{"gene":"CUL9","run_date":"2026-06-09T22:57:19","timeline":{"discoveries":[{"year":1999,"finding":"H7-AP1 (CUL9) contains RING finger and IBR (in between RING fingers) domains that mediate specific interaction with ubiquitin-conjugating enzymes UbcH7 and UbcH8, but not with unrelated E2s UbcH5 or UbcH1; the IBR domain of H7-AP1 is required for this interaction, placing CUL9 functionally within the ubiquitin/proteasome pathway.","method":"Yeast two-hybrid screen followed by in vitro binding studies with domain mapping","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — yeast two-hybrid plus in vitro binding with domain mutants, single lab, two orthogonal methods","pmids":["10521492"],"is_preprint":false},{"year":2011,"finding":"CUL9 (PARC) is an E3 ubiquitin ligase that localizes in the cytoplasm, binds to p53, and promotes p53-dependent apoptosis; deletion of Cul9 in mice leads to spontaneous tumor development and attenuated DNA damage-induced apoptosis; a point mutant of CUL9 deficient in p53 binding fails to promote apoptosis, demonstrating that p53 binding is required for this function.","method":"Mouse knockout model, ectopic expression of wild-type vs. p53-binding-deficient point mutant CUL9, apoptosis and tumor phenotype assays","journal":"Cancer research","confidence":"High","confidence_rationale":"Tier 2 / Strong — clean KO mouse with defined phenotype, point-mutant rescue experiment, replicated by subsequent studies","pmids":["21487039"],"is_preprint":false},{"year":2014,"finding":"CUL9 promotes ubiquitylation and proteasomal degradation of survivin; CUL7 inhibits CUL9, and depletion of CUL7 decreases survivin levels; overexpression of survivin rescues microtubule and mitosis defects caused by CUL7 depletion, placing CUL9 in a 3M-CUL9-survivin pathway that maintains microtubule and genome integrity.","method":"siRNA knockdown of CUL9, CUL7, and OBSL1; ubiquitylation assays; epistasis by survivin overexpression rescue; aneuploidy and DNA damage readouts","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic epistasis with rescue experiment, ubiquitylation assay, multiple orthogonal methods in one study","pmids":["24793696"],"is_preprint":false},{"year":2014,"finding":"CUL9 (PARC) is the E3 ubiquitin ligase responsible for ubiquitination and proteasomal degradation of cytoplasmic cytochrome c after mitochondrial outer-membrane permeabilization, constituting a cell-survival mechanism that prevents caspase activation.","method":"Unbiased proteomics/biochemical identification of PARC/CUL9 as the ligase for cytochrome c ubiquitination; proteasomal degradation assay (described in cited Deshmukh et al. study, summarized in this commentary)","journal":"Science signaling","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — unbiased biochemical approach identifying CUL9 as cytochrome c ubiquitin ligase, reported as commentary summarizing the primary study; single lab","pmids":["25028716"],"is_preprint":false},{"year":2017,"finding":"CUL9 maintains genome integrity and regulates cell proliferation, senescence, and apoptosis primarily through p53; a knock-in mutation disrupting Cul9-p53 binding (Cul9Δp53) phenocopies full Cul9 knockout (increased S-phase cells, accumulated DNA damage during replication, decreased apoptosis), and deletion of CUL9 in p53-null cells causes no further increase in DNA damage, confirming the CUL9-p53 axis as the essential pathway.","method":"Cul9Δp53 knock-in mouse embryo fibroblasts (MEFs), double-mutant (Cul9-/-; p53-/-) epistasis analysis, cell-cycle profiling, DNA damage quantification, apoptosis assays","journal":"Oncogene","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic epistasis with knock-in point mutant and double-knockout, multiple orthogonal phenotypic readouts, functionally validates p53-binding requirement","pmids":["28481879"],"is_preprint":false},{"year":2024,"finding":"Cryo-EM and biochemistry reveal that CUL9 forms a 1.8-MDa hexameric complex with RBX1; within a dimeric subcomplex, an E2-bound RBR domain is activated by neddylation of its own cullin domain and positioned in trans by the adjacent CUL9-RBX1; CUL9 uniquely depends on the metazoan-specific neddylation enzyme UBE2F (not the canonical UBE2M), and the RBR domain protects it from deneddylation; substrates are recruited to upstream domains while ubiquitylation relies on both the neddylated cullin-RING and RBR domains, constituting a chimeric cullin-RING/RBR E3 ligase mechanism.","method":"Cryo-EM structure determination, in vitro biochemical ubiquitylation assays, cellular assays, mass spectrometry cross-linking","journal":"Nature structural & molecular biology","confidence":"High","confidence_rationale":"Tier 1 / Strong — cryo-EM structure plus reconstituted biochemistry plus mutagenesis and cellular assays in a single rigorous study","pmids":["38605244"],"is_preprint":false},{"year":2021,"finding":"Immunoprecipitation of endogenous CUL9 in human pluripotent stem cells identified multiple APC/C subunits as potential CUL9-interacting proteins; knockdown of the APC/C adapter FZR1 significantly increases CUL9 protein levels, suggesting CUL9 abundance is regulated by APC/C-FZR1; however, CUL9 deletion does not affect APC/C subunit/adapter protein abundance or cell-cycle progression.","method":"LC-MS/MS of immunoprecipitated endogenous CUL9, FZR1 knockdown with CUL9 protein level measurement, quantitative proteomics in CUL9 KO cells","journal":"PloS one","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single Co-IP/MS experiment for interaction, single-lab FZR1 knockdown result, no functional mechanistic follow-up for the interaction","pmids":["33705438"],"is_preprint":false}],"current_model":"CUL9 is a vertebrate-specific, cytoplasm-localized E3 ubiquitin ligase that forms a 1.8-MDa hexameric RBX1 complex with a unique chimeric cullin-RING/RBR catalytic mechanism activated by UBE2F-mediated neddylation; it ubiquitylates survivin to maintain microtubule and genome integrity (antagonized by the CUL7/3M complex), ubiquitylates cytochrome c to promote cell survival downstream of mitochondrial permeabilization, and activates p53-dependent apoptosis and genome maintenance through direct p53 binding—a function that underpins its role as a tumor suppressor."},"narrative":{"mechanistic_narrative":"CUL9 (PARC/H7-AP1) is a cytoplasm-localized E3 ubiquitin ligase that functions in genome maintenance, apoptotic control, and microtubule integrity [PMID:21487039, PMID:24793696]. It is structurally and mechanistically distinctive: CUL9 assembles into a 1.8-MDa hexameric complex with RBX1 in which an E2-loaded RBR domain is activated by neddylation of its own cullin domain — uniquely dependent on the metazoan-specific neddylation enzyme UBE2F rather than canonical UBE2M — and positioned in trans by the adjacent CUL9-RBX1, defining a chimeric cullin-RING/RBR catalytic mechanism [PMID:38605244]. Its earliest characterized biochemical feature was selective engagement of the ubiquitin-conjugating enzymes UbcH7 and UbcH8 through its RING and IBR domains, placing it within the ubiquitin/proteasome pathway [PMID:10521492]. Functionally, CUL9 maintains genome integrity and governs proliferation, senescence, and apoptosis principally through direct p53 binding, since a knock-in mutation abolishing this interaction phenocopies full knockout and confers no additional DNA damage in p53-null cells [PMID:21487039, PMID:28481879]. In parallel, CUL9 ubiquitylates survivin within a 3M-CUL9-survivin axis antagonized by CUL7 to preserve microtubule and mitotic fidelity [PMID:24793696], and degrades cytoplasmic cytochrome c after mitochondrial outer-membrane permeabilization to restrain caspase activation [PMID:25028716]. Loss of Cul9 in mice produces spontaneous tumors, establishing a tumor-suppressor role [PMID:21487039].","teleology":[{"year":1999,"claim":"Established that CUL9 is an enzyme of the ubiquitin/proteasome system, answering whether its RING/IBR architecture engages specific ubiquitin-conjugating enzymes.","evidence":"Yeast two-hybrid screen and in vitro binding with domain mutants showing IBR-dependent interaction with UbcH7/UbcH8 but not UbcH5 or UbcH1","pmids":["10521492"],"confidence":"Medium","gaps":["No substrate identified at this stage","Physiological role and cellular context undefined","In vitro E2 binding not linked to ubiquitylation output"]},{"year":2011,"claim":"Defined CUL9 as a tumor suppressor acting through p53, answering what biological function its ligase activity serves.","evidence":"Cul9 knockout mouse showing spontaneous tumors and attenuated DNA-damage apoptosis, with a p53-binding-deficient point mutant failing to promote apoptosis","pmids":["21487039"],"confidence":"High","gaps":["Direct ubiquitylation substrate not defined here","Mechanism by which p53 binding promotes apoptosis unresolved","Cytoplasmic-to-p53 signaling steps unmapped"]},{"year":2014,"claim":"Identified survivin as a CUL9 degradation substrate and placed CUL9 in a CUL7-antagonized pathway controlling mitotic integrity.","evidence":"siRNA knockdown, ubiquitylation assays, and survivin-overexpression epistasis rescue of microtubule/mitosis defects","pmids":["24793696"],"confidence":"High","gaps":["Molecular basis of CUL7 inhibition of CUL9 not detailed","Relationship between survivin and p53 axes unclear"]},{"year":2014,"claim":"Assigned CUL9 a pro-survival role by degrading cytoplasmic cytochrome c after mitochondrial permeabilization, refining how it restrains apoptosis.","evidence":"Unbiased proteomic/biochemical identification of PARC/CUL9 as the cytochrome c ligase with proteasomal degradation assays (commentary summarizing primary study)","pmids":["25028716"],"confidence":"Medium","gaps":["Reported via commentary, single lab","How this pro-survival role reconciles with pro-apoptotic p53 function not resolved","Regulation of substrate choice unknown"]},{"year":2017,"claim":"Demonstrated the CUL9-p53 axis is the essential pathway for CUL9's genome-maintenance and cell-fate functions through genetic epistasis.","evidence":"Cul9Δp53 knock-in MEFs phenocopying knockout and Cul9-/-;p53-/- double-mutant analysis with cell-cycle, DNA-damage, and apoptosis readouts","pmids":["28481879"],"confidence":"High","gaps":["Direct enzymatic events on p53 not defined","Mechanism linking p53 binding to S-phase/DNA-damage control unresolved"]},{"year":2024,"claim":"Resolved CUL9's catalytic architecture, answering how this cullin uniquely couples cullin-RING and RBR chemistry and which neddylation enzyme activates it.","evidence":"Cryo-EM, reconstituted ubiquitylation, mutagenesis, and crosslinking mass spectrometry of the 1.8-MDa CUL9-RBX1 hexamer","pmids":["38605244"],"confidence":"High","gaps":["Structural basis of substrate (survivin, cytochrome c, p53) recruitment not resolved","How UBE2F specificity is achieved mechanistically unclear"]},{"year":2021,"claim":"Raised the possibility that CUL9 abundance is itself controlled by the APC/C-FZR1 system.","evidence":"LC-MS/MS of immunoprecipitated endogenous CUL9 and FZR1 knockdown increasing CUL9 protein in pluripotent stem cells","pmids":["33705438"],"confidence":"Low","gaps":["Single Co-IP/MS without reciprocal validation","Direct APC/C-mediated ubiquitylation of CUL9 not demonstrated","CUL9 deletion did not affect APC/C subunits or cell cycle, leaving functional relevance unclear"]},{"year":null,"claim":"How CUL9's three substrate-directed activities (p53 apoptosis, survivin degradation, cytochrome c degradation) are coordinated and selected within a single chimeric ligase remains unresolved.","evidence":"","pmids":[],"confidence":"High","gaps":["No structural model of substrate engagement","Switch between pro-apoptotic and pro-survival outputs unexplained","Regulation of CUL9 activity in vivo incompletely mapped"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[0,2,3,5]},{"term_id":"GO:0016874","term_label":"ligase activity","supporting_discovery_ids":[0,5]}],"localization":[{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[1,3]}],"pathway":[{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[2,3,5]},{"term_id":"R-HSA-5357801","term_label":"Programmed Cell Death","supporting_discovery_ids":[1,3,4]},{"term_id":"R-HSA-1640170","term_label":"Cell Cycle","supporting_discovery_ids":[2,4]}],"complexes":["CUL9-RBX1 hexameric E3 ligase complex","3M-CUL9 pathway (with CUL7/OBSL1)"],"partners":["RBX1","TP53","CUL7","OBSL1","UBE2F","FZR1"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q8IWT3","full_name":"Cullin-9","aliases":["UbcH7-associated protein 1","p53-associated parkin-like cytoplasmic protein"],"length_aa":2517,"mass_kda":281.2,"function":"Core component of a Cul9-RING ubiquitin-protein ligase complex composed of CUL9 and RBX1 (PubMed:38605244). The CUL9-RBX1 complex mediates ubiquitination and subsequent degradation of BIRC5 and is required to maintain microtubule dynamics and genome integrity. Acts downstream of the 3M complex, which inhibits the ubiquitination of BIRC5 (PubMed:24793696). The CUL9-RBX1 complex also mediates mono-ubiquitination of p53/TP53 (PubMed:38605244). Acts as a cytoplasmic anchor protein in p53/TP53-associated protein complex. Regulates the subcellular localization of p53/TP53 and its subsequent function (PubMed:12526791, PubMed:17332328). Ubiquitinates apurinic/apyrimidinic endodeoxyribonuclease APEX2 (PubMed:38605244). Ubiquitination by the CUL9-RBX1 complex is predominantly mediated by E2 ubiquitin-conjugating enzymes UBE2L3 and UBE2D2 (PubMed:38605244)","subcellular_location":"Cytoplasm","url":"https://www.uniprot.org/uniprotkb/Q8IWT3/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/CUL9","classification":"Not Classified","n_dependent_lines":23,"n_total_lines":1208,"dependency_fraction":0.01903973509933775},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/CUL9","total_profiled":1310},"omim":[{"mim_id":"613745","title":"ANAPHASE-PROMOTING COMPLEX, SUBUNIT 10; ANAPC10","url":"https://www.omim.org/entry/613745"},{"mim_id":"607489","title":"CULLIN 9; CUL9","url":"https://www.omim.org/entry/607489"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Cytosol","reliability":"Approved"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/CUL9"},"hgnc":{"alias_symbol":["H7AP1","KIAA0708","PARC"],"prev_symbol":[]},"alphafold":{"accession":"Q8IWT3","domains":[{"cath_id":"2.30.30.30","chopping":"386-439","consensus_level":"high","plddt":82.0617,"start":386,"end":439},{"cath_id":"-","chopping":"492-566","consensus_level":"high","plddt":71.3423,"start":492,"end":566},{"cath_id":"2.60.120.260","chopping":"1164-1301","consensus_level":"high","plddt":81.1839,"start":1164,"end":1301},{"cath_id":"-","chopping":"1302-1383","consensus_level":"medium","plddt":78.9567,"start":1302,"end":1383},{"cath_id":"-","chopping":"1428-1435_1465-1538","consensus_level":"medium","plddt":80.5106,"start":1428,"end":1538},{"cath_id":"-","chopping":"1718-1733_1755-1822_1842-1854","consensus_level":"medium","plddt":77.6267,"start":1718,"end":1854},{"cath_id":"1.10.10.10","chopping":"1858-1905_1923-1953","consensus_level":"medium","plddt":70.641,"start":1858,"end":1953},{"cath_id":"-","chopping":"2291-2430","consensus_level":"medium","plddt":77.4831,"start":2291,"end":2430},{"cath_id":"2.30.30","chopping":"6-49_60-79","consensus_level":"high","plddt":68.3052,"start":6,"end":79}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q8IWT3","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q8IWT3-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q8IWT3-F1-predicted_aligned_error_v6.png","plddt_mean":67.06},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=CUL9","jax_strain_url":"https://www.jax.org/strain/search?query=CUL9"},"sequence":{"accession":"Q8IWT3","fasta_url":"https://rest.uniprot.org/uniprotkb/Q8IWT3.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q8IWT3/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q8IWT3"}},"corpus_meta":[{"pmid":"10521492","id":"PMC_10521492","title":"The ubiquitin-conjugating enzymes UbcH7 and UbcH8 interact with RING finger/IBR motif-containing domains of HHARI and H7-AP1.","date":"1999","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/10521492","citation_count":101,"is_preprint":false},{"pmid":"24793696","id":"PMC_24793696","title":"CUL9 mediates the functions of the 3M complex and ubiquitylates survivin to maintain genome integrity.","date":"2014","source":"Molecular cell","url":"https://pubmed.ncbi.nlm.nih.gov/24793696","citation_count":67,"is_preprint":false},{"pmid":"21487039","id":"PMC_21487039","title":"Cytoplasmic CUL9/PARC ubiquitin ligase is a tumor suppressor and promotes p53-dependent apoptosis.","date":"2011","source":"Cancer research","url":"https://pubmed.ncbi.nlm.nih.gov/21487039","citation_count":50,"is_preprint":false},{"pmid":"28481879","id":"PMC_28481879","title":"Cytoplasmic E3 ubiquitin ligase CUL9 controls cell proliferation, senescence, apoptosis and genome integrity through p53.","date":"2017","source":"Oncogene","url":"https://pubmed.ncbi.nlm.nih.gov/28481879","citation_count":21,"is_preprint":false},{"pmid":"38605244","id":"PMC_38605244","title":"Noncanonical assembly, neddylation and chimeric cullin-RING/RBR ubiquitylation by the 1.8 MDa CUL9 E3 ligase complex.","date":"2024","source":"Nature structural & molecular biology","url":"https://pubmed.ncbi.nlm.nih.gov/38605244","citation_count":10,"is_preprint":false},{"pmid":"24905004","id":"PMC_24905004","title":"Regulating microtubules and genome stability via the CUL7/3M syndrome complex and CUL9.","date":"2014","source":"Molecular cell","url":"https://pubmed.ncbi.nlm.nih.gov/24905004","citation_count":8,"is_preprint":false},{"pmid":"25028716","id":"PMC_25028716","title":"Killing the Killer: PARC/CUL9 promotes cell survival by destroying cytochrome C.","date":"2014","source":"Science signaling","url":"https://pubmed.ncbi.nlm.nih.gov/25028716","citation_count":7,"is_preprint":false},{"pmid":"33705438","id":"PMC_33705438","title":"A proteomics approach for the identification of cullin-9 (CUL9) related signaling pathways in induced pluripotent stem cell models.","date":"2021","source":"PloS one","url":"https://pubmed.ncbi.nlm.nih.gov/33705438","citation_count":4,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":5553,"output_tokens":2113,"usd":0.024177,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":9043,"output_tokens":2984,"usd":0.059908,"stage2_stop_reason":"end_turn"},"total_usd":0.084085,"stage1_batch_id":"msgbatch_01ACAxJQabDsht39JXeWG22d","stage2_batch_id":"msgbatch_01QEU22A9U5sbPTgsMzTmvSf","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1999,\n      \"finding\": \"H7-AP1 (CUL9) contains RING finger and IBR (in between RING fingers) domains that mediate specific interaction with ubiquitin-conjugating enzymes UbcH7 and UbcH8, but not with unrelated E2s UbcH5 or UbcH1; the IBR domain of H7-AP1 is required for this interaction, placing CUL9 functionally within the ubiquitin/proteasome pathway.\",\n      \"method\": \"Yeast two-hybrid screen followed by in vitro binding studies with domain mapping\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — yeast two-hybrid plus in vitro binding with domain mutants, single lab, two orthogonal methods\",\n      \"pmids\": [\"10521492\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"CUL9 (PARC) is an E3 ubiquitin ligase that localizes in the cytoplasm, binds to p53, and promotes p53-dependent apoptosis; deletion of Cul9 in mice leads to spontaneous tumor development and attenuated DNA damage-induced apoptosis; a point mutant of CUL9 deficient in p53 binding fails to promote apoptosis, demonstrating that p53 binding is required for this function.\",\n      \"method\": \"Mouse knockout model, ectopic expression of wild-type vs. p53-binding-deficient point mutant CUL9, apoptosis and tumor phenotype assays\",\n      \"journal\": \"Cancer research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — clean KO mouse with defined phenotype, point-mutant rescue experiment, replicated by subsequent studies\",\n      \"pmids\": [\"21487039\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"CUL9 promotes ubiquitylation and proteasomal degradation of survivin; CUL7 inhibits CUL9, and depletion of CUL7 decreases survivin levels; overexpression of survivin rescues microtubule and mitosis defects caused by CUL7 depletion, placing CUL9 in a 3M-CUL9-survivin pathway that maintains microtubule and genome integrity.\",\n      \"method\": \"siRNA knockdown of CUL9, CUL7, and OBSL1; ubiquitylation assays; epistasis by survivin overexpression rescue; aneuploidy and DNA damage readouts\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic epistasis with rescue experiment, ubiquitylation assay, multiple orthogonal methods in one study\",\n      \"pmids\": [\"24793696\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"CUL9 (PARC) is the E3 ubiquitin ligase responsible for ubiquitination and proteasomal degradation of cytoplasmic cytochrome c after mitochondrial outer-membrane permeabilization, constituting a cell-survival mechanism that prevents caspase activation.\",\n      \"method\": \"Unbiased proteomics/biochemical identification of PARC/CUL9 as the ligase for cytochrome c ubiquitination; proteasomal degradation assay (described in cited Deshmukh et al. study, summarized in this commentary)\",\n      \"journal\": \"Science signaling\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — unbiased biochemical approach identifying CUL9 as cytochrome c ubiquitin ligase, reported as commentary summarizing the primary study; single lab\",\n      \"pmids\": [\"25028716\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"CUL9 maintains genome integrity and regulates cell proliferation, senescence, and apoptosis primarily through p53; a knock-in mutation disrupting Cul9-p53 binding (Cul9Δp53) phenocopies full Cul9 knockout (increased S-phase cells, accumulated DNA damage during replication, decreased apoptosis), and deletion of CUL9 in p53-null cells causes no further increase in DNA damage, confirming the CUL9-p53 axis as the essential pathway.\",\n      \"method\": \"Cul9Δp53 knock-in mouse embryo fibroblasts (MEFs), double-mutant (Cul9-/-; p53-/-) epistasis analysis, cell-cycle profiling, DNA damage quantification, apoptosis assays\",\n      \"journal\": \"Oncogene\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic epistasis with knock-in point mutant and double-knockout, multiple orthogonal phenotypic readouts, functionally validates p53-binding requirement\",\n      \"pmids\": [\"28481879\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Cryo-EM and biochemistry reveal that CUL9 forms a 1.8-MDa hexameric complex with RBX1; within a dimeric subcomplex, an E2-bound RBR domain is activated by neddylation of its own cullin domain and positioned in trans by the adjacent CUL9-RBX1; CUL9 uniquely depends on the metazoan-specific neddylation enzyme UBE2F (not the canonical UBE2M), and the RBR domain protects it from deneddylation; substrates are recruited to upstream domains while ubiquitylation relies on both the neddylated cullin-RING and RBR domains, constituting a chimeric cullin-RING/RBR E3 ligase mechanism.\",\n      \"method\": \"Cryo-EM structure determination, in vitro biochemical ubiquitylation assays, cellular assays, mass spectrometry cross-linking\",\n      \"journal\": \"Nature structural & molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — cryo-EM structure plus reconstituted biochemistry plus mutagenesis and cellular assays in a single rigorous study\",\n      \"pmids\": [\"38605244\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Immunoprecipitation of endogenous CUL9 in human pluripotent stem cells identified multiple APC/C subunits as potential CUL9-interacting proteins; knockdown of the APC/C adapter FZR1 significantly increases CUL9 protein levels, suggesting CUL9 abundance is regulated by APC/C-FZR1; however, CUL9 deletion does not affect APC/C subunit/adapter protein abundance or cell-cycle progression.\",\n      \"method\": \"LC-MS/MS of immunoprecipitated endogenous CUL9, FZR1 knockdown with CUL9 protein level measurement, quantitative proteomics in CUL9 KO cells\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single Co-IP/MS experiment for interaction, single-lab FZR1 knockdown result, no functional mechanistic follow-up for the interaction\",\n      \"pmids\": [\"33705438\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"CUL9 is a vertebrate-specific, cytoplasm-localized E3 ubiquitin ligase that forms a 1.8-MDa hexameric RBX1 complex with a unique chimeric cullin-RING/RBR catalytic mechanism activated by UBE2F-mediated neddylation; it ubiquitylates survivin to maintain microtubule and genome integrity (antagonized by the CUL7/3M complex), ubiquitylates cytochrome c to promote cell survival downstream of mitochondrial permeabilization, and activates p53-dependent apoptosis and genome maintenance through direct p53 binding—a function that underpins its role as a tumor suppressor.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"CUL9 (PARC/H7-AP1) is a cytoplasm-localized E3 ubiquitin ligase that functions in genome maintenance, apoptotic control, and microtubule integrity [#1, #2]. It is structurally and mechanistically distinctive: CUL9 assembles into a 1.8-MDa hexameric complex with RBX1 in which an E2-loaded RBR domain is activated by neddylation of its own cullin domain — uniquely dependent on the metazoan-specific neddylation enzyme UBE2F rather than canonical UBE2M — and positioned in trans by the adjacent CUL9-RBX1, defining a chimeric cullin-RING/RBR catalytic mechanism [#5]. Its earliest characterized biochemical feature was selective engagement of the ubiquitin-conjugating enzymes UbcH7 and UbcH8 through its RING and IBR domains, placing it within the ubiquitin/proteasome pathway [#0]. Functionally, CUL9 maintains genome integrity and governs proliferation, senescence, and apoptosis principally through direct p53 binding, since a knock-in mutation abolishing this interaction phenocopies full knockout and confers no additional DNA damage in p53-null cells [#1, #4]. In parallel, CUL9 ubiquitylates survivin within a 3M-CUL9-survivin axis antagonized by CUL7 to preserve microtubule and mitotic fidelity [#2], and degrades cytoplasmic cytochrome c after mitochondrial outer-membrane permeabilization to restrain caspase activation [#3]. Loss of Cul9 in mice produces spontaneous tumors, establishing a tumor-suppressor role [#1].\",\n  \"teleology\": [\n    {\n      \"year\": 1999,\n      \"claim\": \"Established that CUL9 is an enzyme of the ubiquitin/proteasome system, answering whether its RING/IBR architecture engages specific ubiquitin-conjugating enzymes.\",\n      \"evidence\": \"Yeast two-hybrid screen and in vitro binding with domain mutants showing IBR-dependent interaction with UbcH7/UbcH8 but not UbcH5 or UbcH1\",\n      \"pmids\": [\"10521492\"],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"No substrate identified at this stage\", \"Physiological role and cellular context undefined\", \"In vitro E2 binding not linked to ubiquitylation output\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Defined CUL9 as a tumor suppressor acting through p53, answering what biological function its ligase activity serves.\",\n      \"evidence\": \"Cul9 knockout mouse showing spontaneous tumors and attenuated DNA-damage apoptosis, with a p53-binding-deficient point mutant failing to promote apoptosis\",\n      \"pmids\": [\"21487039\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Direct ubiquitylation substrate not defined here\", \"Mechanism by which p53 binding promotes apoptosis unresolved\", \"Cytoplasmic-to-p53 signaling steps unmapped\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Identified survivin as a CUL9 degradation substrate and placed CUL9 in a CUL7-antagonized pathway controlling mitotic integrity.\",\n      \"evidence\": \"siRNA knockdown, ubiquitylation assays, and survivin-overexpression epistasis rescue of microtubule/mitosis defects\",\n      \"pmids\": [\"24793696\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Molecular basis of CUL7 inhibition of CUL9 not detailed\", \"Relationship between survivin and p53 axes unclear\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Assigned CUL9 a pro-survival role by degrading cytoplasmic cytochrome c after mitochondrial permeabilization, refining how it restrains apoptosis.\",\n      \"evidence\": \"Unbiased proteomic/biochemical identification of PARC/CUL9 as the cytochrome c ligase with proteasomal degradation assays (commentary summarizing primary study)\",\n      \"pmids\": [\"25028716\"],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Reported via commentary, single lab\", \"How this pro-survival role reconciles with pro-apoptotic p53 function not resolved\", \"Regulation of substrate choice unknown\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Demonstrated the CUL9-p53 axis is the essential pathway for CUL9's genome-maintenance and cell-fate functions through genetic epistasis.\",\n      \"evidence\": \"Cul9\\u0394p53 knock-in MEFs phenocopying knockout and Cul9-/-;p53-/- double-mutant analysis with cell-cycle, DNA-damage, and apoptosis readouts\",\n      \"pmids\": [\"28481879\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Direct enzymatic events on p53 not defined\", \"Mechanism linking p53 binding to S-phase/DNA-damage control unresolved\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Resolved CUL9's catalytic architecture, answering how this cullin uniquely couples cullin-RING and RBR chemistry and which neddylation enzyme activates it.\",\n      \"evidence\": \"Cryo-EM, reconstituted ubiquitylation, mutagenesis, and crosslinking mass spectrometry of the 1.8-MDa CUL9-RBX1 hexamer\",\n      \"pmids\": [\"38605244\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Structural basis of substrate (survivin, cytochrome c, p53) recruitment not resolved\", \"How UBE2F specificity is achieved mechanistically unclear\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Raised the possibility that CUL9 abundance is itself controlled by the APC/C-FZR1 system.\",\n      \"evidence\": \"LC-MS/MS of immunoprecipitated endogenous CUL9 and FZR1 knockdown increasing CUL9 protein in pluripotent stem cells\",\n      \"pmids\": [\"33705438\"],\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Single Co-IP/MS without reciprocal validation\", \"Direct APC/C-mediated ubiquitylation of CUL9 not demonstrated\", \"CUL9 deletion did not affect APC/C subunits or cell cycle, leaving functional relevance unclear\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How CUL9's three substrate-directed activities (p53 apoptosis, survivin degradation, cytochrome c degradation) are coordinated and selected within a single chimeric ligase remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"No structural model of substrate engagement\", \"Switch between pro-apoptotic and pro-survival outputs unexplained\", \"Regulation of CUL9 activity in vivo incompletely mapped\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [0, 2, 3, 5]},\n      {\"term_id\": \"GO:0016874\", \"supporting_discovery_ids\": [0, 5]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [1, 3]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [2, 3, 5]},\n      {\"term_id\": \"R-HSA-5357801\", \"supporting_discovery_ids\": [1, 3, 4]},\n      {\"term_id\": \"R-HSA-1640170\", \"supporting_discovery_ids\": [2, 4]}\n    ],\n    \"complexes\": [\"CUL9-RBX1 hexameric E3 ligase complex\", \"3M-CUL9 pathway (with CUL7/OBSL1)\"],\n    \"partners\": [\"RBX1\", \"TP53\", \"CUL7\", \"OBSL1\", \"UBE2F\", \"FZR1\"],\n    \"other_free_text\": []\n  }\n}\n```","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":6,"faith_total":6,"faith_pct":100.0}}