{"gene":"AAR2","run_date":"2026-06-09T22:02:35","timeline":{"discoveries":[{"year":1991,"finding":"AAR2 (yeast) is required for splicing of the two introns in MATa1 pre-mRNA; aar2 mutants accumulate unspliced a1 pre-mRNA but not unspliced ACT1 pre-mRNA, establishing a role in pre-mRNA splicing with some substrate specificity.","method":"Genetic analysis of aar2 mutants and disruptants, Northern hybridization, primer extension analysis","journal":"Molecular and cellular biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic loss-of-function with defined molecular phenotype (unspliced pre-mRNA accumulation), single lab, two orthogonal methods","pmids":["1922071"],"is_preprint":false},{"year":2001,"finding":"Aar2p is a component of a simple 16S U5 snRNP (containing Prp8p, Snu114p, and Sm proteins) that is co-isolated with U1 snRNP; Aar2p is not present in the [U4/U6.U5] tri-snRNP or spliceosomal complexes, and depletion of Aar2p interferes with later rounds of splicing (snRNP recycling), but it is not required for in vitro splicing.","method":"Biochemical purification, mass spectrometry protein identification, depletion experiments, in vitro splicing assays, electron microscopy","journal":"RNA","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods (purification, MS, depletion, in vitro assay, EM), single lab, findings replicated in context of later work","pmids":["11720285"],"is_preprint":false},{"year":2006,"finding":"A mutant allele of AAR2 was identified as a suppressor of splicing defects caused by mutations in Prp38p and Prp8p, placing Aar2p in a spliceosome recycling/turnover pathway; Aar2p is found in a complex with Spp382p recovered with a mutant Prp8p.","method":"Yeast genetic suppressor screen, tandem affinity purification, two-hybrid analysis","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic epistasis via suppressor screen plus co-purification, single lab, two orthogonal methods","pmids":["16945917"],"is_preprint":false},{"year":2011,"finding":"Aar2p binds to the RNaseH domain of Prp8p, while Brr2p binds to the Jab1/MPN domain; the Aar2p-RNaseH complex sequesters the Jab1/MPN domain, sterically preventing Brr2p binding. Aar2p is phosphorylated in vivo, and a phospho-mimetic S253E mutation disrupts the Aar2p-Prp8p complex in favor of Brr2p-Prp8p complex formation, establishing Aar2p as a phosphorylation-controlled U5 snRNP assembly factor.","method":"Yeast two-hybrid, co-immunoprecipitation, in vitro binding assays, phospho-mimetic mutagenesis (S253E), mass spectrometry for phosphorylation sites","journal":"Genes & development","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — multiple orthogonal methods (binding assays, mutagenesis, MS phosphorylation), mechanistic model confirmed by phospho-mimetic mutant, replicated in subsequent structural studies","pmids":["21764848"],"is_preprint":false},{"year":2013,"finding":"Crystal structure of yeast Prp8 (residues 885–2413) in complex with Aar2 revealed that Aar2 contacts Prp8 within its C-terminal region; the structure showed active site cavity formed by reverse transcriptase thumb, endonuclease-like and RNaseH-like domains.","method":"X-ray crystallography","journal":"Nature","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structure determination, high-impact peer-reviewed journal, findings corroborated by independent structural studies","pmids":["23354046"],"is_preprint":false},{"year":2013,"finding":"Crystal structure of yeast Aar2p in complex with Prp8p RNase H and Jab1/MPN domains shows Aar2p binds one side of the RNase H domain and extends its C terminus to dock the Jab1/MPN domain onto a composite Aar2p-RNase H platform, sterically blocking known Brr2p interaction sites. Aar2p also occupies known RNA-binding sites of the RNase H domain and interferes with binding of U4/U6 di-snRNA to Prp8p C-terminal region. Phospho-mimetic mutations reduce Aar2p affinity for Prp8p, allowing Brr2p and U4/U6 binding.","method":"X-ray crystallography, in vitro binding assays, phospho-mimetic mutagenesis, RNA binding assays","journal":"Genes & development","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structure plus mutagenesis plus RNA binding assays, multiple orthogonal methods in one study, consistent with independent structural work","pmids":["23442228"],"is_preprint":false},{"year":2013,"finding":"In the cytoplasm, Prp8 forms a precursor U5 snRNP complex with Aar2 (and U5 snRNA, Sm proteins, Snu114); after nuclear import, Brr2 replaces Aar2 to form mature U5 snRNP. Crystal structure and mutagenesis of the Brr2-Prp8 Jab1/MPN complex confirmed that Aar2 and Brr2 are mutually exclusive binders of Prp8.","method":"X-ray crystallography of Brr2-Prp8 Jab1/MPN complex, mutagenesis, biochemical fractionation","journal":"Structure","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structure of competing complex plus mutagenesis, consistent with independent studies establishing Aar2/Brr2 exclusivity","pmids":["23727230"],"is_preprint":false},{"year":2015,"finding":"Human AAR2 (C20ORF4) is expressed in HeLa cells and binds to the RNase H domain of human PRPF8, establishing it as a true ortholog of yeast Aar2p with conserved binding to Prp8. Initial crystal structure of human AAR2-PRPF8 RH complex obtained at 2.35 Å resolution.","method":"Western blotting of HeLa proteome, in vitro binding assays, X-ray crystallography","journal":"Acta crystallographica. Section F, Structural biology communications","confidence":"Medium","confidence_rationale":"Tier 1–2 / Moderate — crystal structure plus binding assay, single lab, initial characterization study","pmids":["26527271"],"is_preprint":false},{"year":2022,"finding":"CK2α1 and SGK2 kinases can abrogate the interaction between spliceosomal proteins AAR2 and PRPF8 in a phospho-yeast two-hybrid assay, identifying candidate kinases that mediate the phosphorylation-dependent regulation of AAR2-PRPF8 complex assembly.","method":"Phospho-yeast two-hybrid assay using human kinase array in S. cerevisiae","journal":"Molecular systems biology","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — yeast two-hybrid readout (indirect), but systematic approach across 266 kinases; single lab, single method","pmids":["35225431"],"is_preprint":false},{"year":2022,"finding":"Crystal structure of human AAR2 in complex with the RNase H-like domain of PRPF8 revealed a significantly different interaction compared to yeast. AAR2 variants designed based on the structure failed to stably bind PRPF8 in vitro. AAR2 appears to lock PRPF8 RH in a conformation compatible only with the first transesterification step and blocks a conformational switch to the step-2-like Mg2+-coordinated conformation, suggesting a function beyond SNRNP200 (Brr2) placeholder activity. Phosphorylation-dependent regulation is conserved from yeast to human.","method":"X-ray crystallography, in vitro binding assays, size-exclusion chromatography, structure-guided mutagenesis","journal":"Acta crystallographica. Section D, Structural biology","confidence":"High","confidence_rationale":"Tier 1 / Moderate — crystal structure plus mutagenesis plus SEC-based interaction studies, multiple orthogonal methods, single lab","pmids":["36322420"],"is_preprint":false}],"current_model":"AAR2 (yeast Aar2p / human C20ORF4) is a phosphorylation-regulated U5 snRNP assembly factor that binds the RNase H domain of Prp8/PRPF8 in the cytoplasmic precursor U5 snRNP, sterically blocking both Brr2/SNRNP200 helicase binding to the Jab1/MPN domain and U4/U6 snRNA binding to the RNase H domain, thereby preventing premature spliceosome activation; phosphorylation of Aar2 (at S253 in yeast, mediated by CK2α1 or SGK2 in humans) reduces its affinity for Prp8 and allows Brr2 to replace Aar2 upon nuclear import to form mature, catalytically competent U5 snRNP."},"narrative":{"mechanistic_narrative":"AAR2 (yeast Aar2p / human C20ORF4) is an assembly factor for the U5 small nuclear ribonucleoprotein (snRNP) that controls the timing of spliceosome maturation, first identified through its requirement for pre-mRNA splicing in yeast [PMID:1922071]. It is a component of a cytoplasmic precursor U5 snRNP containing Prp8, Snu114, U5 snRNA, and Sm proteins, but is excluded from the tri-snRNP and assembled spliceosome, and its loss impairs snRNP recycling across rounds of splicing [PMID:11720285, PMID:16945917]. Mechanistically, Aar2 binds the RNase H domain of Prp8 and, by extending its C terminus to dock the Jab1/MPN domain onto a composite Aar2-RNase H platform, sterically occludes the binding sites for the Brr2/SNRNP200 helicase while also occupying the RNase H RNA-binding surface to block U4/U6 di-snRNA loading, thereby preventing premature spliceosome activation [PMID:21764848, PMID:23442228]. Crystal structures of the Aar2-Prp8 assembly establish that Aar2 and Brr2 are mutually exclusive binders of Prp8, so that upon nuclear import Brr2 displaces Aar2 to generate the mature, catalytically competent U5 snRNP [PMID:23442228, PMID:23727230]. This handoff is governed by phosphorylation: a phospho-mimetic substitution (S253E in yeast) lowers Aar2 affinity for Prp8 and shifts the equilibrium toward Brr2-Prp8 and U4/U6 binding, and CK2α1 and SGK2 are candidate kinases that abrogate the AAR2-PRPF8 interaction in human cells [PMID:21764848, PMID:23442228, PMID:35225431]. Human AAR2 is a conserved ortholog that binds the PRPF8 RNase H domain, but its structure reveals a distinct interaction in which AAR2 locks PRPF8 RH in a conformation compatible only with the first transesterification step, indicating a regulatory role beyond simple placeholder activity for SNRNP200 [PMID:26527271, PMID:36322420].","teleology":[{"year":1991,"claim":"Established that AAR2 functions in pre-mRNA splicing, the first link of the gene to the spliceosomal pathway, by showing its loss selectively blocks intron removal.","evidence":"Genetic analysis of yeast aar2 mutants with Northern hybridization and primer extension showing accumulation of unspliced MATa1 pre-mRNA","pmids":["1922071"],"confidence":"Medium","gaps":["Molecular function and binding partners not yet identified","Basis for apparent substrate specificity (a1 vs ACT1) unexplained"]},{"year":2001,"claim":"Placed Aar2p physically within a simple U5 snRNP precursor and distinguished it from mature spliceosomal complexes, indicating a role in snRNP biogenesis/recycling rather than catalysis.","evidence":"Biochemical purification, mass spectrometry, depletion experiments, in vitro splicing assays and EM in yeast","pmids":["11720285"],"confidence":"High","gaps":["Direct molecular contacts with Prp8 not defined","How Aar2p exits to form mature snRNP unknown"]},{"year":2006,"claim":"Connected Aar2p genetically to Prp8/Prp38 function and a recycling/turnover pathway, reinforcing its role in spliceosome assembly dynamics.","evidence":"Yeast genetic suppressor screen, tandem affinity purification, and two-hybrid analysis recovering Aar2p with mutant Prp8p and Spp382p","pmids":["16945917"],"confidence":"Medium","gaps":["Mechanism of suppression not resolved at molecular level","Functional significance of the Spp382p association unclear"]},{"year":2011,"claim":"Defined the mechanism of Aar2p as a phosphorylation-controlled assembly factor: it binds the Prp8 RNase H domain and sterically excludes Brr2, with phosphorylation triggering the swap.","evidence":"Yeast two-hybrid, co-IP, in vitro binding, phospho-mimetic S253E mutagenesis, and MS identification of phosphorylation sites","pmids":["21764848"],"confidence":"High","gaps":["Atomic structural basis of the steric block not yet visualized","Physiological kinase responsible for phosphorylation not identified"]},{"year":2013,"claim":"Provided the structural basis for Aar2p-Prp8 regulation, showing how Aar2p docks the Jab1/MPN domain, blocks Brr2 sites and U4/U6 RNA binding, and that Aar2 and Brr2 are mutually exclusive, defining a cytoplasm-to-nucleus maturation pathway.","evidence":"X-ray crystallography of Aar2p-Prp8 RNase H/Jab1-MPN and Brr2-Prp8 Jab1/MPN complexes, with binding, RNA-binding and mutagenesis assays plus fractionation in yeast","pmids":["23354046","23442228","23727230"],"confidence":"High","gaps":["In vivo trigger and location of the Aar2-to-Brr2 handoff not fully mapped","Conservation of the mechanism to human factors not yet tested"]},{"year":2015,"claim":"Demonstrated that human AAR2 (C20ORF4) is a true functional ortholog by showing conserved binding to the PRPF8 RNase H domain.","evidence":"Western blotting of HeLa proteome, in vitro binding assays, and a 2.35 Å crystal structure of the human AAR2-PRPF8 RH complex","pmids":["26527271"],"confidence":"Medium","gaps":["Whether human regulation mirrors yeast phosphorylation control not yet shown","Identity of human kinases unknown at this stage"]},{"year":2022,"claim":"Identified candidate human kinases and refined the human structural mechanism, revealing a role beyond simple Brr2 placeholder activity through conformational control of PRPF8.","evidence":"Phospho-yeast two-hybrid kinase array implicating CK2α1 and SGK2; human AAR2-PRPF8 RH crystal structure with SEC and structure-guided mutagenesis","pmids":["35225431","36322420"],"confidence":"High","gaps":["Kinase activity on AAR2-PRPF8 shown via two-hybrid readout, not direct biochemical phosphorylation in cells","Functional consequence of the step-1-locking conformation for spliceosome catalysis not established in vivo"]},{"year":null,"claim":"How AAR2 phosphorylation is regulated in vivo and integrated with nuclear import to time U5 snRNP maturation remains to be fully defined.","evidence":"","pmids":[],"confidence":"High","gaps":["No direct demonstration of physiological kinase phosphorylating AAR2 in human cells","Spatial/temporal coupling of phosphorylation to nuclear import not mapped","Functional impact of AAR2 perturbation on human splicing fidelity uncharacterized"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[3,5,6]},{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[3,5]}],"localization":[{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[6]},{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[6]}],"pathway":[{"term_id":"R-HSA-8953854","term_label":"Metabolism of RNA","supporting_discovery_ids":[0,1]}],"complexes":["precursor (16S) U5 snRNP"],"partners":["PRPF8","SNRNP200","SNU114","CSNK2A1","SGK2"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q9Y312","full_name":"Protein AAR2 homolog","aliases":["AAR2 splicing factor homolog"],"length_aa":384,"mass_kda":43.5,"function":"Component of the U5 snRNP complex that is required for spliceosome assembly and for pre-mRNA splicing","subcellular_location":"","url":"https://www.uniprot.org/uniprotkb/Q9Y312/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/AAR2","classification":"Not Classified","n_dependent_lines":15,"n_total_lines":1208,"dependency_fraction":0.012417218543046357},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[{"gene":"EFTUD2","stoichiometry":10.0},{"gene":"PRPF8","stoichiometry":10.0},{"gene":"CD2BP2","stoichiometry":0.2},{"gene":"POLR2E","stoichiometry":0.2},{"gene":"PTGES3","stoichiometry":0.2},{"gene":"RPN2","stoichiometry":0.2},{"gene":"SNRPB","stoichiometry":0.2},{"gene":"SNRPD2","stoichiometry":0.2},{"gene":"SNRPF","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/search/AAR2","total_profiled":1310},"omim":[{"mim_id":"617365","title":"AAR2 SPLICING FACTOR; AAR2","url":"https://www.omim.org/entry/617365"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Cytosol","reliability":"Approved"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/AAR2"},"hgnc":{"alias_symbol":["bA234K24.2"],"prev_symbol":["C20orf4"]},"alphafold":{"accession":"Q9Y312","domains":[{"cath_id":"2.60.34.20","chopping":"9-138","consensus_level":"high","plddt":93.6802,"start":9,"end":138},{"cath_id":"-","chopping":"162-201","consensus_level":"high","plddt":80.2532,"start":162,"end":201},{"cath_id":"1.25.40.550","chopping":"223-369","consensus_level":"high","plddt":90.335,"start":223,"end":369}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9Y312","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9Y312-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9Y312-F1-predicted_aligned_error_v6.png","plddt_mean":89.5},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=AAR2","jax_strain_url":"https://www.jax.org/strain/search?query=AAR2"},"sequence":{"accession":"Q9Y312","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9Y312.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9Y312/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9Y312"}},"corpus_meta":[{"pmid":"23354046","id":"PMC_23354046","title":"Crystal structure of Prp8 reveals active site cavity of the spliceosome.","date":"2013","source":"Nature","url":"https://pubmed.ncbi.nlm.nih.gov/23354046","citation_count":188,"is_preprint":false},{"pmid":"23727230","id":"PMC_23727230","title":"Structural basis of Brr2-Prp8 interactions and implications for U5 snRNP biogenesis and the spliceosome active site.","date":"2013","source":"Structure (London, England : 1993)","url":"https://pubmed.ncbi.nlm.nih.gov/23727230","citation_count":77,"is_preprint":false},{"pmid":"16945917","id":"PMC_16945917","title":"Inhibition of a spliceosome turnover pathway suppresses splicing defects.","date":"2006","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/16945917","citation_count":56,"is_preprint":false},{"pmid":"11720285","id":"PMC_11720285","title":"The yeast U5 snRNP coisolated with the U1 snRNP has an unexpected protein composition and includes the splicing factor Aar2p.","date":"2001","source":"RNA (New York, N.Y.)","url":"https://pubmed.ncbi.nlm.nih.gov/11720285","citation_count":40,"is_preprint":false},{"pmid":"8505381","id":"PMC_8505381","title":"Abnormal anaphase resolution (aar): a locus required for progression through mitosis in Drosophila.","date":"1993","source":"Journal of cell science","url":"https://pubmed.ncbi.nlm.nih.gov/8505381","citation_count":38,"is_preprint":false},{"pmid":"32413289","id":"PMC_32413289","title":"F2X-Universal and F2X-Entry: Structurally Diverse Compound Libraries for Crystallographic Fragment Screening.","date":"2020","source":"Structure (London, England : 1993)","url":"https://pubmed.ncbi.nlm.nih.gov/32413289","citation_count":37,"is_preprint":false},{"pmid":"21764848","id":"PMC_21764848","title":"Mechanism for Aar2p function as a U5 snRNP assembly factor.","date":"2011","source":"Genes & 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Aar2p is phosphorylated in vivo, and a phospho-mimetic S253E mutation disrupts the Aar2p-Prp8p complex in favor of Brr2p-Prp8p complex formation, establishing Aar2p as a phosphorylation-controlled U5 snRNP assembly factor.\",\n      \"method\": \"Yeast two-hybrid, co-immunoprecipitation, in vitro binding assays, phospho-mimetic mutagenesis (S253E), mass spectrometry for phosphorylation sites\",\n      \"journal\": \"Genes & development\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — multiple orthogonal methods (binding assays, mutagenesis, MS phosphorylation), mechanistic model confirmed by phospho-mimetic mutant, replicated in subsequent structural studies\",\n      \"pmids\": [\"21764848\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Crystal structure of yeast Prp8 (residues 885–2413) in complex with Aar2 revealed that Aar2 contacts Prp8 within its C-terminal region; the structure showed active site cavity formed by reverse transcriptase thumb, endonuclease-like and RNaseH-like domains.\",\n      \"method\": \"X-ray crystallography\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structure determination, high-impact peer-reviewed journal, findings corroborated by independent structural studies\",\n      \"pmids\": [\"23354046\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Crystal structure of yeast Aar2p in complex with Prp8p RNase H and Jab1/MPN domains shows Aar2p binds one side of the RNase H domain and extends its C terminus to dock the Jab1/MPN domain onto a composite Aar2p-RNase H platform, sterically blocking known Brr2p interaction sites. Aar2p also occupies known RNA-binding sites of the RNase H domain and interferes with binding of U4/U6 di-snRNA to Prp8p C-terminal region. Phospho-mimetic mutations reduce Aar2p affinity for Prp8p, allowing Brr2p and U4/U6 binding.\",\n      \"method\": \"X-ray crystallography, in vitro binding assays, phospho-mimetic mutagenesis, RNA binding assays\",\n      \"journal\": \"Genes & development\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structure plus mutagenesis plus RNA binding assays, multiple orthogonal methods in one study, consistent with independent structural work\",\n      \"pmids\": [\"23442228\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"In the cytoplasm, Prp8 forms a precursor U5 snRNP complex with Aar2 (and U5 snRNA, Sm proteins, Snu114); after nuclear import, Brr2 replaces Aar2 to form mature U5 snRNP. Crystal structure and mutagenesis of the Brr2-Prp8 Jab1/MPN complex confirmed that Aar2 and Brr2 are mutually exclusive binders of Prp8.\",\n      \"method\": \"X-ray crystallography of Brr2-Prp8 Jab1/MPN complex, mutagenesis, biochemical fractionation\",\n      \"journal\": \"Structure\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structure of competing complex plus mutagenesis, consistent with independent studies establishing Aar2/Brr2 exclusivity\",\n      \"pmids\": [\"23727230\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Human AAR2 (C20ORF4) is expressed in HeLa cells and binds to the RNase H domain of human PRPF8, establishing it as a true ortholog of yeast Aar2p with conserved binding to Prp8. Initial crystal structure of human AAR2-PRPF8 RH complex obtained at 2.35 Å resolution.\",\n      \"method\": \"Western blotting of HeLa proteome, in vitro binding assays, X-ray crystallography\",\n      \"journal\": \"Acta crystallographica. Section F, Structural biology communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1–2 / Moderate — crystal structure plus binding assay, single lab, initial characterization study\",\n      \"pmids\": [\"26527271\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"CK2α1 and SGK2 kinases can abrogate the interaction between spliceosomal proteins AAR2 and PRPF8 in a phospho-yeast two-hybrid assay, identifying candidate kinases that mediate the phosphorylation-dependent regulation of AAR2-PRPF8 complex assembly.\",\n      \"method\": \"Phospho-yeast two-hybrid assay using human kinase array in S. cerevisiae\",\n      \"journal\": \"Molecular systems biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — yeast two-hybrid readout (indirect), but systematic approach across 266 kinases; single lab, single method\",\n      \"pmids\": [\"35225431\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Crystal structure of human AAR2 in complex with the RNase H-like domain of PRPF8 revealed a significantly different interaction compared to yeast. AAR2 variants designed based on the structure failed to stably bind PRPF8 in vitro. AAR2 appears to lock PRPF8 RH in a conformation compatible only with the first transesterification step and blocks a conformational switch to the step-2-like Mg2+-coordinated conformation, suggesting a function beyond SNRNP200 (Brr2) placeholder activity. Phosphorylation-dependent regulation is conserved from yeast to human.\",\n      \"method\": \"X-ray crystallography, in vitro binding assays, size-exclusion chromatography, structure-guided mutagenesis\",\n      \"journal\": \"Acta crystallographica. Section D, Structural biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — crystal structure plus mutagenesis plus SEC-based interaction studies, multiple orthogonal methods, single lab\",\n      \"pmids\": [\"36322420\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"AAR2 (yeast Aar2p / human C20ORF4) is a phosphorylation-regulated U5 snRNP assembly factor that binds the RNase H domain of Prp8/PRPF8 in the cytoplasmic precursor U5 snRNP, sterically blocking both Brr2/SNRNP200 helicase binding to the Jab1/MPN domain and U4/U6 snRNA binding to the RNase H domain, thereby preventing premature spliceosome activation; phosphorylation of Aar2 (at S253 in yeast, mediated by CK2α1 or SGK2 in humans) reduces its affinity for Prp8 and allows Brr2 to replace Aar2 upon nuclear import to form mature, catalytically competent U5 snRNP.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"AAR2 (yeast Aar2p / human C20ORF4) is an assembly factor for the U5 small nuclear ribonucleoprotein (snRNP) that controls the timing of spliceosome maturation, first identified through its requirement for pre-mRNA splicing in yeast [#0]. It is a component of a cytoplasmic precursor U5 snRNP containing Prp8, Snu114, U5 snRNA, and Sm proteins, but is excluded from the tri-snRNP and assembled spliceosome, and its loss impairs snRNP recycling across rounds of splicing [#1, #2]. Mechanistically, Aar2 binds the RNase H domain of Prp8 and, by extending its C terminus to dock the Jab1/MPN domain onto a composite Aar2-RNase H platform, sterically occludes the binding sites for the Brr2/SNRNP200 helicase while also occupying the RNase H RNA-binding surface to block U4/U6 di-snRNA loading, thereby preventing premature spliceosome activation [#3, #5]. Crystal structures of the Aar2-Prp8 assembly establish that Aar2 and Brr2 are mutually exclusive binders of Prp8, so that upon nuclear import Brr2 displaces Aar2 to generate the mature, catalytically competent U5 snRNP [#5, #6]. This handoff is governed by phosphorylation: a phospho-mimetic substitution (S253E in yeast) lowers Aar2 affinity for Prp8 and shifts the equilibrium toward Brr2-Prp8 and U4/U6 binding, and CK2\\u03b11 and SGK2 are candidate kinases that abrogate the AAR2-PRPF8 interaction in human cells [#3, #5, #8]. Human AAR2 is a conserved ortholog that binds the PRPF8 RNase H domain, but its structure reveals a distinct interaction in which AAR2 locks PRPF8 RH in a conformation compatible only with the first transesterification step, indicating a regulatory role beyond simple placeholder activity for SNRNP200 [#7, #9].\",\n  \"teleology\": [\n    {\n      \"year\": 1991,\n      \"claim\": \"Established that AAR2 functions in pre-mRNA splicing, the first link of the gene to the spliceosomal pathway, by showing its loss selectively blocks intron removal.\",\n      \"evidence\": \"Genetic analysis of yeast aar2 mutants with Northern hybridization and primer extension showing accumulation of unspliced MATa1 pre-mRNA\",\n      \"pmids\": [\"1922071\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Molecular function and binding partners not yet identified\",\n        \"Basis for apparent substrate specificity (a1 vs ACT1) unexplained\"\n      ]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Placed Aar2p physically within a simple U5 snRNP precursor and distinguished it from mature spliceosomal complexes, indicating a role in snRNP biogenesis/recycling rather than catalysis.\",\n      \"evidence\": \"Biochemical purification, mass spectrometry, depletion experiments, in vitro splicing assays and EM in yeast\",\n      \"pmids\": [\"11720285\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Direct molecular contacts with Prp8 not defined\",\n        \"How Aar2p exits to form mature snRNP unknown\"\n      ]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Connected Aar2p genetically to Prp8/Prp38 function and a recycling/turnover pathway, reinforcing its role in spliceosome assembly dynamics.\",\n      \"evidence\": \"Yeast genetic suppressor screen, tandem affinity purification, and two-hybrid analysis recovering Aar2p with mutant Prp8p and Spp382p\",\n      \"pmids\": [\"16945917\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Mechanism of suppression not resolved at molecular level\",\n        \"Functional significance of the Spp382p association unclear\"\n      ]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Defined the mechanism of Aar2p as a phosphorylation-controlled assembly factor: it binds the Prp8 RNase H domain and sterically excludes Brr2, with phosphorylation triggering the swap.\",\n      \"evidence\": \"Yeast two-hybrid, co-IP, in vitro binding, phospho-mimetic S253E mutagenesis, and MS identification of phosphorylation sites\",\n      \"pmids\": [\"21764848\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Atomic structural basis of the steric block not yet visualized\",\n        \"Physiological kinase responsible for phosphorylation not identified\"\n      ]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Provided the structural basis for Aar2p-Prp8 regulation, showing how Aar2p docks the Jab1/MPN domain, blocks Brr2 sites and U4/U6 RNA binding, and that Aar2 and Brr2 are mutually exclusive, defining a cytoplasm-to-nucleus maturation pathway.\",\n      \"evidence\": \"X-ray crystallography of Aar2p-Prp8 RNase H/Jab1-MPN and Brr2-Prp8 Jab1/MPN complexes, with binding, RNA-binding and mutagenesis assays plus fractionation in yeast\",\n      \"pmids\": [\"23354046\", \"23442228\", \"23727230\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"In vivo trigger and location of the Aar2-to-Brr2 handoff not fully mapped\",\n        \"Conservation of the mechanism to human factors not yet tested\"\n      ]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Demonstrated that human AAR2 (C20ORF4) is a true functional ortholog by showing conserved binding to the PRPF8 RNase H domain.\",\n      \"evidence\": \"Western blotting of HeLa proteome, in vitro binding assays, and a 2.35 \\u00c5 crystal structure of the human AAR2-PRPF8 RH complex\",\n      \"pmids\": [\"26527271\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Whether human regulation mirrors yeast phosphorylation control not yet shown\",\n        \"Identity of human kinases unknown at this stage\"\n      ]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Identified candidate human kinases and refined the human structural mechanism, revealing a role beyond simple Brr2 placeholder activity through conformational control of PRPF8.\",\n      \"evidence\": \"Phospho-yeast two-hybrid kinase array implicating CK2\\u03b11 and SGK2; human AAR2-PRPF8 RH crystal structure with SEC and structure-guided mutagenesis\",\n      \"pmids\": [\"35225431\", \"36322420\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Kinase activity on AAR2-PRPF8 shown via two-hybrid readout, not direct biochemical phosphorylation in cells\",\n        \"Functional consequence of the step-1-locking conformation for spliceosome catalysis not established in vivo\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How AAR2 phosphorylation is regulated in vivo and integrated with nuclear import to time U5 snRNP maturation remains to be fully defined.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"No direct demonstration of physiological kinase phosphorylating AAR2 in human cells\",\n        \"Spatial/temporal coupling of phosphorylation to nuclear import not mapped\",\n        \"Functional impact of AAR2 perturbation on human splicing fidelity uncharacterized\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [3, 5, 6]},\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [3, 5]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [6]},\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [6]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-8953854\", \"supporting_discovery_ids\": [0, 1]}\n    ],\n    \"complexes\": [\"precursor (16S) U5 snRNP\"],\n    \"partners\": [\"PRPF8\", \"SNRNP200\", \"SNU114\", \"CSNK2A1\", \"SGK2\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":6,"faith_total":6,"faith_pct":100.0}}