{"gene":"BUB1B","run_date":"2026-04-28T17:12:38","timeline":{"discoveries":[{"year":1999,"finding":"hBUBR1 is an essential mitotic checkpoint kinase that localizes to kinetochores during mitosis, monitors CENP-E-dependent kinetochore activities, and associates with the cyclosome/APC in mitotically arrested cells but not in interphase cells. Its kinase activity is stimulated upon spindle disruption.","method":"Immunoprecipitation, kinase assays, dominant-negative expression, cell synchronization","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods (Co-IP, kinase assay, dominant-negative, localization) with functional phenotype","pmids":["10477750"],"is_preprint":false},{"year":2001,"finding":"BubR1-Bub3 complex purified from mitotic human cells inhibits APC/C(Cdc20) in a Mad2-independent manner by blocking Cdc20 binding to APC; this inhibition does not require BubR1 kinase activity. BubR1 also interacts with the mitotic motor CENP-E.","method":"Biochemical purification, in vitro APC/C activity assay, immunoprecipitation, Xenopus egg extract reconstitution","journal":"Developmental cell","confidence":"High","confidence_rationale":"Tier 1 — in vitro reconstitution with purified components, multiple orthogonal assays","pmids":["11702782"],"is_preprint":false},{"year":2002,"finding":"BubR1 directly binds Cdc20 and inhibits APC/C activation independently of its kinase activity; BubR1 is 12-fold more potent than Mad2 as an APC/C inhibitor, and BubR1 and Mad2 synergistically inhibit Cdc20-APC at physiological concentrations by mutually promoting each other's binding to Cdc20.","method":"In vitro APC/C ubiquitylation assay, recombinant protein binding, quantitative analysis","journal":"Molecular biology of the cell","confidence":"High","confidence_rationale":"Tier 1 — in vitro reconstitution with quantitative analysis, replicated findings","pmids":["11907259"],"is_preprint":false},{"year":2003,"finding":"CENP-E directly activates BubR1 kinase activity; microtubule capture by the CENP-E motor domain silences BubR1 kinase activity in a ternary BubR1-CENP-E-microtubule complex, establishing CENP-E as the signal transducer that links kinetochore microtubule capture to checkpoint silencing.","method":"Purified component reconstitution with CENP-E and BubR1, Xenopus egg extracts, kinase assays","journal":"Cell","confidence":"High","confidence_rationale":"Tier 1 — in vitro reconstitution with purified components, multiple assay systems","pmids":["12859900"],"is_preprint":false},{"year":2003,"finding":"Aurora B kinase activity is required for kinetochore localization of BubR1 (and Mad2 and CENP-E), for phosphorylation of BubR1 on mitotic entry, and for rebinding of BubR1 to metaphase kinetochores after reduction in centromeric tension. BubR1 is also required for chromosome alignment, not only spindle checkpoint function.","method":"Aurora kinase inhibitor (ZM447439), RNA interference, immunofluorescence, Western blot","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 2 — chemical inhibition plus RNAi with multiple cellular phenotype readouts","pmids":["12719470"],"is_preprint":false},{"year":2004,"finding":"BubR1 depletion in human cells causes failure to form stable kinetochore-microtubule attachments (not failure to bind microtubules), increases phosphorylation of the Aurora kinase substrate CENP-A at kinetochores, and this defect is rescued by Aurora kinase inhibition.","method":"RNAi depletion, live imaging, immunofluorescence, phospho-specific antibody","journal":"Nature cell biology","confidence":"High","confidence_rationale":"Tier 2 — RNAi with mechanistic rescue, phospho-substrate readout","pmids":["15592459"],"is_preprint":false},{"year":2005,"finding":"Microtubule capture by the CENP-E motor domain constitutively silences BubR1 kinase activity in vitro using purified BubR1, CENP-E, and microtubules, confirming a ternary complex-based silencing mechanism.","method":"In vitro kinase assay with purified BubR1, CENP-E, and microtubules","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 1 — in vitro reconstitution with purified components","pmids":["16144904"],"is_preprint":false},{"year":2007,"finding":"BubR1 is phosphorylated by Cdk1 on T620, which recruits Plk1 to BubR1; Plk1 then phosphorylates BubR1 on S676. S676 phosphorylation is tension-sensitive: it is present during prometaphase but dephosphorylated at metaphase upon establishment of sister chromatid tension. This phosphorylation is required for stable kinetochore-microtubule interactions and chromosome alignment.","method":"In vitro kinase assay, quantitative mass spectrometry, phospho-specific antibody, mutagenesis","journal":"Genes & development","confidence":"High","confidence_rationale":"Tier 1 — in vitro kinase assay plus in vivo phospho-site identification by MS and mutagenesis","pmids":["17785528"],"is_preprint":false},{"year":2007,"finding":"BubR1 colocalizes and physically interacts with Plk1 at kinetochores of unaligned chromosomes; Plk1 phosphorylates BubR1 in vitro at two consensus sites in its kinase domain, and phosphomimetic BubR1 rescues chromosome alignment defects in BubR1-deficient cells, demonstrating Plk1 facilitates chromosome alignment through BubR1.","method":"Co-immunoprecipitation, in vitro kinase assay, RNAi, rescue with phosphomimetic mutants","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1–2 — in vitro kinase assay combined with mutagenesis and functional rescue","pmids":["17376779"],"is_preprint":false},{"year":2007,"finding":"BubR1 interacts with APC/EB1 microtubule plus-end tracking proteins; BubR1 directly phosphorylates APC in vitro and forms a ternary complex with APC and microtubules, providing a link between BubR1 kinase activity and stable kinetochore-microtubule attachment.","method":"Immunodepletion from Xenopus egg extracts, in vitro kinase assay with purified components, Co-immunoprecipitation","journal":"The Journal of cell biology","confidence":"Medium","confidence_rationale":"Tier 1–2 — in vitro phosphorylation assay and egg extract work, single lab","pmids":["17709426"],"is_preprint":false},{"year":2010,"finding":"BUBR1 has two functionally distinct domains: KEN boxes are required for spindle assembly checkpoint (SAC) function and Cdc20/APC-C binding, while multiple proline-directed phosphorylation sites are required for chromosome-spindle attachment but not SAC. The GLEBS motif is required for Bub3 interaction, kinetochore localization, and both SAC and chromosome congression functions.","method":"In vivo phosphorylation mapping by mass spectrometry, mutagenesis, Co-immunoprecipitation, functional assays","journal":"Journal of cell science","confidence":"High","confidence_rationale":"Tier 1–2 — phosphosite mapping by MS combined with systematic mutagenesis and functional readouts","pmids":["20016069"],"is_preprint":false},{"year":2012,"finding":"BUBR1 is an evolutionary pseudokinase in vertebrates: its kinase catalytic motifs are degenerate and dispensable for error-free chromosome segregation, but residues that interact with ATP are essential for conformational stability of the protein.","method":"Evolutionary analysis, mutagenesis, in vivo complementation assay, structural modeling","journal":"Developmental cell","confidence":"High","confidence_rationale":"Tier 1–2 — mutagenesis with functional complementation, evolutionary analysis across species","pmids":["22698286"],"is_preprint":false},{"year":2012,"finding":"A conserved kinetochore-attachment regulatory domain (KARD) in BUBR1 is phosphorylated by PLK1, promoting direct interaction of BUBR1 with the PP2A-B56α phosphatase subunit. This recruits PP2A-B56α to kinetochores, counteracting Aurora B activity there and stabilizing kinetochore-microtubule attachments.","method":"Mutagenesis, Co-immunoprecipitation, kinetochore fractionation, phospho-substrate assays, RNAi","journal":"Developmental cell","confidence":"High","confidence_rationale":"Tier 2 — reciprocal Co-IP, multiple orthogonal methods, mechanistic model validated by rescue","pmids":["23079597"],"is_preprint":false},{"year":2012,"finding":"CENP-E-dependent BubR1 autophosphorylation occurs at unattached kinetochores and is required for full-strength mitotic checkpoint and chromosome alignment; a non-phosphorylatable BubR1 mutant causes chromosome misalignment and reduces Aurora B-mediated Ndc80 phosphorylation at kinetochores.","method":"Autophosphorylation assay, non-phosphorylatable and phosphomimetic mutants, immunofluorescence, RNAi","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 1–2 — in vitro kinase assay for autophosphorylation, mutagenesis with functional rescue","pmids":["22801780"],"is_preprint":false},{"year":2013,"finding":"BubR1 directly binds B56 family PP2A regulatory subunits through a conserved motif phosphorylated by Cdk1 and Plk1; two hydrophobic residues flanking S670 are required for B56 binding; BubR1 targets a pool of B56-PP2A to kinetochores, and mutation of these residues impairs metaphase plate formation.","method":"In vitro binding assay, mutagenesis, kinetochore fractionation, phosphorylation assays","journal":"Journal of cell science","confidence":"High","confidence_rationale":"Tier 1–2 — direct binding assay with mutagenesis and localization studies, replicated by another lab","pmids":["23345399"],"is_preprint":false},{"year":2014,"finding":"SIRT2 deacetylates BubR1 at lysine-668 to maintain BubR1 protein stability; with age, declining NAD+ reduces SIRT2 activity, leading to CBP acetyltransferase-mediated acetylation of K668 and BubR1 degradation. Overexpression of SIRT2 or NMN treatment restores BubR1 levels in vivo.","method":"Acetylation site mutagenesis, SIRT2 overexpression/knockout, in vivo mouse studies, biochemical assays","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 2 — site-specific acetylation mapping with mutagenesis and in vivo rescue","pmids":["24825348"],"is_preprint":false},{"year":2014,"finding":"Bub3 promotes mitotic checkpoint signaling through two distinct mechanisms: enhancing BubR1 binding to unattached kinetochores and stimulating BubR1's N-terminal Cdc20-binding domain interaction with a site on Cdc20 exposed by initial Mad2 binding, generating the final BubR1-Bub3-Cdc20 mitotic checkpoint complex that inhibits APC/C.","method":"In vitro reconstitution, cell-based assays, mutagenesis","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 — in vitro reconstitution combined with cell-based validation","pmids":["25246557"],"is_preprint":false},{"year":2015,"finding":"BubR1 kinetochore localization relies on direct heterodimerization with Bub1 at a pseudo-symmetric interface; grafting a short Bub1 motif onto BubR1 promotes Bub1-independent kinetochore recruitment of BubR1 but cannot sustain checkpoint function, uncoupling localization from signaling.","method":"Mutagenesis, chimeric protein expression, in vitro binding assays, functional complementation","journal":"eLife","confidence":"High","confidence_rationale":"Tier 1–2 — gain-of-function chimera plus mutagenesis with functional uncoupling","pmids":["25611342"],"is_preprint":false},{"year":2015,"finding":"Two distinct pools of BubR1/Bub3 exist at kinetochores: a major pool dependent on Bub1 interaction required for chromosome alignment but not SAC, and a minor pool binding directly to phosphorylated MELT repeats on KNL1 required for efficient incorporation into checkpoint complexes and SAC signaling.","method":"Defined BubR1/Bub3 mutants, kinetochore recruitment assays, checkpoint activity assays","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 2 — defined mutants uncoupling two pools with distinct functional readouts","pmids":["27457023"],"is_preprint":false},{"year":2016,"finding":"Crystal structure of PP2A B56-BubR1 complex shows that a conserved BubR1 LxxIxE motif binds to a groove in the concave side of B56 HEAT repeats 3 and 4, distant from the C subunit binding site, providing structural basis for B56-directed PP2A recruitment to kinetochores via BubR1.","method":"X-ray crystallography at 1.8 Å resolution, biochemical validation","journal":"Protein & cell","confidence":"High","confidence_rationale":"Tier 1 — crystal structure with biochemical validation","pmids":["27350047"],"is_preprint":false},{"year":2010,"finding":"The N-terminal TPR domain of BubR1 (crystal structure at 1.8 Å) contains a shallow groove on its concave surface that mediates interaction with kinetochore protein Blinkin; BubR1 also directly interacts with Bub1 via regions C-terminal to their TPR domains, providing a mechanism for Bub1-dependent kinetochore recruitment of BubR1.","method":"X-ray crystallography, in vitro binding assays, mutagenesis","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — crystal structure with in vitro binding validation","pmids":["20220147"],"is_preprint":false},{"year":2000,"finding":"BUBR1 directly interacts with p55CDC/hCdc20 (yeast two-hybrid and GST pulldown), and this interaction is enhanced upon spindle checkpoint activation by nocodazole. BUBR1 phosphorylates p55CDC in vitro, and APC components CDC16 and CDC27 preferentially associate with BUBR1 in mitotically arrested cells.","method":"Yeast two-hybrid, GST pulldown, co-immunoprecipitation, in vitro kinase assay","journal":"Oncogene","confidence":"Medium","confidence_rationale":"Tier 2–3 — multiple binding assays plus in vitro kinase activity, single lab","pmids":["11030144"],"is_preprint":false},{"year":2011,"finding":"BUBR1 is required for primary cilia formation in vertebrates; in G0 phase, BUBR1 mediates ubiquitin-dependent proteasomal degradation of CDC20 and maintains APC/C-CDH1 activity that regulates dishevelled levels for ciliogenesis. Morpholino knockdown of bubr1 in medaka caused ciliary dysfunction and defects in cerebellar development and left-right asymmetry.","method":"Morpholino knockdown (medaka), biochemical assays, conditional knockdown in mammalian cells","journal":"Human molecular genetics","confidence":"Medium","confidence_rationale":"Tier 2–3 — in vivo knockdown plus biochemical mechanism, single lab","pmids":["21389084"],"is_preprint":false},{"year":2006,"finding":"BubR1 interacts with PARP-1 (identified by affinity pulldown and mass spectrometry, confirmed by reciprocal Co-IP); BubR1 deficiency compromises retention of intact PARP-1 upon DNA damage, linking the spindle checkpoint to the DNA damage response pathway.","method":"Affinity pulldown with mass spectrometry, reciprocal co-immunoprecipitation, BubR1 knockout MEFs","journal":"Oncogene","confidence":"Medium","confidence_rationale":"Tier 3 — Co-IP with MS identification, single lab, limited mechanistic follow-up","pmids":["16449973"],"is_preprint":false},{"year":2003,"finding":"BCSG1 (breast cancer-specific gene 1/gamma-synuclein) directly binds BubR1 (identified by yeast two-hybrid, confirmed by Co-IP and GST pulldown) and promotes BubR1 proteasomal degradation, impairing the mitotic checkpoint.","method":"Yeast two-hybrid, Co-immunoprecipitation, GST pulldown, proteasome inhibitor rescue","journal":"Oncogene","confidence":"Low","confidence_rationale":"Tier 3 — single Co-IP/pulldown with limited mechanistic dissection","pmids":["14576821"],"is_preprint":false},{"year":2017,"finding":"Sirt2-mediated deacetylation of BubR1 at lysine-243 regulates meiotic spindle assembly checkpoint and kinetochore-microtubule attachments in mouse oocytes; acetylation-mimetic BubR1-K243Q recapitulates Sirt2-knockdown meiotic defects, and non-acetylatable BubR1-K243R partially rescues meiotic deficits in aged oocytes.","method":"Oocyte microinjection, acetylation site mutagenesis, live imaging, immunofluorescence","journal":"Aging cell","confidence":"Medium","confidence_rationale":"Tier 2 — site-specific mutagenesis with functional rescue in oocytes, single lab","pmids":["29067790"],"is_preprint":false},{"year":2020,"finding":"The basic C-terminal helix of BubR1 is necessary (but not sufficient) for direct interaction with the acidic kinetochore-targeting domain of CENP-E; BubR1 is required for recruitment of CENP-E to kinetochores for chromosome alignment, defining the molecular basis of the BubR1-CENP-E axis.","method":"Mutagenesis, Co-immunoprecipitation, kinetochore localization assays, RNAi rescue","journal":"Journal of cell science","confidence":"Medium","confidence_rationale":"Tier 2 — domain mutagenesis with functional rescue, single lab","pmids":["32665320"],"is_preprint":false},{"year":2021,"finding":"Increased BUB1B expression promotes mutagenic non-homologous end joining (NHEJ) DNA repair; phosphorylated ATM physically interacts with BUB1B after ionizing radiation, and resistance to DNA double-strand breaks conferred by elevated BUB1B is ATM-dependent.","method":"CRISPR/Cas9-based DNA repair assay, Co-immunoprecipitation, ATM inhibitor rescue","journal":"Oncogene","confidence":"Medium","confidence_rationale":"Tier 2–3 — Co-IP plus functional DNA repair assay, single lab","pmids":["34545188"],"is_preprint":false},{"year":2015,"finding":"Distinct Bub1 domains separately mediate kinetochore localization of RZZ complex and BubR1; Bub1 (not Zwint) is required for RZZ recruitment; a specific Bub1 region mediates direct BubR1 binding for its kinetochore localization, and removal of the BubR1-recruiting Bub1 region paradoxically increases checkpoint strength.","method":"Domain deletion analysis, RNAi, kinetochore localization assays","journal":"Nature communications","confidence":"Medium","confidence_rationale":"Tier 2 — domain dissection with functional phenotype readouts, single lab","pmids":["26031201"],"is_preprint":false}],"current_model":"BubR1 (encoded by BUB1B) is a pseudokinase-containing scaffold that operates as a central spindle assembly checkpoint (SAC) effector by: (1) forming a Bub3-BubR1-Cdc20 mitotic checkpoint complex that directly inhibits APC/C(Cdc20) through KEN-box–mediated Cdc20 binding; (2) localizing to kinetochores via direct heterodimerization with Bub1 and/or binding to phospho-MELT repeats on KNL1; (3) being activated as a kinase by CENP-E at unattached kinetochores and silenced upon CENP-E-mediated microtubule capture; (4) recruiting PP2A-B56α phosphatase to kinetochores through a PLK1/CDK1-phosphorylated LxxIxE/KARD motif to counteract Aurora B and stabilize kinetochore-microtubule attachments; and (5) being regulated post-translationally by PLK1 phosphorylation (promoting chromosome alignment), SIRT2 deacetylation (maintaining protein stability), and Aurora B (governing kinetochore localization and tension sensing)."},"narrative":{"teleology":[{"year":1999,"claim":"Establishing that BubR1 is a mitotic checkpoint kinase that localizes to kinetochores and associates with APC/C answered the question of whether a second BUB1-family kinase participates directly in checkpoint signaling in human cells.","evidence":"Co-IP, kinase assays, dominant-negative expression in synchronized human cells","pmids":["10477750"],"confidence":"High","gaps":["Substrate identity of kinase activity unknown","Whether kinase activity is required for checkpoint function unclear","Mechanism of APC/C interaction undefined"]},{"year":2001,"claim":"Demonstrating that purified BubR1–Bub3 directly inhibits APC/C^Cdc20 independently of Mad2 and without requiring kinase activity resolved a key question about how the checkpoint signal converges on APC/C and revealed a kinase-independent inhibitory mechanism.","evidence":"Biochemical purification and in vitro APC/C assays with purified BubR1–Bub3, Xenopus extract reconstitution","pmids":["11702782","11907259"],"confidence":"High","gaps":["How Mad2 and BubR1 cooperate mechanistically at the structural level","Identity of critical BubR1 residues for Cdc20 interaction"]},{"year":2003,"claim":"Identification of CENP-E as the activating ligand for BubR1 kinase activity—and silencing of that activity upon microtubule capture—solved the problem of how kinetochore attachment status is transduced into checkpoint signaling.","evidence":"In vitro reconstitution with purified BubR1, CENP-E, and microtubules; Xenopus egg extracts","pmids":["12859900","16144904"],"confidence":"High","gaps":["In vivo significance of BubR1 kinase activity vs. scaffolding role contested","Substrates of BubR1 kinase in checkpoint silencing not fully identified"]},{"year":2003,"claim":"Showing that Aurora B activity is required for BubR1 kinetochore localization and its rebinding upon loss of tension established Aurora B as an upstream regulator linking tension sensing to checkpoint protein dynamics.","evidence":"Aurora kinase inhibitor (ZM447439), RNAi, immunofluorescence in human cells","pmids":["12719470"],"confidence":"High","gaps":["Whether Aurora B directly phosphorylates BubR1 or acts indirectly","Precise phosphosites responsible for localization unresolved"]},{"year":2004,"claim":"Demonstrating that BubR1 depletion causes unstable (not absent) kinetochore–microtubule attachments rescued by Aurora B inhibition revealed that BubR1 functions to counterbalance Aurora B destabilization, beyond its SAC role.","evidence":"RNAi depletion, live imaging, phospho-CENP-A detection in human cells","pmids":["15592459"],"confidence":"High","gaps":["Molecular mechanism by which BubR1 opposes Aurora B not yet identified"]},{"year":2007,"claim":"Mapping CDK1-dependent PLK1 recruitment to BubR1 (T620→S676 phosphorylation cascade) and showing tension-dependent dephosphorylation of S676 answered how PLK1 is targeted to BubR1 and how this phosphorylation governs chromosome alignment.","evidence":"In vitro kinase assay, mass spectrometry, phospho-specific antibodies, mutagenesis with functional rescue","pmids":["17785528","17376779"],"confidence":"High","gaps":["Identity of the phosphatase responsible for S676 dephosphorylation at metaphase","How PLK1 phosphorylation of BubR1 mechanistically stabilizes attachments"]},{"year":2010,"claim":"Systematic mutagenesis separating KEN-box (SAC), GLEBS motif (kinetochore recruitment via Bub3), and proline-directed phosphosites (chromosome alignment) established BubR1 as a modular scaffold with genetically separable checkpoint and attachment functions.","evidence":"Mass spectrometry-based phosphomapping, systematic mutagenesis with functional readouts in human cells","pmids":["20016069","20220147"],"confidence":"High","gaps":["How the TPR domain coordinates Blinkin/Bub1 binding at the structural level"]},{"year":2012,"claim":"Three concurrent studies resolved longstanding questions: BubR1 is a vertebrate pseudokinase dispensable for catalysis but requiring ATP binding for stability; PLK1 phosphorylation of the KARD motif directly recruits PP2A-B56α to kinetochores to oppose Aurora B; and CENP-E-dependent autophosphorylation contributes to checkpoint strength.","evidence":"Evolutionary analysis with mutagenesis and in vivo complementation; reciprocal Co-IP and phospho-substrate assays; autophosphorylation assays with non-phosphorylatable mutants","pmids":["22698286","23079597","22801780"],"confidence":"High","gaps":["Whether residual kinase activity has any physiological role in specific contexts","How PP2A-B56α recruitment is regulated during the metaphase-anaphase transition"]},{"year":2014,"claim":"Discovery that SIRT2 deacetylates BubR1-K668 to maintain its protein stability, and that age-related NAD+ decline leads to CBP-mediated acetylation and degradation of BubR1, established a post-translational mechanism linking metabolic aging to checkpoint erosion.","evidence":"Acetylation site mutagenesis, SIRT2 overexpression/knockout, NMN supplementation in mice","pmids":["24825348"],"confidence":"High","gaps":["Whether acetylation-dependent degradation uses a specific E3 ligase","Relevance to human aging not directly tested"]},{"year":2015,"claim":"Reconstitution and chimera experiments demonstrated that BubR1 reaches kinetochores primarily through direct heterodimerization with Bub1 at a pseudo-symmetric interface, while a minor Bub3-dependent pool on KNL1 MELT repeats is specifically required for SAC signaling, uncoupling localization from checkpoint function.","evidence":"Chimeric protein expression, defined mutants, kinetochore recruitment and checkpoint assays","pmids":["25611342","27457023","26031201"],"confidence":"High","gaps":["How the minor MELT-bound pool is selectively channeled into MCC assembly","Stoichiometry of the two pools at individual kinetochores unknown"]},{"year":2016,"claim":"Crystal structure of the PP2A-B56–BubR1 complex at 1.8 Å revealed the LxxIxE motif docking into a groove on B56 HEAT repeats 3–4, providing the atomic basis for how BubR1 recruits PP2A to kinetochores.","evidence":"X-ray crystallography with biochemical validation","pmids":["27350047"],"confidence":"High","gaps":["Structural basis for phosphorylation-dependent regulation of the LxxIxE motif binding","Full-length BubR1 structure at kinetochore not resolved"]},{"year":2020,"claim":"Mapping the BubR1 C-terminal basic helix as necessary for CENP-E interaction defined the molecular interface through which BubR1 recruits CENP-E to kinetochores for chromosome congression.","evidence":"Domain mutagenesis, Co-IP, kinetochore localization assays with RNAi rescue","pmids":["32665320"],"confidence":"Medium","gaps":["Structural detail of the BubR1–CENP-E interface lacking","Whether other proteins bridge or stabilize this interaction in vivo"]},{"year":2021,"claim":"Finding that elevated BUB1B promotes mutagenic NHEJ and physically interacts with phospho-ATM after irradiation raised the possibility that BubR1 has a non-mitotic role in the DNA damage response.","evidence":"CRISPR-based DNA repair assay, Co-IP of pATM–BUB1B, ATM inhibitor rescue","pmids":["34545188"],"confidence":"Medium","gaps":["Mechanism by which BubR1 promotes NHEJ is unknown","Whether the pATM interaction is direct or mediated by a bridging factor","Not independently replicated"]},{"year":null,"claim":"Key unresolved questions include the full structural basis of BubR1 at the kinetochore (no full-length structure in context of the KMN network), whether residual catalytic activity has any physiological role, how the two kinetochore pools are dynamically regulated during error correction, and the in vivo relevance of non-mitotic functions (ciliogenesis, DNA repair).","evidence":"","pmids":[],"confidence":"Low","gaps":["No full-length BubR1 structure in kinetochore context","Physiological significance of pseudokinase catalytic activity unresolved","Non-mitotic roles lack independent replication"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[1,2,10,12,16]},{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[12,14,19]},{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[0,3,13]}],"localization":[{"term_id":"GO:0005694","term_label":"chromosome","supporting_discovery_ids":[0,4,12,17,18]},{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[1,2]}],"pathway":[{"term_id":"R-HSA-1640170","term_label":"Cell Cycle","supporting_discovery_ids":[0,1,2,4,10,16,18]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[7,8,12]},{"term_id":"R-HSA-1643685","term_label":"Disease","supporting_discovery_ids":[27]}],"complexes":["Mitotic checkpoint complex (MCC)","BubR1–Bub3","PP2A-B56α–BubR1"],"partners":["CDC20","BUB3","BUB1","CENPE","PLK1","PPP2R5A","CASC5","MAD2L1"],"other_free_text":[]},"mechanistic_narrative":"BUB1B encodes BubR1, a pseudokinase scaffold that serves dual roles in mitotic fidelity: enforcing the spindle assembly checkpoint (SAC) and stabilizing kinetochore–microtubule attachments. BubR1 inhibits APC/C^Cdc20 through direct KEN-box–mediated binding of Cdc20, acting in concert with Bub3 and Mad2 to generate the mitotic checkpoint complex; this inhibitory activity is independent of its kinase domain, which is catalytically degenerate in vertebrates yet essential for protein stability [PMID:11702782, PMID:11907259, PMID:22698286]. BubR1 localizes to kinetochores via two routes—heterodimerization with Bub1 (the major pool, required for chromosome alignment) and direct Bub3-dependent binding to phospho-MELT repeats on KNL1 (a minor pool required for SAC signaling)—and recruits PP2A-B56α through a PLK1/CDK1-phosphorylated LxxIxE/KARD motif, counteracting Aurora B to stabilize kinetochore–microtubule attachments [PMID:23079597, PMID:27350047, PMID:27457023]. BubR1 protein levels are maintained by SIRT2-mediated deacetylation at K668, and its checkpoint-silencing is coupled to CENP-E–dependent microtubule capture at kinetochores [PMID:24825348, PMID:12859900]."},"prefetch_data":{"uniprot":{"accession":"O60566","full_name":"Mitotic checkpoint serine/threonine-protein kinase BUB1 beta","aliases":["MAD3/BUB1-related protein kinase","hBUBR1","Mitotic checkpoint kinase MAD3L","Protein SSK1"],"length_aa":1050,"mass_kda":119.5,"function":"Essential component of the mitotic checkpoint. Required for normal mitosis progression. The mitotic checkpoint delays anaphase until all chromosomes are properly attached to the mitotic spindle. One of its checkpoint functions is to inhibit the activity of the anaphase-promoting complex/cyclosome (APC/C) by blocking the binding of CDC20 to APC/C, independently of its kinase activity. The other is to monitor kinetochore activities that depend on the kinetochore motor CENPE. Required for kinetochore localization of CENPE. Negatively regulates PLK1 activity in interphase cells and suppresses centrosome amplification. Also implicated in triggering apoptosis in polyploid cells that exit aberrantly from mitotic arrest. 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distinct kinetochore pools of BubR1 ensure accurate chromosome segregation.","date":"2016","source":"Nature communications","url":"https://pubmed.ncbi.nlm.nih.gov/27457023","citation_count":30,"is_preprint":false},{"pmid":"16182441","id":"PMC_16182441","title":"Comparative genomic hybridization and BUB1B mutation analyses in childhood cancers associated with mosaic variegated aneuploidy syndrome.","date":"2005","source":"Cancer letters","url":"https://pubmed.ncbi.nlm.nih.gov/16182441","citation_count":29,"is_preprint":false},{"pmid":"17570218","id":"PMC_17570218","title":"APC inactivation associates with abnormal mitosis completion and concomitant BUB1B/MAD2L1 up-regulation.","date":"2007","source":"Gastroenterology","url":"https://pubmed.ncbi.nlm.nih.gov/17570218","citation_count":27,"is_preprint":false},{"pmid":"17525528","id":"PMC_17525528","title":"BubR1 and CENP-E have antagonistic effects upon the stability of microtubule-kinetochore attachments in Drosophila S2 cell mitosis.","date":"2007","source":"Cell cycle (Georgetown, Tex.)","url":"https://pubmed.ncbi.nlm.nih.gov/17525528","citation_count":27,"is_preprint":false},{"pmid":"26863027","id":"PMC_26863027","title":"How the Pathogenic Fungus Alternaria alternata Copes with Stress via the Response Regulators SSK1 and SHO1.","date":"2016","source":"PloS one","url":"https://pubmed.ncbi.nlm.nih.gov/26863027","citation_count":26,"is_preprint":false},{"pmid":"35184951","id":"PMC_35184951","title":"The spindle checkpoint proteins BUB1 and BUBR1: (SLiM)ming down to the basics.","date":"2022","source":"Trends in biochemical sciences","url":"https://pubmed.ncbi.nlm.nih.gov/35184951","citation_count":25,"is_preprint":false},{"pmid":"19618914","id":"PMC_19618914","title":"Effects of osmolytes on the SLN1-YPD1-SSK1 phosphorelay system from Saccharomyces cerevisiae.","date":"2009","source":"Biochemistry","url":"https://pubmed.ncbi.nlm.nih.gov/19618914","citation_count":25,"is_preprint":false},{"pmid":"27679546","id":"PMC_27679546","title":"Association between polymorphisms in segregation genes BUB1B and TTK and gastric cancer risk.","date":"2016","source":"Radiology and oncology","url":"https://pubmed.ncbi.nlm.nih.gov/27679546","citation_count":24,"is_preprint":false},{"pmid":"26553926","id":"PMC_26553926","title":"Bub3-BubR1-dependent sequestration of Cdc20Fizzy at DNA breaks facilitates the correct segregation of broken chromosomes.","date":"2015","source":"The Journal of cell biology","url":"https://pubmed.ncbi.nlm.nih.gov/26553926","citation_count":24,"is_preprint":false},{"pmid":"18440313","id":"PMC_18440313","title":"Chromosome damage in mitosis induces BubR1 activation and prometaphase arrest.","date":"2008","source":"FEBS letters","url":"https://pubmed.ncbi.nlm.nih.gov/18440313","citation_count":23,"is_preprint":false},{"pmid":"27922816","id":"PMC_27922816","title":"The progeroid gene BubR1 regulates axon myelination and motor function.","date":"2016","source":"Aging","url":"https://pubmed.ncbi.nlm.nih.gov/27922816","citation_count":22,"is_preprint":false},{"pmid":"20040022","id":"PMC_20040022","title":"Expression of mitotic checkpoint proteins BUB1B and MAD2L1 in salivary duct carcinomas.","date":"2009","source":"Journal of oral pathology & medicine : official publication of the International Association of Oral Pathologists and the American Academy of Oral Pathology","url":"https://pubmed.ncbi.nlm.nih.gov/20040022","citation_count":22,"is_preprint":false},{"pmid":"20421417","id":"PMC_20421417","title":"Recruitment of Cdc20 to the kinetochore requires BubR1 but not Mad2 in Drosophila melanogaster.","date":"2010","source":"Molecular and cellular biology","url":"https://pubmed.ncbi.nlm.nih.gov/20421417","citation_count":22,"is_preprint":false},{"pmid":"18932004","id":"PMC_18932004","title":"Gradual reduction of BUBR1 protein levels results in premature sister-chromatid separation then in aneuploidy.","date":"2008","source":"Human genetics","url":"https://pubmed.ncbi.nlm.nih.gov/18932004","citation_count":21,"is_preprint":false},{"pmid":"18710370","id":"PMC_18710370","title":"Ajuba: a new microtubule-associated protein that interacts with BUBR1 and Aurora B at kinetochores in metaphase.","date":"2009","source":"Biology of the cell","url":"https://pubmed.ncbi.nlm.nih.gov/18710370","citation_count":21,"is_preprint":false},{"pmid":"21550059","id":"PMC_21550059","title":"Reduced proliferation of aged human vascular smooth muscle cells--role of oxygen-derived free radicals and BubR1 expression.","date":"2011","source":"The Journal of surgical research","url":"https://pubmed.ncbi.nlm.nih.gov/21550059","citation_count":20,"is_preprint":false},{"pmid":"25698537","id":"PMC_25698537","title":"BUB1B promotes multiple myeloma cell proliferation through CDC20/CCNB axis.","date":"2015","source":"Medical oncology (Northwood, London, England)","url":"https://pubmed.ncbi.nlm.nih.gov/25698537","citation_count":19,"is_preprint":false},{"pmid":"30668728","id":"PMC_30668728","title":"Nearly complete deletion of BubR1 causes microcephaly through shortened mitosis and massive cell death.","date":"2019","source":"Human molecular genetics","url":"https://pubmed.ncbi.nlm.nih.gov/30668728","citation_count":19,"is_preprint":false},{"pmid":"29100415","id":"PMC_29100415","title":"The E2F activators control multiple mitotic regulators and maintain genomic integrity through Sgo1 and BubR1.","date":"2017","source":"Oncotarget","url":"https://pubmed.ncbi.nlm.nih.gov/29100415","citation_count":19,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":46333,"output_tokens":6467,"usd":0.118002},"stage2":{"model":"claude-opus-4-6","input_tokens":10162,"output_tokens":3378,"usd":0.20289},"total_usd":0.320892,"stage1_batch_id":"msgbatch_01TqNM6p6FsoMBT2jzQTwKBr","stage2_batch_id":"msgbatch_01Ya473eco8sTHbvRymQaU8o","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1999,\n      \"finding\": \"hBUBR1 is an essential mitotic checkpoint kinase that localizes to kinetochores during mitosis, monitors CENP-E-dependent kinetochore activities, and associates with the cyclosome/APC in mitotically arrested cells but not in interphase cells. Its kinase activity is stimulated upon spindle disruption.\",\n      \"method\": \"Immunoprecipitation, kinase assays, dominant-negative expression, cell synchronization\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (Co-IP, kinase assay, dominant-negative, localization) with functional phenotype\",\n      \"pmids\": [\"10477750\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"BubR1-Bub3 complex purified from mitotic human cells inhibits APC/C(Cdc20) in a Mad2-independent manner by blocking Cdc20 binding to APC; this inhibition does not require BubR1 kinase activity. BubR1 also interacts with the mitotic motor CENP-E.\",\n      \"method\": \"Biochemical purification, in vitro APC/C activity assay, immunoprecipitation, Xenopus egg extract reconstitution\",\n      \"journal\": \"Developmental cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro reconstitution with purified components, multiple orthogonal assays\",\n      \"pmids\": [\"11702782\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"BubR1 directly binds Cdc20 and inhibits APC/C activation independently of its kinase activity; BubR1 is 12-fold more potent than Mad2 as an APC/C inhibitor, and BubR1 and Mad2 synergistically inhibit Cdc20-APC at physiological concentrations by mutually promoting each other's binding to Cdc20.\",\n      \"method\": \"In vitro APC/C ubiquitylation assay, recombinant protein binding, quantitative analysis\",\n      \"journal\": \"Molecular biology of the cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro reconstitution with quantitative analysis, replicated findings\",\n      \"pmids\": [\"11907259\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"CENP-E directly activates BubR1 kinase activity; microtubule capture by the CENP-E motor domain silences BubR1 kinase activity in a ternary BubR1-CENP-E-microtubule complex, establishing CENP-E as the signal transducer that links kinetochore microtubule capture to checkpoint silencing.\",\n      \"method\": \"Purified component reconstitution with CENP-E and BubR1, Xenopus egg extracts, kinase assays\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro reconstitution with purified components, multiple assay systems\",\n      \"pmids\": [\"12859900\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Aurora B kinase activity is required for kinetochore localization of BubR1 (and Mad2 and CENP-E), for phosphorylation of BubR1 on mitotic entry, and for rebinding of BubR1 to metaphase kinetochores after reduction in centromeric tension. BubR1 is also required for chromosome alignment, not only spindle checkpoint function.\",\n      \"method\": \"Aurora kinase inhibitor (ZM447439), RNA interference, immunofluorescence, Western blot\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — chemical inhibition plus RNAi with multiple cellular phenotype readouts\",\n      \"pmids\": [\"12719470\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"BubR1 depletion in human cells causes failure to form stable kinetochore-microtubule attachments (not failure to bind microtubules), increases phosphorylation of the Aurora kinase substrate CENP-A at kinetochores, and this defect is rescued by Aurora kinase inhibition.\",\n      \"method\": \"RNAi depletion, live imaging, immunofluorescence, phospho-specific antibody\",\n      \"journal\": \"Nature cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — RNAi with mechanistic rescue, phospho-substrate readout\",\n      \"pmids\": [\"15592459\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Microtubule capture by the CENP-E motor domain constitutively silences BubR1 kinase activity in vitro using purified BubR1, CENP-E, and microtubules, confirming a ternary complex-based silencing mechanism.\",\n      \"method\": \"In vitro kinase assay with purified BubR1, CENP-E, and microtubules\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro reconstitution with purified components\",\n      \"pmids\": [\"16144904\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"BubR1 is phosphorylated by Cdk1 on T620, which recruits Plk1 to BubR1; Plk1 then phosphorylates BubR1 on S676. S676 phosphorylation is tension-sensitive: it is present during prometaphase but dephosphorylated at metaphase upon establishment of sister chromatid tension. This phosphorylation is required for stable kinetochore-microtubule interactions and chromosome alignment.\",\n      \"method\": \"In vitro kinase assay, quantitative mass spectrometry, phospho-specific antibody, mutagenesis\",\n      \"journal\": \"Genes & development\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro kinase assay plus in vivo phospho-site identification by MS and mutagenesis\",\n      \"pmids\": [\"17785528\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"BubR1 colocalizes and physically interacts with Plk1 at kinetochores of unaligned chromosomes; Plk1 phosphorylates BubR1 in vitro at two consensus sites in its kinase domain, and phosphomimetic BubR1 rescues chromosome alignment defects in BubR1-deficient cells, demonstrating Plk1 facilitates chromosome alignment through BubR1.\",\n      \"method\": \"Co-immunoprecipitation, in vitro kinase assay, RNAi, rescue with phosphomimetic mutants\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — in vitro kinase assay combined with mutagenesis and functional rescue\",\n      \"pmids\": [\"17376779\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"BubR1 interacts with APC/EB1 microtubule plus-end tracking proteins; BubR1 directly phosphorylates APC in vitro and forms a ternary complex with APC and microtubules, providing a link between BubR1 kinase activity and stable kinetochore-microtubule attachment.\",\n      \"method\": \"Immunodepletion from Xenopus egg extracts, in vitro kinase assay with purified components, Co-immunoprecipitation\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1–2 — in vitro phosphorylation assay and egg extract work, single lab\",\n      \"pmids\": [\"17709426\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"BUBR1 has two functionally distinct domains: KEN boxes are required for spindle assembly checkpoint (SAC) function and Cdc20/APC-C binding, while multiple proline-directed phosphorylation sites are required for chromosome-spindle attachment but not SAC. The GLEBS motif is required for Bub3 interaction, kinetochore localization, and both SAC and chromosome congression functions.\",\n      \"method\": \"In vivo phosphorylation mapping by mass spectrometry, mutagenesis, Co-immunoprecipitation, functional assays\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — phosphosite mapping by MS combined with systematic mutagenesis and functional readouts\",\n      \"pmids\": [\"20016069\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"BUBR1 is an evolutionary pseudokinase in vertebrates: its kinase catalytic motifs are degenerate and dispensable for error-free chromosome segregation, but residues that interact with ATP are essential for conformational stability of the protein.\",\n      \"method\": \"Evolutionary analysis, mutagenesis, in vivo complementation assay, structural modeling\",\n      \"journal\": \"Developmental cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — mutagenesis with functional complementation, evolutionary analysis across species\",\n      \"pmids\": [\"22698286\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"A conserved kinetochore-attachment regulatory domain (KARD) in BUBR1 is phosphorylated by PLK1, promoting direct interaction of BUBR1 with the PP2A-B56α phosphatase subunit. This recruits PP2A-B56α to kinetochores, counteracting Aurora B activity there and stabilizing kinetochore-microtubule attachments.\",\n      \"method\": \"Mutagenesis, Co-immunoprecipitation, kinetochore fractionation, phospho-substrate assays, RNAi\",\n      \"journal\": \"Developmental cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal Co-IP, multiple orthogonal methods, mechanistic model validated by rescue\",\n      \"pmids\": [\"23079597\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"CENP-E-dependent BubR1 autophosphorylation occurs at unattached kinetochores and is required for full-strength mitotic checkpoint and chromosome alignment; a non-phosphorylatable BubR1 mutant causes chromosome misalignment and reduces Aurora B-mediated Ndc80 phosphorylation at kinetochores.\",\n      \"method\": \"Autophosphorylation assay, non-phosphorylatable and phosphomimetic mutants, immunofluorescence, RNAi\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — in vitro kinase assay for autophosphorylation, mutagenesis with functional rescue\",\n      \"pmids\": [\"22801780\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"BubR1 directly binds B56 family PP2A regulatory subunits through a conserved motif phosphorylated by Cdk1 and Plk1; two hydrophobic residues flanking S670 are required for B56 binding; BubR1 targets a pool of B56-PP2A to kinetochores, and mutation of these residues impairs metaphase plate formation.\",\n      \"method\": \"In vitro binding assay, mutagenesis, kinetochore fractionation, phosphorylation assays\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — direct binding assay with mutagenesis and localization studies, replicated by another lab\",\n      \"pmids\": [\"23345399\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"SIRT2 deacetylates BubR1 at lysine-668 to maintain BubR1 protein stability; with age, declining NAD+ reduces SIRT2 activity, leading to CBP acetyltransferase-mediated acetylation of K668 and BubR1 degradation. Overexpression of SIRT2 or NMN treatment restores BubR1 levels in vivo.\",\n      \"method\": \"Acetylation site mutagenesis, SIRT2 overexpression/knockout, in vivo mouse studies, biochemical assays\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — site-specific acetylation mapping with mutagenesis and in vivo rescue\",\n      \"pmids\": [\"24825348\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Bub3 promotes mitotic checkpoint signaling through two distinct mechanisms: enhancing BubR1 binding to unattached kinetochores and stimulating BubR1's N-terminal Cdc20-binding domain interaction with a site on Cdc20 exposed by initial Mad2 binding, generating the final BubR1-Bub3-Cdc20 mitotic checkpoint complex that inhibits APC/C.\",\n      \"method\": \"In vitro reconstitution, cell-based assays, mutagenesis\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro reconstitution combined with cell-based validation\",\n      \"pmids\": [\"25246557\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"BubR1 kinetochore localization relies on direct heterodimerization with Bub1 at a pseudo-symmetric interface; grafting a short Bub1 motif onto BubR1 promotes Bub1-independent kinetochore recruitment of BubR1 but cannot sustain checkpoint function, uncoupling localization from signaling.\",\n      \"method\": \"Mutagenesis, chimeric protein expression, in vitro binding assays, functional complementation\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — gain-of-function chimera plus mutagenesis with functional uncoupling\",\n      \"pmids\": [\"25611342\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Two distinct pools of BubR1/Bub3 exist at kinetochores: a major pool dependent on Bub1 interaction required for chromosome alignment but not SAC, and a minor pool binding directly to phosphorylated MELT repeats on KNL1 required for efficient incorporation into checkpoint complexes and SAC signaling.\",\n      \"method\": \"Defined BubR1/Bub3 mutants, kinetochore recruitment assays, checkpoint activity assays\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — defined mutants uncoupling two pools with distinct functional readouts\",\n      \"pmids\": [\"27457023\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Crystal structure of PP2A B56-BubR1 complex shows that a conserved BubR1 LxxIxE motif binds to a groove in the concave side of B56 HEAT repeats 3 and 4, distant from the C subunit binding site, providing structural basis for B56-directed PP2A recruitment to kinetochores via BubR1.\",\n      \"method\": \"X-ray crystallography at 1.8 Å resolution, biochemical validation\",\n      \"journal\": \"Protein & cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — crystal structure with biochemical validation\",\n      \"pmids\": [\"27350047\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"The N-terminal TPR domain of BubR1 (crystal structure at 1.8 Å) contains a shallow groove on its concave surface that mediates interaction with kinetochore protein Blinkin; BubR1 also directly interacts with Bub1 via regions C-terminal to their TPR domains, providing a mechanism for Bub1-dependent kinetochore recruitment of BubR1.\",\n      \"method\": \"X-ray crystallography, in vitro binding assays, mutagenesis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — crystal structure with in vitro binding validation\",\n      \"pmids\": [\"20220147\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"BUBR1 directly interacts with p55CDC/hCdc20 (yeast two-hybrid and GST pulldown), and this interaction is enhanced upon spindle checkpoint activation by nocodazole. BUBR1 phosphorylates p55CDC in vitro, and APC components CDC16 and CDC27 preferentially associate with BUBR1 in mitotically arrested cells.\",\n      \"method\": \"Yeast two-hybrid, GST pulldown, co-immunoprecipitation, in vitro kinase assay\",\n      \"journal\": \"Oncogene\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — multiple binding assays plus in vitro kinase activity, single lab\",\n      \"pmids\": [\"11030144\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"BUBR1 is required for primary cilia formation in vertebrates; in G0 phase, BUBR1 mediates ubiquitin-dependent proteasomal degradation of CDC20 and maintains APC/C-CDH1 activity that regulates dishevelled levels for ciliogenesis. Morpholino knockdown of bubr1 in medaka caused ciliary dysfunction and defects in cerebellar development and left-right asymmetry.\",\n      \"method\": \"Morpholino knockdown (medaka), biochemical assays, conditional knockdown in mammalian cells\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — in vivo knockdown plus biochemical mechanism, single lab\",\n      \"pmids\": [\"21389084\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"BubR1 interacts with PARP-1 (identified by affinity pulldown and mass spectrometry, confirmed by reciprocal Co-IP); BubR1 deficiency compromises retention of intact PARP-1 upon DNA damage, linking the spindle checkpoint to the DNA damage response pathway.\",\n      \"method\": \"Affinity pulldown with mass spectrometry, reciprocal co-immunoprecipitation, BubR1 knockout MEFs\",\n      \"journal\": \"Oncogene\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — Co-IP with MS identification, single lab, limited mechanistic follow-up\",\n      \"pmids\": [\"16449973\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"BCSG1 (breast cancer-specific gene 1/gamma-synuclein) directly binds BubR1 (identified by yeast two-hybrid, confirmed by Co-IP and GST pulldown) and promotes BubR1 proteasomal degradation, impairing the mitotic checkpoint.\",\n      \"method\": \"Yeast two-hybrid, Co-immunoprecipitation, GST pulldown, proteasome inhibitor rescue\",\n      \"journal\": \"Oncogene\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — single Co-IP/pulldown with limited mechanistic dissection\",\n      \"pmids\": [\"14576821\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Sirt2-mediated deacetylation of BubR1 at lysine-243 regulates meiotic spindle assembly checkpoint and kinetochore-microtubule attachments in mouse oocytes; acetylation-mimetic BubR1-K243Q recapitulates Sirt2-knockdown meiotic defects, and non-acetylatable BubR1-K243R partially rescues meiotic deficits in aged oocytes.\",\n      \"method\": \"Oocyte microinjection, acetylation site mutagenesis, live imaging, immunofluorescence\",\n      \"journal\": \"Aging cell\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — site-specific mutagenesis with functional rescue in oocytes, single lab\",\n      \"pmids\": [\"29067790\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"The basic C-terminal helix of BubR1 is necessary (but not sufficient) for direct interaction with the acidic kinetochore-targeting domain of CENP-E; BubR1 is required for recruitment of CENP-E to kinetochores for chromosome alignment, defining the molecular basis of the BubR1-CENP-E axis.\",\n      \"method\": \"Mutagenesis, Co-immunoprecipitation, kinetochore localization assays, RNAi rescue\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — domain mutagenesis with functional rescue, single lab\",\n      \"pmids\": [\"32665320\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Increased BUB1B expression promotes mutagenic non-homologous end joining (NHEJ) DNA repair; phosphorylated ATM physically interacts with BUB1B after ionizing radiation, and resistance to DNA double-strand breaks conferred by elevated BUB1B is ATM-dependent.\",\n      \"method\": \"CRISPR/Cas9-based DNA repair assay, Co-immunoprecipitation, ATM inhibitor rescue\",\n      \"journal\": \"Oncogene\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — Co-IP plus functional DNA repair assay, single lab\",\n      \"pmids\": [\"34545188\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Distinct Bub1 domains separately mediate kinetochore localization of RZZ complex and BubR1; Bub1 (not Zwint) is required for RZZ recruitment; a specific Bub1 region mediates direct BubR1 binding for its kinetochore localization, and removal of the BubR1-recruiting Bub1 region paradoxically increases checkpoint strength.\",\n      \"method\": \"Domain deletion analysis, RNAi, kinetochore localization assays\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — domain dissection with functional phenotype readouts, single lab\",\n      \"pmids\": [\"26031201\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"BubR1 (encoded by BUB1B) is a pseudokinase-containing scaffold that operates as a central spindle assembly checkpoint (SAC) effector by: (1) forming a Bub3-BubR1-Cdc20 mitotic checkpoint complex that directly inhibits APC/C(Cdc20) through KEN-box–mediated Cdc20 binding; (2) localizing to kinetochores via direct heterodimerization with Bub1 and/or binding to phospho-MELT repeats on KNL1; (3) being activated as a kinase by CENP-E at unattached kinetochores and silenced upon CENP-E-mediated microtubule capture; (4) recruiting PP2A-B56α phosphatase to kinetochores through a PLK1/CDK1-phosphorylated LxxIxE/KARD motif to counteract Aurora B and stabilize kinetochore-microtubule attachments; and (5) being regulated post-translationally by PLK1 phosphorylation (promoting chromosome alignment), SIRT2 deacetylation (maintaining protein stability), and Aurora B (governing kinetochore localization and tension sensing).\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"BUB1B encodes BubR1, a pseudokinase scaffold that serves dual roles in mitotic fidelity: enforcing the spindle assembly checkpoint (SAC) and stabilizing kinetochore–microtubule attachments. BubR1 inhibits APC/C^Cdc20 through direct KEN-box–mediated binding of Cdc20, acting in concert with Bub3 and Mad2 to generate the mitotic checkpoint complex; this inhibitory activity is independent of its kinase domain, which is catalytically degenerate in vertebrates yet essential for protein stability [PMID:11702782, PMID:11907259, PMID:22698286]. BubR1 localizes to kinetochores via two routes—heterodimerization with Bub1 (the major pool, required for chromosome alignment) and direct Bub3-dependent binding to phospho-MELT repeats on KNL1 (a minor pool required for SAC signaling)—and recruits PP2A-B56α through a PLK1/CDK1-phosphorylated LxxIxE/KARD motif, counteracting Aurora B to stabilize kinetochore–microtubule attachments [PMID:23079597, PMID:27350047, PMID:27457023]. BubR1 protein levels are maintained by SIRT2-mediated deacetylation at K668, and its checkpoint-silencing is coupled to CENP-E–dependent microtubule capture at kinetochores [PMID:24825348, PMID:12859900].\",\n  \"teleology\": [\n    {\n      \"year\": 1999,\n      \"claim\": \"Establishing that BubR1 is a mitotic checkpoint kinase that localizes to kinetochores and associates with APC/C answered the question of whether a second BUB1-family kinase participates directly in checkpoint signaling in human cells.\",\n      \"evidence\": \"Co-IP, kinase assays, dominant-negative expression in synchronized human cells\",\n      \"pmids\": [\"10477750\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Substrate identity of kinase activity unknown\", \"Whether kinase activity is required for checkpoint function unclear\", \"Mechanism of APC/C interaction undefined\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Demonstrating that purified BubR1–Bub3 directly inhibits APC/C^Cdc20 independently of Mad2 and without requiring kinase activity resolved a key question about how the checkpoint signal converges on APC/C and revealed a kinase-independent inhibitory mechanism.\",\n      \"evidence\": \"Biochemical purification and in vitro APC/C assays with purified BubR1–Bub3, Xenopus extract reconstitution\",\n      \"pmids\": [\"11702782\", \"11907259\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How Mad2 and BubR1 cooperate mechanistically at the structural level\", \"Identity of critical BubR1 residues for Cdc20 interaction\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Identification of CENP-E as the activating ligand for BubR1 kinase activity—and silencing of that activity upon microtubule capture—solved the problem of how kinetochore attachment status is transduced into checkpoint signaling.\",\n      \"evidence\": \"In vitro reconstitution with purified BubR1, CENP-E, and microtubules; Xenopus egg extracts\",\n      \"pmids\": [\"12859900\", \"16144904\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo significance of BubR1 kinase activity vs. scaffolding role contested\", \"Substrates of BubR1 kinase in checkpoint silencing not fully identified\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Showing that Aurora B activity is required for BubR1 kinetochore localization and its rebinding upon loss of tension established Aurora B as an upstream regulator linking tension sensing to checkpoint protein dynamics.\",\n      \"evidence\": \"Aurora kinase inhibitor (ZM447439), RNAi, immunofluorescence in human cells\",\n      \"pmids\": [\"12719470\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether Aurora B directly phosphorylates BubR1 or acts indirectly\", \"Precise phosphosites responsible for localization unresolved\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Demonstrating that BubR1 depletion causes unstable (not absent) kinetochore–microtubule attachments rescued by Aurora B inhibition revealed that BubR1 functions to counterbalance Aurora B destabilization, beyond its SAC role.\",\n      \"evidence\": \"RNAi depletion, live imaging, phospho-CENP-A detection in human cells\",\n      \"pmids\": [\"15592459\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular mechanism by which BubR1 opposes Aurora B not yet identified\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Mapping CDK1-dependent PLK1 recruitment to BubR1 (T620→S676 phosphorylation cascade) and showing tension-dependent dephosphorylation of S676 answered how PLK1 is targeted to BubR1 and how this phosphorylation governs chromosome alignment.\",\n      \"evidence\": \"In vitro kinase assay, mass spectrometry, phospho-specific antibodies, mutagenesis with functional rescue\",\n      \"pmids\": [\"17785528\", \"17376779\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Identity of the phosphatase responsible for S676 dephosphorylation at metaphase\", \"How PLK1 phosphorylation of BubR1 mechanistically stabilizes attachments\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Systematic mutagenesis separating KEN-box (SAC), GLEBS motif (kinetochore recruitment via Bub3), and proline-directed phosphosites (chromosome alignment) established BubR1 as a modular scaffold with genetically separable checkpoint and attachment functions.\",\n      \"evidence\": \"Mass spectrometry-based phosphomapping, systematic mutagenesis with functional readouts in human cells\",\n      \"pmids\": [\"20016069\", \"20220147\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How the TPR domain coordinates Blinkin/Bub1 binding at the structural level\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Three concurrent studies resolved longstanding questions: BubR1 is a vertebrate pseudokinase dispensable for catalysis but requiring ATP binding for stability; PLK1 phosphorylation of the KARD motif directly recruits PP2A-B56α to kinetochores to oppose Aurora B; and CENP-E-dependent autophosphorylation contributes to checkpoint strength.\",\n      \"evidence\": \"Evolutionary analysis with mutagenesis and in vivo complementation; reciprocal Co-IP and phospho-substrate assays; autophosphorylation assays with non-phosphorylatable mutants\",\n      \"pmids\": [\"22698286\", \"23079597\", \"22801780\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether residual kinase activity has any physiological role in specific contexts\", \"How PP2A-B56α recruitment is regulated during the metaphase-anaphase transition\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Discovery that SIRT2 deacetylates BubR1-K668 to maintain its protein stability, and that age-related NAD+ decline leads to CBP-mediated acetylation and degradation of BubR1, established a post-translational mechanism linking metabolic aging to checkpoint erosion.\",\n      \"evidence\": \"Acetylation site mutagenesis, SIRT2 overexpression/knockout, NMN supplementation in mice\",\n      \"pmids\": [\"24825348\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether acetylation-dependent degradation uses a specific E3 ligase\", \"Relevance to human aging not directly tested\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Reconstitution and chimera experiments demonstrated that BubR1 reaches kinetochores primarily through direct heterodimerization with Bub1 at a pseudo-symmetric interface, while a minor Bub3-dependent pool on KNL1 MELT repeats is specifically required for SAC signaling, uncoupling localization from checkpoint function.\",\n      \"evidence\": \"Chimeric protein expression, defined mutants, kinetochore recruitment and checkpoint assays\",\n      \"pmids\": [\"25611342\", \"27457023\", \"26031201\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How the minor MELT-bound pool is selectively channeled into MCC assembly\", \"Stoichiometry of the two pools at individual kinetochores unknown\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Crystal structure of the PP2A-B56–BubR1 complex at 1.8 Å revealed the LxxIxE motif docking into a groove on B56 HEAT repeats 3–4, providing the atomic basis for how BubR1 recruits PP2A to kinetochores.\",\n      \"evidence\": \"X-ray crystallography with biochemical validation\",\n      \"pmids\": [\"27350047\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis for phosphorylation-dependent regulation of the LxxIxE motif binding\", \"Full-length BubR1 structure at kinetochore not resolved\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Mapping the BubR1 C-terminal basic helix as necessary for CENP-E interaction defined the molecular interface through which BubR1 recruits CENP-E to kinetochores for chromosome congression.\",\n      \"evidence\": \"Domain mutagenesis, Co-IP, kinetochore localization assays with RNAi rescue\",\n      \"pmids\": [\"32665320\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Structural detail of the BubR1–CENP-E interface lacking\", \"Whether other proteins bridge or stabilize this interaction in vivo\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Finding that elevated BUB1B promotes mutagenic NHEJ and physically interacts with phospho-ATM after irradiation raised the possibility that BubR1 has a non-mitotic role in the DNA damage response.\",\n      \"evidence\": \"CRISPR-based DNA repair assay, Co-IP of pATM–BUB1B, ATM inhibitor rescue\",\n      \"pmids\": [\"34545188\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism by which BubR1 promotes NHEJ is unknown\", \"Whether the pATM interaction is direct or mediated by a bridging factor\", \"Not independently replicated\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the full structural basis of BubR1 at the kinetochore (no full-length structure in context of the KMN network), whether residual catalytic activity has any physiological role, how the two kinetochore pools are dynamically regulated during error correction, and the in vivo relevance of non-mitotic functions (ciliogenesis, DNA repair).\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No full-length BubR1 structure in kinetochore context\", \"Physiological significance of pseudokinase catalytic activity unresolved\", \"Non-mitotic roles lack independent replication\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [1, 2, 10, 12, 16]},\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [12, 14, 19]},\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [0, 3, 13]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005694\", \"supporting_discovery_ids\": [0, 4, 12, 17, 18]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [1, 2]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1640170\", \"supporting_discovery_ids\": [0, 1, 2, 4, 10, 16, 18]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [7, 8, 12]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [27]}\n    ],\n    \"complexes\": [\n      \"Mitotic checkpoint complex (MCC)\",\n      \"BubR1–Bub3\",\n      \"PP2A-B56α–BubR1\"\n    ],\n    \"partners\": [\n      \"CDC20\",\n      \"BUB3\",\n      \"BUB1\",\n      \"CENPE\",\n      \"PLK1\",\n      \"PPP2R5A\",\n      \"CASC5\",\n      \"MAD2L1\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}