{"gene":"SNRPA","run_date":"2026-06-10T07:46:37","timeline":{"discoveries":[{"year":1994,"finding":"Crystal structure of the N-terminal RNA-binding domain of U1A complexed with a 21-nucleotide RNA hairpin (hairpin II of U1 snRNA) at 1.92 Å resolution revealed that the 10-nucleotide RNA loop binds to the surface of the beta-sheet as an open structure, with the AUUGCAC sequence interacting extensively with conserved RNP1 and RNP2 motifs and the C-terminal extension via base stacking with aromatic side chains and direct/water-mediated hydrogen bonds.","method":"X-ray crystallography at 1.92 Å resolution","journal":"Nature","confidence":"High","confidence_rationale":"Tier 1 / Strong — high-resolution crystal structure with detailed atomic contacts, foundational paper replicated and built upon by many subsequent studies","pmids":["7984237"],"is_preprint":false},{"year":1989,"finding":"The RNA binding site for U1A on U1 snRNA is hairpin II (positions 48-91), with the conserved loop sequence critical for interaction. The region of U1A required for RNA binding consists of an ~80 amino acid RNP motif plus flanking residues; point mutations in the most conserved RNP1 and RNP2 regions abolish RNA binding.","method":"In vitro RNA binding assays, point mutagenesis, deletion analysis","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods (binding assays, mutagenesis, deletion analysis), foundational mapping paper replicated by later structural studies","pmids":["2531658"],"is_preprint":false},{"year":1990,"finding":"Binding specificity between U1A/U1 snRNA and U2B''/U2 snRNA pairs is determined by two nucleotides in the RNA and eight amino acids in the protein; exchange of these residues reverses specificity. U1A binds U1 snRNA independently, whereas U2B'' requires accessory protein U2A' for specific binding to U2 snRNA.","method":"In vitro RNA binding assays with chimeric protein and RNA mutants","journal":"Nature","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal specificity swap experiments with chimeric proteins and RNAs, replicated in subsequent studies","pmids":["2140872"],"is_preprint":false},{"year":1991,"finding":"Molecular contacts between U1A protein and U1 snRNA identified by mutagenesis: Thr11→Val and Asn15→Val in RNP2 abolished binding; Tyr13→Phe and Asn16→Val substantially reduced binding. Arg52 in RNP1 forms a salt bridge with RNA phosphates. Ethylation protection showed phosphates of the 3' two-thirds of loop II and the 5' stem contact U1A. A→G and G→A replacements in loop II substantially reduced binding.","method":"Site-directed mutagenesis combined with filter binding and ethylation protection of U1 RNA","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro mutagenesis plus chemical probing, multiple orthogonal methods, consistent with crystal structure","pmids":["1833186"],"is_preprint":false},{"year":1993,"finding":"U1A protein autoregulates its own mRNA production by binding two sites in the conserved 47 nt region of the 3' UTR of its own pre-mRNA, inhibiting polyadenylation. Overexpression of U1A in mouse cells downregulates endogenous U1A mRNA; a single U1A molecule binding is insufficient for efficient polyadenylation inhibition.","method":"In vitro binding assays, in vivo overexpression, in vitro polyadenylation assays, structure probing, mutational analysis of pre-mRNA","journal":"Cell / The EMBO journal","confidence":"High","confidence_rationale":"Tier 2 / Strong — combined in vitro and in vivo functional data, replicated across multiple studies and organisms","pmids":["8458082","8262062"],"is_preprint":false},{"year":1994,"finding":"U1A protein bound to its own pre-mRNA directly interacts with mammalian poly(A) polymerase (PAP) and inhibits both specific and nonspecific polyadenylation. This inhibition does not prevent CPSF binding or pre-mRNA cleavage. Domains in both U1A (identified by mutagenesis) and the C-terminus of PAP are required; the interaction is specific to mammalian (not yeast) PAP.","method":"In vitro polyadenylation assays, domain mutagenesis, in vitro protein-protein interaction (direct binding of U1A to PAP)","journal":"Cell","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — in vitro reconstitution of inhibition, domain mapping by mutagenesis, direct protein-protein interaction demonstrated, replicated and extended by subsequent studies","pmids":["8313473"],"is_preprint":false},{"year":1992,"finding":"U1A nuclear localization requires an active transport process independent of U1 snRNA binding; the nuclear localization signal is an unusually large sequence element between amino acids 94 and 204. U1A shuttles between nucleus and cytoplasm, and its intracellular distribution is determined by the number of free RNA binding sites available in each compartment.","method":"Microinjection of deletion mutants into Xenopus oocytes, transfection assays, introduction of competing RNA sequences","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 2 / Moderate — direct localization experiments with deletion mutants and RNA competition, multiple functional readouts","pmids":["1618898"],"is_preprint":false},{"year":1996,"finding":"The solution NMR structure of U1A N-terminal RBD (residues 2-117) shows that the C-terminal helix C lies across the beta-sheet in the free protein (acting as a 'lid'), making hydrophobic contacts that stabilize the protein. Upon RNA binding, helix C rotates ~135° to allow Tyr13, Phe56 and Gln54 to stack with RNA bases, stabilized in the new position by hydrophobic interactions and a Ser91-Thr11 hydrogen bond.","method":"Multi-dimensional heteronuclear NMR solution structure determination with functional validation","journal":"Journal of molecular biology","confidence":"High","confidence_rationale":"Tier 1 / Strong — NMR solution structure with conformational change mechanism described, consistent with crystal structure data and validated by subsequent studies","pmids":["8609632"],"is_preprint":false},{"year":1997,"finding":"The carboxy-terminal 20 amino acids of vertebrate PAP are essential for its inhibition by the U1A-RNA complex; transfer of these 20 residues to yeast PAP confers U1A-mediated inhibition. A GST fusion of these PAP residues interacts in vitro with an RNA-U1A complex containing two U1A molecules but not one, explaining the requirement for two U1A-binding sites. U1A amino acids 103-119 are required for PAP inhibition; a multimeric U1A peptide spanning this region uncouples splicing and 3'-end formation in vitro.","method":"In vitro PAP inhibition assays with domain mutants, yeast PAP chimera experiments, GST pulldown, in vitro splicing/polyadenylation uncoupling assay","journal":"Genes & development","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — reconstituted in vitro inhibition, reciprocal domain-swap to yeast PAP, multiple orthogonal assays in one study","pmids":["9087430"],"is_preprint":false},{"year":1997,"finding":"The NMR structure of the U1A RBD1-polyadenylation inhibitory element (PIE RNA) complex at 1.08 Å interface precision shows that the same heptanucleotide AUUGCAC is recognized in two different structural contexts (hairpin and internal loop) by identical protein contacts; a protein dimer forms via C-terminal contacts that are RNA binding-dependent, providing the structural basis for cooperativity and PAP inhibition.","method":"NMR structure determination of U1A-3'UTR complex, model-building based on crystal structure of hairpin complex","journal":"Structure / Journal of biomolecular NMR","confidence":"High","confidence_rationale":"Tier 1 / Moderate — NMR structure plus crystallographic model, consistent with biochemical data, single lab but multiple methods","pmids":["8736559","9566313"],"is_preprint":false},{"year":2000,"finding":"NMR structure of the 38 kDa complex of two U1A proteins bound to PIE RNA shows that cooperativity of U1A binding depends on helix C, which undergoes a conformational change upon RNA binding. This same helix C is adjacent to the PAP-interacting domain, ensuring that PAP inhibition can only occur when U1A is RNA-bound, linking cooperative RNA binding to PAP inhibition.","method":"NMR structure determination of trimolecular 38 kDa complex","journal":"Nature structural biology","confidence":"High","confidence_rationale":"Tier 1 / Strong — NMR structure of the full autoregulatory complex with mechanistic explanation for cooperativity and PAP inhibition coupling","pmids":["10742179"],"is_preprint":false},{"year":2000,"finding":"U1A homodimerizes via two separate regions in the N-terminal 115 residues (one at aa 103-115) even when RNA binding is abolished. Mutation of the aa 103-115 dimerization region abolishes cooperative binding of two U1A molecules to PIE RNA and inhibition of polyadenylation, but does not affect single-molecule binding to PIE RNA.","method":"Yeast two-hybrid, in vitro coselection, gel mobility shift, polyadenylation inhibition assays with point and deletion mutants","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods (yeast two-hybrid, in vitro coselection, EMSA, polyadenylation assay), clear domain mapping","pmids":["10688667"],"is_preprint":false},{"year":2001,"finding":"The nuclear import of U1A is mediated by importin alpha/beta and Ran; the NLS is located within residues 100-144. U1A was shown to bind the C-terminal portion of importin alpha. Nuclear accumulation in intact cells is Ran-dependent and inhibited by the importin beta-binding domain of importin alpha.","method":"Microinjection of deletion mutants into BHK21 cells, in vitro nuclear import assay with recombinant importins, in vivo Ran inhibition","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Moderate — in vivo and in vitro nuclear import assays with deletion mutants and direct binding to importin alpha demonstrated, two orthogonal methods","pmids":["11278401"],"is_preprint":false},{"year":2004,"finding":"U1A binds two AUGCN(1-3)C motifs within a 29-nucleotide sequence between the two downstream GU-rich regions of the IgM heavy-chain secretory poly(A) site, inhibits binding of CstF 64K to GU-rich elements, and inhibits cleavage at the secretory poly(A) site.","method":"In vitro RNA binding assays, polyadenylation cleavage assays, competition binding experiments","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 2 / Moderate — direct binding demonstrated, functional inhibition of cleavage shown, mechanism of CstF displacement established","pmids":["15226420"],"is_preprint":false},{"year":2006,"finding":"A snRNP-free form of U1A (SF-A) exists in human cells as part of a novel complex containing PSF, p54nrb, and p68; this complex promotes pre-mRNA cleavage during polyadenylation. p54nrb is specifically critical for the cleavage function, as shown by immunodepletion/reconstitution experiments.","method":"Tandem affinity purification, mass spectrometry, immunodepletion/reconstitution of in vitro polyadenylation, Co-IP","journal":"RNA","confidence":"High","confidence_rationale":"Tier 2 / Moderate — TAP purification with MS identification, functional reconstitution experiments, reciprocal immunoprecipitation","pmids":["16373496"],"is_preprint":false},{"year":1997,"finding":"Identification of a novel snRNP-free U1A complex (SF-A) in which U1A is not associated with U1 snRNA or other snRNP components, but co-sediments and co-immunoprecipitates with non-snRNP proteins of ~58, 59, 63, 65, and 105 kDa. Anti-SF-A antibodies significantly diminish polyadenylation in vitro, indicating a functional role in this process.","method":"Sucrose density gradient fractionation, immunoprecipitation with conformation-specific monoclonal antibody (MAb 12E12), in vitro polyadenylation","journal":"RNA","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reciprocal immunoprecipitation and functional antibody inhibition, single lab","pmids":["9404895"],"is_preprint":false},{"year":1998,"finding":"The largest component of the SF-A complex, p105, is PSF (polypyrimidine-tract binding protein-associated splicing factor). PSF co-purifies and co-immunoprecipitates with SF-A from 293T cell nucleoplasm and interacts with SF-A in vitro. Anti-SF-A antibodies inhibit both splicing and polyadenylation in a coupled in vitro reaction.","method":"Co-immunoprecipitation, in vitro binding assay, in vitro coupled splicing/polyadenylation assay","journal":"RNA","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reciprocal Co-IP plus in vitro functional inhibition, single lab","pmids":["9848648"],"is_preprint":false},{"year":2013,"finding":"U1A binds directly and with high affinity and specificity to the SMN 3'-UTR adjacent to the polyadenylation site, independent of U1 snRNP. This binding inhibits polyadenylation of SMN pre-mRNA by specifically inhibiting 3' cleavage by CPSF. Excess U1A over U1 snRNA causes inhibition of SMN polyadenylation and decreases SMN protein levels.","method":"In vitro RNA binding assays, in vitro polyadenylation/cleavage assays, overexpression experiments with SMN protein level readout","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Moderate — direct binding established, CPSF-specific cleavage inhibition demonstrated in vitro, functional consequence (SMN protein level) shown in vivo","pmids":["24362020"],"is_preprint":false},{"year":2007,"finding":"A conserved U1 site in the U1A gene 3'UTR acts in synergy with PIE to inhibit U1A expression via nuclear polyadenylation inhibition (poly(A) tail addition). The two elements function as a bipartite repressor: the U1 site is normally suppressed by a base-pairing mechanism, but together they form a ternary complex that inhibits polyadenylation without simply stabilizing factor binding.","method":"In vitro and in vivo reporter assays, mutational analysis of 3'UTR, in vitro polyadenylation assays","journal":"RNA","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — functional dissection using mutants in vivo and in vitro, novel mechanism for bipartite regulation identified, single lab","pmids":["17942741"],"is_preprint":false},{"year":2019,"finding":"SAM68 directly interacts with U1A through SAM68's C-terminal tyrosine-rich (YY) domain binding the RRM1 domain of U1A; this interaction promotes U1 snRNP recruitment to the 5' splice site of mTor intron 5. Deletion of the U1A-SAM68 interaction domain or mutation of SAM68-binding sites in mTor intron 5 abrogates U1A recruitment and leads to premature intron 5 termination and polyadenylation.","method":"Co-immunoprecipitation, deletion mutagenesis, splicing assays, polyadenylation assays, in vivo and in vitro binding","journal":"Nucleic acids research","confidence":"High","confidence_rationale":"Tier 2 / Moderate — direct interaction mapped by domain deletion, functional consequence on splicing and polyadenylation demonstrated in vitro and in vivo, multiple orthogonal methods","pmids":["30767021"],"is_preprint":false},{"year":2006,"finding":"Non-snRNP U1A levels decrease during B-cell differentiation, releasing the IgM secretory poly(A) site from repression. Undifferentiated B cells have more total U1A and a greater proportion is non-snRNP-associated, which directly inhibits poly(A) addition. The inhibitory effect is proportional to the amount of available U1A recovered by immunopurification.","method":"Flow cytometry, exhaustive immunoprecipitation, cold competitor RNA oligo de-repression of polyadenylation in nuclear extracts, in vitro poly(A) polymerase inhibition assay","journal":"RNA","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct functional link between non-snRNP U1A levels and poly(A) site inhibition demonstrated using competitor RNA and purified U1A fractions, single lab","pmids":["16373497"],"is_preprint":false},{"year":2001,"finding":"U1A-RNA complex formation involves two mechanistically distinct steps: an initial rapid association driven by electrostatic interactions (slowed by charge neutralization or high salt) and a subsequent 'locking' step driven by close-range hydrogen bonding and stacking interactions (reflected in dissociation rate). Single amino acid substitutions can selectively perturb either step.","method":"Surface plasmon resonance (real-time biomolecular interaction analysis) with U1A mutants and RNA mutants, salt dependence experiments","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Moderate — quantitative kinetic analysis by SPR with panel of mutants, two-step mechanism mechanistically dissected, replicated in subsequent SPR studies","pmids":["11297556"],"is_preprint":false},{"year":1995,"finding":"In vitro phage-display selection identified Leu-49 as a critical residue that disproportionately affects the rates of binding and release of U1A from U1 RNA; it appears to lock the protein onto the RNA. Three other residues in the mutagenized region also proved important for specific RNA binding.","method":"Phage-display combinatorial library with affinity selection (in vitro genetic selection), RNA binding assays","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — novel in vitro selection method identifies mechanistically important residue, single lab, single method type","pmids":["8524863"],"is_preprint":false},{"year":1997,"finding":"The NMR structure of U1A C-terminal RNA-binding domain (RBD2) shows it folds as a canonical alpha/beta sandwich but does not bind U1, U2, or U5 snRNA, RNA hairpins, or random sequence RNAs. The two RBDs of the full-length protein exhibit uncorrelated motion due to a highly flexible linker region.","method":"NMR structure determination, RNA binding assays with multiple RNA substrates, fluorescence polarization","journal":"Journal of molecular biology / Biochemistry / Protein science","confidence":"High","confidence_rationale":"Tier 1 / Moderate — NMR structure plus negative functional RNA binding data confirmed by multiple RNA substrates, consistent across two studies","pmids":["7723028","9265619"],"is_preprint":false},{"year":1998,"finding":"A single leucine residue in U1A (Leu-44) is critical for the intrinsic specificity of U1A for the U1hpII loop sequence over U2hpIV. U2A' protein enables U2B'' to discriminate loop sequences but not stem sequences; U2A' can also promote heterospecific U1A binding to U2hpIV but requires ~500-fold higher concentration due to preferential U2A' binding to U2B''.","method":"In vitro RNA binding assays with chimeric proteins, competition assays, site-directed mutagenesis","journal":"RNA","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — mutational analysis with quantitative binding measurements, single lab, clear specificity determinant identified","pmids":["9814759"],"is_preprint":false},{"year":2006,"finding":"Positively charged residues close to the RNA-binding site (Lys20, Lys22, Lys23) play distinct roles: Lys20 predominantly aids association while Lys22 and Lys23 are important for complex stability. Residues away from the binding site (Arg7, Lys60, Arg70) have minimal effect on either association or stability, demonstrating position-dependent roles of electrostatic interactions in the 'lure and lock' binding model.","method":"Surface plasmon resonance kinetics, salt dependence experiments, molecular dynamics simulation","journal":"Nucleic acids research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — SPR kinetics with panel of mutants plus MD simulation, single lab, mechanistically informative dissection of electrostatic roles","pmids":["16407334"],"is_preprint":false},{"year":2013,"finding":"The C-terminal helix (helix C) of U1A RRM1 occludes the RNA-binding surface in the free protein. Truncation or disruption of helix C increases the association rate (by exposing the binding surface) but simultaneously reduces complex stability (loss of locking). The quadruple stacking interaction (involving helix C residues) makes a minor kinetic contribution compared to the intraprotein hydrogen bonds formed when helix C relocates.","method":"Surface plasmon resonance kinetics with truncation and point mutants of helix C","journal":"Nucleic acids research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — systematic SPR kinetic analysis with panel of helix C mutants, mechanistic dissection of association vs. locking steps, single lab","pmids":["23703211"],"is_preprint":false},{"year":2020,"finding":"SNRPA binds directly to the G-quadruplex structure within the 5'UTR of BAG-1 mRNA. Label-free RNA pulldown from colorectal cancer cell protein extracts followed by LC-MS/MS identified SNRPA; direct binding was confirmed, and knockdown experiments suggested SNRPA modulates BAG-1 expression levels.","method":"RNA G-quadruplex pulldown with mutant control, LC-MS/MS, confirmation of direct binding, siRNA knockdown","journal":"Biochimie","confidence":"Medium","confidence_rationale":"Tier 3 / Weak — pulldown/MS identification with direct binding confirmation, single lab, limited mechanistic follow-up","pmids":["32629040"],"is_preprint":false},{"year":2024,"finding":"SNRPA controls alternative splicing of ERCC1 exon 8; its depletion causes ERCC1 exon 8 skipping and reduced ERCC1-XPF complex formation, reducing DNA damage repair. IGF2BP1 (m6A reader) and ELAVL1 (RNA stabilizer) bind SNRPA mRNA, with ELAVL1 promoting SNRPA expression and thereby cisplatin resistance in an SNRPA-dependent manner.","method":"CRISPR/Cas9 knockout and shRNA knockdown, overexpression, splicing assays, Co-IP for ERCC1-XPF complex, gene ontology analysis, mouse xenograft model","journal":"Advanced science","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — CRISPR/KO with specific splicing and complex formation readouts, in vivo xenograft validation, upstream m6A regulation mechanism identified, single lab","pmids":["39555714"],"is_preprint":false}],"current_model":"SNRPA (U1A) is a core U1 snRNP component that uses its N-terminal RRM domain to bind the AUUGCAC loop of U1 snRNA hairpin II with picomolar affinity via a two-step 'lure and lock' mechanism: electrostatic interactions mediate rapid association, after which C-terminal helix C repositions to lock the complex through stacking and hydrogen bond networks; in excess over U1 snRNA, free U1A cooperatively binds two sites in the 3'UTR PIE element of its own pre-mRNA and directly contacts the C-terminus of poly(A) polymerase to inhibit polyadenylation in a U1A-mRNA-bound, dimerization-dependent manner; U1A also exists in a non-snRNP SF-A complex with PSF and p54nrb that promotes pre-mRNA 3' cleavage, inhibits the IgM secretory poly(A) site, regulates SMN 3' processing, and interacts with SAM68 to modulate U1 snRNP recruitment to 5' splice sites; nuclear import of U1A is mediated by importin alpha/beta and Ran, and the protein shuttles between nucleus and cytoplasm with distribution governed by available RNA binding sites; additionally, SNRPA controls alternative splicing of ERCC1 exon 8 and binds BAG-1 mRNA G-quadruplex, with its own mRNA stability regulated by IGF2BP1/ELAVL1 via m6A."},"narrative":{"mechanistic_narrative":"SNRPA (U1A) is an RRM-containing RNA-binding protein that functions both as a core component of the U1 snRNP and as a sequence-specific regulator of 3'-end processing and alternative splicing [PMID:2531658, PMID:8458082, PMID:8262062]. Its N-terminal RRM recognizes the AUUGCAC loop of U1 snRNA hairpin II through conserved RNP1 and RNP2 motifs that contact the RNA via base stacking with aromatic side chains and hydrogen-bond networks [PMID:7984237, PMID:1833186], with binding specificity over the U2B''/U2 snRNA pair dictated by a small set of protein and RNA residues [PMID:2140872, PMID:9814759]. Recognition proceeds by a two-step 'lure and lock' mechanism: rapid electrostatic association followed by a locking step in which the C-terminal helix C, which occludes the binding surface in the free protein, rotates onto the RNA to stabilize the complex through stacking and intraprotein hydrogen bonds [PMID:8609632, PMID:11297556, PMID:23703211]. When U1A is in excess over U1 snRNA, free protein autoregulates its own expression by cooperatively binding two sites in the 3'UTR PIE element of its own pre-mRNA and directly contacting the C-terminus of poly(A) polymerase to inhibit polyadenylation; both cooperativity and PAP inhibition depend on helix C-mediated dimerization, coupling RNA binding to inhibition [PMID:8458082, PMID:8262062, PMID:8313473, PMID:9087430, PMID:10742179, PMID:10688667]. This regulatory activity extends to other transcripts, where U1A binds 3'UTR elements to inhibit cleavage and polyadenylation of the IgM secretory poly(A) site by displacing CstF and of SMN pre-mRNA by blocking CPSF cleavage [PMID:15226420, PMID:24362020]. A snRNP-free pool of U1A (SF-A) assembles with PSF and p54nrb/p68 to promote pre-mRNA cleavage during polyadenylation [PMID:16373496, PMID:9848648], and U1A also interacts with SAM68 to promote U1 snRNP recruitment to 5' splice sites [PMID:30767021] and controls alternative splicing of ERCC1 exon 8 to support ERCC1-XPF complex formation and DNA damage repair [PMID:39555714]. Nuclear import of U1A is mediated by importin alpha/beta and Ran, and the protein shuttles between nucleus and cytoplasm with its distribution set by the number of available RNA binding sites [PMID:1618898, PMID:11278401].","teleology":[{"year":1989,"claim":"Establishing that U1A recognizes a defined RNA element via a discrete RNP motif converted U1A from a snRNP subunit into a tractable sequence-specific RNA-binding protein.","evidence":"in vitro RNA binding, point mutagenesis and deletion analysis mapping the U1 snRNA hairpin II site and the RNP motif","pmids":["2531658"],"confidence":"High","gaps":["No atomic-resolution description of contacts","Functional consequence of binding beyond snRNP assembly not addressed"]},{"year":1990,"claim":"Defining the few protein and RNA residues that distinguish U1A/U1 from U2B''/U2 explained how closely related RRM proteins achieve target specificity and showed U1A binds independently of accessory proteins.","evidence":"reciprocal specificity-swap experiments with chimeric proteins and RNAs","pmids":["2140872"],"confidence":"High","gaps":["Single critical specificity residue not yet resolved","Structural basis of discrimination not shown"]},{"year":1994,"claim":"High-resolution structures of the U1A RRM-hairpin II complex and of the free RBD revealed the atomic contacts and the helix C conformational switch underlying recognition.","evidence":"X-ray crystallography at 1.92 Å and multidimensional heteronuclear NMR of the free and bound RBD","pmids":["7984237","8609632"],"confidence":"High","gaps":["Kinetic ordering of association vs locking not yet defined","Behavior of the second RRM (RBD2) unresolved"]},{"year":1994,"claim":"Identifying U1A autoregulation through PIE-element binding and direct PAP inhibition established a non-snRNP function and a feedback loop controlling U1A levels.","evidence":"in vitro/in vivo polyadenylation assays, overexpression, domain mutagenesis, and direct U1A-PAP binding","pmids":["8458082","8262062","8313473"],"confidence":"High","gaps":["Why two U1A molecules are required not yet structurally explained","Whether other mRNAs are similarly regulated unknown at this stage"]},{"year":1997,"claim":"Mapping the PAP C-terminal 20 residues and U1A residues 103-119 as the inhibitory interface, and showing two bound U1A molecules are needed, mechanistically linked cooperative RNA binding to polyadenylation control.","evidence":"in vitro PAP inhibition assays, yeast PAP chimera, GST pulldown, splicing/polyadenylation uncoupling","pmids":["9087430"],"confidence":"High","gaps":["Structural basis of the dimer-PAP contact not yet visualized","In vivo relevance across cell types not established"]},{"year":2000,"claim":"NMR structures of the two-U1A:PIE complex and dimerization mapping showed that helix C drives both RNA-binding cooperativity and ensures PAP inhibition occurs only when U1A is RNA-bound.","evidence":"NMR structure of the 38 kDa trimolecular complex; yeast two-hybrid, coselection, EMSA and polyadenylation assays with dimerization mutants","pmids":["10742179","10688667"],"confidence":"High","gaps":["Dynamics of dimer assembly in cells not measured","Regulation of free vs snRNP-bound U1A partitioning unresolved"]},{"year":2001,"claim":"Dissecting binding into an electrostatic 'lure' and a hydrogen-bond/stacking 'lock' step provided a kinetic framework for how the RRM achieves both speed and stability.","evidence":"surface plasmon resonance kinetics with protein and RNA mutants and salt-dependence experiments","pmids":["11297556"],"confidence":"High","gaps":["Single mutants only partially separate the steps","Generalizability to other RRM-RNA pairs not tested here"]},{"year":2001,"claim":"Defining importin alpha/beta- and Ran-dependent nuclear import and an internal NLS clarified how U1A localizes, complementing earlier work showing distribution follows available RNA binding sites.","evidence":"microinjection of deletion mutants, in vitro import assays with recombinant importins, and in vivo Ran inhibition; earlier Xenopus oocyte and RNA-competition assays","pmids":["11278401","1618898"],"confidence":"High","gaps":["Regulation of shuttling under physiological conditions unclear","Cytoplasmic functions of shuttling U1A not defined"]},{"year":2006,"claim":"Purification of the snRNP-free SF-A complex containing PSF and p54nrb/p68, and its requirement for pre-mRNA cleavage, distinguished a positive 3'-processing role for non-snRNP U1A from its inhibitory autoregulatory role.","evidence":"TAP purification with MS, immunodepletion/reconstitution of in vitro polyadenylation, Co-IP; earlier sucrose gradient and conformation-specific antibody work","pmids":["16373496","9404895","9848648"],"confidence":"High","gaps":["How U1A partitions between SF-A and inhibitory pools not established","Substrate selectivity of SF-A-promoted cleavage unknown"]},{"year":2006,"claim":"Linking declining non-snRNP U1A to release of the IgM secretory poly(A) site during B-cell differentiation gave a physiological setting for U1A-mediated cleavage inhibition through CstF displacement.","evidence":"flow cytometry, exhaustive immunoprecipitation, competitor-RNA de-repression and PAP inhibition assays; earlier mapping of AUGCN motifs and CstF inhibition","pmids":["16373497","15226420"],"confidence":"Medium","gaps":["Single lab; in vivo causality during differentiation not genetically proven","Breadth of poly(A) sites regulated this way unknown"]},{"year":2013,"claim":"Demonstrating direct U1A binding to the SMN 3'UTR with CPSF-specific cleavage inhibition and reduced SMN protein extended U1A's regulatory reach to a disease-relevant transcript.","evidence":"in vitro RNA binding and cleavage assays plus overexpression with SMN protein readout","pmids":["24362020"],"confidence":"High","gaps":["Physiological conditions favoring this inhibition not defined","Endogenous regulation of SMN by U1A in neurons not shown"]},{"year":2019,"claim":"Identifying a SAM68-U1A interaction that promotes U1 snRNP recruitment to a 5' splice site showed U1A participates in splice-site selection and intron retention/premature polyadenylation control.","evidence":"Co-IP, domain-deletion mapping, splicing and polyadenylation assays on mTor intron 5","pmids":["30767021"],"confidence":"High","gaps":["Genome-wide scope of SAM68-U1A-regulated targets unknown","Mechanism of snRNP recruitment beyond the contact not detailed"]},{"year":2024,"claim":"Showing SNRPA controls ERCC1 exon 8 splicing to support ERCC1-XPF complex formation and DNA repair, with its own mRNA regulated by m6A reader IGF2BP1 and stabilizer ELAVL1, connected U1A to a cancer-relevant splicing and drug-resistance axis.","evidence":"CRISPR knockout/shRNA, splicing assays, Co-IP for ERCC1-XPF, and mouse xenograft","pmids":["39555714"],"confidence":"Medium","gaps":["Single lab; direct mechanism of exon 8 recognition not mapped","Generality of U1A-regulated alternative splicing not established"]},{"year":null,"claim":"How the cell dynamically partitions U1A among the U1 snRNP, SF-A, and free inhibitory pools to coordinate splicing and 3'-end processing genome-wide remains unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No quantitative model of pool partitioning in vivo","Transcriptome-wide map of direct U1A regulatory targets incomplete","Signals controlling free vs snRNP-bound U1A levels unknown"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0003723","term_label":"RNA binding","supporting_discovery_ids":[0,1,2,3,4,13,17,27]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[5,8,13,17]},{"term_id":"GO:0140110","term_label":"transcription regulator activity","supporting_discovery_ids":[4,19,28]}],"localization":[{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[6,12]},{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[6]}],"pathway":[{"term_id":"R-HSA-8953854","term_label":"Metabolism of RNA","supporting_discovery_ids":[4,14,19,28]},{"term_id":"R-HSA-74160","term_label":"Gene expression (Transcription)","supporting_discovery_ids":[4,13,17]}],"complexes":["U1 snRNP","SF-A (U1A-PSF-p54nrb-p68)"],"partners":["PAPOLA","PSF/SFPQ","NONO","SAM68/KHDRBS1","ERCC1","IGF2BP1","ELAVL1"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"P09012","full_name":"U1 small nuclear ribonucleoprotein A","aliases":[],"length_aa":282,"mass_kda":31.3,"function":"Component of the spliceosomal U1 snRNP, which is essential for recognition of the pre-mRNA 5' splice-site and the subsequent assembly of the spliceosome. 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analysis of SNF, the Drosophila U1A/U2B\" homolog: identification of dispensable and indispensable motifs for both snRNP assembly and function in vivo.","date":"1999","source":"RNA (New York, N.Y.)","url":"https://pubmed.ncbi.nlm.nih.gov/10580472","citation_count":21,"is_preprint":false},{"pmid":"18154282","id":"PMC_18154282","title":"Prediction of salt and mutational effects on the association rate of U1A protein and U1 small nuclear RNA stem/loop II.","date":"2007","source":"The journal of physical chemistry. B","url":"https://pubmed.ncbi.nlm.nih.gov/18154282","citation_count":21,"is_preprint":false},{"pmid":"24497193","id":"PMC_24497193","title":"Structure-function analysis of the Yhc1 subunit of yeast U1 snRNP and genetic interactions of Yhc1 with Mud2, Nam8, Mud1, Tgs1, U1 snRNA, SmD3 and Prp28.","date":"2014","source":"Nucleic acids research","url":"https://pubmed.ncbi.nlm.nih.gov/24497193","citation_count":20,"is_preprint":false},{"pmid":"3027665","id":"PMC_3027665","title":"Functional, developmentally expressed genes for mouse U1a and U1b snRNAs contain both conserved and non-conserved transcription signals.","date":"1986","source":"Nucleic acids research","url":"https://pubmed.ncbi.nlm.nih.gov/3027665","citation_count":20,"is_preprint":false},{"pmid":"2833197","id":"PMC_2833197","title":"Characterization of an iron sensitive Mud1 mutant in E. coli lacking the ribonucleotide reductase subunit B2.","date":"1988","source":"Archives of microbiology","url":"https://pubmed.ncbi.nlm.nih.gov/2833197","citation_count":19,"is_preprint":false},{"pmid":"16698546","id":"PMC_16698546","title":"High-resolution structural validation of the computational redesign of human U1A protein.","date":"2006","source":"Structure (London, England : 1993)","url":"https://pubmed.ncbi.nlm.nih.gov/16698546","citation_count":18,"is_preprint":false},{"pmid":"36715182","id":"PMC_36715182","title":"Splicing factor SNRPA associated with microvascular invasion promotes hepatocellular carcinoma metastasis through activating NOTCH1/Snail pathway and is mediated by circSEC62/miR-625-5p axis.","date":"2023","source":"Environmental toxicology","url":"https://pubmed.ncbi.nlm.nih.gov/36715182","citation_count":17,"is_preprint":false},{"pmid":"23796518","id":"PMC_23796518","title":"Resurrection of an Urbilaterian U1A/U2B″/SNF protein.","date":"2013","source":"Journal of molecular biology","url":"https://pubmed.ncbi.nlm.nih.gov/23796518","citation_count":17,"is_preprint":false},{"pmid":"11454059","id":"PMC_11454059","title":"Characterization of self-T-cell response and antigenic determinants of U1A protein with bone marrow-derived dendritic cells in NZB x NZW F1 mice.","date":"2001","source":"Immunology","url":"https://pubmed.ncbi.nlm.nih.gov/11454059","citation_count":17,"is_preprint":false},{"pmid":"12876372","id":"PMC_12876372","title":"U1A RNA-binding domain at 1.8 A resolution.","date":"2003","source":"Acta crystallographica. Section D, Biological crystallography","url":"https://pubmed.ncbi.nlm.nih.gov/12876372","citation_count":16,"is_preprint":false},{"pmid":"11278401","id":"PMC_11278401","title":"Nuclear import of the U1A splicesome protein is mediated by importin alpha /beta and Ran in living mammalian cells.","date":"2001","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/11278401","citation_count":16,"is_preprint":false},{"pmid":"12617996","id":"PMC_12617996","title":"The snRNP-associated U1A levels change following IL-6 stimulation of human B-cells.","date":"2003","source":"Molecular immunology","url":"https://pubmed.ncbi.nlm.nih.gov/12617996","citation_count":16,"is_preprint":false},{"pmid":"10329165","id":"PMC_10329165","title":"Binding of U1A protein to the 3' untranslated region of its pre-mRNA.","date":"1999","source":"Journal of molecular biology","url":"https://pubmed.ncbi.nlm.nih.gov/10329165","citation_count":16,"is_preprint":false},{"pmid":"12270158","id":"PMC_12270158","title":"Inhibition of the U1A-RNA complex by an aminoacridine derivative.","date":"2002","source":"Bioorganic & medicinal chemistry letters","url":"https://pubmed.ncbi.nlm.nih.gov/12270158","citation_count":15,"is_preprint":false},{"pmid":"12100720","id":"PMC_12100720","title":"In vivo tolerance breakdown with dendritic cells pulsed with U1A protein in non-autoimmune mice: the induction of a high level of autoantibodies but not renal pathological changes.","date":"2002","source":"Immunology","url":"https://pubmed.ncbi.nlm.nih.gov/12100720","citation_count":15,"is_preprint":false},{"pmid":"10548057","id":"PMC_10548057","title":"Global and local dynamics of the human U1A protein determined by tryptophan fluorescence.","date":"1999","source":"Protein science : a publication of the Protein Society","url":"https://pubmed.ncbi.nlm.nih.gov/10548057","citation_count":14,"is_preprint":false},{"pmid":"23703211","id":"PMC_23703211","title":"The role of the C-terminal helix of U1A protein in the interaction with U1hpII RNA.","date":"2013","source":"Nucleic acids research","url":"https://pubmed.ncbi.nlm.nih.gov/23703211","citation_count":14,"is_preprint":false},{"pmid":"21419778","id":"PMC_21419778","title":"Multistep kinetics of the U1A-SL2 RNA complex dissociation.","date":"2011","source":"Journal of molecular biology","url":"https://pubmed.ncbi.nlm.nih.gov/21419778","citation_count":13,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":40988,"output_tokens":7349,"usd":0.116599,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":16650,"output_tokens":4494,"usd":0.0978,"stage2_stop_reason":"end_turn"},"total_usd":0.214399,"stage1_batch_id":"msgbatch_013LWZZT6MWaqB9qyTSeQBmU","stage2_batch_id":"msgbatch_0194ZzQaTGNpruoNJBbEMbt3","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1994,\n      \"finding\": \"Crystal structure of the N-terminal RNA-binding domain of U1A complexed with a 21-nucleotide RNA hairpin (hairpin II of U1 snRNA) at 1.92 Å resolution revealed that the 10-nucleotide RNA loop binds to the surface of the beta-sheet as an open structure, with the AUUGCAC sequence interacting extensively with conserved RNP1 and RNP2 motifs and the C-terminal extension via base stacking with aromatic side chains and direct/water-mediated hydrogen bonds.\",\n      \"method\": \"X-ray crystallography at 1.92 Å resolution\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — high-resolution crystal structure with detailed atomic contacts, foundational paper replicated and built upon by many subsequent studies\",\n      \"pmids\": [\"7984237\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1989,\n      \"finding\": \"The RNA binding site for U1A on U1 snRNA is hairpin II (positions 48-91), with the conserved loop sequence critical for interaction. The region of U1A required for RNA binding consists of an ~80 amino acid RNP motif plus flanking residues; point mutations in the most conserved RNP1 and RNP2 regions abolish RNA binding.\",\n      \"method\": \"In vitro RNA binding assays, point mutagenesis, deletion analysis\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods (binding assays, mutagenesis, deletion analysis), foundational mapping paper replicated by later structural studies\",\n      \"pmids\": [\"2531658\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1990,\n      \"finding\": \"Binding specificity between U1A/U1 snRNA and U2B''/U2 snRNA pairs is determined by two nucleotides in the RNA and eight amino acids in the protein; exchange of these residues reverses specificity. U1A binds U1 snRNA independently, whereas U2B'' requires accessory protein U2A' for specific binding to U2 snRNA.\",\n      \"method\": \"In vitro RNA binding assays with chimeric protein and RNA mutants\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal specificity swap experiments with chimeric proteins and RNAs, replicated in subsequent studies\",\n      \"pmids\": [\"2140872\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1991,\n      \"finding\": \"Molecular contacts between U1A protein and U1 snRNA identified by mutagenesis: Thr11→Val and Asn15→Val in RNP2 abolished binding; Tyr13→Phe and Asn16→Val substantially reduced binding. Arg52 in RNP1 forms a salt bridge with RNA phosphates. Ethylation protection showed phosphates of the 3' two-thirds of loop II and the 5' stem contact U1A. A→G and G→A replacements in loop II substantially reduced binding.\",\n      \"method\": \"Site-directed mutagenesis combined with filter binding and ethylation protection of U1 RNA\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — in vitro mutagenesis plus chemical probing, multiple orthogonal methods, consistent with crystal structure\",\n      \"pmids\": [\"1833186\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1993,\n      \"finding\": \"U1A protein autoregulates its own mRNA production by binding two sites in the conserved 47 nt region of the 3' UTR of its own pre-mRNA, inhibiting polyadenylation. Overexpression of U1A in mouse cells downregulates endogenous U1A mRNA; a single U1A molecule binding is insufficient for efficient polyadenylation inhibition.\",\n      \"method\": \"In vitro binding assays, in vivo overexpression, in vitro polyadenylation assays, structure probing, mutational analysis of pre-mRNA\",\n      \"journal\": \"Cell / The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — combined in vitro and in vivo functional data, replicated across multiple studies and organisms\",\n      \"pmids\": [\"8458082\", \"8262062\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1994,\n      \"finding\": \"U1A protein bound to its own pre-mRNA directly interacts with mammalian poly(A) polymerase (PAP) and inhibits both specific and nonspecific polyadenylation. This inhibition does not prevent CPSF binding or pre-mRNA cleavage. Domains in both U1A (identified by mutagenesis) and the C-terminus of PAP are required; the interaction is specific to mammalian (not yeast) PAP.\",\n      \"method\": \"In vitro polyadenylation assays, domain mutagenesis, in vitro protein-protein interaction (direct binding of U1A to PAP)\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — in vitro reconstitution of inhibition, domain mapping by mutagenesis, direct protein-protein interaction demonstrated, replicated and extended by subsequent studies\",\n      \"pmids\": [\"8313473\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1992,\n      \"finding\": \"U1A nuclear localization requires an active transport process independent of U1 snRNA binding; the nuclear localization signal is an unusually large sequence element between amino acids 94 and 204. U1A shuttles between nucleus and cytoplasm, and its intracellular distribution is determined by the number of free RNA binding sites available in each compartment.\",\n      \"method\": \"Microinjection of deletion mutants into Xenopus oocytes, transfection assays, introduction of competing RNA sequences\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct localization experiments with deletion mutants and RNA competition, multiple functional readouts\",\n      \"pmids\": [\"1618898\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"The solution NMR structure of U1A N-terminal RBD (residues 2-117) shows that the C-terminal helix C lies across the beta-sheet in the free protein (acting as a 'lid'), making hydrophobic contacts that stabilize the protein. Upon RNA binding, helix C rotates ~135° to allow Tyr13, Phe56 and Gln54 to stack with RNA bases, stabilized in the new position by hydrophobic interactions and a Ser91-Thr11 hydrogen bond.\",\n      \"method\": \"Multi-dimensional heteronuclear NMR solution structure determination with functional validation\",\n      \"journal\": \"Journal of molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — NMR solution structure with conformational change mechanism described, consistent with crystal structure data and validated by subsequent studies\",\n      \"pmids\": [\"8609632\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"The carboxy-terminal 20 amino acids of vertebrate PAP are essential for its inhibition by the U1A-RNA complex; transfer of these 20 residues to yeast PAP confers U1A-mediated inhibition. A GST fusion of these PAP residues interacts in vitro with an RNA-U1A complex containing two U1A molecules but not one, explaining the requirement for two U1A-binding sites. U1A amino acids 103-119 are required for PAP inhibition; a multimeric U1A peptide spanning this region uncouples splicing and 3'-end formation in vitro.\",\n      \"method\": \"In vitro PAP inhibition assays with domain mutants, yeast PAP chimera experiments, GST pulldown, in vitro splicing/polyadenylation uncoupling assay\",\n      \"journal\": \"Genes & development\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — reconstituted in vitro inhibition, reciprocal domain-swap to yeast PAP, multiple orthogonal assays in one study\",\n      \"pmids\": [\"9087430\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"The NMR structure of the U1A RBD1-polyadenylation inhibitory element (PIE RNA) complex at 1.08 Å interface precision shows that the same heptanucleotide AUUGCAC is recognized in two different structural contexts (hairpin and internal loop) by identical protein contacts; a protein dimer forms via C-terminal contacts that are RNA binding-dependent, providing the structural basis for cooperativity and PAP inhibition.\",\n      \"method\": \"NMR structure determination of U1A-3'UTR complex, model-building based on crystal structure of hairpin complex\",\n      \"journal\": \"Structure / Journal of biomolecular NMR\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — NMR structure plus crystallographic model, consistent with biochemical data, single lab but multiple methods\",\n      \"pmids\": [\"8736559\", \"9566313\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"NMR structure of the 38 kDa complex of two U1A proteins bound to PIE RNA shows that cooperativity of U1A binding depends on helix C, which undergoes a conformational change upon RNA binding. This same helix C is adjacent to the PAP-interacting domain, ensuring that PAP inhibition can only occur when U1A is RNA-bound, linking cooperative RNA binding to PAP inhibition.\",\n      \"method\": \"NMR structure determination of trimolecular 38 kDa complex\",\n      \"journal\": \"Nature structural biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — NMR structure of the full autoregulatory complex with mechanistic explanation for cooperativity and PAP inhibition coupling\",\n      \"pmids\": [\"10742179\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"U1A homodimerizes via two separate regions in the N-terminal 115 residues (one at aa 103-115) even when RNA binding is abolished. Mutation of the aa 103-115 dimerization region abolishes cooperative binding of two U1A molecules to PIE RNA and inhibition of polyadenylation, but does not affect single-molecule binding to PIE RNA.\",\n      \"method\": \"Yeast two-hybrid, in vitro coselection, gel mobility shift, polyadenylation inhibition assays with point and deletion mutants\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods (yeast two-hybrid, in vitro coselection, EMSA, polyadenylation assay), clear domain mapping\",\n      \"pmids\": [\"10688667\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"The nuclear import of U1A is mediated by importin alpha/beta and Ran; the NLS is located within residues 100-144. U1A was shown to bind the C-terminal portion of importin alpha. Nuclear accumulation in intact cells is Ran-dependent and inhibited by the importin beta-binding domain of importin alpha.\",\n      \"method\": \"Microinjection of deletion mutants into BHK21 cells, in vitro nuclear import assay with recombinant importins, in vivo Ran inhibition\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vivo and in vitro nuclear import assays with deletion mutants and direct binding to importin alpha demonstrated, two orthogonal methods\",\n      \"pmids\": [\"11278401\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"U1A binds two AUGCN(1-3)C motifs within a 29-nucleotide sequence between the two downstream GU-rich regions of the IgM heavy-chain secretory poly(A) site, inhibits binding of CstF 64K to GU-rich elements, and inhibits cleavage at the secretory poly(A) site.\",\n      \"method\": \"In vitro RNA binding assays, polyadenylation cleavage assays, competition binding experiments\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct binding demonstrated, functional inhibition of cleavage shown, mechanism of CstF displacement established\",\n      \"pmids\": [\"15226420\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"A snRNP-free form of U1A (SF-A) exists in human cells as part of a novel complex containing PSF, p54nrb, and p68; this complex promotes pre-mRNA cleavage during polyadenylation. p54nrb is specifically critical for the cleavage function, as shown by immunodepletion/reconstitution experiments.\",\n      \"method\": \"Tandem affinity purification, mass spectrometry, immunodepletion/reconstitution of in vitro polyadenylation, Co-IP\",\n      \"journal\": \"RNA\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — TAP purification with MS identification, functional reconstitution experiments, reciprocal immunoprecipitation\",\n      \"pmids\": [\"16373496\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"Identification of a novel snRNP-free U1A complex (SF-A) in which U1A is not associated with U1 snRNA or other snRNP components, but co-sediments and co-immunoprecipitates with non-snRNP proteins of ~58, 59, 63, 65, and 105 kDa. Anti-SF-A antibodies significantly diminish polyadenylation in vitro, indicating a functional role in this process.\",\n      \"method\": \"Sucrose density gradient fractionation, immunoprecipitation with conformation-specific monoclonal antibody (MAb 12E12), in vitro polyadenylation\",\n      \"journal\": \"RNA\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal immunoprecipitation and functional antibody inhibition, single lab\",\n      \"pmids\": [\"9404895\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"The largest component of the SF-A complex, p105, is PSF (polypyrimidine-tract binding protein-associated splicing factor). PSF co-purifies and co-immunoprecipitates with SF-A from 293T cell nucleoplasm and interacts with SF-A in vitro. Anti-SF-A antibodies inhibit both splicing and polyadenylation in a coupled in vitro reaction.\",\n      \"method\": \"Co-immunoprecipitation, in vitro binding assay, in vitro coupled splicing/polyadenylation assay\",\n      \"journal\": \"RNA\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal Co-IP plus in vitro functional inhibition, single lab\",\n      \"pmids\": [\"9848648\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"U1A binds directly and with high affinity and specificity to the SMN 3'-UTR adjacent to the polyadenylation site, independent of U1 snRNP. This binding inhibits polyadenylation of SMN pre-mRNA by specifically inhibiting 3' cleavage by CPSF. Excess U1A over U1 snRNA causes inhibition of SMN polyadenylation and decreases SMN protein levels.\",\n      \"method\": \"In vitro RNA binding assays, in vitro polyadenylation/cleavage assays, overexpression experiments with SMN protein level readout\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct binding established, CPSF-specific cleavage inhibition demonstrated in vitro, functional consequence (SMN protein level) shown in vivo\",\n      \"pmids\": [\"24362020\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"A conserved U1 site in the U1A gene 3'UTR acts in synergy with PIE to inhibit U1A expression via nuclear polyadenylation inhibition (poly(A) tail addition). The two elements function as a bipartite repressor: the U1 site is normally suppressed by a base-pairing mechanism, but together they form a ternary complex that inhibits polyadenylation without simply stabilizing factor binding.\",\n      \"method\": \"In vitro and in vivo reporter assays, mutational analysis of 3'UTR, in vitro polyadenylation assays\",\n      \"journal\": \"RNA\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — functional dissection using mutants in vivo and in vitro, novel mechanism for bipartite regulation identified, single lab\",\n      \"pmids\": [\"17942741\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"SAM68 directly interacts with U1A through SAM68's C-terminal tyrosine-rich (YY) domain binding the RRM1 domain of U1A; this interaction promotes U1 snRNP recruitment to the 5' splice site of mTor intron 5. Deletion of the U1A-SAM68 interaction domain or mutation of SAM68-binding sites in mTor intron 5 abrogates U1A recruitment and leads to premature intron 5 termination and polyadenylation.\",\n      \"method\": \"Co-immunoprecipitation, deletion mutagenesis, splicing assays, polyadenylation assays, in vivo and in vitro binding\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct interaction mapped by domain deletion, functional consequence on splicing and polyadenylation demonstrated in vitro and in vivo, multiple orthogonal methods\",\n      \"pmids\": [\"30767021\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Non-snRNP U1A levels decrease during B-cell differentiation, releasing the IgM secretory poly(A) site from repression. Undifferentiated B cells have more total U1A and a greater proportion is non-snRNP-associated, which directly inhibits poly(A) addition. The inhibitory effect is proportional to the amount of available U1A recovered by immunopurification.\",\n      \"method\": \"Flow cytometry, exhaustive immunoprecipitation, cold competitor RNA oligo de-repression of polyadenylation in nuclear extracts, in vitro poly(A) polymerase inhibition assay\",\n      \"journal\": \"RNA\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct functional link between non-snRNP U1A levels and poly(A) site inhibition demonstrated using competitor RNA and purified U1A fractions, single lab\",\n      \"pmids\": [\"16373497\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"U1A-RNA complex formation involves two mechanistically distinct steps: an initial rapid association driven by electrostatic interactions (slowed by charge neutralization or high salt) and a subsequent 'locking' step driven by close-range hydrogen bonding and stacking interactions (reflected in dissociation rate). Single amino acid substitutions can selectively perturb either step.\",\n      \"method\": \"Surface plasmon resonance (real-time biomolecular interaction analysis) with U1A mutants and RNA mutants, salt dependence experiments\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — quantitative kinetic analysis by SPR with panel of mutants, two-step mechanism mechanistically dissected, replicated in subsequent SPR studies\",\n      \"pmids\": [\"11297556\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"In vitro phage-display selection identified Leu-49 as a critical residue that disproportionately affects the rates of binding and release of U1A from U1 RNA; it appears to lock the protein onto the RNA. Three other residues in the mutagenized region also proved important for specific RNA binding.\",\n      \"method\": \"Phage-display combinatorial library with affinity selection (in vitro genetic selection), RNA binding assays\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — novel in vitro selection method identifies mechanistically important residue, single lab, single method type\",\n      \"pmids\": [\"8524863\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"The NMR structure of U1A C-terminal RNA-binding domain (RBD2) shows it folds as a canonical alpha/beta sandwich but does not bind U1, U2, or U5 snRNA, RNA hairpins, or random sequence RNAs. The two RBDs of the full-length protein exhibit uncorrelated motion due to a highly flexible linker region.\",\n      \"method\": \"NMR structure determination, RNA binding assays with multiple RNA substrates, fluorescence polarization\",\n      \"journal\": \"Journal of molecular biology / Biochemistry / Protein science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — NMR structure plus negative functional RNA binding data confirmed by multiple RNA substrates, consistent across two studies\",\n      \"pmids\": [\"7723028\", \"9265619\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"A single leucine residue in U1A (Leu-44) is critical for the intrinsic specificity of U1A for the U1hpII loop sequence over U2hpIV. U2A' protein enables U2B'' to discriminate loop sequences but not stem sequences; U2A' can also promote heterospecific U1A binding to U2hpIV but requires ~500-fold higher concentration due to preferential U2A' binding to U2B''.\",\n      \"method\": \"In vitro RNA binding assays with chimeric proteins, competition assays, site-directed mutagenesis\",\n      \"journal\": \"RNA\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — mutational analysis with quantitative binding measurements, single lab, clear specificity determinant identified\",\n      \"pmids\": [\"9814759\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Positively charged residues close to the RNA-binding site (Lys20, Lys22, Lys23) play distinct roles: Lys20 predominantly aids association while Lys22 and Lys23 are important for complex stability. Residues away from the binding site (Arg7, Lys60, Arg70) have minimal effect on either association or stability, demonstrating position-dependent roles of electrostatic interactions in the 'lure and lock' binding model.\",\n      \"method\": \"Surface plasmon resonance kinetics, salt dependence experiments, molecular dynamics simulation\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — SPR kinetics with panel of mutants plus MD simulation, single lab, mechanistically informative dissection of electrostatic roles\",\n      \"pmids\": [\"16407334\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"The C-terminal helix (helix C) of U1A RRM1 occludes the RNA-binding surface in the free protein. Truncation or disruption of helix C increases the association rate (by exposing the binding surface) but simultaneously reduces complex stability (loss of locking). The quadruple stacking interaction (involving helix C residues) makes a minor kinetic contribution compared to the intraprotein hydrogen bonds formed when helix C relocates.\",\n      \"method\": \"Surface plasmon resonance kinetics with truncation and point mutants of helix C\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — systematic SPR kinetic analysis with panel of helix C mutants, mechanistic dissection of association vs. locking steps, single lab\",\n      \"pmids\": [\"23703211\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"SNRPA binds directly to the G-quadruplex structure within the 5'UTR of BAG-1 mRNA. Label-free RNA pulldown from colorectal cancer cell protein extracts followed by LC-MS/MS identified SNRPA; direct binding was confirmed, and knockdown experiments suggested SNRPA modulates BAG-1 expression levels.\",\n      \"method\": \"RNA G-quadruplex pulldown with mutant control, LC-MS/MS, confirmation of direct binding, siRNA knockdown\",\n      \"journal\": \"Biochimie\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Weak — pulldown/MS identification with direct binding confirmation, single lab, limited mechanistic follow-up\",\n      \"pmids\": [\"32629040\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"SNRPA controls alternative splicing of ERCC1 exon 8; its depletion causes ERCC1 exon 8 skipping and reduced ERCC1-XPF complex formation, reducing DNA damage repair. IGF2BP1 (m6A reader) and ELAVL1 (RNA stabilizer) bind SNRPA mRNA, with ELAVL1 promoting SNRPA expression and thereby cisplatin resistance in an SNRPA-dependent manner.\",\n      \"method\": \"CRISPR/Cas9 knockout and shRNA knockdown, overexpression, splicing assays, Co-IP for ERCC1-XPF complex, gene ontology analysis, mouse xenograft model\",\n      \"journal\": \"Advanced science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — CRISPR/KO with specific splicing and complex formation readouts, in vivo xenograft validation, upstream m6A regulation mechanism identified, single lab\",\n      \"pmids\": [\"39555714\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"SNRPA (U1A) is a core U1 snRNP component that uses its N-terminal RRM domain to bind the AUUGCAC loop of U1 snRNA hairpin II with picomolar affinity via a two-step 'lure and lock' mechanism: electrostatic interactions mediate rapid association, after which C-terminal helix C repositions to lock the complex through stacking and hydrogen bond networks; in excess over U1 snRNA, free U1A cooperatively binds two sites in the 3'UTR PIE element of its own pre-mRNA and directly contacts the C-terminus of poly(A) polymerase to inhibit polyadenylation in a U1A-mRNA-bound, dimerization-dependent manner; U1A also exists in a non-snRNP SF-A complex with PSF and p54nrb that promotes pre-mRNA 3' cleavage, inhibits the IgM secretory poly(A) site, regulates SMN 3' processing, and interacts with SAM68 to modulate U1 snRNP recruitment to 5' splice sites; nuclear import of U1A is mediated by importin alpha/beta and Ran, and the protein shuttles between nucleus and cytoplasm with distribution governed by available RNA binding sites; additionally, SNRPA controls alternative splicing of ERCC1 exon 8 and binds BAG-1 mRNA G-quadruplex, with its own mRNA stability regulated by IGF2BP1/ELAVL1 via m6A.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"SNRPA (U1A) is an RRM-containing RNA-binding protein that functions both as a core component of the U1 snRNP and as a sequence-specific regulator of 3'-end processing and alternative splicing [#1, #4]. Its N-terminal RRM recognizes the AUUGCAC loop of U1 snRNA hairpin II through conserved RNP1 and RNP2 motifs that contact the RNA via base stacking with aromatic side chains and hydrogen-bond networks [#0, #3], with binding specificity over the U2B''/U2 snRNA pair dictated by a small set of protein and RNA residues [#2, #24]. Recognition proceeds by a two-step 'lure and lock' mechanism: rapid electrostatic association followed by a locking step in which the C-terminal helix C, which occludes the binding surface in the free protein, rotates onto the RNA to stabilize the complex through stacking and intraprotein hydrogen bonds [#7, #21, #26]. When U1A is in excess over U1 snRNA, free protein autoregulates its own expression by cooperatively binding two sites in the 3'UTR PIE element of its own pre-mRNA and directly contacting the C-terminus of poly(A) polymerase to inhibit polyadenylation; both cooperativity and PAP inhibition depend on helix C-mediated dimerization, coupling RNA binding to inhibition [#4, #5, #8, #10, #11]. This regulatory activity extends to other transcripts, where U1A binds 3'UTR elements to inhibit cleavage and polyadenylation of the IgM secretory poly(A) site by displacing CstF and of SMN pre-mRNA by blocking CPSF cleavage [#13, #17]. A snRNP-free pool of U1A (SF-A) assembles with PSF and p54nrb/p68 to promote pre-mRNA cleavage during polyadenylation [#14, #16], and U1A also interacts with SAM68 to promote U1 snRNP recruitment to 5' splice sites [#19] and controls alternative splicing of ERCC1 exon 8 to support ERCC1-XPF complex formation and DNA damage repair [#28]. Nuclear import of U1A is mediated by importin alpha/beta and Ran, and the protein shuttles between nucleus and cytoplasm with its distribution set by the number of available RNA binding sites [#6, #12].\",\n  \"teleology\": [\n    {\n      \"year\": 1989,\n      \"claim\": \"Establishing that U1A recognizes a defined RNA element via a discrete RNP motif converted U1A from a snRNP subunit into a tractable sequence-specific RNA-binding protein.\",\n      \"evidence\": \"in vitro RNA binding, point mutagenesis and deletion analysis mapping the U1 snRNA hairpin II site and the RNP motif\",\n      \"pmids\": [\"2531658\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No atomic-resolution description of contacts\", \"Functional consequence of binding beyond snRNP assembly not addressed\"]\n    },\n    {\n      \"year\": 1990,\n      \"claim\": \"Defining the few protein and RNA residues that distinguish U1A/U1 from U2B''/U2 explained how closely related RRM proteins achieve target specificity and showed U1A binds independently of accessory proteins.\",\n      \"evidence\": \"reciprocal specificity-swap experiments with chimeric proteins and RNAs\",\n      \"pmids\": [\"2140872\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Single critical specificity residue not yet resolved\", \"Structural basis of discrimination not shown\"]\n    },\n    {\n      \"year\": 1994,\n      \"claim\": \"High-resolution structures of the U1A RRM-hairpin II complex and of the free RBD revealed the atomic contacts and the helix C conformational switch underlying recognition.\",\n      \"evidence\": \"X-ray crystallography at 1.92 Å and multidimensional heteronuclear NMR of the free and bound RBD\",\n      \"pmids\": [\"7984237\", \"8609632\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Kinetic ordering of association vs locking not yet defined\", \"Behavior of the second RRM (RBD2) unresolved\"]\n    },\n    {\n      \"year\": 1994,\n      \"claim\": \"Identifying U1A autoregulation through PIE-element binding and direct PAP inhibition established a non-snRNP function and a feedback loop controlling U1A levels.\",\n      \"evidence\": \"in vitro/in vivo polyadenylation assays, overexpression, domain mutagenesis, and direct U1A-PAP binding\",\n      \"pmids\": [\"8458082\", \"8262062\", \"8313473\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Why two U1A molecules are required not yet structurally explained\", \"Whether other mRNAs are similarly regulated unknown at this stage\"]\n    },\n    {\n      \"year\": 1997,\n      \"claim\": \"Mapping the PAP C-terminal 20 residues and U1A residues 103-119 as the inhibitory interface, and showing two bound U1A molecules are needed, mechanistically linked cooperative RNA binding to polyadenylation control.\",\n      \"evidence\": \"in vitro PAP inhibition assays, yeast PAP chimera, GST pulldown, splicing/polyadenylation uncoupling\",\n      \"pmids\": [\"9087430\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of the dimer-PAP contact not yet visualized\", \"In vivo relevance across cell types not established\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"NMR structures of the two-U1A:PIE complex and dimerization mapping showed that helix C drives both RNA-binding cooperativity and ensures PAP inhibition occurs only when U1A is RNA-bound.\",\n      \"evidence\": \"NMR structure of the 38 kDa trimolecular complex; yeast two-hybrid, coselection, EMSA and polyadenylation assays with dimerization mutants\",\n      \"pmids\": [\"10742179\", \"10688667\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Dynamics of dimer assembly in cells not measured\", \"Regulation of free vs snRNP-bound U1A partitioning unresolved\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Dissecting binding into an electrostatic 'lure' and a hydrogen-bond/stacking 'lock' step provided a kinetic framework for how the RRM achieves both speed and stability.\",\n      \"evidence\": \"surface plasmon resonance kinetics with protein and RNA mutants and salt-dependence experiments\",\n      \"pmids\": [\"11297556\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Single mutants only partially separate the steps\", \"Generalizability to other RRM-RNA pairs not tested here\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Defining importin alpha/beta- and Ran-dependent nuclear import and an internal NLS clarified how U1A localizes, complementing earlier work showing distribution follows available RNA binding sites.\",\n      \"evidence\": \"microinjection of deletion mutants, in vitro import assays with recombinant importins, and in vivo Ran inhibition; earlier Xenopus oocyte and RNA-competition assays\",\n      \"pmids\": [\"11278401\", \"1618898\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Regulation of shuttling under physiological conditions unclear\", \"Cytoplasmic functions of shuttling U1A not defined\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Purification of the snRNP-free SF-A complex containing PSF and p54nrb/p68, and its requirement for pre-mRNA cleavage, distinguished a positive 3'-processing role for non-snRNP U1A from its inhibitory autoregulatory role.\",\n      \"evidence\": \"TAP purification with MS, immunodepletion/reconstitution of in vitro polyadenylation, Co-IP; earlier sucrose gradient and conformation-specific antibody work\",\n      \"pmids\": [\"16373496\", \"9404895\", \"9848648\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How U1A partitions between SF-A and inhibitory pools not established\", \"Substrate selectivity of SF-A-promoted cleavage unknown\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Linking declining non-snRNP U1A to release of the IgM secretory poly(A) site during B-cell differentiation gave a physiological setting for U1A-mediated cleavage inhibition through CstF displacement.\",\n      \"evidence\": \"flow cytometry, exhaustive immunoprecipitation, competitor-RNA de-repression and PAP inhibition assays; earlier mapping of AUGCN motifs and CstF inhibition\",\n      \"pmids\": [\"16373497\", \"15226420\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab; in vivo causality during differentiation not genetically proven\", \"Breadth of poly(A) sites regulated this way unknown\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Demonstrating direct U1A binding to the SMN 3'UTR with CPSF-specific cleavage inhibition and reduced SMN protein extended U1A's regulatory reach to a disease-relevant transcript.\",\n      \"evidence\": \"in vitro RNA binding and cleavage assays plus overexpression with SMN protein readout\",\n      \"pmids\": [\"24362020\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Physiological conditions favoring this inhibition not defined\", \"Endogenous regulation of SMN by U1A in neurons not shown\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Identifying a SAM68-U1A interaction that promotes U1 snRNP recruitment to a 5' splice site showed U1A participates in splice-site selection and intron retention/premature polyadenylation control.\",\n      \"evidence\": \"Co-IP, domain-deletion mapping, splicing and polyadenylation assays on mTor intron 5\",\n      \"pmids\": [\"30767021\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Genome-wide scope of SAM68-U1A-regulated targets unknown\", \"Mechanism of snRNP recruitment beyond the contact not detailed\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Showing SNRPA controls ERCC1 exon 8 splicing to support ERCC1-XPF complex formation and DNA repair, with its own mRNA regulated by m6A reader IGF2BP1 and stabilizer ELAVL1, connected U1A to a cancer-relevant splicing and drug-resistance axis.\",\n      \"evidence\": \"CRISPR knockout/shRNA, splicing assays, Co-IP for ERCC1-XPF, and mouse xenograft\",\n      \"pmids\": [\"39555714\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab; direct mechanism of exon 8 recognition not mapped\", \"Generality of U1A-regulated alternative splicing not established\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How the cell dynamically partitions U1A among the U1 snRNP, SF-A, and free inhibitory pools to coordinate splicing and 3'-end processing genome-wide remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No quantitative model of pool partitioning in vivo\", \"Transcriptome-wide map of direct U1A regulatory targets incomplete\", \"Signals controlling free vs snRNP-bound U1A levels unknown\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0003723\", \"supporting_discovery_ids\": [0, 1, 2, 3, 4, 13, 17, 27]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [5, 8, 13, 17]},\n      {\"term_id\": \"GO:0140110\", \"supporting_discovery_ids\": [4, 19, 28]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [6, 12]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [6]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-8953854\", \"supporting_discovery_ids\": [4, 14, 19, 28]},\n      {\"term_id\": \"R-HSA-74160\", \"supporting_discovery_ids\": [4, 13, 17]}\n    ],\n    \"complexes\": [\"U1 snRNP\", \"SF-A (U1A-PSF-p54nrb-p68)\"],\n    \"partners\": [\"PAPOLA\", \"PSF/SFPQ\", \"NONO\", \"SAM68/KHDRBS1\", \"ERCC1\", \"IGF2BP1\", \"ELAVL1\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":7,"faith_total":7,"faith_pct":100.0}}