downstream of a priming E3, recognizing both N-degron and pre-existing mono-ubiquitin to clear orphan complex subunits.","evidence":"Cellular epistasis and in vitro reconstitution (preprint)","pmids":["bio_10.1101_2024.08.07.607117"],"confidence":"High","gaps":["Identity of all priming E3s unknown","Generality of the two-mark logic across substrates not exhaustively tested"]},{"year":2024,"claim":"Refined substrate-recognition specificity, showing UBR4 uniquely reads MetAP2-generated position-3 Arg/Lys N-degrons, distinguishing it from UBR1/UBR2.","evidence":"CRISPR KO of N-recognins, reporter assays, MetAP2 inhibition (preprint)","pmids":["bio_10.1101_2024.10.03.616566"],"confidence":"Medium","gaps":["Endogenous substrates bearing this degron not enumerated","Preprint awaiting peer review"]},{"year":2024,"claim":"Added MLH1, procaspase-8, and BIRC2 as substrates whose stability is set by competing protective factors, linking UBR4 to mismatch repair, apoptosis, and cancer.","evidence":"Co-IP, domain mapping, ubiquitination assays, N-terminomics, KO/KD epistasis, drug-response assays","pmids":["39032648","39099191","38538582"],"confidence":"Medium","gaps":["Whether UBR4 directly ubiquitinates each target or acts in larger complexes not fully resolved","Single-lab assignments per substrate"]},{"year":2025,"claim":"Provided the integrated structure of the ~1.3 MDa UBR4-KCMF1-CALM1 ring and defined UBR4's E4 chain-elongating function with KCMF1 as a substrate filter.","evidence":"Cryo-EM, in vitro reconstitution, epistasis","pmids":["40875847"],"confidence":"High","gaps":["Dynamics of flexible catalytic units during processive chain elongation not captured","How the central arena selects among substrates structurally undefined"]},{"year":2025,"claim":"Connected UBR4 to mitochondrial quality control, showing its loss impairs mitophagy and drives senescence with tumor-suppressive consequences.","evidence":"CRISPR KO, senescence and mitophagy assays, xenografts, antioxidant rescue","pmids":["40531870"],"confidence":"Medium","gaps":["Direct mitophagy substrate of UBR4 not identified","Relationship to its endolysosomal roles unclear"]},{"year":2026,"claim":"Extended UBR4's roles to stress-response and receptor turnover: degradation of HRI constrains the integrated stress response, EGFR routing to degradation, and a CaMKK2-scaffolded proteasome-targeting mechanism.","evidence":"Tissue-specific KO mouse (HRI/ISR/mitophagy), siRNA/Co-IP/MS (EGFR), Co-IP-MS and kinase assays (CaMKK2, preprint)","pmids":["41548766","41491123","42182340"],"confidence":"Medium","gaps":["Whether HRI and EGFR are direct N-degron substrates not established","CaMKK2 scaffolding model from preprint awaiting peer review"]},{"year":null,"claim":"How UBR4 partitions between proteasomal degradation, endolysosomal trafficking, autophagy/mitophagy, and degradation-independent scaffolding roles within a single megadalton assembly remains unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No unified model linking N-degron/E4 catalysis to non-degradative trafficking functions","Full endogenous substrate census incomplete","Tissue-specific cofactor configurations of the ring complex undefined"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[0,1,2,6,13,16,17]},{"term_id":"GO:0016874","term_label":"ligase activity","supporting_discovery_ids":[0,1,3,6]},{"term_id":"GO:0031386","term_label":"protein tag activity","supporting_discovery_ids":[0,1,2]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[5,19]}],"localization":[{"term_id":"GO:0005768","term_label":"endosome","supporting_discovery_ids":[9,11,12]},{"term_id":"GO:0005764","term_label":"lysosome","supporting_discovery_ids":[9,27]},{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[1,0]}],"pathway":[{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[0,1,2,3]},{"term_id":"R-HSA-5653656","term_label":"Vesicle-mediated transport","supporting_discovery_ids":[11,12,14,22]},{"term_id":"R-HSA-9612973","term_label":"Autophagy","supporting_discovery_ids":[3,21,23]},{"term_id":"R-HSA-9909396","term_label":"Circadian clock","supporting_discovery_ids":[14,24]}],"complexes":["UBR4-KCMF1-calmodulin ring complex","UBR4-KCMF1-RAD6 module"],"partners":["KCMF1","CALM1","UBE2A","UBE2B","UBE2L6","RAD6","STAT2","PMS2"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q5T4S7","full_name":"E3 ubiquitin-protein ligase UBR4","aliases":["600 kDa retinoblastoma protein-associated factor","p600","N-recognin-4","Retinoblastoma-associated factor of 600 kDa","RBAF600"],"length_aa":5183,"mass_kda":573.8,"function":"E3 ubiquitin-protein ligase involved in different protein quality control pathways in the cytoplasm (PubMed:25582440, PubMed:29033132, PubMed:34893540, PubMed:37891180, PubMed:38030679, PubMed:38182926, PubMed:38297121). Component of the N-end rule pathway: ubiquitinates proteins bearing specific N-terminal residues that are destabilizing according to the N-end rule, leading to their degradation (PubMed:34893540, PubMed:37891180, PubMed:38030679). Recognizes both type-1 and type-2 N-degrons, containing positively charged amino acids (Arg, Lys and His) and bulky and hydrophobic amino acids, respectively (PubMed:38030679). Does not ubiquitinate proteins that are acetylated at the N-terminus (PubMed:37891180). Together with UBR5, part of a cytoplasm protein quality control pathway that prevents protein aggregation by catalyzing assembly of heterotypic 'Lys-11'-/'Lys-48'-linked branched ubiquitin chains on aggregated proteins, leading to substrate recognition by the segregase p97/VCP and degradation by the proteasome: UBR4 probably synthesizes mixed chains containing multiple linkages, while UBR5 is likely branching multiple 'Lys-48'-linked chains of substrates initially modified (PubMed:29033132). Together with KCMF1, part of a protein quality control pathway that catalyzes ubiquitination and degradation of proteins that have been oxidized in response to reactive oxygen species (ROS): recognizes proteins with an Arg-CysO3(H) degron at the N-terminus, and mediates assembly of heterotypic 'Lys-63'-/'Lys-27'-linked branched ubiquitin chains on oxidized proteins, leading to their degradation by autophagy (PubMed:34893540). Catalytic component of the SIFI complex, a multiprotein complex required to inhibit the mitochondrial stress response after a specific stress event has been resolved: ubiquitinates and degrades (1) components of the HRI-mediated signaling of the integrated stress response, such as DELE1 and EIF2AK1/HRI, as well as (2) unimported mitochondrial precursors (PubMed:38297121). Within the SIFI complex, UBR4 initiates ubiquitin chain that are further elongated or branched by KCMF1 (PubMed:38297121). Mediates ubiquitination of ACLY, leading to its subsequent degradation (PubMed:23932781). Together with clathrin, forms meshwork structures involved in membrane morphogenesis and cytoskeletal organization (PubMed:16214886)","subcellular_location":"Cytoplasm; Cytoplasm, cytoskeleton; Nucleus","url":"https://www.uniprot.org/uniprotkb/Q5T4S7/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":true,"resolved_as":"","url":"https://depmap.org/portal/gene/UBR4","classification":"Common Essential","n_dependent_lines":1131,"n_total_lines":1208,"dependency_fraction":0.9362582781456954},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[{"gene":"CALM1","stoichiometry":0.2},{"gene":"CALM2","stoichiometry":0.2},{"gene":"CALM3","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/search/UBR4","total_profiled":1310},"omim":[{"mim_id":"616055","title":"EPISODIC ATAXIA, TYPE 8; EA8","url":"https://www.omim.org/entry/616055"},{"mim_id":"615741","title":"DAP3-BINDING CELL DEATH ENHANCER 1; DELE1","url":"https://www.omim.org/entry/615741"},{"mim_id":"614719","title":"POTASSIUM CHANNEL MODULATORY FACTOR 1; KCMF1","url":"https://www.omim.org/entry/614719"},{"mim_id":"613635","title":"EUKARYOTIC TRANSLATION INITIATION FACTOR 2-ALPHA KINASE 1; EIF2AK1","url":"https://www.omim.org/entry/613635"},{"mim_id":"609890","title":"UBIQUITIN PROTEIN LIGASE E3 COMPONENT N-RECOGNIN 4; UBR4","url":"https://www.omim.org/entry/609890"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Supported","locations":[{"location":"Nucleoplasm","reliability":"Supported"},{"location":"Centrosome","reliability":"Additional"},{"location":"Cytosol","reliability":"Additional"},{"location":"Acrosome","reliability":"Additional"},{"location":"Perinuclear theca","reliability":"Additional"},{"location":"Flagellar centriole","reliability":"Additional"},{"location":"Principal piece","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/UBR4"},"hgnc":{"alias_symbol":["KIAA1307","KIAA0462","RBAF600","p600"],"prev_symbol":["ZUBR1"]},"alphafold":{"accession":"Q5T4S7","domains":[],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q5T4S7","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q5T4S7-6-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q5T4S7-6-F1-predicted_aligned_error_v6.png","plddt_mean":85.25},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=UBR4","jax_strain_url":"https://www.jax.org/strain/search?query=UBR4"},"sequence":{"accession":"Q5T4S7","fasta_url":"https://rest.uniprot.org/uniprotkb/Q5T4S7.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q5T4S7/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q5T4S7"}},"corpus_meta":[{"pmid":"26651948","id":"PMC_26651948","title":"Meta- and Orthogonal Integration of Influenza \"OMICs\" Data Defines a Role for UBR4 in Virus 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EJHG","url":"https://pubmed.ncbi.nlm.nih.gov/23982692","citation_count":67,"is_preprint":false},{"pmid":"31365869","id":"PMC_31365869","title":"A Key Role for the Ubiquitin Ligase UBR4 in Myofiber Hypertrophy in Drosophila and Mice.","date":"2019","source":"Cell reports","url":"https://pubmed.ncbi.nlm.nih.gov/31365869","citation_count":65,"is_preprint":false},{"pmid":"33658508","id":"PMC_33658508","title":"Antagonistic control of myofiber size and muscle protein quality control by the ubiquitin ligase UBR4 during aging.","date":"2021","source":"Nature communications","url":"https://pubmed.ncbi.nlm.nih.gov/33658508","citation_count":58,"is_preprint":false},{"pmid":"26792178","id":"PMC_26792178","title":"The ubiquitin ligase Ubr4 controls stability of podocin/MEC-2 supercomplexes.","date":"2016","source":"Human molecular genetics","url":"https://pubmed.ncbi.nlm.nih.gov/26792178","citation_count":48,"is_preprint":false},{"pmid":"25582440","id":"PMC_25582440","title":"KCMF1 (potassium 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fever virus production.","date":"2022","source":"Virology","url":"https://pubmed.ncbi.nlm.nih.gov/35032865","citation_count":4,"is_preprint":false},{"pmid":"37529615","id":"PMC_37529615","title":"UBR4 deficiency causes male sterility and testis abnormal in Drosophila.","date":"2023","source":"Frontiers in endocrinology","url":"https://pubmed.ncbi.nlm.nih.gov/37529615","citation_count":2,"is_preprint":false},{"pmid":"40374006","id":"PMC_40374006","title":"The interaction of UBR4, LRP1, and OPHN1 in refractory epilepsy: Drosophila model to investigate the oligogenic effect on epilepsy.","date":"2025","source":"Neurobiology of disease","url":"https://pubmed.ncbi.nlm.nih.gov/40374006","citation_count":1,"is_preprint":false},{"pmid":"41548766","id":"PMC_41548766","title":"UBR4 attenuates cisplatin-induced acute kidney injury by regulating the HRI-ISR axis.","date":"2026","source":"Free radical biology & 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comprising a 'hemiRING' zinc finger, a helical-rich UBR zinc-finger interacting (UZI) subdomain, and an N-terminal affinity region that recruits cognate E2 conjugating enzymes UBE2A and UBE2B. The UZI subdomain allosterically activates the Ub-loaded E2 (E2~Ub). The hemiRING specifically recognizes UBE2A/UBE2B through determinants revealed by crystal structure of the E2-E3 complex.\",\n      \"method\": \"Crystal/cryo-EM structure of E2-E3 complex, in vitro ubiquitination assays, mutagenesis\",\n      \"journal\": \"Nature structural & molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — atomic-resolution structure combined with in vitro functional assays and mutagenesis in a single rigorous study\",\n      \"pmids\": [\"38182926\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Cryo-EM analysis of UBR4 in complex with cofactors KCMF1 and CALM1 reveals a 1.3-megadalton ring structure with a central substrate-binding arena and flexibly attached catalytic units. UBR4 acts as an E4 ubiquitin chain-elongating ligase, extending K48-linked ubiquitin chains on pre-ubiquitinated substrates. Efficient substrate targeting requires both preubiquitination and specific N-degrons, with KCMF1 acting as a substrate filter.\",\n      \"method\": \"Cryo-electron microscopy, in vitro ubiquitination reconstitution, epistasis analysis\",\n      \"journal\": \"Science (New York, N.Y.)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — cryo-EM structure plus reconstitution in vitro with epistasis in a single rigorous study\",\n      \"pmids\": [\"40875847\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"The UBR4-KCMF1 ubiquitin ligase complex acts downstream of a priming E3 that mono-ubiquitinates orphan subunits; UBR4 then recognizes both the orphan protein and its mono-ubiquitin mark and builds K48-linked poly-ubiquitin chains for proteasomal degradation. This convergence point is required for efficient degradation of orphan subunits from the chaperonin, proteasome cap, proteasome core, and a protein targeting complex.\",\n      \"method\": \"Epistasis analysis in cells, in vitro reconstitution assays\",\n      \"journal\": \"bioRxiv (preprint)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro reconstitution plus cellular epistasis, preprint single lab but two orthogonal methods\",\n      \"pmids\": [\"bio_10.1101_2024.08.07.607117\"],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"UBR4 (p600) recognizes N-degrons through its UBR box as an N-recognin of the N-end rule pathway, mediating ubiquitylation-dependent proteolysis. UBR4-deficient mice die during embryogenesis with impaired yolk sac vascular development. In the yolk sac endoderm, UBR4 regulates autophagic flux including LC3 lipidation/activation and autophagic double-membrane structure formation.\",\n      \"method\": \"Genetic knockout mouse model, autophagic flux assays, LC3 lipidation assays\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — clean KO with defined developmental and cellular phenotypes, multiple orthogonal readouts\",\n      \"pmids\": [\"23431188\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Dengue virus NS5 protein bridges STAT2 and UBR4, recruiting UBR4 to promote proteasome-dependent STAT2 degradation. Processed (but not unprocessed) NS5 preferentially binds UBR4. UBR4 knockdown reduces DENV-mediated STAT2 degradation and impairs viral replication in IFN-I competent cells.\",\n      \"method\": \"Co-immunoprecipitation, siRNA knockdown, proteasome inhibitor assays, viral replication assays\",\n      \"journal\": \"PLoS pathogens\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal Co-IP, siRNA KD with functional viral and STAT2 degradation readouts, multiple orthogonal methods\",\n      \"pmids\": [\"23555265\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Structural analysis of the UBR box of UBR4 reveals it recognizes type-2 N-terminal amino acids (containing aromatic rings) and type-1 N-terminal arginine through two phenylalanines on a hydrophobic surface, a distinct mechanism from other UBR box-containing proteins. The second ligand residue binding mechanism was also characterized.\",\n      \"method\": \"X-ray crystallography / structural analysis, binding assays\",\n      \"journal\": \"Communications biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — crystal structure with functional binding characterization, single lab but atomic-resolution structural data\",\n      \"pmids\": [\"38030679\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Loss of UBR4 in Drosophila and mouse myofibers promotes hypertrophy via decreased ubiquitination and degradation of the HAT1/RBBP4/RBBP7 histone-binding complex. RNAi screening in Drosophila identified UBR4 as an N-end rule ubiquitin ligase that limits myofiber size.\",\n      \"method\": \"RNAi screening in Drosophila, mouse KO, ubiquitination assays, proteomics (mass spectrometry)\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — loss-of-function in two organisms with defined substrate identification by mass spectrometry and ubiquitination assays\",\n      \"pmids\": [\"31365869\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"UBR4 levels increase in skeletal muscle with aging and UBR4 increases proteasome proteolytic activity. Muscle-specific UBR4 loss rescues age-associated myofiber atrophy in mice but reduces specific force and accelerates protein quality control decline, demonstrating that UBR4 antithetically regulates myofiber size and muscle protein quality control.\",\n      \"method\": \"Muscle-specific KO mouse model, proteasome activity assays, in vivo muscle function measurements\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — muscle-specific KO with multiple orthogonal functional phenotypic readouts in mice and Drosophila\",\n      \"pmids\": [\"33658508\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"UBR4 associates with influenza A virus M2 protein (identified by affinity purification/mass spectrometry and Co-IP) and promotes apical transport of viral proteins; UBR4 knockdown impairs viral budding.\",\n      \"method\": \"Affinity purification-mass spectrometry, Co-immunoprecipitation, RNAi knockdown, viral protein localization assays\",\n      \"journal\": \"Cell host & microbe\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Co-IP plus functional knockdown with viral phenotypic readout, single study\",\n      \"pmids\": [\"26651948\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"UBR4 forms a complex with KCMF1 and the E2 enzyme RAD6. NMR and in vitro interaction mapping showed KCMF1 C-terminus binds directly to RAD6, while KCMF1 N-terminal domains interact with UBR4. KCMF1 and RAD6 colocalize at late endosomes and lysosomes. RAD6A mutations (R7W, R11Q) found in X-linked intellectual disability specifically disrupt interaction with KCMF1 and UBR4.\",\n      \"method\": \"Affinity purification-mass spectrometry, NMR, in vivo and in vitro interaction mapping, co-localization microscopy\",\n      \"journal\": \"Molecular & cellular proteomics : MCP\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — NMR structure plus in vitro interaction mapping plus cellular colocalization, single lab with multiple orthogonal methods\",\n      \"pmids\": [\"25582440\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"In Drosophila, POE/UBR4 acts as an E3 ubiquitin ligase together with E2 UBC6 and co-factor UFD4 to target MAPK for proteasomal degradation, opposing the deubiquitinase USP47. KCMF1 was identified as another key component modulating MAPK levels through this complex. Genetic interaction screening (RNAi-based epistasis) established the pathway order.\",\n      \"method\": \"RNAi-based genetic interaction screen (epistasis), Drosophila genetics, MAPK half-life measurements\",\n      \"journal\": \"PLoS biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — epistasis by RNAi screen with defined pathway order, single lab\",\n      \"pmids\": [\"27552662\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"UBR4 is recruited to maturing endosomes through interaction with Ca2+-bound calmodulin, and this endosomal recruitment is essential for biogenesis of early endosomes (EEs) and trafficking of endocytosed cargo. Loss of UBR4 disrupts endosome-lysosome pathway function in mouse embryos.\",\n      \"method\": \"Co-immunoprecipitation (calmodulin interaction), cellular fractionation, live-cell imaging, mouse embryo analysis, KO phenotyping\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Co-IP interaction, KO with defined endosomal phenotype, single lab\",\n      \"pmids\": [\"30111582\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"UBR4-deficient mice die around E9.5-10.5 with defects in neurogenesis and cardiovascular development correlated with depletion of plasma membrane proteins and impaired multivesicular body (MVB) biogenesis, suggesting UBR4 governs proteome-wide turnover of cell surface proteins through the endosomal pathway.\",\n      \"method\": \"Conditional KO mouse model (UBR box deletion), proteomics of plasma membrane fractions, MVB biogenesis assays\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — KO mouse with proteomics and MVB phenotype, single lab\",\n      \"pmids\": [\"30157281\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"The ubiquitin ligase UBR4 is a key component of the podocin interactome in podocytes and glomeruli (identified by purification and MS). UBR4 co-localizes with podocin and controls its ubiquitylation at K301 and a second lysine, regulating stability and disassembly of podocin/MEC-2 multimeric complexes. In C. elegans, Ubr4 degrades mislocalized MEC-2 multimers.\",\n      \"method\": \"Affinity purification-mass spectrometry, co-immunoprecipitation, ubiquitylomics, C. elegans genetics, molecular dynamics simulations\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — AP-MS identification, site-specific ubiquitylomics, C. elegans in vivo validation, multiple orthogonal methods\",\n      \"pmids\": [\"26792178\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"UBR4/POE ubiquitin ligase promotes neuropeptide trafficking in clock neurons by maintaining expression of Coronin 7, which is required for budding of Golgi-derived transport vesicles. Ubr4-deficient mice show resistance to jetlag; poe knockdown flies are prone to arrhythmicity due to reduced axonal trafficking of circadian neuropeptides.\",\n      \"method\": \"Mouse KO, Drosophila RNAi, live imaging, neuropeptide trafficking assays, Golgi export assays\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — KO/KD in two organisms with defined molecular mechanism (Coronin 7 regulation), multiple orthogonal readouts\",\n      \"pmids\": [\"35332162\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"UBR4 ubiquitinates YAP at its substrate to promote YAP degradation and nuclear-cytoplasmic translocation, thereby reducing endometrial fibrosis. P65 (NF-κB) binds the UBR4 promoter to transcriptionally upregulate UBR4 expression in menstrual blood stromal cells.\",\n      \"method\": \"siRNA knockdown, ubiquitination assays, nuclear/cytoplasmic fractionation, chromatin immunoprecipitation (P65 on UBR4 promoter), co-immunoprecipitation\",\n      \"journal\": \"Journal of nanobiotechnology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — ubiquitination assay plus ChIP plus fractionation, single lab\",\n      \"pmids\": [\"37644565\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"UBR4 (E3 ligase) cooperates with UBE2L6 (E2) to ubiquitinate EZH2 at K381, promoting its degradation. UHRF1-mediated CpG methylation downregulates UBE2L6 in low-pigmented melanoma cells, stabilizing EZH2. This UHRF1/UBE2L6/UBR4 axis controls EZH2 abundance and melanocytic differentiation states.\",\n      \"method\": \"Co-immunoprecipitation, mass spectrometry, ubiquitination assays, siRNA knockdown, animal studies\",\n      \"journal\": \"Oncogene\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Co-IP plus site-specific ubiquitination assays plus animal xenograft validation, single lab\",\n      \"pmids\": [\"36906655\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"UBR4 promotes proteasomal degradation of MLH1 through its ubiquitin ligase activity; the region of MLH1 encompassing amino acids 516-650 is crucial for UBR4-dependent degradation. PMS2 binding to this segment shields MLH1 from UBR4-mediated degradation. The deubiquitylase USP5 opposes UBR4 to stabilize MLH1. UBR4 or USP5 deficiency affects cellular response to 6-TG, linking them to mismatch repair.\",\n      \"method\": \"Co-immunoprecipitation, siRNA knockdown, ubiquitination assays, domain-mapping (deletion constructs), 6-TG sensitivity assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Co-IP, domain mapping, functional drug response assay, single lab\",\n      \"pmids\": [\"39032648\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"NatB (N-terminal acetyltransferase B)-mediated N-terminal acetylation protects procaspase-8 from UBR4 (and UBR1) Arg/N-recognin-mediated degradation. In Naa20-/- MEFs lacking NatB catalytic activity, procaspase-8 is degraded via UBR4, reducing responsiveness to extrinsic apoptotic stimuli. UBR4 silencing in Naa20-/- cells partially restores procaspase-8 levels and apoptotic response.\",\n      \"method\": \"MEF KO (Naa20-/-), quantitative N-terminomics, label-free quantification, targeted proteomics, siRNA knockdown of UBR4, apoptosis assays\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic KO combined with siRNA epistasis and proteomics, single lab with multiple methods\",\n      \"pmids\": [\"39099191\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"UBR4 exclusively (not UBR1 or UBR2) targets N-degrons bearing position-3 arginine or lysine residues after MetAP2-mediated co-translational cleavage of N-terminal methionine preceding second-position threonine or valines. This MetAP2-dependent Arg/N-degron pathway is a distinct substrate-recognition mechanism for UBR4.\",\n      \"method\": \"Reporter assays, CRISPR-Cas9 knockout of N-recognins (UBR1, UBR2, UBR4), MetAP2 inhibition\",\n      \"journal\": \"bioRxiv (preprint)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — CRISPR KO epistasis with reporter assays demonstrating UBR4 specificity, preprint single lab\",\n      \"pmids\": [\"bio_10.1101_2024.10.03.616566\"],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"NAP1L1 protects BIRC2 from UBR4-mediated ubiquitin-proteasome degradation. NAP1L1 knockdown promotes UBR4-dependent ubiquitination and degradation of BIRC2, inhibiting hepatocellular carcinoma cell proliferation and apoptotic escape. UBR4 was identified as the intermediate molecule linking NAP1L1 and BIRC2 by Co-IP and mass spectrometry.\",\n      \"method\": \"Co-immunoprecipitation, mass spectrometry, siRNA knockdown, ubiquitination assays\",\n      \"journal\": \"Cell death discovery\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Co-IP plus mass spectrometry plus functional KD assays, single lab\",\n      \"pmids\": [\"38538582\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"UBR4 promotes ubiquitination and proteasomal degradation of the kinase HRI, thereby constraining integrated stress response (ISR) overactivation and alleviating ISR-mediated inhibition of mitophagy in proximal tubular epithelial cells. Tubule-specific Ubr4 deficiency exacerbates cisplatin-induced kidney injury by allowing HRI-mediated ISR overactivation.\",\n      \"method\": \"Tubule-specific KO mouse model, ubiquitination assays, HRI protein stability assays, ISR pathway readouts, mitophagy assays\",\n      \"journal\": \"Free radical biology & medicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — tissue-specific KO with defined substrate (HRI) and pathway rescue, single lab\",\n      \"pmids\": [\"41548766\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"UBR4 mediates clathrin-dependent EGFR endocytosis and degradation in colorectal cancer cells. UBR4 knockdown promotes EGFR recycling rather than degradation, enhancing cell proliferation and reducing apoptosis in response to cetuximab.\",\n      \"method\": \"siRNA knockdown, co-immunoprecipitation, tandem mass spectrometry, immunofluorescence, cell viability and apoptosis assays\",\n      \"journal\": \"Journal of gastroenterology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Co-IP/MS identification plus functional siRNA KD with multiple cellular readouts, single lab\",\n      \"pmids\": [\"41491123\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"UBR4 knockout in A549 lung cancer cells induces cellular senescence with defective mitochondria caused by impaired mitophagy. Restoration of UBR4 or antioxidant treatment reverses the ΔUBR4 phenotypes, suggesting UBR4 regulates mitochondrial quality control to prevent senescence-associated tumor suppression.\",\n      \"method\": \"CRISPR KO, senescence assays, mitophagy flux assays, xenograft mouse models, antioxidant rescue experiments\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — CRISPR KO with multiple phenotypic and rescue readouts in vitro and in vivo, single lab\",\n      \"pmids\": [\"40531870\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"In the murine SCN, UBR4 protein expression is time-of-day-dependent and light-inducible. UBR4 is localized exclusively to arginine vasopressin (AVP)-expressing neurons of the SCN shell, as determined by immunofluorescence microscopy. Photic stimulation in the early subjective night increases the number of UBR4-expressing SCN cells.\",\n      \"method\": \"Proteome-wide SCN screen, immunofluorescence microscopy, temporal expression profiling\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — direct localization by immunofluorescence with temporal functional correlation, replicated across time points but single lab\",\n      \"pmids\": [\"25084275\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"UBR4 depletion reduces RVFV titers and viral RNA production, identifying UBR4 as a host factor required for Rift Valley fever virus replication. UBR4 was identified as a Gn glycoprotein-interacting protein by proteomics-based approach using V5-epitope tagged virus.\",\n      \"method\": \"Proteomics (affinity purification-mass spectrometry), siRNA knockdown, viral titer assays, viral RNA quantification\",\n      \"journal\": \"Virology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — AP-MS identification plus functional KD with viral replication readout, single lab\",\n      \"pmids\": [\"35032865\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"CaMKK2 co-immunoprecipitates with UBR4 and the 19S proteasome regulatory particle (RP) in chondrocytes; CaMKK2 scaffolds UBR4 and 19S RP around polyubiquitinated proteins such as Sox9 for proteasomal degradation, independently of its kinase activity, while CaMKK2 kinase activity phosphorylates Psmc5 (19S ATPase) to enhance proteasome activity.\",\n      \"method\": \"Co-immunoprecipitation-mass spectrometry, kinase assays, phosphosite identification by mass spectrometry, proteasome activity assays, kinase-dead mutants\",\n      \"journal\": \"bioRxiv (preprint)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Co-IP/MS with kinase assay and proteasome activity readout, preprint single lab with multiple orthogonal methods\",\n      \"pmids\": [\"42182340\"],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"In IgG4-RD, a UBR4 variant prevents lysosomal degradation of the phosphatase CD45 in T cells, contributing to T cell hyperresponsiveness. This was established in patient-derived T cells carrying the specific UBR4 variant.\",\n      \"method\": \"Patient cell-based assays, CD45 protein stability measurements, lysosomal degradation assays\",\n      \"journal\": \"The Journal of clinical investigation\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single variant analysis in patient cells, no in vitro reconstitution or biochemical mechanism established\",\n      \"pmids\": [\"38885295\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"UBR4 is a large (~574 kDa) E3/E4 ubiquitin ligase of the N-degron pathway that assembles into a ~1.3 MDa ring complex with KCMF1 and calmodulin; its hemiRING catalytic module specifically recruits E2s UBE2A/UBE2B to extend K48-linked ubiquitin chains on substrates bearing N-degrons or preexisting mono-ubiquitin marks, thereby targeting diverse substrates—including orphan protein complex subunits, STAT2, podocin, MAPK, EZH2, MLH1, BIRC2, HRI, and the HAT1/RBBP4/RBBP7 histone-binding complex—for proteasomal degradation, while also regulating endosomal/lysosomal trafficking, autophagy, neuropeptide secretion, and muscle proteostasis.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"UBR4 is a large E3/E4 ubiquitin ligase of the N-degron (N-end rule) pathway that directs ubiquitin-proteasome and endolysosomal turnover of a broad set of substrates across development, proteostasis, and quality control [#3, #6]. Its catalytic core comprises a hemiRING zinc finger and a UZI subdomain that specifically recruit and allosterically activate the cognate E2 enzymes UBE2A/UBE2B (RAD6), determining E2 selectivity [#0, #9]. Substrate selection operates through a UBR box that recognizes type-1 (N-terminal Arg) and type-2 (aromatic) N-degrons via a distinct two-phenylalanine surface, with specialized recognition of MetAP2-generated position-3 Arg/Lys degrons [#5, #19]. UBR4 assembles with KCMF1 and calmodulin into a ~1.3 MDa ring with a central substrate arena; it functions predominantly as an E4 chain-elongating ligase that extends K48-linked chains on substrates already bearing priming mono-ubiquitin and an N-degron, with KCMF1 acting as a substrate filter — a logic that drives clearance of orphan subunits from the chaperonin, proteasome cap/core, and targeting complexes [#1, #2]. Through these activities UBR4 degrades diverse targets including the HAT1/RBBP4/RBBP7 complex to limit myofiber size [#6], podocin multimers [#13], MAPK [#10], EZH2 [#16], MLH1 [#17], YAP [#15], BIRC2 [#20], and the stress kinase HRI [#21], and is exploited by Dengue NS5 to degrade STAT2 [#4]. Independently of degradation, UBR4 governs endosomal/lysosomal trafficking via calmodulin-dependent recruitment to maturing endosomes, multivesicular body biogenesis, autophagic flux, mitophagy-linked mitochondrial quality control, and Golgi-derived neuropeptide secretion [#3, #11, #12, #14, #23]. UBR4 loss is embryonic lethal in mice with vascular, neural, and cardiovascular defects tied to surface-proteome turnover [#3, #12].\",\n  \"teleology\": [\n    {\n      \"year\": 2013,\n      \"claim\": \"Established UBR4 as a bona fide N-recognin of the N-end rule pathway and tied it to autophagy and embryonic development, defining its core biological role.\",\n      \"evidence\": \"UBR knockout mouse with developmental phenotyping and LC3 lipidation/autophagic flux assays\",\n      \"pmids\": [\"23431188\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not identify direct degradation substrates\", \"Link between N-degron recognition and autophagy regulation left mechanistically unresolved\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Showed UBR4 can be hijacked by a viral adaptor to degrade a host substrate, demonstrating substrate recruitment can occur via bridging proteins rather than intrinsic N-degron recognition alone.\",\n      \"evidence\": \"Reciprocal Co-IP, siRNA knockdown, proteasome inhibition, and viral replication assays with Dengue NS5/STAT2\",\n      \"pmids\": [\"23555265\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether STAT2 carries an N-degron or is recruited solely via NS5 bridging not resolved\", \"Direct ubiquitination of STAT2 by UBR4 not reconstituted\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Defined the UBR4-KCMF1-RAD6 module architecture and connected it to endolysosomal compartments and X-linked intellectual disability through RAD6A mutations.\",\n      \"evidence\": \"AP-MS, NMR, in vitro interaction mapping, and colocalization microscopy\",\n      \"pmids\": [\"25582440\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Substrates targeted by this complex at endosomes/lysosomes not identified\", \"Catalytic logic of the assembly not yet defined\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Identified specific physiological substrates and trafficking roles — podocin multimers and a MAPK degradation pathway — broadening UBR4 from a developmental factor to a substrate-specific ligase.\",\n      \"evidence\": \"AP-MS/ubiquitylomics with C. elegans validation (podocin); RNAi epistasis in Drosophila (MAPK/UBC6/UFD4/KCMF1)\",\n      \"pmids\": [\"26792178\", \"27552662\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Site-specific ubiquitination logic generalizability across substrates unknown\", \"Mammalian conservation of the MAPK pathway not established\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Mechanistically linked UBR4 to endosome biogenesis and surface-proteome turnover via calmodulin-dependent recruitment, explaining embryonic lethality through trafficking failure.\",\n      \"evidence\": \"Co-IP (calmodulin), fractionation, live imaging, KO mouse embryo proteomics and MVB assays\",\n      \"pmids\": [\"30111582\", \"30157281\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether endosomal roles require ligase activity or are scaffolding functions unclear\", \"Direct surface-protein substrates not individually validated\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Demonstrated that UBR4-mediated degradation of the HAT1/RBBP4/RBBP7 complex limits myofiber size, establishing a conserved proteostatic role in muscle.\",\n      \"evidence\": \"Drosophila RNAi screen, mouse KO, ubiquitination assays, mass spectrometry\",\n      \"pmids\": [\"31365869\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How histone-binding complex turnover controls hypertrophy mechanistically unresolved\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Revealed an antithetical role in which UBR4 boosts proteasome activity and protein quality control even as it restrains myofiber size, complicating its role in age-associated atrophy.\",\n      \"evidence\": \"Muscle-specific KO mouse, proteasome activity and in vivo force measurements\",\n      \"pmids\": [\"33658508\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular basis for UBR4 enhancement of proteasome activity not defined\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Showed a degradation-independent trafficking function: UBR4 maintains Coronin 7 to drive Golgi-derived neuropeptide vesicle budding and circadian neuropeptide transport.\",\n      \"evidence\": \"Mouse KO, Drosophila RNAi, live imaging, Golgi export and neuropeptide trafficking assays\",\n      \"pmids\": [\"35332162\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether Coronin 7 is a UBR4 ubiquitination substrate or stabilized by another route not resolved\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Resolved the structural basis of UBR box N-degron recognition, showing a distinct two-phenylalanine mechanism for type-1 and type-2 residues.\",\n      \"evidence\": \"X-ray crystallography of the UBR box with binding assays\",\n      \"pmids\": [\"38030679\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structure does not connect N-degron binding to catalytic chain assembly\", \"Second-ligand binding contribution to substrate selection in vivo unverified\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Expanded the substrate repertoire to disease-relevant targets — EZH2 (with E2 UBE2L6) and YAP — and showed UBR4 itself is transcriptionally controlled by NF-kB.\",\n      \"evidence\": \"Co-IP, site-specific ubiquitination assays, fractionation, ChIP, animal studies\",\n      \"pmids\": [\"36906655\", \"37644565\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether these substrates are recognized via N-degron or other determinants unclear\", \"Single-lab substrate assignments without structural validation\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Defined the dedicated catalytic E3 module (hemiRING + UZI) and its E2 selectivity, providing the structural mechanism of UBE2A/UBE2B recruitment and activation.\",\n      \"evidence\": \"Crystal/cryo-EM of E2-E3 complex, in vitro ubiquitination, mutagenesis\",\n      \"pmids\": [\"38182926\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How the catalytic module is positioned relative to substrates in the full assembly not resolved in this study\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Established UBR4 as a convergence point acting downstream of a priming E3, recognizing both N-degron and pre-existing mono-ubiquitin to clear orphan complex subunits.\",\n      \"evidence\": \"Cellular epistasis and in vitro reconstitution (preprint)\",\n      \"pmids\": [\"bio_10.1101_2024.08.07.607117\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Identity of all priming E3s unknown\", \"Generality of the two-mark logic across substrates not exhaustively tested\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Refined substrate-recognition specificity, showing UBR4 uniquely reads MetAP2-generated position-3 Arg/Lys N-degrons, distinguishing it from UBR1/UBR2.\",\n      \"evidence\": \"CRISPR KO of N-recognins, reporter assays, MetAP2 inhibition (preprint)\",\n      \"pmids\": [\"bio_10.1101_2024.10.03.616566\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Endogenous substrates bearing this degron not enumerated\", \"Preprint awaiting peer review\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Added MLH1, procaspase-8, and BIRC2 as substrates whose stability is set by competing protective factors, linking UBR4 to mismatch repair, apoptosis, and cancer.\",\n      \"evidence\": \"Co-IP, domain mapping, ubiquitination assays, N-terminomics, KO/KD epistasis, drug-response assays\",\n      \"pmids\": [\"39032648\", \"39099191\", \"38538582\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether UBR4 directly ubiquitinates each target or acts in larger complexes not fully resolved\", \"Single-lab assignments per substrate\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Provided the integrated structure of the ~1.3 MDa UBR4-KCMF1-CALM1 ring and defined UBR4's E4 chain-elongating function with KCMF1 as a substrate filter.\",\n      \"evidence\": \"Cryo-EM, in vitro reconstitution, epistasis\",\n      \"pmids\": [\"40875847\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Dynamics of flexible catalytic units during processive chain elongation not captured\", \"How the central arena selects among substrates structurally undefined\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Connected UBR4 to mitochondrial quality control, showing its loss impairs mitophagy and drives senescence with tumor-suppressive consequences.\",\n      \"evidence\": \"CRISPR KO, senescence and mitophagy assays, xenografts, antioxidant rescue\",\n      \"pmids\": [\"40531870\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct mitophagy substrate of UBR4 not identified\", \"Relationship to its endolysosomal roles unclear\"]\n    },\n    {\n      \"year\": 2026,\n      \"claim\": \"Extended UBR4's roles to stress-response and receptor turnover: degradation of HRI constrains the integrated stress response, EGFR routing to degradation, and a CaMKK2-scaffolded proteasome-targeting mechanism.\",\n      \"evidence\": \"Tissue-specific KO mouse (HRI/ISR/mitophagy), siRNA/Co-IP/MS (EGFR), Co-IP-MS and kinase assays (CaMKK2, preprint)\",\n      \"pmids\": [\"41548766\", \"41491123\", \"42182340\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether HRI and EGFR are direct N-degron substrates not established\", \"CaMKK2 scaffolding model from preprint awaiting peer review\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How UBR4 partitions between proteasomal degradation, endolysosomal trafficking, autophagy/mitophagy, and degradation-independent scaffolding roles within a single megadalton assembly remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No unified model linking N-degron/E4 catalysis to non-degradative trafficking functions\", \"Full endogenous substrate census incomplete\", \"Tissue-specific cofactor configurations of the ring complex undefined\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [0, 1, 2, 6, 13, 16, 17]},\n      {\"term_id\": \"GO:0016874\", \"supporting_discovery_ids\": [0, 1, 3, 6]},\n      {\"term_id\": \"GO:0031386\", \"supporting_discovery_ids\": [0, 1, 2]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [5, 19]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005768\", \"supporting_discovery_ids\": [9, 11, 12]},\n      {\"term_id\": \"GO:0005764\", \"supporting_discovery_ids\": [9, 27]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [1, 0]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [0, 1, 2, 3]},\n      {\"term_id\": \"R-HSA-5653656\", \"supporting_discovery_ids\": [11, 12, 14, 22]},\n      {\"term_id\": \"R-HSA-9612973\", \"supporting_discovery_ids\": [3, 21, 23]},\n      {\"term_id\": \"R-HSA-9909396\", \"supporting_discovery_ids\": [14, 24]}\n    ],\n    \"complexes\": [\"UBR4-KCMF1-calmodulin ring complex\", \"UBR4-KCMF1-RAD6 module\"],\n    \"partners\": [\"KCMF1\", \"CALM1\", \"UBE2A\", \"UBE2B\", \"UBE2L6\", \"RAD6\", \"STAT2\", \"PMS2\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":7,"faith_total":7,"faith_pct":100.0}}