Affinage

MED21

Mediator of RNA polymerase II transcription subunit 21 · UniProt Q13503

Length
144 aa
Mass
15.6 kDa
Annotated
2026-04-28
34 papers in source corpus 15 papers cited in narrative 15 extracted findings

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

MED21 (SRB7) is an essential, ubiquitously expressed subunit of the Mediator middle module that functions as a central structural and regulatory node coupling RNA polymerase II to transcriptional activators and repressors. MED21 forms a four-helix-bundle heterodimer with MED7, connected by a flexible hinge whose integrity is required for Mediator–Pol II holoenzyme assembly; it also contacts MED6, bridging the middle and head modules, and interacts with the co-repressor Tup1 (PMID:15710619, PMID:27821593, PMID:16758199). Beyond transcription initiation, MED21 promotes Pol II elongation through coding regions and facilitates histone displacement, and in mammalian keratinocytes it coactivates the vitamin D receptor to coordinate proliferation and differentiation (PMID:22377631, PMID:20520624). Disruption of MED21 is lethal in murine embryonic stem cells, establishing it as essential for mammalian embryonic development (PMID:10500093).

Mechanistic history

Synthesis pass · year-by-year structured walk · 10 steps
  1. 1996 High

    Establishing that MED21 is a bona fide subunit of a mammalian RNA Pol II holoenzyme resolved whether yeast Mediator components have functional human counterparts and showed the holoenzyme is more responsive to activators than core Pol II.

    Evidence Immunopurification with anti-SRB7 antibodies co-precipitated RNAPII, TFIIE, and TFIIH; purified complex supported activator-enhanced transcription in vitro (human cell extracts)

    PMID:8598913 PMID:8871557

    Open questions at the time
    • Precise position of MED21 within the Mediator architecture unknown
    • No direct demonstration of MED21 contribution distinct from other subunits
  2. 1998 High

    Discovery that MED21-containing complexes (NAT/SMCC) can both repress and activate Pol II transcription depending on context established the dual regulatory capacity of the Mediator and revealed CTD-kinase-dependent repression distinct from TFIIH.

    Evidence Biochemical purification of NAT and Mediator complexes with CTD kinase assays and reconstituted in vitro transcription (human)

    PMID:10024883 PMID:9671713 PMID:9734358

    Open questions at the time
    • How MED21 itself contributes to switching between activation and repression was unclear
    • No structural data on MED21 contacts within the complex
  3. 1999 High

    Gene knockout demonstrated that MED21 is essential for murine cell viability and embryonic development, and that MED21 associates exclusively with high-molecular-weight Pol II complexes, confirming it functions only within the Mediator.

    Evidence Targeted gene disruption in mouse ES cells; Northern blot showing ubiquitous expression; co-fractionation of Srb7 with large Pol II complexes

    PMID:10500093 PMID:9933582

    Open questions at the time
    • Stage-specific developmental role not characterized
    • Molecular basis of lethality unresolved
  4. 2005 High

    Solving the MED7·MED21 heterodimer crystal structure revealed the four-helix bundle and flexible hinge architecture, identifying protein-binding surfaces that organize the middle module and bridge to the head module via MED6.

    Evidence X-ray crystallography at 3.0 Å resolution with protein-protein interaction mapping (yeast proteins)

    PMID:15710619

    Open questions at the time
    • Structure of the full middle module in complex with MED21 not yet determined
    • Functional significance of the hinge flexibility not tested
  5. 2006 Medium

    Systematic interaction mapping showed MED21 contacts MED7, MED4, MED10, MED6, and the co-repressor Tup1, with the N-terminal residues 2–8 critical for assembly, establishing MED21 as a molecular interaction hub within Mediator.

    Evidence Two-hybrid, co-immunoprecipitation of recombinant proteins from insect cells and E. coli, high-copy suppressor screen (yeast)

    PMID:16758199

    Open questions at the time
    • Direct structural basis of Tup1–MED21 interaction not resolved
    • Whether Tup1 interaction is conserved in mammals is untested
  6. 2012 Medium

    Demonstrating that MED21 mutations block Pol II elongation and histone displacement without impairing promoter recruitment expanded the Mediator's role beyond initiation to post-initiation/elongation control.

    Evidence ChIP for Pol II occupancy across coding regions, 6-azauracil sensitivity, genetic epistasis with elongation factors (yeast med21 mutants)

    PMID:22377631

    Open questions at the time
    • Mechanism by which MED21/middle module facilitates elongation is unclear
    • Not confirmed in mammalian system
  7. 2013 Medium

    Cross-linking mass spectrometry placed MED7/MED21 at the center of an extended, flexible middle-module tetramer with MED4/MED9, refining the architectural model of how MED21 organizes this module.

    Evidence Lysine–lysine chemical cross-linking with mass spectrometry and homology modeling (yeast middle module)

    PMID:23939621

    Open questions at the time
    • Model resolution limited by cross-link distance constraints
    • Dynamic conformational states not captured
  8. 2016 High

    Point mutations in the MED7–MED21 hinge proved it is specifically required for Mediator–Pol II holoenzyme formation, directly linking the hinge flexibility observed crystallographically to a functional requirement for Pol II engagement.

    Evidence Site-directed mutagenesis of hinge residues, co-purification/affinity assays, and electron microscopy of human Mediator

    PMID:27821593

    Open questions at the time
    • Atomic-resolution structure of hinge mutant Mediator not available
    • Whether hinge conformational changes are regulated in vivo is unknown
  9. 2010 Medium

    MED21 knockdown in keratinocytes revealed a specific physiological role in VDR-dependent transcription controlling proliferation and differentiation, linking a structural Mediator subunit to a defined receptor coactivation program.

    Evidence siRNA knockdown of MED21 in human keratinocytes with proliferation, differentiation, and gene expression readouts

    PMID:20520624

    Open questions at the time
    • Whether effect is direct VDR coactivation vs. general transcription impairment not fully distinguished
    • In vivo relevance in skin biology not tested
  10. 2014 Medium

    MED21 knockdown impaired HIV-1 replication at a post-integration step, extending MED21's role to viral transcription regulation and identifying Mediator as a host dependency factor for HIV.

    Evidence siRNA knockdown with HIV-1 replication and transcript quantification assays (human cells)

    PMID:25100719

    Open questions at the time
    • Direct vs. indirect role in Tat-dependent transcription not resolved
    • Single-lab finding without independent replication

Open questions

Synthesis pass · forward-looking unresolved questions
  • Key open questions include how MED21 conformational dynamics are regulated in vivo to switch between activation and repression, the structural basis of co-repressor (Tup1/TLE) recruitment through MED21, and whether MED21's elongation function operates through the same hinge-dependent mechanism that controls holoenzyme assembly.
  • No high-resolution structure of MED21 within the complete human Mediator–Pol II–PIC complex
  • Mechanism of MED21-dependent elongation control unresolved
  • In vivo gene-specific versus global transcription contributions of MED21 not systematically mapped

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0140110 transcription regulator activity 5 GO:0005198 structural molecule activity 3
Localization
GO:0005634 nucleus 3
Pathway
R-HSA-74160 Gene expression (Transcription) 6 R-HSA-1266738 Developmental Biology 1
Partners
CCNCCDK8MED10MED4MED6MED7MED9
Complex memberships
Mediator complex (middle module)RNA Pol II holoenzyme

Evidence

Reading pass · 15 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
1998 Murine MED21 (SRB7) co-purifies as part of a mammalian Mediator complex that binds to the RNA polymerase II C-terminal domain (CTD) and stimulates phosphorylation of the CTD by TFIIH, identifying it as a subunit of the mammalian transcriptional Mediator. Biochemical purification, peptide sequencing, CTD binding and phosphorylation assays Proceedings of the National Academy of Sciences of the United States of America High 9671713
1996 Human SRB7 (MED21) protein is a component of a mammalian RNA polymerase II holoenzyme; antibodies against human SRB7 were used to purify a complex containing RNAPII, TFIIE, and TFIIH that is more responsive to transcriptional activators than core RNAPII. Immunopurification with anti-SRB7 antibodies, transcription assays Nature High 8598913
1998 Human SRB7 (MED21) is a subunit of the NAT complex (containing homologs of yeast Srb7, Srb10, Srb11, Rgr1, and Med6) that represses activated transcription; the complex phosphorylates the CTD of RNAPII at residues distinct from those phosphorylated by TFIIH, and interacts with RNAPII in a manner not mediated by the CTD but precluded by CTD phosphorylation. Biochemical purification, CTD kinase assay, co-immunoprecipitation with RNAPII Molecular cell High 9734358
1999 Human SRB7 (MED21) is a component of the SMCC complex that can repress activator-dependent transcription or synergistically enhance it at limiting TFIIH; the complex shows direct activator interactions and can act independently of the RNAPII CTD. Biochemical purification, in vitro transcription assays, activator interaction assays Molecular cell High 10024883
1999 Murine Srb7 (MED21) gene is single-copy and ubiquitously expressed; disruption in embryonic stem cells revealed it is essential for cell viability and murine embryonic development, and murine Srb7 associates exclusively with high molecular weight forms of RNA polymerase II in extracts. Northern blot, gene disruption (KO) in ES cells, co-fractionation with RNAPII Genes & development High 10500093
1999 Human SRB7 (MED21) is an integral component of an RNA polymerase II-SRB complex: anti-SRB7 antibody immunoprecipitates hTRFP and RNAPII, and reciprocally anti-hTRFP immunoprecipitates RNAPII and SRB7; the complex supports basal transcription and enhances Gal4-VP16-activated transcription in the presence of PC4. Reciprocal co-immunoprecipitation, in vitro transcription assay The Journal of biological chemistry High 9933582
1996 Human SRB7 (MED21) co-fractionates with CDK8, cyclin C, and E1A/VP16-associated CTD kinase activity in a ~1.5 MDa complex consistent with the RNAPII holoenzyme, indicating MED21 is part of the holoenzyme complex that viral transactivators interact with. Gel filtration chromatography, co-fractionation, immunoblotting Nucleic acids research Medium 8871557
2005 The MED7·MED21 (Med7·Srb7) heterodimer structure was solved at 3.0 Å by X-ray crystallography, revealing a four-helix bundle domain and a coiled-coil protrusion connected by a flexible hinge; four putative protein-binding sites allow assembly of the Mediator middle module and binding of MED6, which bridges to the Mediator head module. X-ray crystallography (3.0 Å), protein-protein interaction assays The Journal of biological chemistry High 15710619
2013 A 3D model of the Mediator middle module places the MED7/MED21 heterodimer as part of a central tetramer with Med4/Med9, flanked by Med10 and Med31, with the module being highly extended and flexible, based on lysine-lysine cross-links mapped by mass spectrometry. Chemical cross-linking mass spectrometry, homology modeling Nucleic acids research Medium 23939621
2016 The integrity of the conserved hinge in the MED21-MED7 heterodimer is required for human Mediator binding to RNA polymerase II to form the holoenzyme; point mutations in the hinge leave core Mediator intact but cause increased disorder of the middle module and markedly reduced affinity for Pol II. Biochemistry (co-purification, affinity assays), site-directed mutagenesis, electron microscopy The Journal of biological chemistry High 27821593
2002 In yeast, LexA-Srb7 (Med21) fusion is a cryptic transcriptional activator that becomes active in the absence of Srb8, Srb10, Srb11, or Sin4, and is stably associated with Med4 and Med8 when incorporated into Mediator, suggesting functional interactions within the complex. Transcriptional activation assays with LexA fusions, co-immunoprecipitation of tagged proteins The Journal of biological chemistry Medium 12468546
2006 Yeast Med21 (Srb7) interacts with Mediator subunits Med7, Med10, and Med4 (confirmed by co-immunoprecipitation of tagged proteins from insect cells and E. coli), and these interactions depend strongly on amino acid residues 2-8 of Med21; Med21 also interacts with Med6 and the co-repressor Tup1, suggesting Med21 serves as a molecular switchboard integrating signals. Two-hybrid assay, co-immunoprecipitation of recombinant tagged proteins from insect cells and E. coli, high-copy suppressor screen Molecular genetics and genomics : MGG Medium 16758199
2012 Yeast Med21 mutations (ewe alleles) severely impair heat-shock-induced expression of HSP82 by blocking Pol II elongation through the coding region without impairing Pol II recruitment to the promoter; med21 mutations also impair histone displacement from promoter and coding regions and reduce Pol II processivity on GAL1-regulated genes, implicating the middle module in regulating post-initiation steps. Genetic epistasis, chromatin immunoprecipitation (ChIP), 6-azauracil sensitivity assay Genetics Medium 22377631
2010 MED21 (human) promotes keratinocyte proliferation and differentiation as part of the DRIP/Mediator complex coactivating vitamin D receptor (VDR); siRNA knockdown of MED21 caused hyperproliferation accompanied by increases in cyclin D1 and Gli mRNA, and defects in calcium-induced differentiation with decreased differentiation markers and impaired E-cadherin membrane translocation. VDR affinity bead purification, mass spectrometry, siRNA knockdown with proliferation and differentiation phenotypic readouts The Journal of investigative dermatology Medium 20520624
2014 siRNA-mediated knockdown of human MED21 significantly impairs HIV-1 replication at a post-integration step, implicating MED21 in regulation of HIV transcription. siRNA knockdown, HIV replication assays, early/late transcript quantification The Journal of biological chemistry Medium 25100719

Source papers

Stage 0 corpus · 34 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
1998 Mammalian mediator of transcriptional regulation and its possible role as an end-point of signal transduction pathways. Proceedings of the National Academy of Sciences of the United States of America 258 9671713
1999 A novel human SRB/MED-containing cofactor complex, SMCC, involved in transcription regulation. Molecular cell 232 10024883
1998 NAT, a human complex containing Srb polypeptides that functions as a negative regulator of activated transcription. Molecular cell 194 9734358
2009 HISTONE MONOUBIQUITINATION1 interacts with a subunit of the mediator complex and regulates defense against necrotrophic fungal pathogens in Arabidopsis. The Plant cell 193 19286969
1996 A mammalian SRB protein associated with an RNA polymerase II holoenzyme. Nature 131 8598913
2008 Malleable machines in transcription regulation: the mediator complex. PLoS computational biology 102 19096501
2003 Quantitative analysis of binding of transcription factor complex to biotinylated DNA probe by a streptavidin-agarose pulldown assay. Analytical biochemistry 66 14622953
2006 A subunit of the mediator complex regulates vertebrate neuronal development. Proceedings of the National Academy of Sciences of the United States of America 63 17088561
2005 A conserved mediator hinge revealed in the structure of the MED7.MED21 (Med7.Srb7) heterodimer. The Journal of biological chemistry 61 15710619
1999 Ubiquitous expression and embryonic requirement for RNA polymerase II coactivator subunit Srb7 in mice. Genes & development 59 10500093
2021 Repression by the Arabidopsis TOPLESS corepressor requires association with the core mediator complex. eLife 55 34075876
2003 Recruitment of Tup1-Ssn6 by yeast hypoxic genes and chromatin-independent exclusion of TATA binding protein. Eukaryotic cell 55 14665463
1996 Viral transactivators E1A and VP16 interact with a large complex that is associated with CTD kinase activity and contains CDK8. Nucleic acids research 49 8871557
2000 Artificial recruitment of TFIID, but not RNA polymerase II holoenzyme, activates transcription in mammalian cells. Molecular and cellular biology 41 10825198
2003 Characterization of mutations that are synthetic lethal with pol3-13, a mutated allele of DNA polymerase delta in Saccharomyces cerevisiae. Current genetics 40 12759774
2013 The function of the Mediator complex in plant immunity. Plant signaling & behavior 37 23299323
2013 Model of the Mediator middle module based on protein cross-linking. Nucleic acids research 37 23939621
2012 Role of Mediator in regulating Pol II elongation and nucleosome displacement in Saccharomyces cerevisiae. Genetics 32 22377631
2010 The transcriptional coactivator DRIP/mediator complex is involved in vitamin D receptor function and regulates keratinocyte proliferation and differentiation. The Journal of investigative dermatology 30 20520624
2014 Characterization of the influence of mediator complex in HIV-1 transcription. The Journal of biological chemistry 27 25100719
2016 Role for the MED21-MED7 Hinge in Assembly of the Mediator-RNA Polymerase II Holoenzyme. The Journal of biological chemistry 24 27821593
2002 Functional interactions within yeast mediator and evidence of differential subunit modifications. The Journal of biological chemistry 22 12468546
2006 Functional and physical interactions within the middle domain of the yeast mediator. Molecular genetics and genomics : MGG 17 16758199
2019 Whole Exome and Transcriptome RNA-Sequencing Model for the Diagnosis of Prostate Cancer. ACS omega 12 31956794
1999 The human homologue of Drosophila TRF-proximal protein is associated with an RNA polymerase II-SRB complex. The Journal of biological chemistry 12 9933582
2018 Theileria annulata Cyclophilin1 (TaCyp1) Interacts With Host Cell MED21. Frontiers in microbiology 8 30559736
2023 Screening of Lipid Metabolism-Related Genes as Diagnostic Indicators in Chronic Obstructive Pulmonary Disease. International journal of chronic obstructive pulmonary disease 7 38046983
2024 Quantitative proteomics reveals the mechanism of endoplasmic reticulum stress-mediated pulmonary fibrosis in mice. Heliyon 2 39640640
1999 Genomics of the human genes encoding four TAFII subunits of TFIID, the three subunits of TFIIA, as well as CDK8 and SURB7. Somatic cell and molecular genetics 2 11441538
2025 A Hot-Swappable Genetic Switch: Building an Inducible and Trackable Functional Assay for the Essential Gene MEDIATOR 21. ACS synthetic biology 1 40340410
2024 A Hot-Swappable Genetic Switch: Building an inducible and trackable functional assay for the essential gene MEDIATOR 21. bioRxiv : the preprint server for biology 1 39763940
2019 Highlighted role of VEGFA in follow up of celiac disease. Gastroenterology and hepatology from bed to bench 1 31528310
2026 Identification and validation of mitochondrial metabolism-ralated biomarkers in coronary heart disease. Medicine 0 41686635
2026 PsAvh109 suppresses SA-triggered immunity by mimicking TPL function to disrupt mediator complex assembly. Nature communications 0 41986337