{"gene":"MRTO4","run_date":"2026-06-10T02:59:51","timeline":{"discoveries":[{"year":2009,"finding":"Yvh1 (dual-specificity phosphatase) is required for the release of Mrt4 from pre-60S subunits in the cytoplasm; deletion of YVH1 causes Mrt4 to persist on cytoplasmic pre-60S subunits. A mutation in Mrt4 at the protein-RNA interface (G68D) bypasses the requirement for Yvh1, indicating Yvh1 acts by disrupting the Mrt4-rRNA interaction. Pre-60S subunits associated with Yvh1 contain Rpl12 but lack both Mrt4 and P0, suggesting a linear assembly order: Yvh1 binds pre-60S to displace Mrt4, then P0 loads to assemble the mature stalk, then Yvh1 is released.","method":"Genetic deletion/mutation analysis, sucrose gradient fractionation, co-immunoprecipitation, suppressor mutation mapping (Mrt4-G68D bypasses Yvh1 requirement)","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal genetic epistasis, biochemical fractionation, and suppressor mutation analysis, replicated across two independent labs (PMID:19797078 and PMID:19789271) with orthogonal methods","pmids":["19797078"],"is_preprint":false},{"year":2009,"finding":"Mrt4 and P0 compete for binding to the same site (the 25S rRNA GAR domain) on the large ribosomal subunit and cannot bind simultaneously. Mrt4 defines medium pre-60S particles in the nucleus/nucleoplasm, while P0 is found only in late/cytoplasmic pre-60S complexes. Loss of Mrt4 causes P0 to assemble prematurely onto nuclear (medium) pre-60S particles; loss of P0 causes Mrt4 to relocate to the cytoplasm within aberrant 60S subunits. Thus Mrt4 controls the timing and position of P0 assembly, and P0 is required for Mrt4 release.","method":"TAP-tag purification, sucrose gradient sedimentation, fluorescence microscopy, genetic depletion (conditional null mutants)","journal":"Nucleic acids research","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods (TAP purification, gradient fractionation, fluorescence microscopy, conditional mutants), independently consistent with PMID:19797078","pmids":["19789271"],"is_preprint":false},{"year":2009,"finding":"Mrt4 and P0 bind to the same site on the 25S rRNA. A chimera containing the N-terminal 137 amino acids of Mrt4 fused to the C-terminal 190 amino acids of P0 can partially complement a P0 null mutant and associates with ribosomes, though with weaker binding than wild-type P0. Ribosomes bearing the chimera contain less P1/P2 and show altered L12 interaction, indicating that the N-terminal domain of Mrt4 directs rRNA binding while differences in the C-terminal region affect stalk protein interactions.","method":"Chimeric protein complementation assay, ribosome co-sedimentation, molecular dynamics simulation, yeast genetics","journal":"Nucleic acids research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — chimeric protein functional rescue and ribosome association assay in single lab, supported by molecular modeling","pmids":["19346338"],"is_preprint":false},{"year":1999,"finding":"Temperature-sensitive mutation in MRT4 causes defects in mRNA decay of several mRNAs without impairing protein synthesis, identifying Mrt4 as a factor with a role in mRNA turnover independent of general translation.","method":"Genetic complementation, mRNA decay assays (Northern blot/pulse-chase), temperature-sensitive mutant characterization","journal":"Genetics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct genetic and biochemical demonstration of mRNA decay defect with translation-intact controls, single lab","pmids":["10471698"],"is_preprint":false},{"year":2010,"finding":"Human MRTO4 (hMrt4) localizes predominantly to the nucleolar compartment (unlike cytoplasmic P0/P1/P2) and binds to the same site on the large ribosomal subunit as P0 but cannot functionally replace P0. Nucleolar accumulation of hMrt4 depends on interaction with rRNA rather than a specific nucleolus-targeting signal, while nuclear import requires a short sequence in the N-terminal domain. Treatment with actinomycin D or leptomycin B confirmed nucleus-cytoplasm shuttling capacity and trans-acting role in ribosome maturation.","method":"Fluorescence microscopy, biochemical fractionation, inhibitor treatments (actinomycin D, leptomycin B), chimeric protein domain-swap analysis","journal":"The international journal of biochemistry & cell biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct localization experiments with functional inhibitors and domain-swap constructs, single lab, multiple orthogonal approaches","pmids":["20083226"],"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 the subcellular distribution of hMrt4 under normal conditions but affects its molecular behavior during actinomycin D-induced nucleolar stress, identifying the C-terminal region as a regulatory element linking ribosome biogenesis to the stress response pathway.","method":"In vivo phosphorylation assay, in vitro kinase assay (CK2), site-directed mutagenesis of S229/S233/S235, fluorescence microscopy under stress conditions","journal":"The international journal of biochemistry & cell biology","confidence":"Medium","confidence_rationale":"Tier 1–2 / Moderate — in vitro kinase assay with site-specific mutagenesis plus in vivo phosphorylation detection and functional stress assay, single lab","pmids":["26494001"],"is_preprint":false},{"year":2011,"finding":"Genetic interaction between Yvh1 and Mrt4 is essential for normal glycogen accumulation, mRNA decay, and induction of early meiotic genes (IME2, SPO13, HOP1). The Mrt4(G68D) suppressor allele (mutation at the rRNA-binding interface) restores all of these Yvh1-deletion phenotypes, indicating they are downstream consequences of the Yvh1-Mrt4 ribosome assembly axis rather than separate Yvh1 functions.","method":"Genetic suppressor analysis, mRNA decay assays, glycogen accumulation assay, sporulation gene expression analysis, dominant suppressor screen","journal":"Journal of biochemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple phenotypic readouts rescued by single suppressor allele, epistasis clearly placed, single lab","pmids":["21474464"],"is_preprint":false},{"year":2024,"finding":"MRTO4 promotes glycolysis in hepatocellular carcinoma cells and accelerates HCC progression by inhibiting the glycolytic enzyme ALDOB. Knockdown of MRTO4 reduced glycolysis, proliferation, and invasion while promoting apoptosis; this mechanism was attributed to MRTO4-mediated suppression of ALDOB.","method":"RT-qPCR, Western blotting, CCK8, TUNEL, clone formation assay, Transwell assay, ELISA, immunohistochemistry, siRNA knockdown","journal":"Medical science monitor","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single lab, cellular knockdown with phenotypic readouts but no direct biochemical demonstration of MRTO4-ALDOB interaction mechanism; ALDOB inhibition inferred rather than reconstituted","pmids":["38778508"],"is_preprint":false},{"year":2026,"finding":"Mrt4 binds rRNA through cysteines C96 and C189 on Candida albicans Mrt4 (CaMrt4); covalent engagement of both residues by a fumaramidmycin-derived compound selectively inhibits CaMrt4-rRNA interaction and disrupts fungal ribosomal assembly. Human MRTO4-rRNA interaction is not inhibited by this compound, establishing a structural/mechanistic distinction between fungal and human Mrt4 at the rRNA-binding interface.","method":"Activity- and inactive-based protein profiling (AIBPP), chemical-genetic profiling, fluorescence polarization assay, in vivo Galleria mellonella and murine candidiasis models","journal":"Nature communications","confidence":"Medium","confidence_rationale":"Tier 1–2 / Moderate — fluorescence polarization directly measures Mrt4-rRNA binding disruption; covalent site identification by chemical proteomics; in vivo validation; single lab but multiple orthogonal methods","pmids":["41781388"],"is_preprint":false}],"current_model":"MRTO4/Mrt4 is a nuclear/nucleolar paralogue of the ribosomal stalk protein P0 that occupies the same rRNA-binding site (25S rRNA GAR domain) on pre-60S subunits during ribosome assembly; it is displaced from cytoplasmic pre-60S particles by the dual-specificity phosphatase Yvh1 (acting at the Mrt4-rRNA interface), after which P0 loads to assemble the mature stalk — a process also linked to mRNA decay, glycogen metabolism, and stress-responsive phosphorylation of Mrt4's C-terminal domain by CK2."},"narrative":{"mechanistic_narrative":"MRTO4 (Mrt4) is a nuclear/nucleolar paralogue of the ribosomal stalk protein P0 that acts as a trans-acting assembly factor occupying the P0 site on the 25S rRNA GAR domain of pre-60S subunits during large ribosomal subunit biogenesis [PMID:19789271, PMID:19346338]. Mrt4 and P0 compete for the same rRNA site and cannot bind simultaneously, so Mrt4 defines nuclear/nucleoplasmic medium pre-60S particles and reserves the stalk position until P0 loads in late/cytoplasmic complexes; loss of Mrt4 allows premature P0 assembly onto nuclear particles, while loss of P0 causes Mrt4 to escape to the cytoplasm in aberrant 60S subunits [PMID:19789271]. In the cytoplasm the dual-specificity phosphatase Yvh1 displaces Mrt4 by disrupting the Mrt4-rRNA interface — a step bypassed by the interface mutation Mrt4-G68D — after which P0 loads to build the mature stalk and Yvh1 is released, defining an ordered, linear stalk-assembly pathway [PMID:19797078]. The N-terminal domain of Mrt4 directs rRNA binding and nuclear import, and its nucleolar accumulation depends on rRNA interaction rather than a dedicated targeting signal [PMID:19346338, PMID:20083226]; the human protein localizes to the nucleolus, binds the P0 site, and shuttles between nucleus and cytoplasm but cannot functionally replace P0 [PMID:20083226]. The acidic C-terminal extension is a regulatory element: CK2 phosphorylates serines S229/S233/S235 of human MRTO4, modulating its behavior during nucleolar stress and linking ribosome biogenesis to the stress response [PMID:26494001]. Through the Yvh1-Mrt4 assembly axis, Mrt4 function is also coupled to mRNA decay and to downstream physiological outputs including glycogen accumulation and meiotic gene induction [PMID:10471698, PMID:21474464].","teleology":[{"year":1999,"claim":"Established the first functional role for Mrt4, showing it acts in mRNA turnover separable from general translation.","evidence":"Temperature-sensitive MRT4 mutant with mRNA decay assays and translation-intact controls in yeast","pmids":["10471698"],"confidence":"Medium","gaps":["Did not link the decay defect to a molecular mechanism","Ribosome assembly role not yet recognized"]},{"year":2009,"claim":"Resolved how Mrt4 and P0 relate on the ribosome, showing they compete for the same 25S rRNA GAR site and that Mrt4 controls the timing and position of P0 loading during 60S maturation.","evidence":"TAP-tag purification, sucrose gradient sedimentation, fluorescence microscopy, and conditional null mutants in yeast","pmids":["19789271"],"confidence":"High","gaps":["Did not identify the displacement machinery beyond P0 dependence","Structural detail of the shared binding site inferred indirectly"]},{"year":2009,"claim":"Identified the molecular trigger for Mrt4 removal, placing Yvh1 as the factor that disrupts the Mrt4-rRNA interface to permit P0 loading in an ordered cytoplasmic step.","evidence":"Genetic deletion/mutation, sucrose gradient fractionation, co-IP, and suppressor mapping (Mrt4-G68D bypass) in yeast, replicated across labs","pmids":["19797078"],"confidence":"High","gaps":["Whether Yvh1 phosphatase activity per se drives release was not directly demonstrated","No structural model of the Yvh1-pre-60S engagement"]},{"year":2009,"claim":"Dissected the domain architecture, showing the Mrt4 N-terminal region directs rRNA binding while C-terminal differences from P0 govern stalk protein (P1/P2/L12) interactions.","evidence":"Mrt4-P0 chimeric complementation, ribosome co-sedimentation, and molecular dynamics simulation in yeast","pmids":["19346338"],"confidence":"Medium","gaps":["Chimera only partially complemented P0 loss","Precise C-terminal residues controlling stalk assembly not mapped"]},{"year":2010,"claim":"Extended the model to the human protein, establishing hMRTO4 as a nucleolar, rRNA-dependent shuttling factor that occupies the P0 site but cannot substitute for P0.","evidence":"Fluorescence microscopy, fractionation, actinomycin D / leptomycin B inhibitor treatments, and domain-swap constructs in human cells","pmids":["20083226"],"confidence":"Medium","gaps":["Human counterpart of Yvh1-mediated release not directly tested","Functional consequences of failure to replace P0 not characterized"]},{"year":2011,"claim":"Connected the Yvh1-Mrt4 assembly axis to physiological outputs, showing glycogen accumulation, mRNA decay, and meiotic gene induction are downstream of this single ribosome-assembly interaction.","evidence":"Genetic suppressor analysis (Mrt4-G68D rescue), mRNA decay, glycogen, and sporulation gene assays in yeast","pmids":["21474464"],"confidence":"Medium","gaps":["Mechanistic chain from ribosome assembly to glycogen/meiotic phenotypes not delineated","Single lab"]},{"year":2015,"claim":"Identified a regulatory layer on human MRTO4, mapping CK2 phosphorylation of its acidic C-terminus and linking it to nucleolar stress behavior.","evidence":"In vivo phosphorylation, in vitro CK2 kinase assay, S229/S233/S235 mutagenesis, and stress-condition microscopy in human cells","pmids":["26494001"],"confidence":"Medium","gaps":["Functional consequence of phosphorylation under normal conditions not observed","Downstream effectors of stress-induced phosphorylation unknown"]},{"year":2024,"claim":"Proposed a disease-associated role, linking MRTO4 to glycolysis and hepatocellular carcinoma progression via suppression of ALDOB.","evidence":"siRNA knockdown with proliferation, invasion, apoptosis, and glycolysis readouts plus immunohistochemistry in HCC cells","pmids":["38778508"],"confidence":"Low","gaps":["No direct biochemical demonstration of MRTO4-ALDOB interaction; inhibition inferred not reconstituted","Relationship to ribosome-assembly function untested","Single lab"]},{"year":2026,"claim":"Defined the rRNA-binding interface chemically and established a fungal-versus-human distinction exploitable for selective antifungal targeting.","evidence":"Activity-based protein profiling, fluorescence polarization of Mrt4-rRNA binding, and in vivo candidiasis models for Candida albicans Mrt4 (cysteines C96/C189)","pmids":["41781388"],"confidence":"Medium","gaps":["Structural basis of the human resistance to the covalent compound not resolved","Human MRTO4 cysteine usage at the interface not directly mapped"]},{"year":null,"claim":"How human MRTO4 release is executed and whether its non-ribosomal roles (mRNA decay, stress response, cancer metabolism) are direct or downstream of ribosome biogenesis remains unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No human Yvh1-equivalent release step directly demonstrated","Causal link between assembly role and metabolic/cancer phenotypes unestablished","No high-resolution structure of human Mrt4 on pre-60S"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0003723","term_label":"RNA binding","supporting_discovery_ids":[1,2,4,8]},{"term_id":"GO:0005198","term_label":"structural molecule activity","supporting_discovery_ids":[1,2,4]}],"localization":[{"term_id":"GO:0005730","term_label":"nucleolus","supporting_discovery_ids":[4]},{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[1,4]},{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[0,1,4]}],"pathway":[{"term_id":"R-HSA-8953854","term_label":"Metabolism of RNA","supporting_discovery_ids":[0,1]},{"term_id":"R-HSA-1852241","term_label":"Organelle biogenesis and maintenance","supporting_discovery_ids":[0,1,4]}],"complexes":["pre-60S ribosomal particle"],"partners":["YVH1","RPP0","RPL12"],"other_free_text":[]}},"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 stalk assembly requires the dual-specificity phosphatase Yvh1 for the exchange of Mrt4 with P0.","date":"2009","source":"The Journal of cell biology","url":"https://pubmed.ncbi.nlm.nih.gov/19797078","citation_count":97,"is_preprint":false},{"pmid":"19789271","id":"PMC_19789271","title":"Role and dynamics of the ribosomal protein P0 and its related trans-acting factor Mrt4 during ribosome assembly in Saccharomyces cerevisiae.","date":"2009","source":"Nucleic acids research","url":"https://pubmed.ncbi.nlm.nih.gov/19789271","citation_count":64,"is_preprint":false},{"pmid":"10471698","id":"PMC_10471698","title":"Temperature-sensitive mutations in the Saccharomyces cerevisiae MRT4, GRC5, SLA2 and THS1 genes result in defects in mRNA turnover.","date":"1999","source":"Genetics","url":"https://pubmed.ncbi.nlm.nih.gov/10471698","citation_count":54,"is_preprint":false},{"pmid":"19346338","id":"PMC_19346338","title":"The amino terminal domain from Mrt4 protein can functionally replace the RNA binding domain of the ribosomal P0 protein.","date":"2009","source":"Nucleic acids research","url":"https://pubmed.ncbi.nlm.nih.gov/19346338","citation_count":44,"is_preprint":false},{"pmid":"20083226","id":"PMC_20083226","title":"Subcellular localization of ribosomal P0-like protein MRT4 is determined by its N-terminal domain.","date":"2010","source":"The international journal of biochemistry & cell biology","url":"https://pubmed.ncbi.nlm.nih.gov/20083226","citation_count":18,"is_preprint":false},{"pmid":"26494001","id":"PMC_26494001","title":"Molecular behavior of human Mrt4 protein, MRTO4, in stress conditions is regulated by its C-terminal region.","date":"2015","source":"The international journal of biochemistry & cell biology","url":"https://pubmed.ncbi.nlm.nih.gov/26494001","citation_count":9,"is_preprint":false},{"pmid":"21474464","id":"PMC_21474464","title":"Genetic interactions of ribosome maturation factors Yvh1 and Mrt4 influence mRNA decay, glycogen accumulation, and the expression of early meiotic genes in Saccharomyces cerevisiae.","date":"2011","source":"Journal of biochemistry","url":"https://pubmed.ncbi.nlm.nih.gov/21474464","citation_count":9,"is_preprint":false},{"pmid":"19819847","id":"PMC_19819847","title":"[Influence of Tripterygium wilfordii on the expression of spermiogenesis related genes Herc4, Ipo11 and Mrto4 in mice].","date":"2009","source":"Yi chuan = Hereditas","url":"https://pubmed.ncbi.nlm.nih.gov/19819847","citation_count":8,"is_preprint":false},{"pmid":"38778508","id":"PMC_38778508","title":"MRTO4 Enhances Glycolysis to Facilitate HCC Progression by Inhibiting ALDOB.","date":"2024","source":"Medical science monitor : international medical journal of experimental and clinical research","url":"https://pubmed.ncbi.nlm.nih.gov/38778508","citation_count":7,"is_preprint":false},{"pmid":"35387555","id":"PMC_35387555","title":"Recombinant expression and biophysical characterization of Mrt4 protein that involved in mRNA turnover and ribosome assembly from Saccharomyces cerevisiae.","date":"2022","source":"Bioengineered","url":"https://pubmed.ncbi.nlm.nih.gov/35387555","citation_count":5,"is_preprint":false},{"pmid":"41364147","id":"PMC_41364147","title":"Integrated multi-omics analysis identifies DARS2, MRTO4, and MRPL37 as novel biomarkers and potential therapeutic targets for bladder cancer.","date":"2025","source":"Discover oncology","url":"https://pubmed.ncbi.nlm.nih.gov/41364147","citation_count":1,"is_preprint":false},{"pmid":"39969382","id":"PMC_39969382","title":"Roles of WDR12 and MRTO4 genes in colorectal cancer.","date":"2024","source":"Medicine","url":"https://pubmed.ncbi.nlm.nih.gov/39969382","citation_count":0,"is_preprint":false},{"pmid":"41781388","id":"PMC_41781388","title":"Inhibiting Mrt4-rRNA interaction with fumaramidmycin-based derivatives as an antifungal strategy.","date":"2026","source":"Nature communications","url":"https://pubmed.ncbi.nlm.nih.gov/41781388","citation_count":0,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":8361,"output_tokens":2720,"usd":0.032941,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":10035,"output_tokens":3341,"usd":0.06685,"stage2_stop_reason":"end_turn"},"total_usd":0.099791,"stage1_batch_id":"msgbatch_01BbTCMyrRPyhtEq6b1MdwCN","stage2_batch_id":"msgbatch_011dEed5bbxYvTAaQWKJevxu","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2009,\n      \"finding\": \"Yvh1 (dual-specificity phosphatase) is required for the release of Mrt4 from pre-60S subunits in the cytoplasm; deletion of YVH1 causes Mrt4 to persist on cytoplasmic pre-60S subunits. A mutation in Mrt4 at the protein-RNA interface (G68D) bypasses the requirement for Yvh1, indicating Yvh1 acts by disrupting the Mrt4-rRNA interaction. Pre-60S subunits associated with Yvh1 contain Rpl12 but lack both Mrt4 and P0, suggesting a linear assembly order: Yvh1 binds pre-60S to displace Mrt4, then P0 loads to assemble the mature stalk, then Yvh1 is released.\",\n      \"method\": \"Genetic deletion/mutation analysis, sucrose gradient fractionation, co-immunoprecipitation, suppressor mutation mapping (Mrt4-G68D bypasses Yvh1 requirement)\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal genetic epistasis, biochemical fractionation, and suppressor mutation analysis, replicated across two independent labs (PMID:19797078 and PMID:19789271) with orthogonal methods\",\n      \"pmids\": [\"19797078\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Mrt4 and P0 compete for binding to the same site (the 25S rRNA GAR domain) on the large ribosomal subunit and cannot bind simultaneously. Mrt4 defines medium pre-60S particles in the nucleus/nucleoplasm, while P0 is found only in late/cytoplasmic pre-60S complexes. Loss of Mrt4 causes P0 to assemble prematurely onto nuclear (medium) pre-60S particles; loss of P0 causes Mrt4 to relocate to the cytoplasm within aberrant 60S subunits. Thus Mrt4 controls the timing and position of P0 assembly, and P0 is required for Mrt4 release.\",\n      \"method\": \"TAP-tag purification, sucrose gradient sedimentation, fluorescence microscopy, genetic depletion (conditional null mutants)\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods (TAP purification, gradient fractionation, fluorescence microscopy, conditional mutants), independently consistent with PMID:19797078\",\n      \"pmids\": [\"19789271\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Mrt4 and P0 bind to the same site on the 25S rRNA. A chimera containing the N-terminal 137 amino acids of Mrt4 fused to the C-terminal 190 amino acids of P0 can partially complement a P0 null mutant and associates with ribosomes, though with weaker binding than wild-type P0. Ribosomes bearing the chimera contain less P1/P2 and show altered L12 interaction, indicating that the N-terminal domain of Mrt4 directs rRNA binding while differences in the C-terminal region affect stalk protein interactions.\",\n      \"method\": \"Chimeric protein complementation assay, ribosome co-sedimentation, molecular dynamics simulation, yeast genetics\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — chimeric protein functional rescue and ribosome association assay in single lab, supported by molecular modeling\",\n      \"pmids\": [\"19346338\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"Temperature-sensitive mutation in MRT4 causes defects in mRNA decay of several mRNAs without impairing protein synthesis, identifying Mrt4 as a factor with a role in mRNA turnover independent of general translation.\",\n      \"method\": \"Genetic complementation, mRNA decay assays (Northern blot/pulse-chase), temperature-sensitive mutant characterization\",\n      \"journal\": \"Genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct genetic and biochemical demonstration of mRNA decay defect with translation-intact controls, single lab\",\n      \"pmids\": [\"10471698\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Human MRTO4 (hMrt4) localizes predominantly to the nucleolar compartment (unlike cytoplasmic P0/P1/P2) and binds to the same site on the large ribosomal subunit as P0 but cannot functionally replace P0. Nucleolar accumulation of hMrt4 depends on interaction with rRNA rather than a specific nucleolus-targeting signal, while nuclear import requires a short sequence in the N-terminal domain. Treatment with actinomycin D or leptomycin B confirmed nucleus-cytoplasm shuttling capacity and trans-acting role in ribosome maturation.\",\n      \"method\": \"Fluorescence microscopy, biochemical fractionation, inhibitor treatments (actinomycin D, leptomycin B), chimeric protein domain-swap analysis\",\n      \"journal\": \"The international journal of biochemistry & cell biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct localization experiments with functional inhibitors and domain-swap constructs, single lab, multiple orthogonal approaches\",\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 acidic C-terminal extension are phosphorylated by CK2 kinase in vitro. This phosphorylation does not alter the subcellular distribution of hMrt4 under normal conditions but affects its molecular behavior during actinomycin D-induced nucleolar stress, identifying the C-terminal region as a regulatory element linking ribosome biogenesis to the stress response pathway.\",\n      \"method\": \"In vivo phosphorylation assay, in vitro kinase assay (CK2), site-directed mutagenesis of S229/S233/S235, fluorescence microscopy under stress conditions\",\n      \"journal\": \"The international journal of biochemistry & cell biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1–2 / Moderate — in vitro kinase assay with site-specific mutagenesis plus in vivo phosphorylation detection and functional stress assay, single lab\",\n      \"pmids\": [\"26494001\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Genetic interaction between Yvh1 and Mrt4 is essential for normal glycogen accumulation, mRNA decay, and induction of early meiotic genes (IME2, SPO13, HOP1). The Mrt4(G68D) suppressor allele (mutation at the rRNA-binding interface) restores all of these Yvh1-deletion phenotypes, indicating they are downstream consequences of the Yvh1-Mrt4 ribosome assembly axis rather than separate Yvh1 functions.\",\n      \"method\": \"Genetic suppressor analysis, mRNA decay assays, glycogen accumulation assay, sporulation gene expression analysis, dominant suppressor screen\",\n      \"journal\": \"Journal of biochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple phenotypic readouts rescued by single suppressor allele, epistasis clearly placed, single lab\",\n      \"pmids\": [\"21474464\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"MRTO4 promotes glycolysis in hepatocellular carcinoma cells and accelerates HCC progression by inhibiting the glycolytic enzyme ALDOB. Knockdown of MRTO4 reduced glycolysis, proliferation, and invasion while promoting apoptosis; this mechanism was attributed to MRTO4-mediated suppression of ALDOB.\",\n      \"method\": \"RT-qPCR, Western blotting, CCK8, TUNEL, clone formation assay, Transwell assay, ELISA, immunohistochemistry, siRNA knockdown\",\n      \"journal\": \"Medical science monitor\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single lab, cellular knockdown with phenotypic readouts but no direct biochemical demonstration of MRTO4-ALDOB interaction mechanism; ALDOB inhibition inferred rather than reconstituted\",\n      \"pmids\": [\"38778508\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"Mrt4 binds rRNA through cysteines C96 and C189 on Candida albicans Mrt4 (CaMrt4); covalent engagement of both residues by a fumaramidmycin-derived compound selectively inhibits CaMrt4-rRNA interaction and disrupts fungal ribosomal assembly. Human MRTO4-rRNA interaction is not inhibited by this compound, establishing a structural/mechanistic distinction between fungal and human Mrt4 at the rRNA-binding interface.\",\n      \"method\": \"Activity- 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\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1–2 / Moderate — fluorescence polarization directly measures Mrt4-rRNA binding disruption; covalent site identification by chemical proteomics; in vivo validation; single lab but multiple orthogonal methods\",\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 occupies the same rRNA-binding site (25S rRNA GAR domain) on pre-60S subunits during ribosome assembly; it is displaced from cytoplasmic pre-60S particles by the dual-specificity phosphatase Yvh1 (acting at the Mrt4-rRNA interface), after which P0 loads to assemble the mature stalk — a process also linked to mRNA decay, glycogen metabolism, and stress-responsive phosphorylation of Mrt4's C-terminal domain by CK2.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"MRTO4 (Mrt4) is a nuclear/nucleolar paralogue of the ribosomal stalk protein P0 that acts as a trans-acting assembly factor occupying the P0 site on the 25S rRNA GAR domain of pre-60S subunits during large ribosomal subunit biogenesis [#1, #2]. Mrt4 and P0 compete for the same rRNA site and cannot bind simultaneously, so Mrt4 defines nuclear/nucleoplasmic medium pre-60S particles and reserves the stalk position until P0 loads in late/cytoplasmic complexes; loss of Mrt4 allows premature P0 assembly onto nuclear particles, while loss of P0 causes Mrt4 to escape to the cytoplasm in aberrant 60S subunits [#1]. In the cytoplasm the dual-specificity phosphatase Yvh1 displaces Mrt4 by disrupting the Mrt4-rRNA interface — a step bypassed by the interface mutation Mrt4-G68D — after which P0 loads to build the mature stalk and Yvh1 is released, defining an ordered, linear stalk-assembly pathway [#0]. The N-terminal domain of Mrt4 directs rRNA binding and nuclear import, and its nucleolar accumulation depends on rRNA interaction rather than a dedicated targeting signal [#2, #4]; the human protein localizes to the nucleolus, binds the P0 site, and shuttles between nucleus and cytoplasm but cannot functionally replace P0 [#4]. The acidic C-terminal extension is a regulatory element: CK2 phosphorylates serines S229/S233/S235 of human MRTO4, modulating its behavior during nucleolar stress and linking ribosome biogenesis to the stress response [#5]. Through the Yvh1-Mrt4 assembly axis, Mrt4 function is also coupled to mRNA decay and to downstream physiological outputs including glycogen accumulation and meiotic gene induction [#3, #6].\",\n  \"teleology\": [\n    {\n      \"year\": 1999,\n      \"claim\": \"Established the first functional role for Mrt4, showing it acts in mRNA turnover separable from general translation.\",\n      \"evidence\": \"Temperature-sensitive MRT4 mutant with mRNA decay assays and translation-intact controls in yeast\",\n      \"pmids\": [\"10471698\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Did not link the decay defect to a molecular mechanism\", \"Ribosome assembly role not yet recognized\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Resolved how Mrt4 and P0 relate on the ribosome, showing they compete for the same 25S rRNA GAR site and that Mrt4 controls the timing and position of P0 loading during 60S maturation.\",\n      \"evidence\": \"TAP-tag purification, sucrose gradient sedimentation, fluorescence microscopy, and conditional null mutants in yeast\",\n      \"pmids\": [\"19789271\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not identify the displacement machinery beyond P0 dependence\", \"Structural detail of the shared binding site inferred indirectly\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Identified the molecular trigger for Mrt4 removal, placing Yvh1 as the factor that disrupts the Mrt4-rRNA interface to permit P0 loading in an ordered cytoplasmic step.\",\n      \"evidence\": \"Genetic deletion/mutation, sucrose gradient fractionation, co-IP, and suppressor mapping (Mrt4-G68D bypass) in yeast, replicated across labs\",\n      \"pmids\": [\"19797078\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether Yvh1 phosphatase activity per se drives release was not directly demonstrated\", \"No structural model of the Yvh1-pre-60S engagement\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Dissected the domain architecture, showing the Mrt4 N-terminal region directs rRNA binding while C-terminal differences from P0 govern stalk protein (P1/P2/L12) interactions.\",\n      \"evidence\": \"Mrt4-P0 chimeric complementation, ribosome co-sedimentation, and molecular dynamics simulation in yeast\",\n      \"pmids\": [\"19346338\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Chimera only partially complemented P0 loss\", \"Precise C-terminal residues controlling stalk assembly not mapped\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Extended the model to the human protein, establishing hMRTO4 as a nucleolar, rRNA-dependent shuttling factor that occupies the P0 site but cannot substitute for P0.\",\n      \"evidence\": \"Fluorescence microscopy, fractionation, actinomycin D / leptomycin B inhibitor treatments, and domain-swap constructs in human cells\",\n      \"pmids\": [\"20083226\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Human counterpart of Yvh1-mediated release not directly tested\", \"Functional consequences of failure to replace P0 not characterized\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Connected the Yvh1-Mrt4 assembly axis to physiological outputs, showing glycogen accumulation, mRNA decay, and meiotic gene induction are downstream of this single ribosome-assembly interaction.\",\n      \"evidence\": \"Genetic suppressor analysis (Mrt4-G68D rescue), mRNA decay, glycogen, and sporulation gene assays in yeast\",\n      \"pmids\": [\"21474464\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanistic chain from ribosome assembly to glycogen/meiotic phenotypes not delineated\", \"Single lab\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Identified a regulatory layer on human MRTO4, mapping CK2 phosphorylation of its acidic C-terminus and linking it to nucleolar stress behavior.\",\n      \"evidence\": \"In vivo phosphorylation, in vitro CK2 kinase assay, S229/S233/S235 mutagenesis, and stress-condition microscopy in human cells\",\n      \"pmids\": [\"26494001\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Functional consequence of phosphorylation under normal conditions not observed\", \"Downstream effectors of stress-induced phosphorylation unknown\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Proposed a disease-associated role, linking MRTO4 to glycolysis and hepatocellular carcinoma progression via suppression of ALDOB.\",\n      \"evidence\": \"siRNA knockdown with proliferation, invasion, apoptosis, and glycolysis readouts plus immunohistochemistry in HCC cells\",\n      \"pmids\": [\"38778508\"],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No direct biochemical demonstration of MRTO4-ALDOB interaction; inhibition inferred not reconstituted\", \"Relationship to ribosome-assembly function untested\", \"Single lab\"]\n    },\n    {\n      \"year\": 2026,\n      \"claim\": \"Defined the rRNA-binding interface chemically and established a fungal-versus-human distinction exploitable for selective antifungal targeting.\",\n      \"evidence\": \"Activity-based protein profiling, fluorescence polarization of Mrt4-rRNA binding, and in vivo candidiasis models for Candida albicans Mrt4 (cysteines C96/C189)\",\n      \"pmids\": [\"41781388\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Structural basis of the human resistance to the covalent compound not resolved\", \"Human MRTO4 cysteine usage at the interface not directly mapped\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How human MRTO4 release is executed and whether its non-ribosomal roles (mRNA decay, stress response, cancer metabolism) are direct or downstream of ribosome biogenesis remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No human Yvh1-equivalent release step directly demonstrated\", \"Causal link between assembly role and metabolic/cancer phenotypes unestablished\", \"No high-resolution structure of human Mrt4 on pre-60S\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0003723\", \"supporting_discovery_ids\": [1, 2, 4, 8]},\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [1, 2, 4]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005730\", \"supporting_discovery_ids\": [4]},\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [1, 4]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [0, 1, 4]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-8953854\", \"supporting_discovery_ids\": [0, 1]},\n      {\"term_id\": \"R-HSA-1852241\", \"supporting_discovery_ids\": [0, 1, 4]}\n    ],\n    \"complexes\": [\n      \"pre-60S ribosomal particle\"\n    ],\n    \"partners\": [\n      \"YVH1\",\n      \"RPP0\",\n      \"RPL12\"\n    ],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":5,"faith_total":6,"faith_pct":83.33333333333333}}