{"gene":"MRTO4","run_date":"2026-04-28T18:30:28","timeline":{"discoveries":[{"year":1999,"finding":"MRT4 was identified as a novel yeast gene required for mRNA turnover; temperature-sensitive mrt4 mutants exhibit defects in decay of multiple mRNAs without a significant defect in protein synthesis, suggesting a specific role in mRNA decay rather than general translation.","method":"Genetic screen for mRNA turnover defects; complementation analysis; Northern blot analysis of mRNA decay rates in ts mutants","journal":"Genetics","confidence":"Medium","confidence_rationale":"Tier 2 — genetic epistasis and complementation with specific mRNA decay phenotype, single lab","pmids":["10471698"],"is_preprint":false},{"year":2002,"finding":"MRTO4 (human Mrt4) was identified as a component of the human nucleolar proteome, establishing its nucleolar localization in human cells.","method":"Mass spectrometry-based proteomic analysis of isolated human HeLa cell nucleoli","journal":"Current biology : CB","confidence":"Medium","confidence_rationale":"Tier 2 — direct biochemical fractionation and MS identification, but no functional follow-up in this paper","pmids":["11790298"],"is_preprint":false},{"year":2009,"finding":"Mrt4 is a nuclear/nucleolar paralogue of the ribosomal stalk protein P0 that binds the same site on 25S rRNA (GAR domain); Mrt4 and P0 cannot bind simultaneously to ribosomes, and Mrt4 controls the position and timing of P0 assembly into pre-60S particles. Loss of Mrt4 causes 60S subunit deficits due to impaired 27S rRNA processing.","method":"TAP-tag purification of pre-60S complexes; sucrose gradient sedimentation; rRNA processing analysis; genetic depletion of P0 and Mrt4","journal":"Nucleic acids research","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods (TAP, gradients, rRNA processing), replicated across two independent studies","pmids":["19789271"],"is_preprint":false},{"year":2009,"finding":"The dual-specificity phosphatase Yvh1 is required for release of Mrt4 from pre-60S subunits in the cytoplasm; Yvh1 binds pre-60S to displace Mrt4, after which P0 loads to assemble the mature stalk and Yvh1 is released. A Mrt4 mutation at the protein-RNA interface (bypassing Yvh1 requirement) confirmed the mechanism.","method":"Genetic deletion of YVH1; sucrose gradient sedimentation; TAP purification of pre-60S particles; mutagenesis of Mrt4 RNA-binding interface","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 2 — genetic epistasis combined with mutagenesis bypass and biochemical fractionation, strongly replicated by companion paper","pmids":["19797078"],"is_preprint":false},{"year":2009,"finding":"The N-terminal domain (first 137 aa) of Mrt4 is sufficient for rRNA binding and can partially replace the RNA-binding domain of P0; a Mrt4-P0 chimera partially complements P0 absence but shows weaker ribosome binding and reduced association of P1/P2 and L12 stalk components. Molecular dynamics simulations showed both proteins bind similarly to rRNA but differ in L12 interaction.","method":"Chimeric protein construction; ribosome co-sedimentation; genetic complementation of conditional P0 null mutant; molecular dynamics simulation","journal":"Nucleic acids research","confidence":"High","confidence_rationale":"Tier 1–2 — in vitro/cellular reconstitution with mutagenesis/chimera plus computational validation","pmids":["19346338"],"is_preprint":false},{"year":2010,"finding":"Human MRTO4 localizes predominantly to the nucleolus (unlike cytoplasmic P0/P1/P2) and binds the same site on the large ribosomal subunit as P0 but cannot functionally complement P0 absence. Nuclear import of hMrt4 depends on a short N-terminal sequence, and its nucleolar retention occurs via interaction with rRNA rather than a dedicated nucleolus localization signal. Actinomycin D and leptomycin B experiments show hMrt4 shuttles between nucleus and cytoplasm and acts as a trans-acting ribosome maturation factor.","method":"Fluorescence microscopy of hybrid hMrt4-P0 proteins; biochemical fractionation; pharmacological inhibition with actinomycin D and leptomycin B; ribosome co-sedimentation","journal":"The international journal of biochemistry & cell biology","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods (live imaging, fractionation, inhibitors, functional complementation) in single study","pmids":["20083226"],"is_preprint":false},{"year":2010,"finding":"RNAi knockdown of the human MRTO4 ortholog in a systematic screen confirmed its functional contribution to 60S ribosomal subunit biogenesis in human cells, consistent with its yeast role.","method":"Systematic RNAi screen using fluorescent ribosomal protein reporters and automated image analysis in human cells","journal":"PLoS biology","confidence":"Medium","confidence_rationale":"Tier 2 — systematic RNAi with imaging readout, but MRTO4 is one of 153 hits, no deep mechanistic follow-up","pmids":["21048991"],"is_preprint":false},{"year":2011,"finding":"Genetic interaction analysis in yeast showed that the Mrt4(G68D) mutation (at the protein-RNA interface) suppresses all phenotypes caused by YVH1 deletion, including defects in rRNA biogenesis, mRNA decay, glycogen accumulation, and induction of sporulation genes IME2, SPO13, and HOP1, demonstrating that the Mrt4-Yvh1 interaction is essential for these diverse cellular functions.","method":"Dominant suppressor genetic screen; yeast growth assays; rRNA processing analysis; mRNA decay assays; glycogen staining; sporulation gene expression analysis","journal":"Journal of biochemistry","confidence":"Medium","confidence_rationale":"Tier 2 — genetic epistasis with multiple phenotypic readouts, single lab","pmids":["21474464"],"is_preprint":false},{"year":2012,"finding":"MRTO4 was identified as a component of the human mRNA-bound proteome by UV crosslinking and oligo(dT) capture in HeLa cells, establishing it as an RNA-binding protein with direct mRNA contacts.","method":"Interactome capture — UV crosslinking of RBPs to mRNA followed by oligo(dT) purification and quantitative mass spectrometry","journal":"Cell","confidence":"Medium","confidence_rationale":"Tier 2 — direct covalent crosslinking-based biochemical identification, large systematic study","pmids":["22658674"],"is_preprint":false},{"year":2015,"finding":"Human MRTO4 undergoes phosphorylation in vivo; serines S229, S233, and S235 within its acidic C-terminal extension are phosphorylated by CK2 kinase in vitro. This phosphorylation does not alter steady-state subcellular distribution but regulates MRTO4 molecular behavior during actinomycin D-induced nucleolar stress, implicating the C-terminal region as a regulatory element linking ribosome biogenesis to stress response.","method":"In vivo phosphorylation detection; in vitro CK2 kinase assay with phosphosite mutagenesis; fluorescence microscopy under nucleolar stress conditions","journal":"The international journal of biochemistry & cell biology","confidence":"Medium","confidence_rationale":"Tier 2 — in vitro kinase assay with site-directed mutagenesis plus in vivo validation, single lab","pmids":["26494001"],"is_preprint":false},{"year":2024,"finding":"MRTO4 promotes glycolysis in hepatocellular carcinoma cells by inhibiting ALDOB (aldolase B); knockdown of MRTO4 reduces glycolytic flux, proliferation, and invasion while increasing apoptosis, and overexpression has opposite effects, placing MRTO4 upstream of ALDOB in metabolic reprogramming.","method":"RT-qPCR; Western blotting; CCK8 proliferation assay; TUNEL apoptosis assay; clone formation; Transwell invasion assay; ELISA; glycolysis measurements; loss- and gain-of-function experiments","journal":"Medical science monitor","confidence":"Medium","confidence_rationale":"Tier 3 — multiple cellular assays with defined phenotype and pathway placement, single lab, no in vitro reconstitution","pmids":["38778508"],"is_preprint":false},{"year":2026,"finding":"Fungal Mrt4 (CaMrt4) binds rRNA through cysteine residues C96 and C189; cis-fumaramidmycin-derived compounds selectively inhibit CaMrt4-rRNA interaction by covalently engaging both cysteines, disrupting ribosome assembly specifically in fungi without affecting human MRTO4-rRNA interaction, validating Mrt4-rRNA binding as an antifungal target.","method":"Active-and-inactive-based protein profiling (AIBPP); chemical-genetic profiling; fluorescence polarization assay; in vivo Galleria mellonella and murine candidiasis models","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1–2 — covalent target engagement biochemistry, fluorescence polarization binding assay, multiple in vivo models, identifies specific residues","pmids":["41781388"],"is_preprint":false}],"current_model":"MRTO4/Mrt4 is a nuclear/nucleolar paralogue of the ribosomal stalk protein P0 that binds the 25S/28S rRNA GAR domain to occupy the P0 site on assembling pre-60S particles; it is displaced from cytoplasmic pre-60S subunits by the dual-specificity phosphatase Yvh1, enabling P0 to load and complete stalk assembly, with its N-terminal domain mediating rRNA binding, its C-terminal serines (S229/S233/S235) phosphorylated by CK2 to regulate behavior under nucleolar stress, and an additional cytoplasmic function in promoting glycolysis via ALDOB inhibition in cancer cells."},"narrative":{"teleology":[{"year":1999,"claim":"The initial identification of MRT4 linked it to mRNA turnover rather than ribosome biogenesis, establishing that loss of this gene specifically impairs mRNA decay in yeast.","evidence":"Genetic screen with ts mutants; Northern blot mRNA decay assays in S. cerevisiae","pmids":["10471698"],"confidence":"Medium","gaps":["Mechanism connecting MRT4 to mRNA decay was unknown","No direct biochemical activity assigned","Relationship to ribosome function not yet tested"]},{"year":2002,"claim":"Proteomic cataloguing placed human MRTO4 in the nucleolus, suggesting a role in ribosome biogenesis or nucleolar RNA metabolism rather than exclusively cytoplasmic mRNA decay.","evidence":"Mass spectrometry of purified HeLa nucleoli","pmids":["11790298"],"confidence":"Medium","gaps":["No functional characterization accompanied the localization data","Relationship to yeast Mrt4 function not established"]},{"year":2009,"claim":"Three concurrent studies resolved MRTO4/Mrt4's core mechanism: it is a P0 paralogue that occupies the same rRNA-binding site on pre-60S particles, its N-terminal domain mediates rRNA binding, and the phosphatase Yvh1 displaces it from cytoplasmic pre-60S subunits to enable P0 loading and stalk maturation.","evidence":"TAP purification of pre-60S complexes; sucrose gradient sedimentation; chimeric Mrt4-P0 complementation; Yvh1 deletion and Mrt4 bypass mutagenesis; molecular dynamics simulation","pmids":["19789271","19797078","19346338"],"confidence":"High","gaps":["Structural basis of Yvh1-mediated displacement not resolved at atomic level","Whether Yvh1 catalytic activity or physical binding drives Mrt4 release was uncertain","Human-specific validation of the Yvh1-MRTO4 axis not yet performed"]},{"year":2010,"claim":"Human MRTO4 was confirmed as a nucleolar, nucleocytoplasmic-shuttling ribosome maturation factor that binds the large subunit like P0 but cannot substitute for it, and systematic RNAi validated its role in 60S biogenesis in human cells.","evidence":"Fluorescence microscopy with actinomycin D and leptomycin B; ribosome co-sedimentation; systematic RNAi screen with ribosomal protein reporters","pmids":["20083226","21048991"],"confidence":"High","gaps":["Mechanism of nucleolar retention via rRNA versus protein-protein interactions not fully dissected","Contribution to specific pre-rRNA processing steps in human cells unknown"]},{"year":2011,"claim":"The Mrt4-Yvh1 interaction was shown to underlie diverse YVH1 deletion phenotypes including mRNA decay, glycogen accumulation, and sporulation gene induction, broadening the physiological scope of Mrt4 displacement beyond ribosome assembly.","evidence":"Dominant suppressor screen with Mrt4(G68D); multiple phenotypic assays in S. cerevisiae","pmids":["21474464"],"confidence":"Medium","gaps":["Whether these pleiotropic phenotypes arise from defective ribosome maturation or separate Mrt4 functions was unresolved","No direct test of stalk composition in suppressed strains"]},{"year":2012,"claim":"UV crosslinking-based interactome capture demonstrated that MRTO4 directly contacts poly(A)+ mRNA in human cells, consistent with an RNA-binding function extending beyond rRNA.","evidence":"UV crosslinking and oligo(dT) capture followed by quantitative mass spectrometry in HeLa cells","pmids":["22658674"],"confidence":"Medium","gaps":["Identity of bound mRNA targets unknown","Functional consequence of mRNA binding not tested"]},{"year":2015,"claim":"CK2-mediated phosphorylation of the MRTO4 C-terminal acidic extension (S229/S233/S235) was identified as a regulatory modification that modulates MRTO4 behavior during nucleolar stress without altering steady-state localization.","evidence":"In vitro CK2 kinase assay with phosphosite mutagenesis; in vivo phosphorylation detection; fluorescence microscopy under actinomycin D stress","pmids":["26494001"],"confidence":"Medium","gaps":["Downstream effectors of phosphorylation unknown","Whether phosphorylation affects rRNA or pre-60S binding affinity not tested","In vivo kinase responsible not confirmed by knockdown/inhibition"]},{"year":2024,"claim":"A non-ribosomal function was described in hepatocellular carcinoma, where MRTO4 promotes glycolysis and tumor phenotypes by inhibiting ALDOB, expanding MRTO4's roles to metabolic reprogramming.","evidence":"Loss- and gain-of-function experiments with glycolysis measurements, proliferation, apoptosis, and invasion assays in HCC cell lines","pmids":["38778508"],"confidence":"Medium","gaps":["Mechanism of ALDOB inhibition (direct binding vs. indirect) not established","Whether this function is independent of ribosome biogenesis activity is unclear","Single-lab finding without in vivo tumor model validation"]},{"year":2026,"claim":"Chemical biology targeting of fungal Mrt4's rRNA-binding interface identified species-selective covalent inhibitors engaging C96/C189, validating the Mrt4-rRNA interaction as druggable and revealing that human MRTO4 is insensitive to these compounds due to structural divergence.","evidence":"AIBPP chemical proteomics; fluorescence polarization binding assay; Galleria mellonella and murine candidiasis infection models","pmids":["41781388"],"confidence":"High","gaps":["No high-resolution co-crystal structure of inhibitor-Mrt4 complex","Whether human MRTO4 is targetable by alternative scaffolds is unexplored"]},{"year":null,"claim":"Key open questions include the structural basis of Yvh1-mediated MRTO4 displacement in human cells, whether MRTO4's mRNA-binding activity serves a function beyond ribosome assembly, and the molecular mechanism linking MRTO4 to ALDOB inhibition and glycolytic regulation.","evidence":"","pmids":[],"confidence":"Low","gaps":["No atomic-resolution structure of human MRTO4 on a pre-60S particle","mRNA targets of MRTO4 remain unidentified","ALDOB inhibition mechanism (direct vs. indirect) unresolved"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0003723","term_label":"RNA binding","supporting_discovery_ids":[2,4,5,8,11]},{"term_id":"GO:0005198","term_label":"structural molecule activity","supporting_discovery_ids":[2,3,5]}],"localization":[{"term_id":"GO:0005730","term_label":"nucleolus","supporting_discovery_ids":[1,5]},{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[2,5]},{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[3,5]}],"pathway":[{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[2,3,5,6]},{"term_id":"R-HSA-8953854","term_label":"Metabolism of RNA","supporting_discovery_ids":[2,5,6]}],"complexes":["pre-60S ribosomal particle"],"partners":["RPSA","YVH1","RPLP0","ALDOB","CSNK2A1"],"other_free_text":[]},"mechanistic_narrative":"MRTO4 is a trans-acting ribosome assembly factor that occupies the ribosomal P0-binding site on pre-60S particles to regulate the timing and fidelity of stalk assembly during large ribosomal subunit biogenesis. As a structural paralogue of P0, MRTO4 binds the 25S/28S rRNA GAR domain through its N-terminal domain, preventing premature P0 loading; in the cytoplasm, the phosphatase Yvh1 displaces MRTO4 from pre-60S subunits to allow P0 incorporation and mature stalk formation [PMID:19346338, PMID:19797078, PMID:20083226]. MRTO4 localizes predominantly to the nucleolus via rRNA interaction, shuttles between nucleus and cytoplasm, and its C-terminal serines are phosphorylated by CK2 to modulate its behavior during nucleolar stress [PMID:20083226, PMID:26494001]. An additional cytoplasmic role has been described in hepatocellular carcinoma cells, where MRTO4 promotes glycolysis by inhibiting aldolase B (ALDOB) [PMID:38778508]."},"prefetch_data":{"uniprot":{"accession":"Q9UKD2","full_name":"mRNA turnover protein 4 homolog","aliases":["Ribosome assembly factor MRTO4"],"length_aa":239,"mass_kda":27.6,"function":"Component of the ribosome assembly machinery. Nuclear paralog of the ribosomal protein P0, it binds pre-60S subunits at an early stage of assembly in the nucleolus, and is replaced by P0 in cytoplasmic pre-60S subunits and mature 80S ribosomes","subcellular_location":"Nucleus, nucleolus; Cytoplasm","url":"https://www.uniprot.org/uniprotkb/Q9UKD2/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":true,"resolved_as":"","url":"https://depmap.org/portal/gene/MRTO4","classification":"Common Essential","n_dependent_lines":1141,"n_total_lines":1208,"dependency_fraction":0.9445364238410596},"opencell":{"profiled":true,"resolved_as":"","ensg_id":"ENSG00000053372","cell_line_id":"CID001001","localizations":[{"compartment":"nucleolus_gc","grade":3}],"interactors":[{"gene":"CAD","stoichiometry":10.0},{"gene":"RPL37A","stoichiometry":10.0}],"url":"https://opencell.sf.czbiohub.org/target/CID001001","total_profiled":1310},"omim":[{"mim_id":"620476","title":"MRT4 HOMOLOG, RIBOSOME MATURATION FACTOR; MRTO4","url":"https://www.omim.org/entry/620476"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Nucleoplasm","reliability":"Approved"},{"location":"Nuclear membrane","reliability":"Approved"},{"location":"Nucleoli","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/MRTO4"},"hgnc":{"alias_symbol":["dJ657E11.4","MRT4"],"prev_symbol":["C1orf33"]},"alphafold":{"accession":"Q9UKD2","domains":[{"cath_id":"3.30.70.1730","chopping":"25-124_198-216","consensus_level":"high","plddt":95.7546,"start":25,"end":216},{"cath_id":"3.90.105.20","chopping":"128-194","consensus_level":"high","plddt":96.7222,"start":128,"end":194}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9UKD2","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9UKD2-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9UKD2-F1-predicted_aligned_error_v6.png","plddt_mean":89.38},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=MRTO4","jax_strain_url":"https://www.jax.org/strain/search?query=MRTO4"},"sequence":{"accession":"Q9UKD2","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9UKD2.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9UKD2/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9UKD2"}},"corpus_meta":[{"pmid":"19797078","id":"PMC_19797078","title":"Ribosome 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biology","url":"https://pubmed.ncbi.nlm.nih.gov/32989298","citation_count":129,"is_preprint":false,"source_track":"gene2pubmed"}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":7590,"output_tokens":2528,"usd":0.030345},"stage2":{"model":"claude-opus-4-6","input_tokens":5861,"output_tokens":2355,"usd":0.13227},"total_usd":0.364633,"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":19209,"output_tokens":3630,"usd":0.056038},"round2_rules_fired":"R3","round2_stage2":{"model":"claude-opus-4-6","input_tokens":6549,"output_tokens":2583,"usd":0.14598}},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1999,\n      \"finding\": \"MRT4 was identified as a factor involved in mRNA turnover; temperature-sensitive mutations in MRT4 caused defects in mRNA decay without impairing protein synthesis, suggesting a specific role in mRNA degradation distinct from general translation.\",\n      \"method\": \"Genetic screen, complementation analysis, mRNA decay assays in temperature-sensitive yeast mutants\",\n      \"journal\": \"Genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic/biochemical in yeast, single lab, multiple mRNA substrates tested\",\n      \"pmids\": [\"10471698\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Mrt4 is a nuclear paralogue of P0 (ribosomal stalk protein) that binds the same site on 25S rRNA (GAR domain) as P0; Mrt4 and P0 cannot bind ribosomes simultaneously, establishing Mrt4 as a placeholder that occupies the P0 binding site on pre-60S subunits.\",\n      \"method\": \"Co-immunoprecipitation, ribosome sedimentation, chimeric protein complementation, molecular dynamics simulation\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — replicated across two independent labs (PMID 19346338, 19789271) with orthogonal methods\",\n      \"pmids\": [\"19346338\", \"19789271\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"The dual-specificity phosphatase Yvh1 is required for release of Mrt4 from pre-60S subunits in the cytoplasm; Yvh1 binds pre-60S subunits to displace Mrt4, after which P0 loads onto the subunit to assemble the mature stalk, and Yvh1 is then released.\",\n      \"method\": \"Genetic deletion analysis, co-immunoprecipitation, sucrose gradient sedimentation, suppressor mutation mapping\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — two independent labs (PMID 19797078, 19789271) with reciprocal epistasis and multiple orthogonal methods\",\n      \"pmids\": [\"19797078\", \"19789271\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"A mutation in Mrt4 at the protein-RNA interface (G68D) bypasses the requirement for Yvh1, indicating that Yvh1 functions to disrupt the Mrt4-rRNA interaction to release Mrt4 from pre-60S particles.\",\n      \"method\": \"Suppressor mutation screen, in vivo complementation in yvh1Δ yeast\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — mechanistic epistasis confirmed by two independent labs (PMID 19797078, 21474464)\",\n      \"pmids\": [\"19797078\", \"21474464\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Mrt4 controls the position and timing of P0 assembly: loss of Mrt4 allows P0 to assemble into medium pre-60S (nuclear) particles prematurely, while loss of P0 causes Mrt4 to accumulate in the cytoplasm within aberrant 60S subunits, demonstrating that P0 is required for cytoplasmic release of Mrt4.\",\n      \"method\": \"TAP-tagging, sucrose gradient sedimentation, co-immunoprecipitation, fluorescence microscopy in yeast deletion mutants\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods, reciprocal genetic perturbations, consistent with independent findings\",\n      \"pmids\": [\"19789271\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"The first 137 amino acids of Mrt4 are sufficient to bind the same rRNA site as P0 on the large ribosomal subunit; a chimera of Mrt4's N-terminal domain fused to P0's C-terminal domain partially complements P0 deletion, demonstrating that the Mrt4 N-terminal domain mediates rRNA binding.\",\n      \"method\": \"Chimeric protein construction, ribosome co-sedimentation, functional complementation in yeast\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — domain-swap mutagenesis with functional readout and molecular dynamics simulation\",\n      \"pmids\": [\"19346338\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Human MRTO4 (hMrt4) localizes predominantly to the nucleolar compartment via interaction with rRNA, not via a specific nucleolus localization signal; nuclear import depends on a short sequence in the N-terminal domain; hMrt4 cannot functionally replace P0 despite binding the same ribosomal site.\",\n      \"method\": \"Fluorescence microscopy, biochemical fractionation, inhibitor treatments (actinomycin D, leptomycin B), hybrid protein analysis\",\n      \"journal\": \"The international journal of biochemistry & cell biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct localization with functional inhibitors, single lab\",\n      \"pmids\": [\"20083226\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Human MRTO4 undergoes phosphorylation in vivo; serines S229, S233, and S235 within its C-terminal acidic extension are phosphorylated by CK2 kinase in vitro; this phosphorylation does not alter normal subcellular distribution but affects MRTO4 behavior during nucleolar stress induced by actinomycin D.\",\n      \"method\": \"In vitro kinase assay with CK2, in vivo phosphorylation detection, site-directed mutagenesis, fluorescence microscopy under nucleolar stress\",\n      \"journal\": \"The international journal of biochemistry & cell biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — in vitro kinase assay plus in vivo validation, single lab\",\n      \"pmids\": [\"26494001\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"The Mrt4(G68D) mutation, which reduces Mrt4-rRNA binding, restores not only ribosome biogenesis but also glycogen accumulation, mRNA decay, and sporulation gene induction defects caused by YVH1 deletion, placing Yvh1-Mrt4 ribosome maturation upstream of these cellular processes.\",\n      \"method\": \"Genetic suppressor analysis, glycogen accumulation assays, mRNA decay assays, gene expression analysis in yeast\",\n      \"journal\": \"Journal of biochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis with multiple phenotypic readouts, single lab\",\n      \"pmids\": [\"21474464\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"MRTO4 promotes glycolysis and HCC progression by inhibiting ALDOB; knockdown or overexpression experiments coupled with glycolysis assays defined this mechanistic axis.\",\n      \"method\": \"RT-qPCR, Western blotting, CCK8, TUNEL, clone formation, Transwell assay, ELISA, immunohistochemistry\",\n      \"journal\": \"Medical science monitor\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — single lab, no direct binding or reconstitution of MRTO4-ALDOB interaction\",\n      \"pmids\": [\"38778508\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"Fungal Mrt4 (CaMrt4) binds rRNA via cysteines C96 and C189; covalent small molecules targeting these residues selectively inhibit CaMrt4-rRNA interaction and disrupt ribosomal assembly in fungi, with no equivalent activity on human MRTO4-rRNA interaction, validating the rRNA-binding interface as a druggable and functionally essential site.\",\n      \"method\": \"Activity-based protein profiling (AIBPP), chemical-genetic profiling, fluorescence polarization assay, antifungal efficacy in larvae and murine models\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1-2 — biochemical binding assay plus chemical-genetic profiling and in vivo validation, but single study\",\n      \"pmids\": [\"41781388\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"MRTO4/Mrt4 is a nuclear paralogue of ribosomal stalk protein P0 that occupies the P0-binding site on 25S rRNA within pre-60S particles to serve as a placeholder during ribosome assembly; in the cytoplasm, the dual-specificity phosphatase Yvh1 binds pre-60S subunits to displace Mrt4 (by disrupting its rRNA interaction), enabling P0 to load and form the mature stalk, after which Yvh1 dissociates; in human cells, MRTO4 localizes to the nucleolus via rRNA interaction with its N-terminal domain mediating nuclear import, and its C-terminal serines are phosphorylated by CK2 to regulate its behavior under nucleolar stress.\"\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\nI'll classify each paper before extracting discoveries.\n\n**Classification:**\n\nRETRIEVED PAPERS:\n1. PMID:19797078 - KEEP (yeast Mrt4, ortholog, ribosome assembly mechanism)\n2. PMID:19789271 - KEEP (yeast Mrt4, ortholog, ribosome assembly mechanism)\n3. PMID:10471698 - KEEP (yeast MRT4, original discovery in mRNA turnover/ribosome)\n4. PMID:19346338 - KEEP (yeast Mrt4/P0 chimera, rRNA binding mechanism)\n5. PMID:20083226 - KEEP (human Mrt4/MRTO4, localization and function)\n6. PMID:26494001 - KEEP (human MRTO4, phosphorylation by CK2)\n7. PMID:21474464 - KEEP (yeast Mrt4/Yvh1 genetic interactions)\n8. PMID:19819847 - EXCLUDE (expression study only, no mechanistic findings)\n9. PMID:38778508 - KEEP (human MRTO4, glycolysis/ALDOB mechanism)\n10. PMID:35387555 - KEEP (yeast Mrt4 biophysical characterization)\n11. PMID:39969382 - EXCLUDE (bioinformatics/expression, no mechanism)\n12. PMID:41364147 - EXCLUDE (bioinformatics/expression, no mechanism)\n13. PMID:41781388 - KEEP (Mrt4-rRNA interaction, antifungal mechanism)\n\nADDITIONAL PAPERS (gene2pubmed):\n- Most are large proteomics/interactome screens or unrelated topics. Papers mentioning MRTO4 in large screens (BioPlex, interactome, nucleolus proteomics) provide localization/interaction data worth noting if MRTO4 is specifically called out.\n- PMID:22658674 - KEEP (mRNA interactome capture - MRTO4 identified as RBP)\n- PMID:11790298 - KEEP (nucleolar proteomics - MRTO4 identified in nucleolus)\n- PMID:12429849 - KEEP (nucleolar proteomics)\n- PMID:21048991 - KEEP (RNAi screen for ribosome biogenesis, human homologs)\n- PMID:26186194, 28514442, 33961781 - KEEP (BioPlex interactome, MRTO4 interactions)\n- Others (Ki-67, AURORA-A, EED, integrins, UFMylation, mitochondria, etc.) - EXCLUDE (unrelated to MRTO4)\n\n```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1999,\n      \"finding\": \"MRT4 was identified as a novel yeast gene required for mRNA turnover; temperature-sensitive mrt4 mutants exhibit defects in decay of multiple mRNAs without a significant defect in protein synthesis, suggesting a specific role in mRNA decay rather than general translation.\",\n      \"method\": \"Genetic screen for mRNA turnover defects; complementation analysis; Northern blot analysis of mRNA decay rates in ts mutants\",\n      \"journal\": \"Genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis and complementation with specific mRNA decay phenotype, single lab\",\n      \"pmids\": [\"10471698\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"MRTO4 (human Mrt4) was identified as a component of the human nucleolar proteome, establishing its nucleolar localization in human cells.\",\n      \"method\": \"Mass spectrometry-based proteomic analysis of isolated human HeLa cell nucleoli\",\n      \"journal\": \"Current biology : CB\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct biochemical fractionation and MS identification, but no functional follow-up in this paper\",\n      \"pmids\": [\"11790298\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Mrt4 is a nuclear/nucleolar paralogue of the ribosomal stalk protein P0 that binds the same site on 25S rRNA (GAR domain); Mrt4 and P0 cannot bind simultaneously to ribosomes, and Mrt4 controls the position and timing of P0 assembly into pre-60S particles. Loss of Mrt4 causes 60S subunit deficits due to impaired 27S rRNA processing.\",\n      \"method\": \"TAP-tag purification of pre-60S complexes; sucrose gradient sedimentation; rRNA processing analysis; genetic depletion of P0 and Mrt4\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (TAP, gradients, rRNA processing), replicated across two independent studies\",\n      \"pmids\": [\"19789271\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"The dual-specificity phosphatase Yvh1 is required for release of Mrt4 from pre-60S subunits in the cytoplasm; Yvh1 binds pre-60S to displace Mrt4, after which P0 loads to assemble the mature stalk and Yvh1 is released. A Mrt4 mutation at the protein-RNA interface (bypassing Yvh1 requirement) confirmed the mechanism.\",\n      \"method\": \"Genetic deletion of YVH1; sucrose gradient sedimentation; TAP purification of pre-60S particles; mutagenesis of Mrt4 RNA-binding interface\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis combined with mutagenesis bypass and biochemical fractionation, strongly replicated by companion paper\",\n      \"pmids\": [\"19797078\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"The N-terminal domain (first 137 aa) of Mrt4 is sufficient for rRNA binding and can partially replace the RNA-binding domain of P0; a Mrt4-P0 chimera partially complements P0 absence but shows weaker ribosome binding and reduced association of P1/P2 and L12 stalk components. Molecular dynamics simulations showed both proteins bind similarly to rRNA but differ in L12 interaction.\",\n      \"method\": \"Chimeric protein construction; ribosome co-sedimentation; genetic complementation of conditional P0 null mutant; molecular dynamics simulation\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — in vitro/cellular reconstitution with mutagenesis/chimera plus computational validation\",\n      \"pmids\": [\"19346338\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Human MRTO4 localizes predominantly to the nucleolus (unlike cytoplasmic P0/P1/P2) and binds the same site on the large ribosomal subunit as P0 but cannot functionally complement P0 absence. Nuclear import of hMrt4 depends on a short N-terminal sequence, and its nucleolar retention occurs via interaction with rRNA rather than a dedicated nucleolus localization signal. Actinomycin D and leptomycin B experiments show hMrt4 shuttles between nucleus and cytoplasm and acts as a trans-acting ribosome maturation factor.\",\n      \"method\": \"Fluorescence microscopy of hybrid hMrt4-P0 proteins; biochemical fractionation; pharmacological inhibition with actinomycin D and leptomycin B; ribosome co-sedimentation\",\n      \"journal\": \"The international journal of biochemistry & cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (live imaging, fractionation, inhibitors, functional complementation) in single study\",\n      \"pmids\": [\"20083226\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"RNAi knockdown of the human MRTO4 ortholog in a systematic screen confirmed its functional contribution to 60S ribosomal subunit biogenesis in human cells, consistent with its yeast role.\",\n      \"method\": \"Systematic RNAi screen using fluorescent ribosomal protein reporters and automated image analysis in human cells\",\n      \"journal\": \"PLoS biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — systematic RNAi with imaging readout, but MRTO4 is one of 153 hits, no deep mechanistic follow-up\",\n      \"pmids\": [\"21048991\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Genetic interaction analysis in yeast showed that the Mrt4(G68D) mutation (at the protein-RNA interface) suppresses all phenotypes caused by YVH1 deletion, including defects in rRNA biogenesis, mRNA decay, glycogen accumulation, and induction of sporulation genes IME2, SPO13, and HOP1, demonstrating that the Mrt4-Yvh1 interaction is essential for these diverse cellular functions.\",\n      \"method\": \"Dominant suppressor genetic screen; yeast growth assays; rRNA processing analysis; mRNA decay assays; glycogen staining; sporulation gene expression analysis\",\n      \"journal\": \"Journal of biochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis with multiple phenotypic readouts, single lab\",\n      \"pmids\": [\"21474464\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"MRTO4 was identified as a component of the human mRNA-bound proteome by UV crosslinking and oligo(dT) capture in HeLa cells, establishing it as an RNA-binding protein with direct mRNA contacts.\",\n      \"method\": \"Interactome capture — UV crosslinking of RBPs to mRNA followed by oligo(dT) purification and quantitative mass spectrometry\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct covalent crosslinking-based biochemical identification, large systematic study\",\n      \"pmids\": [\"22658674\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Human MRTO4 undergoes phosphorylation in vivo; serines S229, S233, and S235 within its acidic C-terminal extension are phosphorylated by CK2 kinase in vitro. This phosphorylation does not alter steady-state subcellular distribution but regulates MRTO4 molecular behavior during actinomycin D-induced nucleolar stress, implicating the C-terminal region as a regulatory element linking ribosome biogenesis to stress response.\",\n      \"method\": \"In vivo phosphorylation detection; in vitro CK2 kinase assay with phosphosite mutagenesis; fluorescence microscopy under nucleolar stress conditions\",\n      \"journal\": \"The international journal of biochemistry & cell biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — in vitro kinase assay with site-directed mutagenesis plus in vivo validation, single lab\",\n      \"pmids\": [\"26494001\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"MRTO4 promotes glycolysis in hepatocellular carcinoma cells by inhibiting ALDOB (aldolase B); knockdown of MRTO4 reduces glycolytic flux, proliferation, and invasion while increasing apoptosis, and overexpression has opposite effects, placing MRTO4 upstream of ALDOB in metabolic reprogramming.\",\n      \"method\": \"RT-qPCR; Western blotting; CCK8 proliferation assay; TUNEL apoptosis assay; clone formation; Transwell invasion assay; ELISA; glycolysis measurements; loss- and gain-of-function experiments\",\n      \"journal\": \"Medical science monitor\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — multiple cellular assays with defined phenotype and pathway placement, single lab, no in vitro reconstitution\",\n      \"pmids\": [\"38778508\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"Fungal Mrt4 (CaMrt4) binds rRNA through cysteine residues C96 and C189; cis-fumaramidmycin-derived compounds selectively inhibit CaMrt4-rRNA interaction by covalently engaging both cysteines, disrupting ribosome assembly specifically in fungi without affecting human MRTO4-rRNA interaction, validating Mrt4-rRNA binding as an antifungal target.\",\n      \"method\": \"Active-and-inactive-based protein profiling (AIBPP); chemical-genetic profiling; fluorescence polarization assay; in vivo Galleria mellonella and murine candidiasis models\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — covalent target engagement biochemistry, fluorescence polarization binding assay, multiple in vivo models, identifies specific residues\",\n      \"pmids\": [\"41781388\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"MRTO4/Mrt4 is a nuclear/nucleolar paralogue of the ribosomal stalk protein P0 that binds the 25S/28S rRNA GAR domain to occupy the P0 site on assembling pre-60S particles; it is displaced from cytoplasmic pre-60S subunits by the dual-specificity phosphatase Yvh1, enabling P0 to load and complete stalk assembly, with its N-terminal domain mediating rRNA binding, its C-terminal serines (S229/S233/S235) phosphorylated by CK2 to regulate behavior under nucleolar stress, and an additional cytoplasmic function in promoting glycolysis via ALDOB inhibition in cancer cells.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"MRTO4 (Mrt4 in yeast) is a ribosome assembly factor that functions as a placeholder on pre-60S ribosomal subunits, occupying the P0 (ribosomal stalk protein) binding site on 25S/28S rRNA to prevent premature P0 loading during nuclear stages of large subunit maturation [PMID:19346338, PMID:19789271]. In the cytoplasm, the dual-specificity phosphatase Yvh1 binds pre-60S particles and disrupts the Mrt4–rRNA interaction, releasing Mrt4 and enabling P0 assembly into the mature ribosomal stalk; a suppressor mutation (G68D) at the Mrt4–rRNA interface bypasses the Yvh1 requirement, confirming that Yvh1 acts by destabilizing this protein–RNA contact [PMID:19797078, PMID:21474464]. Human MRTO4 localizes to the nucleolus through its rRNA interaction, with nuclear import mediated by an N-terminal sequence and C-terminal serines phosphorylated by CK2 to modulate its behavior during nucleolar stress [PMID:20083226, PMID:26494001]. The rRNA-binding interface of fungal Mrt4, mediated by specific cysteine residues, is druggable by covalent small molecules that selectively disrupt ribosome assembly in fungi without affecting human MRTO4 [PMID:41781388].\",\n  \"teleology\": [\n    {\n      \"year\": 1999,\n      \"claim\": \"MRT4 was first linked to mRNA turnover through a genetic screen, raising the question of whether it directly participates in mRNA decay or influences it indirectly through another pathway.\",\n      \"evidence\": \"Temperature-sensitive yeast mutants showing mRNA decay defects without impaired protein synthesis\",\n      \"pmids\": [\"10471698\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"No biochemical activity in mRNA decay was demonstrated\",\n        \"Later work suggested the mRNA decay phenotype may be secondary to ribosome biogenesis defects\"\n      ]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Multiple independent studies established that Mrt4 is a structural paralogue of ribosomal stalk protein P0 that occupies the same 25S rRNA binding site on pre-60S particles, resolving its molecular identity as a ribosome assembly placeholder rather than a direct mRNA decay factor.\",\n      \"evidence\": \"Co-immunoprecipitation, ribosome co-sedimentation, chimeric protein complementation, and molecular dynamics simulation across two independent labs\",\n      \"pmids\": [\"19346338\", \"19789271\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Structural resolution of the Mrt4–rRNA complex was not yet available\",\n        \"Whether the placeholder function is conserved in human cells was not directly tested\"\n      ]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"The cytoplasmic phosphatase Yvh1 was identified as the factor that displaces Mrt4 from pre-60S subunits, enabling P0 loading and stalk assembly, thereby defining the ordered relay mechanism of 60S maturation.\",\n      \"evidence\": \"Genetic deletion, co-immunoprecipitation, suppressor mutation (Mrt4-G68D) mapping at the rRNA interface, confirmed by two independent groups\",\n      \"pmids\": [\"19797078\", \"19789271\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"The structural basis for how Yvh1 disrupts Mrt4–rRNA binding was unknown\",\n        \"Whether Yvh1's phosphatase catalytic activity is required for displacement was not resolved\"\n      ]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Domain-mapping showed the first 137 residues of Mrt4 are sufficient for rRNA binding, and chimeric Mrt4-P0 proteins partially complement P0 loss, establishing which domains mediate placeholder function versus stalk-specific activity.\",\n      \"evidence\": \"Chimeric protein construction with ribosome co-sedimentation and functional complementation in yeast\",\n      \"pmids\": [\"19346338\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"The residues critical for rRNA contact were mapped only in part\",\n        \"Why chimeric complementation was only partial was not explained\"\n      ]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Characterization of human MRTO4 established that it localizes to the nucleolus through rRNA interaction, with nuclear import dependent on its N-terminal domain, extending the placeholder model to mammalian cells.\",\n      \"evidence\": \"Fluorescence microscopy, biochemical fractionation, actinomycin D and leptomycin B inhibitor treatments in human cells\",\n      \"pmids\": [\"20083226\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Whether the Yvh1-mediated release mechanism is conserved in human cells was not tested\",\n        \"The human pre-60S particle associations of MRTO4 were not biochemically characterized\"\n      ]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"The Mrt4-G68D suppressor mutation rescued not only ribosome biogenesis but also glycogen accumulation, mRNA decay, and sporulation defects of yvh1Δ cells, demonstrating that these pleiotropic phenotypes are downstream consequences of defective ribosome maturation rather than independent Yvh1 functions.\",\n      \"evidence\": \"Genetic suppressor analysis with glycogen, mRNA decay, and gene expression assays in yeast\",\n      \"pmids\": [\"21474464\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"The precise signaling pathways connecting ribosome maturation to glycogen and mRNA metabolism were not delineated\",\n        \"Single-lab study without independent replication\"\n      ]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Identification of CK2-mediated phosphorylation at C-terminal serines of human MRTO4 revealed a regulatory modification that modulates MRTO4 behavior specifically under nucleolar stress, adding a post-translational regulatory layer to the placeholder model.\",\n      \"evidence\": \"In vitro CK2 kinase assay, in vivo phosphorylation detection, site-directed mutagenesis, fluorescence microscopy under actinomycin D stress\",\n      \"pmids\": [\"26494001\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"How phosphorylation mechanistically alters MRTO4 function or interactions during stress was not determined\",\n        \"Whether CK2 phosphorylation affects Mrt4 release from pre-60S particles is unknown\"\n      ]\n    },\n    {\n      \"year\": 2026,\n      \"claim\": \"Chemical targeting of fungal Mrt4 via cysteines C96 and C189 at the rRNA-binding interface validated this surface as functionally essential and druggable, while demonstrating species-selectivity over human MRTO4.\",\n      \"evidence\": \"Activity-based protein profiling, fluorescence polarization binding assay, chemical-genetic profiling, and antifungal efficacy in larvae and murine models\",\n      \"pmids\": [\"41781388\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Single study; independent replication of antifungal efficacy is needed\",\n        \"The structural basis for species selectivity between fungal and human Mrt4 is not resolved at atomic level\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key open questions include the high-resolution structure of MRTO4 bound to a human pre-60S particle, whether the Yvh1-mediated displacement mechanism is conserved in mammals, and how CK2 phosphorylation integrates with ribosome maturation and nucleolar stress responses.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\n        \"No cryo-EM or crystal structure of MRTO4 on a human pre-60S particle\",\n        \"Yvh1/DUSP12 role in human MRTO4 displacement not experimentally tested\",\n        \"Functional significance of CK2 phosphorylation for ribosome maturation is unresolved\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0003723\", \"supporting_discovery_ids\": [1, 5, 6, 10]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005730\", \"supporting_discovery_ids\": [6]},\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [1, 4, 6]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [1, 2, 4]},\n      {\"term_id\": \"R-HSA-1852241\", \"supporting_discovery_ids\": [1, 2, 3, 4]}\n    ],\n    \"complexes\": [\n      \"pre-60S ribosomal subunit\"\n    ],\n    \"partners\": [\n      \"RPP0\",\n      \"YVH1\",\n      \"CSNK2A1\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"mechanistic_narrative\": \"MRTO4 is a trans-acting ribosome assembly factor that occupies the ribosomal P0-binding site on pre-60S particles to regulate the timing and fidelity of stalk assembly during large ribosomal subunit biogenesis. As a structural paralogue of P0, MRTO4 binds the 25S/28S rRNA GAR domain through its N-terminal domain, preventing premature P0 loading; in the cytoplasm, the phosphatase Yvh1 displaces MRTO4 from pre-60S subunits to allow P0 incorporation and mature stalk formation [PMID:19346338, PMID:19797078, PMID:20083226]. MRTO4 localizes predominantly to the nucleolus via rRNA interaction, shuttles between nucleus and cytoplasm, and its C-terminal serines are phosphorylated by CK2 to modulate its behavior during nucleolar stress [PMID:20083226, PMID:26494001]. An additional cytoplasmic role has been described in hepatocellular carcinoma cells, where MRTO4 promotes glycolysis by inhibiting aldolase B (ALDOB) [PMID:38778508].\",\n  \"teleology\": [\n    {\n      \"year\": 1999,\n      \"claim\": \"The initial identification of MRT4 linked it to mRNA turnover rather than ribosome biogenesis, establishing that loss of this gene specifically impairs mRNA decay in yeast.\",\n      \"evidence\": \"Genetic screen with ts mutants; Northern blot mRNA decay assays in S. cerevisiae\",\n      \"pmids\": [\"10471698\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Mechanism connecting MRT4 to mRNA decay was unknown\",\n        \"No direct biochemical activity assigned\",\n        \"Relationship to ribosome function not yet tested\"\n      ]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Proteomic cataloguing placed human MRTO4 in the nucleolus, suggesting a role in ribosome biogenesis or nucleolar RNA metabolism rather than exclusively cytoplasmic mRNA decay.\",\n      \"evidence\": \"Mass spectrometry of purified HeLa nucleoli\",\n      \"pmids\": [\"11790298\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"No functional characterization accompanied the localization data\",\n        \"Relationship to yeast Mrt4 function not established\"\n      ]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Three concurrent studies resolved MRTO4/Mrt4's core mechanism: it is a P0 paralogue that occupies the same rRNA-binding site on pre-60S particles, its N-terminal domain mediates rRNA binding, and the phosphatase Yvh1 displaces it from cytoplasmic pre-60S subunits to enable P0 loading and stalk maturation.\",\n      \"evidence\": \"TAP purification of pre-60S complexes; sucrose gradient sedimentation; chimeric Mrt4-P0 complementation; Yvh1 deletion and Mrt4 bypass mutagenesis; molecular dynamics simulation\",\n      \"pmids\": [\"19789271\", \"19797078\", \"19346338\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Structural basis of Yvh1-mediated displacement not resolved at atomic level\",\n        \"Whether Yvh1 catalytic activity or physical binding drives Mrt4 release was uncertain\",\n        \"Human-specific validation of the Yvh1-MRTO4 axis not yet performed\"\n      ]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Human MRTO4 was confirmed as a nucleolar, nucleocytoplasmic-shuttling ribosome maturation factor that binds the large subunit like P0 but cannot substitute for it, and systematic RNAi validated its role in 60S biogenesis in human cells.\",\n      \"evidence\": \"Fluorescence microscopy with actinomycin D and leptomycin B; ribosome co-sedimentation; systematic RNAi screen with ribosomal protein reporters\",\n      \"pmids\": [\"20083226\", \"21048991\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Mechanism of nucleolar retention via rRNA versus protein-protein interactions not fully dissected\",\n        \"Contribution to specific pre-rRNA processing steps in human cells unknown\"\n      ]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"The Mrt4-Yvh1 interaction was shown to underlie diverse YVH1 deletion phenotypes including mRNA decay, glycogen accumulation, and sporulation gene induction, broadening the physiological scope of Mrt4 displacement beyond ribosome assembly.\",\n      \"evidence\": \"Dominant suppressor screen with Mrt4(G68D); multiple phenotypic assays in S. cerevisiae\",\n      \"pmids\": [\"21474464\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Whether these pleiotropic phenotypes arise from defective ribosome maturation or separate Mrt4 functions was unresolved\",\n        \"No direct test of stalk composition in suppressed strains\"\n      ]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"UV crosslinking-based interactome capture demonstrated that MRTO4 directly contacts poly(A)+ mRNA in human cells, consistent with an RNA-binding function extending beyond rRNA.\",\n      \"evidence\": \"UV crosslinking and oligo(dT) capture followed by quantitative mass spectrometry in HeLa cells\",\n      \"pmids\": [\"22658674\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Identity of bound mRNA targets unknown\",\n        \"Functional consequence of mRNA binding not tested\"\n      ]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"CK2-mediated phosphorylation of the MRTO4 C-terminal acidic extension (S229/S233/S235) was identified as a regulatory modification that modulates MRTO4 behavior during nucleolar stress without altering steady-state localization.\",\n      \"evidence\": \"In vitro CK2 kinase assay with phosphosite mutagenesis; in vivo phosphorylation detection; fluorescence microscopy under actinomycin D stress\",\n      \"pmids\": [\"26494001\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Downstream effectors of phosphorylation unknown\",\n        \"Whether phosphorylation affects rRNA or pre-60S binding affinity not tested\",\n        \"In vivo kinase responsible not confirmed by knockdown/inhibition\"\n      ]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"A non-ribosomal function was described in hepatocellular carcinoma, where MRTO4 promotes glycolysis and tumor phenotypes by inhibiting ALDOB, expanding MRTO4's roles to metabolic reprogramming.\",\n      \"evidence\": \"Loss- and gain-of-function experiments with glycolysis measurements, proliferation, apoptosis, and invasion assays in HCC cell lines\",\n      \"pmids\": [\"38778508\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Mechanism of ALDOB inhibition (direct binding vs. indirect) not established\",\n        \"Whether this function is independent of ribosome biogenesis activity is unclear\",\n        \"Single-lab finding without in vivo tumor model validation\"\n      ]\n    },\n    {\n      \"year\": 2026,\n      \"claim\": \"Chemical biology targeting of fungal Mrt4's rRNA-binding interface identified species-selective covalent inhibitors engaging C96/C189, validating the Mrt4-rRNA interaction as druggable and revealing that human MRTO4 is insensitive to these compounds due to structural divergence.\",\n      \"evidence\": \"AIBPP chemical proteomics; fluorescence polarization binding assay; Galleria mellonella and murine candidiasis infection models\",\n      \"pmids\": [\"41781388\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"No high-resolution co-crystal structure of inhibitor-Mrt4 complex\",\n        \"Whether human MRTO4 is targetable by alternative scaffolds is unexplored\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key open questions include the structural basis of Yvh1-mediated MRTO4 displacement in human cells, whether MRTO4's mRNA-binding activity serves a function beyond ribosome assembly, and the molecular mechanism linking MRTO4 to ALDOB inhibition and glycolytic regulation.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\n        \"No atomic-resolution structure of human MRTO4 on a pre-60S particle\",\n        \"mRNA targets of MRTO4 remain unidentified\",\n        \"ALDOB inhibition mechanism (direct vs. indirect) unresolved\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0003723\", \"supporting_discovery_ids\": [2, 4, 5, 8, 11]},\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [2, 3, 5]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005730\", \"supporting_discovery_ids\": [1, 5]},\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [2, 5]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [3, 5]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [2, 3, 5, 6]},\n      {\"term_id\": \"R-HSA-8953854\", \"supporting_discovery_ids\": [2, 5, 6]}\n    ],\n    \"complexes\": [\n      \"pre-60S ribosomal particle\"\n    ],\n    \"partners\": [\n      \"RPSA\",\n      \"YVH1\",\n      \"RPLP0\",\n      \"ALDOB\",\n      \"CSNK2A1\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}