{"gene":"MED20","run_date":"2026-06-10T02:59:50","timeline":{"discoveries":[{"year":2006,"finding":"Med8, Med18, and Med20 form a subcomplex (Med8/18/20) within the Mediator head module. X-ray crystallography revealed that Med18 and Med20 adopt related beta-barrel folds. The N-terminal domain of Med8 binds TBP (TATA box-binding protein) in vitro. A conserved protein-interaction face on the Med8C/18/20 submodule includes sites altered by srb mutations that counteract defects from RNA Pol II truncation, supporting a positive role in initiation-complex formation.","method":"X-ray crystallography, in vitro TBP-binding assay, in vivo genetic analysis of srb mutations","journal":"Nature structural & molecular biology","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structure combined with in vitro binding assay and in vivo genetic validation in a single rigorous study","pmids":["16964259"],"is_preprint":false},{"year":2009,"finding":"Med8, Med18, and Med20 are interdependent for proper folding and trimer complex formation. Renaturation experiments showed that the concurrent presence of all three subunits is required to form the correctly folded trimeric complex; pairwise combinations yield distinct subcomplexes with different folding states.","method":"Immunoprecipitation, far-UV circular dichroism, fluorescence spectroscopy on recombinantly expressed and denatured/renatured proteins","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 / Moderate — multiple orthogonal biophysical methods (CD, fluorescence, IP) in a single focused study on reconstituted proteins","pmids":["19934057"],"is_preprint":false},{"year":1996,"finding":"Yeast SRB2 (MED20 ortholog) is required for DNA repair of MMS-induced damage; srb2 null cells are sensitive to prolonged MMS treatment, and an srb2 point mutation (Gly150Asp) suppresses hyperrecombination between direct repeats caused by hpr1Δ, linking SRB2-dependent transcription complex function to DNA repair and recombination.","method":"Genetic suppressor analysis, srb2 null and point-mutant allele construction, MMS sensitivity assay, recombination frequency measurement","journal":"Genetics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — clean genetic epistasis with multiple alleles and two distinct phenotypic readouts (recombination suppression and MMS sensitivity) in a single lab study","pmids":["8844143"],"is_preprint":false},{"year":2008,"finding":"Genetic interaction screens in S. cerevisiae identified MED20 (head module subunit) as required for transcriptional repression of ribosomal protein (RP) genes following rapamycin treatment; med20Δ shows synthetic sick/lethal interaction with maf1Δ, placing Med20 in a parallel pathway to Maf1 for negative regulation of RP mRNA synthesis.","method":"Genome-wide synthetic sick/lethal genetic screen, gene expression profiling (microarray), rapamycin treatment assays","journal":"PLoS genetics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic epistasis via SSL screen confirmed by gene expression profiling, single lab with two orthogonal approaches","pmids":["18604275"],"is_preprint":false},{"year":2015,"finding":"Loss of Med20 in fission yeast leads to accumulation of aberrant, polyadenylated readthrough tRNA transcripts targeted for exosome-mediated degradation, and similarly affects snRNA, snoRNA, and rRNA; this implicates Med20-containing Mediator in a regulatory pathway controlling Pol III-dependent transcription fidelity.","method":"Gene deletion (med20Δ), Northern blotting/RNA analysis, polyadenylate RNA purification, exosome mutant epistasis","journal":"Biochimica et biophysica acta","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — loss-of-function with direct RNA phenotype readout and genetic epistasis with exosome mutants, single lab","pmids":["26608234"],"is_preprint":false},{"year":2017,"finding":"The RES complex (Bud13p/Snu17p/Pml1p) controls Med20 protein levels by promoting efficient splicing and nuclear retention of MED20 pre-mRNA; loss of bud13Δ or snu17Δ causes MED20 pre-mRNA splicing defects, accumulation of unspliced transcript targeted by NMD, and reduced Med20 levels that underlie temperature-sensitive growth defects.","method":"Genetic deletion analysis, RT-PCR/splicing assays, NMD pathway inactivation epistasis, growth phenotype rescue","journal":"RNA biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic epistasis with NMD pathway, direct splicing assay, and phenotypic rescue, single lab","pmids":["28277935"],"is_preprint":false},{"year":2019,"finding":"MED20 is essential for early mouse embryogenesis; Med20 mutant blastocysts fail to hatch from the zona pellucida and show ectopic NANOG expression in the trophectoderm, indicating that MED20 is required for proper trophoblast specification and epiblast-trophectoderm boundary formation.","method":"Mouse knockout (Med20 null), blastocyst outgrowth assay, immunofluorescence for lineage markers (NANOG, CDX2, GATA6), cell death assays","journal":"Reproduction (Cambridge, England)","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — clean KO with defined cellular phenotype and multiple lineage marker readouts, single lab","pmids":["30571656"],"is_preprint":false},{"year":2021,"finding":"MED20 is a substrate of the CRL4-WDTC1 E3 ubiquitin ligase complex; WDTC1 overexpression leads to MED20 degradation while depletion of WDTC1 or CUL4A/B causes MED20 accumulation. MED20 promotes adipogenesis by bridging C/EBPβ and RNA Pol II at the PPARγ promoter to drive its transcription, as shown by ChIP-seq.","method":"Affinity purification/candidate screening, co-immunoprecipitation, overexpression/knockdown/knockout in preadipocytes, non-degradable mutant rescue, ChIP-seq, brown adipose tissue-specific Med20 knockout mouse","journal":"Cell reports","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — multiple orthogonal methods (AP, Co-IP, KO mouse, ChIP-seq, mutant rescue) in a single comprehensive study identifying E3 ligase, ubiquitination substrate relationship, and transcriptional mechanism","pmids":["34233190"],"is_preprint":false},{"year":2025,"finding":"Med20 loss in Schwann cells induces ferroptosis and impairs peripheral nervous system myelination. Mechanistically, Med20 transcriptionally activates DDB1, which suppresses ferroptosis by regulating HO-1 (Hmox1) through a DDB1-UHRF1-BACH1-Hmox1 axis and by directly ubiquitinating HO-1 protein. Ferroptosis inhibitor Fer-1 or HO-1 inhibitor ZnPP restores myelination in Med20-deficient mice.","method":"Schwann cell-specific Med20 knockout mouse, ferroptosis assays, ChIP, co-immunoprecipitation, ubiquitination assay, pharmacological rescue (Fer-1, ZnPP), gene expression analysis","journal":"Cell reports","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — conditional KO with defined cellular phenotype, mechanistic pathway delineated by ChIP and Co-IP, pharmacological rescue, single lab","pmids":["41108685"],"is_preprint":false}],"current_model":"MED20 is a head-module subunit of the Mediator complex that forms an interdependent trimeric subcomplex with Med8 and Med18 (adopting a beta-barrel fold), contributes to TBP binding and RNA Pol II initiation-complex assembly, represses ribosomal protein gene transcription, and—at the cellular level—acts as a substrate of the CRL4-WDTC1 E3 ubiquitin ligase to control adipogenesis by bridging C/EBPβ and Pol II at the PPARγ promoter, while in Schwann cells it regulates myelination by transcriptionally activating DDB1 to suppress ferroptosis via a DDB1-UHRF1-BACH1-HO-1 axis."},"narrative":{"mechanistic_narrative":"MED20 is a subunit of the Mediator head module that participates in RNA polymerase II transcription initiation by assembling, together with Med8 and Med18, into an interdependent trimeric submodule in which Med18 and Med20 adopt related beta-barrel folds and the Med8 N-terminus contacts the TATA box-binding protein [PMID:16964259, PMID:19934057]. The three subunits are mutually required for correct folding and trimer formation, and the submodule presents a conserved interaction face whose mutation counteracts defects from Pol II truncation, marking a positive role in initiation-complex assembly [PMID:16964259, PMID:19934057]. In yeast, MED20 contributes to negative regulation of ribosomal protein gene transcription in a pathway parallel to Maf1 [PMID:18604275], and its loss perturbs Pol III-dependent transcript fidelity, causing accumulation of aberrant readthrough tRNA, snRNA, snoRNA, and rRNA species that are degraded by the exosome [PMID:26608234]. In mammals, MED20 is essential for early embryogenesis, with mutant blastocysts failing to hatch and showing disrupted trophoblast specification [PMID:30571656]. MED20 is a substrate of the CRL4-WDTC1 E3 ubiquitin ligase, and through this controlled abundance it drives adipogenesis by bridging C/EBPβ and Pol II at the PPARγ promoter [PMID:34233190]. In Schwann cells, MED20 transcriptionally activates DDB1 to suppress ferroptosis via a DDB1-UHRF1-BACH1-HO-1 axis, thereby supporting peripheral nerve myelination [PMID:41108685].","teleology":[{"year":1996,"claim":"Before its biochemical role was defined, the MED20 ortholog SRB2 was linked to transcription-coupled genome maintenance, showing that this Mediator-associated factor influences DNA repair and recombination outcomes.","evidence":"Genetic suppressor analysis with srb2 null and point-mutant alleles, MMS sensitivity and recombination frequency assays in yeast","pmids":["8844143"],"confidence":"Medium","gaps":["Does not establish a direct biochemical activity for SRB2/MED20","Connection to repair is genetic and may be indirect through transcription complex function"]},{"year":2006,"claim":"To define how MED20 contributes to Mediator architecture, structural and binding work showed it forms a Med8/18/20 head submodule with a beta-barrel fold and a conserved interaction face supporting initiation-complex formation.","evidence":"X-ray crystallography, in vitro TBP-binding assay, and in vivo srb mutation analysis","pmids":["16964259"],"confidence":"High","gaps":["TBP binding maps to Med8, not directly to MED20","Does not resolve MED20's contribution within the holo-Mediator/Pol II initiation complex"]},{"year":2009,"claim":"To test whether the three subunits assemble independently, renaturation experiments established that Med8, Med18, and Med20 are mutually required for correct folding and trimer formation.","evidence":"IP, far-UV circular dichroism, and fluorescence spectroscopy on denatured/renatured recombinant proteins","pmids":["19934057"],"confidence":"High","gaps":["Performed on isolated recombinant proteins, not within intact Mediator","Does not address assembly order in vivo"]},{"year":2008,"claim":"To place MED20 in a regulatory context, genetic screens showed it is required for repression of ribosomal protein genes upon rapamycin treatment in a pathway parallel to Maf1.","evidence":"Genome-wide synthetic sick/lethal screen with expression profiling in S. cerevisiae","pmids":["18604275"],"confidence":"Medium","gaps":["Genetic, not biochemical, definition of the repression mechanism","Relationship between head-module function and RP gene repression unresolved"]},{"year":2015,"claim":"Extending MED20 function beyond Pol II, loss-of-function in fission yeast linked Med20-containing Mediator to Pol III transcript fidelity.","evidence":"med20Δ deletion with Northern/RNA analysis, polyadenylated RNA purification, and exosome mutant epistasis","pmids":["26608234"],"confidence":"Medium","gaps":["Mechanism by which Mediator influences Pol III transcripts is unclear","Direct versus indirect effect not distinguished"]},{"year":2019,"claim":"To assess the organismal requirement for MED20, mouse knockout established it is essential for early embryogenesis and proper trophoblast specification.","evidence":"Med20-null mouse, blastocyst outgrowth assay, and lineage-marker immunofluorescence","pmids":["30571656"],"confidence":"Medium","gaps":["Does not identify the transcriptional targets underlying the lineage defect","Cannot separate global Mediator loss from a specific MED20 function"]},{"year":2021,"claim":"To explain how MED20 abundance is controlled and used in a differentiation program, work identified it as a CRL4-WDTC1 substrate that bridges C/EBPβ and Pol II at the PPARγ promoter to drive adipogenesis.","evidence":"Affinity purification, Co-IP, degradation/rescue assays, ChIP-seq, and brown adipose-specific Med20 knockout mouse","pmids":["34233190"],"confidence":"High","gaps":["Ubiquitination site and degradation kinetics not fully mapped","Generality of MED20 promoter-bridging beyond PPARγ unknown"]},{"year":2025,"claim":"To define a tissue-specific role, Schwann cell studies showed MED20 transcriptionally activates DDB1 to suppress ferroptosis and support peripheral myelination.","evidence":"Schwann cell-specific Med20 knockout mouse, ferroptosis assays, ChIP, Co-IP, ubiquitination assay, and pharmacological rescue (Fer-1, ZnPP)","pmids":["41108685"],"confidence":"Medium","gaps":["Whether DDB1 activation reflects direct MED20 promoter occupancy or Mediator-wide effect is not separated","Single-lab characterization of the DDB1-UHRF1-BACH1-HO-1 axis"]},{"year":null,"claim":"How the conserved structural role of MED20 in the Mediator head module mechanistically connects to its diverse context-specific outputs (RP gene repression, Pol III fidelity, adipogenesis, ferroptosis suppression) remains unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No structural model of MED20 within an active promoter-bound complex driving these target genes","Unclear which functions are Mediator-intrinsic versus moonlighting"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140110","term_label":"transcription regulator activity","supporting_discovery_ids":[7,8]},{"term_id":"GO:0005198","term_label":"structural molecule activity","supporting_discovery_ids":[0,1]},{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[7]}],"localization":[{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[0,7]}],"pathway":[{"term_id":"R-HSA-74160","term_label":"Gene expression (Transcription)","supporting_discovery_ids":[0,7]},{"term_id":"R-HSA-1266738","term_label":"Developmental Biology","supporting_discovery_ids":[6,8]}],"complexes":["Mediator complex (head module)","Med8/Med18/Med20 submodule"],"partners":["MED8","MED18","TBP","WDTC1","CEBPB"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q9H944","full_name":"Mediator of RNA polymerase II transcription subunit 20","aliases":["Mediator complex subunit 20","TRF-proximal protein homolog","hTRFP"],"length_aa":212,"mass_kda":23.2,"function":"Component of the Mediator complex, a coactivator involved in the regulated transcription of nearly all RNA polymerase II-dependent genes. Mediator functions as a bridge to convey information from gene-specific regulatory proteins to the basal RNA polymerase II transcription machinery. Mediator is recruited to promoters by direct interactions with regulatory proteins and serves as a scaffold for the assembly of a functional preinitiation complex with RNA polymerase II and the general transcription factors","subcellular_location":"Nucleus","url":"https://www.uniprot.org/uniprotkb/Q9H944/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":true,"resolved_as":"","url":"https://depmap.org/portal/gene/MED20","classification":"Common Essential","n_dependent_lines":1163,"n_total_lines":1208,"dependency_fraction":0.9627483443708609},"opencell":{"profiled":true,"resolved_as":"","ensg_id":"ENSG00000124641","cell_line_id":"CID000240","localizations":[{"compartment":"nuclear_punctae","grade":3},{"compartment":"nucleoplasm","grade":3}],"interactors":[{"gene":"MED10","stoichiometry":10.0},{"gene":"MED11","stoichiometry":10.0},{"gene":"MED14","stoichiometry":10.0},{"gene":"MED17","stoichiometry":10.0},{"gene":"MED18","stoichiometry":10.0},{"gene":"MED19","stoichiometry":10.0},{"gene":"MED27","stoichiometry":10.0},{"gene":"MED9","stoichiometry":10.0},{"gene":"MED31","stoichiometry":10.0},{"gene":"MED30","stoichiometry":10.0}],"url":"https://opencell.sf.czbiohub.org/target/CID000240","total_profiled":1310},"omim":[{"mim_id":"612915","title":"MEDIATOR COMPLEX SUBUNIT 20; MED20","url":"https://www.omim.org/entry/612915"},{"mim_id":"612914","title":"MEDIATOR COMPLEX SUBUNIT 29; MED29","url":"https://www.omim.org/entry/612914"},{"mim_id":"612384","title":"MEDIATOR COMPLEX SUBUNIT 18; MED18","url":"https://www.omim.org/entry/612384"},{"mim_id":"600045","title":"DNA DAMAGE-BINDING PROTEIN 1; DDB1","url":"https://www.omim.org/entry/600045"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/MED20"},"hgnc":{"alias_symbol":["DKFZp586D2223","PRO0213","SRB2"],"prev_symbol":["TRFP"]},"alphafold":{"accession":"Q9H944","domains":[{"cath_id":"2.40.320","chopping":"2-203","consensus_level":"medium","plddt":93.7687,"start":2,"end":203}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9H944","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9H944-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9H944-F1-predicted_aligned_error_v6.png","plddt_mean":91.62},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=MED20","jax_strain_url":"https://www.jax.org/strain/search?query=MED20"},"sequence":{"accession":"Q9H944","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9H944.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9H944/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9H944"}},"corpus_meta":[{"pmid":"29627764","id":"PMC_29627764","title":"Dynamic role of the transmembrane glycoprotein CD36 (SR-B2) in cellular fatty acid uptake and utilization.","date":"2018","source":"Journal of lipid research","url":"https://pubmed.ncbi.nlm.nih.gov/29627764","citation_count":243,"is_preprint":false},{"pmid":"16964259","id":"PMC_16964259","title":"Structure and TBP binding of the Mediator head subcomplex Med8-Med18-Med20.","date":"2006","source":"Nature structural & molecular biology","url":"https://pubmed.ncbi.nlm.nih.gov/16964259","citation_count":94,"is_preprint":false},{"pmid":"27615427","id":"PMC_27615427","title":"Post-translational modifications of CD36 (SR-B2): Implications for regulation of myocellular fatty acid uptake.","date":"2016","source":"Biochimica et biophysica acta","url":"https://pubmed.ncbi.nlm.nih.gov/27615427","citation_count":70,"is_preprint":false},{"pmid":"35125400","id":"PMC_35125400","title":"CD36 (SR-B2) as master regulator of cellular fatty acid homeostasis.","date":"2022","source":"Current opinion in lipidology","url":"https://pubmed.ncbi.nlm.nih.gov/35125400","citation_count":64,"is_preprint":false},{"pmid":"29931157","id":"PMC_29931157","title":"SRB-2: a promiscuous rainbow aptamer for live-cell RNA imaging.","date":"2018","source":"Nucleic acids research","url":"https://pubmed.ncbi.nlm.nih.gov/29931157","citation_count":58,"is_preprint":false},{"pmid":"32821722","id":"PMC_32821722","title":"CD36 (SR-B2) as a Target to Treat Lipid Overload-Induced Cardiac Dysfunction.","date":"2020","source":"Journal of lipid and atherosclerosis","url":"https://pubmed.ncbi.nlm.nih.gov/32821722","citation_count":35,"is_preprint":false},{"pmid":"8844143","id":"PMC_8844143","title":"Mutations in the yeast SRB2 general transcription factor suppress hpr1-induced recombination and show defects in DNA repair.","date":"1996","source":"Genetics","url":"https://pubmed.ncbi.nlm.nih.gov/8844143","citation_count":27,"is_preprint":false},{"pmid":"25446406","id":"PMC_25446406","title":"MED20 mutation associated with infantile basal ganglia degeneration and brain atrophy.","date":"2014","source":"European journal of pediatrics","url":"https://pubmed.ncbi.nlm.nih.gov/25446406","citation_count":19,"is_preprint":false},{"pmid":"30571656","id":"PMC_30571656","title":"MED20 is essential for early embryogenesis and regulates NANOG expression.","date":"2019","source":"Reproduction (Cambridge, England)","url":"https://pubmed.ncbi.nlm.nih.gov/30571656","citation_count":17,"is_preprint":false},{"pmid":"34233190","id":"PMC_34233190","title":"The Mediator subunit MED20 organizes the early adipogenic complex to promote development of adipose tissues and diet-induced obesity.","date":"2021","source":"Cell reports","url":"https://pubmed.ncbi.nlm.nih.gov/34233190","citation_count":16,"is_preprint":false},{"pmid":"18604275","id":"PMC_18604275","title":"Genetic interactions of MAF1 identify a role for Med20 in transcriptional repression of ribosomal protein genes.","date":"2008","source":"PLoS genetics","url":"https://pubmed.ncbi.nlm.nih.gov/18604275","citation_count":12,"is_preprint":false},{"pmid":"28948682","id":"PMC_28948682","title":"A novel Cre-inducible knock-in ARL13B-tRFP fusion cilium reporter.","date":"2017","source":"Genesis (New York, N.Y. : 2000)","url":"https://pubmed.ncbi.nlm.nih.gov/28948682","citation_count":10,"is_preprint":false},{"pmid":"26608234","id":"PMC_26608234","title":"Loss of the Mediator subunit Med20 affects transcription of tRNA and other non-coding RNA genes in fission yeast.","date":"2015","source":"Biochimica et biophysica acta","url":"https://pubmed.ncbi.nlm.nih.gov/26608234","citation_count":10,"is_preprint":false},{"pmid":"19934057","id":"PMC_19934057","title":"Med8, Med18, and Med20 subunits of the Mediator head domain are interdependent upon each other for folding and complex formation.","date":"2009","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/19934057","citation_count":7,"is_preprint":false},{"pmid":"28277935","id":"PMC_28277935","title":"The pre-mRNA retention and splicing complex controls expression of the Mediator subunit Med20.","date":"2017","source":"RNA biology","url":"https://pubmed.ncbi.nlm.nih.gov/28277935","citation_count":5,"is_preprint":false},{"pmid":"30673728","id":"PMC_30673728","title":"Fluorescent labelling of membrane fatty acid transporter CD36 (SR-B2) in the extracellular loop.","date":"2019","source":"PloS one","url":"https://pubmed.ncbi.nlm.nih.gov/30673728","citation_count":4,"is_preprint":false},{"pmid":"38168634","id":"PMC_38168634","title":"Cellular localization and potential ligands of a novel scavenger receptor class B/CD36 protein homolog (Pt-SRB2) identified in the marine crab, Portunustrituberculatus.","date":"2024","source":"Fish & shellfish immunology","url":"https://pubmed.ncbi.nlm.nih.gov/38168634","citation_count":2,"is_preprint":false},{"pmid":"41108685","id":"PMC_41108685","title":"Med20 regulates myelination in the peripheral nervous system by modulating ferroptosis of Schwann cells.","date":"2025","source":"Cell reports","url":"https://pubmed.ncbi.nlm.nih.gov/41108685","citation_count":0,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":9555,"output_tokens":2482,"usd":0.032947,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":9687,"output_tokens":2919,"usd":0.060705,"stage2_stop_reason":"end_turn"},"total_usd":0.093652,"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\": 2006,\n      \"finding\": \"Med8, Med18, and Med20 form a subcomplex (Med8/18/20) within the Mediator head module. X-ray crystallography revealed that Med18 and Med20 adopt related beta-barrel folds. The N-terminal domain of Med8 binds TBP (TATA box-binding protein) in vitro. A conserved protein-interaction face on the Med8C/18/20 submodule includes sites altered by srb mutations that counteract defects from RNA Pol II truncation, supporting a positive role in initiation-complex formation.\",\n      \"method\": \"X-ray crystallography, in vitro TBP-binding assay, in vivo genetic analysis of srb mutations\",\n      \"journal\": \"Nature structural & molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structure combined with in vitro binding assay and in vivo genetic validation in a single rigorous study\",\n      \"pmids\": [\"16964259\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Med8, Med18, and Med20 are interdependent for proper folding and trimer complex formation. Renaturation experiments showed that the concurrent presence of all three subunits is required to form the correctly folded trimeric complex; pairwise combinations yield distinct subcomplexes with different folding states.\",\n      \"method\": \"Immunoprecipitation, far-UV circular dichroism, fluorescence spectroscopy on recombinantly expressed and denatured/renatured proteins\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — multiple orthogonal biophysical methods (CD, fluorescence, IP) in a single focused study on reconstituted proteins\",\n      \"pmids\": [\"19934057\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"Yeast SRB2 (MED20 ortholog) is required for DNA repair of MMS-induced damage; srb2 null cells are sensitive to prolonged MMS treatment, and an srb2 point mutation (Gly150Asp) suppresses hyperrecombination between direct repeats caused by hpr1Δ, linking SRB2-dependent transcription complex function to DNA repair and recombination.\",\n      \"method\": \"Genetic suppressor analysis, srb2 null and point-mutant allele construction, MMS sensitivity assay, recombination frequency measurement\",\n      \"journal\": \"Genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — clean genetic epistasis with multiple alleles and two distinct phenotypic readouts (recombination suppression and MMS sensitivity) in a single lab study\",\n      \"pmids\": [\"8844143\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Genetic interaction screens in S. cerevisiae identified MED20 (head module subunit) as required for transcriptional repression of ribosomal protein (RP) genes following rapamycin treatment; med20Δ shows synthetic sick/lethal interaction with maf1Δ, placing Med20 in a parallel pathway to Maf1 for negative regulation of RP mRNA synthesis.\",\n      \"method\": \"Genome-wide synthetic sick/lethal genetic screen, gene expression profiling (microarray), rapamycin treatment assays\",\n      \"journal\": \"PLoS genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic epistasis via SSL screen confirmed by gene expression profiling, single lab with two orthogonal approaches\",\n      \"pmids\": [\"18604275\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Loss of Med20 in fission yeast leads to accumulation of aberrant, polyadenylated readthrough tRNA transcripts targeted for exosome-mediated degradation, and similarly affects snRNA, snoRNA, and rRNA; this implicates Med20-containing Mediator in a regulatory pathway controlling Pol III-dependent transcription fidelity.\",\n      \"method\": \"Gene deletion (med20Δ), Northern blotting/RNA analysis, polyadenylate RNA purification, exosome mutant epistasis\",\n      \"journal\": \"Biochimica et biophysica acta\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — loss-of-function with direct RNA phenotype readout and genetic epistasis with exosome mutants, single lab\",\n      \"pmids\": [\"26608234\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"The RES complex (Bud13p/Snu17p/Pml1p) controls Med20 protein levels by promoting efficient splicing and nuclear retention of MED20 pre-mRNA; loss of bud13Δ or snu17Δ causes MED20 pre-mRNA splicing defects, accumulation of unspliced transcript targeted by NMD, and reduced Med20 levels that underlie temperature-sensitive growth defects.\",\n      \"method\": \"Genetic deletion analysis, RT-PCR/splicing assays, NMD pathway inactivation epistasis, growth phenotype rescue\",\n      \"journal\": \"RNA biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic epistasis with NMD pathway, direct splicing assay, and phenotypic rescue, single lab\",\n      \"pmids\": [\"28277935\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"MED20 is essential for early mouse embryogenesis; Med20 mutant blastocysts fail to hatch from the zona pellucida and show ectopic NANOG expression in the trophectoderm, indicating that MED20 is required for proper trophoblast specification and epiblast-trophectoderm boundary formation.\",\n      \"method\": \"Mouse knockout (Med20 null), blastocyst outgrowth assay, immunofluorescence for lineage markers (NANOG, CDX2, GATA6), cell death assays\",\n      \"journal\": \"Reproduction (Cambridge, England)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — clean KO with defined cellular phenotype and multiple lineage marker readouts, single lab\",\n      \"pmids\": [\"30571656\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"MED20 is a substrate of the CRL4-WDTC1 E3 ubiquitin ligase complex; WDTC1 overexpression leads to MED20 degradation while depletion of WDTC1 or CUL4A/B causes MED20 accumulation. MED20 promotes adipogenesis by bridging C/EBPβ and RNA Pol II at the PPARγ promoter to drive its transcription, as shown by ChIP-seq.\",\n      \"method\": \"Affinity purification/candidate screening, co-immunoprecipitation, overexpression/knockdown/knockout in preadipocytes, non-degradable mutant rescue, ChIP-seq, brown adipose tissue-specific Med20 knockout mouse\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — multiple orthogonal methods (AP, Co-IP, KO mouse, ChIP-seq, mutant rescue) in a single comprehensive study identifying E3 ligase, ubiquitination substrate relationship, and transcriptional mechanism\",\n      \"pmids\": [\"34233190\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Med20 loss in Schwann cells induces ferroptosis and impairs peripheral nervous system myelination. Mechanistically, Med20 transcriptionally activates DDB1, which suppresses ferroptosis by regulating HO-1 (Hmox1) through a DDB1-UHRF1-BACH1-Hmox1 axis and by directly ubiquitinating HO-1 protein. Ferroptosis inhibitor Fer-1 or HO-1 inhibitor ZnPP restores myelination in Med20-deficient mice.\",\n      \"method\": \"Schwann cell-specific Med20 knockout mouse, ferroptosis assays, ChIP, co-immunoprecipitation, ubiquitination assay, pharmacological rescue (Fer-1, ZnPP), gene expression analysis\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — conditional KO with defined cellular phenotype, mechanistic pathway delineated by ChIP and Co-IP, pharmacological rescue, single lab\",\n      \"pmids\": [\"41108685\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"MED20 is a head-module subunit of the Mediator complex that forms an interdependent trimeric subcomplex with Med8 and Med18 (adopting a beta-barrel fold), contributes to TBP binding and RNA Pol II initiation-complex assembly, represses ribosomal protein gene transcription, and—at the cellular level—acts as a substrate of the CRL4-WDTC1 E3 ubiquitin ligase to control adipogenesis by bridging C/EBPβ and Pol II at the PPARγ promoter, while in Schwann cells it regulates myelination by transcriptionally activating DDB1 to suppress ferroptosis via a DDB1-UHRF1-BACH1-HO-1 axis.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"MED20 is a subunit of the Mediator head module that participates in RNA polymerase II transcription initiation by assembling, together with Med8 and Med18, into an interdependent trimeric submodule in which Med18 and Med20 adopt related beta-barrel folds and the Med8 N-terminus contacts the TATA box-binding protein [#0, #1]. The three subunits are mutually required for correct folding and trimer formation, and the submodule presents a conserved interaction face whose mutation counteracts defects from Pol II truncation, marking a positive role in initiation-complex assembly [#0, #1]. In yeast, MED20 contributes to negative regulation of ribosomal protein gene transcription in a pathway parallel to Maf1 [#3], and its loss perturbs Pol III-dependent transcript fidelity, causing accumulation of aberrant readthrough tRNA, snRNA, snoRNA, and rRNA species that are degraded by the exosome [#4]. In mammals, MED20 is essential for early embryogenesis, with mutant blastocysts failing to hatch and showing disrupted trophoblast specification [#6]. MED20 is a substrate of the CRL4-WDTC1 E3 ubiquitin ligase, and through this controlled abundance it drives adipogenesis by bridging C/EBPβ and Pol II at the PPARγ promoter [#7]. In Schwann cells, MED20 transcriptionally activates DDB1 to suppress ferroptosis via a DDB1-UHRF1-BACH1-HO-1 axis, thereby supporting peripheral nerve myelination [#8].\",\n  \"teleology\": [\n    {\n      \"year\": 1996,\n      \"claim\": \"Before its biochemical role was defined, the MED20 ortholog SRB2 was linked to transcription-coupled genome maintenance, showing that this Mediator-associated factor influences DNA repair and recombination outcomes.\",\n      \"evidence\": \"Genetic suppressor analysis with srb2 null and point-mutant alleles, MMS sensitivity and recombination frequency assays in yeast\",\n      \"pmids\": [\"8844143\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Does not establish a direct biochemical activity for SRB2/MED20\", \"Connection to repair is genetic and may be indirect through transcription complex function\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"To define how MED20 contributes to Mediator architecture, structural and binding work showed it forms a Med8/18/20 head submodule with a beta-barrel fold and a conserved interaction face supporting initiation-complex formation.\",\n      \"evidence\": \"X-ray crystallography, in vitro TBP-binding assay, and in vivo srb mutation analysis\",\n      \"pmids\": [\"16964259\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"TBP binding maps to Med8, not directly to MED20\", \"Does not resolve MED20's contribution within the holo-Mediator/Pol II initiation complex\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"To test whether the three subunits assemble independently, renaturation experiments established that Med8, Med18, and Med20 are mutually required for correct folding and trimer formation.\",\n      \"evidence\": \"IP, far-UV circular dichroism, and fluorescence spectroscopy on denatured/renatured recombinant proteins\",\n      \"pmids\": [\"19934057\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Performed on isolated recombinant proteins, not within intact Mediator\", \"Does not address assembly order in vivo\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"To place MED20 in a regulatory context, genetic screens showed it is required for repression of ribosomal protein genes upon rapamycin treatment in a pathway parallel to Maf1.\",\n      \"evidence\": \"Genome-wide synthetic sick/lethal screen with expression profiling in S. cerevisiae\",\n      \"pmids\": [\"18604275\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Genetic, not biochemical, definition of the repression mechanism\", \"Relationship between head-module function and RP gene repression unresolved\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Extending MED20 function beyond Pol II, loss-of-function in fission yeast linked Med20-containing Mediator to Pol III transcript fidelity.\",\n      \"evidence\": \"med20Δ deletion with Northern/RNA analysis, polyadenylated RNA purification, and exosome mutant epistasis\",\n      \"pmids\": [\"26608234\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism by which Mediator influences Pol III transcripts is unclear\", \"Direct versus indirect effect not distinguished\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"To assess the organismal requirement for MED20, mouse knockout established it is essential for early embryogenesis and proper trophoblast specification.\",\n      \"evidence\": \"Med20-null mouse, blastocyst outgrowth assay, and lineage-marker immunofluorescence\",\n      \"pmids\": [\"30571656\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Does not identify the transcriptional targets underlying the lineage defect\", \"Cannot separate global Mediator loss from a specific MED20 function\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"To explain how MED20 abundance is controlled and used in a differentiation program, work identified it as a CRL4-WDTC1 substrate that bridges C/EBPβ and Pol II at the PPARγ promoter to drive adipogenesis.\",\n      \"evidence\": \"Affinity purification, Co-IP, degradation/rescue assays, ChIP-seq, and brown adipose-specific Med20 knockout mouse\",\n      \"pmids\": [\"34233190\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Ubiquitination site and degradation kinetics not fully mapped\", \"Generality of MED20 promoter-bridging beyond PPARγ unknown\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"To define a tissue-specific role, Schwann cell studies showed MED20 transcriptionally activates DDB1 to suppress ferroptosis and support peripheral myelination.\",\n      \"evidence\": \"Schwann cell-specific Med20 knockout mouse, ferroptosis assays, ChIP, Co-IP, ubiquitination assay, and pharmacological rescue (Fer-1, ZnPP)\",\n      \"pmids\": [\"41108685\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether DDB1 activation reflects direct MED20 promoter occupancy or Mediator-wide effect is not separated\", \"Single-lab characterization of the DDB1-UHRF1-BACH1-HO-1 axis\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How the conserved structural role of MED20 in the Mediator head module mechanistically connects to its diverse context-specific outputs (RP gene repression, Pol III fidelity, adipogenesis, ferroptosis suppression) remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No structural model of MED20 within an active promoter-bound complex driving these target genes\", \"Unclear which functions are Mediator-intrinsic versus moonlighting\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140110\", \"supporting_discovery_ids\": [7, 8]},\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [0, 1]},\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [7]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [0, 7]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-74160\", \"supporting_discovery_ids\": [0, 7]},\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [6, 8]}\n    ],\n    \"complexes\": [\"Mediator complex (head module)\", \"Med8/Med18/Med20 submodule\"],\n    \"partners\": [\"MED8\", \"MED18\", \"TBP\", \"WDTC1\", \"CEBPB\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":6,"faith_total":6,"faith_pct":100.0}}