{"gene":"MRPL20","run_date":"2026-04-28T18:30:28","timeline":{"discoveries":[{"year":1990,"finding":"The yeast MRP-L20 gene (ortholog of human MRPL20) was cloned and shown to encode a 22.3-kDa mitochondrial large ribosomal subunit protein with an 18-amino-acid N-terminal presequence. Gene disruption experiments demonstrated that MRP-L20 is essential for mitochondrial function and that its absence causes instability of mitochondrial DNA.","method":"Gene cloning by oligonucleotide hybridization, DNA sequencing, gene disruption experiments, Southern blot","journal":"Nucleic acids research","confidence":"High","confidence_rationale":"Tier 2 — gene disruption with defined phenotypic readout (loss of mitochondrial function and mtDNA instability), replicated by genomic analyses","pmids":["2183197"],"is_preprint":false},{"year":2001,"finding":"MRPL20 (homolog of E. coli L20) was identified as one of 48 distinct proteins constituting the human mitochondrial large (39S) ribosomal subunit, established by proteolytic digestion of whole 39S subunits followed by liquid chromatography-mass spectrometry and EST database searching.","method":"LC-MS/MS peptide sequencing of purified 39S mitoribosomal subunit","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — direct biochemical identification from purified ribosomal subunit with MS validation","pmids":["11551941"],"is_preprint":false},{"year":2014,"finding":"MRPL20 was resolved as a structural component of the human mitochondrial large ribosomal subunit (mt-LSU) at 3.4 Å resolution by cryo-EM, confirming its integral role in the 39S subunit architecture.","method":"Single-particle cryo-EM of purified human mt-LSU to 3.4 Å resolution","journal":"Science (New York, N.Y.)","confidence":"High","confidence_rationale":"Tier 1 — high-resolution cryo-EM structure with direct visualization of protein complement","pmids":["25278503"],"is_preprint":false},{"year":2016,"finding":"MRPL20 was identified as essential for oxidative phosphorylation (OXPHOS) in human cells; its loss causes cell death in galactose medium (which forces OXPHOS dependency), establishing it as a required gene for mitochondrial translation and OXPHOS.","method":"Genome-wide CRISPR death screen selecting dying cells in galactose medium","journal":"Cell metabolism","confidence":"High","confidence_rationale":"Tier 2 — genome-wide genetic screen with defined metabolic phenotype, high-confidence hit","pmids":["27667664"],"is_preprint":false},{"year":2017,"finding":"Cryo-EM structures of two late-stage assembly intermediates of the human mt-LSU reveal the timing of MRPL20 incorporation during ribosomal maturation and demonstrate the structural context of mt-LSU biogenesis.","method":"Cryo-EM of native mt-LSU assembly intermediates isolated from human cells (~3 Å resolution)","journal":"Nature structural & molecular biology","confidence":"High","confidence_rationale":"Tier 1 — high-resolution cryo-EM structures of native assembly intermediates with direct structural visualization","pmids":["28892042"],"is_preprint":false},{"year":2020,"finding":"BioID proximity interaction mapping in a high-density mitochondrial network confirmed MRPL20's localization and proximity interactions within the mitochondrial compartment, placing it in a functional cluster consistent with the mitoribosomal large subunit.","method":"BioID proximity-dependent biotinylation with 100 mitochondrial bait proteins followed by mass spectrometry","journal":"Cell metabolism","confidence":"Medium","confidence_rationale":"Tier 2 — systematic proximity labeling, but MRPL20 is a prey rather than bait","pmids":["32877691"],"is_preprint":false},{"year":2021,"finding":"OpenCell endogenous tagging confirmed MRPL20 localizes to mitochondria in human cells by live-cell confocal imaging of an endogenously GFP-tagged protein, and mass spectrometry identified its mitoribosome co-complex interactions.","method":"CRISPR-based endogenous GFP tagging, confocal live-cell imaging, affinity purification mass spectrometry","journal":"Science (New York, N.Y.)","confidence":"High","confidence_rationale":"Tier 1–2 — endogenous tagging with direct imaging and AP-MS, multiple orthogonal methods","pmids":["35271311"],"is_preprint":false},{"year":2021,"finding":"MRPL20 was quantified as part of the high-confidence human mitochondrial proteome (MitoCoP), with defined protein abundance and half-life dynamics, establishing it as a stably expressed mitochondrial protein across cellular contexts.","method":"Quantitative mass spectrometry of mitochondrial preparations with turnover measurements","journal":"Cell metabolism","confidence":"Medium","confidence_rationale":"Tier 2 — quantitative proteomics with defined abundance and turnover metrics","pmids":["34800366"],"is_preprint":false},{"year":2025,"finding":"Under copper stress (cuproptosis induction), OXA1L levels in mitochondria are reduced, which disrupts MRPL20 import into the mitochondrial matrix and impairs oxidative phosphorylation complex I synthesis. This mitochondrial dysfunction triggers nuclear translocation of MRPL20, which activates the mitochondrial unfolded protein response (UPRmt) as an adaptive mechanism to restore mitochondrial proteostasis during early cuproptosis.","method":"Spatio-temporal mass spectrometry (STMS) of subcellular proteome dynamics, genetic functional screen, copper stress experiments in living cells","journal":"bioRxiv","confidence":"Medium","confidence_rationale":"Tier 2 — novel subcellular proteomics approach with genetic screen validation, but preprint not yet peer-reviewed","pmids":["bio_10.1101_2025.07.01.662679"],"is_preprint":true}],"current_model":"MRPL20 is an essential structural protein of the human mitochondrial large (39S) ribosomal subunit required for mitochondrial translation and oxidative phosphorylation; its import into mitochondria depends on OXA1L, and under copper stress its import is disrupted leading to nuclear translocation that activates the mitochondrial unfolded protein response, while its absence causes mitochondrial dysfunction and mtDNA instability."},"narrative":{"teleology":[{"year":1990,"claim":"Establishing that the MRP-L20 gene product is essential for mitochondrial function answered whether this ribosomal protein is dispensable or critical, revealing that its loss destabilizes mitochondrial DNA and abolishes respiratory competence.","evidence":"Gene disruption of yeast MRP-L20 ortholog with Southern blot and growth phenotype analysis","pmids":["2183197"],"confidence":"High","gaps":["Mechanism linking loss of a single ribosomal subunit to mtDNA instability was not elucidated","No data on the human ortholog at this stage"]},{"year":2001,"claim":"Direct biochemical identification of MRPL20 as a bona fide subunit of the purified human 39S mitoribosomal particle established its assignment to the large subunit proteome, moving beyond sequence homology.","evidence":"LC-MS/MS peptide sequencing of purified human 39S subunits","pmids":["11551941"],"confidence":"High","gaps":["Precise position within the 39S architecture was unknown","Functional necessity in human cells was not tested"]},{"year":2014,"claim":"High-resolution cryo-EM visualization of MRPL20 within the mt-LSU at 3.4 Å resolved its structural context and contacts within the subunit, defining its architectural role.","evidence":"Single-particle cryo-EM of purified human mt-LSU","pmids":["25278503"],"confidence":"High","gaps":["When MRPL20 is incorporated during subunit assembly was not addressed","Whether the protein has functions beyond a structural scaffold was unknown"]},{"year":2016,"claim":"A genome-wide CRISPR screen demonstrated that MRPL20 is essential for oxidative phosphorylation in human cells, providing the first direct human genetic evidence for its functional requirement.","evidence":"CRISPR death screen selecting cells dying under galactose (OXPHOS-dependent) culture","pmids":["27667664"],"confidence":"High","gaps":["Whether MRPL20 loss affects specific OXPHOS complexes differentially was not resolved","No patient mutations linked to disease at this point"]},{"year":2017,"claim":"Cryo-EM of native mt-LSU assembly intermediates revealed the timing of MRPL20 incorporation, placing it in the late-stage maturation pathway and clarifying subunit biogenesis order.","evidence":"Cryo-EM of two native late-stage mt-LSU assembly intermediates from human cells (~3 Å)","pmids":["28892042"],"confidence":"High","gaps":["Identity of specific assembly factors chaperoning MRPL20 incorporation was not determined","Kinetics of incorporation in living cells were not measured"]},{"year":2020,"claim":"Proximity labeling placed MRPL20 within a mitochondrial interaction cluster consistent with the mt-LSU, orthogonally validating its compartment assignment and local interactome.","evidence":"BioID proximity-dependent biotinylation with 100 mitochondrial baits followed by MS","pmids":["32877691"],"confidence":"Medium","gaps":["MRPL20 was a prey, not a bait; its direct proximity partners were not exhaustively mapped","Proximity data do not distinguish direct from indirect contacts"]},{"year":2021,"claim":"Endogenous GFP tagging confirmed mitochondrial localization in live human cells and AP-MS validated co-complex interactions, providing the highest-quality localization and interaction data for the native protein.","evidence":"CRISPR endogenous GFP tagging with confocal imaging and affinity purification mass spectrometry","pmids":["35271311"],"confidence":"High","gaps":["Sub-mitochondrial localization (matrix vs. inner membrane association) was not resolved by imaging","Stoichiometry of MRPL20 relative to other mt-LSU subunits was not quantified"]},{"year":2025,"claim":"Discovery that copper stress reduces OXA1L, blocking MRPL20 mitochondrial import and causing its nuclear translocation to activate the UPRmt, established a non-canonical signaling role for a ribosomal protein as a stress-responsive retrograde signal.","evidence":"Spatio-temporal subcellular mass spectrometry and genetic screen under copper stress (preprint)","pmids":["bio_10.1101_2025.07.01.662679"],"confidence":"Medium","gaps":["Preprint not yet peer-reviewed; independent replication needed","Mechanism by which nuclear MRPL20 activates UPRmt transcription is not defined","Whether MRPL20 nuclear translocation occurs under other mitochondrial stresses is unknown"]},{"year":null,"claim":"It remains unknown whether MRPL20 directly contacts DNA or transcription factors in the nucleus to activate UPRmt, whether human loss-of-function mutations cause mitochondrial disease, and what determines the switch between its structural role in the mitoribosome and its signaling function.","evidence":"","pmids":[],"confidence":"Low","gaps":["No patient mutations or Mendelian disease association reported","Nuclear binding partners and transcriptional targets uncharacterized","Whether MRPL20 nuclear function is conserved in model organisms is untested"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0005198","term_label":"structural molecule activity","supporting_discovery_ids":[1,2,4]}],"localization":[{"term_id":"GO:0005739","term_label":"mitochondrion","supporting_discovery_ids":[0,1,2,5,6,7]},{"term_id":"GO:0005840","term_label":"ribosome","supporting_discovery_ids":[1,2,4,6]}],"pathway":[{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[1,2,3,4]}],"complexes":["mitochondrial large ribosomal subunit (39S)"],"partners":["OXA1L"],"other_free_text":[]},"mechanistic_narrative":"MRPL20 is a structural component of the human mitochondrial large (39S) ribosomal subunit essential for mitochondrial translation and oxidative phosphorylation. The protein was biochemically identified as one of 48 distinct subunits of the 39S particle and resolved at 3.4 Å within the mt-LSU by cryo-EM, with subsequent structures capturing its incorporation during late-stage ribosomal assembly [PMID:11551941, PMID:25278503, PMID:28892042]. Disruption of its yeast ortholog causes loss of mitochondrial function and mitochondrial DNA instability, and genome-wide CRISPR screening in human cells confirmed it is required for OXPHOS-dependent viability [PMID:2183197, PMID:27667664]. Import of MRPL20 into the mitochondrial matrix depends on OXA1L, and under copper stress its import is disrupted, leading to nuclear translocation that activates the mitochondrial unfolded protein response [PMID:bio_10.1101_2025.07.01.662679]."},"prefetch_data":{"uniprot":{"accession":"Q9BYC9","full_name":"Large ribosomal subunit protein bL20m","aliases":["39S ribosomal protein L20, mitochondrial","L20mt","MRP-L20"],"length_aa":149,"mass_kda":17.4,"function":"","subcellular_location":"Mitochondrion","url":"https://www.uniprot.org/uniprotkb/Q9BYC9/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/MRPL20","classification":"Not Classified","n_dependent_lines":503,"n_total_lines":1208,"dependency_fraction":0.4163907284768212},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[{"gene":"CTPS1","stoichiometry":0.2},{"gene":"IPO5","stoichiometry":0.2},{"gene":"LSM14A","stoichiometry":0.2},{"gene":"NPM1","stoichiometry":0.2},{"gene":"PSME3","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/search/MRPL20","total_profiled":1310},"omim":[{"mim_id":"611833","title":"MITOCHONDRIAL RIBOSOMAL PROTEIN L20; MRPL20","url":"https://www.omim.org/entry/611833"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Supported","locations":[{"location":"Mitochondria","reliability":"Supported"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/MRPL20"},"hgnc":{"alias_symbol":["FLJ10024","bL20m"],"prev_symbol":[]},"alphafold":{"accession":"Q9BYC9","domains":[{"cath_id":"1.10.1900.20","chopping":"68-132","consensus_level":"medium","plddt":96.1955,"start":68,"end":132}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9BYC9","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9BYC9-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9BYC9-F1-predicted_aligned_error_v6.png","plddt_mean":91.0},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=MRPL20","jax_strain_url":"https://www.jax.org/strain/search?query=MRPL20"},"sequence":{"accession":"Q9BYC9","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9BYC9.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9BYC9/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9BYC9"}},"corpus_meta":[{"pmid":"32987154","id":"PMC_32987154","title":"Expression analysis 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Analysis of the complement of ribosomal proteins present.","date":"2001","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/11551941","citation_count":216,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"29568061","id":"PMC_29568061","title":"An AP-MS- and BioID-compatible MAC-tag enables comprehensive mapping of protein interactions and subcellular localizations.","date":"2018","source":"Nature communications","url":"https://pubmed.ncbi.nlm.nih.gov/29568061","citation_count":201,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"20186120","id":"PMC_20186120","title":"A functional peptidyl-tRNA hydrolase, ICT1, has been recruited into the human mitochondrial ribosome.","date":"2010","source":"The EMBO journal","url":"https://pubmed.ncbi.nlm.nih.gov/20186120","citation_count":153,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"18461144","id":"PMC_18461144","title":"Systematic identification of mRNAs recruited to argonaute 2 by specific microRNAs and corresponding changes in transcript abundance.","date":"2008","source":"PloS one","url":"https://pubmed.ncbi.nlm.nih.gov/18461144","citation_count":148,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"32877691","id":"PMC_32877691","title":"A High-Density Human Mitochondrial Proximity Interaction Network.","date":"2020","source":"Cell metabolism","url":"https://pubmed.ncbi.nlm.nih.gov/32877691","citation_count":148,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"16710414","id":"PMC_16710414","title":"The DNA sequence and biological annotation of human chromosome 1.","date":"2006","source":"Nature","url":"https://pubmed.ncbi.nlm.nih.gov/16710414","citation_count":144,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"31871319","id":"PMC_31871319","title":"Mapping the proximity interaction network of the Rho-family GTPases reveals signalling pathways and regulatory mechanisms.","date":"2019","source":"Nature cell biology","url":"https://pubmed.ncbi.nlm.nih.gov/31871319","citation_count":137,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"28892042","id":"PMC_28892042","title":"Structures of the human mitochondrial ribosome in native states of assembly.","date":"2017","source":"Nature structural & molecular biology","url":"https://pubmed.ncbi.nlm.nih.gov/28892042","citation_count":136,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"27591049","id":"PMC_27591049","title":"SPATA2 Links CYLD to LUBAC, Activates CYLD, and Controls LUBAC Signaling.","date":"2016","source":"Molecular cell","url":"https://pubmed.ncbi.nlm.nih.gov/27591049","citation_count":134,"is_preprint":false,"source_track":"gene2pubmed"}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":6248,"output_tokens":794,"usd":0.015327},"stage2":{"model":"claude-opus-4-6","input_tokens":3975,"output_tokens":1267,"usd":0.077325},"total_usd":0.260857,"stage1_batch_id":"msgbatch_01Mgbec4vM6c9vXht7JVJzss","stage2_batch_id":"msgbatch_01JkuuH5RB2BdJNsgnhjbC6s","note":"batch pricing = 50% of standard","round2_stage1":{"model":"claude-sonnet-4-6","input_tokens":18252,"output_tokens":2999,"usd":0.04987},"round2_rules_fired":"R3","round2_stage2":{"model":"claude-opus-4-6","input_tokens":5438,"output_tokens":2068,"usd":0.118335}},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1990,\n      \"finding\": \"Yeast MRP-L20 encodes a 22.3-kDa mitochondrial large ribosomal subunit protein with an 18-amino-acid presequence (mitochondrial targeting signal). Gene disruption experiments showed that YmL20 is essential for mitochondrial function, and its absence causes instability of mitochondrial DNA.\",\n      \"method\": \"Gene cloning, sequencing, and gene disruption experiments in Saccharomyces cerevisiae\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean gene disruption with defined functional phenotype (loss of mitochondrial function, mtDNA instability), foundational paper with 40 citations\",\n      \"pmids\": [\"2183197\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Each mitochondrial ribosomal protein, including MRPL20, has a unique and essential role in the mitoribosome complex with no functional redundancy; Mrp20 knockout results in early embryonic lethality in mice, and expression is ubiquitous throughout early embryogenesis with little stage or tissue specificity.\",\n      \"method\": \"Mouse developmental expression analysis and knockout phenotyping (embryonic lethality screen)\",\n      \"journal\": \"Gene expression patterns : GEP\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — KO phenotype with defined lethality readout, but mechanistic detail is limited to essentiality without molecular pathway placement\",\n      \"pmids\": [\"32987154\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"During copper stress (cuproptosis), OXA1L levels in mitochondria are reduced, which disrupts MRPL20 import into mitochondria, impairs oxidative phosphorylation complex I synthesis, and promotes MRPL20 nuclear translocation. Nuclear MRPL20 triggers the mitochondrial unfolded protein response (mtUPR) as an adaptive mechanism to restore mitochondrial proteostasis.\",\n      \"method\": \"Spatio-temporal mass spectrometry (STMS), genetic functional screen, subcellular fractionation/proteomics in living cells\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — novel mechanistic pathway with proteomic and genetic screen support, but preprint without peer review\",\n      \"pmids\": [\"bio_10.1101_2025.07.01.662679\"],\n      \"is_preprint\": true\n    }\n  ],\n  \"current_model\": \"MRPL20 is a nuclear-encoded mitochondrial large ribosomal subunit protein that is imported into mitochondria via an N-terminal presequence in an OXA1L-dependent manner, where it is essential for mitoribosome integrity and oxidative phosphorylation complex I synthesis; under copper stress, impaired mitochondrial import leads to MRPL20 nuclear translocation, which activates the mitochondrial unfolded protein response as an adaptive proteostasis mechanism.\"\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\nI'll classify each paper first, then extract discoveries only from KEEP papers.\n\n**Classification:**\n\n- Paper [1] (PMID:32987154): KEEP — expression analysis of Mrp genes including MRPL20 in mice\n- Paper [2] (PMID:24098497): EXCLUDE — breast cancer biomarker/survival study, MRPL20 mentioned as part of a gene signature only\n- Paper [3] (PMID:2183197): KEEP — yeast ortholog MRP-L20 cloning and functional characterization\n- Paper [4] (PMID:8203164): KEEP — yeast chromosome sequence, MRP-L20 gene mapping context\n- Paper [5] (PMID:35988669): EXCLUDE — case (B) alt-locus product: lncRNA MRPL20-AS1, not the protein\n- Paper [6] (PMID:36824437): EXCLUDE — DCM biomarker study, expression-based only\n- Paper [7] (PMID:39529627): EXCLUDE — MRPL20 mentioned as hub gene in correlation analysis only\n- Paper [8] (PMID:41749276): EXCLUDE — MRPL20 mentioned in epigenome association, no mechanistic findings\n- Paper [9] (PMID:1481572): KEEP — yeast gene mapping, MRP-L20 chromosomal location\n- Paper [10] (bio_10.1101_2025.07.01.662679): KEEP — PREPRINT: direct mechanistic findings about MRPL20 in cuproptosis context\n\n**Gene2pubmed papers:**\n- PMID:11181995: EXCLUDE — human genome sequence paper, no specific MRPL20 mechanism\n- PMID:16169070: EXCLUDE — general interactome screen, no specific MRPL20 mechanism\n- PMID:12477932: EXCLUDE — cDNA collection paper\n- PMID:26186194: EXCLUDE — general interactome (BioPlex), no specific MRPL20 mechanism extracted\n- PMID:28514442: EXCLUDE — general interactome\n- PMID:26496610: EXCLUDE — general interactome\n- PMID:22681889: EXCLUDE — general RNA-binding proteome\n- PMID:29507755: EXCLUDE — m6A methylation, MRPL20 not specifically studied\n- PMID:14702039: EXCLUDE — cDNA sequencing\n- PMID:33961781: EXCLUDE — general interactome\n- PMID:22939629: EXCLUDE — general complex mapping\n- PMID:21873635: EXCLUDE — GO annotation methodology\n- PMID:15489334: EXCLUDE — MGC cDNA project\n- PMID:35271311: KEEP — OpenCell: direct localization of MRPL20 by endogenous tagging and imaging\n- PMID:26344197: KEEP — metazoan complexes: co-fractionation data placing MRPL20 in mitoribosome complex\n- PMID:34079125: KEEP — BioID proximity map: subcellular localization\n- PMID:25278503: KEEP — cryo-EM structure of human mt-LSU including MRPL20\n- PMID:27667664: KEEP — CRISPR screen showing MRPL20 essential for OXPHOS\n- PMID:27023846: KEEP — review of mitoribosome structure/function with structural data\n- PMID:11551941: KEEP — mass spectrometry identification of MRPL20 in human mt-LSU\n- PMID:29568061: EXCLUDE — general MAC-tag methodology\n- PMID:20186120: EXCLUDE — about ICT1/mitoribosome, MRPL20 not specifically studied\n- PMID:18461144: EXCLUDE — Ago2/miRNA, MRPL20 not specifically studied\n- PMID:32877691: KEEP — BioID mitochondrial proximity network including MRPL20\n- PMID:16710414: EXCLUDE — chromosome 1 annotation\n- PMID:31871319: EXCLUDE — Rho GTPase network\n- PMID:28892042: KEEP — cryo-EM structures of mt-LSU assembly intermediates\n- PMID:27591049: EXCLUDE — LUBAC/CYLD signaling\n- PMID:34800366: KEEP — mitochondrial proteome with MRPL20 dynamics\n- PMID:21900206: EXCLUDE — signal transduction network\n- PMID:22939629: EXCLUDE — already classified\n\n```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1990,\n      \"finding\": \"The yeast MRP-L20 gene (ortholog of human MRPL20) was cloned and shown to encode a 22.3-kDa mitochondrial large ribosomal subunit protein with an 18-amino-acid N-terminal presequence. Gene disruption experiments demonstrated that MRP-L20 is essential for mitochondrial function and that its absence causes instability of mitochondrial DNA.\",\n      \"method\": \"Gene cloning by oligonucleotide hybridization, DNA sequencing, gene disruption experiments, Southern blot\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — gene disruption with defined phenotypic readout (loss of mitochondrial function and mtDNA instability), replicated by genomic analyses\",\n      \"pmids\": [\"2183197\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"MRPL20 (homolog of E. coli L20) was identified as one of 48 distinct proteins constituting the human mitochondrial large (39S) ribosomal subunit, established by proteolytic digestion of whole 39S subunits followed by liquid chromatography-mass spectrometry and EST database searching.\",\n      \"method\": \"LC-MS/MS peptide sequencing of purified 39S mitoribosomal subunit\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — direct biochemical identification from purified ribosomal subunit with MS validation\",\n      \"pmids\": [\"11551941\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"MRPL20 was resolved as a structural component of the human mitochondrial large ribosomal subunit (mt-LSU) at 3.4 Å resolution by cryo-EM, confirming its integral role in the 39S subunit architecture.\",\n      \"method\": \"Single-particle cryo-EM of purified human mt-LSU to 3.4 Å resolution\",\n      \"journal\": \"Science (New York, N.Y.)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — high-resolution cryo-EM structure with direct visualization of protein complement\",\n      \"pmids\": [\"25278503\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"MRPL20 was identified as essential for oxidative phosphorylation (OXPHOS) in human cells; its loss causes cell death in galactose medium (which forces OXPHOS dependency), establishing it as a required gene for mitochondrial translation and OXPHOS.\",\n      \"method\": \"Genome-wide CRISPR death screen selecting dying cells in galactose medium\",\n      \"journal\": \"Cell metabolism\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genome-wide genetic screen with defined metabolic phenotype, high-confidence hit\",\n      \"pmids\": [\"27667664\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Cryo-EM structures of two late-stage assembly intermediates of the human mt-LSU reveal the timing of MRPL20 incorporation during ribosomal maturation and demonstrate the structural context of mt-LSU biogenesis.\",\n      \"method\": \"Cryo-EM of native mt-LSU assembly intermediates isolated from human cells (~3 Å resolution)\",\n      \"journal\": \"Nature structural & molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — high-resolution cryo-EM structures of native assembly intermediates with direct structural visualization\",\n      \"pmids\": [\"28892042\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"BioID proximity interaction mapping in a high-density mitochondrial network confirmed MRPL20's localization and proximity interactions within the mitochondrial compartment, placing it in a functional cluster consistent with the mitoribosomal large subunit.\",\n      \"method\": \"BioID proximity-dependent biotinylation with 100 mitochondrial bait proteins followed by mass spectrometry\",\n      \"journal\": \"Cell metabolism\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — systematic proximity labeling, but MRPL20 is a prey rather than bait\",\n      \"pmids\": [\"32877691\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"OpenCell endogenous tagging confirmed MRPL20 localizes to mitochondria in human cells by live-cell confocal imaging of an endogenously GFP-tagged protein, and mass spectrometry identified its mitoribosome co-complex interactions.\",\n      \"method\": \"CRISPR-based endogenous GFP tagging, confocal live-cell imaging, affinity purification mass spectrometry\",\n      \"journal\": \"Science (New York, N.Y.)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — endogenous tagging with direct imaging and AP-MS, multiple orthogonal methods\",\n      \"pmids\": [\"35271311\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"MRPL20 was quantified as part of the high-confidence human mitochondrial proteome (MitoCoP), with defined protein abundance and half-life dynamics, establishing it as a stably expressed mitochondrial protein across cellular contexts.\",\n      \"method\": \"Quantitative mass spectrometry of mitochondrial preparations with turnover measurements\",\n      \"journal\": \"Cell metabolism\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — quantitative proteomics with defined abundance and turnover metrics\",\n      \"pmids\": [\"34800366\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Under copper stress (cuproptosis induction), OXA1L levels in mitochondria are reduced, which disrupts MRPL20 import into the mitochondrial matrix and impairs oxidative phosphorylation complex I synthesis. This mitochondrial dysfunction triggers nuclear translocation of MRPL20, which activates the mitochondrial unfolded protein response (UPRmt) as an adaptive mechanism to restore mitochondrial proteostasis during early cuproptosis.\",\n      \"method\": \"Spatio-temporal mass spectrometry (STMS) of subcellular proteome dynamics, genetic functional screen, copper stress experiments in living cells\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — novel subcellular proteomics approach with genetic screen validation, but preprint not yet peer-reviewed\",\n      \"pmids\": [\"bio_10.1101_2025.07.01.662679\"],\n      \"is_preprint\": true\n    }\n  ],\n  \"current_model\": \"MRPL20 is an essential structural protein of the human mitochondrial large (39S) ribosomal subunit required for mitochondrial translation and oxidative phosphorylation; its import into mitochondria depends on OXA1L, and under copper stress its import is disrupted leading to nuclear translocation that activates the mitochondrial unfolded protein response, while its absence causes mitochondrial dysfunction and mtDNA instability.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"MRPL20 is a nuclear-encoded component of the mitochondrial large ribosomal subunit that is essential for mitoribosome integrity, oxidative phosphorylation complex I synthesis, and mitochondrial DNA stability [PMID:2183197, PMID:32987154]. MRPL20 is imported into mitochondria via an N-terminal presequence in an OXA1L-dependent manner; under copper stress, reduced OXA1L levels impair MRPL20 mitochondrial import, leading to its nuclear translocation where it activates the mitochondrial unfolded protein response (mtUPR) as an adaptive proteostasis mechanism [bio_10.1101_2025.07.01.662679]. Loss of MRPL20 causes early embryonic lethality in mice, reflecting its non-redundant role in mitoribosome function [PMID:32987154].\",\n  \"teleology\": [\n    {\n      \"year\": 1990,\n      \"claim\": \"The identification of yeast MRP-L20 as a mitochondrial large ribosomal subunit protein with an N-terminal targeting presequence established it as a nuclear-encoded, mitochondrially-imported component essential for mitochondrial function and mtDNA stability.\",\n      \"evidence\": \"Gene cloning, sequencing, and gene disruption in S. cerevisiae\",\n      \"pmids\": [\"2183197\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Mechanism by which loss of YmL20 destabilizes mtDNA was not determined\",\n        \"No mammalian ortholog characterized at this point\",\n        \"Import pathway and chaperone requirements not defined\"\n      ]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Demonstration that mouse Mrpl20 knockout causes early embryonic lethality established that MRPL20 is individually essential with no functional redundancy among mitoribosomal proteins, and its ubiquitous embryonic expression indicated a housekeeping role rather than tissue-specific function.\",\n      \"evidence\": \"Mouse knockout phenotyping and developmental expression analysis\",\n      \"pmids\": [\"32987154\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Molecular basis of lethality (e.g., which OXPHOS complexes are affected) not dissected\",\n        \"No conditional or tissue-specific knockout to assess postnatal roles\",\n        \"No direct biochemical assay of mitoribosome assembly in the knockout\"\n      ]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Revealing that copper stress reduces OXA1L, thereby blocking MRPL20 mitochondrial import and redirecting it to the nucleus where it activates the mtUPR, established a moonlighting signaling role for MRPL20 linking mitochondrial import failure to a transcriptional proteostasis response.\",\n      \"evidence\": \"Spatio-temporal mass spectrometry, genetic functional screen, and subcellular fractionation/proteomics in living cells (preprint)\",\n      \"pmids\": [\"bio_10.1101_2025.07.01.662679\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Preprint not yet peer-reviewed; key findings await independent validation\",\n        \"Nuclear targets or transcriptional partners through which MRPL20 activates mtUPR are not identified\",\n        \"Whether MRPL20 nuclear translocation occurs under stresses other than copper overload is unknown\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"It remains unknown how nuclear MRPL20 mechanistically activates the mtUPR—specifically, whether it acts as a transcriptional regulator, adaptor, or signaling intermediate—and whether this moonlighting function is conserved across species.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\n        \"No direct DNA-binding or transcription factor interaction data for nuclear MRPL20\",\n        \"No structural model of MRPL20 outside the mitoribosome context\",\n        \"Conservation of the OXA1L–MRPL20–mtUPR axis beyond mammalian cells not tested\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [0, 1]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005739\", \"supporting_discovery_ids\": [0, 1, 2]},\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [2]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"GO:0005840\", \"supporting_discovery_ids\": [0, 1]},\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [0, 1, 2]}\n    ],\n    \"complexes\": [\n      \"mitochondrial large ribosomal subunit (mt-LSU)\"\n    ],\n    \"partners\": [\n      \"OXA1L\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"mechanistic_narrative\": \"MRPL20 is a structural component of the human mitochondrial large (39S) ribosomal subunit essential for mitochondrial translation and oxidative phosphorylation. The protein was biochemically identified as one of 48 distinct subunits of the 39S particle and resolved at 3.4 Å within the mt-LSU by cryo-EM, with subsequent structures capturing its incorporation during late-stage ribosomal assembly [PMID:11551941, PMID:25278503, PMID:28892042]. Disruption of its yeast ortholog causes loss of mitochondrial function and mitochondrial DNA instability, and genome-wide CRISPR screening in human cells confirmed it is required for OXPHOS-dependent viability [PMID:2183197, PMID:27667664]. Import of MRPL20 into the mitochondrial matrix depends on OXA1L, and under copper stress its import is disrupted, leading to nuclear translocation that activates the mitochondrial unfolded protein response [PMID:bio_10.1101_2025.07.01.662679].\",\n  \"teleology\": [\n    {\n      \"year\": 1990,\n      \"claim\": \"Establishing that the MRP-L20 gene product is essential for mitochondrial function answered whether this ribosomal protein is dispensable or critical, revealing that its loss destabilizes mitochondrial DNA and abolishes respiratory competence.\",\n      \"evidence\": \"Gene disruption of yeast MRP-L20 ortholog with Southern blot and growth phenotype analysis\",\n      \"pmids\": [\"2183197\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Mechanism linking loss of a single ribosomal subunit to mtDNA instability was not elucidated\",\n        \"No data on the human ortholog at this stage\"\n      ]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Direct biochemical identification of MRPL20 as a bona fide subunit of the purified human 39S mitoribosomal particle established its assignment to the large subunit proteome, moving beyond sequence homology.\",\n      \"evidence\": \"LC-MS/MS peptide sequencing of purified human 39S subunits\",\n      \"pmids\": [\"11551941\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Precise position within the 39S architecture was unknown\",\n        \"Functional necessity in human cells was not tested\"\n      ]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"High-resolution cryo-EM visualization of MRPL20 within the mt-LSU at 3.4 Å resolved its structural context and contacts within the subunit, defining its architectural role.\",\n      \"evidence\": \"Single-particle cryo-EM of purified human mt-LSU\",\n      \"pmids\": [\"25278503\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"When MRPL20 is incorporated during subunit assembly was not addressed\",\n        \"Whether the protein has functions beyond a structural scaffold was unknown\"\n      ]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"A genome-wide CRISPR screen demonstrated that MRPL20 is essential for oxidative phosphorylation in human cells, providing the first direct human genetic evidence for its functional requirement.\",\n      \"evidence\": \"CRISPR death screen selecting cells dying under galactose (OXPHOS-dependent) culture\",\n      \"pmids\": [\"27667664\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Whether MRPL20 loss affects specific OXPHOS complexes differentially was not resolved\",\n        \"No patient mutations linked to disease at this point\"\n      ]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Cryo-EM of native mt-LSU assembly intermediates revealed the timing of MRPL20 incorporation, placing it in the late-stage maturation pathway and clarifying subunit biogenesis order.\",\n      \"evidence\": \"Cryo-EM of two native late-stage mt-LSU assembly intermediates from human cells (~3 Å)\",\n      \"pmids\": [\"28892042\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Identity of specific assembly factors chaperoning MRPL20 incorporation was not determined\",\n        \"Kinetics of incorporation in living cells were not measured\"\n      ]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Proximity labeling placed MRPL20 within a mitochondrial interaction cluster consistent with the mt-LSU, orthogonally validating its compartment assignment and local interactome.\",\n      \"evidence\": \"BioID proximity-dependent biotinylation with 100 mitochondrial baits followed by MS\",\n      \"pmids\": [\"32877691\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"MRPL20 was a prey, not a bait; its direct proximity partners were not exhaustively mapped\",\n        \"Proximity data do not distinguish direct from indirect contacts\"\n      ]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Endogenous GFP tagging confirmed mitochondrial localization in live human cells and AP-MS validated co-complex interactions, providing the highest-quality localization and interaction data for the native protein.\",\n      \"evidence\": \"CRISPR endogenous GFP tagging with confocal imaging and affinity purification mass spectrometry\",\n      \"pmids\": [\"35271311\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Sub-mitochondrial localization (matrix vs. inner membrane association) was not resolved by imaging\",\n        \"Stoichiometry of MRPL20 relative to other mt-LSU subunits was not quantified\"\n      ]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Discovery that copper stress reduces OXA1L, blocking MRPL20 mitochondrial import and causing its nuclear translocation to activate the UPRmt, established a non-canonical signaling role for a ribosomal protein as a stress-responsive retrograde signal.\",\n      \"evidence\": \"Spatio-temporal subcellular mass spectrometry and genetic screen under copper stress (preprint)\",\n      \"pmids\": [\"bio_10.1101_2025.07.01.662679\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Preprint not yet peer-reviewed; independent replication needed\",\n        \"Mechanism by which nuclear MRPL20 activates UPRmt transcription is not defined\",\n        \"Whether MRPL20 nuclear translocation occurs under other mitochondrial stresses is unknown\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"It remains unknown whether MRPL20 directly contacts DNA or transcription factors in the nucleus to activate UPRmt, whether human loss-of-function mutations cause mitochondrial disease, and what determines the switch between its structural role in the mitoribosome and its signaling function.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\n        \"No patient mutations or Mendelian disease association reported\",\n        \"Nuclear binding partners and transcriptional targets uncharacterized\",\n        \"Whether MRPL20 nuclear function is conserved in model organisms is untested\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\n        \"term_id\": \"GO:0005198\",\n        \"supporting_discovery_ids\": [1, 2, 4]\n      }\n    ],\n    \"localization\": [\n      {\n        \"term_id\": \"GO:0005739\",\n        \"supporting_discovery_ids\": [0, 1, 2, 5, 6, 7]\n      },\n      {\n        \"term_id\": \"GO:0005840\",\n        \"supporting_discovery_ids\": [1, 2, 4, 6]\n      }\n    ],\n    \"pathway\": [\n      {\n        \"term_id\": \"R-HSA-392499\",\n        \"supporting_discovery_ids\": [1, 2, 3, 4]\n      }\n    ],\n    \"complexes\": [\n      \"mitochondrial large ribosomal subunit (39S)\"\n    ],\n    \"partners\": [\n      \"OXA1L\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}