{"gene":"MED18","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 with two submodules: the N-terminal domain of Med8 binds TBP in vitro, while the C-terminal Med8/Med18/Med20 submodule reveals that Med18 and Med20 adopt related beta-barrel folds. A conserved putative protein-interaction face on this submodule includes sites altered by srb mutations that counteract defects from Pol II truncation, supporting a positive role in initiation-complex formation.","method":"X-ray crystallography, in vitro TBP-binding assay, genetic epistasis (srb mutations)","journal":"Nature structural & molecular biology","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structure combined with in vitro binding assay and genetic validation in a single rigorous study","pmids":["16964259"],"is_preprint":false},{"year":2003,"finding":"The mammalian homologue of yeast Srb5 (MED18) is a bona fide subunit of the mammalian Mediator complex, identified by tandem mass spectrometry of purified Med8-containing fractions, with direct pairwise binding partners among known mammalian Mediator subunits determined biochemically.","method":"Tandem mass spectrometry of purified complex, direct biochemical pairwise binding assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Strong — biochemical purification plus MS identification plus pairwise binding experiments; foundational identification of the mammalian subunit","pmids":["12584197"],"is_preprint":false},{"year":2009,"finding":"Med8, Med18, and Med20 are interdependent for proper folding and trimer complex formation; the concurrent presence of all three subunits during renaturation is required for proper folding, although each can form soluble monomers and pairwise subcomplexes individually.","method":"Immunoprecipitation, far-UV circular dichroism, fluorescence spectroscopy on recombinantly expressed and 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 study with reconstitution approach","pmids":["19934057"],"is_preprint":false},{"year":2011,"finding":"Yeast Srb5/Med18 is required for proper termination of transcription of a subset of genes (INO1, CHA1); in srb5-null cells, TBP and TFIIB recruitment to promoters is unaffected, but cross-linking of cleavage-polyadenylation factors Rna15 and Pta1 to the 3' end of genes is compromised, RNA Pol II accumulates near and beyond 3' ends, and transcription readthrough occurs.","method":"Chromatin immunoprecipitation (ChIP), transcription run-on analysis, srb5 deletion strain analysis","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple methods (ChIP, transcription run-on) in a single lab with clear mechanistic readouts","pmids":["21921038"],"is_preprint":false},{"year":2013,"finding":"Srb5/Med18-mediated transcription termination operates through gene looping: Srb5/Med18 physically interacts with the CF1 (cleavage factor 1) complex (but not the cleavage and polyadenylation factor complex) via co-immunoprecipitation; this interaction and Srb5/Med18 cross-linking to 3' ends of genes is abolished in the looping-defective sua7-1 strain; chromosome conformation capture shows looped gene architecture is disrupted in srb5-null cells; CTD-Ser2 phosphorylation is unaffected by Srb5 loss.","method":"Co-immunoprecipitation, ChIP, chromosome conformation capture (3C), analysis of looping-defective sua7-1 mutant","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal methods (Co-IP, ChIP, 3C, genetic epistasis) in a single lab","pmids":["23476016"],"is_preprint":false},{"year":2009,"finding":"The SRB5/MED18 gene was identified as a multicopy suppressor of the Rpb11 L111A point mutation in yeast, establishing a functional interaction between Med18 and the Rpb11 subunit of the RNA Pol II core enzyme.","method":"Multicopy suppressor screen, genetic suppression analysis","journal":"Bioorganicheskaia khimiia","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single genetic suppressor screen, single lab, no biochemical validation of direct interaction reported in abstract","pmids":["19928061"],"is_preprint":false},{"year":2014,"finding":"Human MED18 (hMED18) functions in transcriptional repression: siRNA-mediated depletion of hMED18 augments transcription of target genes (RAM/FAM103A1, DMT1/SLC11A2); hMED18 and middle module subunit hMED1 are lost from target gene promoters upon hMED18 depletion while CDK8 remains bound, indicating a repression mechanism mediated through CDK8 and a positive role for the free CDK/cyclin module in transcriptional activation. This repression is independent of HDAC activity (TSA- and nicotinamide-insensitive).","method":"siRNA knockdown, ChIP at target promoters, HDAC inhibitor treatment","journal":"Genes to cells : devoted to molecular & cellular mechanisms","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — siRNA KD with defined promoter occupancy ChIP readout and pharmacological controls, single lab","pmids":["24840924"],"is_preprint":false}],"current_model":"MED18 (Srb5) is a head-module subunit of the Mediator complex that adopts a beta-barrel fold, requires co-folding with Med8 and Med20 for proper trimer assembly, participates in initiation-complex formation via TBP binding (through Med8), and also facilitates transcription termination at a subset of genes by physically interacting with the CF1 cleavage factor complex in a gene-looping-dependent manner; in human cells, hMED18 acts as a transcriptional repressor at specific promoters through a CDK8-dependent mechanism that is independent of HDAC activity."},"narrative":{"mechanistic_narrative":"MED18 is a head-module subunit of the Mediator complex that contributes both to transcription initiation and, at a subset of genes, to transcription termination [PMID:16964259, PMID:21921038]. Within the head module it forms a Med8/Med18/Med20 subcomplex in which Med18 adopts a beta-barrel fold; proper trimer assembly requires the concurrent co-folding of all three subunits, and the submodule presents a conserved protein-interaction face whose mutation counteracts defects from Pol II truncation, linking it to initiation-complex formation through Med8-mediated TBP binding [PMID:16964259, PMID:19934057]. The mammalian ortholog is a bona fide Mediator subunit with defined pairwise contacts to other Mediator subunits [PMID:12584197]. Beyond initiation, yeast Srb5/Med18 is required for proper termination at a subset of genes (INO1, CHA1): its loss leaves TBP and TFIIB promoter recruitment intact but compromises recruitment of cleavage-polyadenylation factors to gene 3' ends, causing Pol II accumulation and readthrough [PMID:21921038]. This termination role operates through gene looping, as Med18 physically interacts with the CF1 cleavage factor complex and this interaction, along with looped gene architecture, is abolished in looping-defective cells [PMID:23476016]. In human cells, MED18 also acts as a transcriptional repressor at specific promoters through a CDK8-dependent mechanism that is independent of HDAC activity [PMID:24840924].","teleology":[{"year":2003,"claim":"Establishing that the mammalian Srb5 homolog is a genuine Mediator subunit was the prerequisite for studying its function in human transcription.","evidence":"tandem mass spectrometry of purified Med8-containing Mediator fractions with pairwise binding assays","pmids":["12584197"],"confidence":"High","gaps":["Does not define the structural organization of the subunit within the head module","No functional consequence of the subunit assignment tested"]},{"year":2006,"claim":"The crystal structure resolved how Med18 is organized within the head module and tied it mechanistically to initiation-complex formation.","evidence":"X-ray crystallography of the Med8/Med18/Med20 submodule, in vitro TBP-binding assay, and srb mutation epistasis","pmids":["16964259"],"confidence":"High","gaps":["TBP binding is attributed to the Med8 N-terminus, not Med18 directly","Does not address roles outside initiation"]},{"year":2009,"claim":"Reconstitution defined the assembly logic of the head submodule, showing Med8, Med18, and Med20 must co-fold to form a stable trimer.","evidence":"immunoprecipitation, far-UV circular dichroism, and fluorescence spectroscopy on recombinant renatured proteins","pmids":["19934057"],"confidence":"High","gaps":["In vitro renaturation may not recapitulate in vivo assembly chaperones","Does not connect folding interdependence to a transcriptional output"]},{"year":2009,"claim":"A genetic screen linked Med18 function to the Pol II core enzyme via Rpb11, hinting at a Mediator–polymerase functional coupling.","evidence":"multicopy suppressor screen of the Rpb11 L111A mutation in yeast","pmids":["19928061"],"confidence":"Low","gaps":["Genetic suppression only; no biochemical evidence of a direct Med18–Rpb11 interaction","Single lab, not independently validated"]},{"year":2011,"claim":"Med18 was assigned an unexpected role in transcription termination distinct from its initiation function, showing it is required for 3'-end processing factor recruitment at specific genes.","evidence":"ChIP and transcription run-on analysis in srb5-null yeast at INO1 and CHA1","pmids":["21921038"],"confidence":"Medium","gaps":["Mechanism by which a head-module subunit influences 3'-end events left unexplained","Limited to a subset of genes"]},{"year":2013,"claim":"Gene looping was identified as the mechanism connecting Med18 to termination, via a physical interaction with the CF1 cleavage factor complex.","evidence":"co-IP, ChIP, chromosome conformation capture, and the looping-defective sua7-1 mutant in yeast","pmids":["23476016"],"confidence":"Medium","gaps":["Interaction specificity to CF1 versus CPF needs deeper structural definition","Does not establish whether human MED18 performs the same looping role"]},{"year":2014,"claim":"Human MED18 was shown to act as a CDK8-dependent transcriptional repressor at specific promoters, distinguishing its mammalian role from the positive initiation function.","evidence":"siRNA knockdown, promoter ChIP, and HDAC inhibitor controls at RAM/FAM103A1 and DMT1/SLC11A2 in human cells","pmids":["24840924"],"confidence":"Medium","gaps":["Direct molecular basis of CDK8-dependent repression unresolved","Generality beyond the tested target promoters unknown"]},{"year":null,"claim":"How Med18's initiation, termination/looping, and CDK8-dependent repression activities are integrated across organisms and gene classes remains unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No structural model of Med18 engaging CF1 or CDK8","Conservation of the looping/termination role in human cells untested","Determinants of gene-selective repression versus activation unknown"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140110","term_label":"transcription regulator activity","supporting_discovery_ids":[0,6]},{"term_id":"GO:0005198","term_label":"structural molecule activity","supporting_discovery_ids":[0,2]}],"localization":[{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[3,6]}],"pathway":[{"term_id":"R-HSA-74160","term_label":"Gene expression (Transcription)","supporting_discovery_ids":[0,3,6]}],"complexes":["Mediator complex","Mediator head module (Med8/Med18/Med20 submodule)"],"partners":["MED8","MED20","TBP","CDK8","MED1"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q9BUE0","full_name":"Mediator of RNA polymerase II transcription subunit 18","aliases":["Mediator complex subunit 18","p28b"],"length_aa":208,"mass_kda":23.7,"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/Q9BUE0/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":true,"resolved_as":"","url":"https://depmap.org/portal/gene/MED18","classification":"Common Essential","n_dependent_lines":1089,"n_total_lines":1208,"dependency_fraction":0.9014900662251656},"opencell":{"profiled":true,"resolved_as":"","ensg_id":"ENSG00000130772","cell_line_id":"CID000239","localizations":[{"compartment":"nuclear_punctae","grade":3},{"compartment":"nucleoplasm","grade":3}],"interactors":[{"gene":"ANKRD39","stoichiometry":10.0},{"gene":"CKAP2","stoichiometry":10.0},{"gene":"CLASP1","stoichiometry":10.0},{"gene":"FXR1","stoichiometry":10.0},{"gene":"MED10","stoichiometry":10.0},{"gene":"MED11","stoichiometry":10.0},{"gene":"MED14","stoichiometry":10.0},{"gene":"MED20","stoichiometry":10.0},{"gene":"MED28","stoichiometry":10.0},{"gene":"MED8","stoichiometry":10.0}],"url":"https://opencell.sf.czbiohub.org/target/CID000239","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":"612382","title":"MEDIATOR COMPLEX SUBUNIT 10; MED10","url":"https://www.omim.org/entry/612382"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Nucleoplasm","reliability":"Approved"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/MED18"},"hgnc":{"alias_symbol":["FLJ20045","p28b","SRB5"],"prev_symbol":[]},"alphafold":{"accession":"Q9BUE0","domains":[{"cath_id":"2.40.320.10","chopping":"18-193","consensus_level":"medium","plddt":94.0611,"start":18,"end":193}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9BUE0","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9BUE0-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9BUE0-F1-predicted_aligned_error_v6.png","plddt_mean":89.5},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=MED18","jax_strain_url":"https://www.jax.org/strain/search?query=MED18"},"sequence":{"accession":"Q9BUE0","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9BUE0.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9BUE0/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9BUE0"}},"corpus_meta":[{"pmid":"24451981","id":"PMC_24451981","title":"MED18 interaction with distinct transcription factors regulates multiple plant functions.","date":"2014","source":"Nature communications","url":"https://pubmed.ncbi.nlm.nih.gov/24451981","citation_count":128,"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":"31534128","id":"PMC_31534128","title":"Long non-coding RNA SNHG3 promotes progression of gastric cancer by regulating neighboring MED18 gene methylation.","date":"2019","source":"Cell death & disease","url":"https://pubmed.ncbi.nlm.nih.gov/31534128","citation_count":78,"is_preprint":false},{"pmid":"12584197","id":"PMC_12584197","title":"Identification of mammalian Mediator subunits with similarities to yeast Mediator subunits Srb5, Srb6, Med11, and Rox3.","date":"2003","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/12584197","citation_count":48,"is_preprint":false},{"pmid":"21921038","id":"PMC_21921038","title":"Novel role for mediator complex subunit Srb5/Med18 in termination of transcription.","date":"2011","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/21921038","citation_count":46,"is_preprint":false},{"pmid":"23476016","id":"PMC_23476016","title":"Srb5/Med18-mediated termination of transcription is dependent on gene looping.","date":"2013","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/23476016","citation_count":33,"is_preprint":false},{"pmid":"30003619","id":"PMC_30003619","title":"Developmental role of the tomato Mediator complex subunit MED18 in pollen ontogeny.","date":"2018","source":"The Plant journal : for cell and molecular biology","url":"https://pubmed.ncbi.nlm.nih.gov/30003619","citation_count":21,"is_preprint":false},{"pmid":"36198124","id":"PMC_36198124","title":"Sulfate-Dependent Anaerobic Degradation of Herbicide Acetochlor by a Sulfate-Reducing Bacterium Cupidesulfovibrio sp. SRB-5.","date":"2022","source":"Journal of agricultural and food chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/36198124","citation_count":12,"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":"24840924","id":"PMC_24840924","title":"Mediator MED18 subunit plays a negative role in transcription via the CDK/cyclin module.","date":"2014","source":"Genes to cells : devoted to molecular & cellular mechanisms","url":"https://pubmed.ncbi.nlm.nih.gov/24840924","citation_count":7,"is_preprint":false},{"pmid":"19928061","id":"PMC_19928061","title":"[The functional interaction of an RNA polymerase II Rpb11 subunit with the Med18 subunit (Srb5) of the Saccharomyces cerevisiae mediator complex].","date":"2009","source":"Bioorganicheskaia khimiia","url":"https://pubmed.ncbi.nlm.nih.gov/19928061","citation_count":4,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":6689,"output_tokens":1874,"usd":0.024088,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":8720,"output_tokens":2383,"usd":0.051588,"stage2_stop_reason":"end_turn"},"total_usd":0.075676,"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 with two submodules: the N-terminal domain of Med8 binds TBP in vitro, while the C-terminal Med8/Med18/Med20 submodule reveals that Med18 and Med20 adopt related beta-barrel folds. A conserved putative protein-interaction face on this submodule includes sites altered by srb mutations that counteract defects from Pol II truncation, supporting a positive role in initiation-complex formation.\",\n      \"method\": \"X-ray crystallography, in vitro TBP-binding assay, genetic epistasis (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 genetic validation in a single rigorous study\",\n      \"pmids\": [\"16964259\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"The mammalian homologue of yeast Srb5 (MED18) is a bona fide subunit of the mammalian Mediator complex, identified by tandem mass spectrometry of purified Med8-containing fractions, with direct pairwise binding partners among known mammalian Mediator subunits determined biochemically.\",\n      \"method\": \"Tandem mass spectrometry of purified complex, direct biochemical pairwise binding assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — biochemical purification plus MS identification plus pairwise binding experiments; foundational identification of the mammalian subunit\",\n      \"pmids\": [\"12584197\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Med8, Med18, and Med20 are interdependent for proper folding and trimer complex formation; the concurrent presence of all three subunits during renaturation is required for proper folding, although each can form soluble monomers and pairwise subcomplexes individually.\",\n      \"method\": \"Immunoprecipitation, far-UV circular dichroism, fluorescence spectroscopy on recombinantly expressed and 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 study with reconstitution approach\",\n      \"pmids\": [\"19934057\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Yeast Srb5/Med18 is required for proper termination of transcription of a subset of genes (INO1, CHA1); in srb5-null cells, TBP and TFIIB recruitment to promoters is unaffected, but cross-linking of cleavage-polyadenylation factors Rna15 and Pta1 to the 3' end of genes is compromised, RNA Pol II accumulates near and beyond 3' ends, and transcription readthrough occurs.\",\n      \"method\": \"Chromatin immunoprecipitation (ChIP), transcription run-on analysis, srb5 deletion strain analysis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple methods (ChIP, transcription run-on) in a single lab with clear mechanistic readouts\",\n      \"pmids\": [\"21921038\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Srb5/Med18-mediated transcription termination operates through gene looping: Srb5/Med18 physically interacts with the CF1 (cleavage factor 1) complex (but not the cleavage and polyadenylation factor complex) via co-immunoprecipitation; this interaction and Srb5/Med18 cross-linking to 3' ends of genes is abolished in the looping-defective sua7-1 strain; chromosome conformation capture shows looped gene architecture is disrupted in srb5-null cells; CTD-Ser2 phosphorylation is unaffected by Srb5 loss.\",\n      \"method\": \"Co-immunoprecipitation, ChIP, chromosome conformation capture (3C), analysis of looping-defective sua7-1 mutant\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal methods (Co-IP, ChIP, 3C, genetic epistasis) in a single lab\",\n      \"pmids\": [\"23476016\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"The SRB5/MED18 gene was identified as a multicopy suppressor of the Rpb11 L111A point mutation in yeast, establishing a functional interaction between Med18 and the Rpb11 subunit of the RNA Pol II core enzyme.\",\n      \"method\": \"Multicopy suppressor screen, genetic suppression analysis\",\n      \"journal\": \"Bioorganicheskaia khimiia\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single genetic suppressor screen, single lab, no biochemical validation of direct interaction reported in abstract\",\n      \"pmids\": [\"19928061\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Human MED18 (hMED18) functions in transcriptional repression: siRNA-mediated depletion of hMED18 augments transcription of target genes (RAM/FAM103A1, DMT1/SLC11A2); hMED18 and middle module subunit hMED1 are lost from target gene promoters upon hMED18 depletion while CDK8 remains bound, indicating a repression mechanism mediated through CDK8 and a positive role for the free CDK/cyclin module in transcriptional activation. This repression is independent of HDAC activity (TSA- and nicotinamide-insensitive).\",\n      \"method\": \"siRNA knockdown, ChIP at target promoters, HDAC inhibitor treatment\",\n      \"journal\": \"Genes to cells : devoted to molecular & cellular mechanisms\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — siRNA KD with defined promoter occupancy ChIP readout and pharmacological controls, single lab\",\n      \"pmids\": [\"24840924\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"MED18 (Srb5) is a head-module subunit of the Mediator complex that adopts a beta-barrel fold, requires co-folding with Med8 and Med20 for proper trimer assembly, participates in initiation-complex formation via TBP binding (through Med8), and also facilitates transcription termination at a subset of genes by physically interacting with the CF1 cleavage factor complex in a gene-looping-dependent manner; in human cells, hMED18 acts as a transcriptional repressor at specific promoters through a CDK8-dependent mechanism that is independent of HDAC activity.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"MED18 is a head-module subunit of the Mediator complex that contributes both to transcription initiation and, at a subset of genes, to transcription termination [#0, #3]. Within the head module it forms a Med8/Med18/Med20 subcomplex in which Med18 adopts a beta-barrel fold; proper trimer assembly requires the concurrent co-folding of all three subunits, and the submodule presents a conserved protein-interaction face whose mutation counteracts defects from Pol II truncation, linking it to initiation-complex formation through Med8-mediated TBP binding [#0, #2]. The mammalian ortholog is a bona fide Mediator subunit with defined pairwise contacts to other Mediator subunits [#1]. Beyond initiation, yeast Srb5/Med18 is required for proper termination at a subset of genes (INO1, CHA1): its loss leaves TBP and TFIIB promoter recruitment intact but compromises recruitment of cleavage-polyadenylation factors to gene 3' ends, causing Pol II accumulation and readthrough [#3]. This termination role operates through gene looping, as Med18 physically interacts with the CF1 cleavage factor complex and this interaction, along with looped gene architecture, is abolished in looping-defective cells [#4]. In human cells, MED18 also acts as a transcriptional repressor at specific promoters through a CDK8-dependent mechanism that is independent of HDAC activity [#6].\",\n  \"teleology\": [\n    {\n      \"year\": 2003,\n      \"claim\": \"Establishing that the mammalian Srb5 homolog is a genuine Mediator subunit was the prerequisite for studying its function in human transcription.\",\n      \"evidence\": \"tandem mass spectrometry of purified Med8-containing Mediator fractions with pairwise binding assays\",\n      \"pmids\": [\"12584197\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Does not define the structural organization of the subunit within the head module\", \"No functional consequence of the subunit assignment tested\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"The crystal structure resolved how Med18 is organized within the head module and tied it mechanistically to initiation-complex formation.\",\n      \"evidence\": \"X-ray crystallography of the Med8/Med18/Med20 submodule, in vitro TBP-binding assay, and srb mutation epistasis\",\n      \"pmids\": [\"16964259\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"TBP binding is attributed to the Med8 N-terminus, not Med18 directly\", \"Does not address roles outside initiation\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Reconstitution defined the assembly logic of the head submodule, showing Med8, Med18, and Med20 must co-fold to form a stable trimer.\",\n      \"evidence\": \"immunoprecipitation, far-UV circular dichroism, and fluorescence spectroscopy on recombinant renatured proteins\",\n      \"pmids\": [\"19934057\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vitro renaturation may not recapitulate in vivo assembly chaperones\", \"Does not connect folding interdependence to a transcriptional output\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"A genetic screen linked Med18 function to the Pol II core enzyme via Rpb11, hinting at a Mediator–polymerase functional coupling.\",\n      \"evidence\": \"multicopy suppressor screen of the Rpb11 L111A mutation in yeast\",\n      \"pmids\": [\"19928061\"],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"Genetic suppression only; no biochemical evidence of a direct Med18–Rpb11 interaction\", \"Single lab, not independently validated\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Med18 was assigned an unexpected role in transcription termination distinct from its initiation function, showing it is required for 3'-end processing factor recruitment at specific genes.\",\n      \"evidence\": \"ChIP and transcription run-on analysis in srb5-null yeast at INO1 and CHA1\",\n      \"pmids\": [\"21921038\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism by which a head-module subunit influences 3'-end events left unexplained\", \"Limited to a subset of genes\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Gene looping was identified as the mechanism connecting Med18 to termination, via a physical interaction with the CF1 cleavage factor complex.\",\n      \"evidence\": \"co-IP, ChIP, chromosome conformation capture, and the looping-defective sua7-1 mutant in yeast\",\n      \"pmids\": [\"23476016\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Interaction specificity to CF1 versus CPF needs deeper structural definition\", \"Does not establish whether human MED18 performs the same looping role\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Human MED18 was shown to act as a CDK8-dependent transcriptional repressor at specific promoters, distinguishing its mammalian role from the positive initiation function.\",\n      \"evidence\": \"siRNA knockdown, promoter ChIP, and HDAC inhibitor controls at RAM/FAM103A1 and DMT1/SLC11A2 in human cells\",\n      \"pmids\": [\"24840924\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct molecular basis of CDK8-dependent repression unresolved\", \"Generality beyond the tested target promoters unknown\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How Med18's initiation, termination/looping, and CDK8-dependent repression activities are integrated across organisms and gene classes remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No structural model of Med18 engaging CF1 or CDK8\", \"Conservation of the looping/termination role in human cells untested\", \"Determinants of gene-selective repression versus activation unknown\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140110\", \"supporting_discovery_ids\": [0, 6]},\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [0, 2]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [3, 6]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-74160\", \"supporting_discovery_ids\": [0, 3, 6]}\n    ],\n    \"complexes\": [\"Mediator complex\", \"Mediator head module (Med8/Med18/Med20 submodule)\"],\n    \"partners\": [\"MED8\", \"MED20\", \"TBP\", \"CDK8\", \"MED1\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":6,"faith_total":6,"faith_pct":100.0}}