{"gene":"SNAPC2","run_date":"2026-04-28T20:42:08","timeline":{"discoveries":[{"year":1995,"finding":"SNAPc (containing SNAP43/SNAPC2, SNAP45, SNAP50, and TBP) was identified as a TBP-TAF complex required for transcription of both RNA polymerase II and III snRNA genes, binding specifically to the proximal sequence element (PSE) in their core promoters.","method":"Biochemical purification, transcription assays, DNA binding assays","journal":"Nature","confidence":"High","confidence_rationale":"Tier 1 — reconstituted complex with functional transcription assays, foundational paper with 128 citations","pmids":["7715707"],"is_preprint":false},{"year":1996,"finding":"SNAP43 (SNAPC2) interacts with SNAP50 in co-immunoprecipitation experiments but not with SNAP45 or TBP, providing initial architecture of the SNAPc complex.","method":"Co-immunoprecipitation","journal":"The EMBO journal","confidence":"Medium","confidence_rationale":"Tier 3 — single co-IP experiment defining SNAP43-SNAP50 interaction","pmids":["9003788"],"is_preprint":false},{"year":1998,"finding":"SNAPc was reconstituted from five recombinant subunits — SNAP43 (SNAPC2), SNAP45, SNAP50, SNAP190, and the newly identified SNAP19 — and the recombinant complex binds specifically to the PSE and directs both RNA polymerase II and III snRNA gene transcription.","method":"Recombinant protein reconstitution, PSE-binding assay, in vitro transcription","journal":"Genes & development","confidence":"High","confidence_rationale":"Tier 1 — full reconstitution of complex with functional assays, replicated across labs","pmids":["9732265"],"is_preprint":false},{"year":2000,"finding":"Detailed subunit-subunit contact map of SNAPc defined by domain deletion analysis showed that SNAP43 (SNAPC2) participates in specific protein-protein contacts within the complex; complexes containing only the minimal interaction domains still bind specifically to the PSE.","method":"Domain deletion analysis, co-immunoprecipitation, PSE binding assays","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 — systematic domain mapping with DNA-binding functional validation","pmids":["11056176"],"is_preprint":false},{"year":2006,"finding":"A partial SNAPc composed of SNAP190 (1-505), SNAP50, SNAP43 (SNAPC2), and SNAP19 co-expressed in E. coli binds PSE DNA specifically, recruits TBP to U6 promoter DNA, and supports transcription of both U1 and U6 snRNA genes by RNA polymerases II and III respectively.","method":"Recombinant co-expression in E. coli, DNA binding assay, TBP recruitment assay, in vitro transcription reconstitution","journal":"Protein expression and purification","confidence":"High","confidence_rationale":"Tier 1 — reconstitution and functional transcription assays with defined subunits","pmids":["16603380"],"is_preprint":false},{"year":2011,"finding":"In zebrafish, the snapc4(s445) mutation truncates the C-terminus of Snapc4, deleting the domain responsible for interaction with Snapc2 (the vertebrate-specific SNAPc subunit); snapc2 knockdown similarly disrupts the intrahepatic biliary network by causing apoptosis of biliary epithelial cells, and only a subset of snRNA transcripts are altered, demonstrating that the Snapc2–Snapc4 physical interaction is required for expression of a subset of snRNAs.","method":"Genetic mutant analysis in zebrafish, morpholino knockdown, snRNA quantification, biliary phenotype assay","journal":"Developmental biology","confidence":"Medium","confidence_rationale":"Tier 2 — genetic epistasis with defined molecular interaction domain and phenotypic readout, zebrafish ortholog","pmids":["22222761"],"is_preprint":false},{"year":2025,"finding":"Cryo-EM structures of the full-length SNAPc-containing Pol III pre-initiation complex on the U6 snRNA promoter (open and melting states, 3.2–4.2 Å) combined with crosslinking mass spectrometry localize SNAPC2 and SNAPC5 near the promoter DNA, revealing the structural basis for SNAPc engagement within Pol III and Pol II PICs.","method":"Cryo-EM structure determination, crosslinking mass spectrometry","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1 — high-resolution cryo-EM structure with crosslinking MS orthogonal validation","pmids":["39747245"],"is_preprint":false}],"current_model":"SNAPC2 (SNAP43) is a core subunit of the five-subunit SNAPc transcription factor complex that binds the proximal sequence element (PSE) of both RNA polymerase II and III snRNA gene promoters; it interacts directly with SNAP50 within the complex and with SNAPC4 (SNAP190) through a C-terminal domain, and cryo-EM structures show SNAPC2 is positioned near promoter DNA in the pre-initiation complex, collectively enabling SNAPc to recruit the appropriate RNA polymerase and TBP to initiate snRNA transcription."},"narrative":{"teleology":[{"year":1995,"claim":"Identification of SNAPC2 as a subunit of a novel TBP-containing complex (SNAPc) that binds the PSE and is required for both Pol II and Pol III snRNA transcription established the fundamental function of the gene.","evidence":"Biochemical purification coupled with in vitro transcription and DNA binding assays in human cell extracts","pmids":["7715707"],"confidence":"High","gaps":["Subunit-subunit contacts within SNAPc were unknown","Whether SNAPC2 contacts DNA directly was not determined","The number of SNAPc subunits was incomplete (SNAP19 not yet identified)"]},{"year":1996,"claim":"Demonstrating that SNAPC2 physically interacts with SNAP50 but not with SNAP45 or TBP provided the first internal architecture map of the complex.","evidence":"Co-immunoprecipitation of individual SNAPc subunits","pmids":["9003788"],"confidence":"Medium","gaps":["Single co-IP approach without reciprocal validation or orthogonal method","The interaction domain within SNAPC2 was not mapped","Interaction with SNAP190 was not tested"]},{"year":1998,"claim":"Reconstitution of a five-subunit SNAPc from recombinant proteins that bound the PSE and supported snRNA transcription proved that SNAPC2, together with SNAP19, SNAP45, SNAP50, and SNAP190, constitutes the minimal complex.","evidence":"Recombinant co-expression, PSE-binding assay, and in vitro transcription reconstitution","pmids":["9732265"],"confidence":"High","gaps":["The contribution of each subunit to DNA binding versus transcription activation was not resolved","No structural information on subunit arrangement"]},{"year":2000,"claim":"Systematic domain deletion mapping defined the minimal protein–protein contact surfaces within SNAPc, showing that SNAPC2 makes specific contacts required for PSE binding.","evidence":"Domain deletion analysis with co-immunoprecipitation and PSE binding assays","pmids":["11056176"],"confidence":"Medium","gaps":["Precise residue-level contacts were not determined","Whether SNAPC2 contributes to DNA contact versus complex stability was unresolved"]},{"year":2006,"claim":"A minimal four-subunit sub-complex including SNAPC2 was sufficient for PSE binding, TBP recruitment, and transcription by both Pol II and Pol III, establishing that SNAP45 is dispensable for core promoter recognition.","evidence":"Recombinant co-expression in E. coli, DNA binding, TBP recruitment, and in vitro transcription assays","pmids":["16603380"],"confidence":"High","gaps":["How SNAP45 contributes to Pol III-specific transcription in vivo remained unclear","No structural data on subunit positioning on DNA"]},{"year":2011,"claim":"Genetic evidence in zebrafish demonstrated that the SNAPC2–SNAPC4 interaction is required in vivo for a subset of snRNA transcripts and for biliary epithelial cell survival, revealing tissue-specific consequences of SNAPc disruption.","evidence":"Zebrafish snapc4 truncation mutant and snapc2 morpholino knockdown with snRNA quantification and biliary phenotype analysis","pmids":["22222761"],"confidence":"Medium","gaps":["Mechanism by which only a subset of snRNAs is affected is unknown","Whether this tissue-specific phenotype extends to mammals was not tested","Morpholino knockdown can have off-target effects"]},{"year":2025,"claim":"High-resolution cryo-EM structures of the SNAPc-containing Pol III PIC on the U6 promoter located SNAPC2 near promoter DNA, providing the first structural framework for how SNAPc subunits engage the transcription machinery.","evidence":"Cryo-EM at 3.2–4.2 Å resolution with crosslinking mass spectrometry validation","pmids":["39747245"],"confidence":"High","gaps":["Whether SNAPC2 directly contacts DNA or acts solely through protein–protein interactions is not resolved at current resolution","The equivalent Pol II PIC structure with SNAPc has not been determined at comparable resolution"]},{"year":null,"claim":"It remains unknown how SNAPc discriminates between Pol II- and Pol III-transcribed snRNA promoters at the mechanistic level, and what role SNAPC2 specifically plays in this switch.","evidence":"","pmids":[],"confidence":"High","gaps":["No mutational or structural data distinguishing SNAPC2's role in Pol II versus Pol III promoter selectivity","No high-resolution structure of SNAPC2 in the Pol II snRNA PIC for comparison","Post-translational regulation of SNAPC2 is uncharacterized"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0003677","term_label":"DNA binding","supporting_discovery_ids":[0,2,4,6]},{"term_id":"GO:0005198","term_label":"structural molecule activity","supporting_discovery_ids":[2,3,4]}],"localization":[{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[0,6]}],"pathway":[{"term_id":"R-HSA-74160","term_label":"Gene expression (Transcription)","supporting_discovery_ids":[0,2,4,6]}],"complexes":["SNAPc"],"partners":["SNAPC1","SNAPC4","SNAPC3","SNAPC5","TBP"],"other_free_text":[]},"mechanistic_narrative":"SNAPC2 (SNAP43) is a core subunit of the five-subunit small nuclear RNA-activating protein complex (SNAPc), which binds the proximal sequence element (PSE) of snRNA gene promoters and is required for transcription by both RNA polymerase II and RNA polymerase III [PMID:7715707, PMID:9732265]. Within SNAPc, SNAPC2 directly interacts with SNAP50 and with the C-terminal domain of SNAPC4 (SNAP190), contacts that are essential for PSE-specific DNA binding and for TBP recruitment to snRNA promoters [PMID:9003788, PMID:11056176, PMID:16603380]. Cryo-EM structures of the Pol III pre-initiation complex on the U6 snRNA promoter position SNAPC2 near promoter DNA, providing a structural basis for its role in snRNA transcription initiation [PMID:39747245]. In zebrafish, disruption of the Snapc2–Snapc4 interaction selectively reduces a subset of snRNA transcripts and causes apoptosis of biliary epithelial cells, indicating tissue-specific requirements for SNAPc integrity [PMID:22222761]."},"prefetch_data":{"uniprot":{"accession":"Q13487","full_name":"snRNA-activating protein complex subunit 2","aliases":["Proximal sequence element-binding transcription factor subunit delta","PSE-binding factor subunit delta","PTF subunit delta","Small nuclear RNA-activating complex polypeptide 2","snRNA-activating protein complex 45 kDa subunit","SNAPc 45 kDa subunit"],"length_aa":334,"mass_kda":35.6,"function":"Part of the SNAPc complex required for the transcription of both RNA polymerase II and III small-nuclear RNA genes. Binds to the proximal sequence element (PSE), a non-TATA-box basal promoter element common to these 2 types of genes. Recruits TBP and BRF2 to the U6 snRNA TATA box","subcellular_location":"Nucleus","url":"https://www.uniprot.org/uniprotkb/Q13487/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":true,"resolved_as":"","url":"https://depmap.org/portal/gene/SNAPC2","classification":"Common Essential","n_dependent_lines":1202,"n_total_lines":1208,"dependency_fraction":0.9950331125827815},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/SNAPC2","total_profiled":1310},"omim":[{"mim_id":"605979","title":"SMALL NUCLEAR RNA-ACTIVATING PROTEIN COMPLEX, POLYPEPTIDE 5; SNAPC5","url":"https://www.omim.org/entry/605979"},{"mim_id":"605076","title":"SMALL NUCLEAR RNA-ACTIVATING PROTEIN COMPLEX, POLYPEPTIDE 2; SNAPC2","url":"https://www.omim.org/entry/605076"},{"mim_id":"602777","title":"SMALL NUCLEAR RNA-ACTIVATING PROTEIN COMPLEX, POLYPEPTIDE 4; SNAPC4","url":"https://www.omim.org/entry/602777"},{"mim_id":"600591","title":"SMALL NUCLEAR RNA-ACTIVATING PROTEIN COMPLEX, POLYPEPTIDE 1; SNAPC1","url":"https://www.omim.org/entry/600591"},{"mim_id":"176399","title":"PREGNANCY-SPECIFIC BETA-1-GLYCOPROTEIN 10, PSEUDOGENE; PSG10P","url":"https://www.omim.org/entry/176399"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Supported","locations":[{"location":"Nucleoplasm","reliability":"Supported"},{"location":"Nuclear bodies","reliability":"Supported"},{"location":"Cytosol","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/SNAPC2"},"hgnc":{"alias_symbol":["SNAP45","PTFdelta"],"prev_symbol":[]},"alphafold":{"accession":"Q13487","domains":[{"cath_id":"-","chopping":"207-259","consensus_level":"medium","plddt":86.5215,"start":207,"end":259},{"cath_id":"1.10.10","chopping":"25-88","consensus_level":"medium","plddt":77.5553,"start":25,"end":88}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q13487","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q13487-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q13487-F1-predicted_aligned_error_v6.png","plddt_mean":66.0},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=SNAPC2","jax_strain_url":"https://www.jax.org/strain/search?query=SNAPC2"},"sequence":{"accession":"Q13487","fasta_url":"https://rest.uniprot.org/uniprotkb/Q13487.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q13487/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q13487"}},"corpus_meta":[{"pmid":"7715707","id":"PMC_7715707","title":"A TBP-TAF complex required for transcription of human snRNA genes by RNA polymerase II and III.","date":"1995","source":"Nature","url":"https://pubmed.ncbi.nlm.nih.gov/7715707","citation_count":128,"is_preprint":false},{"pmid":"9418884","id":"PMC_9418884","title":"The large subunit of basal transcription factor SNAPc is a Myb domain protein that interacts with Oct-1.","date":"1998","source":"Molecular and cellular biology","url":"https://pubmed.ncbi.nlm.nih.gov/9418884","citation_count":82,"is_preprint":false},{"pmid":"9732265","id":"PMC_9732265","title":"SNAP19 mediates the assembly of a functional core promoter complex (SNAPc) shared by RNA polymerases II and III.","date":"1998","source":"Genes & development","url":"https://pubmed.ncbi.nlm.nih.gov/9732265","citation_count":73,"is_preprint":false},{"pmid":"8633057","id":"PMC_8633057","title":"The SNAP45 subunit of the small nuclear RNA (snRNA) activating protein complex is required for RNA polymerase II and III snRNA gene transcription and interacts with the TATA box binding protein.","date":"1996","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/8633057","citation_count":52,"is_preprint":false},{"pmid":"9003788","id":"PMC_9003788","title":"Cloning and characterization of SNAP50, a subunit of the snRNA-activating protein complex SNAPc.","date":"1996","source":"The EMBO journal","url":"https://pubmed.ncbi.nlm.nih.gov/9003788","citation_count":51,"is_preprint":false},{"pmid":"11056176","id":"PMC_11056176","title":"A map of protein-protein contacts within the small nuclear RNA-activating protein complex SNAPc.","date":"2000","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/11056176","citation_count":41,"is_preprint":false},{"pmid":"31552087","id":"PMC_31552087","title":"Machine Learning Classifiers for Endometriosis Using Transcriptomics and Methylomics Data.","date":"2019","source":"Frontiers in genetics","url":"https://pubmed.ncbi.nlm.nih.gov/31552087","citation_count":37,"is_preprint":false},{"pmid":"12391172","id":"PMC_12391172","title":"Redundant cooperative interactions for assembly of a human U6 transcription initiation complex.","date":"2002","source":"Molecular and cellular biology","url":"https://pubmed.ncbi.nlm.nih.gov/12391172","citation_count":34,"is_preprint":false},{"pmid":"22222761","id":"PMC_22222761","title":"Mutation of zebrafish Snapc4 is associated with loss of the intrahepatic biliary network.","date":"2011","source":"Developmental biology","url":"https://pubmed.ncbi.nlm.nih.gov/22222761","citation_count":18,"is_preprint":false},{"pmid":"27610895","id":"PMC_27610895","title":"Association between genes on chromosome 19p13.2 and panic disorder.","date":"2016","source":"Psychiatric genetics","url":"https://pubmed.ncbi.nlm.nih.gov/27610895","citation_count":7,"is_preprint":false},{"pmid":"16603380","id":"PMC_16603380","title":"Co-expression of multiple subunits enables recombinant SNAPC assembly and function for transcription by human RNA polymerases II and III.","date":"2006","source":"Protein expression and purification","url":"https://pubmed.ncbi.nlm.nih.gov/16603380","citation_count":6,"is_preprint":false},{"pmid":"18356157","id":"PMC_18356157","title":"Mitotic functions for SNAP45, a subunit of the small nuclear RNA-activating protein complex SNAPc.","date":"2008","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/18356157","citation_count":5,"is_preprint":false},{"pmid":"39747245","id":"PMC_39747245","title":"Structural insights into distinct mechanisms of RNA polymerase II and III recruitment to snRNA promoters.","date":"2025","source":"Nature communications","url":"https://pubmed.ncbi.nlm.nih.gov/39747245","citation_count":4,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":6930,"output_tokens":1563,"usd":0.022117},"stage2":{"model":"claude-opus-4-6","input_tokens":4832,"output_tokens":1968,"usd":0.11004},"total_usd":0.132157,"stage1_batch_id":"msgbatch_011rkcQdVV1RaFq9XhcG6df6","stage2_batch_id":"msgbatch_01RzgrJafdF4EWHhj5mNi6dh","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1995,\n      \"finding\": \"SNAPc (containing SNAP43/SNAPC2, SNAP45, SNAP50, and TBP) was identified as a TBP-TAF complex required for transcription of both RNA polymerase II and III snRNA genes, binding specifically to the proximal sequence element (PSE) in their core promoters.\",\n      \"method\": \"Biochemical purification, transcription assays, DNA binding assays\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstituted complex with functional transcription assays, foundational paper with 128 citations\",\n      \"pmids\": [\"7715707\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"SNAP43 (SNAPC2) interacts with SNAP50 in co-immunoprecipitation experiments but not with SNAP45 or TBP, providing initial architecture of the SNAPc complex.\",\n      \"method\": \"Co-immunoprecipitation\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — single co-IP experiment defining SNAP43-SNAP50 interaction\",\n      \"pmids\": [\"9003788\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"SNAPc was reconstituted from five recombinant subunits — SNAP43 (SNAPC2), SNAP45, SNAP50, SNAP190, and the newly identified SNAP19 — and the recombinant complex binds specifically to the PSE and directs both RNA polymerase II and III snRNA gene transcription.\",\n      \"method\": \"Recombinant protein reconstitution, PSE-binding assay, in vitro transcription\",\n      \"journal\": \"Genes & development\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — full reconstitution of complex with functional assays, replicated across labs\",\n      \"pmids\": [\"9732265\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"Detailed subunit-subunit contact map of SNAPc defined by domain deletion analysis showed that SNAP43 (SNAPC2) participates in specific protein-protein contacts within the complex; complexes containing only the minimal interaction domains still bind specifically to the PSE.\",\n      \"method\": \"Domain deletion analysis, co-immunoprecipitation, PSE binding assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — systematic domain mapping with DNA-binding functional validation\",\n      \"pmids\": [\"11056176\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"A partial SNAPc composed of SNAP190 (1-505), SNAP50, SNAP43 (SNAPC2), and SNAP19 co-expressed in E. coli binds PSE DNA specifically, recruits TBP to U6 promoter DNA, and supports transcription of both U1 and U6 snRNA genes by RNA polymerases II and III respectively.\",\n      \"method\": \"Recombinant co-expression in E. coli, DNA binding assay, TBP recruitment assay, in vitro transcription reconstitution\",\n      \"journal\": \"Protein expression and purification\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstitution and functional transcription assays with defined subunits\",\n      \"pmids\": [\"16603380\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"In zebrafish, the snapc4(s445) mutation truncates the C-terminus of Snapc4, deleting the domain responsible for interaction with Snapc2 (the vertebrate-specific SNAPc subunit); snapc2 knockdown similarly disrupts the intrahepatic biliary network by causing apoptosis of biliary epithelial cells, and only a subset of snRNA transcripts are altered, demonstrating that the Snapc2–Snapc4 physical interaction is required for expression of a subset of snRNAs.\",\n      \"method\": \"Genetic mutant analysis in zebrafish, morpholino knockdown, snRNA quantification, biliary phenotype assay\",\n      \"journal\": \"Developmental biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis with defined molecular interaction domain and phenotypic readout, zebrafish ortholog\",\n      \"pmids\": [\"22222761\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Cryo-EM structures of the full-length SNAPc-containing Pol III pre-initiation complex on the U6 snRNA promoter (open and melting states, 3.2–4.2 Å) combined with crosslinking mass spectrometry localize SNAPC2 and SNAPC5 near the promoter DNA, revealing the structural basis for SNAPc engagement within Pol III and Pol II PICs.\",\n      \"method\": \"Cryo-EM structure determination, crosslinking mass spectrometry\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — high-resolution cryo-EM structure with crosslinking MS orthogonal validation\",\n      \"pmids\": [\"39747245\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"SNAPC2 (SNAP43) is a core subunit of the five-subunit SNAPc transcription factor complex that binds the proximal sequence element (PSE) of both RNA polymerase II and III snRNA gene promoters; it interacts directly with SNAP50 within the complex and with SNAPC4 (SNAP190) through a C-terminal domain, and cryo-EM structures show SNAPC2 is positioned near promoter DNA in the pre-initiation complex, collectively enabling SNAPc to recruit the appropriate RNA polymerase and TBP to initiate snRNA transcription.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"SNAPC2 (SNAP43) is a core subunit of the five-subunit small nuclear RNA-activating protein complex (SNAPc), which binds the proximal sequence element (PSE) of snRNA gene promoters and is required for transcription by both RNA polymerase II and RNA polymerase III [PMID:7715707, PMID:9732265]. Within SNAPc, SNAPC2 directly interacts with SNAP50 and with the C-terminal domain of SNAPC4 (SNAP190), contacts that are essential for PSE-specific DNA binding and for TBP recruitment to snRNA promoters [PMID:9003788, PMID:11056176, PMID:16603380]. Cryo-EM structures of the Pol III pre-initiation complex on the U6 snRNA promoter position SNAPC2 near promoter DNA, providing a structural basis for its role in snRNA transcription initiation [PMID:39747245]. In zebrafish, disruption of the Snapc2–Snapc4 interaction selectively reduces a subset of snRNA transcripts and causes apoptosis of biliary epithelial cells, indicating tissue-specific requirements for SNAPc integrity [PMID:22222761].\",\n  \"teleology\": [\n    {\n      \"year\": 1995,\n      \"claim\": \"Identification of SNAPC2 as a subunit of a novel TBP-containing complex (SNAPc) that binds the PSE and is required for both Pol II and Pol III snRNA transcription established the fundamental function of the gene.\",\n      \"evidence\": \"Biochemical purification coupled with in vitro transcription and DNA binding assays in human cell extracts\",\n      \"pmids\": [\"7715707\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Subunit-subunit contacts within SNAPc were unknown\",\n        \"Whether SNAPC2 contacts DNA directly was not determined\",\n        \"The number of SNAPc subunits was incomplete (SNAP19 not yet identified)\"\n      ]\n    },\n    {\n      \"year\": 1996,\n      \"claim\": \"Demonstrating that SNAPC2 physically interacts with SNAP50 but not with SNAP45 or TBP provided the first internal architecture map of the complex.\",\n      \"evidence\": \"Co-immunoprecipitation of individual SNAPc subunits\",\n      \"pmids\": [\"9003788\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Single co-IP approach without reciprocal validation or orthogonal method\",\n        \"The interaction domain within SNAPC2 was not mapped\",\n        \"Interaction with SNAP190 was not tested\"\n      ]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Reconstitution of a five-subunit SNAPc from recombinant proteins that bound the PSE and supported snRNA transcription proved that SNAPC2, together with SNAP19, SNAP45, SNAP50, and SNAP190, constitutes the minimal complex.\",\n      \"evidence\": \"Recombinant co-expression, PSE-binding assay, and in vitro transcription reconstitution\",\n      \"pmids\": [\"9732265\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"The contribution of each subunit to DNA binding versus transcription activation was not resolved\",\n        \"No structural information on subunit arrangement\"\n      ]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"Systematic domain deletion mapping defined the minimal protein–protein contact surfaces within SNAPc, showing that SNAPC2 makes specific contacts required for PSE binding.\",\n      \"evidence\": \"Domain deletion analysis with co-immunoprecipitation and PSE binding assays\",\n      \"pmids\": [\"11056176\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Precise residue-level contacts were not determined\",\n        \"Whether SNAPC2 contributes to DNA contact versus complex stability was unresolved\"\n      ]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"A minimal four-subunit sub-complex including SNAPC2 was sufficient for PSE binding, TBP recruitment, and transcription by both Pol II and Pol III, establishing that SNAP45 is dispensable for core promoter recognition.\",\n      \"evidence\": \"Recombinant co-expression in E. coli, DNA binding, TBP recruitment, and in vitro transcription assays\",\n      \"pmids\": [\"16603380\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"How SNAP45 contributes to Pol III-specific transcription in vivo remained unclear\",\n        \"No structural data on subunit positioning on DNA\"\n      ]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Genetic evidence in zebrafish demonstrated that the SNAPC2–SNAPC4 interaction is required in vivo for a subset of snRNA transcripts and for biliary epithelial cell survival, revealing tissue-specific consequences of SNAPc disruption.\",\n      \"evidence\": \"Zebrafish snapc4 truncation mutant and snapc2 morpholino knockdown with snRNA quantification and biliary phenotype analysis\",\n      \"pmids\": [\"22222761\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Mechanism by which only a subset of snRNAs is affected is unknown\",\n        \"Whether this tissue-specific phenotype extends to mammals was not tested\",\n        \"Morpholino knockdown can have off-target effects\"\n      ]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"High-resolution cryo-EM structures of the SNAPc-containing Pol III PIC on the U6 promoter located SNAPC2 near promoter DNA, providing the first structural framework for how SNAPc subunits engage the transcription machinery.\",\n      \"evidence\": \"Cryo-EM at 3.2–4.2 Å resolution with crosslinking mass spectrometry validation\",\n      \"pmids\": [\"39747245\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Whether SNAPC2 directly contacts DNA or acts solely through protein–protein interactions is not resolved at current resolution\",\n        \"The equivalent Pol II PIC structure with SNAPc has not been determined at comparable resolution\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"It remains unknown how SNAPc discriminates between Pol II- and Pol III-transcribed snRNA promoters at the mechanistic level, and what role SNAPC2 specifically plays in this switch.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"No mutational or structural data distinguishing SNAPC2's role in Pol II versus Pol III promoter selectivity\",\n        \"No high-resolution structure of SNAPC2 in the Pol II snRNA PIC for comparison\",\n        \"Post-translational regulation of SNAPC2 is uncharacterized\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0003677\", \"supporting_discovery_ids\": [0, 2, 4, 6]},\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [2, 3, 4]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [0, 6]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-74160\", \"supporting_discovery_ids\": [0, 2, 4, 6]}\n    ],\n    \"complexes\": [\n      \"SNAPc\"\n    ],\n    \"partners\": [\n      \"SNAPC1\",\n      \"SNAPC4\",\n      \"SNAPC3\",\n      \"SNAPC5\",\n      \"TBP\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}