{"gene":"SNAPC3","run_date":"2026-06-10T07:46:37","timeline":{"discoveries":[{"year":1995,"finding":"SNAP50 (SNAPC3) is a subunit of SNAPc, a TBP-TAF complex that binds specifically to the proximal sequence element (PSE) and is required for transcription of both RNA polymerase II and III snRNA genes.","method":"Biochemical purification, transcription assays in vitro","journal":"Nature","confidence":"High","confidence_rationale":"Tier 1 / Strong — biochemical reconstitution of complex, functional transcription assays, replicated across multiple studies","pmids":["7715707"],"is_preprint":false},{"year":1996,"finding":"SNAP50 (SNAPC3) contains two potential zinc finger motifs and directly contacts DNA within the SNAPc-PSE complex, as shown by UV cross-linking; antibody depletion of SNAP50 inhibits both RNA polymerase II and III snRNA gene transcription in vitro. SNAP50 interacts with SNAP43 (SNAPC1) by co-immunoprecipitation but not with SNAP45 or TBP.","method":"cDNA cloning, UV cross-linking, co-immunoprecipitation, antibody depletion/transcription assays","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — multiple orthogonal methods (UV cross-linking, co-IP, antibody depletion with functional readout) in the defining characterization paper","pmids":["9003788"],"is_preprint":false},{"year":1998,"finding":"SNAPc can be reconstituted from five recombinant subunits (SNAP43, SNAP45, SNAP50, SNAP190, and SNAP19); this recombinant complex binds specifically to the PSE and directs both RNA polymerase II and III snRNA gene transcription, establishing SNAP50 as an essential core subunit.","method":"Recombinant protein reconstitution, PSE binding assay, in vitro transcription","journal":"Genes & development","confidence":"High","confidence_rationale":"Tier 1 / Strong — full reconstitution from defined recombinant subunits with functional transcription readout","pmids":["9732265"],"is_preprint":false},{"year":2000,"finding":"Subunit-subunit interaction mapping within SNAPc revealed specific domains required for SNAP50 to associate with other subunits; complexes containing only the mapped interaction domains retain specific PSE binding.","method":"Co-immunoprecipitation, deletion/domain mapping, PSE binding assays","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — domain mapping with functional PSE binding confirmation, single lab","pmids":["11056176"],"is_preprint":false},{"year":2002,"finding":"A mini-SNAPc composed of SNAP43, SNAP50, and the N-terminal third of SNAP190 binds cooperatively with TBP to the core U6 promoter and supports transcription; SNAP50 participates in cooperative TBP recruitment to the U6 TATA box.","method":"Recombinant mini-complex assembly, TBP recruitment assays, in vitro transcription","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 1 / Moderate — reconstituted mini-complex with defined subunits, functional transcription and TBP recruitment assays, single lab but multiple orthogonal methods","pmids":["12391172"],"is_preprint":false},{"year":2002,"finding":"The 57 kDa subunit of the trypanosome PBP-1 transcription factor complex is orthologous to human SNAP50, indicating an evolutionarily conserved role for SNAP50-like subunits in snRNA gene transcription across large evolutionary distances.","method":"Biochemical purification, gene cloning, sequence/structural homology analysis","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — purification and cloning with functional context, ortholog inference supported by domain conservation","pmids":["12486231"],"is_preprint":false},{"year":2006,"finding":"The SNAP50 zinc finger domain contains 15 cysteine and histidine residues in two potential zinc coordination arrangements, but binds only a single zinc atom; eight residues are critical for DNA binding by SNAPc, four of which are also essential for both U1 (RNA Pol II) and U6 (RNA Pol III) transcription. Defects in DNA binding caused by mutations in four residues can be suppressed by cooperative DNA binding with TFIIIB.","method":"Alanine-scanning mutagenesis, metal binding studies, PSE DNA binding assays, in vitro transcription","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — systematic active-site mutagenesis combined with metal binding and functional transcription assays, multiple orthogonal methods in a single rigorous study","pmids":["16901896"],"is_preprint":false},{"year":2006,"finding":"A partial SNAPc comprising SNAP190(1-505), SNAP50, SNAP43, and SNAP19 co-expressed in E. coli binds PSE specifically, recruits TBP to U6 promoter DNA, and supports transcription of both human U1 and U6 snRNA genes by RNA polymerases II and III.","method":"Bacterial co-expression, PSE binding assay, TBP recruitment assay, reconstituted in vitro transcription","journal":"Protein expression and purification","confidence":"High","confidence_rationale":"Tier 1 / Moderate — reconstitution with defined subunits in bacteria, functional PSE binding and transcription assays, multiple orthogonal methods","pmids":["16603380"],"is_preprint":false},{"year":2022,"finding":"Cryo-EM structure of human mini-SNAPc (N-terminal domain of SNAP190, SNAP50, and SNAP43) bound to the U6-1 PSE at 3.49 Å resolution reveals that SNAP50 contributes three important motifs involved in both major groove and minor groove recognition of the PSE, acting in coordination with the SNAP190 Myb domain, with a 'wrap-around' binding mode.","method":"Cryo-electron microscopy structure determination, structural analysis of protein-DNA contacts","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1 / Strong — cryo-EM structure at near-atomic resolution with direct visualization of SNAP50-DNA contacts and mechanistic interpretation","pmids":["36369505"],"is_preprint":false},{"year":2025,"finding":"SUMOylation-deficient SNAPC1 (2KR mutant) retains the ability to interact with SNAPC3 (SNAP50) but shows impaired interaction with SNAPC4, indicating that SNAPC3 interaction with SNAPC1 is independent of SNAPC1 SUMOylation status.","method":"Endogenous tagging of SNAPC3 and SNAPC4, co-immunoprecipitation with SUMOylation-deficient SNAPC1 mutant","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reciprocal endogenous-tag co-IP with defined mutant, single lab, functional context established","pmids":["40956881"],"is_preprint":false}],"current_model":"SNAPC3 (SNAP50) is an essential DNA-contacting subunit of the five-subunit SNAPc general transcription factor that binds the proximal sequence element (PSE) at snRNA promoters through an unorthodox zinc finger domain (with eight critical cysteine/histidine residues and a single bound zinc atom), cooperates with the SNAP190 Myb domain for major- and minor-groove PSE recognition (as revealed by cryo-EM structure), participates in cooperative TBP recruitment to the U6 TATA box, and is required for transcription of both RNA polymerase II and III snRNA genes; within the complex, SNAP50 directly interacts with SNAP43 (SNAPC1) in a manner that is independent of SNAPC1 SUMOylation."},"narrative":{"mechanistic_narrative":"SNAPC3 (SNAP50) is an essential DNA-contacting core subunit of the snRNA-activating protein complex (SNAPc), a TBP-containing general transcription factor that binds the proximal sequence element (PSE) of snRNA promoters and is required for transcription of both RNA polymerase II and III snRNA genes [PMID:7715707, PMID:9732265]. Within the assembled five-subunit complex, SNAP50 makes direct sequence-specific contacts with PSE DNA through an unorthodox zinc finger domain that, despite containing 15 candidate cysteine/histidine residues across two potential coordination arrangements, binds only a single zinc atom; eight of these residues are critical for SNAPc DNA binding and a subset is essential for both U1 (Pol II) and U6 (Pol III) transcription [PMID:9003788, PMID:16901896]. SNAP50 recognizes the PSE through both major- and minor-groove contacts in coordination with the SNAP190 Myb domain in a 'wrap-around' binding mode, and participates in the cooperative recruitment of TBP to the U6 TATA box [PMID:12391172, PMID:36369505]. SNAP50 directly associates with SNAP43 (SNAPC1), an interaction that is independent of SNAPC1 SUMOylation status [PMID:9003788, PMID:40956881]. The role of SNAP50-like subunits in snRNA gene transcription is evolutionarily conserved [PMID:12486231].","teleology":[{"year":1995,"claim":"Establishing that snRNA promoters require a dedicated factor answered how PSE-driven genes are transcribed by two different polymerases, identifying SNAP50 as a subunit of the PSE-binding SNAPc complex.","evidence":"Biochemical purification and in vitro transcription assays","pmids":["7715707"],"confidence":"High","gaps":["Did not resolve which subunit contacts DNA","Subunit stoichiometry and individual roles undefined"]},{"year":1996,"claim":"Cloning and cross-linking determined that SNAP50 itself directly contacts PSE DNA and is functionally required, distinguishing it from purely scaffolding subunits.","evidence":"cDNA cloning, UV cross-linking, antibody depletion with in vitro transcription readout, and co-immunoprecipitation","pmids":["9003788"],"confidence":"High","gaps":["Zinc finger metal coordination not directly demonstrated","Specific DNA-contacting residues not mapped"]},{"year":1998,"claim":"Full reconstitution from five recombinant subunits established SNAP50 as an essential core subunit rather than a co-purifying contaminant.","evidence":"Recombinant five-subunit reconstitution with PSE binding and in vitro transcription","pmids":["9732265"],"confidence":"High","gaps":["Did not map subunit-subunit contact surfaces","No structural detail of DNA recognition"]},{"year":2000,"claim":"Domain mapping defined the minimal SNAP50 regions required for assembly into a PSE-binding-competent complex, clarifying the architecture of subunit interactions.","evidence":"Co-immunoprecipitation, deletion/domain mapping, PSE binding assays","pmids":["11056176"],"confidence":"Medium","gaps":["Single-lab mapping without structural confirmation","Interaction interfaces inferred, not visualized"]},{"year":2002,"claim":"A minimal SNAP43/SNAP50/SNAP190 complex showed SNAP50 participates in cooperative TBP recruitment to the U6 TATA box, linking PSE binding to TATA-element promoter engagement.","evidence":"Recombinant mini-complex assembly, TBP recruitment assays, in vitro transcription","pmids":["12391172"],"confidence":"High","gaps":["Mechanism of SNAP50-TBP cooperativity not structurally defined"]},{"year":2002,"claim":"Identification of an orthologous 57 kDa subunit in trypanosome PBP-1 demonstrated that the SNAP50 function in snRNA transcription is deeply conserved.","evidence":"Biochemical purification, gene cloning, sequence/structural homology analysis","pmids":["12486231"],"confidence":"Medium","gaps":["Functional equivalence inferred from homology","Conservation of DNA-contact residues not tested"]},{"year":2006,"claim":"Systematic mutagenesis defined the SNAP50 zinc finger as an unorthodox single-zinc domain and identified the residues critical for DNA binding versus transcription, mechanistically separating these functions.","evidence":"Alanine-scanning mutagenesis, metal binding studies, PSE DNA binding and in vitro transcription assays","pmids":["16901896","16603380"],"confidence":"High","gaps":["Three-dimensional fold of the zinc finger not yet resolved","Suppression by TFIIIB cooperativity mechanism unclear"]},{"year":2022,"claim":"The cryo-EM structure of mini-SNAPc on the PSE revealed how SNAP50 reads DNA, showing three motifs engaging both grooves alongside the SNAP190 Myb domain in a wrap-around mode.","evidence":"Cryo-EM structure of mini-SNAPc-PSE at 3.49 Å","pmids":["36369505"],"confidence":"High","gaps":["Structure limited to a partial complex lacking SNAP45 and SNAP19","TBP/TATA-bound state not captured"]},{"year":2025,"claim":"Endogenous-tag co-IP with a SUMOylation-deficient SNAPC1 showed SNAP50-SNAPC1 contact is SUMO-independent while SNAPC4 interaction is SUMO-sensitive, refining how post-translational modification shapes complex assembly.","evidence":"Endogenous tagging and co-immunoprecipitation with SUMOylation-deficient SNAPC1 (2KR) mutant","pmids":["40956881"],"confidence":"Medium","gaps":["Single-lab finding","Functional consequence of SUMO-dependent SNAPC4 interaction on transcription not established"]},{"year":null,"claim":"How SNAP50-mediated PSE recognition is integrated with full holo-SNAPc assembly and differential recruitment of Pol II versus Pol III machinery in vivo remains open.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No structure of the complete five-subunit SNAPc on promoter DNA","Determinants of Pol II versus Pol III specificity downstream of SNAP50 binding unresolved"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0003677","term_label":"DNA binding","supporting_discovery_ids":[1,6,8]},{"term_id":"GO:0140110","term_label":"transcription regulator activity","supporting_discovery_ids":[0,2,4]}],"localization":[{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[0,2]}],"pathway":[{"term_id":"R-HSA-74160","term_label":"Gene expression (Transcription)","supporting_discovery_ids":[0,2,4]}],"complexes":["SNAPc"],"partners":["SNAPC1","SNAP190","TBP","SNAPC4"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q92966","full_name":"snRNA-activating protein complex subunit 3","aliases":["Proximal sequence element-binding transcription factor subunit beta","PSE-binding factor subunit beta","PTF subunit beta","Small nuclear RNA-activating complex polypeptide 3","snRNA-activating protein complex 50 kDa subunit","SNAPc 50 kDa subunit"],"length_aa":411,"mass_kda":46.8,"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/Q92966/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":true,"resolved_as":"","url":"https://depmap.org/portal/gene/SNAPC3","classification":"Common Essential","n_dependent_lines":1183,"n_total_lines":1208,"dependency_fraction":0.9793046357615894},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/SNAPC3","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":"602777","title":"SMALL NUCLEAR RNA-ACTIVATING PROTEIN COMPLEX, POLYPEPTIDE 4; SNAPC4","url":"https://www.omim.org/entry/602777"},{"mim_id":"602348","title":"SMALL NUCLEAR RNA-ACTIVATING PROTEIN COMPLEX, POLYPEPTIDE 3; SNAPC3","url":"https://www.omim.org/entry/602348"},{"mim_id":"600591","title":"SMALL NUCLEAR RNA-ACTIVATING PROTEIN COMPLEX, POLYPEPTIDE 1; SNAPC1","url":"https://www.omim.org/entry/600591"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Supported","locations":[{"location":"Nucleoplasm","reliability":"Supported"},{"location":"Nuclear bodies","reliability":"Supported"},{"location":"Nucleoli","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/SNAPC3"},"hgnc":{"alias_symbol":["SNAP50","PTFbeta","MGC33124","MGC132011"],"prev_symbol":[]},"alphafold":{"accession":"Q92966","domains":[{"cath_id":"3.10.290","chopping":"170-229_239-345","consensus_level":"high","plddt":91.4979,"start":170,"end":345},{"cath_id":"-","chopping":"349-411","consensus_level":"medium","plddt":92.869,"start":349,"end":411}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q92966","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q92966-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q92966-F1-predicted_aligned_error_v6.png","plddt_mean":84.06},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=SNAPC3","jax_strain_url":"https://www.jax.org/strain/search?query=SNAPC3"},"sequence":{"accession":"Q92966","fasta_url":"https://rest.uniprot.org/uniprotkb/Q92966.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q92966/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q92966"}},"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":"19356248","id":"PMC_19356248","title":"Histone acetylations mark origins of polycistronic transcription in Leishmania major.","date":"2009","source":"BMC genomics","url":"https://pubmed.ncbi.nlm.nih.gov/19356248","citation_count":115,"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 & 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\"confidence_rationale\": \"Tier 1 / Strong — biochemical reconstitution of complex, functional transcription assays, replicated across multiple studies\",\n      \"pmids\": [\"7715707\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"SNAP50 (SNAPC3) contains two potential zinc finger motifs and directly contacts DNA within the SNAPc-PSE complex, as shown by UV cross-linking; antibody depletion of SNAP50 inhibits both RNA polymerase II and III snRNA gene transcription in vitro. SNAP50 interacts with SNAP43 (SNAPC1) by co-immunoprecipitation but not with SNAP45 or TBP.\",\n      \"method\": \"cDNA cloning, UV cross-linking, co-immunoprecipitation, antibody depletion/transcription assays\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — multiple orthogonal methods (UV cross-linking, co-IP, antibody depletion with functional readout) in the defining characterization paper\",\n      \"pmids\": [\"9003788\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"SNAPc can be reconstituted from five recombinant subunits (SNAP43, SNAP45, SNAP50, SNAP190, and SNAP19); this recombinant complex binds specifically to the PSE and directs both RNA polymerase II and III snRNA gene transcription, establishing SNAP50 as an essential core subunit.\",\n      \"method\": \"Recombinant protein reconstitution, PSE binding assay, in vitro transcription\",\n      \"journal\": \"Genes & development\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — full reconstitution from defined recombinant subunits with functional transcription readout\",\n      \"pmids\": [\"9732265\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"Subunit-subunit interaction mapping within SNAPc revealed specific domains required for SNAP50 to associate with other subunits; complexes containing only the mapped interaction domains retain specific PSE binding.\",\n      \"method\": \"Co-immunoprecipitation, deletion/domain mapping, PSE binding assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — domain mapping with functional PSE binding confirmation, single lab\",\n      \"pmids\": [\"11056176\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"A mini-SNAPc composed of SNAP43, SNAP50, and the N-terminal third of SNAP190 binds cooperatively with TBP to the core U6 promoter and supports transcription; SNAP50 participates in cooperative TBP recruitment to the U6 TATA box.\",\n      \"method\": \"Recombinant mini-complex assembly, TBP recruitment assays, in vitro transcription\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — reconstituted mini-complex with defined subunits, functional transcription and TBP recruitment assays, single lab but multiple orthogonal methods\",\n      \"pmids\": [\"12391172\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"The 57 kDa subunit of the trypanosome PBP-1 transcription factor complex is orthologous to human SNAP50, indicating an evolutionarily conserved role for SNAP50-like subunits in snRNA gene transcription across large evolutionary distances.\",\n      \"method\": \"Biochemical purification, gene cloning, sequence/structural homology analysis\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — purification and cloning with functional context, ortholog inference supported by domain conservation\",\n      \"pmids\": [\"12486231\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"The SNAP50 zinc finger domain contains 15 cysteine and histidine residues in two potential zinc coordination arrangements, but binds only a single zinc atom; eight residues are critical for DNA binding by SNAPc, four of which are also essential for both U1 (RNA Pol II) and U6 (RNA Pol III) transcription. Defects in DNA binding caused by mutations in four residues can be suppressed by cooperative DNA binding with TFIIIB.\",\n      \"method\": \"Alanine-scanning mutagenesis, metal binding studies, PSE DNA binding assays, in vitro transcription\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — systematic active-site mutagenesis combined with metal binding and functional transcription assays, multiple orthogonal methods in a single rigorous study\",\n      \"pmids\": [\"16901896\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"A partial SNAPc comprising SNAP190(1-505), SNAP50, SNAP43, and SNAP19 co-expressed in E. coli binds PSE specifically, recruits TBP to U6 promoter DNA, and supports transcription of both human U1 and U6 snRNA genes by RNA polymerases II and III.\",\n      \"method\": \"Bacterial co-expression, PSE binding assay, TBP recruitment assay, reconstituted in vitro transcription\",\n      \"journal\": \"Protein expression and purification\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — reconstitution with defined subunits in bacteria, functional PSE binding and transcription assays, multiple orthogonal methods\",\n      \"pmids\": [\"16603380\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Cryo-EM structure of human mini-SNAPc (N-terminal domain of SNAP190, SNAP50, and SNAP43) bound to the U6-1 PSE at 3.49 Å resolution reveals that SNAP50 contributes three important motifs involved in both major groove and minor groove recognition of the PSE, acting in coordination with the SNAP190 Myb domain, with a 'wrap-around' binding mode.\",\n      \"method\": \"Cryo-electron microscopy structure determination, structural analysis of protein-DNA contacts\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — cryo-EM structure at near-atomic resolution with direct visualization of SNAP50-DNA contacts and mechanistic interpretation\",\n      \"pmids\": [\"36369505\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"SUMOylation-deficient SNAPC1 (2KR mutant) retains the ability to interact with SNAPC3 (SNAP50) but shows impaired interaction with SNAPC4, indicating that SNAPC3 interaction with SNAPC1 is independent of SNAPC1 SUMOylation status.\",\n      \"method\": \"Endogenous tagging of SNAPC3 and SNAPC4, co-immunoprecipitation with SUMOylation-deficient SNAPC1 mutant\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal endogenous-tag co-IP with defined mutant, single lab, functional context established\",\n      \"pmids\": [\"40956881\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"SNAPC3 (SNAP50) is an essential DNA-contacting subunit of the five-subunit SNAPc general transcription factor that binds the proximal sequence element (PSE) at snRNA promoters through an unorthodox zinc finger domain (with eight critical cysteine/histidine residues and a single bound zinc atom), cooperates with the SNAP190 Myb domain for major- and minor-groove PSE recognition (as revealed by cryo-EM structure), participates in cooperative TBP recruitment to the U6 TATA box, and is required for transcription of both RNA polymerase II and III snRNA genes; within the complex, SNAP50 directly interacts with SNAP43 (SNAPC1) in a manner that is independent of SNAPC1 SUMOylation.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"SNAPC3 (SNAP50) is an essential DNA-contacting core subunit of the snRNA-activating protein complex (SNAPc), a TBP-containing general transcription factor that binds the proximal sequence element (PSE) of snRNA promoters and is required for transcription of both RNA polymerase II and III snRNA genes [#0, #2]. Within the assembled five-subunit complex, SNAP50 makes direct sequence-specific contacts with PSE DNA through an unorthodox zinc finger domain that, despite containing 15 candidate cysteine/histidine residues across two potential coordination arrangements, binds only a single zinc atom; eight of these residues are critical for SNAPc DNA binding and a subset is essential for both U1 (Pol II) and U6 (Pol III) transcription [#1, #6]. SNAP50 recognizes the PSE through both major- and minor-groove contacts in coordination with the SNAP190 Myb domain in a 'wrap-around' binding mode, and participates in the cooperative recruitment of TBP to the U6 TATA box [#4, #8]. SNAP50 directly associates with SNAP43 (SNAPC1), an interaction that is independent of SNAPC1 SUMOylation status [#1, #9]. The role of SNAP50-like subunits in snRNA gene transcription is evolutionarily conserved [#5].\",\n  \"teleology\": [\n    {\n      \"year\": 1995,\n      \"claim\": \"Establishing that snRNA promoters require a dedicated factor answered how PSE-driven genes are transcribed by two different polymerases, identifying SNAP50 as a subunit of the PSE-binding SNAPc complex.\",\n      \"evidence\": \"Biochemical purification and in vitro transcription assays\",\n      \"pmids\": [\"7715707\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not resolve which subunit contacts DNA\", \"Subunit stoichiometry and individual roles undefined\"]\n    },\n    {\n      \"year\": 1996,\n      \"claim\": \"Cloning and cross-linking determined that SNAP50 itself directly contacts PSE DNA and is functionally required, distinguishing it from purely scaffolding subunits.\",\n      \"evidence\": \"cDNA cloning, UV cross-linking, antibody depletion with in vitro transcription readout, and co-immunoprecipitation\",\n      \"pmids\": [\"9003788\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Zinc finger metal coordination not directly demonstrated\", \"Specific DNA-contacting residues not mapped\"]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Full reconstitution from five recombinant subunits established SNAP50 as an essential core subunit rather than a co-purifying contaminant.\",\n      \"evidence\": \"Recombinant five-subunit reconstitution with PSE binding and in vitro transcription\",\n      \"pmids\": [\"9732265\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not map subunit-subunit contact surfaces\", \"No structural detail of DNA recognition\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"Domain mapping defined the minimal SNAP50 regions required for assembly into a PSE-binding-competent complex, clarifying the architecture of subunit interactions.\",\n      \"evidence\": \"Co-immunoprecipitation, deletion/domain mapping, PSE binding assays\",\n      \"pmids\": [\"11056176\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab mapping without structural confirmation\", \"Interaction interfaces inferred, not visualized\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"A minimal SNAP43/SNAP50/SNAP190 complex showed SNAP50 participates in cooperative TBP recruitment to the U6 TATA box, linking PSE binding to TATA-element promoter engagement.\",\n      \"evidence\": \"Recombinant mini-complex assembly, TBP recruitment assays, in vitro transcription\",\n      \"pmids\": [\"12391172\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism of SNAP50-TBP cooperativity not structurally defined\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Identification of an orthologous 57 kDa subunit in trypanosome PBP-1 demonstrated that the SNAP50 function in snRNA transcription is deeply conserved.\",\n      \"evidence\": \"Biochemical purification, gene cloning, sequence/structural homology analysis\",\n      \"pmids\": [\"12486231\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Functional equivalence inferred from homology\", \"Conservation of DNA-contact residues not tested\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Systematic mutagenesis defined the SNAP50 zinc finger as an unorthodox single-zinc domain and identified the residues critical for DNA binding versus transcription, mechanistically separating these functions.\",\n      \"evidence\": \"Alanine-scanning mutagenesis, metal binding studies, PSE DNA binding and in vitro transcription assays\",\n      \"pmids\": [\"16901896\", \"16603380\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Three-dimensional fold of the zinc finger not yet resolved\", \"Suppression by TFIIIB cooperativity mechanism unclear\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"The cryo-EM structure of mini-SNAPc on the PSE revealed how SNAP50 reads DNA, showing three motifs engaging both grooves alongside the SNAP190 Myb domain in a wrap-around mode.\",\n      \"evidence\": \"Cryo-EM structure of mini-SNAPc-PSE at 3.49 Å\",\n      \"pmids\": [\"36369505\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structure limited to a partial complex lacking SNAP45 and SNAP19\", \"TBP/TATA-bound state not captured\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Endogenous-tag co-IP with a SUMOylation-deficient SNAPC1 showed SNAP50-SNAPC1 contact is SUMO-independent while SNAPC4 interaction is SUMO-sensitive, refining how post-translational modification shapes complex assembly.\",\n      \"evidence\": \"Endogenous tagging and co-immunoprecipitation with SUMOylation-deficient SNAPC1 (2KR) mutant\",\n      \"pmids\": [\"40956881\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab finding\", \"Functional consequence of SUMO-dependent SNAPC4 interaction on transcription not established\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How SNAP50-mediated PSE recognition is integrated with full holo-SNAPc assembly and differential recruitment of Pol II versus Pol III machinery in vivo remains open.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No structure of the complete five-subunit SNAPc on promoter DNA\", \"Determinants of Pol II versus Pol III specificity downstream of SNAP50 binding unresolved\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0003677\", \"supporting_discovery_ids\": [1, 6, 8]},\n      {\"term_id\": \"GO:0140110\", \"supporting_discovery_ids\": [0, 2, 4]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [0, 2]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-74160\", \"supporting_discovery_ids\": [0, 2, 4]}\n    ],\n    \"complexes\": [\"SNAPc\"],\n    \"partners\": [\"SNAPC1\", \"SNAP190\", \"TBP\", \"SNAPC4\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"tie","faith_supported":5,"faith_total":5,"faith_pct":100.0}}