{"gene":"SNU13","run_date":"2026-04-28T20:42:08","timeline":{"discoveries":[{"year":2000,"finding":"Snu13p (yeast)/15.5K (human) was identified as a component of the U4/U6.U5 tri-snRNP that also associates with box C/D snoRNAs. The box C/D motif can be folded into a stem-internal loop-stem structure almost identical to the 15.5K binding site in U4 snRNA, and binds Snu13p/15.5K in vitro. Genetic depletion of Snu13p in yeast causes a major defect in RNA metabolism, establishing a shared core RNP structure between box C/D snoRNPs and the spliceosomal U4 snRNP.","method":"In vitro binding assay, yeast genetic depletion, structural RNA analysis, mass spectrometry","journal":"Cell","confidence":"High","confidence_rationale":"Tier 1–2 — reconstituted in vitro binding, genetic depletion with defined phenotype, replicated by multiple subsequent labs","pmids":["11081632"],"is_preprint":false},{"year":2002,"finding":"15.5K (SNU13) binding to the box C/D motif is essential for the hierarchical assembly of the remaining snoRNP proteins (NOP56, NOP58, fibrillarin, TIP48, TIP49) onto U14 snoRNA in vitro. The conserved stem II sequence of the box C/D motif (absent in U4 5' stem-loop) determines binding specificity of NOP56, NOP58, and fibrillarin, and is essential for nucleolar localization of U14 snoRNA in HeLa cells.","method":"In vitro RNP assembly assay, HeLa cell microinjection, mutational analysis of RNA elements","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 1–2 — in vitro reconstitution combined with cell-based localization assay and mutational dissection","pmids":["12417735"],"is_preprint":false},{"year":2002,"finding":"The association of hU3-55K with the U3 snoRNA box B/C motif is dependent on prior binding of 15.5K to the box B/C motif, as well as on a conserved flanking RNA structure. hU3-55K directly cross-links to the U3BC RNA, and the box B/C motif adopts a K-turn-like structure similar to the U4 5' stem-loop and box C/D motif.","method":"In vitro assembly assay, UV cross-linking, RNA structural analysis","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1–2 — in vitro reconstitution with multiple orthogonal methods (assembly, cross-linking, structural analysis)","pmids":["12381732"],"is_preprint":false},{"year":2003,"finding":"The 15.5K/Snu13p protein binds independently to both the C'/D and B/C motifs of U3 snoRNA. The B/C motif has a higher affinity for Snu13p than the C'/D motif despite having a highly reduced stem I (1 bp). Conservation of residues 2 and 3 in the B/C K-turn bulge is more important for Snu13p binding and U3 snoRNA function than corresponding residues in the C'/D K-turn, demonstrating that binding of 15.5K/Snu13p to both motifs is essential for U3 snoRNP assembly and activity.","method":"Chemical probing of in vitro reconstituted RNA/protein complexes, gel-shift assays, phylogenetic analysis, yeast functional assays with mutant U3 snoRNA","journal":"RNA (New York, N.Y.)","confidence":"High","confidence_rationale":"Tier 1–2 — multiple orthogonal biochemical and genetic methods, in vitro and in vivo validation","pmids":["12810916"],"is_preprint":false},{"year":2005,"finding":"The 15.5K protein folds its cognate K-turn RNA through an induced-fit mechanism: the free U4 snRNA 5' stem-loop lacks stable secondary structure and undergoes conformational fluctuations (loss of G-A base pairs, opening of the K-turn) that are suppressed upon 15.5K binding. Protein binding stabilizes specific RNA conformational states including canonical G-C base pairs and the kinked RNA backbone.","method":"Molecular dynamics simulation, chemical RNA modification experiments (biochemical probing of free vs. protein-bound RNA)","journal":"RNA (New York, N.Y.)","confidence":"Medium","confidence_rationale":"Tier 2–3 — computational simulation validated by chemical probing, single study","pmids":["15659359"],"is_preprint":false},{"year":2006,"finding":"15.5K mediates assembly of the U4/U6 snRNP, box C/D snoRNP, and U3 box B/C RNP through both RNA-induced conformational changes and direct protein-protein contacts. Surface mutagenesis of 15.5K demonstrated that each of the three distinct RNP complexes requires a distinct set of surface regions on 15.5K for assembly. Direct protein-protein interaction between 15.5K and hU3-55K was demonstrated, confirming that protein-protein contacts are essential alongside RNA-mediated interactions.","method":"Surface mutagenesis of 15.5K, in vitro RNP assembly assays, direct protein-protein interaction assay","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 1–2 — systematic mutagenesis with multiple RNP assembly assays and direct protein interaction demonstration","pmids":["16782898"],"is_preprint":false},{"year":2007,"finding":"The solution NMR structure of free 15.5K (SNU13) shows a well-structured protein with conformational flexibility primarily in the alpha3 helix. 15N NMR relaxation and H/D exchange experiments reveal fast time-scale motions and limited intermediate/slow motions, suggesting that local flexibility in alpha3 contributes to K-turn RNA recognition and binding.","method":"Solution NMR structure determination, 15N NMR relaxation, H/D exchange","journal":"Biochemistry","confidence":"Medium","confidence_rationale":"Tier 1 — NMR structure of free protein with dynamics characterization, single study without functional mutagenesis validation","pmids":["18044964"],"is_preprint":false},{"year":2013,"finding":"Snu13p/15.5K directly interacts with the assembly factor Rsa1p/NUFIP. NMR structure determination and docking identified a specific interface where residues R249, R246, K250 in Rsa1p and E72, D73 in Snu13p form electrostatic interactions shielded by hydrophobic residues, with W253 of Rsa1p inserted into a hydrophobic cavity of Snu13p. This interaction is mutually exclusive with active snoRNP protein interactions (based on archaeal sRNP structural templates). Individual mutations in yeast impair both cell growth and snoRNP formation, demonstrating that Rsa1p/NUFIP prevents premature activity of pre-snoRNPs.","method":"NMR structure determination, molecular docking, biophysical assays, yeast mutagenesis with growth and snoRNP formation readouts, imaging","journal":"Nucleic acids research","confidence":"High","confidence_rationale":"Tier 1–2 — NMR structure, docking model, multiple orthogonal biophysical methods, and in vivo genetic validation in yeast","pmids":["24234454"],"is_preprint":false},{"year":2016,"finding":"X-ray crystallographic analysis of Snu13 from the minimalist spliceosome of Cyanidioschyzon merolae (CmSnu13) revealed near-identical three-dimensional structure to yeast and human Snu13, demonstrating that Snu13 structure is deeply conserved. Evidence for conservation of Snu13 function in the C. merolae splicing pathway was also presented.","method":"X-ray crystallography","journal":"Protein science","confidence":"Medium","confidence_rationale":"Tier 1 — crystal structure, single study, functional conservation inferred rather than directly tested by mutagenesis","pmids":["26833716"],"is_preprint":false},{"year":2019,"finding":"The free yeast U14 C/D box snoRNA K-turn motif (kt-U14) adopts a pre-folded conformation similar to the protein-bound K-turn even in the absence of divalent ions, displaying weak hydrogen bonds in U•U and G•A tandem base pairs. Snu13p binding stabilizes and rigidifies this conformation, establishing kt-U14 as the first reference example for canonical C/D box snoRNA K-turn motifs.","method":"Solution NMR structure determination of free RNA, protein-binding NMR experiments","journal":"Biochimie","confidence":"Medium","confidence_rationale":"Tier 1 — NMR structure of free RNA with Snu13p binding characterization, single study","pmids":["30914254"],"is_preprint":false},{"year":2025,"finding":"Small molecule ligands that bind the K-turn region of the yeast U4 snRNA were identified and one analog (compound 22) inhibits the binding of U4 snRNA to Snu13 in biochemical assays with an IC50 of 3.2 ± 0.4 µM, demonstrating that the Snu13–U4 snRNA interaction can be pharmacologically disrupted.","method":"Small molecule microarray screening, biophysical binding assays, deltaSHAPE RNA structure probing, MD simulations, biochemical inhibition assay","journal":"bioRxiv","confidence":"Medium","confidence_rationale":"Tier 2 — multiple orthogonal biophysical and biochemical methods, preprint not yet peer-reviewed","pmids":["bio_10.1101_2025.10.10.681657"],"is_preprint":true}],"current_model":"SNU13 (human 15.5K / yeast Snu13p) is a K-turn RNA-binding protein that nucleates the hierarchical assembly of three distinct RNP complexes—the spliceosomal U4/U6 snRNP, box C/D snoRNPs, and the U3 snoRNP box B/C complex—by binding K-turn motifs through an induced-fit mechanism and then recruiting complex-specific proteins (NOP56, NOP58, fibrillarin for snoRNPs; hPrp31/61K for U4/U6) via distinct protein-protein contact surfaces; its interaction with the assembly factor Rsa1p/NUFIP (via a defined electrostatic interface involving E72/D73 of Snu13p) prevents premature snoRNP activation until mature complex formation is complete."},"narrative":{"teleology":[{"year":2000,"claim":"The discovery that Snu13p/15.5K binds both U4 snRNA and box C/D snoRNAs via a shared stem-internal loop-stem motif established SNU13 as a dual-function RNP core protein bridging splicing and rRNA modification machineries.","evidence":"In vitro binding assays, mass spectrometry of U4/U6.U5 tri-snRNP, yeast genetic depletion with RNA metabolism phenotype","pmids":["11081632"],"confidence":"High","gaps":["Mechanism of 15.5K recruitment to RNA not yet defined at atomic level","Whether 15.5K binding alone is sufficient for downstream protein assembly unknown"]},{"year":2002,"claim":"Reconstitution experiments demonstrated that 15.5K binding is the obligate first step in hierarchical box C/D snoRNP assembly and also nucleates U3 snoRNP box B/C complex formation, revealing a general organizational principle for K-turn-containing RNPs.","evidence":"In vitro RNP assembly on U14 snoRNA with sequential protein addition; UV cross-linking and assembly assays on U3 snoRNA box B/C motif in vitro; HeLa cell microinjection for nucleolar localization","pmids":["12417735","12381732"],"confidence":"High","gaps":["Whether 15.5K contacts downstream snoRNP proteins directly or only via RNA-mediated conformational changes was unresolved","Stoichiometry of 15.5K binding on U3 snoRNA (two K-turns) not fully characterized"]},{"year":2003,"claim":"Detailed analysis of the two K-turn motifs in U3 snoRNA showed that Snu13p binds both independently with different affinities, and that both binding events are required for U3 snoRNP function, establishing that multi-site K-turn recognition is functionally essential.","evidence":"Chemical probing of reconstituted complexes, gel-shift assays, phylogenetic analysis, yeast U3 mutant functional assays","pmids":["12810916"],"confidence":"High","gaps":["Structural basis for differential affinity of the two K-turns not resolved","Whether simultaneous occupancy of both sites by Snu13p is required in vivo unknown"]},{"year":2005,"claim":"Molecular dynamics and chemical probing revealed that 15.5K folds the K-turn RNA through an induced-fit mechanism, resolving the question of whether the RNA is pre-folded or actively structured by the protein.","evidence":"Molecular dynamics simulation of free and bound U4 5' stem-loop, chemical modification experiments","pmids":["15659359"],"confidence":"Medium","gaps":["Induced-fit conclusion based on computational simulation with limited experimental benchmarks","Whether the same induced-fit mechanism applies to box C/D K-turns not tested"]},{"year":2006,"claim":"Systematic surface mutagenesis of 15.5K demonstrated that it uses distinct protein surfaces to nucleate each of its three RNP complexes and that direct protein-protein contacts (not only RNA-mediated effects) are essential, explaining how one protein can serve as a platform for divergent assemblies.","evidence":"Surface mutagenesis of 15.5K tested in U4/U6, box C/D snoRNP, and U3 box B/C in vitro assembly assays; direct 15.5K–hU3-55K interaction assay","pmids":["16782898"],"confidence":"High","gaps":["Atomic-resolution structures of complete ternary complexes lacking","Whether surface utilization changes in the context of fully assembled particles unknown"]},{"year":2007,"claim":"The solution NMR structure of free 15.5K identified conformational flexibility in the alpha3 helix, suggesting a structural basis for adaptability in K-turn recognition.","evidence":"Solution NMR structure, 15N relaxation, H/D exchange on free human 15.5K protein","pmids":["18044964"],"confidence":"Medium","gaps":["No corresponding NMR structure of the RNA-bound form for direct comparison","Functional relevance of alpha3 flexibility not validated by mutagenesis"]},{"year":2013,"claim":"Structural and genetic characterization of the Snu13p–Rsa1p/NUFIP interface revealed a regulated assembly checkpoint: the Rsa1p binding surface on Snu13p overlaps with mature snoRNP protein contacts, preventing premature snoRNP activation until assembly factor release.","evidence":"NMR structure and molecular docking of Snu13p–Rsa1p complex, electrostatic interface mutagenesis, yeast growth and snoRNP formation assays","pmids":["24234454"],"confidence":"High","gaps":["Trigger for Rsa1p/NUFIP release during maturation not identified","Whether this checkpoint operates identically for all box C/D snoRNPs unknown"]},{"year":2019,"claim":"NMR of the free U14 K-turn motif showed it adopts a pre-folded conformation even without Snu13p or divalent ions, partially revising the earlier induced-fit model by showing that some C/D box K-turns are intrinsically structured before protein binding.","evidence":"Solution NMR of free kt-U14 RNA, NMR-monitored Snu13p binding","pmids":["30914254"],"confidence":"Medium","gaps":["Generalizability to other C/D box K-turns not tested","Quantitative comparison of pre-folding extent across different K-turn substrates lacking"]},{"year":null,"claim":"Key unresolved questions include the precise trigger for Rsa1p/NUFIP release during snoRNP maturation, whether the induced-fit versus pre-folded K-turn distinction has functional consequences for assembly kinetics in vivo, and whether pharmacological disruption of the Snu13–K-turn interface can be achieved with cellular activity.","evidence":"","pmids":[],"confidence":"Low","gaps":["No in vivo kinetic measurements of snoRNP assembly steps","No cell-active small molecule inhibitors of Snu13–RNA interaction reported in peer-reviewed literature","Full cryo-EM or crystal structures of complete human box C/D snoRNP with 15.5K in native context not described in this timeline"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0003723","term_label":"RNA binding","supporting_discovery_ids":[0,1,2,3,4,9]},{"term_id":"GO:0005198","term_label":"structural molecule activity","supporting_discovery_ids":[1,5,7]}],"localization":[{"term_id":"GO:0005730","term_label":"nucleolus","supporting_discovery_ids":[1]},{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[1,5]}],"pathway":[{"term_id":"R-HSA-8953854","term_label":"Metabolism of RNA","supporting_discovery_ids":[0,1,2,3,5,7]}],"complexes":["U4/U6 snRNP","box C/D snoRNP","U3 snoRNP"],"partners":["NOP56","NOP58","FBL","PRPF31","NUFIP1","U3-55K"],"other_free_text":[]},"mechanistic_narrative":"SNU13 (15.5K in human, Snu13p in yeast) is a K-turn RNA-binding protein that serves as the nucleation factor for the hierarchical assembly of three distinct RNP complexes: the spliceosomal U4/U6 snRNP, box C/D snoRNPs, and the U3 snoRNP box B/C complex [PMID:11081632, PMID:12417735, PMID:12381732]. It recognizes K-turn motifs in its target RNAs through an induced-fit mechanism that stabilizes the kinked RNA backbone, and it presents distinct protein-protein contact surfaces to recruit complex-specific partners such as NOP56, NOP58, fibrillarin, and hPrp31 for snoRNPs and U4/U6, respectively [PMID:16782898, PMID:15659359]. Its interaction with the assembly factor Rsa1p/NUFIP, mediated by an electrostatic interface centered on E72/D73 of Snu13p, is mutually exclusive with mature snoRNP protein contacts and prevents premature snoRNP activation [PMID:24234454]. The three-dimensional fold of SNU13 is deeply conserved from red algae to humans, and the Snu13–U4 snRNA interface has been shown to be pharmacologically targetable [PMID:26833716]."},"prefetch_data":{"uniprot":{"accession":"P55769","full_name":"NHP2-like protein 1","aliases":["High mobility group-like nuclear protein 2 homolog 1","OTK27","SNU13 homolog","hSNU13","U4/U6.U5 small nuclear ribonucleoprotein SNU13","U4/U6.U5 tri-snRNP 15.5 kDa protein"],"length_aa":128,"mass_kda":14.2,"function":"Part of the small subunit (SSU) processome, first precursor of the small eukaryotic ribosomal subunit. During the assembly of the SSU processome in the nucleolus, many ribosome biogenesis factors, an RNA chaperone and ribosomal proteins associate with the nascent pre-rRNA and work in concert to generate RNA folding, modifications, rearrangements and cleavage as well as targeted degradation of pre-ribosomal RNA by the RNA exosome (PubMed:34516797). Involved in pre-mRNA splicing as component of the spliceosome (PubMed:28781166). Binds to the 5'-stem-loop of U4 snRNA and thereby contributes to spliceosome assembly (PubMed:10545122, PubMed:17412961). The protein undergoes a conformational change upon RNA-binding (PubMed:10545122, PubMed:17412961, PubMed:28781166). Core component of box C/D small nucleolar ribonucleoprotein (snoRNP) complexes that function in methylation of multiple sites on ribosomal RNAs (rRNAs) and messenger RNAs (mRNAs) (PubMed:39570315)","subcellular_location":"Nucleus; Nucleus, nucleolus","url":"https://www.uniprot.org/uniprotkb/P55769/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":true,"resolved_as":"","url":"https://depmap.org/portal/gene/SNU13","classification":"Common Essential","n_dependent_lines":1207,"n_total_lines":1208,"dependency_fraction":0.9991721854304636},"opencell":{"profiled":true,"resolved_as":"","ensg_id":"ENSG00000100138","cell_line_id":"CID001053","localizations":[{"compartment":"nuclear_punctae","grade":3},{"compartment":"nucleolus_fc_dfc","grade":3}],"interactors":[{"gene":"NOP58","stoichiometry":10.0},{"gene":"RAB11A","stoichiometry":4.0},{"gene":"SNRPF","stoichiometry":4.0},{"gene":"CALD1","stoichiometry":0.2},{"gene":"CPSF6","stoichiometry":0.2},{"gene":"FBL","stoichiometry":0.2},{"gene":"POLR2A","stoichiometry":0.2},{"gene":"RBM39","stoichiometry":0.2},{"gene":"RBM42","stoichiometry":0.2},{"gene":"SNRPA","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/target/CID001053","total_profiled":1310},"omim":[{"mim_id":"620473","title":"ZINC FINGER HIT DOMAIN-CONTAINING PROTEIN 6; ZNHIT6","url":"https://www.omim.org/entry/620473"},{"mim_id":"620013","title":"RIBOSOMAL RNA-PROCESSING 9, U3 SMALL NUCLEOLAR RNA-BINDING PROTEIN; RRP9","url":"https://www.omim.org/entry/620013"},{"mim_id":"616663","title":"SMALL NUCLEOLAR RNA, C/D BOX, 118; SNORD118","url":"https://www.omim.org/entry/616663"},{"mim_id":"614561","title":"LEUKOENCEPHALOPATHY, BRAIN CALCIFICATIONS, AND CYSTS; LCC","url":"https://www.omim.org/entry/614561"},{"mim_id":"601304","title":"SMALL NUCLEAR RIBONUCLEOPROTEIN (U4/U6.U5), SNU13 HOMOLOG; SNU13","url":"https://www.omim.org/entry/601304"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/SNU13"},"hgnc":{"alias_symbol":["FA-1","SPAG12","SNRNP15-5","15.5K"],"prev_symbol":["SSFA1","NHP2L1"]},"alphafold":{"accession":"P55769","domains":[{"cath_id":"3.30.1330.30","chopping":"15-124","consensus_level":"high","plddt":97.1237,"start":15,"end":124}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/P55769","model_url":"https://alphafold.ebi.ac.uk/files/AF-P55769-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-P55769-F1-predicted_aligned_error_v6.png","plddt_mean":94.88},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=SNU13","jax_strain_url":"https://www.jax.org/strain/search?query=SNU13"},"sequence":{"accession":"P55769","fasta_url":"https://rest.uniprot.org/uniprotkb/P55769.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/P55769/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/P55769"}},"corpus_meta":[{"pmid":"12417735","id":"PMC_12417735","title":"Conserved stem II of the box C/D motif is essential for nucleolar localization and is required, along with the 15.5K protein, for the hierarchical assembly of the box C/D snoRNP.","date":"2002","source":"Molecular and cellular biology","url":"https://pubmed.ncbi.nlm.nih.gov/12417735","citation_count":173,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"3316276","id":"PMC_3316276","title":"Involvement of fertilization antigen (FA-1) in involuntary immunoinfertility in humans.","date":"1987","source":"The Journal of clinical investigation","url":"https://pubmed.ncbi.nlm.nih.gov/3316276","citation_count":78,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"3224746","id":"PMC_3224746","title":"Two fetal antigens (FA-1 and FA-2) and endometrial proteins (PP12 and PP14) isolated from amniotic fluid; preliminary observations in fetal and maternal tissues.","date":"1988","source":"European journal of obstetrics, gynecology, and reproductive biology","url":"https://pubmed.ncbi.nlm.nih.gov/3224746","citation_count":75,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"12810916","id":"PMC_12810916","title":"A structural, phylogenetic, and functional study of 15.5-kD/Snu13 protein binding on U3 small nucleolar RNA.","date":"2003","source":"RNA (New York, N.Y.)","url":"https://pubmed.ncbi.nlm.nih.gov/12810916","citation_count":58,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"7534465","id":"PMC_7534465","title":"Molecular identities of human sperm proteins that bind human zona pellucida: nature of sperm-zona interaction, tyrosine kinase activity, and involvement of FA-1.","date":"1994","source":"Molecular reproduction and development","url":"https://pubmed.ncbi.nlm.nih.gov/7534465","citation_count":50,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"15006749","id":"PMC_15006749","title":"Characterization of the arginine deiminase operon of Streptococcus rattus FA-1.","date":"2004","source":"Applied and environmental microbiology","url":"https://pubmed.ncbi.nlm.nih.gov/15006749","citation_count":44,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"2469607","id":"PMC_2469607","title":"Two fetal antigens (FA-1 and FA-2) and endometrial proteins (PP12 and PP14) isolated from amniotic fluid: localisation in the fetus and adult female genital tract.","date":"1989","source":"European journal of obstetrics, gynecology, and reproductive biology","url":"https://pubmed.ncbi.nlm.nih.gov/2469607","citation_count":43,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"16782898","id":"PMC_16782898","title":"Protein-protein and protein-RNA contacts both contribute to the 15.5K-mediated assembly of the U4/U6 snRNP and the box C/D snoRNPs.","date":"2006","source":"Molecular and cellular biology","url":"https://pubmed.ncbi.nlm.nih.gov/16782898","citation_count":41,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"3625608","id":"PMC_3625608","title":"The fertilization antigen (FA-1) causes a reduction of fertility in actively immunized female rabbits.","date":"1987","source":"Journal of reproductive immunology","url":"https://pubmed.ncbi.nlm.nih.gov/3625608","citation_count":39,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"28464500","id":"PMC_28464500","title":"CsI Pre-Intercalation in the Inorganic Framework for Efficient and Stable FA1-x Csx PbI3 (Cl) Perovskite Solar Cells.","date":"2017","source":"Small (Weinheim an der Bergstrasse, Germany)","url":"https://pubmed.ncbi.nlm.nih.gov/28464500","citation_count":39,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"12381732","id":"PMC_12381732","title":"The hU3-55K protein requires 15.5K binding to the box B/C motif as well as flanking RNA elements for its association with the U3 small nucleolar RNA in Vitro.","date":"2002","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/12381732","citation_count":38,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"15659359","id":"PMC_15659359","title":"The snRNP 15.5K protein folds its cognate K-turn RNA: a combined theoretical and biochemical study.","date":"2005","source":"RNA (New York, N.Y.)","url":"https://pubmed.ncbi.nlm.nih.gov/15659359","citation_count":37,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"24234454","id":"PMC_24234454","title":"Characterization of the interaction between protein Snu13p/15.5K and the Rsa1p/NUFIP factor and demonstration of its functional importance for snoRNP assembly.","date":"2013","source":"Nucleic acids research","url":"https://pubmed.ncbi.nlm.nih.gov/24234454","citation_count":33,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"2673845","id":"PMC_2673845","title":"The detection in human sera of antisperm antibodies reactive with FA-1, an evolutionarily conserved antigen, and with murine spermatozoa.","date":"1989","source":"Fertility and sterility","url":"https://pubmed.ncbi.nlm.nih.gov/2673845","citation_count":31,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"2318530","id":"PMC_2318530","title":"Isolation, characterization, and inhibition kinetics of enolase from Streptococcus rattus FA-1.","date":"1990","source":"Infection and immunity","url":"https://pubmed.ncbi.nlm.nih.gov/2318530","citation_count":29,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"1875328","id":"PMC_1875328","title":"Human spermatozoal FA-1 binds with ZP3 of porcine zona pellucida.","date":"1991","source":"Journal of reproductive immunology","url":"https://pubmed.ncbi.nlm.nih.gov/1875328","citation_count":29,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"2372399","id":"PMC_2372399","title":"Antibodies to sperm surface fertilization antigen (FA-1): their specificities and site of 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The conserved stem II of the box C/D motif (distinct from U4 snRNA) determines specific recruitment of NOP56, NOP58, fibrillarin, TIP48, and TIP49 but not 15.5K itself, and is required for nucleolar localization of U14 snoRNA.\",\n      \"method\": \"In vitro assembly assay with purified proteins and RNA; HeLa cell microinjection for nucleolar localization; mutational analysis of stem II\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — in vitro reconstitution with multiple RNA/protein combinations plus cell-based localization assay, replicated across multiple constructs\",\n      \"pmids\": [\"12417735\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"The 15.5K/Snu13p protein independently binds two K-turn motifs in U3 snoRNA (B/C and C'/D motifs). Snu13p has higher affinity for the B/C motif than the C'/D motif. The B/C K-turn lacks a well-formed stem I in naked RNA but is stabilized upon protein binding. Conservation of specific residues at positions 2 and 3 in the B/C K-turn is more critical for Snu13p binding and U3 function than corresponding residues in the C'/D K-turn. Binding of 15.5K/Snu13p to both motifs is required for U3 snoRNP assembly and activity.\",\n      \"method\": \"Chemical probing of in vitro reconstituted RNA-protein complexes; gel-shift assays; phylogenetic analysis; yeast functional assays with truncated U3 snoRNA carrying base substitutions\",\n      \"journal\": \"RNA (New York, N.Y.)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — multiple orthogonal methods including chemical probing, gel-shift, and in vivo yeast functional assays\",\n      \"pmids\": [\"12810916\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"hU3-55K association with U3 snoRNA is dependent on prior binding of 15.5K to the box B/C motif; 15.5K binding to box B/C creates a binding platform that enables hU3-55K recruitment. A conserved RNA structure flanking the box B/C motif is additionally required for hU3-55K binding. hU3-55K directly cross-links to the U3BC RNA.\",\n      \"method\": \"In vitro assembly assay; UV cross-linking; mutational analysis of box B/C flanking sequences\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal in vitro assembly with defined dependencies plus direct cross-linking\",\n      \"pmids\": [\"12381732\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"15.5K mediates RNP assembly not only through RNA-induced conformational changes (induced fit/K-turn folding) but also through direct protein-protein interactions. Distinct surface regions of 15.5K are required for assembly of each of the three RNP complexes (U4/U6 snRNP, box C/D snoRNP, U3 B/C RNP). Direct protein-protein interaction between hU3-55K and 15.5K was demonstrated.\",\n      \"method\": \"Surface mutagenesis of 15.5K followed by in vitro RNP assembly assays; direct protein-protein interaction assays (pulldown/co-IP)\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — systematic mutagenesis of protein surface combined with functional RNP assembly assays and direct interaction demonstration\",\n      \"pmids\": [\"16782898\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"The 15.5K protein folds its cognate K-turn RNA via an induced-fit mechanism: the free U4 snRNA 5' stem-loop is flexible and lacks stable secondary structure elements (G-A base pairs, K-turn), and protein binding stabilizes the K-turn conformation. Conformational transitions including loss of G-A base pairs and opening of the K-turn occur only in the free RNA.\",\n      \"method\": \"Molecular dynamics simulations; chemical RNA modification experiments; NMR-based structural analysis\",\n      \"journal\": \"RNA (New York, N.Y.)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — combined computational and biochemical (chemical probing) approaches with experimental validation of simulation predictions\",\n      \"pmids\": [\"15659359\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Snu13p/15.5K directly interacts with the assembly factor Rsa1p/NUFIP. The interface involves electrostatic interactions between R249, R246, K250 in Rsa1p and E72, D73 in Snu13p, with W253 of Rsa1p inserted into a hydrophobic cavity of Snu13p. This interaction is mutually exclusive with Snu13p interactions in the active snoRNP (based on archaeal sRNP structures), suggesting Rsa1p/NUFIP prevents premature snoRNP activity during assembly. Mutations at the interface impair cell growth and snoRNP formation in yeast.\",\n      \"method\": \"NMR structure determination; docking; biophysical interaction assays; yeast mutagenesis with growth and snoRNP formation readouts; imaging approaches\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — NMR structure of interface, functional mutagenesis in yeast, and multiple orthogonal biophysical methods\",\n      \"pmids\": [\"24234454\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"The solution NMR structure of free 15.5K protein shows it is well-structured with conformational flexibility specifically in the alpha3 helix region. Fast timescale motions and limited intermediate/slow motions in the free protein may contribute to K-turn RNA recognition and binding.\",\n      \"method\": \"Solution NMR structure determination; 15N NMR relaxation; H/D exchange experiments\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 — NMR structure of free protein with relaxation analysis, single study\",\n      \"pmids\": [\"18044964\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"X-ray crystallographic analysis of Snu13 from the red alga Cyanidioschyzon merolae (CmSnu13) shows near-identical three-dimensional structure to yeast and human Snu13, confirming conservation of the protein fold across species. Evidence for conservation of Snu13 function in algal pre-mRNA splicing was presented.\",\n      \"method\": \"X-ray crystallography; biochemical functional assays\",\n      \"journal\": \"Protein science : a publication of the Protein Society\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 — crystal structure, but primarily a structural conservation study from a single lab\",\n      \"pmids\": [\"26833716\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Snu13p binds the K-turn motif of yeast U14 snoRNA (kt-U14) and stabilizes its structure. The free kt-U14 RNA adopts a pre-folded conformation similar to the protein-bound K-turn even without divalent ions (a 'weak' K-turn), displaying weak U•U and G•A tandem base pairs, and this conformation is further stabilized upon Snu13p binding.\",\n      \"method\": \"NMR solution structure of free kt-U14 RNA; binding assays with Snu13p\",\n      \"journal\": \"Biochimie\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 — NMR structure of RNA, but functional consequences of pre-folding not deeply dissected, single lab\",\n      \"pmids\": [\"30914254\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Small molecules can inhibit binding of U4 snRNA to Snu13 by targeting the K-turn region of the yeast U4/U6 snRNA 5' stem-loop. One compound (analog 22) binds the K-turn region with KD = 3.9 µM and inhibits U4-Snu13 interaction with IC50 = 3.2 µM in biochemical assays.\",\n      \"method\": \"Small molecule microarray screening; biophysical binding assays; deltaSHAPE chemical probing; MD simulations; biochemical inhibition assays\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (SMM, biophysics, SHAPE, MD) but preprint, not yet peer-reviewed\",\n      \"pmids\": [\"bio_10.1101_2025.10.10.681657\"],\n      \"is_preprint\": true\n    }\n  ],\n  \"current_model\": \"SNU13 (15.5K in humans, Snu13p in yeast) is a K-turn RNA-binding protein that uses an induced-fit mechanism to fold and bind the 5' stem-loop of U4 snRNA and the box C/D and B/C motifs of snoRNAs, thereby nucleating hierarchical assembly of the U4/U6 snRNP, box C/D snoRNPs, and the U3 snoRNP through a combination of RNA conformational changes and direct protein-protein contacts with complex-specific proteins (NOP56, NOP58, fibrillarin, hU3-55K) and the assembly factor Rsa1p/NUFIP, which scaffolds pre-snoRNPs and prevents premature activity until active snoRNPs are formed.\"\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2000,\n      \"finding\": \"Snu13p (yeast)/15.5K (human) was identified as a component of the U4/U6.U5 tri-snRNP that also associates with box C/D snoRNAs. The box C/D motif can be folded into a stem-internal loop-stem structure almost identical to the 15.5K binding site in U4 snRNA, and binds Snu13p/15.5K in vitro. Genetic depletion of Snu13p in yeast causes a major defect in RNA metabolism, establishing a shared core RNP structure between box C/D snoRNPs and the spliceosomal U4 snRNP.\",\n      \"method\": \"In vitro binding assay, yeast genetic depletion, structural RNA analysis, mass spectrometry\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — reconstituted in vitro binding, genetic depletion with defined phenotype, replicated by multiple subsequent labs\",\n      \"pmids\": [\"11081632\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"15.5K (SNU13) binding to the box C/D motif is essential for the hierarchical assembly of the remaining snoRNP proteins (NOP56, NOP58, fibrillarin, TIP48, TIP49) onto U14 snoRNA in vitro. The conserved stem II sequence of the box C/D motif (absent in U4 5' stem-loop) determines binding specificity of NOP56, NOP58, and fibrillarin, and is essential for nucleolar localization of U14 snoRNA in HeLa cells.\",\n      \"method\": \"In vitro RNP assembly assay, HeLa cell microinjection, mutational analysis of RNA elements\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — in vitro reconstitution combined with cell-based localization assay and mutational dissection\",\n      \"pmids\": [\"12417735\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"The association of hU3-55K with the U3 snoRNA box B/C motif is dependent on prior binding of 15.5K to the box B/C motif, as well as on a conserved flanking RNA structure. hU3-55K directly cross-links to the U3BC RNA, and the box B/C motif adopts a K-turn-like structure similar to the U4 5' stem-loop and box C/D motif.\",\n      \"method\": \"In vitro assembly assay, UV cross-linking, RNA structural analysis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — in vitro reconstitution with multiple orthogonal methods (assembly, cross-linking, structural analysis)\",\n      \"pmids\": [\"12381732\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"The 15.5K/Snu13p protein binds independently to both the C'/D and B/C motifs of U3 snoRNA. The B/C motif has a higher affinity for Snu13p than the C'/D motif despite having a highly reduced stem I (1 bp). Conservation of residues 2 and 3 in the B/C K-turn bulge is more important for Snu13p binding and U3 snoRNA function than corresponding residues in the C'/D K-turn, demonstrating that binding of 15.5K/Snu13p to both motifs is essential for U3 snoRNP assembly and activity.\",\n      \"method\": \"Chemical probing of in vitro reconstituted RNA/protein complexes, gel-shift assays, phylogenetic analysis, yeast functional assays with mutant U3 snoRNA\",\n      \"journal\": \"RNA (New York, N.Y.)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — multiple orthogonal biochemical and genetic methods, in vitro and in vivo validation\",\n      \"pmids\": [\"12810916\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"The 15.5K protein folds its cognate K-turn RNA through an induced-fit mechanism: the free U4 snRNA 5' stem-loop lacks stable secondary structure and undergoes conformational fluctuations (loss of G-A base pairs, opening of the K-turn) that are suppressed upon 15.5K binding. Protein binding stabilizes specific RNA conformational states including canonical G-C base pairs and the kinked RNA backbone.\",\n      \"method\": \"Molecular dynamics simulation, chemical RNA modification experiments (biochemical probing of free vs. protein-bound RNA)\",\n      \"journal\": \"RNA (New York, N.Y.)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — computational simulation validated by chemical probing, single study\",\n      \"pmids\": [\"15659359\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"15.5K mediates assembly of the U4/U6 snRNP, box C/D snoRNP, and U3 box B/C RNP through both RNA-induced conformational changes and direct protein-protein contacts. Surface mutagenesis of 15.5K demonstrated that each of the three distinct RNP complexes requires a distinct set of surface regions on 15.5K for assembly. Direct protein-protein interaction between 15.5K and hU3-55K was demonstrated, confirming that protein-protein contacts are essential alongside RNA-mediated interactions.\",\n      \"method\": \"Surface mutagenesis of 15.5K, in vitro RNP assembly assays, direct protein-protein interaction assay\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — systematic mutagenesis with multiple RNP assembly assays and direct protein interaction demonstration\",\n      \"pmids\": [\"16782898\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"The solution NMR structure of free 15.5K (SNU13) shows a well-structured protein with conformational flexibility primarily in the alpha3 helix. 15N NMR relaxation and H/D exchange experiments reveal fast time-scale motions and limited intermediate/slow motions, suggesting that local flexibility in alpha3 contributes to K-turn RNA recognition and binding.\",\n      \"method\": \"Solution NMR structure determination, 15N NMR relaxation, H/D exchange\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 — NMR structure of free protein with dynamics characterization, single study without functional mutagenesis validation\",\n      \"pmids\": [\"18044964\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Snu13p/15.5K directly interacts with the assembly factor Rsa1p/NUFIP. NMR structure determination and docking identified a specific interface where residues R249, R246, K250 in Rsa1p and E72, D73 in Snu13p form electrostatic interactions shielded by hydrophobic residues, with W253 of Rsa1p inserted into a hydrophobic cavity of Snu13p. This interaction is mutually exclusive with active snoRNP protein interactions (based on archaeal sRNP structural templates). Individual mutations in yeast impair both cell growth and snoRNP formation, demonstrating that Rsa1p/NUFIP prevents premature activity of pre-snoRNPs.\",\n      \"method\": \"NMR structure determination, molecular docking, biophysical assays, yeast mutagenesis with growth and snoRNP formation readouts, imaging\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — NMR structure, docking model, multiple orthogonal biophysical methods, and in vivo genetic validation in yeast\",\n      \"pmids\": [\"24234454\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"X-ray crystallographic analysis of Snu13 from the minimalist spliceosome of Cyanidioschyzon merolae (CmSnu13) revealed near-identical three-dimensional structure to yeast and human Snu13, demonstrating that Snu13 structure is deeply conserved. Evidence for conservation of Snu13 function in the C. merolae splicing pathway was also presented.\",\n      \"method\": \"X-ray crystallography\",\n      \"journal\": \"Protein science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 — crystal structure, single study, functional conservation inferred rather than directly tested by mutagenesis\",\n      \"pmids\": [\"26833716\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"The free yeast U14 C/D box snoRNA K-turn motif (kt-U14) adopts a pre-folded conformation similar to the protein-bound K-turn even in the absence of divalent ions, displaying weak hydrogen bonds in U•U and G•A tandem base pairs. Snu13p binding stabilizes and rigidifies this conformation, establishing kt-U14 as the first reference example for canonical C/D box snoRNA K-turn motifs.\",\n      \"method\": \"Solution NMR structure determination of free RNA, protein-binding NMR experiments\",\n      \"journal\": \"Biochimie\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 — NMR structure of free RNA with Snu13p binding characterization, single study\",\n      \"pmids\": [\"30914254\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Small molecule ligands that bind the K-turn region of the yeast U4 snRNA were identified and one analog (compound 22) inhibits the binding of U4 snRNA to Snu13 in biochemical assays with an IC50 of 3.2 ± 0.4 µM, demonstrating that the Snu13–U4 snRNA interaction can be pharmacologically disrupted.\",\n      \"method\": \"Small molecule microarray screening, biophysical binding assays, deltaSHAPE RNA structure probing, MD simulations, biochemical inhibition assay\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal biophysical and biochemical methods, preprint not yet peer-reviewed\",\n      \"pmids\": [\"bio_10.1101_2025.10.10.681657\"],\n      \"is_preprint\": true\n    }\n  ],\n  \"current_model\": \"SNU13 (human 15.5K / yeast Snu13p) is a K-turn RNA-binding protein that nucleates the hierarchical assembly of three distinct RNP complexes—the spliceosomal U4/U6 snRNP, box C/D snoRNPs, and the U3 snoRNP box B/C complex—by binding K-turn motifs through an induced-fit mechanism and then recruiting complex-specific proteins (NOP56, NOP58, fibrillarin for snoRNPs; hPrp31/61K for U4/U6) via distinct protein-protein contact surfaces; its interaction with the assembly factor Rsa1p/NUFIP (via a defined electrostatic interface involving E72/D73 of Snu13p) prevents premature snoRNP activation until mature complex formation is complete.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"SNU13 (15.5K in humans, Snu13p in yeast) is a K-turn RNA-binding protein that serves as the nucleation factor for assembly of the U4/U6 snRNP, box C/D snoRNPs, and the U3 snoRNP. It directly binds K-turn motifs in the 5′ stem-loop of U4 snRNA and in the box C/D and B/C motifs of snoRNAs via an induced-fit mechanism that stabilizes the K-turn conformation from a flexible or pre-folded RNA state, and this initial binding event is a prerequisite for hierarchical recruitment of complex-specific proteins including NOP56, NOP58, fibrillarin, and hU3-55K [PMID:12417735, PMID:15659359, PMID:12810916]. Distinct surface regions of SNU13 mediate RNA-induced conformational changes and direct protein–protein contacts required for each of the three RNP complexes, with hU3-55K binding depending on both prior SNU13 occupancy of box B/C and flanking RNA structural elements [PMID:16782898, PMID:12381732]. During snoRNP biogenesis, SNU13 interacts with the assembly factor Rsa1p/NUFIP through a surface that overlaps with the active snoRNP interface, thereby preventing premature catalytic activity until assembly is complete [PMID:24234454].\",\n  \"teleology\": [\n    {\n      \"year\": 2002,\n      \"claim\": \"Establishing that 15.5K/SNU13 is the primary RNA-binding initiator of box C/D snoRNP assembly resolved how the hierarchical recruitment of NOP56, NOP58, fibrillarin, and other proteins is nucleated on snoRNAs and U4 snRNA.\",\n      \"evidence\": \"In vitro reconstitution with purified proteins and RNA mutants; HeLa cell microinjection for nucleolar localization\",\n      \"pmids\": [\"12417735\", \"12381732\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"The structural basis for how stem II of the box C/D motif distinguishes snoRNP-specific from U4-specific protein recruitment was not resolved\",\n        \"Contributions of protein–protein versus RNA–protein contacts to downstream assembly were not separated\"\n      ]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Demonstrating that SNU13 independently recognizes two K-turn motifs in U3 snoRNA with different affinities, and that both are required for U3 function, explained how a single protein nucleates a bipartite RNP scaffold on U3.\",\n      \"evidence\": \"Chemical probing of reconstituted complexes; gel-shift assays; yeast functional assays with U3 mutants\",\n      \"pmids\": [\"12810916\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"How the differential affinity for B/C versus C′/D K-turns is translated into ordered assembly was not determined\",\n        \"The structural basis for the incomplete stem I in the B/C K-turn before protein binding was unknown\"\n      ]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Showing that the free U4 snRNA K-turn is flexible and that 15.5K folds it via induced fit established the mechanism of RNA conformational selection versus folding in K-turn recognition.\",\n      \"evidence\": \"Molecular dynamics simulations validated by chemical modification and NMR-based analysis of free and bound RNA\",\n      \"pmids\": [\"15659359\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Whether all K-turn substrates (snoRNAs) use the same induced-fit pathway or some are pre-folded was unresolved\",\n        \"Kinetic parameters of the folding-upon-binding process were not measured\"\n      ]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Systematic surface mutagenesis revealed that distinct protein faces of 15.5K mediate assembly of U4/U6, box C/D, and U3 RNPs, separating RNA-folding from protein–protein interaction functions and explaining how one protein can nucleate three different complexes.\",\n      \"evidence\": \"Surface mutagenesis of 15.5K with in vitro RNP assembly assays; pulldown demonstrating direct 15.5K–hU3-55K interaction\",\n      \"pmids\": [\"16782898\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Atomic-resolution structures of the 15.5K interfaces with each partner complex were not available\",\n        \"Whether the same surface logic applies in vivo under competitive conditions was untested\"\n      ]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Determining the NMR structure of the Snu13p–Rsa1p interface and showing its overlap with the active snoRNP binding surface established that the assembly chaperone NUFIP/Rsa1p prevents premature snoRNP activity by occupying the same site on SNU13 used in the mature complex.\",\n      \"evidence\": \"NMR structure of interface; yeast mutagenesis with growth and snoRNP formation readouts; biophysical interaction assays\",\n      \"pmids\": [\"24234454\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"The hand-off mechanism from Rsa1p/NUFIP to NOP proteins during maturation was not captured\",\n        \"Whether NUFIP occupancy regulates SNU13 on all snoRNA substrates equally was not addressed\"\n      ]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Solving the NMR structure of free yeast U14 snoRNA K-turn revealed a pre-folded 'weak' K-turn conformation, demonstrating that not all SNU13 substrates start from a fully unfolded state and refining the induced-fit model.\",\n      \"evidence\": \"NMR solution structure of free kt-U14 RNA; Snu13p binding assays\",\n      \"pmids\": [\"30914254\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Functional consequences of pre-folding for assembly kinetics or snoRNP stability were not tested\",\n        \"Whether weak K-turn pre-folding correlates with binding affinity across all snoRNA targets was not determined\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How SNU13 transitions between assembly factor-bound, RNA-bound, and mature RNP-integrated states in vivo, and whether its different K-turn substrates compete for a limiting pool of the protein, remain open questions.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"No in vivo kinetic or stoichiometric measurements of SNU13 partitioning among its RNA targets\",\n        \"No high-resolution structure of a complete human box C/D snoRNP with SNU13\",\n        \"Regulatory inputs controlling SNU13 availability or activity are unknown\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0003723\", \"supporting_discovery_ids\": [0, 1, 4, 8]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005730\", \"supporting_discovery_ids\": [0]},\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [0, 1]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-8953854\", \"supporting_discovery_ids\": [0, 1, 2, 4]}\n    ],\n    \"complexes\": [\n      \"U4/U6 snRNP\",\n      \"box C/D snoRNP\",\n      \"U3 snoRNP\"\n    ],\n    \"partners\": [\n      \"NOP56\",\n      \"NOP58\",\n      \"FBL\",\n      \"NUFIP1\",\n      \"U3-55K\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"mechanistic_narrative\": \"SNU13 (15.5K in human, Snu13p in yeast) is a K-turn RNA-binding protein that serves as the nucleation factor for the hierarchical assembly of three distinct RNP complexes: the spliceosomal U4/U6 snRNP, box C/D snoRNPs, and the U3 snoRNP box B/C complex [PMID:11081632, PMID:12417735, PMID:12381732]. It recognizes K-turn motifs in its target RNAs through an induced-fit mechanism that stabilizes the kinked RNA backbone, and it presents distinct protein-protein contact surfaces to recruit complex-specific partners such as NOP56, NOP58, fibrillarin, and hPrp31 for snoRNPs and U4/U6, respectively [PMID:16782898, PMID:15659359]. Its interaction with the assembly factor Rsa1p/NUFIP, mediated by an electrostatic interface centered on E72/D73 of Snu13p, is mutually exclusive with mature snoRNP protein contacts and prevents premature snoRNP activation [PMID:24234454]. The three-dimensional fold of SNU13 is deeply conserved from red algae to humans, and the Snu13–U4 snRNA interface has been shown to be pharmacologically targetable [PMID:26833716].\",\n  \"teleology\": [\n    {\n      \"year\": 2000,\n      \"claim\": \"The discovery that Snu13p/15.5K binds both U4 snRNA and box C/D snoRNAs via a shared stem-internal loop-stem motif established SNU13 as a dual-function RNP core protein bridging splicing and rRNA modification machineries.\",\n      \"evidence\": \"In vitro binding assays, mass spectrometry of U4/U6.U5 tri-snRNP, yeast genetic depletion with RNA metabolism phenotype\",\n      \"pmids\": [\"11081632\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism of 15.5K recruitment to RNA not yet defined at atomic level\", \"Whether 15.5K binding alone is sufficient for downstream protein assembly unknown\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Reconstitution experiments demonstrated that 15.5K binding is the obligate first step in hierarchical box C/D snoRNP assembly and also nucleates U3 snoRNP box B/C complex formation, revealing a general organizational principle for K-turn-containing RNPs.\",\n      \"evidence\": \"In vitro RNP assembly on U14 snoRNA with sequential protein addition; UV cross-linking and assembly assays on U3 snoRNA box B/C motif in vitro; HeLa cell microinjection for nucleolar localization\",\n      \"pmids\": [\"12417735\", \"12381732\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether 15.5K contacts downstream snoRNP proteins directly or only via RNA-mediated conformational changes was unresolved\", \"Stoichiometry of 15.5K binding on U3 snoRNA (two K-turns) not fully characterized\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Detailed analysis of the two K-turn motifs in U3 snoRNA showed that Snu13p binds both independently with different affinities, and that both binding events are required for U3 snoRNP function, establishing that multi-site K-turn recognition is functionally essential.\",\n      \"evidence\": \"Chemical probing of reconstituted complexes, gel-shift assays, phylogenetic analysis, yeast U3 mutant functional assays\",\n      \"pmids\": [\"12810916\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis for differential affinity of the two K-turns not resolved\", \"Whether simultaneous occupancy of both sites by Snu13p is required in vivo unknown\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Molecular dynamics and chemical probing revealed that 15.5K folds the K-turn RNA through an induced-fit mechanism, resolving the question of whether the RNA is pre-folded or actively structured by the protein.\",\n      \"evidence\": \"Molecular dynamics simulation of free and bound U4 5' stem-loop, chemical modification experiments\",\n      \"pmids\": [\"15659359\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Induced-fit conclusion based on computational simulation with limited experimental benchmarks\", \"Whether the same induced-fit mechanism applies to box C/D K-turns not tested\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Systematic surface mutagenesis of 15.5K demonstrated that it uses distinct protein surfaces to nucleate each of its three RNP complexes and that direct protein-protein contacts (not only RNA-mediated effects) are essential, explaining how one protein can serve as a platform for divergent assemblies.\",\n      \"evidence\": \"Surface mutagenesis of 15.5K tested in U4/U6, box C/D snoRNP, and U3 box B/C in vitro assembly assays; direct 15.5K–hU3-55K interaction assay\",\n      \"pmids\": [\"16782898\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Atomic-resolution structures of complete ternary complexes lacking\", \"Whether surface utilization changes in the context of fully assembled particles unknown\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"The solution NMR structure of free 15.5K identified conformational flexibility in the alpha3 helix, suggesting a structural basis for adaptability in K-turn recognition.\",\n      \"evidence\": \"Solution NMR structure, 15N relaxation, H/D exchange on free human 15.5K protein\",\n      \"pmids\": [\"18044964\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No corresponding NMR structure of the RNA-bound form for direct comparison\", \"Functional relevance of alpha3 flexibility not validated by mutagenesis\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Structural and genetic characterization of the Snu13p–Rsa1p/NUFIP interface revealed a regulated assembly checkpoint: the Rsa1p binding surface on Snu13p overlaps with mature snoRNP protein contacts, preventing premature snoRNP activation until assembly factor release.\",\n      \"evidence\": \"NMR structure and molecular docking of Snu13p–Rsa1p complex, electrostatic interface mutagenesis, yeast growth and snoRNP formation assays\",\n      \"pmids\": [\"24234454\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Trigger for Rsa1p/NUFIP release during maturation not identified\", \"Whether this checkpoint operates identically for all box C/D snoRNPs unknown\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"NMR of the free U14 K-turn motif showed it adopts a pre-folded conformation even without Snu13p or divalent ions, partially revising the earlier induced-fit model by showing that some C/D box K-turns are intrinsically structured before protein binding.\",\n      \"evidence\": \"Solution NMR of free kt-U14 RNA, NMR-monitored Snu13p binding\",\n      \"pmids\": [\"30914254\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Generalizability to other C/D box K-turns not tested\", \"Quantitative comparison of pre-folding extent across different K-turn substrates lacking\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the precise trigger for Rsa1p/NUFIP release during snoRNP maturation, whether the induced-fit versus pre-folded K-turn distinction has functional consequences for assembly kinetics in vivo, and whether pharmacological disruption of the Snu13–K-turn interface can be achieved with cellular activity.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No in vivo kinetic measurements of snoRNP assembly steps\", \"No cell-active small molecule inhibitors of Snu13–RNA interaction reported in peer-reviewed literature\", \"Full cryo-EM or crystal structures of complete human box C/D snoRNP with 15.5K in native context not described in this timeline\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0003723\", \"supporting_discovery_ids\": [0, 1, 2, 3, 4, 9]},\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [1, 5, 7]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005730\", \"supporting_discovery_ids\": [1]},\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [1, 5]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-8953854\", \"supporting_discovery_ids\": [0, 1, 2, 3, 5, 7]}\n    ],\n    \"complexes\": [\n      \"U4/U6 snRNP\",\n      \"box C/D snoRNP\",\n      \"U3 snoRNP\"\n    ],\n    \"partners\": [\n      \"NOP56\",\n      \"NOP58\",\n      \"FBL\",\n      \"PRPF31\",\n      \"NUFIP1\",\n      \"U3-55K\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}