{"gene":"POLR2H","run_date":"2026-06-10T06:43:35","timeline":{"discoveries":[{"year":1990,"finding":"RPB8 (POLR2H) is a shared subunit present in all three yeast nuclear RNA polymerases (I, II, and III); the gene is single-copy, essential for viability, and immunoprecipitation of RPB6 co-precipitates proteins consistent with RNA Pol I, II, and III subunits, confirming shared incorporation.","method":"Gene isolation/sequencing, immunoprecipitation, sporulation/tetrad analysis","journal":"Genes & development","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal immunoprecipitation plus genetic viability assay, replicated across multiple subsequent studies in multiple organisms","pmids":["2186966"],"is_preprint":false},{"year":1995,"finding":"Human RPB8 (hRPB17/POLR2H) functionally complements the S. cerevisiae rpb8-Δ1 null mutant, demonstrating cross-species functional conservation, though with a strong thermosensitive phenotype indicating partial functional divergence.","method":"Heterospecific complementation of yeast null mutant","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — direct genetic complementation in null mutant, replicated for multiple subunits in same study with clear positive/negative controls","pmids":["7651387"],"is_preprint":false},{"year":1998,"finding":"The solution structure of yeast RPB8 (POLR2H ortholog) was determined by NMR and revealed an eight-stranded antiparallel beta-barrel with a large unstructured omega-loop, classifying it as a novel member of the oligonucleotide/oligosaccharide (OB)-binding protein family.","method":"NMR spectroscopy (solution structure determination)","journal":"Nature structural biology","confidence":"High","confidence_rationale":"Tier 1 / Strong — high-resolution NMR structure, confirmed by subsequent human RPB8 NMR structure","pmids":["9461075"],"is_preprint":false},{"year":2001,"finding":"Rpb8p interacts via its invariant GGLLM motif with a fragment conserved in the largest subunits of RNA polymerases I (Rpa190p), II (Rpb1p), and III (Rpc160p) corresponding to the pore 1 module; defective mutants at the GGLLM motif and two other conserved residues all impair this two-hybrid interaction.","method":"Two-hybrid interaction assay, site-directed mutagenesis","journal":"Molecular and cellular biology","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — two-hybrid plus mutagenesis in single lab, consistent mapping of interaction surface","pmids":["11486042"],"is_preprint":false},{"year":2001,"finding":"Rpb6p and Rpb8p functionally interact in vivo: increased gene dosage of Rpb6p suppresses the conditional growth defect caused by replacing Rpb8p with its human counterpart, and also suppresses an RNA polymerase I mutant (rpa190-G728D) at the pore 1 module.","method":"Genetic suppression (dosage suppressor analysis) in yeast","journal":"Molecular and cellular biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic epistasis/suppression, single lab but multiple alleles tested","pmids":["11486042"],"is_preprint":false},{"year":1998,"finding":"S. pombe Rpb8 cannot substitute for S. cerevisiae ABC14.5 (Rpb8) in vivo; amino acid differences in the N-terminal 67 residues are responsible for this functional distinction. Overexpression of Rpc1 (C160, largest RNAP III subunit) allows S. pombe rpb8 to functionally replace ABC14.5, revealing a specific genetic interaction between Rpb8 and the RNAP III largest subunit. S. pombe rpb8 selectively impairs RNA polymerase III but not RNA polymerase I complex assembly.","method":"Heterospecific complementation, dosage suppressor analysis, biochemical assembly analysis","journal":"Nucleic acids research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic complementation with multiple alleles plus biochemical assembly evidence, single lab","pmids":["9927738"],"is_preprint":false},{"year":2000,"finding":"In assembly experiments using baculovirus-expressed subunits, Rpb8 was incorporated into a seven-subunit complex containing Rpb1, Rpb2, Rpb3, Rpb5, Rpb7, Rpb8, and Rpb11. Rpb11 enhances Rpb3-Rpb8 interaction, and Rpb8 in turn enhances Rpb1-Rpb3 interaction, defining a sequential assembly mechanism.","method":"Recombinant co-expression in insect cells, GST and His-tag affinity chromatography","journal":"Nucleic acids research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reconstitution in insect cells with pairwise validation, single lab","pmids":["10648788"],"is_preprint":false},{"year":1998,"finding":"Far-Western blot analysis of S. pombe RNA polymerase II showed Rpb8 weakly interacts with Rpb5 and/or Rpb11 (a 15-kDa subunit), in addition to direct contacts with large subunits Rpb1 and Rpb2. Cross-linking experiments confirmed Rpb6-Rpb8 direct contact within the assembled polymerase.","method":"Far-Western blot, protein-protein cross-linking with bifunctional cross-linkers","journal":"Journal of molecular biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — two orthogonal methods (far-Western and cross-linking), single lab","pmids":["9642054"],"is_preprint":false},{"year":2006,"finding":"The three-dimensional solution structure of human RPB8 (hRPB8/POLR2H) was determined by NMR, revealing an eight-stranded beta-barrel, six short helices, and an unstructured Omega-loop belonging to the OB-fold family. hRPB8 has a more clustered positively charged interface with RPB1 compared to yeast RPB8. hRPB8 binds single-stranded DNA nonspecifically, with identified residues involved in binding and conformational changes upon DNA binding.","method":"NMR spectroscopy (structure determination), biochemical DNA-binding assays, NMR titration","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — NMR structure combined with biochemical binding assays and dynamic characterization, multiple orthogonal methods in one study","pmids":["16632472"],"is_preprint":false},{"year":2007,"finding":"BRCA1-BARD1 ubiquitin ligase complex mediates polyubiquitination of RPB8 (POLR2H) in response to DNA damage (UV irradiation and epirubicin). RPB8 physically interacts with BRCA1-BARD1. This ubiquitination does not destabilize RPB8 but increases soluble RPB8. Substitution of five lysine residues abolishing ubiquitination while preserving polymerase activity results in UV hypersensitivity and upregulated caspase activity in stable HeLa lines, indicating ubiquitination is required for cell survival after DNA damage.","method":"Proteomics screen, Co-immunoprecipitation, in vitro and in vivo ubiquitination assay, site-directed mutagenesis (K→R), stable cell line phenotyping, RNAi knockdown","journal":"Cancer research","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — in vitro and in vivo ubiquitination reconstitution, mutagenesis, and functional phenotype in cells; multiple orthogonal methods","pmids":["17283126"],"is_preprint":false},{"year":2010,"finding":"POLR2H (Rpb8) and Rpb1 form a cytoplasmic assembly intermediate associated with the HSP90 cochaperone hSpagh (RPAP3) and the R2TP/Prefoldin-like complex during RNA polymerase II biogenesis. HSP90 activity stabilizes incompletely assembled Rpb1 in the cytoplasm, and nuclear import of Rpb1 requires the presence of all subunits, indicating quality-control coupling of assembly to nuclear import.","method":"MS-based quantitative proteomics, characterization of assembly intermediates, HSP90 inhibition, nuclear fractionation, fluorescence microscopy","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 2 / Strong — quantitative MS proteomics plus pharmacological and genetic perturbation of assembly, replicated with multiple subunits and polymerases","pmids":["20864038"],"is_preprint":false},{"year":2012,"finding":"POLR2H (HaloTag-fused) simultaneously captures human RNA polymerases I, II, and III as confirmed by quantitative proteomics, identifying all RNAP core subunits and novel interacting partners, and the fusion protein reports POLR2H subcellular localization in live cells.","method":"HaloTag affinity purification, label-free quantitative proteomics, cellular imaging","journal":"Journal of proteome research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — affinity purification with quantitative proteomics confirms shared subunit membership in all three polymerases; single lab","pmids":["22149079"],"is_preprint":false},{"year":2000,"finding":"Immunofluorescence with monoclonal antibodies against RPB8 (shared by all three polymerases) reveals its localization to nucleoli and nucleoplasm in human cells, with redistribution upon treatment with transcriptional inhibitors DRB and actinomycin D, linking RPB8 localization to active transcription.","method":"Immunofluorescence with validated monoclonal antibodies, transcriptional inhibitor treatment","journal":"Experimental cell research","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — immunofluorescence with functionally validated antibodies, replicated across inhibitor conditions","pmids":["10623476"],"is_preprint":false},{"year":2025,"finding":"GLS1 (glutaminase 1) acts as a chaperone that interacts with POLR2H and POLR2E subunits to modulate the RNA pol II complex in the nucleus; overexpression of POLR2H (full-length but not truncated variants) abolishes GLS1's protective effects against alcohol-induced hepatic steatosis, placing POLR2H as a functional mediator of GLS1-dependent RNA pol II complex regulation.","method":"Co-immunoprecipitation (POLR2H-GLS1 interaction), hepatic overexpression/knockdown in vivo, truncation mutant analysis","journal":"Advanced science","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reciprocal co-IP plus in vivo rescue with truncation mutants in single lab study","pmids":["40936098"],"is_preprint":false}],"current_model":"POLR2H (RPB8) is a structurally characterized OB-fold subunit shared by all three nuclear RNA polymerases (I, II, III) that contacts the pore-1 module of the largest polymerase subunit via its conserved GGLLM motif, participates in cytoplasmic RNA Pol II assembly as part of an HSP90/R2TP-dependent quality-control pathway, and is polyubiquitinated by BRCA1-BARD1 in response to DNA damage to promote cell survival—a modification that increases soluble RPB8 without causing its degradation."},"narrative":{"mechanistic_narrative":"POLR2H (RPB8/hRPB17) is an essential, single-copy subunit shared by all three nuclear RNA polymerases (I, II, and III), incorporated into each enzyme as a structural component required for viability [PMID:2186966, PMID:22149079]. It adopts an eight-stranded antiparallel beta-barrel with an unstructured omega-loop, classifying it as an OB-fold protein, and the human protein binds single-stranded DNA nonspecifically [PMID:9461075, PMID:16632472]. Within the polymerase, RPB8 docks onto the pore-1 module of the largest subunit (Rpb1/Rpa190/Rpc160) through its invariant GGLLM motif, and functionally cooperates with RPB6, with which it makes direct contact in the assembled enzyme [PMID:11486042, PMID:9642054]. During RNA polymerase II biogenesis it forms a cytoplasmic assembly intermediate with Rpb1 that engages the HSP90 cochaperone RPAP3 and the R2TP/Prefoldin-like complex, coupling subunit assembly to a quality-control step that licenses nuclear import [PMID:20864038]. Beyond its constitutive structural role, RPB8 is polyubiquitinated by the BRCA1-BARD1 ubiquitin ligase in response to UV and chemotherapeutic DNA damage; this modification does not trigger degradation but increases soluble RPB8 and is required for cell survival, as ubiquitination-deficient cells become UV-hypersensitive with elevated caspase activity [PMID:17283126]. Assembly of the enzyme follows a defined order in which RPB11 promotes the RPB3-RPB8 interaction and RPB8 in turn enhances the RPB1-RPB3 interaction [PMID:10648788].","teleology":[{"year":1990,"claim":"Established that RPB8 is not a polymerase-II-specific factor but a single essential subunit physically shared by all three nuclear RNA polymerases, defining its core identity.","evidence":"Gene isolation, reciprocal immunoprecipitation, and tetrad viability analysis in S. cerevisiae","pmids":["2186966"],"confidence":"High","gaps":["Does not resolve the structural basis of incorporation","Stoichiometry and contacts within each polymerase not defined"]},{"year":1995,"claim":"Showed the human and yeast subunits are functionally interchangeable, establishing cross-species conservation while revealing partial divergence via a thermosensitive phenotype.","evidence":"Heterospecific complementation of a yeast rpb8 null with human RPB8","pmids":["7651387"],"confidence":"High","gaps":["Molecular basis of the thermosensitivity not identified","Does not map the divergent surfaces"]},{"year":1998,"claim":"Defined the fold of RPB8 as an OB-fold beta-barrel, providing the structural framework for understanding its binding surfaces and a candidate nucleic-acid-binding activity.","evidence":"NMR solution structure of yeast RPB8","pmids":["9461075"],"confidence":"High","gaps":["Function of the omega-loop unresolved","Ligand of the OB-fold not yet demonstrated"]},{"year":1998,"claim":"Mapped RPB8 contacts within the assembled polymerase and revealed species-specific genetic coupling to the RNAP III largest subunit, hinting at differential roles across the three enzymes.","evidence":"Far-Western, cross-linking, and heterospecific complementation/dosage suppression in S. pombe and S. cerevisiae","pmids":["9642054","9927738"],"confidence":"Medium","gaps":["Single-lab cross-linking contacts","Why S. pombe Rpb8 selectively impairs RNAP III assembly is unexplained"]},{"year":2000,"claim":"Reconstituted a sequential assembly pathway showing how RPB8 incorporation is ordered relative to RPB3 and RPB11, and localized the endogenous protein to nucleoli and nucleoplasm in a transcription-dependent manner.","evidence":"Recombinant insect-cell co-expression with affinity chromatography; immunofluorescence with transcriptional inhibitors","pmids":["10648788","10623476"],"confidence":"Medium","gaps":["In vitro assembly may not capture in vivo chaperone requirements","Redistribution mechanism upon inhibition not defined"]},{"year":2001,"claim":"Identified the GGLLM motif as the determinant of RPB8 binding to the pore-1 module of all three large subunits and demonstrated functional interplay with RPB6, linking a specific surface to enzyme function.","evidence":"Two-hybrid assays, site-directed mutagenesis, and dosage-suppressor genetics in yeast","pmids":["11486042"],"confidence":"Medium","gaps":["Two-hybrid interactions not confirmed structurally in context","Mechanism of RPB6 suppression unresolved"]},{"year":2006,"claim":"Determined the human RPB8 structure and demonstrated nonspecific single-stranded DNA binding, assigning a possible nucleic-acid-contacting activity to the human subunit.","evidence":"NMR structure determination, biochemical DNA-binding assays, and NMR titration","pmids":["16632472"],"confidence":"High","gaps":["Physiological relevance of ssDNA binding within the holoenzyme unknown","Sequence preference and in vivo target not established"]},{"year":2007,"claim":"Connected RPB8 to the DNA-damage response, showing BRCA1-BARD1 polyubiquitinates it to increase the soluble pool and promote survival rather than to target it for degradation.","evidence":"Proteomics, Co-IP, in vitro/in vivo ubiquitination, K→R mutagenesis, and stable cell-line phenotyping in HeLa","pmids":["17283126"],"confidence":"High","gaps":["Downstream fate of the soluble ubiquitinated pool unclear","How increased soluble RPB8 promotes survival mechanistically unresolved"]},{"year":2010,"claim":"Placed RPB8 in a cytoplasmic RNA Pol II assembly intermediate coupled to HSP90/RPAP3/R2TP chaperone-driven quality control, defining how assembly is linked to nuclear import.","evidence":"Quantitative MS proteomics, HSP90 inhibition, nuclear fractionation, and microscopy","pmids":["20864038"],"confidence":"High","gaps":["Order of RPB8 entry into the cytoplasmic intermediate not specified","Direct chaperone-RPB8 contacts not mapped"]},{"year":2012,"claim":"Validated RPB8 as a tractable shared bait that simultaneously captures all three human polymerases and reports its live-cell localization, consolidating its pan-polymerase membership in human cells.","evidence":"HaloTag affinity purification with label-free quantitative proteomics and imaging","pmids":["22149079"],"confidence":"Medium","gaps":["Novel partners not functionally characterized","Single-lab dataset"]},{"year":2025,"claim":"Implicated RPB8 as a functional mediator of GLS1-dependent RNA Pol II regulation in a hepatic disease context, expanding its role beyond a passive structural subunit.","evidence":"Reciprocal Co-IP, hepatic overexpression/knockdown in vivo, and truncation-mutant rescue in an alcohol-induced steatosis model","pmids":["40936098"],"confidence":"Medium","gaps":["Mechanism by which POLR2H abolishes GLS1 protection unresolved","Single-lab study in one disease model"]},{"year":null,"claim":"How RPB8's distinct activities — pore-1 anchoring, ssDNA binding, BRCA1-BARD1 ubiquitination, and chaperone-coupled assembly — are integrated to differentially regulate the three polymerases remains unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No unified model linking ubiquitination to assembly/survival","Functional consequence of ssDNA binding in the holoenzyme unknown","Polymerase-specific roles of RPB8 not dissected in human cells"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0005198","term_label":"structural molecule activity","supporting_discovery_ids":[0,2,8]},{"term_id":"GO:0003677","term_label":"DNA binding","supporting_discovery_ids":[8]}],"localization":[{"term_id":"GO:0005730","term_label":"nucleolus","supporting_discovery_ids":[12]},{"term_id":"GO:0005654","term_label":"nucleoplasm","supporting_discovery_ids":[12]},{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[10]}],"pathway":[{"term_id":"R-HSA-74160","term_label":"Gene expression (Transcription)","supporting_discovery_ids":[0,11]},{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[10]},{"term_id":"R-HSA-73894","term_label":"DNA Repair","supporting_discovery_ids":[9]}],"complexes":["RNA polymerase I","RNA polymerase II","RNA polymerase III","R2TP/Prefoldin-like complex"],"partners":["POLR2A","POLR2F","POLR2E","BRCA1","BARD1","RPAP3","GLS1","POLR2C"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"P52434","full_name":"DNA-directed RNA polymerases I, II, and III subunit RPABC3","aliases":["DNA-directed RNA polymerase II subunit H","DNA-directed RNA polymerases I, II, and III 17.1 kDa polypeptide","RPB17","RPB8 homolog","hRPB8"],"length_aa":150,"mass_kda":17.1,"function":"DNA-dependent RNA polymerase catalyzes the transcription of DNA into RNA using the four ribonucleoside triphosphates as substrates. Common component of RNA polymerases I, II and III which synthesize ribosomal RNA precursors, mRNA precursors and many functional non-coding RNAs, and small RNAs, such as 5S rRNA and tRNAs, respectively","subcellular_location":"Nucleus; Nucleus, nucleolus","url":"https://www.uniprot.org/uniprotkb/P52434/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":true,"resolved_as":"","url":"https://depmap.org/portal/gene/POLR2H","classification":"Common Essential","n_dependent_lines":1202,"n_total_lines":1208,"dependency_fraction":0.9950331125827815},"opencell":{"profiled":true,"resolved_as":"","ensg_id":"ENSG00000163882","cell_line_id":"CID000703","localizations":[{"compartment":"nucleolus_fc_dfc","grade":3},{"compartment":"nucleoplasm","grade":1}],"interactors":[{"gene":"POLR1C","stoichiometry":10.0},{"gene":"POLR1D","stoichiometry":10.0},{"gene":"POLR2B","stoichiometry":10.0},{"gene":"POLR2E","stoichiometry":10.0},{"gene":"POLR2F","stoichiometry":10.0},{"gene":"TRMT1L","stoichiometry":10.0},{"gene":"POLR2L","stoichiometry":10.0},{"gene":"PDRG1","stoichiometry":10.0},{"gene":"POLR3B","stoichiometry":10.0},{"gene":"POLR3H","stoichiometry":10.0}],"url":"https://opencell.sf.czbiohub.org/target/CID000703","total_profiled":1310},"omim":[{"mim_id":"606023","title":"POLYMERASE II, RNA, SUBUNIT H; POLR2H","url":"https://www.omim.org/entry/606023"},{"mim_id":"601189","title":"POLYMERASE II, RNA, SUBUNIT L; POLR2L","url":"https://www.omim.org/entry/601189"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Uncertain","locations":[{"location":"Nucleoplasm","reliability":"Uncertain"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/POLR2H"},"hgnc":{"alias_symbol":["RPB8"],"prev_symbol":[]},"alphafold":{"accession":"P52434","domains":[{"cath_id":"2.40.50.140","chopping":"3-147","consensus_level":"medium","plddt":85.432,"start":3,"end":147}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/P52434","model_url":"https://alphafold.ebi.ac.uk/files/AF-P52434-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-P52434-F1-predicted_aligned_error_v6.png","plddt_mean":84.25},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=POLR2H","jax_strain_url":"https://www.jax.org/strain/search?query=POLR2H"},"sequence":{"accession":"P52434","fasta_url":"https://rest.uniprot.org/uniprotkb/P52434.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/P52434/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/P52434"}},"corpus_meta":[{"pmid":"20864038","id":"PMC_20864038","title":"HSP90 and its R2TP/Prefoldin-like 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Part A","url":"https://pubmed.ncbi.nlm.nih.gov/41804662","citation_count":0,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":24580,"output_tokens":3381,"usd":0.062227,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":10908,"output_tokens":3587,"usd":0.072108,"stage2_stop_reason":"end_turn"},"total_usd":0.134335,"stage1_batch_id":"msgbatch_01CNoDbkChHEkgrJnL7BwtGJ","stage2_batch_id":"msgbatch_016Y4x9E9Wg5VQDANj9quSQD","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1990,\n      \"finding\": \"RPB8 (POLR2H) is a shared subunit present in all three yeast nuclear RNA polymerases (I, II, and III); the gene is single-copy, essential for viability, and immunoprecipitation of RPB6 co-precipitates proteins consistent with RNA Pol I, II, and III subunits, confirming shared incorporation.\",\n      \"method\": \"Gene isolation/sequencing, immunoprecipitation, sporulation/tetrad analysis\",\n      \"journal\": \"Genes & development\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal immunoprecipitation plus genetic viability assay, replicated across multiple subsequent studies in multiple organisms\",\n      \"pmids\": [\"2186966\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"Human RPB8 (hRPB17/POLR2H) functionally complements the S. cerevisiae rpb8-Δ1 null mutant, demonstrating cross-species functional conservation, though with a strong thermosensitive phenotype indicating partial functional divergence.\",\n      \"method\": \"Heterospecific complementation of yeast null mutant\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — direct genetic complementation in null mutant, replicated for multiple subunits in same study with clear positive/negative controls\",\n      \"pmids\": [\"7651387\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"The solution structure of yeast RPB8 (POLR2H ortholog) was determined by NMR and revealed an eight-stranded antiparallel beta-barrel with a large unstructured omega-loop, classifying it as a novel member of the oligonucleotide/oligosaccharide (OB)-binding protein family.\",\n      \"method\": \"NMR spectroscopy (solution structure determination)\",\n      \"journal\": \"Nature structural biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — high-resolution NMR structure, confirmed by subsequent human RPB8 NMR structure\",\n      \"pmids\": [\"9461075\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"Rpb8p interacts via its invariant GGLLM motif with a fragment conserved in the largest subunits of RNA polymerases I (Rpa190p), II (Rpb1p), and III (Rpc160p) corresponding to the pore 1 module; defective mutants at the GGLLM motif and two other conserved residues all impair this two-hybrid interaction.\",\n      \"method\": \"Two-hybrid interaction assay, site-directed mutagenesis\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — two-hybrid plus mutagenesis in single lab, consistent mapping of interaction surface\",\n      \"pmids\": [\"11486042\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"Rpb6p and Rpb8p functionally interact in vivo: increased gene dosage of Rpb6p suppresses the conditional growth defect caused by replacing Rpb8p with its human counterpart, and also suppresses an RNA polymerase I mutant (rpa190-G728D) at the pore 1 module.\",\n      \"method\": \"Genetic suppression (dosage suppressor analysis) in yeast\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic epistasis/suppression, single lab but multiple alleles tested\",\n      \"pmids\": [\"11486042\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"S. pombe Rpb8 cannot substitute for S. cerevisiae ABC14.5 (Rpb8) in vivo; amino acid differences in the N-terminal 67 residues are responsible for this functional distinction. Overexpression of Rpc1 (C160, largest RNAP III subunit) allows S. pombe rpb8 to functionally replace ABC14.5, revealing a specific genetic interaction between Rpb8 and the RNAP III largest subunit. S. pombe rpb8 selectively impairs RNA polymerase III but not RNA polymerase I complex assembly.\",\n      \"method\": \"Heterospecific complementation, dosage suppressor analysis, biochemical assembly analysis\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic complementation with multiple alleles plus biochemical assembly evidence, single lab\",\n      \"pmids\": [\"9927738\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"In assembly experiments using baculovirus-expressed subunits, Rpb8 was incorporated into a seven-subunit complex containing Rpb1, Rpb2, Rpb3, Rpb5, Rpb7, Rpb8, and Rpb11. Rpb11 enhances Rpb3-Rpb8 interaction, and Rpb8 in turn enhances Rpb1-Rpb3 interaction, defining a sequential assembly mechanism.\",\n      \"method\": \"Recombinant co-expression in insect cells, GST and His-tag affinity chromatography\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reconstitution in insect cells with pairwise validation, single lab\",\n      \"pmids\": [\"10648788\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"Far-Western blot analysis of S. pombe RNA polymerase II showed Rpb8 weakly interacts with Rpb5 and/or Rpb11 (a 15-kDa subunit), in addition to direct contacts with large subunits Rpb1 and Rpb2. Cross-linking experiments confirmed Rpb6-Rpb8 direct contact within the assembled polymerase.\",\n      \"method\": \"Far-Western blot, protein-protein cross-linking with bifunctional cross-linkers\",\n      \"journal\": \"Journal of molecular biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — two orthogonal methods (far-Western and cross-linking), single lab\",\n      \"pmids\": [\"9642054\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"The three-dimensional solution structure of human RPB8 (hRPB8/POLR2H) was determined by NMR, revealing an eight-stranded beta-barrel, six short helices, and an unstructured Omega-loop belonging to the OB-fold family. hRPB8 has a more clustered positively charged interface with RPB1 compared to yeast RPB8. hRPB8 binds single-stranded DNA nonspecifically, with identified residues involved in binding and conformational changes upon DNA binding.\",\n      \"method\": \"NMR spectroscopy (structure determination), biochemical DNA-binding assays, NMR titration\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — NMR structure combined with biochemical binding assays and dynamic characterization, multiple orthogonal methods in one study\",\n      \"pmids\": [\"16632472\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"BRCA1-BARD1 ubiquitin ligase complex mediates polyubiquitination of RPB8 (POLR2H) in response to DNA damage (UV irradiation and epirubicin). RPB8 physically interacts with BRCA1-BARD1. This ubiquitination does not destabilize RPB8 but increases soluble RPB8. Substitution of five lysine residues abolishing ubiquitination while preserving polymerase activity results in UV hypersensitivity and upregulated caspase activity in stable HeLa lines, indicating ubiquitination is required for cell survival after DNA damage.\",\n      \"method\": \"Proteomics screen, Co-immunoprecipitation, in vitro and in vivo ubiquitination assay, site-directed mutagenesis (K→R), stable cell line phenotyping, RNAi knockdown\",\n      \"journal\": \"Cancer research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — in vitro and in vivo ubiquitination reconstitution, mutagenesis, and functional phenotype in cells; multiple orthogonal methods\",\n      \"pmids\": [\"17283126\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"POLR2H (Rpb8) and Rpb1 form a cytoplasmic assembly intermediate associated with the HSP90 cochaperone hSpagh (RPAP3) and the R2TP/Prefoldin-like complex during RNA polymerase II biogenesis. HSP90 activity stabilizes incompletely assembled Rpb1 in the cytoplasm, and nuclear import of Rpb1 requires the presence of all subunits, indicating quality-control coupling of assembly to nuclear import.\",\n      \"method\": \"MS-based quantitative proteomics, characterization of assembly intermediates, HSP90 inhibition, nuclear fractionation, fluorescence microscopy\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — quantitative MS proteomics plus pharmacological and genetic perturbation of assembly, replicated with multiple subunits and polymerases\",\n      \"pmids\": [\"20864038\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"POLR2H (HaloTag-fused) simultaneously captures human RNA polymerases I, II, and III as confirmed by quantitative proteomics, identifying all RNAP core subunits and novel interacting partners, and the fusion protein reports POLR2H subcellular localization in live cells.\",\n      \"method\": \"HaloTag affinity purification, label-free quantitative proteomics, cellular imaging\",\n      \"journal\": \"Journal of proteome research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — affinity purification with quantitative proteomics confirms shared subunit membership in all three polymerases; single lab\",\n      \"pmids\": [\"22149079\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"Immunofluorescence with monoclonal antibodies against RPB8 (shared by all three polymerases) reveals its localization to nucleoli and nucleoplasm in human cells, with redistribution upon treatment with transcriptional inhibitors DRB and actinomycin D, linking RPB8 localization to active transcription.\",\n      \"method\": \"Immunofluorescence with validated monoclonal antibodies, transcriptional inhibitor treatment\",\n      \"journal\": \"Experimental cell research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — immunofluorescence with functionally validated antibodies, replicated across inhibitor conditions\",\n      \"pmids\": [\"10623476\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"GLS1 (glutaminase 1) acts as a chaperone that interacts with POLR2H and POLR2E subunits to modulate the RNA pol II complex in the nucleus; overexpression of POLR2H (full-length but not truncated variants) abolishes GLS1's protective effects against alcohol-induced hepatic steatosis, placing POLR2H as a functional mediator of GLS1-dependent RNA pol II complex regulation.\",\n      \"method\": \"Co-immunoprecipitation (POLR2H-GLS1 interaction), hepatic overexpression/knockdown in vivo, truncation mutant analysis\",\n      \"journal\": \"Advanced science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal co-IP plus in vivo rescue with truncation mutants in single lab study\",\n      \"pmids\": [\"40936098\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"POLR2H (RPB8) is a structurally characterized OB-fold subunit shared by all three nuclear RNA polymerases (I, II, III) that contacts the pore-1 module of the largest polymerase subunit via its conserved GGLLM motif, participates in cytoplasmic RNA Pol II assembly as part of an HSP90/R2TP-dependent quality-control pathway, and is polyubiquitinated by BRCA1-BARD1 in response to DNA damage to promote cell survival—a modification that increases soluble RPB8 without causing its degradation.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"POLR2H (RPB8/hRPB17) is an essential, single-copy subunit shared by all three nuclear RNA polymerases (I, II, and III), incorporated into each enzyme as a structural component required for viability [#0, #11]. It adopts an eight-stranded antiparallel beta-barrel with an unstructured omega-loop, classifying it as an OB-fold protein, and the human protein binds single-stranded DNA nonspecifically [#2, #8]. Within the polymerase, RPB8 docks onto the pore-1 module of the largest subunit (Rpb1/Rpa190/Rpc160) through its invariant GGLLM motif, and functionally cooperates with RPB6, with which it makes direct contact in the assembled enzyme [#3, #7]. During RNA polymerase II biogenesis it forms a cytoplasmic assembly intermediate with Rpb1 that engages the HSP90 cochaperone RPAP3 and the R2TP/Prefoldin-like complex, coupling subunit assembly to a quality-control step that licenses nuclear import [#10]. Beyond its constitutive structural role, RPB8 is polyubiquitinated by the BRCA1-BARD1 ubiquitin ligase in response to UV and chemotherapeutic DNA damage; this modification does not trigger degradation but increases soluble RPB8 and is required for cell survival, as ubiquitination-deficient cells become UV-hypersensitive with elevated caspase activity [#9]. Assembly of the enzyme follows a defined order in which RPB11 promotes the RPB3-RPB8 interaction and RPB8 in turn enhances the RPB1-RPB3 interaction [#6].\",\n  \"teleology\": [\n    {\n      \"year\": 1990,\n      \"claim\": \"Established that RPB8 is not a polymerase-II-specific factor but a single essential subunit physically shared by all three nuclear RNA polymerases, defining its core identity.\",\n      \"evidence\": \"Gene isolation, reciprocal immunoprecipitation, and tetrad viability analysis in S. cerevisiae\",\n      \"pmids\": [\"2186966\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Does not resolve the structural basis of incorporation\", \"Stoichiometry and contacts within each polymerase not defined\"]\n    },\n    {\n      \"year\": 1995,\n      \"claim\": \"Showed the human and yeast subunits are functionally interchangeable, establishing cross-species conservation while revealing partial divergence via a thermosensitive phenotype.\",\n      \"evidence\": \"Heterospecific complementation of a yeast rpb8 null with human RPB8\",\n      \"pmids\": [\"7651387\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular basis of the thermosensitivity not identified\", \"Does not map the divergent surfaces\"]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Defined the fold of RPB8 as an OB-fold beta-barrel, providing the structural framework for understanding its binding surfaces and a candidate nucleic-acid-binding activity.\",\n      \"evidence\": \"NMR solution structure of yeast RPB8\",\n      \"pmids\": [\"9461075\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Function of the omega-loop unresolved\", \"Ligand of the OB-fold not yet demonstrated\"]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Mapped RPB8 contacts within the assembled polymerase and revealed species-specific genetic coupling to the RNAP III largest subunit, hinting at differential roles across the three enzymes.\",\n      \"evidence\": \"Far-Western, cross-linking, and heterospecific complementation/dosage suppression in S. pombe and S. cerevisiae\",\n      \"pmids\": [\"9642054\", \"9927738\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab cross-linking contacts\", \"Why S. pombe Rpb8 selectively impairs RNAP III assembly is unexplained\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"Reconstituted a sequential assembly pathway showing how RPB8 incorporation is ordered relative to RPB3 and RPB11, and localized the endogenous protein to nucleoli and nucleoplasm in a transcription-dependent manner.\",\n      \"evidence\": \"Recombinant insect-cell co-expression with affinity chromatography; immunofluorescence with transcriptional inhibitors\",\n      \"pmids\": [\"10648788\", \"10623476\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"In vitro assembly may not capture in vivo chaperone requirements\", \"Redistribution mechanism upon inhibition not defined\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Identified the GGLLM motif as the determinant of RPB8 binding to the pore-1 module of all three large subunits and demonstrated functional interplay with RPB6, linking a specific surface to enzyme function.\",\n      \"evidence\": \"Two-hybrid assays, site-directed mutagenesis, and dosage-suppressor genetics in yeast\",\n      \"pmids\": [\"11486042\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Two-hybrid interactions not confirmed structurally in context\", \"Mechanism of RPB6 suppression unresolved\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Determined the human RPB8 structure and demonstrated nonspecific single-stranded DNA binding, assigning a possible nucleic-acid-contacting activity to the human subunit.\",\n      \"evidence\": \"NMR structure determination, biochemical DNA-binding assays, and NMR titration\",\n      \"pmids\": [\"16632472\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Physiological relevance of ssDNA binding within the holoenzyme unknown\", \"Sequence preference and in vivo target not established\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Connected RPB8 to the DNA-damage response, showing BRCA1-BARD1 polyubiquitinates it to increase the soluble pool and promote survival rather than to target it for degradation.\",\n      \"evidence\": \"Proteomics, Co-IP, in vitro/in vivo ubiquitination, K\\u2192R mutagenesis, and stable cell-line phenotyping in HeLa\",\n      \"pmids\": [\"17283126\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Downstream fate of the soluble ubiquitinated pool unclear\", \"How increased soluble RPB8 promotes survival mechanistically unresolved\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Placed RPB8 in a cytoplasmic RNA Pol II assembly intermediate coupled to HSP90/RPAP3/R2TP chaperone-driven quality control, defining how assembly is linked to nuclear import.\",\n      \"evidence\": \"Quantitative MS proteomics, HSP90 inhibition, nuclear fractionation, and microscopy\",\n      \"pmids\": [\"20864038\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Order of RPB8 entry into the cytoplasmic intermediate not specified\", \"Direct chaperone-RPB8 contacts not mapped\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Validated RPB8 as a tractable shared bait that simultaneously captures all three human polymerases and reports its live-cell localization, consolidating its pan-polymerase membership in human cells.\",\n      \"evidence\": \"HaloTag affinity purification with label-free quantitative proteomics and imaging\",\n      \"pmids\": [\"22149079\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Novel partners not functionally characterized\", \"Single-lab dataset\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Implicated RPB8 as a functional mediator of GLS1-dependent RNA Pol II regulation in a hepatic disease context, expanding its role beyond a passive structural subunit.\",\n      \"evidence\": \"Reciprocal Co-IP, hepatic overexpression/knockdown in vivo, and truncation-mutant rescue in an alcohol-induced steatosis model\",\n      \"pmids\": [\"40936098\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism by which POLR2H abolishes GLS1 protection unresolved\", \"Single-lab study in one disease model\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How RPB8's distinct activities — pore-1 anchoring, ssDNA binding, BRCA1-BARD1 ubiquitination, and chaperone-coupled assembly — are integrated to differentially regulate the three polymerases remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No unified model linking ubiquitination to assembly/survival\", \"Functional consequence of ssDNA binding in the holoenzyme unknown\", \"Polymerase-specific roles of RPB8 not dissected in human cells\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [0, 2, 8]},\n      {\"term_id\": \"GO:0003677\", \"supporting_discovery_ids\": [8]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005730\", \"supporting_discovery_ids\": [12]},\n      {\"term_id\": \"GO:0005654\", \"supporting_discovery_ids\": [12]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [10]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-74160\", \"supporting_discovery_ids\": [0, 11]},\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [10]},\n      {\"term_id\": \"R-HSA-73894\", \"supporting_discovery_ids\": [9]}\n    ],\n    \"complexes\": [\"RNA polymerase I\", \"RNA polymerase II\", \"RNA polymerase III\", \"R2TP/Prefoldin-like complex\"],\n    \"partners\": [\"POLR2A\", \"POLR2F\", \"POLR2E\", \"BRCA1\", \"BARD1\", \"RPAP3\", \"GLS1\", \"POLR2C\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":6,"faith_total":6,"faith_pct":100.0}}