{"gene":"SNRNP70","run_date":"2026-06-10T07:46:37","timeline":{"discoveries":[{"year":1989,"finding":"U1-70K binds directly and sequence-specifically to loop I of U1 snRNA; the central RNP consensus sequence (including the conserved eight-amino-acid RNP motif) is necessary and sufficient for this binding, and at least 8 of the 10 bases in loop I are required.","method":"Deletion and mutation analysis of beta-galactosidase/U1-70K fusion proteins and native HeLa U1-70K protein in RNA-binding assays in vitro","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro binding assay with systematic deletion and point mutagenesis of both protein and RNA; rigorous mapping of the binding interface","pmids":["2531275"],"is_preprint":false},{"year":1987,"finding":"Human U1-70K protein binds RNA in vitro; its actual molecular mass is ~52 kDa (not 70 kDa); the gene produces multiple mRNA isoforms via alternative splicing of at least four alternative exon segments, suggesting multiple protein isoforms may exist in vivo.","method":"cDNA cloning, in vitro RNA-binding assay, SDS-PAGE, RNA blotting","journal":"Nucleic acids research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — cDNA cloning combined with in vitro RNA-binding assay and protein size determination; single lab, multiple orthogonal methods","pmids":["2447561"],"is_preprint":false},{"year":1993,"finding":"Phosphorylation state of U1-70K is critical for pre-mRNA splicing: U1 snRNPs bearing thiophosphorylated (dephosphorylation-resistant) 70K fully reconstitute spliceosome assembly but completely block splicing at a pre-catalytic step. An associated kinase activity selectively phosphorylates U1-70K in vitro.","method":"In vitro thiophosphorylation with ATP-γS, depletion-reconstitution of U1 snRNPs in HeLa nuclear splicing extracts, spliceosome assembly assays","journal":"Nature","confidence":"High","confidence_rationale":"Tier 1 / Strong — reconstitution experiment with chemical tool (thiophosphate, phosphatase-resistant) plus functional splicing and assembly readouts; rigorous controls","pmids":["8387646"],"is_preprint":false},{"year":1998,"finding":"U1-70K, when bound to U1 snRNA within U1 snRNP, directly interacts with and inhibits poly(A) polymerase (PAP), thereby suppressing polyadenylation; U1A within the same snRNP does not contribute to this PAP inhibition.","method":"In vitro polyadenylation assays, direct protein-protein interaction assays between U1 70K and PAP","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro biochemical reconstitution with direct interaction assay; mechanistically resolves which subunit mediates inhibition","pmids":["9659922"],"is_preprint":false},{"year":1998,"finding":"The RS domain of U1-70K (residues Arg240–Asp270, containing a repeated Arg-Arg-Arg-Ser-Arg-Ser-Arg-Asp motif) is necessary and sufficient for binding to ASF/SF2 (SRSF1); multiple arginines within this domain are critical for the interaction and are also substrates for phosphorylation by SRPK1.","method":"Yeast two-hybrid, far-Western assay, deletion and point mutagenesis of U1-70K","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reciprocal binding assays (yeast two-hybrid + far-Western) plus mutagenesis; single lab","pmids":["9685421"],"is_preprint":false},{"year":1991,"finding":"The SR protein SF2/ASF contains an RS domain similar to that of U1-70K, and both contain an RNP-type RNA recognition motif; this structural homology implicates the RS domain of U1-70K as part of a conserved interface linking U1 snRNP to SR splicing regulators.","method":"cDNA cloning, recombinant protein expression in bacteria, in vitro splicing assay, sequence comparison","journal":"Cell","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — functional recombinant protein assay and sequence analysis establishing structural homology; the RS domain similarity is observational but the splicing activity is functionally validated","pmids":["1830244"],"is_preprint":false},{"year":2011,"finding":"Early spliceosome (E complex) assembly is mediated by RRM-RRM interaction between SRSF1 and U1-70K; phosphorylation of the SRSF1 RS domain acts as a molecular switch, releasing an intramolecular RRM-RS contact and permitting intermolecular binding to U1-70K RRM, thereby enabling U1 snRNP recruitment to the 5' splice site.","method":"Co-immunoprecipitation, GST pulldown, in vitro E-complex formation assay, specific RRM point mutations that disrupt RRM-RRM interaction, phosphorylation-state analysis","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal biochemical methods (pulldown, Co-IP, E-complex assay) plus mutational validation in a single study, mechanistically defining the switch","pmids":["21536904"],"is_preprint":false},{"year":2021,"finding":"CLK1 phosphorylates Ser-226 in the C terminus of U1-70K, releasing U1-70K from subnuclear granules and enabling it to interact with U1 snRNP and SRSF1; this phosphorylation breaks intramolecular contacts between the C terminus and the RRM, freeing the RRM to bind SRSF1. Subsequent nuclear induction of SRPK1 facilitates CLK1 dissociation from U1-70K, recycling the kinase.","method":"Quantitative proteomic phosphoproteomics, co-immunoprecipitation, imaging of subnuclear localization, in vitro kinase assays, SRPK1 overexpression","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 / Strong — site-specific phosphorylation identified by proteomics and validated biochemically and by imaging; multiple orthogonal methods in one study","pmids":["33811140"],"is_preprint":false},{"year":2024,"finding":"Phosphorylated SRSF1 RS domain interacts with U1-70K BAD1 domain (dominant interaction), while SRSF1 RRM1 interacts with U1-70K RRM (stabilizing interaction); phosphorylation of U1-70K BAD1 inhibits the U1-70K/SRSF1 interaction. BAD1 adopts an α-helical conformation that switches to β-strand/random coil upon RS binding.","method":"Circular dichroism, in vitro binding assays with phosphorylated proteins, in vitro splicing assays, in-cell CRISPR-based saturated domain scanning","journal":"Protein science : a publication of the Protein Society","confidence":"High","confidence_rationale":"Tier 1 / Strong — biophysical (CD), in vitro biochemical, and in-cell CRISPR functional validation; multiple orthogonal methods in one study","pmids":["39023093"],"is_preprint":false},{"year":2015,"finding":"The lysine methyltransferase SETMAR methylates snRNP70 (U1-70K) at lysine 130 in vitro (primarily monomethylation) and in cells, identifying U1-70K as a non-histone substrate of SETMAR.","method":"Quantitative proteomic analysis of methylated lysine (mass spectrometry), in vitro methylation assay, cellular confirmation by mass spectrometry","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vitro methylation assay plus cellular mass spectrometry validation; single lab, two orthogonal methods","pmids":["25795785"],"is_preprint":false},{"year":2009,"finding":"Native mass spectrometry of human U1 snRNP reveals that U1-70K isoforms differentially control subunit dynamics: unstructured, post-translationally modified C-terminal tails of Sm-B/B' and U1-C regulate their dynamic interactions with the Sm core, and these interactions are controlled by binding to different U1-70K isoforms and their phosphorylation status.","method":"Native mass spectrometry of intact U1 snRNP complexes; comparison of native vs. recombinant assemblies","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — native MS of intact complex with isoform comparison; single lab, single method platform","pmids":["19784376"],"is_preprint":false},{"year":2008,"finding":"During early apoptosis, U1-70K undergoes increased phosphorylation at Ser140 (within the RRM), followed by caspase-dependent, PP1-mediated dephosphorylation of other serine residues; the Ser140-phosphorylated form clusters in ectopic RNP-derived structures that are extruded into apoptotic bodies, linking specific phosphorylation events to subcellular redistribution.","method":"Immunofluorescence, phospho-specific antibodies, caspase and PP1 inhibitor treatments, cell fractionation, electron microscopy","journal":"Cell death and differentiation","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple imaging and pharmacological methods establishing PTM-to-localization link; single lab","pmids":["18202700"],"is_preprint":false},{"year":2014,"finding":"U1-70K interacts with the SMN complex in an RNA-independent manner; the SMN complex binding site maps to the unstructured N-terminal tail of U1-70K; U1-70K localizes to nuclear gems and is required for gem integrity.","method":"Co-immunoprecipitation (RNA-independent), deletion mapping of U1-70K, immunofluorescence localization, knockdown rescue experiments","journal":"Journal of cell science","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reciprocal Co-IP with RNase treatment plus deletion mapping and rescue experiments; single lab","pmids":["25052091"],"is_preprint":false},{"year":2019,"finding":"The low-complexity (LC) domain of U1-70K undergoes liquid-liquid phase separation (LLPS) driven by repetitive basic-acidic (ampholytic) motifs, independent of nucleotides; LLPS can transition to aggregation in vitro and in vivo, with the balance determined by the content of ampholytic motifs.","method":"In vitro phase separation assays, in vivo transfection imaging, mutagenesis of basic-acidic motifs","journal":"Science advances","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vitro and in vivo LLPS assays with mutational validation; single lab, two orthogonal systems","pmids":["31723601"],"is_preprint":false},{"year":2014,"finding":"AD brain homogenates induce soluble U1-70K to become Sarkosyl-insoluble in a manner dependent on aggregated protein (not RNA); the C-terminal LC1 and LC2 domains of U1-70K are necessary and sufficient for aggregation; a U1-70K fragment harboring the LC1 domain directly interacts with aggregated U1-70K from AD brain.","method":"Sarkosyl fractionation, proteinase K treatment, recombinant domain deletion analysis, protein cross-linking and mass spectrometry","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple biochemical methods (fractionation, cross-linking MS, domain deletion) in a single study; single lab","pmids":["25355317"],"is_preprint":false},{"year":2014,"finding":"U1-70K is proteolytically cleaved in ~50% of Alzheimer's disease cases to an N-terminal ~40 kDa fragment (N40K); the cleavage site maps to a repetitive hydrophilic domain near Arg300 (±6 residues); expression of N40K causes substantial degeneration of rat primary hippocampal neurons.","method":"LC-MS/MS with stable isotope labeling, Western blotting with recombinant truncation ladder, primary neuron toxicity assay","journal":"Journal of proteome research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — mass spectrometry site mapping validated by recombinant truncation Western blot; functional neuronal toxicity assay; single lab","pmids":["24902715"],"is_preprint":false},{"year":2013,"finding":"U1C protein levels regulate alternative splicing of U1-70K pre-mRNA via a U1C-dependent alternative 3' splice site requiring an adjacent cluster of regulatory 5' splice sites and intact U1 snRNP binding; the non-productive isoform is degraded by NMD, reducing U1-70K mRNA/protein levels and impairing U1C incorporation, establishing a feedback loop controlling U1-70K/U1C homeostasis and U1 snRNP assembly.","method":"RNA-Seq after U1C knockdown, minigene mutational analysis, antisense morpholino splice-site blocking, in vitro binding experiments","journal":"PLoS genetics","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods (RNAseq, minigene mutagenesis, morpholino, in vitro binding) establishing the feedback mechanism; single lab with rigorous controls","pmids":["24146627"],"is_preprint":false},{"year":2001,"finding":"Yeast U1-70K homolog Snp1p physically associates with the essential spliceosomal protein Prp8p (by co-immunoprecipitation), suggesting Snp1p has functions late in spliceosome development; Snp1p also interacts with Exo84p, a subunit of the exocyst secretion complex, which is itself required for pre-mRNA splicing and prespliceosome formation in vitro.","method":"Yeast two-hybrid, co-immunoprecipitation, in vitro splicing assay with temperature-sensitive exo84 mutant, RT-PCR pre-mRNA quantification","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — two-hybrid confirmed by Co-IP; functional in vitro splicing assay with genetic mutant; single lab","pmids":["11425851"],"is_preprint":false},{"year":1995,"finding":"In yeast, the N-terminal domain of Snp1 (yeast U1-70K) is necessary and sufficient for complementation of snp1-null growth and splicing defects and for in vivo association with the U1 snRNP particle; the conserved RRM and glycine-rich domains are dispensable for these functions.","method":"Yeast genetic complementation with deletion alleles of SNP1, in vivo U1 snRNP association assay","journal":"Molecular and cellular biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — systematic deletion genetics with growth and splicing phenotypic readouts; single lab","pmids":["7565787"],"is_preprint":false},{"year":1992,"finding":"The yeast SNP1 gene product binds directly and specifically to the first 47 nucleotides of yeast U1 RNA (including stem-loop I), establishing Snp1p as the yeast ortholog of mammalian U1-70K.","method":"Bacterial expression of SNP1 fusion protein, gel-shift RNA-binding assay","journal":"Nucleic acids research","confidence":"Medium","confidence_rationale":"Tier 1 / Weak — in vitro RNA-binding reconstitution assay; single lab, single method","pmids":["1387202"],"is_preprint":false},{"year":2005,"finding":"The PSI protein (P-element somatic inhibitor) binds U1-70K via two homologous 'A and B box' sequences near its C terminus that interact with a short proline-rich sequence at the C terminus of U1-70K; NMR shows the B box forms an anti-parallel helical hairpin with a hydrophobic cluster of four aromatic residues that contacts the proline-rich region of U1-70K.","method":"NMR structure determination, deletion mapping of both PSI and U1-70K interaction domains","journal":"Journal of molecular biology","confidence":"High","confidence_rationale":"Tier 1 / Moderate — NMR structure with functional domain mapping of both interaction partners; single lab with structural and biochemical validation","pmids":["15990112"],"is_preprint":false},{"year":2004,"finding":"In Drosophila, loss of U1-70K causes embryonic lethality; the arginine-rich RS domain of U1-70K is dispensable for viability and splicing in otherwise wild-type animals, but becomes essential for viability when combined with mutations in another U1 snRNP component, demonstrating a redundant but context-dependent role for the RS domain in U1 snRNP function.","method":"Drosophila genetics: null alleles, RS-domain deletion transgenic rescue, double-mutant analysis","journal":"Genetics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vivo genetic epistasis with null and domain-deletion alleles; double-mutant analysis; single organism/lab","pmids":["15611175"],"is_preprint":false},{"year":2022,"finding":"U1-70K/SNRNP70 localizes to RNA-associated granules in zebrafish axons (cytoplasmic pool); this extra-nuclear SNRNP70 regulates motor axon growth, acetylcholine receptor clustering, and neuromuscular synaptogenesis; it protects a subset of axonal transcripts and regulates splice variants of agrin to control synapse formation.","method":"Live imaging in zebrafish, knockdown experiments, transcript abundance and trafficking measurements in axons, splicing analysis of agrin","journal":"Current biology : CB","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct imaging plus loss-of-function with specific morphological and molecular readouts; single lab","pmids":["36384140"],"is_preprint":false},{"year":2022,"finding":"U1-70K and ZFC3H1 function in the same pathway to retain mRNAs containing 5' splice site motifs (e.g., IPA transcripts) in nuclear speckles, preventing their nuclear export; disassembly of nuclear speckles impairs this nuclear retention.","method":"High-throughput sequencing of cellular fractions, reporter mRNA nuclear retention assays, knockdown of U1-70K and ZFC3H1, nuclear speckle disassembly","journal":"RNA (New York, N.Y.)","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — fractionation sequencing and reporter assays with knockdowns; functional epistasis; single lab","pmids":["35351812"],"is_preprint":false},{"year":2022,"finding":"The extended RRM (eRRM) of U1-70K, including the N-terminal flanking helix (N-helix) and C-terminal intrinsically disordered region (C-IDR), is required for full stability of the U1-70K/SL1 RNA complex; the N-helix strongly contributes to overall binding, while the C-IDR affects the local binding site; all-atom simulations show flanking regions act via collective interactions with RNA rather than through direct hydrogen bond contributions.","method":"Thermal dissociation assays, laser temperature-jump kinetics, long-time all-atom molecular dynamics simulations, truncation analysis","journal":"Nucleic acids research","confidence":"Medium","confidence_rationale":"Tier 1 / Moderate — biophysical binding assays combined with atomistic simulations; single lab, multiple complementary methods","pmids":["35876068"],"is_preprint":false},{"year":2024,"finding":"SNRNP70 directly interacts with CD55 pre-mRNA and modulates its alternative splicing; SNRNP70 overexpression promotes OS cell proliferation and metastasis in vitro, while its depletion reduces these capabilities in vivo.","method":"RNA immunoprecipitation (direct interaction with CD55), splicing analysis, overexpression and knockdown functional assays in vitro and in vivo (mouse xenograft)","journal":"JCI insight","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct RNA-IP showing SNRNP70-CD55 interaction plus functional in vitro/in vivo experiments; single lab","pmids":["39704173"],"is_preprint":false},{"year":2025,"finding":"In yeast, U1 snRNP interaction with RNA polymerase II is mediated predominantly by Prp40 rather than U1-70K (Snp1); residues on yeast U1-70K involved in pol II interaction in humans are not conserved in yeast, and U1-70K makes minimal contribution to U1 snRNP's association with pol II in yeast.","method":"Co-immunoprecipitation of pol II with U1/U2 snRNPs, domain deletion analysis of Prp40, comparison with human U1 snRNP cryo-EM data","journal":"bioRxiv","confidence":"Low","confidence_rationale":"Tier 3 / Weak — preprint, single Co-IP experiment in yeast; negative finding for yeast U1-70K's role; not yet peer-reviewed","pmids":["40909591"],"is_preprint":true}],"current_model":"SNRNP70/U1-70K is the U1 snRNP-specific RNA-binding protein that directly and sequence-specifically contacts loop I of U1 snRNA via its RRM; it recruits SR proteins (especially SRSF1) to the 5' splice site through RRM-RRM and RS/BAD1 domain interactions regulated by CLK1-mediated phosphorylation of Ser-226 and SRPK1-mediated phosphorylation of SRSF1, thereby nucleating early spliceosome (E complex) assembly; it additionally suppresses polyadenylation by directly inhibiting poly(A) polymerase, retains misprocessed mRNAs in nuclear speckles together with ZFC3H1, associates with the SMN complex via its N-terminal tail to maintain nuclear gem integrity, localizes to axonal granules where it regulates local transcript abundance and alternative splicing, undergoes disease-relevant proteolytic cleavage and aggregation through its low-complexity LC1/LC2 domains in Alzheimer's disease, and is a substrate for methylation by SETMAR at Lys-130."},"narrative":{"mechanistic_narrative":"SNRNP70/U1-70K is the U1 snRNP-specific RNA-binding protein that nucleates early spliceosome assembly by directly and sequence-specifically contacting loop I of U1 snRNA through its RNP-consensus RNA recognition motif [PMID:2531275]. Beyond the core RRM, flanking elements of the extended RRM (an N-terminal helix and a C-terminal disordered region) cooperatively stabilize the U1-70K/stem-loop RNA complex [PMID:35876068]. U1-70K bridges U1 snRNP to SR splicing regulators: its RS domain binds SRSF1/ASF (SF2) [PMID:9685421], and at the 5' splice site an RRM-RRM contact between U1-70K and SRSF1, together with a dominant phosphorylated-RS-domain/BAD1-domain interaction, recruits U1 snRNP to enable E-complex formation [PMID:21536904, PMID:39023093]. These interactions are governed by phosphorylation switches: CLK1 phosphorylation of Ser-226 releases U1-70K from subnuclear granules and frees the RRM to engage SRSF1, with SRPK1 recycling CLK1 [PMID:33811140], while phosphorylation of the SRSF1 RS domain releases an intramolecular RRM-RS contact to permit intermolecular binding [PMID:21536904]. The phosphorylation state of U1-70K itself is critical for catalysis, as dephosphorylation-resistant 70K permits spliceosome assembly but blocks splicing at a pre-catalytic step [PMID:8387646]. U1-70K additionally suppresses polyadenylation by directly inhibiting poly(A) polymerase when bound within U1 snRNP [PMID:9659922], and acts with ZFC3H1 to retain 5'-splice-site-containing mRNAs in nuclear speckles, preventing their export [PMID:35351812]. It associates RNA-independently with the SMN complex via its N-terminal tail and is required for nuclear gem integrity [PMID:25052091]. A cytoplasmic axonal pool regulates local transcript abundance and alternative splicing to control motor axon growth and synaptogenesis [PMID:36384140]. Its C-terminal low-complexity domains drive liquid-liquid phase separation that can transition to aggregation [PMID:31723601], and in Alzheimer's disease U1-70K undergoes LC1/LC2-dependent aggregation and proteolytic cleavage to a neurotoxic N40K fragment [PMID:25355317, PMID:24902715].","teleology":[{"year":1987,"claim":"Establishing the molecular identity of U1-70K—an RNA-binding protein of true mass ~52 kDa with multiple alternatively spliced isoforms—provided the foundational reagent and revealed isoform diversity at the gene's outset.","evidence":"cDNA cloning, in vitro RNA-binding, SDS-PAGE and RNA blotting","pmids":["2447561"],"confidence":"Medium","gaps":["Did not map the RNA-binding interface","Functional distinction among isoforms unresolved"]},{"year":1989,"claim":"Mapping the RNA target answered how U1-70K is targeted within U1 snRNP, showing its RNP-motif RRM binds loop I of U1 snRNA directly and sequence-specifically.","evidence":"Deletion and point mutagenesis of fusion and native proteins in in vitro RNA-binding assays","pmids":["2531275"],"confidence":"High","gaps":["No structure of the bound complex","Contribution of flanking regions not addressed"]},{"year":1993,"claim":"Demonstrating that 70K phosphorylation state gates catalysis distinguished spliceosome assembly from activation, placing U1-70K phosphorylation at a pre-catalytic checkpoint.","evidence":"Thiophosphorylation and depletion-reconstitution of U1 snRNPs in HeLa splicing extracts with assembly readouts","pmids":["8387646"],"confidence":"High","gaps":["The responsible kinase not identified at the time","Relevant phosphosites unmapped"]},{"year":1998,"claim":"Identifying the U1-70K RS domain as the SRSF1-binding module and a substrate of SRPK1 connected U1 snRNP to SR-protein splicing regulators through a phosphorylation-controlled interface.","evidence":"Yeast two-hybrid, far-Western, and mutagenesis (idx4); in vitro PAP inhibition and interaction assays (idx3)","pmids":["9685421","9659922"],"confidence":"High","gaps":["Structural basis of RS-domain binding unresolved at this stage","Connection between PAP inhibition and SR-protein recruitment unclear"]},{"year":2011,"claim":"Defining the SRSF1 RRM-U1-70K RRM interaction as a phosphorylation-controlled switch explained mechanistically how SR proteins recruit U1 snRNP to the 5' splice site during E-complex formation.","evidence":"Co-IP, GST pulldown, in vitro E-complex assays, RRM point mutations and phospho-state analysis","pmids":["21536904"],"confidence":"High","gaps":["Did not resolve the dominant RS/BAD1 contact later identified","In vivo dynamics of the switch unaddressed"]},{"year":2021,"claim":"Identifying CLK1 phosphorylation of Ser-226 as the trigger that releases U1-70K from granules answered how the RRM is freed to engage SRSF1, with SRPK1 recycling the kinase.","evidence":"Phosphoproteomics, Co-IP, subnuclear imaging, in vitro kinase assays, SRPK1 overexpression","pmids":["33811140"],"confidence":"High","gaps":["Upstream signals controlling CLK1 activity unknown","Quantitative kinetics of granule release unresolved"]},{"year":2024,"claim":"Resolving the phosphorylated-RS/BAD1 interaction as dominant, with RRM-RRM stabilizing, and a conformational switch in BAD1, refined the architecture and regulation of the U1-70K/SRSF1 interface.","evidence":"Circular dichroism, in vitro binding with phosphoproteins, in vitro splicing, in-cell CRISPR saturated domain scanning","pmids":["39023093"],"confidence":"High","gaps":["High-resolution structure of the assembled interface lacking","Interplay with U1-70K Ser-226 phosphorylation not fully integrated"]},{"year":2022,"claim":"Characterizing the extended RRM showed that disordered flanking regions act collectively to stabilize RNA binding, extending the RNA-recognition determinants beyond the canonical RRM.","evidence":"Thermal dissociation, laser T-jump kinetics, all-atom MD simulations, truncation analysis","pmids":["35876068"],"confidence":"Medium","gaps":["No experimental structure of the eRRM-RNA complex","Cellular consequences of flanking-region truncation untested"]},{"year":2014,"claim":"Mapping the SMN-complex binding to the N-terminal tail and showing a requirement for gem integrity expanded U1-70K's role into snRNP biogenesis bodies.","evidence":"RNA-independent reciprocal Co-IP, deletion mapping, immunofluorescence, knockdown rescue","pmids":["25052091"],"confidence":"Medium","gaps":["Direct vs. indirect SMN contact not distinguished","Mechanism by which U1-70K maintains gem structure unknown"]},{"year":2022,"claim":"Discovery of a cytoplasmic axonal pool established an extra-nuclear function in local transcript regulation and synaptogenesis, broadening U1-70K beyond nuclear splicing.","evidence":"Live imaging and knockdown in zebrafish with axonal transcript, AChR clustering, and agrin splicing readouts","pmids":["36384140"],"confidence":"Medium","gaps":["Mechanism of axonal targeting unknown","Whether axonal splicing occurs locally or upstream unresolved"]},{"year":2022,"claim":"Linking U1-70K with ZFC3H1 in nuclear-speckle retention of 5'-splice-site-containing mRNAs defined a quality-control function preventing export of misprocessed transcripts.","evidence":"Fractionation sequencing, reporter retention assays, knockdowns, speckle disassembly","pmids":["35351812"],"confidence":"Medium","gaps":["Direct vs. indirect U1-70K/ZFC3H1 association not shown","How retained transcripts are subsequently resolved unclear"]},{"year":2014,"claim":"Identifying LC1/LC2-dependent aggregation and proteolytic cleavage to a neurotoxic N40K fragment in Alzheimer's disease connected U1-70K low-complexity domains to neurodegeneration.","evidence":"Sarkosyl fractionation, cross-linking MS, domain deletion (idx14); LC-MS/MS site mapping and primary neuron toxicity assay (idx15)","pmids":["25355317","24902715"],"confidence":"Medium","gaps":["Protease responsible for N40K cleavage unidentified","Causal role of aggregation in disease pathogenesis not established"]},{"year":2019,"claim":"Demonstrating that the low-complexity domain undergoes ampholytic-motif-driven phase separation that can convert to aggregation provided a biophysical basis for both speckle/granule partitioning and pathological aggregation.","evidence":"In vitro and in vivo LLPS assays with mutagenesis of basic-acidic motifs","pmids":["31723601"],"confidence":"Medium","gaps":["Link between LLPS and physiological splicing activity untested","Cellular triggers of the LLPS-to-aggregate transition unknown"]},{"year":2024,"claim":"Showing SNRNP70 directly binds CD55 pre-mRNA and drives osteosarcoma proliferation and metastasis implicated its splicing activity in cancer pathology.","evidence":"RNA-IP, splicing analysis, overexpression/knockdown in vitro and in mouse xenografts","pmids":["39704173"],"confidence":"Medium","gaps":["Whether CD55 splicing change is sufficient for the phenotype untested","Generality across cancer types unknown"]},{"year":2015,"claim":"Identifying SETMAR-mediated monomethylation of Lys-130 added a non-histone PTM to U1-70K, raising the question of how methylation modulates its function.","evidence":"Quantitative methyl-lysine proteomics, in vitro methylation, cellular MS confirmation","pmids":["25795785"],"confidence":"Medium","gaps":["Functional consequence of K130 methylation unknown","Whether methylation affects RNA binding or splicing untested"]},{"year":null,"claim":"How the multiple regulatory layers—phosphorylation switches, methylation, LLPS, SMN/speckle partitioning, and the cytoplasmic axonal pool—are integrated to control U1-70K function in different cellular contexts remains unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No unified structural model of regulated U1 snRNP/SR-protein assembly","Crosstalk between K130 methylation and Ser phosphorylation unexplored","Trigger linking physiological LLPS to pathological aggregation in human brain undefined"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0003723","term_label":"RNA binding","supporting_discovery_ids":[0,24]},{"term_id":"GO:0140110","term_label":"transcription regulator activity","supporting_discovery_ids":[6,25]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[3]},{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[6,4]}],"localization":[{"term_id":"GO:0005654","term_label":"nucleoplasm","supporting_discovery_ids":[7,23]},{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[12]},{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[22]}],"pathway":[{"term_id":"R-HSA-8953854","term_label":"Metabolism of RNA","supporting_discovery_ids":[0,6,16]},{"term_id":"R-HSA-74160","term_label":"Gene expression (Transcription)","supporting_discovery_ids":[6,3]}],"complexes":["U1 snRNP","SMN complex"],"partners":["SRSF1","PAP","ZFC3H1","SMN","CLK1","SRPK1","SETMAR"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"P08621","full_name":"U1 small nuclear ribonucleoprotein 70 kDa","aliases":[],"length_aa":437,"mass_kda":51.6,"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 (PubMed:19325628, PubMed:25555158). SNRNP70 binds to the loop I region of U1-snRNA (PubMed:19325628, PubMed:2467746, PubMed:25555158) Truncated isoforms that lack the RRM domain cannot bind U1-snRNA Truncated isoforms that lack the RRM domain cannot bind U1-snRNA","subcellular_location":"Nucleus speckle; Nucleus, nucleoplasm","url":"https://www.uniprot.org/uniprotkb/P08621/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":true,"resolved_as":"","url":"https://depmap.org/portal/gene/SNRNP70","classification":"Common Essential","n_dependent_lines":1204,"n_total_lines":1208,"dependency_fraction":0.9966887417218543},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[{"gene":"SNRPA","stoichiometry":10.0},{"gene":"SNRPB","stoichiometry":10.0},{"gene":"SNRPC","stoichiometry":10.0},{"gene":"SNRPD2","stoichiometry":10.0},{"gene":"SNRPF","stoichiometry":10.0},{"gene":"TOP1","stoichiometry":4.0},{"gene":"CPSF6","stoichiometry":0.2},{"gene":"DDX21","stoichiometry":0.2},{"gene":"SMN1","stoichiometry":0.2},{"gene":"SNRPB2","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/search/SNRNP70","total_profiled":1310},"omim":[{"mim_id":"620956","title":"ZINC FINGER C3H1 DOMAIN-CONTAINING PROTEIN; ZFC3H1","url":"https://www.omim.org/entry/620956"},{"mim_id":"619631","title":"SMALL NUCLEAR RIBONUCLEOPROTEIN U11/U12, SUBUNIT 35; SNRNP35","url":"https://www.omim.org/entry/619631"},{"mim_id":"619629","title":"SMALL NUCLEAR RIBONUCLEOPROTEIN U4/U6.U5, 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the central RNP consensus sequence (including the conserved eight-amino-acid RNP motif) is necessary and sufficient for this binding, and at least 8 of the 10 bases in loop I are required.\",\n      \"method\": \"Deletion and mutation analysis of beta-galactosidase/U1-70K fusion proteins and native HeLa U1-70K protein in RNA-binding assays in vitro\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — in vitro binding assay with systematic deletion and point mutagenesis of both protein and RNA; rigorous mapping of the binding interface\",\n      \"pmids\": [\"2531275\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1987,\n      \"finding\": \"Human U1-70K protein binds RNA in vitro; its actual molecular mass is ~52 kDa (not 70 kDa); the gene produces multiple mRNA isoforms via alternative splicing of at least four alternative exon segments, suggesting multiple protein isoforms may exist in vivo.\",\n      \"method\": \"cDNA cloning, in vitro RNA-binding assay, SDS-PAGE, RNA blotting\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — cDNA cloning combined with in vitro RNA-binding assay and protein size determination; single lab, multiple orthogonal methods\",\n      \"pmids\": [\"2447561\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1993,\n      \"finding\": \"Phosphorylation state of U1-70K is critical for pre-mRNA splicing: U1 snRNPs bearing thiophosphorylated (dephosphorylation-resistant) 70K fully reconstitute spliceosome assembly but completely block splicing at a pre-catalytic step. An associated kinase activity selectively phosphorylates U1-70K in vitro.\",\n      \"method\": \"In vitro thiophosphorylation with ATP-γS, depletion-reconstitution of U1 snRNPs in HeLa nuclear splicing extracts, spliceosome assembly assays\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — reconstitution experiment with chemical tool (thiophosphate, phosphatase-resistant) plus functional splicing and assembly readouts; rigorous controls\",\n      \"pmids\": [\"8387646\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"U1-70K, when bound to U1 snRNA within U1 snRNP, directly interacts with and inhibits poly(A) polymerase (PAP), thereby suppressing polyadenylation; U1A within the same snRNP does not contribute to this PAP inhibition.\",\n      \"method\": \"In vitro polyadenylation assays, direct protein-protein interaction assays between U1 70K and PAP\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — in vitro biochemical reconstitution with direct interaction assay; mechanistically resolves which subunit mediates inhibition\",\n      \"pmids\": [\"9659922\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"The RS domain of U1-70K (residues Arg240–Asp270, containing a repeated Arg-Arg-Arg-Ser-Arg-Ser-Arg-Asp motif) is necessary and sufficient for binding to ASF/SF2 (SRSF1); multiple arginines within this domain are critical for the interaction and are also substrates for phosphorylation by SRPK1.\",\n      \"method\": \"Yeast two-hybrid, far-Western assay, deletion and point mutagenesis of U1-70K\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal binding assays (yeast two-hybrid + far-Western) plus mutagenesis; single lab\",\n      \"pmids\": [\"9685421\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1991,\n      \"finding\": \"The SR protein SF2/ASF contains an RS domain similar to that of U1-70K, and both contain an RNP-type RNA recognition motif; this structural homology implicates the RS domain of U1-70K as part of a conserved interface linking U1 snRNP to SR splicing regulators.\",\n      \"method\": \"cDNA cloning, recombinant protein expression in bacteria, in vitro splicing assay, sequence comparison\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — functional recombinant protein assay and sequence analysis establishing structural homology; the RS domain similarity is observational but the splicing activity is functionally validated\",\n      \"pmids\": [\"1830244\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Early spliceosome (E complex) assembly is mediated by RRM-RRM interaction between SRSF1 and U1-70K; phosphorylation of the SRSF1 RS domain acts as a molecular switch, releasing an intramolecular RRM-RS contact and permitting intermolecular binding to U1-70K RRM, thereby enabling U1 snRNP recruitment to the 5' splice site.\",\n      \"method\": \"Co-immunoprecipitation, GST pulldown, in vitro E-complex formation assay, specific RRM point mutations that disrupt RRM-RRM interaction, phosphorylation-state analysis\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal biochemical methods (pulldown, Co-IP, E-complex assay) plus mutational validation in a single study, mechanistically defining the switch\",\n      \"pmids\": [\"21536904\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"CLK1 phosphorylates Ser-226 in the C terminus of U1-70K, releasing U1-70K from subnuclear granules and enabling it to interact with U1 snRNP and SRSF1; this phosphorylation breaks intramolecular contacts between the C terminus and the RRM, freeing the RRM to bind SRSF1. Subsequent nuclear induction of SRPK1 facilitates CLK1 dissociation from U1-70K, recycling the kinase.\",\n      \"method\": \"Quantitative proteomic phosphoproteomics, co-immunoprecipitation, imaging of subnuclear localization, in vitro kinase assays, SRPK1 overexpression\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — site-specific phosphorylation identified by proteomics and validated biochemically and by imaging; multiple orthogonal methods in one study\",\n      \"pmids\": [\"33811140\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Phosphorylated SRSF1 RS domain interacts with U1-70K BAD1 domain (dominant interaction), while SRSF1 RRM1 interacts with U1-70K RRM (stabilizing interaction); phosphorylation of U1-70K BAD1 inhibits the U1-70K/SRSF1 interaction. BAD1 adopts an α-helical conformation that switches to β-strand/random coil upon RS binding.\",\n      \"method\": \"Circular dichroism, in vitro binding assays with phosphorylated proteins, in vitro splicing assays, in-cell CRISPR-based saturated domain scanning\",\n      \"journal\": \"Protein science : a publication of the Protein Society\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — biophysical (CD), in vitro biochemical, and in-cell CRISPR functional validation; multiple orthogonal methods in one study\",\n      \"pmids\": [\"39023093\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"The lysine methyltransferase SETMAR methylates snRNP70 (U1-70K) at lysine 130 in vitro (primarily monomethylation) and in cells, identifying U1-70K as a non-histone substrate of SETMAR.\",\n      \"method\": \"Quantitative proteomic analysis of methylated lysine (mass spectrometry), in vitro methylation assay, cellular confirmation by mass spectrometry\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vitro methylation assay plus cellular mass spectrometry validation; single lab, two orthogonal methods\",\n      \"pmids\": [\"25795785\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Native mass spectrometry of human U1 snRNP reveals that U1-70K isoforms differentially control subunit dynamics: unstructured, post-translationally modified C-terminal tails of Sm-B/B' and U1-C regulate their dynamic interactions with the Sm core, and these interactions are controlled by binding to different U1-70K isoforms and their phosphorylation status.\",\n      \"method\": \"Native mass spectrometry of intact U1 snRNP complexes; comparison of native vs. recombinant assemblies\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — native MS of intact complex with isoform comparison; single lab, single method platform\",\n      \"pmids\": [\"19784376\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"During early apoptosis, U1-70K undergoes increased phosphorylation at Ser140 (within the RRM), followed by caspase-dependent, PP1-mediated dephosphorylation of other serine residues; the Ser140-phosphorylated form clusters in ectopic RNP-derived structures that are extruded into apoptotic bodies, linking specific phosphorylation events to subcellular redistribution.\",\n      \"method\": \"Immunofluorescence, phospho-specific antibodies, caspase and PP1 inhibitor treatments, cell fractionation, electron microscopy\",\n      \"journal\": \"Cell death and differentiation\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple imaging and pharmacological methods establishing PTM-to-localization link; single lab\",\n      \"pmids\": [\"18202700\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"U1-70K interacts with the SMN complex in an RNA-independent manner; the SMN complex binding site maps to the unstructured N-terminal tail of U1-70K; U1-70K localizes to nuclear gems and is required for gem integrity.\",\n      \"method\": \"Co-immunoprecipitation (RNA-independent), deletion mapping of U1-70K, immunofluorescence localization, knockdown rescue experiments\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal Co-IP with RNase treatment plus deletion mapping and rescue experiments; single lab\",\n      \"pmids\": [\"25052091\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"The low-complexity (LC) domain of U1-70K undergoes liquid-liquid phase separation (LLPS) driven by repetitive basic-acidic (ampholytic) motifs, independent of nucleotides; LLPS can transition to aggregation in vitro and in vivo, with the balance determined by the content of ampholytic motifs.\",\n      \"method\": \"In vitro phase separation assays, in vivo transfection imaging, mutagenesis of basic-acidic motifs\",\n      \"journal\": \"Science advances\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vitro and in vivo LLPS assays with mutational validation; single lab, two orthogonal systems\",\n      \"pmids\": [\"31723601\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"AD brain homogenates induce soluble U1-70K to become Sarkosyl-insoluble in a manner dependent on aggregated protein (not RNA); the C-terminal LC1 and LC2 domains of U1-70K are necessary and sufficient for aggregation; a U1-70K fragment harboring the LC1 domain directly interacts with aggregated U1-70K from AD brain.\",\n      \"method\": \"Sarkosyl fractionation, proteinase K treatment, recombinant domain deletion analysis, protein cross-linking and mass spectrometry\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple biochemical methods (fractionation, cross-linking MS, domain deletion) in a single study; single lab\",\n      \"pmids\": [\"25355317\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"U1-70K is proteolytically cleaved in ~50% of Alzheimer's disease cases to an N-terminal ~40 kDa fragment (N40K); the cleavage site maps to a repetitive hydrophilic domain near Arg300 (±6 residues); expression of N40K causes substantial degeneration of rat primary hippocampal neurons.\",\n      \"method\": \"LC-MS/MS with stable isotope labeling, Western blotting with recombinant truncation ladder, primary neuron toxicity assay\",\n      \"journal\": \"Journal of proteome research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — mass spectrometry site mapping validated by recombinant truncation Western blot; functional neuronal toxicity assay; single lab\",\n      \"pmids\": [\"24902715\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"U1C protein levels regulate alternative splicing of U1-70K pre-mRNA via a U1C-dependent alternative 3' splice site requiring an adjacent cluster of regulatory 5' splice sites and intact U1 snRNP binding; the non-productive isoform is degraded by NMD, reducing U1-70K mRNA/protein levels and impairing U1C incorporation, establishing a feedback loop controlling U1-70K/U1C homeostasis and U1 snRNP assembly.\",\n      \"method\": \"RNA-Seq after U1C knockdown, minigene mutational analysis, antisense morpholino splice-site blocking, in vitro binding experiments\",\n      \"journal\": \"PLoS genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods (RNAseq, minigene mutagenesis, morpholino, in vitro binding) establishing the feedback mechanism; single lab with rigorous controls\",\n      \"pmids\": [\"24146627\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"Yeast U1-70K homolog Snp1p physically associates with the essential spliceosomal protein Prp8p (by co-immunoprecipitation), suggesting Snp1p has functions late in spliceosome development; Snp1p also interacts with Exo84p, a subunit of the exocyst secretion complex, which is itself required for pre-mRNA splicing and prespliceosome formation in vitro.\",\n      \"method\": \"Yeast two-hybrid, co-immunoprecipitation, in vitro splicing assay with temperature-sensitive exo84 mutant, RT-PCR pre-mRNA quantification\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — two-hybrid confirmed by Co-IP; functional in vitro splicing assay with genetic mutant; single lab\",\n      \"pmids\": [\"11425851\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"In yeast, the N-terminal domain of Snp1 (yeast U1-70K) is necessary and sufficient for complementation of snp1-null growth and splicing defects and for in vivo association with the U1 snRNP particle; the conserved RRM and glycine-rich domains are dispensable for these functions.\",\n      \"method\": \"Yeast genetic complementation with deletion alleles of SNP1, in vivo U1 snRNP association assay\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — systematic deletion genetics with growth and splicing phenotypic readouts; single lab\",\n      \"pmids\": [\"7565787\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1992,\n      \"finding\": \"The yeast SNP1 gene product binds directly and specifically to the first 47 nucleotides of yeast U1 RNA (including stem-loop I), establishing Snp1p as the yeast ortholog of mammalian U1-70K.\",\n      \"method\": \"Bacterial expression of SNP1 fusion protein, gel-shift RNA-binding assay\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Weak — in vitro RNA-binding reconstitution assay; single lab, single method\",\n      \"pmids\": [\"1387202\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"The PSI protein (P-element somatic inhibitor) binds U1-70K via two homologous 'A and B box' sequences near its C terminus that interact with a short proline-rich sequence at the C terminus of U1-70K; NMR shows the B box forms an anti-parallel helical hairpin with a hydrophobic cluster of four aromatic residues that contacts the proline-rich region of U1-70K.\",\n      \"method\": \"NMR structure determination, deletion mapping of both PSI and U1-70K interaction domains\",\n      \"journal\": \"Journal of molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — NMR structure with functional domain mapping of both interaction partners; single lab with structural and biochemical validation\",\n      \"pmids\": [\"15990112\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"In Drosophila, loss of U1-70K causes embryonic lethality; the arginine-rich RS domain of U1-70K is dispensable for viability and splicing in otherwise wild-type animals, but becomes essential for viability when combined with mutations in another U1 snRNP component, demonstrating a redundant but context-dependent role for the RS domain in U1 snRNP function.\",\n      \"method\": \"Drosophila genetics: null alleles, RS-domain deletion transgenic rescue, double-mutant analysis\",\n      \"journal\": \"Genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vivo genetic epistasis with null and domain-deletion alleles; double-mutant analysis; single organism/lab\",\n      \"pmids\": [\"15611175\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"U1-70K/SNRNP70 localizes to RNA-associated granules in zebrafish axons (cytoplasmic pool); this extra-nuclear SNRNP70 regulates motor axon growth, acetylcholine receptor clustering, and neuromuscular synaptogenesis; it protects a subset of axonal transcripts and regulates splice variants of agrin to control synapse formation.\",\n      \"method\": \"Live imaging in zebrafish, knockdown experiments, transcript abundance and trafficking measurements in axons, splicing analysis of agrin\",\n      \"journal\": \"Current biology : CB\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct imaging plus loss-of-function with specific morphological and molecular readouts; single lab\",\n      \"pmids\": [\"36384140\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"U1-70K and ZFC3H1 function in the same pathway to retain mRNAs containing 5' splice site motifs (e.g., IPA transcripts) in nuclear speckles, preventing their nuclear export; disassembly of nuclear speckles impairs this nuclear retention.\",\n      \"method\": \"High-throughput sequencing of cellular fractions, reporter mRNA nuclear retention assays, knockdown of U1-70K and ZFC3H1, nuclear speckle disassembly\",\n      \"journal\": \"RNA (New York, N.Y.)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — fractionation sequencing and reporter assays with knockdowns; functional epistasis; single lab\",\n      \"pmids\": [\"35351812\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"The extended RRM (eRRM) of U1-70K, including the N-terminal flanking helix (N-helix) and C-terminal intrinsically disordered region (C-IDR), is required for full stability of the U1-70K/SL1 RNA complex; the N-helix strongly contributes to overall binding, while the C-IDR affects the local binding site; all-atom simulations show flanking regions act via collective interactions with RNA rather than through direct hydrogen bond contributions.\",\n      \"method\": \"Thermal dissociation assays, laser temperature-jump kinetics, long-time all-atom molecular dynamics simulations, truncation analysis\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — biophysical binding assays combined with atomistic simulations; single lab, multiple complementary methods\",\n      \"pmids\": [\"35876068\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"SNRNP70 directly interacts with CD55 pre-mRNA and modulates its alternative splicing; SNRNP70 overexpression promotes OS cell proliferation and metastasis in vitro, while its depletion reduces these capabilities in vivo.\",\n      \"method\": \"RNA immunoprecipitation (direct interaction with CD55), splicing analysis, overexpression and knockdown functional assays in vitro and in vivo (mouse xenograft)\",\n      \"journal\": \"JCI insight\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct RNA-IP showing SNRNP70-CD55 interaction plus functional in vitro/in vivo experiments; single lab\",\n      \"pmids\": [\"39704173\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"In yeast, U1 snRNP interaction with RNA polymerase II is mediated predominantly by Prp40 rather than U1-70K (Snp1); residues on yeast U1-70K involved in pol II interaction in humans are not conserved in yeast, and U1-70K makes minimal contribution to U1 snRNP's association with pol II in yeast.\",\n      \"method\": \"Co-immunoprecipitation of pol II with U1/U2 snRNPs, domain deletion analysis of Prp40, comparison with human U1 snRNP cryo-EM data\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — preprint, single Co-IP experiment in yeast; negative finding for yeast U1-70K's role; not yet peer-reviewed\",\n      \"pmids\": [\"40909591\"],\n      \"is_preprint\": true\n    }\n  ],\n  \"current_model\": \"SNRNP70/U1-70K is the U1 snRNP-specific RNA-binding protein that directly and sequence-specifically contacts loop I of U1 snRNA via its RRM; it recruits SR proteins (especially SRSF1) to the 5' splice site through RRM-RRM and RS/BAD1 domain interactions regulated by CLK1-mediated phosphorylation of Ser-226 and SRPK1-mediated phosphorylation of SRSF1, thereby nucleating early spliceosome (E complex) assembly; it additionally suppresses polyadenylation by directly inhibiting poly(A) polymerase, retains misprocessed mRNAs in nuclear speckles together with ZFC3H1, associates with the SMN complex via its N-terminal tail to maintain nuclear gem integrity, localizes to axonal granules where it regulates local transcript abundance and alternative splicing, undergoes disease-relevant proteolytic cleavage and aggregation through its low-complexity LC1/LC2 domains in Alzheimer's disease, and is a substrate for methylation by SETMAR at Lys-130.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"SNRNP70/U1-70K is the U1 snRNP-specific RNA-binding protein that nucleates early spliceosome assembly by directly and sequence-specifically contacting loop I of U1 snRNA through its RNP-consensus RNA recognition motif [#0]. Beyond the core RRM, flanking elements of the extended RRM (an N-terminal helix and a C-terminal disordered region) cooperatively stabilize the U1-70K/stem-loop RNA complex [#24]. U1-70K bridges U1 snRNP to SR splicing regulators: its RS domain binds SRSF1/ASF (SF2) [#4], and at the 5' splice site an RRM-RRM contact between U1-70K and SRSF1, together with a dominant phosphorylated-RS-domain/BAD1-domain interaction, recruits U1 snRNP to enable E-complex formation [#6, #8]. These interactions are governed by phosphorylation switches: CLK1 phosphorylation of Ser-226 releases U1-70K from subnuclear granules and frees the RRM to engage SRSF1, with SRPK1 recycling CLK1 [#7], while phosphorylation of the SRSF1 RS domain releases an intramolecular RRM-RS contact to permit intermolecular binding [#6]. The phosphorylation state of U1-70K itself is critical for catalysis, as dephosphorylation-resistant 70K permits spliceosome assembly but blocks splicing at a pre-catalytic step [#2]. U1-70K additionally suppresses polyadenylation by directly inhibiting poly(A) polymerase when bound within U1 snRNP [#3], and acts with ZFC3H1 to retain 5'-splice-site-containing mRNAs in nuclear speckles, preventing their export [#23]. It associates RNA-independently with the SMN complex via its N-terminal tail and is required for nuclear gem integrity [#12]. A cytoplasmic axonal pool regulates local transcript abundance and alternative splicing to control motor axon growth and synaptogenesis [#22]. Its C-terminal low-complexity domains drive liquid-liquid phase separation that can transition to aggregation [#13], and in Alzheimer's disease U1-70K undergoes LC1/LC2-dependent aggregation and proteolytic cleavage to a neurotoxic N40K fragment [#14, #15].\",\n  \"teleology\": [\n    {\n      \"year\": 1987,\n      \"claim\": \"Establishing the molecular identity of U1-70K—an RNA-binding protein of true mass ~52 kDa with multiple alternatively spliced isoforms—provided the foundational reagent and revealed isoform diversity at the gene's outset.\",\n      \"evidence\": \"cDNA cloning, in vitro RNA-binding, SDS-PAGE and RNA blotting\",\n      \"pmids\": [\"2447561\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Did not map the RNA-binding interface\", \"Functional distinction among isoforms unresolved\"]\n    },\n    {\n      \"year\": 1989,\n      \"claim\": \"Mapping the RNA target answered how U1-70K is targeted within U1 snRNP, showing its RNP-motif RRM binds loop I of U1 snRNA directly and sequence-specifically.\",\n      \"evidence\": \"Deletion and point mutagenesis of fusion and native proteins in in vitro RNA-binding assays\",\n      \"pmids\": [\"2531275\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No structure of the bound complex\", \"Contribution of flanking regions not addressed\"]\n    },\n    {\n      \"year\": 1993,\n      \"claim\": \"Demonstrating that 70K phosphorylation state gates catalysis distinguished spliceosome assembly from activation, placing U1-70K phosphorylation at a pre-catalytic checkpoint.\",\n      \"evidence\": \"Thiophosphorylation and depletion-reconstitution of U1 snRNPs in HeLa splicing extracts with assembly readouts\",\n      \"pmids\": [\"8387646\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"The responsible kinase not identified at the time\", \"Relevant phosphosites unmapped\"]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Identifying the U1-70K RS domain as the SRSF1-binding module and a substrate of SRPK1 connected U1 snRNP to SR-protein splicing regulators through a phosphorylation-controlled interface.\",\n      \"evidence\": \"Yeast two-hybrid, far-Western, and mutagenesis (idx4); in vitro PAP inhibition and interaction assays (idx3)\",\n      \"pmids\": [\"9685421\", \"9659922\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of RS-domain binding unresolved at this stage\", \"Connection between PAP inhibition and SR-protein recruitment unclear\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Defining the SRSF1 RRM-U1-70K RRM interaction as a phosphorylation-controlled switch explained mechanistically how SR proteins recruit U1 snRNP to the 5' splice site during E-complex formation.\",\n      \"evidence\": \"Co-IP, GST pulldown, in vitro E-complex assays, RRM point mutations and phospho-state analysis\",\n      \"pmids\": [\"21536904\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not resolve the dominant RS/BAD1 contact later identified\", \"In vivo dynamics of the switch unaddressed\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Identifying CLK1 phosphorylation of Ser-226 as the trigger that releases U1-70K from granules answered how the RRM is freed to engage SRSF1, with SRPK1 recycling the kinase.\",\n      \"evidence\": \"Phosphoproteomics, Co-IP, subnuclear imaging, in vitro kinase assays, SRPK1 overexpression\",\n      \"pmids\": [\"33811140\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Upstream signals controlling CLK1 activity unknown\", \"Quantitative kinetics of granule release unresolved\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Resolving the phosphorylated-RS/BAD1 interaction as dominant, with RRM-RRM stabilizing, and a conformational switch in BAD1, refined the architecture and regulation of the U1-70K/SRSF1 interface.\",\n      \"evidence\": \"Circular dichroism, in vitro binding with phosphoproteins, in vitro splicing, in-cell CRISPR saturated domain scanning\",\n      \"pmids\": [\"39023093\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"High-resolution structure of the assembled interface lacking\", \"Interplay with U1-70K Ser-226 phosphorylation not fully integrated\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Characterizing the extended RRM showed that disordered flanking regions act collectively to stabilize RNA binding, extending the RNA-recognition determinants beyond the canonical RRM.\",\n      \"evidence\": \"Thermal dissociation, laser T-jump kinetics, all-atom MD simulations, truncation analysis\",\n      \"pmids\": [\"35876068\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No experimental structure of the eRRM-RNA complex\", \"Cellular consequences of flanking-region truncation untested\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Mapping the SMN-complex binding to the N-terminal tail and showing a requirement for gem integrity expanded U1-70K's role into snRNP biogenesis bodies.\",\n      \"evidence\": \"RNA-independent reciprocal Co-IP, deletion mapping, immunofluorescence, knockdown rescue\",\n      \"pmids\": [\"25052091\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct vs. indirect SMN contact not distinguished\", \"Mechanism by which U1-70K maintains gem structure unknown\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Discovery of a cytoplasmic axonal pool established an extra-nuclear function in local transcript regulation and synaptogenesis, broadening U1-70K beyond nuclear splicing.\",\n      \"evidence\": \"Live imaging and knockdown in zebrafish with axonal transcript, AChR clustering, and agrin splicing readouts\",\n      \"pmids\": [\"36384140\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism of axonal targeting unknown\", \"Whether axonal splicing occurs locally or upstream unresolved\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Linking U1-70K with ZFC3H1 in nuclear-speckle retention of 5'-splice-site-containing mRNAs defined a quality-control function preventing export of misprocessed transcripts.\",\n      \"evidence\": \"Fractionation sequencing, reporter retention assays, knockdowns, speckle disassembly\",\n      \"pmids\": [\"35351812\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct vs. indirect U1-70K/ZFC3H1 association not shown\", \"How retained transcripts are subsequently resolved unclear\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Identifying LC1/LC2-dependent aggregation and proteolytic cleavage to a neurotoxic N40K fragment in Alzheimer's disease connected U1-70K low-complexity domains to neurodegeneration.\",\n      \"evidence\": \"Sarkosyl fractionation, cross-linking MS, domain deletion (idx14); LC-MS/MS site mapping and primary neuron toxicity assay (idx15)\",\n      \"pmids\": [\"25355317\", \"24902715\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Protease responsible for N40K cleavage unidentified\", \"Causal role of aggregation in disease pathogenesis not established\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Demonstrating that the low-complexity domain undergoes ampholytic-motif-driven phase separation that can convert to aggregation provided a biophysical basis for both speckle/granule partitioning and pathological aggregation.\",\n      \"evidence\": \"In vitro and in vivo LLPS assays with mutagenesis of basic-acidic motifs\",\n      \"pmids\": [\"31723601\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Link between LLPS and physiological splicing activity untested\", \"Cellular triggers of the LLPS-to-aggregate transition unknown\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Showing SNRNP70 directly binds CD55 pre-mRNA and drives osteosarcoma proliferation and metastasis implicated its splicing activity in cancer pathology.\",\n      \"evidence\": \"RNA-IP, splicing analysis, overexpression/knockdown in vitro and in mouse xenografts\",\n      \"pmids\": [\"39704173\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether CD55 splicing change is sufficient for the phenotype untested\", \"Generality across cancer types unknown\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Identifying SETMAR-mediated monomethylation of Lys-130 added a non-histone PTM to U1-70K, raising the question of how methylation modulates its function.\",\n      \"evidence\": \"Quantitative methyl-lysine proteomics, in vitro methylation, cellular MS confirmation\",\n      \"pmids\": [\"25795785\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Functional consequence of K130 methylation unknown\", \"Whether methylation affects RNA binding or splicing untested\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How the multiple regulatory layers—phosphorylation switches, methylation, LLPS, SMN/speckle partitioning, and the cytoplasmic axonal pool—are integrated to control U1-70K function in different cellular contexts remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No unified structural model of regulated U1 snRNP/SR-protein assembly\", \"Crosstalk between K130 methylation and Ser phosphorylation unexplored\", \"Trigger linking physiological LLPS to pathological aggregation in human brain undefined\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0003723\", \"supporting_discovery_ids\": [0, 24]},\n      {\"term_id\": \"GO:0140110\", \"supporting_discovery_ids\": [6, 25]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [3]},\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [6, 4]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005654\", \"supporting_discovery_ids\": [7, 23]},\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [12]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [22]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-8953854\", \"supporting_discovery_ids\": [0, 6, 16]},\n      {\"term_id\": \"R-HSA-74160\", \"supporting_discovery_ids\": [6, 3]}\n    ],\n    \"complexes\": [\"U1 snRNP\", \"SMN complex\"],\n    \"partners\": [\"SRSF1\", \"PAP\", \"ZFC3H1\", \"SMN\", \"CLK1\", \"SRPK1\", \"SETMAR\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":9,"faith_total":9,"faith_pct":100.0}}