{"gene":"GPN2","run_date":"2026-06-10T01:55:21","timeline":{"discoveries":[{"year":2012,"finding":"Yeast Gpn2 binds both Gpn3 and Npa3/Gpn1, forming a network of GPN protein interactions. Temperature-sensitive alleles of GPN2 cause defects in RNA polymerase II nuclear localization and genetic interactions with RNAPII mutants. GPN2 mutants also show RNA polymerase III nuclear localization defects. The nuclear import defect of iwr1Δ (but not gpn2 mutants) is suppressed by NLS fusion to Rpb3, suggesting GPN proteins function upstream of Iwr1 in RNAPII/III biogenesis.","method":"Temperature-sensitive allele genetics, fluorescence microscopy for polymerase localization, genetic interaction/suppression analysis, binding assays","journal":"Genetics","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods (genetic interactions, localization, epistasis with iwr1Δ NLS suppression), replicated across multiple alleles and polymerases","pmids":["23267056"],"is_preprint":false},{"year":2018,"finding":"Gpn2 directly interacts with the RNAPII subunit Rpb12, and also interacts with Rba50 (which itself binds Rpb3). Gpn2 and Rba50 together are required for assembly of the Rpb3 subcomplex; when either is functionally defective, Rpb3 subcomplex assembly is disrupted, blocking overall RNAPII assembly.","method":"Co-immunoprecipitation, pulldown assays, temperature-sensitive mutant analysis of RNAPII subcomplex assembly","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 2 / Moderate — reciprocal Co-IP interactions demonstrated, functional defect in assembly confirmed by independent loss-of-function experiments, single lab with multiple orthogonal methods","pmids":["29661922"],"is_preprint":false},{"year":2021,"finding":"A genome-wide screen of 1350 GFP-tagged nuclear proteins in GPN2 mutant yeast showed that the strongest and most specific mislocalization effects were for RNAPII and RNAPIII subunits, with only a handful of other RNAPII-associated proteins affected. Additionally, Ess1 (an Rpb1 CTD prolyl isomerase) was found to be mislocalized in gpn2 mutants, and disruption of Rpb1-CTD kinases or phosphatases altered Rpb1 nuclear-cytoplasmic distribution, linking CTD modification status to RNAPII nuclear localization downstream of Gpn2.","method":"High-content fluorescence microscopy screen of GFP-tagged nuclear proteome in gpn2 mutant yeast; genetic analysis with CTD kinase/phosphatase mutants","journal":"Cell cycle (Georgetown, Tex.)","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — large-scale localization screen with functional follow-up, single lab, demonstrates exquisite specificity for RNA polymerases","pmids":["34180355"],"is_preprint":false},{"year":2022,"finding":"Rba50 and Gpn2 cooperate to recruit Rpb2 (second largest subunit of RNAPII) during assembly steps following Rpb3 subcomplex formation. Gpn2 facilitates the association of Rba50 and Rpb2. Both gpn2-R347S and rpb2-V1171G variants suppress rba50-3 mutant defects. The Rba50-Gpn2 complex appears to play a similar role in RNAPIII assembly.","method":"Extragenic suppressor mapping, multicopy suppressor screening, rapid depletion of Rba50 followed by co-immunoprecipitation of Rpb3-Rpb2, genetic epistasis","journal":"International journal of biological macromolecules","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic suppressor screen plus biochemical (Co-IP) confirmation of Rba50-Rpb2 association defects, single lab","pmids":["35176321"],"is_preprint":false},{"year":2022,"finding":"Inactivation of Gpn2 (as well as Npa3/Gpn1 and Gpn3) leads to reversible accumulation of RNAPII subunits (Rpb1, Rpb2, Rpb3) in cytoplasmic foci, a stress response termed RNAPII Assembly Stress Response (RASR). These foci are protein-based, nucleic acid-free condensates that resist 1,6-hexanediol dissolution and show dynamic FRAP behavior. Molecular chaperone Hsp82 colocalizes with these foci.","method":"Fluorescence microscopy, FRAP, hexanediol treatment, biochemical fractionation, GFP-tagging of RNAPII subunits in gpn2 mutant yeast","journal":"International journal of biological macromolecules","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal methods (FRAP, hexanediol, fractionation) in a single lab demonstrating cytoplasmic condensate properties","pmids":["35314265"],"is_preprint":false},{"year":2024,"finding":"Specific mutations in Gpn2 (Phe105Tyr and Leu164Pro) confer temperature sensitivity and significantly impair RNAPII assembly. Multicopy suppressor screening identified 31 genes (including PAB1, CDC5, and RGS2) whose overexpression mitigates gpn2ts growth defects, providing functional insights into Gpn2's role in RNAPII assembly.","method":"Large-scale multicopy suppressor screen (>30,000 colonies), temperature-sensitive mutant analysis, site-specific mutagenesis","journal":"PloS one","confidence":"Low","confidence_rationale":"Tier 3 / Weak — suppressor screen identifies genetic interactions but does not establish direct mechanistic relationships for the identified suppressors; single lab, single method per finding","pmids":["39642114"],"is_preprint":false},{"year":2026,"finding":"Inactivation of all three GPN proteins including Gpn2 triggers reversible RNAPII Assembly Stress Response (RASR) foci containing Rpb1, Rpb2, and Rpb3. Hsp82 partially colocalizes with these foci. Oxidative stress (H2O2) increases foci formation, revealing redox sensitivity. Transcriptomic profiling during RASR shows coordinated regulation of ribosome biogenesis genes and metabolic pathways.","method":"Fluorescence microscopy, FRAP, biochemical condensate characterization, RNA-seq transcriptomic profiling, oxidative stress treatment","journal":"International journal of biological macromolecules","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal methods (FRAP, biochemistry, transcriptomics) in single lab extending prior findings on RASR","pmids":["41500282"],"is_preprint":false}],"current_model":"GPN2 encodes a conserved GPN-loop GTPase that functions as an assembly chaperone for RNA polymerase II (and III) biogenesis: it forms a complex with GPN3 and GPN1/Npa3, directly interacts with RNAPII subunit Rpb12, and cooperates with assembly factor Rba50 to facilitate sequential assembly of the Rpb3 subcomplex and subsequent recruitment of Rpb2, acting upstream of the import factor Iwr1 to ensure proper nuclear localization of both RNAPII and RNAPIII; loss of Gpn2 function causes cytoplasmic accumulation of RNAPII/III subunits into reversible, Hsp82-containing condensates as part of a stress response (RASR) triggered by impaired polymerase assembly."},"narrative":{"mechanistic_narrative":"GPN2 encodes a conserved GPN-loop GTPase that acts as an assembly chaperone in the biogenesis of RNA polymerases II and III [PMID:23267056]. It operates within a network of GPN proteins, binding both Gpn3 and Npa3/Gpn1, and temperature-sensitive loss of Gpn2 function causes specific defects in the nuclear localization of RNAPII and RNAPIII, placing GPN proteins upstream of the dedicated import factor Iwr1 [PMID:23267056]. Mechanistically, Gpn2 directly contacts the RNAPII subunit Rpb12 and cooperates with the assembly factor Rba50 to drive ordered subunit assembly: the Gpn2-Rba50 pair is required for formation of the Rpb3 subcomplex and then jointly recruits Rpb2 to the nascent polymerase, with Gpn2 facilitating the Rba50-Rpb2 association [PMID:29661922, PMID:35176321]. A genome-wide localization screen established that this function is exquisitely specific, as RNAPII and RNAPIII subunits are the dominant proteins mislocalized upon Gpn2 inactivation, with CTD modification status linked to RNAPII nuclear distribution downstream of Gpn2 [PMID:34180355]. When Gpn2 (or its partners Gpn1 and Gpn3) is inactivated, unassembled RNAPII subunits accumulate in the cytoplasm as reversible, nucleic-acid-free, Hsp82-containing condensates in a redox-sensitive stress response termed the RNAPII Assembly Stress Response (RASR) [PMID:35314265, PMID:41500282].","teleology":[{"year":2012,"claim":"Established that Gpn2 is part of a GPN protein interaction network and is functionally required for nuclear localization of RNA polymerases II and III, acting upstream of the import factor Iwr1.","evidence":"Temperature-sensitive allele genetics, polymerase localization microscopy, and epistasis with iwr1Δ NLS suppression in yeast","pmids":["23267056"],"confidence":"High","gaps":["Did not define the biochemical step (assembly vs. import) at which Gpn2 acts","No direct RNAPII subunit contact identified","GTPase activity of Gpn2 not linked to function"]},{"year":2018,"claim":"Identified the direct molecular target of Gpn2 in polymerase assembly, showing it contacts Rpb12 and cooperates with Rba50 to build the Rpb3 subcomplex.","evidence":"Co-immunoprecipitation, pulldown, and temperature-sensitive mutant analysis of RNAPII subcomplex assembly in yeast","pmids":["29661922"],"confidence":"High","gaps":["Order of subunit addition relative to Rpb2 not yet resolved","No structural model of the Gpn2-Rba50-Rpb3 intermediate","Role of nucleotide binding/hydrolysis in assembly unaddressed"]},{"year":2021,"claim":"Demonstrated the remarkable substrate specificity of Gpn2, with RNAPII/III subunits as the dominant mislocalized proteins, and connected CTD phosphorylation to RNAPII nuclear distribution.","evidence":"High-content GFP-tagged nuclear proteome localization screen plus CTD kinase/phosphatase mutant analysis in gpn2 mutant yeast","pmids":["34180355"],"confidence":"Medium","gaps":["Mechanistic link between Ess1/CTD status and Gpn2 function not established","Screen reports localization, not direct binding","Specificity determinants on Gpn2 not mapped"]},{"year":2022,"claim":"Resolved a later assembly step, showing Gpn2 and Rba50 cooperate to recruit Rpb2 after Rpb3 subcomplex formation, extending the role to RNAPIII assembly.","evidence":"Extragenic and multicopy suppressor mapping, Rba50 depletion with Rpb3-Rpb2 Co-IP, and genetic epistasis in yeast","pmids":["35176321"],"confidence":"Medium","gaps":["Single lab; reciprocal in vitro reconstitution of Rpb2 recruitment absent","How suppressor variants restore assembly mechanistically unclear","Quantitative kinetics of assembly steps undefined"]},{"year":2022,"claim":"Revealed the cellular consequence of failed assembly: Gpn2 inactivation drives reversible cytoplasmic condensation of unassembled RNAPII subunits (RASR) chaperoned by Hsp82.","evidence":"Fluorescence microscopy, FRAP, 1,6-hexanediol treatment, and biochemical fractionation of GFP-tagged subunits in gpn2 mutant yeast","pmids":["35314265"],"confidence":"Medium","gaps":["Whether RASR is protective or pathological is not resolved","Mechanism of Hsp82 recruitment to foci unknown","Reversibility pathway / disaggregation machinery not identified"]},{"year":2024,"claim":"Mapped specific Gpn2 residues (Phe105Tyr, Leu164Pro) required for RNAPII assembly and surveyed genetic modifiers of gpn2 defects.","evidence":"Site-specific mutagenesis and large-scale multicopy suppressor screen (>30,000 colonies) in yeast","pmids":["39642114"],"confidence":"Low","gaps":["Suppressors (PAB1, CDC5, RGS2) are genetic interactions without established direct mechanism","Structural consequences of the residue substitutions not determined","Functional connection of identified suppressors to assembly untested"]},{"year":2026,"claim":"Extended the RASR model by showing oxidative stress modulates foci formation and that RASR coordinates transcriptional reprogramming of ribosome biogenesis and metabolic genes.","evidence":"Fluorescence microscopy, FRAP, condensate biochemistry, oxidative stress treatment, and RNA-seq transcriptomics in GPN-inactivated yeast","pmids":["41500282"],"confidence":"Medium","gaps":["Redox sensor governing foci formation not identified","Direct causal link between RASR and transcriptomic changes not established","Whether redox sensitivity reflects a normal Gpn2 regulatory mechanism is unknown"]},{"year":null,"claim":"How nucleotide binding/hydrolysis by the GPN-loop GTPase drives the ordered assembly cycle, and how the Gpn2-Gpn1-Gpn3 complex is structurally organized on nascent polymerase, remain unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No structure of the GPN complex bound to RNAPII assembly intermediates","Catalytic cycle of Gpn2 not linked to specific assembly transitions","Function in organisms beyond yeast not characterized in the corpus"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0003924","term_label":"GTPase activity","supporting_discovery_ids":[0]},{"term_id":"GO:0044183","term_label":"protein folding chaperone","supporting_discovery_ids":[1,3]}],"localization":[{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[4]},{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[0]}],"pathway":[{"term_id":"R-HSA-74160","term_label":"Gene expression (Transcription)","supporting_discovery_ids":[0,1]},{"term_id":"R-HSA-8953854","term_label":"Metabolism of RNA","supporting_discovery_ids":[1,3]}],"complexes":["GPN protein complex (Gpn1/Gpn2/Gpn3)","Gpn2-Rba50 assembly module"],"partners":["GPN3","GPN1","RPB12","RBA50","RPB2","RPB3","HSP82"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q9H9Y4","full_name":"GPN-loop GTPase 2","aliases":["ATP-binding domain 1 family member B"],"length_aa":310,"mass_kda":34.6,"function":"Small GTPase involved in the correct assembly of RNA polymerase RNA polymerase II and III (RNAPII and RNAPIII) complexes, ensuring their proper nuclear import","subcellular_location":"Cytoplasm","url":"https://www.uniprot.org/uniprotkb/Q9H9Y4/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":true,"resolved_as":"","url":"https://depmap.org/portal/gene/GPN2","classification":"Common Essential","n_dependent_lines":1201,"n_total_lines":1208,"dependency_fraction":0.9942052980132451},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[{"gene":"POLR2B","stoichiometry":10.0},{"gene":"POLR2C","stoichiometry":10.0},{"gene":"POLR2K","stoichiometry":4.0},{"gene":"POLR2J","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/search/GPN2","total_profiled":1310},"omim":[{"mim_id":"621545","title":"GPN-LOOP GTPase 3; GPN3","url":"https://www.omim.org/entry/621545"},{"mim_id":"621544","title":"GPN-LOOP GTPase 2; GPN2","url":"https://www.omim.org/entry/621544"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/GPN2"},"hgnc":{"alias_symbol":["FLJ10349"],"prev_symbol":["ATPBD1B"]},"alphafold":{"accession":"Q9H9Y4","domains":[{"cath_id":"3.40.50.300","chopping":"10-148_171-183_237-263","consensus_level":"high","plddt":93.7732,"start":10,"end":263}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9H9Y4","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9H9Y4-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9H9Y4-F1-predicted_aligned_error_v6.png","plddt_mean":84.38},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=GPN2","jax_strain_url":"https://www.jax.org/strain/search?query=GPN2"},"sequence":{"accession":"Q9H9Y4","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9H9Y4.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9H9Y4/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9H9Y4"}},"corpus_meta":[{"pmid":"17224219","id":"PMC_17224219","title":"Generation and immunogenicity of novel HIV/AIDS vaccine candidates targeting HIV-1 Env/Gag-Pol-Nef antigens of clade C.","date":"2006","source":"Vaccine","url":"https://pubmed.ncbi.nlm.nih.gov/17224219","citation_count":71,"is_preprint":false},{"pmid":"23267056","id":"PMC_23267056","title":"Biogenesis of RNA polymerases II and III requires the conserved GPN small GTPases in Saccharomyces cerevisiae.","date":"2012","source":"Genetics","url":"https://pubmed.ncbi.nlm.nih.gov/23267056","citation_count":42,"is_preprint":false},{"pmid":"35250176","id":"PMC_35250176","title":"Generation of Dual functional Nanobody-Nanoluciferase Fusion and its potential in Bioluminescence Enzyme Immunoassay for trace Glypican-3 in Serum.","date":"2021","source":"Sensors and actuators. B, Chemical","url":"https://pubmed.ncbi.nlm.nih.gov/35250176","citation_count":26,"is_preprint":false},{"pmid":"29661922","id":"PMC_29661922","title":"Gpn2 and Rba50 Directly Participate in the Assembly of the Rpb3 Subcomplex in the Biogenesis of RNA Polymerase II.","date":"2018","source":"Molecular and cellular biology","url":"https://pubmed.ncbi.nlm.nih.gov/29661922","citation_count":24,"is_preprint":false},{"pmid":"32985767","id":"PMC_32985767","title":"Npa3 interacts with Gpn3 and assembly factor Rba50 for RNA polymerase II biogenesis.","date":"2020","source":"FASEB journal : official publication of the Federation of American Societies for Experimental Biology","url":"https://pubmed.ncbi.nlm.nih.gov/32985767","citation_count":15,"is_preprint":false},{"pmid":"35314265","id":"PMC_35314265","title":"Npa3-Gpn3 cooperate to assemble RNA polymerase II and prevent clump of its subunits in the cytoplasm.","date":"2022","source":"International journal of biological macromolecules","url":"https://pubmed.ncbi.nlm.nih.gov/35314265","citation_count":6,"is_preprint":false},{"pmid":"36190433","id":"PMC_36190433","title":"Rtr1 is required for Rpb1-Rpb2 assembly of RNAPII and prevents their cytoplasmic clump formation.","date":"2022","source":"FASEB journal : official publication of the Federation of American Societies for Experimental Biology","url":"https://pubmed.ncbi.nlm.nih.gov/36190433","citation_count":6,"is_preprint":false},{"pmid":"35176321","id":"PMC_35176321","title":"Rba50 and Gpn2 recruit the second largest subunits for the assembly of RNA polymerase II and III.","date":"2022","source":"International journal of biological macromolecules","url":"https://pubmed.ncbi.nlm.nih.gov/35176321","citation_count":5,"is_preprint":false},{"pmid":"34180355","id":"PMC_34180355","title":"A nuclear proteome localization screen reveals the exquisite specificity of Gpn2 in RNA polymerase biogenesis.","date":"2021","source":"Cell cycle (Georgetown, Tex.)","url":"https://pubmed.ncbi.nlm.nih.gov/34180355","citation_count":3,"is_preprint":false},{"pmid":"39642114","id":"PMC_39642114","title":"Dosage suppressors of gpn2ts mutants and functional insights into the role of Gpn2 in budding yeast.","date":"2024","source":"PloS one","url":"https://pubmed.ncbi.nlm.nih.gov/39642114","citation_count":2,"is_preprint":false},{"pmid":"41500282","id":"PMC_41500282","title":"Reversible cytoplasmic foci of RNA polymerase II subunits serve as proteostatic hubs orchestrating transcriptional reprogramming.","date":"2026","source":"International journal of biological macromolecules","url":"https://pubmed.ncbi.nlm.nih.gov/41500282","citation_count":0,"is_preprint":false},{"pmid":"41244487","id":"PMC_41244487","title":"Proposal of Chemical Inhibitors That Compete with the Binding of RNA Polymerase II Subunits to Essential GTPases GPN Npa3 and Gpn1.","date":"2025","source":"ACS omega","url":"https://pubmed.ncbi.nlm.nih.gov/41244487","citation_count":0,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":7826,"output_tokens":2166,"usd":0.027984,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":9202,"output_tokens":3060,"usd":0.061255,"stage2_stop_reason":"end_turn"},"total_usd":0.089239,"stage1_batch_id":"msgbatch_01DUPACZi7AAWwuZiUjseQF4","stage2_batch_id":"msgbatch_018yt7chE3f48PEFtDwtj6AE","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2012,\n      \"finding\": \"Yeast Gpn2 binds both Gpn3 and Npa3/Gpn1, forming a network of GPN protein interactions. Temperature-sensitive alleles of GPN2 cause defects in RNA polymerase II nuclear localization and genetic interactions with RNAPII mutants. GPN2 mutants also show RNA polymerase III nuclear localization defects. The nuclear import defect of iwr1Δ (but not gpn2 mutants) is suppressed by NLS fusion to Rpb3, suggesting GPN proteins function upstream of Iwr1 in RNAPII/III biogenesis.\",\n      \"method\": \"Temperature-sensitive allele genetics, fluorescence microscopy for polymerase localization, genetic interaction/suppression analysis, binding assays\",\n      \"journal\": \"Genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods (genetic interactions, localization, epistasis with iwr1Δ NLS suppression), replicated across multiple alleles and polymerases\",\n      \"pmids\": [\"23267056\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Gpn2 directly interacts with the RNAPII subunit Rpb12, and also interacts with Rba50 (which itself binds Rpb3). Gpn2 and Rba50 together are required for assembly of the Rpb3 subcomplex; when either is functionally defective, Rpb3 subcomplex assembly is disrupted, blocking overall RNAPII assembly.\",\n      \"method\": \"Co-immunoprecipitation, pulldown assays, temperature-sensitive mutant analysis of RNAPII subcomplex assembly\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal Co-IP interactions demonstrated, functional defect in assembly confirmed by independent loss-of-function experiments, single lab with multiple orthogonal methods\",\n      \"pmids\": [\"29661922\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"A genome-wide screen of 1350 GFP-tagged nuclear proteins in GPN2 mutant yeast showed that the strongest and most specific mislocalization effects were for RNAPII and RNAPIII subunits, with only a handful of other RNAPII-associated proteins affected. Additionally, Ess1 (an Rpb1 CTD prolyl isomerase) was found to be mislocalized in gpn2 mutants, and disruption of Rpb1-CTD kinases or phosphatases altered Rpb1 nuclear-cytoplasmic distribution, linking CTD modification status to RNAPII nuclear localization downstream of Gpn2.\",\n      \"method\": \"High-content fluorescence microscopy screen of GFP-tagged nuclear proteome in gpn2 mutant yeast; genetic analysis with CTD kinase/phosphatase mutants\",\n      \"journal\": \"Cell cycle (Georgetown, Tex.)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — large-scale localization screen with functional follow-up, single lab, demonstrates exquisite specificity for RNA polymerases\",\n      \"pmids\": [\"34180355\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Rba50 and Gpn2 cooperate to recruit Rpb2 (second largest subunit of RNAPII) during assembly steps following Rpb3 subcomplex formation. Gpn2 facilitates the association of Rba50 and Rpb2. Both gpn2-R347S and rpb2-V1171G variants suppress rba50-3 mutant defects. The Rba50-Gpn2 complex appears to play a similar role in RNAPIII assembly.\",\n      \"method\": \"Extragenic suppressor mapping, multicopy suppressor screening, rapid depletion of Rba50 followed by co-immunoprecipitation of Rpb3-Rpb2, genetic epistasis\",\n      \"journal\": \"International journal of biological macromolecules\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic suppressor screen plus biochemical (Co-IP) confirmation of Rba50-Rpb2 association defects, single lab\",\n      \"pmids\": [\"35176321\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Inactivation of Gpn2 (as well as Npa3/Gpn1 and Gpn3) leads to reversible accumulation of RNAPII subunits (Rpb1, Rpb2, Rpb3) in cytoplasmic foci, a stress response termed RNAPII Assembly Stress Response (RASR). These foci are protein-based, nucleic acid-free condensates that resist 1,6-hexanediol dissolution and show dynamic FRAP behavior. Molecular chaperone Hsp82 colocalizes with these foci.\",\n      \"method\": \"Fluorescence microscopy, FRAP, hexanediol treatment, biochemical fractionation, GFP-tagging of RNAPII subunits in gpn2 mutant yeast\",\n      \"journal\": \"International journal of biological macromolecules\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal methods (FRAP, hexanediol, fractionation) in a single lab demonstrating cytoplasmic condensate properties\",\n      \"pmids\": [\"35314265\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Specific mutations in Gpn2 (Phe105Tyr and Leu164Pro) confer temperature sensitivity and significantly impair RNAPII assembly. Multicopy suppressor screening identified 31 genes (including PAB1, CDC5, and RGS2) whose overexpression mitigates gpn2ts growth defects, providing functional insights into Gpn2's role in RNAPII assembly.\",\n      \"method\": \"Large-scale multicopy suppressor screen (>30,000 colonies), temperature-sensitive mutant analysis, site-specific mutagenesis\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — suppressor screen identifies genetic interactions but does not establish direct mechanistic relationships for the identified suppressors; single lab, single method per finding\",\n      \"pmids\": [\"39642114\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"Inactivation of all three GPN proteins including Gpn2 triggers reversible RNAPII Assembly Stress Response (RASR) foci containing Rpb1, Rpb2, and Rpb3. Hsp82 partially colocalizes with these foci. Oxidative stress (H2O2) increases foci formation, revealing redox sensitivity. Transcriptomic profiling during RASR shows coordinated regulation of ribosome biogenesis genes and metabolic pathways.\",\n      \"method\": \"Fluorescence microscopy, FRAP, biochemical condensate characterization, RNA-seq transcriptomic profiling, oxidative stress treatment\",\n      \"journal\": \"International journal of biological macromolecules\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal methods (FRAP, biochemistry, transcriptomics) in single lab extending prior findings on RASR\",\n      \"pmids\": [\"41500282\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"GPN2 encodes a conserved GPN-loop GTPase that functions as an assembly chaperone for RNA polymerase II (and III) biogenesis: it forms a complex with GPN3 and GPN1/Npa3, directly interacts with RNAPII subunit Rpb12, and cooperates with assembly factor Rba50 to facilitate sequential assembly of the Rpb3 subcomplex and subsequent recruitment of Rpb2, acting upstream of the import factor Iwr1 to ensure proper nuclear localization of both RNAPII and RNAPIII; loss of Gpn2 function causes cytoplasmic accumulation of RNAPII/III subunits into reversible, Hsp82-containing condensates as part of a stress response (RASR) triggered by impaired polymerase assembly.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"GPN2 encodes a conserved GPN-loop GTPase that acts as an assembly chaperone in the biogenesis of RNA polymerases II and III [#0]. It operates within a network of GPN proteins, binding both Gpn3 and Npa3/Gpn1, and temperature-sensitive loss of Gpn2 function causes specific defects in the nuclear localization of RNAPII and RNAPIII, placing GPN proteins upstream of the dedicated import factor Iwr1 [#0]. Mechanistically, Gpn2 directly contacts the RNAPII subunit Rpb12 and cooperates with the assembly factor Rba50 to drive ordered subunit assembly: the Gpn2-Rba50 pair is required for formation of the Rpb3 subcomplex and then jointly recruits Rpb2 to the nascent polymerase, with Gpn2 facilitating the Rba50-Rpb2 association [#1, #3]. A genome-wide localization screen established that this function is exquisitely specific, as RNAPII and RNAPIII subunits are the dominant proteins mislocalized upon Gpn2 inactivation, with CTD modification status linked to RNAPII nuclear distribution downstream of Gpn2 [#2]. When Gpn2 (or its partners Gpn1 and Gpn3) is inactivated, unassembled RNAPII subunits accumulate in the cytoplasm as reversible, nucleic-acid-free, Hsp82-containing condensates in a redox-sensitive stress response termed the RNAPII Assembly Stress Response (RASR) [#4, #6].\",\n  \"teleology\": [\n    {\n      \"year\": 2012,\n      \"claim\": \"Established that Gpn2 is part of a GPN protein interaction network and is functionally required for nuclear localization of RNA polymerases II and III, acting upstream of the import factor Iwr1.\",\n      \"evidence\": \"Temperature-sensitive allele genetics, polymerase localization microscopy, and epistasis with iwr1\\u0394 NLS suppression in yeast\",\n      \"pmids\": [\"23267056\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Did not define the biochemical step (assembly vs. import) at which Gpn2 acts\",\n        \"No direct RNAPII subunit contact identified\",\n        \"GTPase activity of Gpn2 not linked to function\"\n      ]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Identified the direct molecular target of Gpn2 in polymerase assembly, showing it contacts Rpb12 and cooperates with Rba50 to build the Rpb3 subcomplex.\",\n      \"evidence\": \"Co-immunoprecipitation, pulldown, and temperature-sensitive mutant analysis of RNAPII subcomplex assembly in yeast\",\n      \"pmids\": [\"29661922\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Order of subunit addition relative to Rpb2 not yet resolved\",\n        \"No structural model of the Gpn2-Rba50-Rpb3 intermediate\",\n        \"Role of nucleotide binding/hydrolysis in assembly unaddressed\"\n      ]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Demonstrated the remarkable substrate specificity of Gpn2, with RNAPII/III subunits as the dominant mislocalized proteins, and connected CTD phosphorylation to RNAPII nuclear distribution.\",\n      \"evidence\": \"High-content GFP-tagged nuclear proteome localization screen plus CTD kinase/phosphatase mutant analysis in gpn2 mutant yeast\",\n      \"pmids\": [\"34180355\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Mechanistic link between Ess1/CTD status and Gpn2 function not established\",\n        \"Screen reports localization, not direct binding\",\n        \"Specificity determinants on Gpn2 not mapped\"\n      ]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Resolved a later assembly step, showing Gpn2 and Rba50 cooperate to recruit Rpb2 after Rpb3 subcomplex formation, extending the role to RNAPIII assembly.\",\n      \"evidence\": \"Extragenic and multicopy suppressor mapping, Rba50 depletion with Rpb3-Rpb2 Co-IP, and genetic epistasis in yeast\",\n      \"pmids\": [\"35176321\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Single lab; reciprocal in vitro reconstitution of Rpb2 recruitment absent\",\n        \"How suppressor variants restore assembly mechanistically unclear\",\n        \"Quantitative kinetics of assembly steps undefined\"\n      ]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Revealed the cellular consequence of failed assembly: Gpn2 inactivation drives reversible cytoplasmic condensation of unassembled RNAPII subunits (RASR) chaperoned by Hsp82.\",\n      \"evidence\": \"Fluorescence microscopy, FRAP, 1,6-hexanediol treatment, and biochemical fractionation of GFP-tagged subunits in gpn2 mutant yeast\",\n      \"pmids\": [\"35314265\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Whether RASR is protective or pathological is not resolved\",\n        \"Mechanism of Hsp82 recruitment to foci unknown\",\n        \"Reversibility pathway / disaggregation machinery not identified\"\n      ]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Mapped specific Gpn2 residues (Phe105Tyr, Leu164Pro) required for RNAPII assembly and surveyed genetic modifiers of gpn2 defects.\",\n      \"evidence\": \"Site-specific mutagenesis and large-scale multicopy suppressor screen (>30,000 colonies) in yeast\",\n      \"pmids\": [\"39642114\"],\n      \"confidence\": \"Low\",\n      \"gaps\": [\n        \"Suppressors (PAB1, CDC5, RGS2) are genetic interactions without established direct mechanism\",\n        \"Structural consequences of the residue substitutions not determined\",\n        \"Functional connection of identified suppressors to assembly untested\"\n      ]\n    },\n    {\n      \"year\": 2026,\n      \"claim\": \"Extended the RASR model by showing oxidative stress modulates foci formation and that RASR coordinates transcriptional reprogramming of ribosome biogenesis and metabolic genes.\",\n      \"evidence\": \"Fluorescence microscopy, FRAP, condensate biochemistry, oxidative stress treatment, and RNA-seq transcriptomics in GPN-inactivated yeast\",\n      \"pmids\": [\"41500282\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Redox sensor governing foci formation not identified\",\n        \"Direct causal link between RASR and transcriptomic changes not established\",\n        \"Whether redox sensitivity reflects a normal Gpn2 regulatory mechanism is unknown\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How nucleotide binding/hydrolysis by the GPN-loop GTPase drives the ordered assembly cycle, and how the Gpn2-Gpn1-Gpn3 complex is structurally organized on nascent polymerase, remain unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"No structure of the GPN complex bound to RNAPII assembly intermediates\",\n        \"Catalytic cycle of Gpn2 not linked to specific assembly transitions\",\n        \"Function in organisms beyond yeast not characterized in the corpus\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0003924\", \"supporting_discovery_ids\": [0]},\n      {\"term_id\": \"GO:0044183\", \"supporting_discovery_ids\": [1, 3]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [4]},\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [0]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-74160\", \"supporting_discovery_ids\": [0, 1]},\n      {\"term_id\": \"R-HSA-8953854\", \"supporting_discovery_ids\": [1, 3]}\n    ],\n    \"complexes\": [\"GPN protein complex (Gpn1/Gpn2/Gpn3)\", \"Gpn2-Rba50 assembly module\"],\n    \"partners\": [\"GPN3\", \"GPN1\", \"RPB12\", \"RBA50\", \"RPB2\", \"RPB3\", \"HSP82\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":5,"faith_total":5,"faith_pct":100.0}}