{"gene":"LSM3","run_date":"2026-06-10T02:59:50","timeline":{"discoveries":[{"year":2013,"finding":"Crystal structure of the heptameric Lsm2-8 complex (including Lsm3) bound to the 3' end of U6 snRNA at 2.8 Å resolution revealed the subunit order Lsm3-2-8-4-7-5-6 in a doughnut-shaped assembly. The four 3'-terminal uridines of U6 snRNA are modularly recognized by Lsm3, Lsm2, Lsm8, and Lsm4; uracil base specificity is conferred by a conserved asparagine residue. The 3'-terminal uracil is sandwiched by His36 and Arg69 of Lsm3 via π-π and cation-π interactions, respectively.","method":"X-ray crystallography at 2.8 Å with associated biochemical assays","journal":"Nature","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structure with atomic detail plus biochemical validation; published in high-impact peer-reviewed journal","pmids":["24240276"],"is_preprint":false},{"year":2013,"finding":"Crystal structure of S. cerevisiae Lsm1-7 at 2.3 Å resolution showed a heptameric ring with subunit order Lsm1-2-3-6-5-7-4. Pat1 recognition by the Lsm1-7 complex is mediated by Lsm2 and Lsm3 (not by the cytoplasm-specific Lsm1 subunit), as revealed by the 3.7 Å structure of Lsm1-7 bound to the C-terminal domain of Pat1.","method":"X-ray crystallography at 2.3 Å (Lsm1-7) and 3.7 Å (Lsm1-7–Pat1C complex)","journal":"Cell reports","confidence":"High","confidence_rationale":"Tier 1 / Strong — two independent crystal structures in one study with functional implications for Pat1 binding site assignment","pmids":["24139796"],"is_preprint":false},{"year":2013,"finding":"Lsm2 and Lsm3 directly bridge the interaction between the Lsm1-7 heptamer and the C-terminus of Pat1 (Pat1C). The crystal structure of the Lsm2-3–Pat1C complex shows three Pat1C molecules surrounding a heptameric ring of Lsm2-3. The Lsm2-3–Pat1C complex and Lsm1-7–Pat1C complex both stimulate mRNA decapping in vitro to a similar extent and exhibit similar RNA-binding preference. Structure-based mutagenesis confirmed the importance of these contacts for decapping activation in vivo.","method":"X-ray crystallography, in vitro decapping assay, RNA-binding assay, structure-guided mutagenesis, in vivo functional assay","journal":"Cell research","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structure plus in vitro reconstitution of decapping activity plus mutagenesis validation in vivo; multiple orthogonal methods in one study","pmids":["24247251"],"is_preprint":false},{"year":2008,"finding":"Crystal structure of yeast Lsm3 reveals a novel octameric (8-subunit) ring organization of the Sm-fold, distinct from the canonical heptameric arrangement. The homomeric Lsm3 octamer can directly recruit Lsm6, Lsm2, and Lsm5 from yeast lysate, and the C-terminal tail of Lsm3 engages in inter-ring β-sheet interactions via specific protein–protein contacts.","method":"X-ray crystallography; pull-down from yeast lysate","journal":"Journal of molecular biology","confidence":"Medium","confidence_rationale":"Tier 1 / Moderate — crystal structure plus pull-down with functionally relevant partners, but pull-down is from crude lysate (single method for recruitment)","pmids":["18329667"],"is_preprint":false},{"year":2012,"finding":"Crystal structure of S. pombe Lsm3 shows it forms a heptamer within the crystal lattice and in solution (confirmed by analytical ultracentrifugation). RNA-binding assays demonstrated that Lsm2/3 together bind oligo(U) RNA, whereas Lsm3 alone does not bind oligo(U), indicating that the complete Lsm2/3 heterodimer is required for RNA binding.","method":"X-ray crystallography, analytical ultracentrifugation, RNA-binding assay","journal":"PloS one","confidence":"High","confidence_rationale":"Tier 1 / Moderate — crystal structure plus in-solution sedimentation plus direct RNA-binding assay; multiple orthogonal methods in one study","pmids":["22615807"],"is_preprint":false},{"year":2018,"finding":"Structure-guided alanine scanning of S. cerevisiae Lsm2-8 residues at RNA-binding sites and intersubunit interfaces identified Lsm3-R69A as the sole lethal mutation among 39 positions tested, consistent with Arg69 of Lsm3 being essential for binding the 3'-terminal UUU of U6 snRNA. Deletion of LSM3 (lsm3Δ) is lethal but is rescued by overexpression of U6 snRNA or U6 snRNP subunit Prp24, indicating that the only essential function of the Lsm2-8 ring is to support U6 snRNA.","method":"Systematic alanine-scanning mutagenesis; yeast genetics (deletion rescue by U6 overexpression or Prp24 overexpression); growth assays","journal":"RNA (New York, N.Y.)","confidence":"High","confidence_rationale":"Tier 2 / Strong — comprehensive mutagenesis across 39 residues plus genetic epistasis (rescue experiments) identifying sole essential function; rigorous controls","pmids":["29615482"],"is_preprint":false},{"year":2008,"finding":"In C. elegans embryos, LSM-3 (along with LSM-1) is recruited to P-bodies specifically in somatic blastomeres, not germline blastomeres. This recruitment requires the LET-711/Not1 subunit of the CCR4-NOT deadenylase complex and correlates spatially and temporally with the onset of maternal mRNA degradation.","method":"Live fluorescence imaging; genetic requirement tested by depleting CCR4-NOT subunit LET-711","journal":"Developmental biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct localization imaging combined with genetic requirement experiment in intact embryos, single study","pmids":["18692039"],"is_preprint":false},{"year":2015,"finding":"In C. elegans, lsm-3 (along with lsm-1) is required for normal development, reproduction, and motility. Under stress conditions, cytoplasmic LSm proteins aggregate into granules in an LSM-1-dependent manner, and lsm-3 is required for processes regulated by the insulin/IGF-1 signaling (IIS) pathway including aging and pathogen resistance.","method":"RNAi knockdown, loss-of-function mutations, DAF-16::GFP reporter assays, stress phenotype assays","journal":"RNA (New York, N.Y.)","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — loss-of-function with multiple defined phenotypic readouts and pathway placement via transcription-factor reporter assay; single lab","pmids":["26150554"],"is_preprint":false},{"year":2024,"finding":"ChIP-seq in S. cerevisiae revealed that Lsm3 co-occupies chromatin with Mediator subunits Med1/Med15 at 86 genes, of which 73 are intron-containing ribosomal protein genes. During late exponential growth, Mediator transitions from gene promoters to 3'-exon positions overlapping Lsm3 binding sites ~250 bp downstream of the last intron-exon boundary. This transition correlates with reduced mRNA levels and reduced splicing ratios for these genes, indicating that Lsm3 and Mediator cooperate to control growth-regulated transcription and splicing of ribosomal protein genes.","method":"ChIP-seq (Lsm3, Med1, Med15); RNA-seq; correlation of chromatin occupancy with mRNA levels and splicing ratios","journal":"Nucleic acids research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — ChIP-seq plus RNA-seq functional readout in single lab; correlation-based mechanism assignment, no direct mutagenesis of the interface","pmids":["38613396"],"is_preprint":false},{"year":2014,"finding":"Knockdown of LSM3 (as well as LSM5 or LSM7) in human cells lengthens the circadian period, placing LSM3 as a regulator of circadian rhythm via its role in RNA processing (alternative splicing) of core clock genes.","method":"siRNA knockdown in human cells; circadian period measurement","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — clean knockdown with defined quantitative phenotype (period lengthening) but precise molecular mechanism within circadian pathway not resolved; single lab","pmids":["25288739"],"is_preprint":false}],"current_model":"LSM3 is a conserved Sm-like protein that assembles into the heptameric Lsm2-8 ring (nuclear, U6 snRNP-associated) and the cytoplasmic Lsm1-7 ring; structurally, Lsm3 sits at a critical position where its His36 and Arg69 residues contact the 3'-terminal uracil of U6 snRNA, and its surface (together with Lsm2) serves as the docking site for the mRNA decapping activator Pat1, thereby linking deadenylation to decapping; in vivo, the sole essential function of the Lsm2-8 ring is to stabilize/recruit U6 snRNA for pre-mRNA splicing, while the cytoplasmic Lsm1-7 complex (where Lsm3 is a non-distinctive shared subunit) promotes P-body assembly and mRNA decay, and chromatin-associated Lsm3 cooperates with Mediator to regulate growth-dependent transcription and splicing of ribosomal protein genes."},"narrative":{"mechanistic_narrative":"LSM3 is a conserved Sm-like protein that functions as a shared subunit of two heptameric Lsm rings linking RNA splicing to mRNA decay [PMID:24240276, PMID:24139796]. In the nuclear Lsm2-8 ring, which adopts a doughnut-shaped assembly with subunit order Lsm3-2-8-4-7-5-6, Lsm3 occupies a critical RNA-contacting position: its His36 and Arg69 sandwich the 3'-terminal uracil of U6 snRNA via π-π and cation-π interactions [PMID:24240276]. This contact is the basis of the complex's sole essential function — structure-guided alanine scanning identified Lsm3-R69A as the only lethal mutation among 39 tested positions, and the lethality of LSM3 deletion is rescued by overexpression of U6 snRNA or the U6 snRNP subunit Prp24, establishing that the Lsm2-8 ring exists in vivo to stabilize and recruit U6 snRNA for pre-mRNA splicing [PMID:29615482]. In the cytoplasmic Lsm1-7 ring, Lsm3 together with Lsm2 forms the docking surface for the C-terminal domain of the decapping activator Pat1; a reconstituted Lsm2-3–Pat1C subcomplex stimulates mRNA decapping in vitro comparably to the full Lsm1-7–Pat1C complex, coupling deadenylation to decapping [PMID:24139796, PMID:24247251]. Consistent with this decay role, LSm3 is recruited to P-bodies in a manner dependent on the CCR4-NOT deadenylase and coincident with maternal mRNA degradation [PMID:18692039], and is required for development, reproduction, and insulin/IGF-1-regulated stress responses [PMID:26150554]. LSM3 additionally co-occupies chromatin with Mediator at intron-containing ribosomal protein genes to control their growth-regulated transcription and splicing [PMID:38613396], and its knockdown lengthens the circadian period through effects on clock-gene RNA processing [PMID:25288739].","teleology":[{"year":2008,"claim":"Before its ring context was resolved, it was unclear how Lsm3 itself oligomerizes and selects partners; the first Lsm3 structure showed it can self-assemble and recruit specific Lsm subunits, hinting at its assembly role.","evidence":"X-ray crystallography of yeast Lsm3 plus pull-down from yeast lysate","pmids":["18329667"],"confidence":"Medium","gaps":["The octameric homomeric ring is a crystallization artifact relative to the physiological heptamer","Recruitment shown only from crude lysate, not reconstituted","Does not address RNA recognition"]},{"year":2008,"claim":"It was unknown whether Lsm3 participates in cytoplasmic mRNA turnover in vivo; imaging in C. elegans embryos placed LSM-3 in P-bodies downstream of deadenylation.","evidence":"Live fluorescence imaging and CCR4-NOT (LET-711) depletion in embryos","pmids":["18692039"],"confidence":"Medium","gaps":["Recruitment is correlative with deadenylation, not mechanistically dissected","Cell-type specificity (soma vs germline) basis unexplained"]},{"year":2012,"claim":"Whether Lsm3 binds RNA alone or only in a complex was unresolved; this work showed the Lsm2/3 heterodimer, not Lsm3 monomer, is the minimal RNA-binding unit.","evidence":"X-ray crystallography, analytical ultracentrifugation, and oligo(U) RNA-binding assays in S. pombe","pmids":["22615807"],"confidence":"High","gaps":["Does not establish base-specific recognition residues","In-crystal heptamer of Lsm3 differs from physiological assembly"]},{"year":2013,"claim":"How the Lsm2-8 ring recognizes U6 snRNA was unknown; the atomic structure defined subunit order and the Lsm3 His36/Arg69 sandwich of the 3'-terminal uracil.","evidence":"2.8 Å crystal structure of Lsm2-8 bound to U6 3' end with biochemical assays","pmids":["24240276"],"confidence":"High","gaps":["Static structure does not address assembly kinetics","Does not test which contacts are essential in vivo"]},{"year":2013,"claim":"The basis for Pat1-dependent coupling of decay to the cytoplasmic ring was unclear; structures showed Lsm2 and Lsm3 — not the cytoplasm-specific Lsm1 — provide the Pat1 docking surface, and the minimal Lsm2-3–Pat1C unit activates decapping.","evidence":"Crystal structures of Lsm1-7, Lsm1-7–Pat1C, and Lsm2-3–Pat1C with in vitro decapping and RNA-binding assays plus in vivo mutagenesis","pmids":["24139796","24247251"],"confidence":"High","gaps":["Stoichiometry of Pat1 binding in the full cellular ring not defined","Does not connect decapping activation to specific transcript classes"]},{"year":2015,"claim":"The organismal consequences of losing lsm-3 were uncharacterized; loss-of-function linked it to development, reproduction, stress granule formation, and insulin/IGF-1 signaling outputs.","evidence":"RNAi and loss-of-function mutants with DAF-16::GFP reporter and stress phenotype assays in C. elegans","pmids":["26150554"],"confidence":"Medium","gaps":["Phenotypes do not isolate splicing vs decay contributions","Direct molecular targets in the IIS pathway not identified"]},{"year":2018,"claim":"Which Lsm2-8 contact is functionally indispensable, and what the ring's essential role is, were open; mutagenesis pinpointed Lsm3-R69 and genetic rescue established U6 support as the sole essential function.","evidence":"Alanine scanning across 39 residues plus deletion rescue by U6 or Prp24 overexpression in S. cerevisiae","pmids":["29615482"],"confidence":"High","gaps":["Does not address non-essential Lsm3 functions in decay or transcription","Rescue assays measure viability, not splicing fidelity directly"]},{"year":2024,"claim":"A chromatin-associated role for Lsm3 was unknown; ChIP-seq revealed it co-occupies ribosomal protein genes with Mediator to control growth-regulated transcription and splicing.","evidence":"ChIP-seq (Lsm3, Med1, Med15) with RNA-seq correlation in S. cerevisiae","pmids":["38613396"],"confidence":"Medium","gaps":["Mechanism is correlative; no mutagenesis of the Lsm3-Mediator interface","Whether chromatin recruitment requires the Lsm2-8 ring is untested"]},{"year":2014,"claim":"Whether Lsm3-dependent RNA processing feeds into physiological rhythms was unknown; knockdown lengthened the circadian period via clock-gene splicing.","evidence":"siRNA knockdown and circadian period measurement in human cells","pmids":["25288739"],"confidence":"Medium","gaps":["Specific clock-gene splicing events not mapped","Cannot separate Lsm2-8 vs Lsm1-7 contribution"]},{"year":null,"claim":"How the same Lsm3 subunit is partitioned between nuclear splicing, cytoplasmic decay, and chromatin/Mediator functions, and whether these reflect distinct ring contexts, remains unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No assay distinguishes which ring drives the chromatin and circadian phenotypes","Regulation of Lsm3 sorting between rings unknown"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0003723","term_label":"RNA binding","supporting_discovery_ids":[0,4]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[2]},{"term_id":"GO:0005198","term_label":"structural molecule activity","supporting_discovery_ids":[0,1]}],"localization":[{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[0,8]},{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[1,6]},{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[5]}],"pathway":[{"term_id":"R-HSA-8953854","term_label":"Metabolism of RNA","supporting_discovery_ids":[0,1,2]}],"complexes":["Lsm2-8 complex","Lsm1-7 complex","P-body"],"partners":["LSM2","PATL1","LSM1","LSM6","LSM5","PRP24"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"P62310","full_name":"U6 snRNA-associated Sm-like protein LSm3","aliases":[],"length_aa":102,"mass_kda":11.8,"function":"Plays a role in pre-mRNA splicing as component of the U4/U6-U5 tri-snRNP complex that is involved in spliceosome assembly, and as component of the precatalytic spliceosome (spliceosome B complex) (PubMed:28781166). The heptameric LSM2-8 complex binds specifically to the 3'-terminal U-tract of U6 snRNA (PubMed:10523320)","subcellular_location":"Nucleus","url":"https://www.uniprot.org/uniprotkb/P62310/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":true,"resolved_as":"","url":"https://depmap.org/portal/gene/LSM3","classification":"Common Essential","n_dependent_lines":1200,"n_total_lines":1208,"dependency_fraction":0.9933774834437086},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[{"gene":"SNRPF","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/search/LSM3","total_profiled":1310},"omim":[{"mim_id":"607288","title":"LSM8 PROTEIN; LSM8","url":"https://www.omim.org/entry/607288"},{"mim_id":"607287","title":"LSM7 PROTEIN; LSM7","url":"https://www.omim.org/entry/607287"},{"mim_id":"607286","title":"LSM6 PROTEIN; LSM6","url":"https://www.omim.org/entry/607286"},{"mim_id":"607285","title":"LSM5 PROTEIN; LSM5","url":"https://www.omim.org/entry/607285"},{"mim_id":"607284","title":"LSM4 PROTEIN; LSM4","url":"https://www.omim.org/entry/607284"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Nucleoli","reliability":"Approved"},{"location":"Nucleoplasm","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/LSM3"},"hgnc":{"alias_symbol":["YLR438C","SMX4","USS2"],"prev_symbol":[]},"alphafold":{"accession":"P62310","domains":[{"cath_id":"2.30.30.100","chopping":"18-95","consensus_level":"high","plddt":95.1185,"start":18,"end":95}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/P62310","model_url":"https://alphafold.ebi.ac.uk/files/AF-P62310-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-P62310-F1-predicted_aligned_error_v6.png","plddt_mean":89.19},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=LSM3","jax_strain_url":"https://www.jax.org/strain/search?query=LSM3"},"sequence":{"accession":"P62310","fasta_url":"https://rest.uniprot.org/uniprotkb/P62310.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/P62310/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/P62310"}},"corpus_meta":[{"pmid":"18692039","id":"PMC_18692039","title":"Processing bodies and 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resolution revealed the subunit order Lsm3-2-8-4-7-5-6 in a doughnut-shaped assembly. The four 3'-terminal uridines of U6 snRNA are modularly recognized by Lsm3, Lsm2, Lsm8, and Lsm4; uracil base specificity is conferred by a conserved asparagine residue. The 3'-terminal uracil is sandwiched by His36 and Arg69 of Lsm3 via π-π and cation-π interactions, respectively.\",\n      \"method\": \"X-ray crystallography at 2.8 Å with associated biochemical assays\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structure with atomic detail plus biochemical validation; published in high-impact peer-reviewed journal\",\n      \"pmids\": [\"24240276\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Crystal structure of S. cerevisiae Lsm1-7 at 2.3 Å resolution showed a heptameric ring with subunit order Lsm1-2-3-6-5-7-4. Pat1 recognition by the Lsm1-7 complex is mediated by Lsm2 and Lsm3 (not by the cytoplasm-specific Lsm1 subunit), as revealed by the 3.7 Å structure of Lsm1-7 bound to the C-terminal domain of Pat1.\",\n      \"method\": \"X-ray crystallography at 2.3 Å (Lsm1-7) and 3.7 Å (Lsm1-7–Pat1C complex)\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — two independent crystal structures in one study with functional implications for Pat1 binding site assignment\",\n      \"pmids\": [\"24139796\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Lsm2 and Lsm3 directly bridge the interaction between the Lsm1-7 heptamer and the C-terminus of Pat1 (Pat1C). The crystal structure of the Lsm2-3–Pat1C complex shows three Pat1C molecules surrounding a heptameric ring of Lsm2-3. The Lsm2-3–Pat1C complex and Lsm1-7–Pat1C complex both stimulate mRNA decapping in vitro to a similar extent and exhibit similar RNA-binding preference. Structure-based mutagenesis confirmed the importance of these contacts for decapping activation in vivo.\",\n      \"method\": \"X-ray crystallography, in vitro decapping assay, RNA-binding assay, structure-guided mutagenesis, in vivo functional assay\",\n      \"journal\": \"Cell research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structure plus in vitro reconstitution of decapping activity plus mutagenesis validation in vivo; multiple orthogonal methods in one study\",\n      \"pmids\": [\"24247251\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Crystal structure of yeast Lsm3 reveals a novel octameric (8-subunit) ring organization of the Sm-fold, distinct from the canonical heptameric arrangement. The homomeric Lsm3 octamer can directly recruit Lsm6, Lsm2, and Lsm5 from yeast lysate, and the C-terminal tail of Lsm3 engages in inter-ring β-sheet interactions via specific protein–protein contacts.\",\n      \"method\": \"X-ray crystallography; pull-down from yeast lysate\",\n      \"journal\": \"Journal of molecular biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — crystal structure plus pull-down with functionally relevant partners, but pull-down is from crude lysate (single method for recruitment)\",\n      \"pmids\": [\"18329667\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Crystal structure of S. pombe Lsm3 shows it forms a heptamer within the crystal lattice and in solution (confirmed by analytical ultracentrifugation). RNA-binding assays demonstrated that Lsm2/3 together bind oligo(U) RNA, whereas Lsm3 alone does not bind oligo(U), indicating that the complete Lsm2/3 heterodimer is required for RNA binding.\",\n      \"method\": \"X-ray crystallography, analytical ultracentrifugation, RNA-binding assay\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — crystal structure plus in-solution sedimentation plus direct RNA-binding assay; multiple orthogonal methods in one study\",\n      \"pmids\": [\"22615807\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Structure-guided alanine scanning of S. cerevisiae Lsm2-8 residues at RNA-binding sites and intersubunit interfaces identified Lsm3-R69A as the sole lethal mutation among 39 positions tested, consistent with Arg69 of Lsm3 being essential for binding the 3'-terminal UUU of U6 snRNA. Deletion of LSM3 (lsm3Δ) is lethal but is rescued by overexpression of U6 snRNA or U6 snRNP subunit Prp24, indicating that the only essential function of the Lsm2-8 ring is to support U6 snRNA.\",\n      \"method\": \"Systematic alanine-scanning mutagenesis; yeast genetics (deletion rescue by U6 overexpression or Prp24 overexpression); growth assays\",\n      \"journal\": \"RNA (New York, N.Y.)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — comprehensive mutagenesis across 39 residues plus genetic epistasis (rescue experiments) identifying sole essential function; rigorous controls\",\n      \"pmids\": [\"29615482\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"In C. elegans embryos, LSM-3 (along with LSM-1) is recruited to P-bodies specifically in somatic blastomeres, not germline blastomeres. This recruitment requires the LET-711/Not1 subunit of the CCR4-NOT deadenylase complex and correlates spatially and temporally with the onset of maternal mRNA degradation.\",\n      \"method\": \"Live fluorescence imaging; genetic requirement tested by depleting CCR4-NOT subunit LET-711\",\n      \"journal\": \"Developmental biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct localization imaging combined with genetic requirement experiment in intact embryos, single study\",\n      \"pmids\": [\"18692039\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"In C. elegans, lsm-3 (along with lsm-1) is required for normal development, reproduction, and motility. Under stress conditions, cytoplasmic LSm proteins aggregate into granules in an LSM-1-dependent manner, and lsm-3 is required for processes regulated by the insulin/IGF-1 signaling (IIS) pathway including aging and pathogen resistance.\",\n      \"method\": \"RNAi knockdown, loss-of-function mutations, DAF-16::GFP reporter assays, stress phenotype assays\",\n      \"journal\": \"RNA (New York, N.Y.)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — loss-of-function with multiple defined phenotypic readouts and pathway placement via transcription-factor reporter assay; single lab\",\n      \"pmids\": [\"26150554\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"ChIP-seq in S. cerevisiae revealed that Lsm3 co-occupies chromatin with Mediator subunits Med1/Med15 at 86 genes, of which 73 are intron-containing ribosomal protein genes. During late exponential growth, Mediator transitions from gene promoters to 3'-exon positions overlapping Lsm3 binding sites ~250 bp downstream of the last intron-exon boundary. This transition correlates with reduced mRNA levels and reduced splicing ratios for these genes, indicating that Lsm3 and Mediator cooperate to control growth-regulated transcription and splicing of ribosomal protein genes.\",\n      \"method\": \"ChIP-seq (Lsm3, Med1, Med15); RNA-seq; correlation of chromatin occupancy with mRNA levels and splicing ratios\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — ChIP-seq plus RNA-seq functional readout in single lab; correlation-based mechanism assignment, no direct mutagenesis of the interface\",\n      \"pmids\": [\"38613396\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Knockdown of LSM3 (as well as LSM5 or LSM7) in human cells lengthens the circadian period, placing LSM3 as a regulator of circadian rhythm via its role in RNA processing (alternative splicing) of core clock genes.\",\n      \"method\": \"siRNA knockdown in human cells; circadian period measurement\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — clean knockdown with defined quantitative phenotype (period lengthening) but precise molecular mechanism within circadian pathway not resolved; single lab\",\n      \"pmids\": [\"25288739\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"LSM3 is a conserved Sm-like protein that assembles into the heptameric Lsm2-8 ring (nuclear, U6 snRNP-associated) and the cytoplasmic Lsm1-7 ring; structurally, Lsm3 sits at a critical position where its His36 and Arg69 residues contact the 3'-terminal uracil of U6 snRNA, and its surface (together with Lsm2) serves as the docking site for the mRNA decapping activator Pat1, thereby linking deadenylation to decapping; in vivo, the sole essential function of the Lsm2-8 ring is to stabilize/recruit U6 snRNA for pre-mRNA splicing, while the cytoplasmic Lsm1-7 complex (where Lsm3 is a non-distinctive shared subunit) promotes P-body assembly and mRNA decay, and chromatin-associated Lsm3 cooperates with Mediator to regulate growth-dependent transcription and splicing of ribosomal protein genes.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"LSM3 is a conserved Sm-like protein that functions as a shared subunit of two heptameric Lsm rings linking RNA splicing to mRNA decay [#0, #1]. In the nuclear Lsm2-8 ring, which adopts a doughnut-shaped assembly with subunit order Lsm3-2-8-4-7-5-6, Lsm3 occupies a critical RNA-contacting position: its His36 and Arg69 sandwich the 3'-terminal uracil of U6 snRNA via π-π and cation-π interactions [#0]. This contact is the basis of the complex's sole essential function — structure-guided alanine scanning identified Lsm3-R69A as the only lethal mutation among 39 tested positions, and the lethality of LSM3 deletion is rescued by overexpression of U6 snRNA or the U6 snRNP subunit Prp24, establishing that the Lsm2-8 ring exists in vivo to stabilize and recruit U6 snRNA for pre-mRNA splicing [#5]. In the cytoplasmic Lsm1-7 ring, Lsm3 together with Lsm2 forms the docking surface for the C-terminal domain of the decapping activator Pat1; a reconstituted Lsm2-3–Pat1C subcomplex stimulates mRNA decapping in vitro comparably to the full Lsm1-7–Pat1C complex, coupling deadenylation to decapping [#1, #2]. Consistent with this decay role, LSm3 is recruited to P-bodies in a manner dependent on the CCR4-NOT deadenylase and coincident with maternal mRNA degradation [#6], and is required for development, reproduction, and insulin/IGF-1-regulated stress responses [#7]. LSM3 additionally co-occupies chromatin with Mediator at intron-containing ribosomal protein genes to control their growth-regulated transcription and splicing [#8], and its knockdown lengthens the circadian period through effects on clock-gene RNA processing [#9].\",\n  \"teleology\": [\n    {\n      \"year\": 2008,\n      \"claim\": \"Before its ring context was resolved, it was unclear how Lsm3 itself oligomerizes and selects partners; the first Lsm3 structure showed it can self-assemble and recruit specific Lsm subunits, hinting at its assembly role.\",\n      \"evidence\": \"X-ray crystallography of yeast Lsm3 plus pull-down from yeast lysate\",\n      \"pmids\": [\"18329667\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"The octameric homomeric ring is a crystallization artifact relative to the physiological heptamer\", \"Recruitment shown only from crude lysate, not reconstituted\", \"Does not address RNA recognition\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"It was unknown whether Lsm3 participates in cytoplasmic mRNA turnover in vivo; imaging in C. elegans embryos placed LSM-3 in P-bodies downstream of deadenylation.\",\n      \"evidence\": \"Live fluorescence imaging and CCR4-NOT (LET-711) depletion in embryos\",\n      \"pmids\": [\"18692039\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Recruitment is correlative with deadenylation, not mechanistically dissected\", \"Cell-type specificity (soma vs germline) basis unexplained\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Whether Lsm3 binds RNA alone or only in a complex was unresolved; this work showed the Lsm2/3 heterodimer, not Lsm3 monomer, is the minimal RNA-binding unit.\",\n      \"evidence\": \"X-ray crystallography, analytical ultracentrifugation, and oligo(U) RNA-binding assays in S. pombe\",\n      \"pmids\": [\"22615807\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Does not establish base-specific recognition residues\", \"In-crystal heptamer of Lsm3 differs from physiological assembly\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"How the Lsm2-8 ring recognizes U6 snRNA was unknown; the atomic structure defined subunit order and the Lsm3 His36/Arg69 sandwich of the 3'-terminal uracil.\",\n      \"evidence\": \"2.8 Å crystal structure of Lsm2-8 bound to U6 3' end with biochemical assays\",\n      \"pmids\": [\"24240276\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Static structure does not address assembly kinetics\", \"Does not test which contacts are essential in vivo\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"The basis for Pat1-dependent coupling of decay to the cytoplasmic ring was unclear; structures showed Lsm2 and Lsm3 — not the cytoplasm-specific Lsm1 — provide the Pat1 docking surface, and the minimal Lsm2-3–Pat1C unit activates decapping.\",\n      \"evidence\": \"Crystal structures of Lsm1-7, Lsm1-7–Pat1C, and Lsm2-3–Pat1C with in vitro decapping and RNA-binding assays plus in vivo mutagenesis\",\n      \"pmids\": [\"24139796\", \"24247251\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Stoichiometry of Pat1 binding in the full cellular ring not defined\", \"Does not connect decapping activation to specific transcript classes\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"The organismal consequences of losing lsm-3 were uncharacterized; loss-of-function linked it to development, reproduction, stress granule formation, and insulin/IGF-1 signaling outputs.\",\n      \"evidence\": \"RNAi and loss-of-function mutants with DAF-16::GFP reporter and stress phenotype assays in C. elegans\",\n      \"pmids\": [\"26150554\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Phenotypes do not isolate splicing vs decay contributions\", \"Direct molecular targets in the IIS pathway not identified\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Which Lsm2-8 contact is functionally indispensable, and what the ring's essential role is, were open; mutagenesis pinpointed Lsm3-R69 and genetic rescue established U6 support as the sole essential function.\",\n      \"evidence\": \"Alanine scanning across 39 residues plus deletion rescue by U6 or Prp24 overexpression in S. cerevisiae\",\n      \"pmids\": [\"29615482\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Does not address non-essential Lsm3 functions in decay or transcription\", \"Rescue assays measure viability, not splicing fidelity directly\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"A chromatin-associated role for Lsm3 was unknown; ChIP-seq revealed it co-occupies ribosomal protein genes with Mediator to control growth-regulated transcription and splicing.\",\n      \"evidence\": \"ChIP-seq (Lsm3, Med1, Med15) with RNA-seq correlation in S. cerevisiae\",\n      \"pmids\": [\"38613396\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism is correlative; no mutagenesis of the Lsm3-Mediator interface\", \"Whether chromatin recruitment requires the Lsm2-8 ring is untested\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Whether Lsm3-dependent RNA processing feeds into physiological rhythms was unknown; knockdown lengthened the circadian period via clock-gene splicing.\",\n      \"evidence\": \"siRNA knockdown and circadian period measurement in human cells\",\n      \"pmids\": [\"25288739\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Specific clock-gene splicing events not mapped\", \"Cannot separate Lsm2-8 vs Lsm1-7 contribution\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How the same Lsm3 subunit is partitioned between nuclear splicing, cytoplasmic decay, and chromatin/Mediator functions, and whether these reflect distinct ring contexts, remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No assay distinguishes which ring drives the chromatin and circadian phenotypes\", \"Regulation of Lsm3 sorting between rings unknown\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0003723\", \"supporting_discovery_ids\": [0, 4]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [2]},\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [0, 1]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [0, 8]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [1, 6]},\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [5]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"GO:0008380\", \"supporting_discovery_ids\": [0, 5]},\n      {\"term_id\": \"R-HSA-8953854\", \"supporting_discovery_ids\": [0, 1, 2]}\n    ],\n    \"complexes\": [\"Lsm2-8 complex\", \"Lsm1-7 complex\", \"P-body\"],\n    \"partners\": [\"LSM2\", \"PATL1\", \"LSM1\", \"LSM6\", \"LSM5\", \"PRP24\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"tie","faith_supported":6,"faith_total":6,"faith_pct":100.0}}