{"gene":"LSM5","run_date":"2026-06-10T02:59:50","timeline":{"discoveries":[{"year":2020,"finding":"Crystal/cryo-EM structures of Lsm1-7 and Lsm2-8 complexes bound to RNA reveal that Lsm5 uniquely recognizes purine bases at the 3' end of RNA substrates, explaining its divergent sequence relative to other Lsm subunits. The Lsm2-8 complex preferentially binds the 2',3' cyclic phosphate end of U6 snRNA, while Lsm1-7 strongly discriminates against cyclic phosphates and binds oligouridylate tracts with terminal purines. Lsm1-7 loads onto RNA from the 3' end, and removal of the Lsm1 carboxy-terminal region allows Lsm1-7 to scan along RNA, suggesting a gated mechanism for accessing internal binding sites.","method":"High-resolution structural determination (four crystal/cryo-EM structures) combined with RNA binding assays","journal":"RNA (New York, N.Y.)","confidence":"High","confidence_rationale":"Tier 1 / Strong — multiple high-resolution structures with functional RNA-binding validation; mechanistic basis for subunit specificity directly demonstrated","pmids":["32518066"],"is_preprint":false},{"year":2001,"finding":"Genetic epistasis in S. cerevisiae showed that deletion of LSM5 causes the La protein homolog Lhp1p to become required for growth, indicating that Lsm5 (as part of the Lsm2-8 complex) acts redundantly with Lhp1p to stabilize nascent U6 snRNA at its 3' end. LSM2 and LSM4, but not LSM5, act as allele-specific low-copy suppressors of lsm8 mutations.","method":"Genetic epistasis (deletion analysis, suppressor screens, growth assays in S. cerevisiae)","journal":"Genetics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — clean genetic epistasis with defined phenotypic readout, single lab, multiple allele combinations tested","pmids":["11333229"],"is_preprint":false},{"year":2018,"finding":"Structure-guided alanine-scanning mutagenesis of the Lsm2-8 ring in S. cerevisiae, including residues in Lsm5's RNA-binding sites and intersubunit interfaces, showed that most single mutations are benign for vegetative growth, but pairwise combinations of benign lsm mutations (including lsm5 alleles) reveal synthetic lethality and growth defects, indicating internal genetic redundancy in the ring. Critically, the lethal single-gene deletion lsm5Δ was rescued by overexpression of U6 snRNA from a high-copy plasmid, demonstrating that the only essential function of Lsm5 (within Lsm2-8) is to support U6 snRNA biogenesis or function.","method":"Alanine scanning mutagenesis, genetic complementation, high-copy suppressor analysis in S. cerevisiae","journal":"RNA (New York, N.Y.)","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — systematic mutagenesis of RNA-binding and interface residues combined with clean genetic rescue experiment; single lab but multiple orthogonal genetic approaches","pmids":["29615482"],"is_preprint":false},{"year":2011,"finding":"The 2.5 Å crystal structure of the LSm5/6/7 sub-complex reveals that Lsm5, Lsm6, and Lsm7 display the canonical Sm fold and arrange into a hexameric ring, constituting an assembly intermediate on the pathway to both the cytoplasmic Lsm1-7 and nuclear Lsm2-8 rings. NMR and pull-down experiments confirmed that the Lsm657 hexameric complex can incorporate Lsm23 to assemble further towards native Lsm rings.","method":"X-ray crystallography (2.5 Å), high-resolution NMR spectroscopy, pull-down assays","journal":"Journal of molecular biology","confidence":"High","confidence_rationale":"Tier 1 / Moderate — crystal structure plus NMR plus pull-down in one study; multiple orthogonal methods establish Lsm5 as part of a defined assembly intermediate","pmids":["22001694"],"is_preprint":false},{"year":2012,"finding":"Crystal structures of the Lsm5/6/7 sub-complex from S. pombe reveal that Lsm5, Lsm6, and Lsm7 form a hexameric ring within the crystal lattice, and analytical ultracentrifugation confirmed the hexameric state in solution. RNA binding assays showed that the Lsm5/6/7 sub-complex binds oligo(U), and structural analysis defined the intersubunit interaction order as Lsm5-Lsm6-Lsm7.","method":"X-ray crystallography, analytical ultracentrifugation, RNA binding assays","journal":"PloS one","confidence":"High","confidence_rationale":"Tier 1 / Moderate — crystal structure with solution-state validation and direct RNA binding assay; multiple orthogonal methods in one study","pmids":["22615807"],"is_preprint":false},{"year":2008,"finding":"Pull-down experiments from yeast lysate demonstrated that homomeric Lsm3 octamers can directly recruit Lsm5 (along with Lsm6 and Lsm2) from yeast lysate, providing evidence for specific protein-protein interactions mediated by the variable loops and termini of Lsm subunits during ring assembly.","method":"Pull-down from yeast lysate using crystallized Lsm3 octamer","journal":"Journal of molecular biology","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — single pull-down method, but structurally grounded and replicated with multiple Lsm proteins in the same study","pmids":["18329667"],"is_preprint":false},{"year":2008,"finding":"In Trypanosoma brucei, TAP-tagged Lsm5 purification and RNAi silencing identified Lsm5 as a component of the heptameric Lsm2-8 complex that binds U6 snRNA. Localization studies showed that Lsm5 (as part of this complex) resides in the nucleus near the nucleolus, and Lsm proteins were not detected in cytoplasmic bodies tagged with YFP-Dhh1.","method":"TAP-tag purification, RNAi silencing, fluorescence localization studies","journal":"Molecular and biochemical parasitology","confidence":"Medium","confidence_rationale":"Tier 2-3 / Moderate — TAP purification plus RNAi plus localization, multiple orthogonal approaches in a single lab; ortholog (trypanosome) context","pmids":["18433897"],"is_preprint":false},{"year":2024,"finding":"Lentiviral knockdown of LSM5 in colon cancer cells suppressed proliferation and promoted apoptosis, and was associated with upregulation of p53, CDKN1A, and TNFRSF10B as assessed by GeneChip assay and Western blotting, placing LSM5 upstream of the p53-CDKN1A-TNFRSF10B apoptotic axis.","method":"Lentiviral shRNA knockdown, proliferation and apoptosis assays, GeneChip transcriptomic profiling, Western blotting","journal":"World journal of gastrointestinal oncology","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single lab, KD with phenotype and downstream pathway markers, but mechanistic placement is indirect (no direct binding or enzymatic evidence); cancer cell line context","pmids":["38994171"],"is_preprint":false}],"current_model":"LSM5 is a core subunit of both the nuclear Lsm2-8 and cytoplasmic Lsm1-7 heteroheptameric ring complexes; within the Lsm2-8 ring it uniquely recognizes purine bases on RNA substrates and its only essential cellular function is to support U6 snRNA biogenesis/stability, while in Lsm1-7 it contributes to mRNA decay; Lsm5 together with Lsm6 and Lsm7 forms a hexameric assembly intermediate that recruits Lsm2/3 en route to the complete rings, and genetic deletion of LSM5 is lethal but fully rescued by U6 snRNA overexpression."},"narrative":{"mechanistic_narrative":"LSM5 is a core subunit of the heteroheptameric Lsm rings that govern small nuclear RNA stability and mRNA decay, functioning within the nuclear Lsm2-8 complex that binds U6 snRNA and the cytoplasmic Lsm1-7 complex [PMID:32518066, PMID:18433897]. Structural studies of both rings bound to RNA show that Lsm5 uniquely recognizes purine bases at the 3' end of RNA substrates, accounting for its divergent sequence relative to the other Lsm subunits; the Lsm2-8 ring preferentially engages the 2',3' cyclic phosphate end of U6 snRNA while Lsm1-7 discriminates against cyclic phosphates and binds oligouridylate tracts with terminal purines [PMID:32518066]. Lsm5 assembles into the mature rings through a defined hexameric intermediate: it adopts the canonical Sm fold and forms an Lsm5-Lsm6-Lsm7 sub-complex that binds oligo(U) and recruits Lsm2/3 toward native ring assembly [PMID:22001694, PMID:22615807]. The sole essential cellular function of Lsm5 within Lsm2-8 is to support U6 snRNA biogenesis or stability: deletion of LSM5 is lethal but is fully rescued by U6 snRNA overexpression, and Lsm5 acts redundantly with the La protein homolog Lhp1p to stabilize the 3' end of nascent U6 snRNA [PMID:11333229, PMID:29615482]. In human colon cancer cells, LSM5 knockdown suppresses proliferation and promotes apoptosis associated with induction of the p53-CDKN1A-TNFRSF10B axis [PMID:38994171].","teleology":[{"year":2001,"claim":"Established that Lsm5's biological role is to stabilize nascent U6 snRNA, defining the functional output of the Lsm2-8 ring through its genetic redundancy with the La protein homolog.","evidence":"Genetic epistasis, deletion analysis and suppressor screens in S. cerevisiae","pmids":["11333229"],"confidence":"Medium","gaps":["Did not resolve the structural basis of U6 recognition","Redundancy with Lhp1p inferred genetically, not by direct biochemistry"]},{"year":2008,"claim":"Showed how Lsm5 is physically recruited during ring assembly and confirmed its membership in the nuclear U6-binding Lsm2-8 complex in a divergent eukaryote, generalizing the assembly logic across species.","evidence":"Pull-down from yeast lysate with a crystallized Lsm3 octamer, and TAP-tag purification plus RNAi and localization in Trypanosoma brucei","pmids":["18329667","18433897"],"confidence":"Medium","gaps":["Pull-down does not define stoichiometry or order of incorporation in vivo","Trypanosome localization is ortholog-based"]},{"year":2012,"claim":"Defined an Lsm5-Lsm6-Lsm7 hexameric assembly intermediate as a shared precursor for both cytoplasmic and nuclear rings, establishing the structural pathway to mature complexes.","evidence":"X-ray crystallography, NMR, analytical ultracentrifugation, pull-down and RNA binding assays (yeast and S. pombe)","pmids":["22001694","22615807"],"confidence":"High","gaps":["Does not establish kinetics or in-cell ordering of assembly","RNA binding by the intermediate not linked to a specific cellular substrate"]},{"year":2018,"claim":"Demonstrated that the single essential function of Lsm5 within Lsm2-8 is to support U6 snRNA, by rescuing the lethal deletion with U6 overexpression and revealing internal genetic redundancy across ring residues.","evidence":"Alanine-scanning mutagenesis, genetic complementation and high-copy U6 suppression in S. cerevisiae","pmids":["29615482"],"confidence":"High","gaps":["Does not address non-essential roles of Lsm5 in mRNA decay","Synthetic lethality mechanism not biochemically resolved"]},{"year":2020,"claim":"Provided the structural mechanism for Lsm5's unique substrate specificity, showing it recognizes 3'-terminal purines and explaining how Lsm2-8 versus Lsm1-7 discriminate between cyclic-phosphate U6 ends and oligouridylate tracts.","evidence":"Four high-resolution crystal/cryo-EM structures of Lsm1-7 and Lsm2-8 bound to RNA, with RNA binding assays","pmids":["32518066"],"confidence":"High","gaps":["Does not define dynamics of 3'-end loading in vivo","Functional consequence of purine recognition for specific transcripts not tested"]},{"year":2024,"claim":"Linked LSM5 to a cellular phenotype in human cancer cells, placing it upstream of a p53-dependent apoptotic axis, though without direct molecular mechanism.","evidence":"Lentiviral shRNA knockdown with proliferation/apoptosis assays, GeneChip profiling and Western blotting in colon cancer cells","pmids":["38994171"],"confidence":"Low","gaps":["Mechanistic placement is indirect with no direct binding or enzymatic evidence","Connection to RNA-processing role of LSM5 unestablished","Single lab, single cancer context"]},{"year":null,"claim":"How Lsm5's RNA-binding and assembly functions in the Lsm1-7 ring contribute mechanistically to mRNA decay, and whether its cancer-cell phenotype derives from its RNA-processing role, remain unresolved.","evidence":"","pmids":[],"confidence":"Low","gaps":["No direct evidence linking Lsm5 to specific mRNA decay substrates","Mechanism connecting LSM5 to the p53 axis unknown"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0003723","term_label":"RNA binding","supporting_discovery_ids":[0,4]},{"term_id":"GO:0005198","term_label":"structural molecule activity","supporting_discovery_ids":[3,4]}],"localization":[{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[6]}],"pathway":[{"term_id":"R-HSA-8953854","term_label":"Metabolism of RNA","supporting_discovery_ids":[0,1,2]}],"complexes":["Lsm2-8 complex","Lsm1-7 complex","Lsm5-6-7 sub-complex"],"partners":["LSM6","LSM7","LSM2","LSM3"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q9Y4Y9","full_name":"U6 snRNA-associated Sm-like protein LSm5","aliases":[],"length_aa":91,"mass_kda":9.9,"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/Q9Y4Y9/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":true,"resolved_as":"","url":"https://depmap.org/portal/gene/LSM5","classification":"Common Essential","n_dependent_lines":1190,"n_total_lines":1208,"dependency_fraction":0.9850993377483444},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[{"gene":"PRPF4B","stoichiometry":10.0},{"gene":"CLTA","stoichiometry":0.2},{"gene":"SNRPC","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/search/LSM5","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":"","locations":[],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/LSM5"},"hgnc":{"alias_symbol":["YER146W"],"prev_symbol":[]},"alphafold":{"accession":"Q9Y4Y9","domains":[{"cath_id":"2.30.30.100","chopping":"26-83","consensus_level":"high","plddt":95.8397,"start":26,"end":83}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9Y4Y9","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9Y4Y9-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9Y4Y9-F1-predicted_aligned_error_v6.png","plddt_mean":90.75},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=LSM5","jax_strain_url":"https://www.jax.org/strain/search?query=LSM5"},"sequence":{"accession":"Q9Y4Y9","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9Y4Y9.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9Y4Y9/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9Y4Y9"}},"corpus_meta":[{"pmid":"23935954","id":"PMC_23935954","title":"Genome-wide association mapping for 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The Lsm2-8 complex preferentially binds the 2',3' cyclic phosphate end of U6 snRNA, while Lsm1-7 strongly discriminates against cyclic phosphates and binds oligouridylate tracts with terminal purines. Lsm1-7 loads onto RNA from the 3' end, and removal of the Lsm1 carboxy-terminal region allows Lsm1-7 to scan along RNA, suggesting a gated mechanism for accessing internal binding sites.\",\n      \"method\": \"High-resolution structural determination (four crystal/cryo-EM structures) combined with RNA binding assays\",\n      \"journal\": \"RNA (New York, N.Y.)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — multiple high-resolution structures with functional RNA-binding validation; mechanistic basis for subunit specificity directly demonstrated\",\n      \"pmids\": [\"32518066\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"Genetic epistasis in S. cerevisiae showed that deletion of LSM5 causes the La protein homolog Lhp1p to become required for growth, indicating that Lsm5 (as part of the Lsm2-8 complex) acts redundantly with Lhp1p to stabilize nascent U6 snRNA at its 3' end. LSM2 and LSM4, but not LSM5, act as allele-specific low-copy suppressors of lsm8 mutations.\",\n      \"method\": \"Genetic epistasis (deletion analysis, suppressor screens, growth assays in S. cerevisiae)\",\n      \"journal\": \"Genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — clean genetic epistasis with defined phenotypic readout, single lab, multiple allele combinations tested\",\n      \"pmids\": [\"11333229\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Structure-guided alanine-scanning mutagenesis of the Lsm2-8 ring in S. cerevisiae, including residues in Lsm5's RNA-binding sites and intersubunit interfaces, showed that most single mutations are benign for vegetative growth, but pairwise combinations of benign lsm mutations (including lsm5 alleles) reveal synthetic lethality and growth defects, indicating internal genetic redundancy in the ring. Critically, the lethal single-gene deletion lsm5Δ was rescued by overexpression of U6 snRNA from a high-copy plasmid, demonstrating that the only essential function of Lsm5 (within Lsm2-8) is to support U6 snRNA biogenesis or function.\",\n      \"method\": \"Alanine scanning mutagenesis, genetic complementation, high-copy suppressor analysis in S. cerevisiae\",\n      \"journal\": \"RNA (New York, N.Y.)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — systematic mutagenesis of RNA-binding and interface residues combined with clean genetic rescue experiment; single lab but multiple orthogonal genetic approaches\",\n      \"pmids\": [\"29615482\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"The 2.5 Å crystal structure of the LSm5/6/7 sub-complex reveals that Lsm5, Lsm6, and Lsm7 display the canonical Sm fold and arrange into a hexameric ring, constituting an assembly intermediate on the pathway to both the cytoplasmic Lsm1-7 and nuclear Lsm2-8 rings. NMR and pull-down experiments confirmed that the Lsm657 hexameric complex can incorporate Lsm23 to assemble further towards native Lsm rings.\",\n      \"method\": \"X-ray crystallography (2.5 Å), high-resolution NMR spectroscopy, pull-down assays\",\n      \"journal\": \"Journal of molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — crystal structure plus NMR plus pull-down in one study; multiple orthogonal methods establish Lsm5 as part of a defined assembly intermediate\",\n      \"pmids\": [\"22001694\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Crystal structures of the Lsm5/6/7 sub-complex from S. pombe reveal that Lsm5, Lsm6, and Lsm7 form a hexameric ring within the crystal lattice, and analytical ultracentrifugation confirmed the hexameric state in solution. RNA binding assays showed that the Lsm5/6/7 sub-complex binds oligo(U), and structural analysis defined the intersubunit interaction order as Lsm5-Lsm6-Lsm7.\",\n      \"method\": \"X-ray crystallography, analytical ultracentrifugation, RNA binding assays\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — crystal structure with solution-state validation and direct RNA binding assay; multiple orthogonal methods in one study\",\n      \"pmids\": [\"22615807\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Pull-down experiments from yeast lysate demonstrated that homomeric Lsm3 octamers can directly recruit Lsm5 (along with Lsm6 and Lsm2) from yeast lysate, providing evidence for specific protein-protein interactions mediated by the variable loops and termini of Lsm subunits during ring assembly.\",\n      \"method\": \"Pull-down from yeast lysate using crystallized Lsm3 octamer\",\n      \"journal\": \"Journal of molecular biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — single pull-down method, but structurally grounded and replicated with multiple Lsm proteins in the same study\",\n      \"pmids\": [\"18329667\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"In Trypanosoma brucei, TAP-tagged Lsm5 purification and RNAi silencing identified Lsm5 as a component of the heptameric Lsm2-8 complex that binds U6 snRNA. Localization studies showed that Lsm5 (as part of this complex) resides in the nucleus near the nucleolus, and Lsm proteins were not detected in cytoplasmic bodies tagged with YFP-Dhh1.\",\n      \"method\": \"TAP-tag purification, RNAi silencing, fluorescence localization studies\",\n      \"journal\": \"Molecular and biochemical parasitology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 / Moderate — TAP purification plus RNAi plus localization, multiple orthogonal approaches in a single lab; ortholog (trypanosome) context\",\n      \"pmids\": [\"18433897\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Lentiviral knockdown of LSM5 in colon cancer cells suppressed proliferation and promoted apoptosis, and was associated with upregulation of p53, CDKN1A, and TNFRSF10B as assessed by GeneChip assay and Western blotting, placing LSM5 upstream of the p53-CDKN1A-TNFRSF10B apoptotic axis.\",\n      \"method\": \"Lentiviral shRNA knockdown, proliferation and apoptosis assays, GeneChip transcriptomic profiling, Western blotting\",\n      \"journal\": \"World journal of gastrointestinal oncology\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single lab, KD with phenotype and downstream pathway markers, but mechanistic placement is indirect (no direct binding or enzymatic evidence); cancer cell line context\",\n      \"pmids\": [\"38994171\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"LSM5 is a core subunit of both the nuclear Lsm2-8 and cytoplasmic Lsm1-7 heteroheptameric ring complexes; within the Lsm2-8 ring it uniquely recognizes purine bases on RNA substrates and its only essential cellular function is to support U6 snRNA biogenesis/stability, while in Lsm1-7 it contributes to mRNA decay; Lsm5 together with Lsm6 and Lsm7 forms a hexameric assembly intermediate that recruits Lsm2/3 en route to the complete rings, and genetic deletion of LSM5 is lethal but fully rescued by U6 snRNA overexpression.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"LSM5 is a core subunit of the heteroheptameric Lsm rings that govern small nuclear RNA stability and mRNA decay, functioning within the nuclear Lsm2-8 complex that binds U6 snRNA and the cytoplasmic Lsm1-7 complex [#0, #6]. Structural studies of both rings bound to RNA show that Lsm5 uniquely recognizes purine bases at the 3' end of RNA substrates, accounting for its divergent sequence relative to the other Lsm subunits; the Lsm2-8 ring preferentially engages the 2',3' cyclic phosphate end of U6 snRNA while Lsm1-7 discriminates against cyclic phosphates and binds oligouridylate tracts with terminal purines [#0]. Lsm5 assembles into the mature rings through a defined hexameric intermediate: it adopts the canonical Sm fold and forms an Lsm5-Lsm6-Lsm7 sub-complex that binds oligo(U) and recruits Lsm2/3 toward native ring assembly [#3, #4]. The sole essential cellular function of Lsm5 within Lsm2-8 is to support U6 snRNA biogenesis or stability: deletion of LSM5 is lethal but is fully rescued by U6 snRNA overexpression, and Lsm5 acts redundantly with the La protein homolog Lhp1p to stabilize the 3' end of nascent U6 snRNA [#1, #2]. In human colon cancer cells, LSM5 knockdown suppresses proliferation and promotes apoptosis associated with induction of the p53-CDKN1A-TNFRSF10B axis [#7].\",\n  \"teleology\": [\n    {\n      \"year\": 2001,\n      \"claim\": \"Established that Lsm5's biological role is to stabilize nascent U6 snRNA, defining the functional output of the Lsm2-8 ring through its genetic redundancy with the La protein homolog.\",\n      \"evidence\": \"Genetic epistasis, deletion analysis and suppressor screens in S. cerevisiae\",\n      \"pmids\": [\"11333229\"],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Did not resolve the structural basis of U6 recognition\", \"Redundancy with Lhp1p inferred genetically, not by direct biochemistry\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Showed how Lsm5 is physically recruited during ring assembly and confirmed its membership in the nuclear U6-binding Lsm2-8 complex in a divergent eukaryote, generalizing the assembly logic across species.\",\n      \"evidence\": \"Pull-down from yeast lysate with a crystallized Lsm3 octamer, and TAP-tag purification plus RNAi and localization in Trypanosoma brucei\",\n      \"pmids\": [\"18329667\", \"18433897\"],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Pull-down does not define stoichiometry or order of incorporation in vivo\", \"Trypanosome localization is ortholog-based\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Defined an Lsm5-Lsm6-Lsm7 hexameric assembly intermediate as a shared precursor for both cytoplasmic and nuclear rings, establishing the structural pathway to mature complexes.\",\n      \"evidence\": \"X-ray crystallography, NMR, analytical ultracentrifugation, pull-down and RNA binding assays (yeast and S. pombe)\",\n      \"pmids\": [\"22001694\", \"22615807\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Does not establish kinetics or in-cell ordering of assembly\", \"RNA binding by the intermediate not linked to a specific cellular substrate\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Demonstrated that the single essential function of Lsm5 within Lsm2-8 is to support U6 snRNA, by rescuing the lethal deletion with U6 overexpression and revealing internal genetic redundancy across ring residues.\",\n      \"evidence\": \"Alanine-scanning mutagenesis, genetic complementation and high-copy U6 suppression in S. cerevisiae\",\n      \"pmids\": [\"29615482\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Does not address non-essential roles of Lsm5 in mRNA decay\", \"Synthetic lethality mechanism not biochemically resolved\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Provided the structural mechanism for Lsm5's unique substrate specificity, showing it recognizes 3'-terminal purines and explaining how Lsm2-8 versus Lsm1-7 discriminate between cyclic-phosphate U6 ends and oligouridylate tracts.\",\n      \"evidence\": \"Four high-resolution crystal/cryo-EM structures of Lsm1-7 and Lsm2-8 bound to RNA, with RNA binding assays\",\n      \"pmids\": [\"32518066\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Does not define dynamics of 3'-end loading in vivo\", \"Functional consequence of purine recognition for specific transcripts not tested\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Linked LSM5 to a cellular phenotype in human cancer cells, placing it upstream of a p53-dependent apoptotic axis, though without direct molecular mechanism.\",\n      \"evidence\": \"Lentiviral shRNA knockdown with proliferation/apoptosis assays, GeneChip profiling and Western blotting in colon cancer cells\",\n      \"pmids\": [\"38994171\"],\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Mechanistic placement is indirect with no direct binding or enzymatic evidence\", \"Connection to RNA-processing role of LSM5 unestablished\", \"Single lab, single cancer context\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How Lsm5's RNA-binding and assembly functions in the Lsm1-7 ring contribute mechanistically to mRNA decay, and whether its cancer-cell phenotype derives from its RNA-processing role, remain unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"No direct evidence linking Lsm5 to specific mRNA decay substrates\", \"Mechanism connecting LSM5 to the p53 axis unknown\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0003723\", \"supporting_discovery_ids\": [0, 4]},\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [3, 4]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [6]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-8953854\", \"supporting_discovery_ids\": [0, 1, 2]}\n    ],\n    \"complexes\": [\"Lsm2-8 complex\", \"Lsm1-7 complex\", \"Lsm5-6-7 sub-complex\"],\n    \"partners\": [\"LSM6\", \"LSM7\", \"LSM2\", \"LSM3\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":5,"faith_total":5,"faith_pct":100.0}}