Affinage

MED8

Mediator of RNA polymerase II transcription subunit 8 · UniProt Q96G25

Length
268 aa
Mass
29.1 kDa
Annotated
2026-06-10
13 papers in source corpus 8 papers cited in narrative 8 extracted findings
Cross-family judge vs UniProt: Affinage preferred faithfulness: 6/6 claims corpus-supported (100%)

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

MED8 is a subunit of the Mediator head module that couples gene-specific regulatory signals to the RNA polymerase II general transcription machinery (PMID:16964259, PMID:9918841). Its conserved N-terminal domain directly binds TBP and is essential in vivo, while its C-terminal region nucleates a structurally defined submodule with Med18 and Med20 — both adopting beta-barrel folds — that is required for proper trimer folding and bears a conserved protein-interaction surface whose mutation counteracts defects from Pol II truncation (PMID:16964259, PMID:19934057). MED8 also engages transcription directly at two levels: it binds defined DNA regulatory elements, covering a (A/C)(A/G)GAAAT motif within the upstream activating sequence of SUC2 and the downstream repressing sequence of HXK2, and it contacts the RNA Pol II CTD (PMID:9918841, PMID:10526178). In fission yeast, MED8 transmits activator input from Ace2 to Pol II through an interaction with Rpb4 mediated by its C-terminal region (PMID:19720063). The protein is targeted by Bunyamwera virus NSs, which binds MED8 to shut down host transcription and antagonize the interferon response (PMID:16973571). In human cancer, MED8 sustains oncogenic transcription: it cooperates with CDK7 to bind and activate PDGFRA super-enhancers in glioma (PMID:42157323), and it stabilizes TRIP4 by reducing its ubiquitination to drive epithelial-mesenchymal transition and sorafenib resistance in hepatocellular carcinoma (PMID:41117311).

Mechanistic history

Synthesis pass · year-by-year structured walk · 6 steps
  1. 1999 Medium

    Established that Med8 is not merely a structural scaffold but contacts both promoter DNA and the polymerase directly, answering how a Mediator subunit might confer gene specificity.

    Evidence Biochemical purification of endogenous yeast Med8, recombinant protein DNA-binding and CTD-binding assays, with AFM visualization of Med8 occupying defined sequence motifs at SUC2 UAS and HXK2 DRS

    PMID:10526178 PMID:9918841

    Open questions at the time
    • Whether sequence-specific DNA binding occurs in the context of intact Mediator in vivo is not resolved
    • Functional consequence of CTD binding not dissected
  2. 2006 High

    Defined the architecture and essential function of the Med8/Med18/Med20 head submodule, showing Med8's N-terminal domain binds TBP and identifying a conserved interaction surface linked to Pol II function.

    Evidence X-ray crystallography of the Med8C/18/20 submodule, in vitro TBP-binding assay, in vivo genetic complementation, and srb-mutation analysis

    PMID:16964259

    Open questions at the time
    • Structure of the TBP-binding N-terminal domain itself not resolved
    • Does not establish how the submodule engages the holo-Mediator or Pol II in vivo
  3. 2006 Medium

    Revealed that MED8 is a host vulnerability exploited by a pathogen, with Bunyamwera NSs targeting it to suppress transcription and interferon responses.

    Evidence Protein interaction mapping and recombinant virus carrying an NSs C-terminal domain deletion with host transcription and interferon antagonism readouts

    PMID:16973571

    Open questions at the time
    • The MED8 surface contacted by NSs is not mapped
    • Whether NSs disrupts Mediator assembly or only its activity is unknown
  4. 2009 Medium

    Showed MED8 acts as a signal conduit from a specific activator to the polymerase, and clarified the folding hierarchy of the head submodule.

    Evidence Co-IP and domain mapping of Med8-Rpb4 and Med8-Ace2 interactions with complementation in S. pombe; in vitro renaturation of recombinant subunits monitored by CD, fluorescence, and IP

    PMID:19720063 PMID:19934057

    Open questions at the time
    • Direct evidence that Ace2 signal physically routes through Med8 to Rpb4 in a single chain is incomplete
    • Folding interdependence shown in vitro; in vivo assembly order not established
  5. 2026 Medium

    Extended MED8 function to human oncogenic transcription, showing cooperation with CDK7 at super-enhancers to sustain a driver oncogene.

    Evidence CUT&TAG, ChIP, co-IP, mass spectrometry, protein fragment complementation, dual-luciferase reporter, plus xenograft and organoid models in glioma

    PMID:42157323

    Open questions at the time
    • Whether MED8-CDK7 cooperation generalizes beyond PDGFRA super-enhancers is untested
    • Single lab without independent replication
  6. 2025 Medium

    Identified a non-canonical, transcription-independent role: MED8 stabilizing a partner protein by limiting its ubiquitination to drive a cancer phenotype.

    Evidence Co-IP, ubiquitination assay, MED8 knockdown/overexpression and TRIP4 rescue in hepatocellular carcinoma cells

    PMID:41117311

    Open questions at the time
    • The E3 ligase MED8 antagonizes on TRIP4 is not identified
    • Whether this function requires Mediator association is unknown
    • Single lab, no independent replication

Open questions

Synthesis pass · forward-looking unresolved questions
  • How MED8's structural role in the Mediator head integrates with its emerging functions in super-enhancer regulation and partner stabilization in human cells remains unresolved.
  • No structure of human MED8 within holo-Mediator
  • No unifying model linking DNA binding, TBP/Rpb4 contacts, and oncogenic super-enhancer activity

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0003677 DNA binding 2 GO:0060090 molecular adaptor activity 2 GO:0140110 transcription regulator activity 2
Localization
GO:0005634 nucleus 2
Pathway
R-HSA-74160 Gene expression (Transcription) 3
Partners
ACE2CDK7MED18MED20RPB4TBPTRIP4
Complex memberships
Mediator head module (Med8/Med18/Med20 submodule)

Evidence

Reading pass · 8 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2006 Med8, Med18, and Med20 form a head subcomplex with two submodules: (1) the N-terminal domain of Med8 binds TBP in vitro and is essential in vivo, and (2) Med8C/Med18/Med20 where Med18 and Med20 adopt related beta-barrel folds. X-ray crystal structure of the Med8C/18/20 submodule revealed a conserved putative protein-interaction face that includes sites altered by srb mutations, which counteract defects from Pol II truncation. X-ray crystallography, in vitro TBP-binding assay, in vivo genetic complementation, structural mutagenesis analysis Nature structural & molecular biology High 16964259
2009 Med8, Med18, and Med20 are interdependent for proper folding and trimer complex formation; all three subunits must be present simultaneously during renaturation to achieve correct folding, though they can also form soluble monomers and pairwise subcomplexes when renatured separately. Immunoprecipitation, far-UV circular dichroism, fluorescence spectroscopy on recombinant denatured/renatured proteins in various combinations Proceedings of the National Academy of Sciences of the United States of America High 19934057
1999 Yeast Med8 (p27) directly binds to regulatory elements of SUC2 (upstream activating sequences) and HXK2 (downstream repressing sequences) genes, as demonstrated by purification of the endogenous protein and in vitro binding of recombinant Med8 expressed in E. coli. Med8 also binds the CTD of RNA polymerase II. Biochemical purification, N-terminal protein sequencing, recombinant protein expression in E. coli, in vitro DNA-binding assay Biochemical and biophysical research communications Medium 9918841
1999 Atomic force microscopy directly visualized Med8 binding to the UAS of the SUC2 promoter and the DRS of the HXK2 gene, showing Med8 fully covers one of two 7 bp motifs (consensus (A/C)(A/G)GAAAT) in each fragment with no preference between the two available sites. Atomic force microscopy (AFM) of DNA-protein complexes FEBS letters Medium 10526178
2006 Bunyamwera virus NSs protein interacts with the MED8 component of Mediator to inhibit host transcription and the interferon response; the interacting domain on NSs maps to the C-terminal region conserved among orthobunyavirus NSs proteins, and deletion of this domain strongly reduced inhibition of host protein expression and abrogated interferon antagonism. Protein interaction mapping, recombinant virus with domain deletion, host transcription and interferon response assays Journal of virology Medium 16973571
2009 In S. pombe, Med8 interacts with Rpb4 (a subunit of RNA polymerase II) and with the transcriptional activator Ace2; the C-terminal region of Med8 is required for the Med8-Rpb4 interaction and can partially complement the sep15-598 mutant, suggesting Med8 transmits regulatory information from Ace2 to Pol II via Rpb4. Genetic epistasis, co-immunoprecipitation, domain-mapping biochemical assays, complementation assays FEBS letters Medium 19720063
2025 Human MED8 interacts with TRIP4, reduces its ubiquitination, and stabilizes TRIP4 protein levels, thereby promoting epithelial-mesenchymal transition and sorafenib resistance in hepatocellular carcinoma cells. Co-immunoprecipitation, ubiquitination assay, MED8 knockdown/overexpression functional assays, TRIP4 overexpression rescue experiment Journal of enzyme inhibition and medicinal chemistry Medium 41117311
2026 Human MED8 cooperates with CDK7 to bind and activate super-enhancers of PDGFRA, sustaining high transcriptional output of this oncogene in glioma; mechanistic evidence from ChIP, CUT&TAG, co-immunoprecipitation, mass spectrometry, protein fragment complementation, and dual-luciferase reporter assays. CUT&TAG, chromatin immunoprecipitation, co-immunoprecipitation, mass spectrometry, protein fragment complementation assay, dual-luciferase reporter assay, xenograft and organoid models Journal of experimental & clinical cancer research Medium 42157323

Source papers

Stage 0 corpus · 13 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2006 Structure and TBP binding of the Mediator head subcomplex Med8-Med18-Med20. Nature structural & molecular biology 94 16964259
2006 Interaction of Bunyamwera Orthobunyavirus NSs protein with mediator protein MED8: a mechanism for inhibiting the interferon response. Journal of virology 72 16973571
1999 Med8, a subunit of the mediator CTD complex of RNA polymerase II, directly binds to regulatory elements of SUC2 and HXK2 genes. Biochemical and biophysical research communications 30 9918841
2000 The Schizosaccharomyces pombe sep15+ gene encodes a protein homologous to the Med8 subunit of the Saccharomyces cerevisiae transcriptional mediator complex. Current genetics 23 11191205
1999 Analysis by atomic force microscopy of Med8 binding to cis-acting regulatory elements of the SUC2 and HXK2 genes of saccharomyces cerevisiae. FEBS letters 23 10526178
2007 Genomic expression patterns in cell separation mutants of Schizosaccharomyces pombe defective in the genes sep10 ( + ) and sep15 ( + ) coding for the Mediator subunits Med31 and Med8. Molecular genetics and genomics : MGG 17 17922236
2009 The Med8 mediator subunit interacts with the Rpb4 subunit of RNA polymerase II and Ace2 transcriptional activator in Schizosaccharomyces pombe. FEBS letters 13 19720063
2011 Knockdown of NtMed8, a Med8-like gene, causes abnormal development of vegetative and floral organs in tobacco (Nicotiana tabacum L.). Plant cell reports 9 21744120
2009 Med8, Med18, and Med20 subunits of the Mediator head domain are interdependent upon each other for folding and complex formation. Proceedings of the National Academy of Sciences of the United States of America 7 19934057
2023 Mediator subunit MED8 interacts with heat shock transcription factor HSF3 to promote fucoxanthin synthesis in the diatom Phaeodactylum tricornutum. The New phytologist 4 38062856
2021 The tomato Mediator subunit MED8 positively regulates plant response to Botrytis cinerea. Journal of plant physiology 3 34601339
2026 Disrupting MED8-dependent epigenetic reprogramming augments avapritinib sensitivity in PDGFRA-driven glioma. Journal of experimental & clinical cancer research : CR 0 42157323
2025 Targeting MED8 enhances sorafenib sensitivity in hepatocellular carcinoma by disrupting epithelial-mesenchymal transition mechanisms. Journal of enzyme inhibition and medicinal chemistry 0 41117311

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