{"gene":"BTBD2","run_date":"2026-06-09T22:02:45","timeline":{"discoveries":[{"year":2002,"finding":"BTBD2 physically interacts with the core domain of human topoisomerase I (TOP1); the interaction was confirmed by co-precipitation assays from HeLa cells, and the TOP1 sequences sufficient to bind BTBD2 were mapped to residues 215–329. Co-incubation of BTBD2 with TOP1 in vitro affected supercoil relaxation and DNA cleavage activities little or not at all.","method":"Two-hybrid screening, co-precipitation (pulldown) from HeLa cells, deletion mapping by two-hybrid and GST-pulldown, in vitro topoisomerase activity assay","journal":"BMC genomics","confidence":"High","confidence_rationale":"Tier 1–2 / Moderate — multiple orthogonal methods (yeast two-hybrid, co-precipitation, GST-pulldown, in vitro enzymatic assay) in a single focused study","pmids":["11818025"],"is_preprint":false},{"year":2003,"finding":"BTBD2 (and BTBD1) colocalize to punctate or elongated cytoplasmic bodies in mouse and human cells. TRIM5δ (a RING-containing RBCC/TRIM family member) colocalizes with BTBD1/2 and serves as a scaffold for assembly of endogenous BTBD1/2 proteins at these bodies; deletion of TRIM5δ's coiled-coil region or mutation of its RING domain abolished colocalization.","method":"Immunofluorescence microscopy, GFP-fusion colocalization, expression of deletion/point mutants of TRIM5δ","journal":"Experimental cell research","confidence":"Medium","confidence_rationale":"Tier 2–3 / Moderate — reciprocal colocalization with domain-mapping mutants in multiple cell lines, single lab","pmids":["12878161"],"is_preprint":false},{"year":2003,"finding":"Epitope-tagged BTBD2 localizes to cytoplasmic bodies; when truncated versions of BTBD2 and TOP1 were co-transfected to direct both proteins to the same compartment (nucleus or cytoplasm), co-localization of BTBD2 and TOP1 was demonstrated in HeLa cells.","method":"Subcellular localization by fluorescence microscopy, co-transfection with compartment-targeting truncation constructs","journal":"BMC genomics","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — localization confirmed with compartment-redirecting truncations in a focused study, single lab","pmids":["11818025"],"is_preprint":false},{"year":2010,"finding":"The interaction of BTBD2 with TOP1 requires human TOP1 residues 236 and 237 (the same residues that enhance HIV-1 virion infectivity). RNAi-mediated knockdown of BTBD2 in AGM and human 293T target cells increased permissiveness to HIV-1 infection two- to three-fold, indicating a modest restriction role.","method":"Mutagenesis of TOP1 residues 236–237, co-immunoprecipitation, RNAi knockdown with HIV-1 infectivity assay","journal":"Virology journal","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — site-directed mutagenesis combined with RNAi/infectivity assay, single lab, two orthogonal methods","pmids":["21092135"],"is_preprint":false},{"year":2016,"finding":"In C. elegans, the BTBD-2 protein functions as an E3 substrate-adaptor component (together with CUL-3, RBX-1, and SIAH-1) downstream of HSF-1 in the ubiquitin–proteasome system to promote linker-cell-type (non-apoptotic) developmental cell death; genetic epistasis placed BTBD-2 in the LCD execution pathway.","method":"Genetic epistasis (C. elegans mutant analysis), molecular-genetic pathway delineation of LCD","journal":"eLife","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic epistasis in C. elegans with multiple pathway components tested, single lab","pmids":["26952214"],"is_preprint":false},{"year":2022,"finding":"BTBD2 (together with BTBD1) acts as an adaptor protein for the E3-RING Cullin 3 (CUL3) ligase complex that ubiquitinates TOP1 cleavage complexes (TOP1-ccs), promoting their proteasomal degradation. NEDDylation of CUL3 activates this pathway, and depletion of CUL3 or inhibition of NEDDylation sensitizes cancer cells to TOP1 inhibitors.","method":"Genomic occupancy analysis, ubiquitination assays, proteasomal degradation assays, siRNA depletion, NEDDylation inhibition with small molecules, cell viability assays","journal":"Science advances","confidence":"High","confidence_rationale":"Tier 1–2 / Moderate — multiple orthogonal methods (ubiquitination assay, degradation assay, knockdown, small-molecule inhibition) in a single focused mechanistic study","pmids":["36490343"],"is_preprint":false},{"year":2024,"finding":"BTBD2 interacts with HNF1β and enables the interaction and activation of Topoisomerase 1 (TOP1) specifically during mitosis, facilitating DNA relaxation around HNF1β mitotic chromatin sites and contributing to chromatin remodeling and gene reactivation after mitotic exit (bookmarking).","method":"Crosslinking-based chromatin immunoprecipitation, protein–protein interaction assay, TOP1 activity assay on mitotic chromatin, mutational analysis of HNF1β–TOP1 interaction domain","journal":"Cell reports","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal methods (crosslinking ChIP, interaction assay, enzymatic assay, mutagenesis) in a single lab study","pmids":["39388351"],"is_preprint":false}],"current_model":"BTBD2 is an adaptor protein that physically bridges Topoisomerase I (TOP1) to the CUL3-RBX1 E3 ubiquitin ligase complex (requiring TOP1 residues 236–237 for the interaction), promoting ubiquitination and proteasomal degradation of trapped TOP1 cleavage complexes in a NEDDylation-dependent manner; it additionally localizes to cytoplasmic bodies where it co-assembles with TRIM5δ, modestly restricts HIV-1 infection, and—during mitosis—bridges HNF1β to TOP1 to drive DNA relaxation at bookmarked chromatin sites, enabling post-mitotic gene reactivation."},"narrative":{"mechanistic_narrative":"BTBD2 is a BTB-domain adaptor protein that couples Topoisomerase I (TOP1) to the cellular ubiquitin–proteasome machinery, governing both the turnover of damaged TOP1 and TOP1-dependent chromatin remodeling [PMID:11818025, PMID:36490343]. It binds the core domain of human TOP1 directly (an interaction mapped to TOP1 residues 215–329 and requiring residues 236–237) without itself altering TOP1's supercoil-relaxation or cleavage activity in vitro [PMID:11818025, PMID:21092135]. Functioning as a substrate-recognition adaptor for the CUL3–RBX1 E3 ubiquitin ligase, BTBD2 (with BTBD1) targets trapped TOP1 cleavage complexes for ubiquitination and proteasomal degradation; NEDDylation of CUL3 activates this pathway, and disabling CUL3 or NEDDylation sensitizes cancer cells to TOP1 inhibitors [PMID:36490343]. The same TOP1-bridging activity is deployed during mitosis, where BTBD2 links HNF1β to TOP1 to drive DNA relaxation at bookmarked chromatin sites and enable post-mitotic gene reactivation [PMID:39388351]. BTBD2 additionally assembles into cytoplasmic bodies scaffolded by TRIM5δ and modestly restricts HIV-1 infection [PMID:12878161, PMID:21092135].","teleology":[{"year":2002,"claim":"Established that BTBD2 is a direct TOP1-binding protein, raising the question of whether it modulates topoisomerase activity or serves another function.","evidence":"Yeast two-hybrid, co-precipitation and GST-pulldown from HeLa cells, with deletion mapping and in vitro topoisomerase assays","pmids":["11818025"],"confidence":"High","gaps":["Did not explain the functional consequence of the interaction, since TOP1 enzymatic activity was unaffected in vitro","No E3 ligase or ubiquitination role yet identified"]},{"year":2003,"claim":"Defined the subcellular context of BTBD2, showing it concentrates in TRIM5δ-scaffolded cytoplasmic bodies and can co-localize with TOP1, indicating a regulated compartmentalization.","evidence":"Immunofluorescence, GFP-fusion colocalization, TRIM5δ domain/point mutants, and compartment-redirecting truncation constructs in mouse and human cells","pmids":["12878161","11818025"],"confidence":"Medium","gaps":["Functional role of cytoplasmic body assembly not established","Whether TOP1 co-localization reflects a physiological pool versus overexpression artifact unclear"]},{"year":2010,"claim":"Connected BTBD2 to a biological readout by pinpointing the TOP1 residues (236–237) required for binding and showing BTBD2 modestly restricts HIV-1 infection.","evidence":"Site-directed mutagenesis of TOP1, co-immunoprecipitation, and RNAi knockdown with HIV-1 infectivity assays in AGM and 293T cells","pmids":["21092135"],"confidence":"Medium","gaps":["Mechanism linking the TOP1 interaction to viral restriction not defined","Effect size was modest (two- to three-fold)"]},{"year":2016,"claim":"Provided the first functional assignment of BTBD2 as a CUL3-pathway substrate adaptor, via its C. elegans ortholog acting in non-apoptotic developmental cell death.","evidence":"Genetic epistasis in C. elegans placing BTBD-2 with CUL-3, RBX-1 and SIAH-1 downstream of HSF-1 in the linker-cell death pathway","pmids":["26952214"],"confidence":"Medium","gaps":["Relevant ubiquitination substrate in the LCD pathway not identified","Conservation of the adaptor role to human BTBD2 not directly tested here"]},{"year":2022,"claim":"Unified the prior observations into a mechanism: BTBD2 serves as a CUL3 substrate adaptor that ubiquitinates trapped TOP1 cleavage complexes for degradation, with therapeutic implications for TOP1 inhibitors.","evidence":"Ubiquitination and proteasomal degradation assays, siRNA depletion, NEDDylation inhibition, and cell viability assays in cancer cells","pmids":["36490343"],"confidence":"High","gaps":["Relative contributions of BTBD1 versus BTBD2 to substrate recognition not separated","Structural basis of TOP1-cc recognition not resolved"]},{"year":2024,"claim":"Extended the TOP1-bridging function to a non-degradative, mitosis-specific role in chromatin bookmarking.","evidence":"Crosslinking ChIP, protein interaction assays, TOP1 activity assays on mitotic chromatin, and mutational analysis of the HNF1β–TOP1 interaction in mitotic cells","pmids":["39388351"],"confidence":"Medium","gaps":["How BTBD2 switches between degradative (CUL3) and activating (HNF1β–TOP1) roles is unclear","Genome-wide scope of HNF1β bookmarking dependent on BTBD2 not defined"]},{"year":null,"claim":"How BTBD2's distinct activities — CUL3-dependent TOP1-cc degradation, mitotic HNF1β–TOP1 activation, cytoplasmic body assembly, and HIV restriction — are coordinated within a single cell remains unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No structural model of BTBD2 bound to TOP1 or CUL3","Regulatory signals that partition BTBD2 between cytoplasmic bodies and nuclear/chromatin functions unknown","Whether cytoplasmic body localization relates mechanistically to the degradation or bookmarking roles is untested"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[0,5,6]},{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[5]}],"localization":[{"term_id":"GO:0031410","term_label":"cytoplasmic vesicle","supporting_discovery_ids":[1,2]},{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[6]}],"pathway":[{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[5]},{"term_id":"R-HSA-73894","term_label":"DNA Repair","supporting_discovery_ids":[5]},{"term_id":"R-HSA-4839726","term_label":"Chromatin organization","supporting_discovery_ids":[6]}],"complexes":["CUL3-RBX1 E3 ubiquitin ligase complex","TRIM5δ cytoplasmic bodies"],"partners":["TOP1","CUL3","RBX1","BTBD1","TRIM5Δ","HNF1Β"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q9BX70","full_name":"BTB/POZ domain-containing protein 2","aliases":[],"length_aa":525,"mass_kda":55.9,"function":"","subcellular_location":"Cytoplasm","url":"https://www.uniprot.org/uniprotkb/Q9BX70/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/BTBD2","classification":"Not Classified","n_dependent_lines":4,"n_total_lines":1208,"dependency_fraction":0.0033112582781456954},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/BTBD2","total_profiled":1310},"omim":[{"mim_id":"608531","title":"BTB/POZ DOMAIN-CONTAINING PROTEIN 2; BTBD2","url":"https://www.omim.org/entry/608531"},{"mim_id":"608530","title":"BTB/POZ DOMAIN-CONTAINING PROTEIN 1; BTBD1","url":"https://www.omim.org/entry/608530"},{"mim_id":"608487","title":"TRIPARTITE MOTIF-CONTAINING PROTEIN 5; TRIM5","url":"https://www.omim.org/entry/608487"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in all","driving_tissues":[{"tissue":"brain","ntpm":159.5}],"url":"https://www.proteinatlas.org/search/BTBD2"},"hgnc":{"alias_symbol":[],"prev_symbol":[]},"alphafold":{"accession":"Q9BX70","domains":[{"cath_id":"3.30.710.10","chopping":"98-217","consensus_level":"high","plddt":93.2198,"start":98,"end":217},{"cath_id":"-","chopping":"279-367","consensus_level":"medium","plddt":92.184,"start":279,"end":367},{"cath_id":"2.60.120.820","chopping":"372-525","consensus_level":"high","plddt":93.3369,"start":372,"end":525}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9BX70","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9BX70-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9BX70-F1-predicted_aligned_error_v6.png","plddt_mean":82.62},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=BTBD2","jax_strain_url":"https://www.jax.org/strain/search?query=BTBD2"},"sequence":{"accession":"Q9BX70","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9BX70.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9BX70/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9BX70"}},"corpus_meta":[{"pmid":"20368557","id":"PMC_20368557","title":"Polymorphisms of LIG4, BTBD2, HMGA2, and RTEL1 genes involved in the double-strand break repair pathway predict glioblastoma survival.","date":"2010","source":"Journal of clinical oncology : official journal of the American Society of Clinical Oncology","url":"https://pubmed.ncbi.nlm.nih.gov/20368557","citation_count":97,"is_preprint":false},{"pmid":"12878161","id":"PMC_12878161","title":"BTBD1 and BTBD2 colocalize to cytoplasmic bodies with the RBCC/tripartite motif protein, TRIM5delta.","date":"2003","source":"Experimental cell research","url":"https://pubmed.ncbi.nlm.nih.gov/12878161","citation_count":89,"is_preprint":false},{"pmid":"11818025","id":"PMC_11818025","title":"Characterization of BTBD1 and BTBD2, two similar BTB-domain-containing Kelch-like proteins that interact with Topoisomerase I.","date":"2002","source":"BMC genomics","url":"https://pubmed.ncbi.nlm.nih.gov/11818025","citation_count":43,"is_preprint":false},{"pmid":"26952214","id":"PMC_26952214","title":"HSF-1 activates the ubiquitin proteasome system to promote non-apoptotic developmental cell death in C. elegans.","date":"2016","source":"eLife","url":"https://pubmed.ncbi.nlm.nih.gov/26952214","citation_count":31,"is_preprint":false},{"pmid":"36490343","id":"PMC_36490343","title":"NEDDylated Cullin 3 mediates the adaptive response to topoisomerase 1 inhibitors.","date":"2022","source":"Science advances","url":"https://pubmed.ncbi.nlm.nih.gov/36490343","citation_count":17,"is_preprint":false},{"pmid":"11179693","id":"PMC_11179693","title":"Identification and characterization of BTBD1, a novel BTB domain containing gene on human chromosome 15q24.","date":"2001","source":"Gene","url":"https://pubmed.ncbi.nlm.nih.gov/11179693","citation_count":15,"is_preprint":false},{"pmid":"12782588","id":"PMC_12782588","title":"Gene and peptide analyses of newly defined lung cancer antigens recognized by HLA-A2402-restricted tumor-specific cytotoxic T lymphocytes.","date":"2003","source":"Cancer research","url":"https://pubmed.ncbi.nlm.nih.gov/12782588","citation_count":14,"is_preprint":false},{"pmid":"39388351","id":"PMC_39388351","title":"HNF1β bookmarking involves Topoisomerase 1 activation and DNA topology relaxation in mitotic chromatin.","date":"2024","source":"Cell reports","url":"https://pubmed.ncbi.nlm.nih.gov/39388351","citation_count":5,"is_preprint":false},{"pmid":"38187229","id":"PMC_38187229","title":"Identification of the transcriptome signatures and immune-inflammatory responses in postmenopausal osteoporosis.","date":"2023","source":"Heliyon","url":"https://pubmed.ncbi.nlm.nih.gov/38187229","citation_count":5,"is_preprint":false},{"pmid":"21092135","id":"PMC_21092135","title":"Human TOP1 residues implicated in species specificity of HIV-1 infection are required for interaction with BTBD2, and RNAi of BTBD2 in old world monkey and human cells increases permissiveness to HIV-1 infection.","date":"2010","source":"Virology journal","url":"https://pubmed.ncbi.nlm.nih.gov/21092135","citation_count":2,"is_preprint":false},{"pmid":"16936789","id":"PMC_16936789","title":"Identification and isolation of a BTB-POZ-containing gene expressed in oocytes and early embryos of the zebrafish Danio rerio.","date":"2006","source":"Genome","url":"https://pubmed.ncbi.nlm.nih.gov/16936789","citation_count":2,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":7285,"output_tokens":1899,"usd":0.02517,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":8861,"output_tokens":2729,"usd":0.056265,"stage2_stop_reason":"end_turn"},"total_usd":0.081435,"stage1_batch_id":"msgbatch_0179Z2pxRYBsXi2xADneafRw","stage2_batch_id":"msgbatch_01LaY9ZaVdgjWArcZo9eqQE5","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2002,\n      \"finding\": \"BTBD2 physically interacts with the core domain of human topoisomerase I (TOP1); the interaction was confirmed by co-precipitation assays from HeLa cells, and the TOP1 sequences sufficient to bind BTBD2 were mapped to residues 215–329. Co-incubation of BTBD2 with TOP1 in vitro affected supercoil relaxation and DNA cleavage activities little or not at all.\",\n      \"method\": \"Two-hybrid screening, co-precipitation (pulldown) from HeLa cells, deletion mapping by two-hybrid and GST-pulldown, in vitro topoisomerase activity assay\",\n      \"journal\": \"BMC genomics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Moderate — multiple orthogonal methods (yeast two-hybrid, co-precipitation, GST-pulldown, in vitro enzymatic assay) in a single focused study\",\n      \"pmids\": [\"11818025\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"BTBD2 (and BTBD1) colocalize to punctate or elongated cytoplasmic bodies in mouse and human cells. TRIM5δ (a RING-containing RBCC/TRIM family member) colocalizes with BTBD1/2 and serves as a scaffold for assembly of endogenous BTBD1/2 proteins at these bodies; deletion of TRIM5δ's coiled-coil region or mutation of its RING domain abolished colocalization.\",\n      \"method\": \"Immunofluorescence microscopy, GFP-fusion colocalization, expression of deletion/point mutants of TRIM5δ\",\n      \"journal\": \"Experimental cell research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 / Moderate — reciprocal colocalization with domain-mapping mutants in multiple cell lines, single lab\",\n      \"pmids\": [\"12878161\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Epitope-tagged BTBD2 localizes to cytoplasmic bodies; when truncated versions of BTBD2 and TOP1 were co-transfected to direct both proteins to the same compartment (nucleus or cytoplasm), co-localization of BTBD2 and TOP1 was demonstrated in HeLa cells.\",\n      \"method\": \"Subcellular localization by fluorescence microscopy, co-transfection with compartment-targeting truncation constructs\",\n      \"journal\": \"BMC genomics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — localization confirmed with compartment-redirecting truncations in a focused study, single lab\",\n      \"pmids\": [\"11818025\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"The interaction of BTBD2 with TOP1 requires human TOP1 residues 236 and 237 (the same residues that enhance HIV-1 virion infectivity). RNAi-mediated knockdown of BTBD2 in AGM and human 293T target cells increased permissiveness to HIV-1 infection two- to three-fold, indicating a modest restriction role.\",\n      \"method\": \"Mutagenesis of TOP1 residues 236–237, co-immunoprecipitation, RNAi knockdown with HIV-1 infectivity assay\",\n      \"journal\": \"Virology journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — site-directed mutagenesis combined with RNAi/infectivity assay, single lab, two orthogonal methods\",\n      \"pmids\": [\"21092135\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"In C. elegans, the BTBD-2 protein functions as an E3 substrate-adaptor component (together with CUL-3, RBX-1, and SIAH-1) downstream of HSF-1 in the ubiquitin–proteasome system to promote linker-cell-type (non-apoptotic) developmental cell death; genetic epistasis placed BTBD-2 in the LCD execution pathway.\",\n      \"method\": \"Genetic epistasis (C. elegans mutant analysis), molecular-genetic pathway delineation of LCD\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic epistasis in C. elegans with multiple pathway components tested, single lab\",\n      \"pmids\": [\"26952214\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"BTBD2 (together with BTBD1) acts as an adaptor protein for the E3-RING Cullin 3 (CUL3) ligase complex that ubiquitinates TOP1 cleavage complexes (TOP1-ccs), promoting their proteasomal degradation. NEDDylation of CUL3 activates this pathway, and depletion of CUL3 or inhibition of NEDDylation sensitizes cancer cells to TOP1 inhibitors.\",\n      \"method\": \"Genomic occupancy analysis, ubiquitination assays, proteasomal degradation assays, siRNA depletion, NEDDylation inhibition with small molecules, cell viability assays\",\n      \"journal\": \"Science advances\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Moderate — multiple orthogonal methods (ubiquitination assay, degradation assay, knockdown, small-molecule inhibition) in a single focused mechanistic study\",\n      \"pmids\": [\"36490343\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"BTBD2 interacts with HNF1β and enables the interaction and activation of Topoisomerase 1 (TOP1) specifically during mitosis, facilitating DNA relaxation around HNF1β mitotic chromatin sites and contributing to chromatin remodeling and gene reactivation after mitotic exit (bookmarking).\",\n      \"method\": \"Crosslinking-based chromatin immunoprecipitation, protein–protein interaction assay, TOP1 activity assay on mitotic chromatin, mutational analysis of HNF1β–TOP1 interaction domain\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal methods (crosslinking ChIP, interaction assay, enzymatic assay, mutagenesis) in a single lab study\",\n      \"pmids\": [\"39388351\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"BTBD2 is an adaptor protein that physically bridges Topoisomerase I (TOP1) to the CUL3-RBX1 E3 ubiquitin ligase complex (requiring TOP1 residues 236–237 for the interaction), promoting ubiquitination and proteasomal degradation of trapped TOP1 cleavage complexes in a NEDDylation-dependent manner; it additionally localizes to cytoplasmic bodies where it co-assembles with TRIM5δ, modestly restricts HIV-1 infection, and—during mitosis—bridges HNF1β to TOP1 to drive DNA relaxation at bookmarked chromatin sites, enabling post-mitotic gene reactivation.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"BTBD2 is a BTB-domain adaptor protein that couples Topoisomerase I (TOP1) to the cellular ubiquitin–proteasome machinery, governing both the turnover of damaged TOP1 and TOP1-dependent chromatin remodeling [#0, #5]. It binds the core domain of human TOP1 directly (an interaction mapped to TOP1 residues 215–329 and requiring residues 236–237) without itself altering TOP1's supercoil-relaxation or cleavage activity in vitro [#0, #3]. Functioning as a substrate-recognition adaptor for the CUL3–RBX1 E3 ubiquitin ligase, BTBD2 (with BTBD1) targets trapped TOP1 cleavage complexes for ubiquitination and proteasomal degradation; NEDDylation of CUL3 activates this pathway, and disabling CUL3 or NEDDylation sensitizes cancer cells to TOP1 inhibitors [#5]. The same TOP1-bridging activity is deployed during mitosis, where BTBD2 links HNF1β to TOP1 to drive DNA relaxation at bookmarked chromatin sites and enable post-mitotic gene reactivation [#6]. BTBD2 additionally assembles into cytoplasmic bodies scaffolded by TRIM5δ and modestly restricts HIV-1 infection [#1, #3].\",\n  \"teleology\": [\n    {\n      \"year\": 2002,\n      \"claim\": \"Established that BTBD2 is a direct TOP1-binding protein, raising the question of whether it modulates topoisomerase activity or serves another function.\",\n      \"evidence\": \"Yeast two-hybrid, co-precipitation and GST-pulldown from HeLa cells, with deletion mapping and in vitro topoisomerase assays\",\n      \"pmids\": [\"11818025\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Did not explain the functional consequence of the interaction, since TOP1 enzymatic activity was unaffected in vitro\",\n        \"No E3 ligase or ubiquitination role yet identified\"\n      ]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Defined the subcellular context of BTBD2, showing it concentrates in TRIM5δ-scaffolded cytoplasmic bodies and can co-localize with TOP1, indicating a regulated compartmentalization.\",\n      \"evidence\": \"Immunofluorescence, GFP-fusion colocalization, TRIM5δ domain/point mutants, and compartment-redirecting truncation constructs in mouse and human cells\",\n      \"pmids\": [\"12878161\", \"11818025\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Functional role of cytoplasmic body assembly not established\",\n        \"Whether TOP1 co-localization reflects a physiological pool versus overexpression artifact unclear\"\n      ]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Connected BTBD2 to a biological readout by pinpointing the TOP1 residues (236–237) required for binding and showing BTBD2 modestly restricts HIV-1 infection.\",\n      \"evidence\": \"Site-directed mutagenesis of TOP1, co-immunoprecipitation, and RNAi knockdown with HIV-1 infectivity assays in AGM and 293T cells\",\n      \"pmids\": [\"21092135\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Mechanism linking the TOP1 interaction to viral restriction not defined\",\n        \"Effect size was modest (two- to three-fold)\"\n      ]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Provided the first functional assignment of BTBD2 as a CUL3-pathway substrate adaptor, via its C. elegans ortholog acting in non-apoptotic developmental cell death.\",\n      \"evidence\": \"Genetic epistasis in C. elegans placing BTBD-2 with CUL-3, RBX-1 and SIAH-1 downstream of HSF-1 in the linker-cell death pathway\",\n      \"pmids\": [\"26952214\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Relevant ubiquitination substrate in the LCD pathway not identified\",\n        \"Conservation of the adaptor role to human BTBD2 not directly tested here\"\n      ]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Unified the prior observations into a mechanism: BTBD2 serves as a CUL3 substrate adaptor that ubiquitinates trapped TOP1 cleavage complexes for degradation, with therapeutic implications for TOP1 inhibitors.\",\n      \"evidence\": \"Ubiquitination and proteasomal degradation assays, siRNA depletion, NEDDylation inhibition, and cell viability assays in cancer cells\",\n      \"pmids\": [\"36490343\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Relative contributions of BTBD1 versus BTBD2 to substrate recognition not separated\",\n        \"Structural basis of TOP1-cc recognition not resolved\"\n      ]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Extended the TOP1-bridging function to a non-degradative, mitosis-specific role in chromatin bookmarking.\",\n      \"evidence\": \"Crosslinking ChIP, protein interaction assays, TOP1 activity assays on mitotic chromatin, and mutational analysis of the HNF1β–TOP1 interaction in mitotic cells\",\n      \"pmids\": [\"39388351\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"How BTBD2 switches between degradative (CUL3) and activating (HNF1β–TOP1) roles is unclear\",\n        \"Genome-wide scope of HNF1β bookmarking dependent on BTBD2 not defined\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How BTBD2's distinct activities — CUL3-dependent TOP1-cc degradation, mitotic HNF1β–TOP1 activation, cytoplasmic body assembly, and HIV restriction — are coordinated within a single cell remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"No structural model of BTBD2 bound to TOP1 or CUL3\",\n        \"Regulatory signals that partition BTBD2 between cytoplasmic bodies and nuclear/chromatin functions unknown\",\n        \"Whether cytoplasmic body localization relates mechanistically to the degradation or bookmarking roles is untested\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [0, 5, 6]},\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [5]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0031410\", \"supporting_discovery_ids\": [1, 2]},\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [6]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [5]},\n      {\"term_id\": \"R-HSA-73894\", \"supporting_discovery_ids\": [5]},\n      {\"term_id\": \"R-HSA-4839726\", \"supporting_discovery_ids\": [6]}\n    ],\n    \"complexes\": [\n      \"CUL3-RBX1 E3 ubiquitin ligase complex\",\n      \"TRIM5\\u03b4 cytoplasmic bodies\"\n    ],\n    \"partners\": [\n      \"TOP1\",\n      \"CUL3\",\n      \"RBX1\",\n      \"BTBD1\",\n      \"TRIM5\\u03b4\",\n      \"HNF1\\u03b2\"\n    ],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"faith_supported":5,"faith_total":5,"faith_pct":100.0}}