{"gene":"UBR2","run_date":"2026-06-10T10:51:56","timeline":{"discoveries":[{"year":2003,"finding":"UBR2 is an E3 ubiquitin ligase of the mammalian N-end rule pathway with substrate-binding properties highly similar to UBR1, recognizing proteins with destabilizing N-terminal residues. UBR2-deficient male mice show spermatocyte arrest between leptotene/zygotene and pachytene with absence of intact synaptonemal complexes and subsequent apoptosis, establishing a required role in male meiosis.","method":"Knockout mouse construction, substrate-binding assays, histological analysis of testes","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic KO with defined cellular phenotype, substrate-binding assays, replicated across strain backgrounds in a single rigorous study","pmids":["14585983"],"is_preprint":false},{"year":2004,"finding":"RECQL4 (mutated in Rothmund-Thomson syndrome) forms a stable complex with UBR1 and UBR2 isolated from HeLa cells. The UBR1/2-bound RECQL4 was not ubiquitylated in vivo and was a long-lived protein; the isolated complex had DNA-stimulated ATPase activity.","method":"Co-immunoprecipitation with anti-RECQL4 antibodies from HeLa cell extracts, in vivo ubiquitylation assay, ATPase activity assay","journal":"Human molecular genetics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reciprocal Co-IP from cells plus functional ATPase assay, single lab, two orthogonal methods","pmids":["15317757"],"is_preprint":false},{"year":2004,"finding":"Yeast Ubr2 ubiquitin ligase mediates ubiquitin-dependent degradation of Rpn4 (a transcriptional activator of proteasome genes) via the ubiquitin-conjugating enzyme Rad6, which directly interacts with Ubr2. Rpn4 was validated as the first physiological substrate of Ubr2 through in vivo and in vitro ubiquitination assays.","method":"In vivo and in vitro ubiquitination assays, genetic deletion, protein stability assays, synthetic growth defect analysis","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — in vitro reconstitution of ubiquitination plus in vivo genetic validation in a single focused study","pmids":["15504724"],"is_preprint":false},{"year":2006,"finding":"UBR1 and UBR2 double-knockout mouse embryos die at midgestation with defects in neurogenesis and cardiovascular development, including reduced proliferation and precocious migration/differentiation of neural progenitor cells; altered expression of D-type cyclins and Notch1 was observed, indicating partially divergent functions between UBR1 and UBR2.","method":"Double-knockout mouse construction, histological and molecular analysis of embryos","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic epistasis via double KO with defined developmental phenotype and molecular markers, rigorous in vivo study","pmids":["16606826"],"is_preprint":false},{"year":2006,"finding":"UBR2-deficient mouse fibroblasts display chromosome fragility, chromosomal bridges, micronuclei, spontaneous chromosomal gaps, hypersensitivity to mitomycin C, and are significantly impaired in homologous recombination repair of double-strand breaks (but show normal non-homologous end joining), establishing a role for UBR2 in maintaining genome integrity via HR repair.","method":"Metaphase chromosome spreads, reporter assay for HR and NHEJ, mitomycin C sensitivity assay, UBR2-/- fibroblast cell lines","journal":"Mutation research","confidence":"High","confidence_rationale":"Tier 2 / Moderate — loss-of-function with multiple defined cellular phenotypes and pathway-specific repair assays, single lab","pmids":["16488448"],"is_preprint":false},{"year":2010,"finding":"UBR2 localizes to meiotic chromatin regions including unsynapsed axial elements and mediates transcriptional silencing by ubiquitinating histone H2A. UBR2 interacts with ubiquitin-conjugating enzyme HR6B, promotes HR6B–H2A interaction and ubiquitin transfer to H2A. UBR2-deficient spermatocytes fail to ubiquitinate H2A and fail to silence genes on unsynapsed X and Y chromosomes.","method":"Chromatin immunofluorescence, Co-immunoprecipitation (UBR2–HR6B, UBR2–H2A), in vitro ubiquitination assay, analysis of UBR2-/- spermatocytes","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — Co-IP, in vitro ubiquitination, localization with functional consequence, KO phenotype, multiple orthogonal methods","pmids":["20080676"],"is_preprint":false},{"year":2010,"finding":"Saccharomyces cerevisiae Ubr1 and Ubr2 ubiquitin ligases promote degradation of unfolded/misfolded cytosolic polypeptides as part of a cytosolic protein quality control pathway. Ubr1 directly interacts with denatured substrates (not native protein), and Hsp70 stimulates polyubiquitination. Loss of Ubr1/Ubr2 suppresses growth arrest from chaperone mutation.","method":"Purified in vitro ubiquitination assay with denatured luciferase, genetic suppressor analysis (double mutant), yeast genetics","journal":"Molecular biology of the cell","confidence":"High","confidence_rationale":"Tier 1 / Moderate — in vitro reconstitution with purified components plus genetic epistasis, single lab with orthogonal methods","pmids":["20462952"],"is_preprint":false},{"year":2010,"finding":"Ubr2 forms a stable protein complex with Tex19.1 (a germ cell-specific protein) in mouse testes. In Ubr2-deficient germ cells, Tex19.1 protein is absent despite normal Tex19.1 transcription, indicating that Ubr2 binding metabolically stabilizes Tex19.1, a function distinct from the conventional N-end rule proteolytic role.","method":"Co-immunoprecipitation from mouse testes, Western blot in Ubr2-/- germ cells, RT-PCR","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — Co-IP plus protein stability analysis in KO, two complementary methods, single lab","pmids":["21103378"],"is_preprint":false},{"year":2010,"finding":"UBR1 and UBR2 are leucine-binding proteins (identified via leucine-immobilized affinity beads). Leucine binds to the substrate-recognition domain of UBR2 and inhibits degradation of N-end rule substrates in vitro. Overexpression of UBR1/UBR2 reduces mTOR-dependent S6K1 phosphorylation, while knockdown increases S6K1 phosphorylation in amino acid-starved cells, identifying UBR1/UBR2 as negative regulators of the leucine-mTOR signaling pathway.","method":"Leucine-affinity pulldown, in vitro N-end rule substrate degradation assay, overexpression and siRNA knockdown with S6K1 phosphorylation readout","journal":"Genes to cells : devoted to molecular & cellular mechanisms","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — affinity pulldown plus in vitro assay plus gain/loss-of-function signaling readout, single lab, multiple methods","pmids":["20298436"],"is_preprint":false},{"year":2012,"finding":"UBR2 functions as a scaffold E3 that promotes HR6B/UbcH2-dependent monoubiquitylation of H2A and H2B (but not H3 or H4) through a mechanism distinct from typical polyubiquitylation. The E3 activity of UBR2 in histone ubiquitylation is allosterically activated by dipeptides bearing destabilizing N-terminal residues. UBR2-deficient spermatocytes show defects in DSB repair and undergo pachytene arrest. UBR2-deficient somatic cells show chromosomal instability and hypersensitivity to DNA damage.","method":"In vitro ubiquitylation assay with histone substrates, dipeptide allosteric activation assay, analysis of UBR2-/- cells (meiotic and somatic), chromosome spread analysis","journal":"PloS one","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — in vitro reconstitution, allosteric activation assay, multiple KO phenotypes in both germ and somatic cells, multiple orthogonal methods","pmids":["22616001"],"is_preprint":false},{"year":2013,"finding":"Tumor cell-induced upregulation of UBR2 in muscle is mediated by p38β MAPK-dependent phosphorylation of C/EBPβ at Thr-188, which enables C/EBPβ binding to a functional responsive element in the UBR2 promoter. Genetic gain/loss-of-function confirmed p38β is sufficient and necessary for UBR2 upregulation; genetic ablation of C/EBPβ blocked UBR2 upregulation in tumor-bearing mice.","method":"Pharmacological inhibitor (SB202190), genetic gain/loss-of-function in C2C12 myotubes, luciferase reporter assay, ChIP-implied C/EBPβ binding, in vivo tumor-bearing mouse model","journal":"FASEB journal","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple genetic and pharmacological interventions, reporter assay, in vitro and in vivo validation, multiple orthogonal methods","pmids":["23568773"],"is_preprint":false},{"year":2013,"finding":"Budding yeast Mub1/Ubr2 ubiquitin ligase complex associates with kinetochore particles through CENP-C (Mif2) and regulates the levels of the outer kinetochore protein Dsn1 via ubiquitylation. Deletion of Mub1/Ubr2 restores levels and viability of a mutant Dsn1 protein, identifying a kinetochore quality control mechanism.","method":"Kinetochore particle purification, Co-IP, genetic deletion, protein level analysis in yeast","journal":"PLoS genetics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — biochemical purification plus genetic loss-of-function, single lab, two orthogonal methods","pmids":["23408894"],"is_preprint":false},{"year":2019,"finding":"UBR2 is required for NLRP1B inflammasome activation by anthrax lethal toxin (LT). LT cleaves NLRP1B after Lys44, generating a fragment with N-terminal leucine that is targeted by UBR2-mediated ubiquitination and degradation, partnering with E2 enzyme UBE2O. This degradation releases the noncovalently-bound C-terminal CARD domain for caspase-1 activation.","method":"Genome-wide siRNA screen, CRISPR-Cas9 KO screen, dual-fluorescence reporter system for ASC speck formation and pyroptosis, biochemical ubiquitination assays","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 2 / Strong — two independent genome-wide screens, biochemical validation, defined mechanism with E2 partner identification, multiple orthogonal approaches","pmids":["31268597"],"is_preprint":false},{"year":2020,"finding":"UBR2 protects cells from caspase-independent cell death (CICD) via the MAPK/Erk pathway. UBR2 downregulation sensitizes cells to CICD while overexpression is protective; this protection is mediated through Erk signaling.","method":"Genome-wide siRNA lethality screen, UBR2 KD/overexpression, pharmacological and genetic manipulation of MAPK/Erk pathway","journal":"Cell death & disease","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genome-wide screen plus gain/loss-of-function with pathway validation, single lab","pmids":["33288741"],"is_preprint":false},{"year":2023,"finding":"UBR1 and UBR2 are key sensors in the ER stress response in mammalian cells. Under normal conditions, UBR1 and UBR2 are polyubiquitinated via Lys48-linked chains and degraded by the 26S proteasome. Under ER stress, UBR1 and UBR2 are stabilized as an adaptive response. Cells lacking both UBR1 and UBR2 are hypersensitive to ER stress-induced apoptosis.","method":"UBR1/UBR2 double knockout cell analysis, ubiquitination linkage analysis (Lys48), protein stability assays under ER stress, apoptosis assays","journal":"Molecules and cells","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — KO cells with defined phenotype plus biochemical ubiquitination analysis, single lab","pmids":["38376480"],"is_preprint":false},{"year":2024,"finding":"DUSP22 phosphatase inhibits UBR2 by dephosphorylating it at specific serine residues, leading to ubiquitin-mediated UBR2 degradation (via the SCF E3 complex via Lys48-linked ubiquitination). UBR2 induces Lys63-linked ubiquitination of Lck at Lys99 and Lys276, followed by Lck Tyr394 phosphorylation and activation during TCR signaling. Knockout of UBR2 attenuates inflammatory phenotypes caused by DUSP22 deletion.","method":"Co-IP (UBR2-Lck interaction), site-directed mapping of ubiquitination (K99, K276) and phosphorylation sites (Ser on UBR2), CRISPR KO of UBR2 in mouse T cells, single-cell RNA sequencing, SLE patient peripheral blood T cell analysis","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal Co-IP, site-specific mutagenesis-level mapping, KO genetic epistasis, in vivo validation, human patient data, multiple orthogonal methods","pmids":["38225265"],"is_preprint":false},{"year":2017,"finding":"UBR2 promotes gastric cancer cell proliferation, migration, and stemness gene expression via the Wnt/β-catenin pathway. Knockdown of UBR2 in MFC cells decreased expression of β-catenin and its downstream targets (CD44, CyclinD1, CyclinD3, C-myc); additional depletion of UBR2 on top of β-catenin depletion showed no further effect on stemness genes, placing UBR2 upstream of β-catenin.","method":"shRNA knockdown of UBR2, Western blot for β-catenin pathway components, epistasis analysis (UBR2 KD + β-catenin depletion), exosome internalization assays","journal":"Stem cells (Dayton, Ohio)","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — loss-of-function with defined pathway readout and epistasis, single lab, indirect pathway placement","pmids":["28895255"],"is_preprint":false},{"year":2025,"finding":"Acetaldehyde treatment of C2C12 myotubes increases UBR2 expression and ubiquitination, and siRNA knockdown of UBR2 prevents acetaldehyde-induced myotube atrophy, establishing a direct role for UBR2 in alcohol-induced type II fast-twitch muscle fiber atrophy.","method":"siRNA knockdown of UBR2 in C2C12 myotubes, acetaldehyde treatment, myotube diameter measurement, ubiquitination assay","journal":"Alcohol, clinical & experimental research","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — loss-of-function with defined cellular phenotype rescue, single lab, single study","pmids":["40511880"],"is_preprint":false}],"current_model":"UBR2 is a RING-type E3 ubiquitin ligase and major recognition component (N-recognin) of the N-end/N-degron proteolytic pathway that targets proteins with destabilizing N-terminal residues for ubiquitination and proteasomal degradation; beyond canonical proteolysis, UBR2 acts as a scaffold E3 promoting HR6B/UbcH2-dependent monoubiquitination of histones H2A and H2B on meiotic and damaged chromatin to mediate transcriptional silencing and DSB repair, mediates NLRP1B inflammasome activation after anthrax lethal toxin cleavage via UBE2O-dependent ubiquitination, activates the Lck kinase during TCR signaling through K63-linked ubiquitination (itself regulated by DUSP22-dependent dephosphorylation and SCF-mediated K48-linked degradation), functions as a negative regulator of leucine-mTOR signaling, is upregulated in catabolic muscle via p38β–C/EBPβ signaling, and—together with UBR1—serves as an ER stress sensor and cytosolic protein quality control E3 ligase."},"narrative":{"mechanistic_narrative":"UBR2 is a RING-type E3 ubiquitin ligase and N-recognin of the N-end rule (N-degron) pathway that recognizes proteins bearing destabilizing N-terminal residues and, with substrate-binding properties closely paralleling UBR1, targets them for ubiquitin-dependent proteasomal degradation [PMID:14585983]; in yeast it acts through the cognate E2 Rad6 to degrade the proteasome-regulatory activator Rpn4 [PMID:15504724] and, together with Ubr1, clears misfolded cytosolic polypeptides in a Hsp70-stimulated quality-control pathway [PMID:20462952]. Beyond canonical proteolysis, UBR2 functions as a scaffold E3 that drives HR6B/UbcH2-dependent monoubiquitination of histones H2A and H2B—an activity allosterically stimulated by dipeptides with destabilizing N-terminal residues—to silence transcription on unsynapsed meiotic chromatin and to support double-strand break repair, with loss producing spermatocyte/pachytene arrest, defective homologous-recombination repair, and chromosomal instability [PMID:20080676, PMID:22616001, PMID:14585983, PMID:16488448]. UBR2 also stabilizes select binding partners rather than degrading them, as for the germ-cell protein Tex19.1 [PMID:21103378], and shapes diverse signaling outputs: it negatively regulates leucine–mTOR signaling as a leucine-binding protein whose substrate domain is leucine-inhibited [PMID:20298436], is required for NLRP1B inflammasome activation by ubiquitinating the lethal-toxin-cleaved NLRP1B N-terminal-leucine fragment via the E2 UBE2O [PMID:31268597], and activates Lck through K63-linked ubiquitination during TCR signaling under control of DUSP22-dependent dephosphorylation and SCF-mediated K48 degradation [PMID:38225265]. UBR2 and UBR1 jointly serve as ER-stress sensors that are stabilized upon stress to protect against apoptosis [PMID:38376480], and UBR2 is transcriptionally induced in catabolic muscle through p38β–C/EBPβ signaling driving fiber atrophy [PMID:23568773, PMID:40511880].","teleology":[{"year":2003,"claim":"Establishing whether UBR2 is a functional N-end rule E3 and what physiological process requires it answered the question of its core enzymatic identity and biological essentiality.","evidence":"Knockout mouse with spermatocyte arrest phenotype plus N-terminal substrate-binding assays","pmids":["14585983"],"confidence":"High","gaps":["Did not identify physiological substrates degraded during meiosis","Mechanism linking N-recognin activity to synaptonemal complex defects unresolved"]},{"year":2004,"claim":"Identification of Rpn4 as the first physiological Ubr2 substrate, degraded via the E2 Rad6, defined the molecular logic of substrate ubiquitination and the Ubr2–E2 partnership.","evidence":"In vitro and in vivo ubiquitination and stability assays with genetic deletion in yeast","pmids":["15504724"],"confidence":"High","gaps":["Mammalian counterpart substrates not addressed","Does not establish whether the same E2 pairing operates in metazoans"]},{"year":2004,"claim":"Finding that UBR1/UBR2 form a stable, non-degradative complex with RECQL4 raised the possibility that UBR2 can act as a chaperone/scaffold distinct from its proteolytic role.","evidence":"Reciprocal Co-IP from HeLa extracts plus DNA-stimulated ATPase assay","pmids":["15317757"],"confidence":"Medium","gaps":["Functional consequence of the complex for RECQL4 activity not defined","Whether binding stabilizes RECQL4 not directly tested"]},{"year":2006,"claim":"Double-knockout embryonic lethality clarified the partially divergent versus redundant roles of UBR1 and UBR2 in development.","evidence":"UBR1/UBR2 double-knockout mouse embryo histology and molecular marker analysis","pmids":["16606826"],"confidence":"High","gaps":["Direct substrates underlying neurogenesis/cardiovascular defects unknown","Cyclin and Notch1 changes are correlative"]},{"year":2006,"claim":"Demonstrating HR-specific repair defects in UBR2-deficient cells connected the ligase to genome maintenance beyond germ cells.","evidence":"Chromosome spreads, HR/NHEJ reporter assays, and mitomycin C sensitivity in UBR2-/- fibroblasts","pmids":["16488448"],"confidence":"High","gaps":["Molecular substrate at DSB sites not identified","Mechanistic link between ubiquitination and HR machinery unresolved"]},{"year":2010,"claim":"Showing UBR2 ubiquitinates H2A through HR6B on unsynapsed chromatin defined a non-proteolytic, chromatin-silencing function and explained the meiotic phenotype.","evidence":"Chromatin immunofluorescence, Co-IP, in vitro ubiquitination, and UBR2-/- spermatocyte analysis","pmids":["20080676"],"confidence":"High","gaps":["How UBR2 is recruited specifically to unsynapsed axial elements unknown"]},{"year":2010,"claim":"Reconstitution of misfolded-protein ubiquitination by Ubr1/Ubr2 with Hsp70 stimulation established a cytosolic protein quality-control role independent of N-terminal residue recognition.","evidence":"Purified in vitro ubiquitination of denatured luciferase plus genetic suppressor analysis in yeast","pmids":["20462952"],"confidence":"High","gaps":["Recognition determinants on denatured substrates not mapped","Mammalian conservation of this activity not shown here"]},{"year":2010,"claim":"Identifying Tex19.1 as a UBR2-stabilized partner showed that UBR2 binding can metabolically protect a substrate rather than destroy it.","evidence":"Co-IP from testes plus Western and RT-PCR in Ubr2-/- germ cells","pmids":["21103378"],"confidence":"Medium","gaps":["Molecular basis of stabilization versus degradation choice unknown","Single-lab observation without reciprocal mechanism"]},{"year":2010,"claim":"Discovery that UBR2 binds leucine and inhibits mTOR-S6K1 signaling linked N-recognin substrate recognition to amino acid sensing.","evidence":"Leucine-affinity pulldown, in vitro substrate degradation, and S6K1 phosphorylation readout under gain/loss-of-function","pmids":["20298436"],"confidence":"Medium","gaps":["Direct ubiquitination target in the mTOR pathway not identified","Whether leucine sensing is physiologically dominant unresolved"]},{"year":2012,"claim":"Defining UBR2 as a dipeptide-allosterically-activated scaffold E3 for H2A/H2B monoubiquitination unified its meiotic silencing and DSB-repair roles with N-degron sensing.","evidence":"In vitro histone ubiquitylation, dipeptide activation assay, and germ/somatic UBR2-/- phenotyping","pmids":["22616001"],"confidence":"High","gaps":["Endogenous allosteric ligand identity in vivo unknown","Structural basis of allosteric activation not solved"]},{"year":2013,"claim":"Mapping the Mub1/Ubr2 complex to kinetochores via CENP-C and its regulation of Dsn1 levels revealed a kinetochore protein quality-control function in yeast.","evidence":"Kinetochore particle purification, Co-IP, and genetic deletion in budding yeast","pmids":["23408894"],"confidence":"Medium","gaps":["Mammalian conservation of kinetochore role untested","Whether Dsn1 ubiquitination is direct not fully established"]},{"year":2013,"claim":"Identifying p38β–C/EBPβ control of the UBR2 promoter explained how catabolic stimuli transcriptionally upregulate UBR2 in muscle.","evidence":"Pharmacologic/genetic p38β manipulation, luciferase reporter, and tumor-bearing mouse models","pmids":["23568773"],"confidence":"High","gaps":["Downstream UBR2 substrates driving muscle wasting not defined"]},{"year":2019,"claim":"Showing UBR2 ubiquitinates the lethal-toxin-cleaved NLRP1B N-terminal-leucine fragment with UBE2O connected N-degron recognition to inflammasome activation.","evidence":"Genome-wide siRNA and CRISPR screens plus biochemical ubiquitination and pyroptosis reporters","pmids":["31268597"],"confidence":"High","gaps":["Why UBE2O rather than the canonical meiotic E2 is used here not explained"]},{"year":2020,"claim":"Linking UBR2 to protection from caspase-independent cell death via Erk extended its role to cell-survival signaling.","evidence":"Genome-wide siRNA lethality screen with UBR2 KD/overexpression and MAPK/Erk pathway manipulation","pmids":["33288741"],"confidence":"Medium","gaps":["Direct ubiquitination substrate in the Erk axis not identified","Mechanism connecting E3 activity to Erk unresolved"]},{"year":2017,"claim":"Placing UBR2 upstream of β-catenin implicated it in Wnt-driven gastric cancer proliferation and stemness.","evidence":"shRNA knockdown with β-catenin pathway readouts and epistasis in gastric cancer cells","pmids":["28895255"],"confidence":"Medium","gaps":["Direct molecular link between UBR2 and β-catenin not defined","Pathway placement is indirect"]},{"year":2024,"claim":"Defining the DUSP22–UBR2–Lck axis showed UBR2 activates TCR signaling through K63-linked Lck ubiquitination and is itself regulated by dephosphorylation and SCF-mediated degradation.","evidence":"Reciprocal Co-IP, site-specific ubiquitination/phosphorylation mapping, T-cell CRISPR KO, scRNA-seq, and SLE patient samples","pmids":["38225265"],"confidence":"High","gaps":["Structural basis of K63 chain assembly on Lck unresolved","Whether this axis operates outside T cells unknown"]},{"year":2023,"claim":"Establishing UBR1/UBR2 as ER-stress sensors that are stabilized to limit apoptosis defined a self-regulatory layer of the ligases under proteostatic stress.","evidence":"UBR1/UBR2 double-knockout cell analysis with Lys48 ubiquitination and apoptosis assays under ER stress","pmids":["38376480"],"confidence":"Medium","gaps":["Sensing mechanism that triggers stabilization not defined","Protective substrates downstream of UBR1/UBR2 not identified"]},{"year":2025,"claim":"Showing UBR2 knockdown prevents acetaldehyde-induced myotube atrophy directly implicated it in alcohol-related muscle fiber loss.","evidence":"siRNA knockdown in C2C12 myotubes with acetaldehyde treatment and atrophy/ubiquitination readouts","pmids":["40511880"],"confidence":"Medium","gaps":["Atrophy-associated substrates of UBR2 not identified","Single-study cellular model only"]},{"year":null,"claim":"The endogenous substrates UBR2 ubiquitinates in most of its mammalian signaling roles (mTOR, Erk survival, Wnt/β-catenin, ER stress, muscle atrophy) remain undefined, as does the structural basis for its switch between proteolytic, scaffold, and substrate-stabilizing modes.","evidence":"","pmids":[],"confidence":"Low","gaps":["No structural model of N-recognin allosteric switching","Direct substrates in survival/cancer/atrophy pathways unidentified","Recruitment determinants to chromatin, kinetochores, and inflammasomes not unified"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0016740","term_label":"transferase activity","supporting_discovery_ids":[0,2,5,9,12,15]},{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[2,5,9,12,15]},{"term_id":"GO:0016874","term_label":"ligase activity","supporting_discovery_ids":[0,2,9]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[7,8]},{"term_id":"GO:0042393","term_label":"histone binding","supporting_discovery_ids":[5,9]}],"localization":[{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[6,8]},{"term_id":"GO:0000228","term_label":"nuclear chromosome","supporting_discovery_ids":[5,9]},{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[5]}],"pathway":[{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[0,2,6,14]},{"term_id":"R-HSA-73894","term_label":"DNA Repair","supporting_discovery_ids":[4,9]},{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[12,15]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[8,13,15,16]},{"term_id":"R-HSA-4839726","term_label":"Chromatin organization","supporting_discovery_ids":[5,9]},{"term_id":"R-HSA-1474165","term_label":"Reproduction","supporting_discovery_ids":[0,5,7]}],"complexes":["Mub1/Ubr2 kinetochore-associated complex","UBR1/UBR2 N-recognin complex"],"partners":["UBR1","RECQL4","HR6B/UBCH2","TEX19.1","UBE2O","NLRP1B","LCK","DUSP22"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q8IWV8","full_name":"E3 ubiquitin-protein ligase UBR2","aliases":["N-recognin-2","Ubiquitin-protein ligase E3-alpha-2","Ubiquitin-protein ligase E3-alpha-II"],"length_aa":1755,"mass_kda":200.5,"function":"E3 ubiquitin-protein ligase which is a component of the N-end rule pathway (PubMed:15548684, PubMed:20835242, PubMed:28392261). Recognizes and binds to proteins bearing specific N-terminal residues (N-degrons) that are destabilizing according to the N-end rule, leading to their ubiquitination and subsequent degradation (PubMed:20835242, PubMed:28392261). 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:20835242, PubMed:28392261). Does not ubiquitinate proteins that are acetylated at the N-terminus (PubMed:20835242). In contrast, it strongly binds methylated N-degrons (PubMed:28392261). Plays a critical role in chromatin inactivation and chromosome-wide transcriptional silencing during meiosis via ubiquitination of histone H2A (By similarity). Binds leucine and is a negative regulator of the leucine-mTOR signaling pathway, thereby controlling cell growth (PubMed:20298436). Required for spermatogenesis, promotes, with Tex19.1, SPO11-dependent recombination foci to accumulate and drive robust homologous chromosome synapsis (By similarity). Polyubiquitinates LINE-1 retrotransposon encoded, LIRE1, which induces degradation, inhibiting LINE-1 retrotransposon mobilization (By similarity). Catalyzes ubiquitination and degradation of the N-terminal part of NLRP1 following NLRP1 activation by pathogens and other damage-associated signals: ubiquitination promotes degradation of the N-terminal part and subsequent release of the cleaved C-terminal part of NLRP1, which polymerizes and forms the NLRP1 inflammasome followed by host cell pyroptosis (By similarity). Plays a role in T-cell receptor signaling by inducing 'Lys-63'-linked ubiquitination of lymphocyte cell-specific kinase LCK (PubMed:38225265). This activity is regulated by DUSP22, which induces 'Lys-48'-linked ubiquitination of UBR2, leading to its proteasomal degradation by SCF E3 ubiquitin-protein ligase complex (PubMed:38225265)","subcellular_location":"Nucleus; Chromosome","url":"https://www.uniprot.org/uniprotkb/Q8IWV8/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/UBR2","classification":"Not Classified","n_dependent_lines":37,"n_total_lines":1208,"dependency_fraction":0.030629139072847682},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/UBR2","total_profiled":1310},"omim":[{"mim_id":"620846","title":"N-TERMINAL GLUTAMINE AMIDASE 1; NTAQ1","url":"https://www.omim.org/entry/620846"},{"mim_id":"620743","title":"SDE2 TELOMERE MAINTENANCE HOMOLOG; SDE2","url":"https://www.omim.org/entry/620743"},{"mim_id":"615367","title":"N-TERMINAL ASPARAGINE AMIDASE; NTAN1","url":"https://www.omim.org/entry/615367"},{"mim_id":"613831","title":"UBIQUITIN PROTEIN LIGASE E3 COMPONENT N-RECOGNIN 3; UBR3","url":"https://www.omim.org/entry/613831"},{"mim_id":"609134","title":"UBIQUITIN-PROTEIN LIGASE E3 COMPONENT N-RECOGNIN 2; UBR2","url":"https://www.omim.org/entry/609134"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Nucleoplasm","reliability":"Approved"},{"location":"Plasma membrane","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in 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UBR2-deficient male mice show spermatocyte arrest between leptotene/zygotene and pachytene with absence of intact synaptonemal complexes and subsequent apoptosis, establishing a required role in male meiosis.\",\n      \"method\": \"Knockout mouse construction, substrate-binding assays, histological analysis of testes\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic KO with defined cellular phenotype, substrate-binding assays, replicated across strain backgrounds in a single rigorous study\",\n      \"pmids\": [\"14585983\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"RECQL4 (mutated in Rothmund-Thomson syndrome) forms a stable complex with UBR1 and UBR2 isolated from HeLa cells. The UBR1/2-bound RECQL4 was not ubiquitylated in vivo and was a long-lived protein; the isolated complex had DNA-stimulated ATPase activity.\",\n      \"method\": \"Co-immunoprecipitation with anti-RECQL4 antibodies from HeLa cell extracts, in vivo ubiquitylation assay, ATPase activity assay\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal Co-IP from cells plus functional ATPase assay, single lab, two orthogonal methods\",\n      \"pmids\": [\"15317757\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"Yeast Ubr2 ubiquitin ligase mediates ubiquitin-dependent degradation of Rpn4 (a transcriptional activator of proteasome genes) via the ubiquitin-conjugating enzyme Rad6, which directly interacts with Ubr2. Rpn4 was validated as the first physiological substrate of Ubr2 through in vivo and in vitro ubiquitination assays.\",\n      \"method\": \"In vivo and in vitro ubiquitination assays, genetic deletion, protein stability assays, synthetic growth defect analysis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro reconstitution of ubiquitination plus in vivo genetic validation in a single focused study\",\n      \"pmids\": [\"15504724\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"UBR1 and UBR2 double-knockout mouse embryos die at midgestation with defects in neurogenesis and cardiovascular development, including reduced proliferation and precocious migration/differentiation of neural progenitor cells; altered expression of D-type cyclins and Notch1 was observed, indicating partially divergent functions between UBR1 and UBR2.\",\n      \"method\": \"Double-knockout mouse construction, histological and molecular analysis of embryos\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic epistasis via double KO with defined developmental phenotype and molecular markers, rigorous in vivo study\",\n      \"pmids\": [\"16606826\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"UBR2-deficient mouse fibroblasts display chromosome fragility, chromosomal bridges, micronuclei, spontaneous chromosomal gaps, hypersensitivity to mitomycin C, and are significantly impaired in homologous recombination repair of double-strand breaks (but show normal non-homologous end joining), establishing a role for UBR2 in maintaining genome integrity via HR repair.\",\n      \"method\": \"Metaphase chromosome spreads, reporter assay for HR and NHEJ, mitomycin C sensitivity assay, UBR2-/- fibroblast cell lines\",\n      \"journal\": \"Mutation research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — loss-of-function with multiple defined cellular phenotypes and pathway-specific repair assays, single lab\",\n      \"pmids\": [\"16488448\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"UBR2 localizes to meiotic chromatin regions including unsynapsed axial elements and mediates transcriptional silencing by ubiquitinating histone H2A. UBR2 interacts with ubiquitin-conjugating enzyme HR6B, promotes HR6B–H2A interaction and ubiquitin transfer to H2A. UBR2-deficient spermatocytes fail to ubiquitinate H2A and fail to silence genes on unsynapsed X and Y chromosomes.\",\n      \"method\": \"Chromatin immunofluorescence, Co-immunoprecipitation (UBR2–HR6B, UBR2–H2A), in vitro ubiquitination assay, analysis of UBR2-/- spermatocytes\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — Co-IP, in vitro ubiquitination, localization with functional consequence, KO phenotype, multiple orthogonal methods\",\n      \"pmids\": [\"20080676\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Saccharomyces cerevisiae Ubr1 and Ubr2 ubiquitin ligases promote degradation of unfolded/misfolded cytosolic polypeptides as part of a cytosolic protein quality control pathway. Ubr1 directly interacts with denatured substrates (not native protein), and Hsp70 stimulates polyubiquitination. Loss of Ubr1/Ubr2 suppresses growth arrest from chaperone mutation.\",\n      \"method\": \"Purified in vitro ubiquitination assay with denatured luciferase, genetic suppressor analysis (double mutant), yeast genetics\",\n      \"journal\": \"Molecular biology of the cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro reconstitution with purified components plus genetic epistasis, single lab with orthogonal methods\",\n      \"pmids\": [\"20462952\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Ubr2 forms a stable protein complex with Tex19.1 (a germ cell-specific protein) in mouse testes. In Ubr2-deficient germ cells, Tex19.1 protein is absent despite normal Tex19.1 transcription, indicating that Ubr2 binding metabolically stabilizes Tex19.1, a function distinct from the conventional N-end rule proteolytic role.\",\n      \"method\": \"Co-immunoprecipitation from mouse testes, Western blot in Ubr2-/- germ cells, RT-PCR\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Co-IP plus protein stability analysis in KO, two complementary methods, single lab\",\n      \"pmids\": [\"21103378\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"UBR1 and UBR2 are leucine-binding proteins (identified via leucine-immobilized affinity beads). Leucine binds to the substrate-recognition domain of UBR2 and inhibits degradation of N-end rule substrates in vitro. Overexpression of UBR1/UBR2 reduces mTOR-dependent S6K1 phosphorylation, while knockdown increases S6K1 phosphorylation in amino acid-starved cells, identifying UBR1/UBR2 as negative regulators of the leucine-mTOR signaling pathway.\",\n      \"method\": \"Leucine-affinity pulldown, in vitro N-end rule substrate degradation assay, overexpression and siRNA knockdown with S6K1 phosphorylation readout\",\n      \"journal\": \"Genes to cells : devoted to molecular & cellular mechanisms\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — affinity pulldown plus in vitro assay plus gain/loss-of-function signaling readout, single lab, multiple methods\",\n      \"pmids\": [\"20298436\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"UBR2 functions as a scaffold E3 that promotes HR6B/UbcH2-dependent monoubiquitylation of H2A and H2B (but not H3 or H4) through a mechanism distinct from typical polyubiquitylation. The E3 activity of UBR2 in histone ubiquitylation is allosterically activated by dipeptides bearing destabilizing N-terminal residues. UBR2-deficient spermatocytes show defects in DSB repair and undergo pachytene arrest. UBR2-deficient somatic cells show chromosomal instability and hypersensitivity to DNA damage.\",\n      \"method\": \"In vitro ubiquitylation assay with histone substrates, dipeptide allosteric activation assay, analysis of UBR2-/- cells (meiotic and somatic), chromosome spread analysis\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — in vitro reconstitution, allosteric activation assay, multiple KO phenotypes in both germ and somatic cells, multiple orthogonal methods\",\n      \"pmids\": [\"22616001\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Tumor cell-induced upregulation of UBR2 in muscle is mediated by p38β MAPK-dependent phosphorylation of C/EBPβ at Thr-188, which enables C/EBPβ binding to a functional responsive element in the UBR2 promoter. Genetic gain/loss-of-function confirmed p38β is sufficient and necessary for UBR2 upregulation; genetic ablation of C/EBPβ blocked UBR2 upregulation in tumor-bearing mice.\",\n      \"method\": \"Pharmacological inhibitor (SB202190), genetic gain/loss-of-function in C2C12 myotubes, luciferase reporter assay, ChIP-implied C/EBPβ binding, in vivo tumor-bearing mouse model\",\n      \"journal\": \"FASEB journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple genetic and pharmacological interventions, reporter assay, in vitro and in vivo validation, multiple orthogonal methods\",\n      \"pmids\": [\"23568773\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Budding yeast Mub1/Ubr2 ubiquitin ligase complex associates with kinetochore particles through CENP-C (Mif2) and regulates the levels of the outer kinetochore protein Dsn1 via ubiquitylation. Deletion of Mub1/Ubr2 restores levels and viability of a mutant Dsn1 protein, identifying a kinetochore quality control mechanism.\",\n      \"method\": \"Kinetochore particle purification, Co-IP, genetic deletion, protein level analysis in yeast\",\n      \"journal\": \"PLoS genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — biochemical purification plus genetic loss-of-function, single lab, two orthogonal methods\",\n      \"pmids\": [\"23408894\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"UBR2 is required for NLRP1B inflammasome activation by anthrax lethal toxin (LT). LT cleaves NLRP1B after Lys44, generating a fragment with N-terminal leucine that is targeted by UBR2-mediated ubiquitination and degradation, partnering with E2 enzyme UBE2O. This degradation releases the noncovalently-bound C-terminal CARD domain for caspase-1 activation.\",\n      \"method\": \"Genome-wide siRNA screen, CRISPR-Cas9 KO screen, dual-fluorescence reporter system for ASC speck formation and pyroptosis, biochemical ubiquitination assays\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — two independent genome-wide screens, biochemical validation, defined mechanism with E2 partner identification, multiple orthogonal approaches\",\n      \"pmids\": [\"31268597\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"UBR2 protects cells from caspase-independent cell death (CICD) via the MAPK/Erk pathway. UBR2 downregulation sensitizes cells to CICD while overexpression is protective; this protection is mediated through Erk signaling.\",\n      \"method\": \"Genome-wide siRNA lethality screen, UBR2 KD/overexpression, pharmacological and genetic manipulation of MAPK/Erk pathway\",\n      \"journal\": \"Cell death & disease\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genome-wide screen plus gain/loss-of-function with pathway validation, single lab\",\n      \"pmids\": [\"33288741\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"UBR1 and UBR2 are key sensors in the ER stress response in mammalian cells. Under normal conditions, UBR1 and UBR2 are polyubiquitinated via Lys48-linked chains and degraded by the 26S proteasome. Under ER stress, UBR1 and UBR2 are stabilized as an adaptive response. Cells lacking both UBR1 and UBR2 are hypersensitive to ER stress-induced apoptosis.\",\n      \"method\": \"UBR1/UBR2 double knockout cell analysis, ubiquitination linkage analysis (Lys48), protein stability assays under ER stress, apoptosis assays\",\n      \"journal\": \"Molecules and cells\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — KO cells with defined phenotype plus biochemical ubiquitination analysis, single lab\",\n      \"pmids\": [\"38376480\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"DUSP22 phosphatase inhibits UBR2 by dephosphorylating it at specific serine residues, leading to ubiquitin-mediated UBR2 degradation (via the SCF E3 complex via Lys48-linked ubiquitination). UBR2 induces Lys63-linked ubiquitination of Lck at Lys99 and Lys276, followed by Lck Tyr394 phosphorylation and activation during TCR signaling. Knockout of UBR2 attenuates inflammatory phenotypes caused by DUSP22 deletion.\",\n      \"method\": \"Co-IP (UBR2-Lck interaction), site-directed mapping of ubiquitination (K99, K276) and phosphorylation sites (Ser on UBR2), CRISPR KO of UBR2 in mouse T cells, single-cell RNA sequencing, SLE patient peripheral blood T cell analysis\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal Co-IP, site-specific mutagenesis-level mapping, KO genetic epistasis, in vivo validation, human patient data, multiple orthogonal methods\",\n      \"pmids\": [\"38225265\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"UBR2 promotes gastric cancer cell proliferation, migration, and stemness gene expression via the Wnt/β-catenin pathway. Knockdown of UBR2 in MFC cells decreased expression of β-catenin and its downstream targets (CD44, CyclinD1, CyclinD3, C-myc); additional depletion of UBR2 on top of β-catenin depletion showed no further effect on stemness genes, placing UBR2 upstream of β-catenin.\",\n      \"method\": \"shRNA knockdown of UBR2, Western blot for β-catenin pathway components, epistasis analysis (UBR2 KD + β-catenin depletion), exosome internalization assays\",\n      \"journal\": \"Stem cells (Dayton, Ohio)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — loss-of-function with defined pathway readout and epistasis, single lab, indirect pathway placement\",\n      \"pmids\": [\"28895255\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Acetaldehyde treatment of C2C12 myotubes increases UBR2 expression and ubiquitination, and siRNA knockdown of UBR2 prevents acetaldehyde-induced myotube atrophy, establishing a direct role for UBR2 in alcohol-induced type II fast-twitch muscle fiber atrophy.\",\n      \"method\": \"siRNA knockdown of UBR2 in C2C12 myotubes, acetaldehyde treatment, myotube diameter measurement, ubiquitination assay\",\n      \"journal\": \"Alcohol, clinical & experimental research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — loss-of-function with defined cellular phenotype rescue, single lab, single study\",\n      \"pmids\": [\"40511880\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"UBR2 is a RING-type E3 ubiquitin ligase and major recognition component (N-recognin) of the N-end/N-degron proteolytic pathway that targets proteins with destabilizing N-terminal residues for ubiquitination and proteasomal degradation; beyond canonical proteolysis, UBR2 acts as a scaffold E3 promoting HR6B/UbcH2-dependent monoubiquitination of histones H2A and H2B on meiotic and damaged chromatin to mediate transcriptional silencing and DSB repair, mediates NLRP1B inflammasome activation after anthrax lethal toxin cleavage via UBE2O-dependent ubiquitination, activates the Lck kinase during TCR signaling through K63-linked ubiquitination (itself regulated by DUSP22-dependent dephosphorylation and SCF-mediated K48-linked degradation), functions as a negative regulator of leucine-mTOR signaling, is upregulated in catabolic muscle via p38β–C/EBPβ signaling, and—together with UBR1—serves as an ER stress sensor and cytosolic protein quality control E3 ligase.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"UBR2 is a RING-type E3 ubiquitin ligase and N-recognin of the N-end rule (N-degron) pathway that recognizes proteins bearing destabilizing N-terminal residues and, with substrate-binding properties closely paralleling UBR1, targets them for ubiquitin-dependent proteasomal degradation [#0]; in yeast it acts through the cognate E2 Rad6 to degrade the proteasome-regulatory activator Rpn4 [#2] and, together with Ubr1, clears misfolded cytosolic polypeptides in a Hsp70-stimulated quality-control pathway [#6]. Beyond canonical proteolysis, UBR2 functions as a scaffold E3 that drives HR6B/UbcH2-dependent monoubiquitination of histones H2A and H2B—an activity allosterically stimulated by dipeptides with destabilizing N-terminal residues—to silence transcription on unsynapsed meiotic chromatin and to support double-strand break repair, with loss producing spermatocyte/pachytene arrest, defective homologous-recombination repair, and chromosomal instability [#5, #9, #0, #4]. UBR2 also stabilizes select binding partners rather than degrading them, as for the germ-cell protein Tex19.1 [#7], and shapes diverse signaling outputs: it negatively regulates leucine–mTOR signaling as a leucine-binding protein whose substrate domain is leucine-inhibited [#8], is required for NLRP1B inflammasome activation by ubiquitinating the lethal-toxin-cleaved NLRP1B N-terminal-leucine fragment via the E2 UBE2O [#12], and activates Lck through K63-linked ubiquitination during TCR signaling under control of DUSP22-dependent dephosphorylation and SCF-mediated K48 degradation [#15]. UBR2 and UBR1 jointly serve as ER-stress sensors that are stabilized upon stress to protect against apoptosis [#14], and UBR2 is transcriptionally induced in catabolic muscle through p38\\u03b2–C/EBP\\u03b2 signaling driving fiber atrophy [#10, #17].\",\n  \"teleology\": [\n    {\n      \"year\": 2003,\n      \"claim\": \"Establishing whether UBR2 is a functional N-end rule E3 and what physiological process requires it answered the question of its core enzymatic identity and biological essentiality.\",\n      \"evidence\": \"Knockout mouse with spermatocyte arrest phenotype plus N-terminal substrate-binding assays\",\n      \"pmids\": [\"14585983\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not identify physiological substrates degraded during meiosis\", \"Mechanism linking N-recognin activity to synaptonemal complex defects unresolved\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Identification of Rpn4 as the first physiological Ubr2 substrate, degraded via the E2 Rad6, defined the molecular logic of substrate ubiquitination and the Ubr2–E2 partnership.\",\n      \"evidence\": \"In vitro and in vivo ubiquitination and stability assays with genetic deletion in yeast\",\n      \"pmids\": [\"15504724\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mammalian counterpart substrates not addressed\", \"Does not establish whether the same E2 pairing operates in metazoans\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Finding that UBR1/UBR2 form a stable, non-degradative complex with RECQL4 raised the possibility that UBR2 can act as a chaperone/scaffold distinct from its proteolytic role.\",\n      \"evidence\": \"Reciprocal Co-IP from HeLa extracts plus DNA-stimulated ATPase assay\",\n      \"pmids\": [\"15317757\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Functional consequence of the complex for RECQL4 activity not defined\", \"Whether binding stabilizes RECQL4 not directly tested\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Double-knockout embryonic lethality clarified the partially divergent versus redundant roles of UBR1 and UBR2 in development.\",\n      \"evidence\": \"UBR1/UBR2 double-knockout mouse embryo histology and molecular marker analysis\",\n      \"pmids\": [\"16606826\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct substrates underlying neurogenesis/cardiovascular defects unknown\", \"Cyclin and Notch1 changes are correlative\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Demonstrating HR-specific repair defects in UBR2-deficient cells connected the ligase to genome maintenance beyond germ cells.\",\n      \"evidence\": \"Chromosome spreads, HR/NHEJ reporter assays, and mitomycin C sensitivity in UBR2-/- fibroblasts\",\n      \"pmids\": [\"16488448\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular substrate at DSB sites not identified\", \"Mechanistic link between ubiquitination and HR machinery unresolved\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Showing UBR2 ubiquitinates H2A through HR6B on unsynapsed chromatin defined a non-proteolytic, chromatin-silencing function and explained the meiotic phenotype.\",\n      \"evidence\": \"Chromatin immunofluorescence, Co-IP, in vitro ubiquitination, and UBR2-/- spermatocyte analysis\",\n      \"pmids\": [\"20080676\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How UBR2 is recruited specifically to unsynapsed axial elements unknown\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Reconstitution of misfolded-protein ubiquitination by Ubr1/Ubr2 with Hsp70 stimulation established a cytosolic protein quality-control role independent of N-terminal residue recognition.\",\n      \"evidence\": \"Purified in vitro ubiquitination of denatured luciferase plus genetic suppressor analysis in yeast\",\n      \"pmids\": [\"20462952\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Recognition determinants on denatured substrates not mapped\", \"Mammalian conservation of this activity not shown here\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Identifying Tex19.1 as a UBR2-stabilized partner showed that UBR2 binding can metabolically protect a substrate rather than destroy it.\",\n      \"evidence\": \"Co-IP from testes plus Western and RT-PCR in Ubr2-/- germ cells\",\n      \"pmids\": [\"21103378\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Molecular basis of stabilization versus degradation choice unknown\", \"Single-lab observation without reciprocal mechanism\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Discovery that UBR2 binds leucine and inhibits mTOR-S6K1 signaling linked N-recognin substrate recognition to amino acid sensing.\",\n      \"evidence\": \"Leucine-affinity pulldown, in vitro substrate degradation, and S6K1 phosphorylation readout under gain/loss-of-function\",\n      \"pmids\": [\"20298436\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct ubiquitination target in the mTOR pathway not identified\", \"Whether leucine sensing is physiologically dominant unresolved\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Defining UBR2 as a dipeptide-allosterically-activated scaffold E3 for H2A/H2B monoubiquitination unified its meiotic silencing and DSB-repair roles with N-degron sensing.\",\n      \"evidence\": \"In vitro histone ubiquitylation, dipeptide activation assay, and germ/somatic UBR2-/- phenotyping\",\n      \"pmids\": [\"22616001\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Endogenous allosteric ligand identity in vivo unknown\", \"Structural basis of allosteric activation not solved\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Mapping the Mub1/Ubr2 complex to kinetochores via CENP-C and its regulation of Dsn1 levels revealed a kinetochore protein quality-control function in yeast.\",\n      \"evidence\": \"Kinetochore particle purification, Co-IP, and genetic deletion in budding yeast\",\n      \"pmids\": [\"23408894\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mammalian conservation of kinetochore role untested\", \"Whether Dsn1 ubiquitination is direct not fully established\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Identifying p38\\u03b2–C/EBP\\u03b2 control of the UBR2 promoter explained how catabolic stimuli transcriptionally upregulate UBR2 in muscle.\",\n      \"evidence\": \"Pharmacologic/genetic p38\\u03b2 manipulation, luciferase reporter, and tumor-bearing mouse models\",\n      \"pmids\": [\"23568773\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Downstream UBR2 substrates driving muscle wasting not defined\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Showing UBR2 ubiquitinates the lethal-toxin-cleaved NLRP1B N-terminal-leucine fragment with UBE2O connected N-degron recognition to inflammasome activation.\",\n      \"evidence\": \"Genome-wide siRNA and CRISPR screens plus biochemical ubiquitination and pyroptosis reporters\",\n      \"pmids\": [\"31268597\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Why UBE2O rather than the canonical meiotic E2 is used here not explained\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Linking UBR2 to protection from caspase-independent cell death via Erk extended its role to cell-survival signaling.\",\n      \"evidence\": \"Genome-wide siRNA lethality screen with UBR2 KD/overexpression and MAPK/Erk pathway manipulation\",\n      \"pmids\": [\"33288741\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct ubiquitination substrate in the Erk axis not identified\", \"Mechanism connecting E3 activity to Erk unresolved\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Placing UBR2 upstream of \\u03b2-catenin implicated it in Wnt-driven gastric cancer proliferation and stemness.\",\n      \"evidence\": \"shRNA knockdown with \\u03b2-catenin pathway readouts and epistasis in gastric cancer cells\",\n      \"pmids\": [\"28895255\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct molecular link between UBR2 and \\u03b2-catenin not defined\", \"Pathway placement is indirect\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Defining the DUSP22–UBR2–Lck axis showed UBR2 activates TCR signaling through K63-linked Lck ubiquitination and is itself regulated by dephosphorylation and SCF-mediated degradation.\",\n      \"evidence\": \"Reciprocal Co-IP, site-specific ubiquitination/phosphorylation mapping, T-cell CRISPR KO, scRNA-seq, and SLE patient samples\",\n      \"pmids\": [\"38225265\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of K63 chain assembly on Lck unresolved\", \"Whether this axis operates outside T cells unknown\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Establishing UBR1/UBR2 as ER-stress sensors that are stabilized to limit apoptosis defined a self-regulatory layer of the ligases under proteostatic stress.\",\n      \"evidence\": \"UBR1/UBR2 double-knockout cell analysis with Lys48 ubiquitination and apoptosis assays under ER stress\",\n      \"pmids\": [\"38376480\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Sensing mechanism that triggers stabilization not defined\", \"Protective substrates downstream of UBR1/UBR2 not identified\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Showing UBR2 knockdown prevents acetaldehyde-induced myotube atrophy directly implicated it in alcohol-related muscle fiber loss.\",\n      \"evidence\": \"siRNA knockdown in C2C12 myotubes with acetaldehyde treatment and atrophy/ubiquitination readouts\",\n      \"pmids\": [\"40511880\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Atrophy-associated substrates of UBR2 not identified\", \"Single-study cellular model only\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The endogenous substrates UBR2 ubiquitinates in most of its mammalian signaling roles (mTOR, Erk survival, Wnt/\\u03b2-catenin, ER stress, muscle atrophy) remain undefined, as does the structural basis for its switch between proteolytic, scaffold, and substrate-stabilizing modes.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No structural model of N-recognin allosteric switching\", \"Direct substrates in survival/cancer/atrophy pathways unidentified\", \"Recruitment determinants to chromatin, kinetochores, and inflammasomes not unified\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0016740\", \"supporting_discovery_ids\": [0, 2, 5, 9, 12, 15]},\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [2, 5, 9, 12, 15]},\n      {\"term_id\": \"GO:0016874\", \"supporting_discovery_ids\": [0, 2, 9]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [7, 8]},\n      {\"term_id\": \"GO:0042393\", \"supporting_discovery_ids\": [5, 9]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [6, 8]},\n      {\"term_id\": \"GO:0000228\", \"supporting_discovery_ids\": [5, 9]},\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [5]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [0, 2, 6, 14]},\n      {\"term_id\": \"R-HSA-73894\", \"supporting_discovery_ids\": [4, 9]},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [12, 15]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [8, 13, 15, 16]},\n      {\"term_id\": \"R-HSA-4839726\", \"supporting_discovery_ids\": [5, 9]},\n      {\"term_id\": \"R-HSA-1474165\", \"supporting_discovery_ids\": [0, 5, 7]}\n    ],\n    \"complexes\": [\n      \"Mub1/Ubr2 kinetochore-associated complex\",\n      \"UBR1/UBR2 N-recognin complex\"\n    ],\n    \"partners\": [\n      \"UBR1\",\n      \"RECQL4\",\n      \"HR6B/UbcH2\",\n      \"Tex19.1\",\n      \"UBE2O\",\n      \"NLRP1B\",\n      \"Lck\",\n      \"DUSP22\"\n    ],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":4,"faith_total":4,"faith_pct":100.0}}