Affinage

MED27

Mediator of RNA polymerase II transcription subunit 27 · UniProt Q6P2C8

Length
311 aa
Mass
35.4 kDa
Annotated
2026-06-10
19 papers in source corpus 12 papers cited in narrative 12 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

MED27 is a tail-module subunit of the Mediator complex that is required for the structural integrity of the entire complex and for the transcriptional programs that drive cell-type specification and organ development (PMID:39209248, PMID:41017421). As a core architectural subunit, loss of MED27 reduces the protein levels of most other Mediator subunits in cardiomyocytes, a destabilizing role that cannot be substituted by MED30 (PMID:39209248); patient-derived MED27 variants likewise destabilize Mediator and impair its chromatin occupancy and chromatin interactions, dysregulating master neurogenic transcription factors and producing cerebellar atrophy and motor deficits in stem-cell and mouse models (PMID:41017421). Work on the yeast ortholog (Hrs1/Med3) established conserved features of this subunit: it binds the Cyc8-Tup1 corepressor to bring repressive activity into Mediator (PMID:10722672), and it is required for incorporation of Med2-module Tlo proteins into the complex (PMID:22562472). The substoichiometric levels of this tail subunit are explained by an autoregulatory loop in which the Mediator kinase Cdk8 phosphorylates Med3, triggering Grr1 ubiquitin-ligase-mediated proteasomal degradation that limits transcription activation (PMID:24550274). In vivo, MED27 directs vertebrate tissue patterning, including retinal cell-type determination (PMID:16582438) and cardiac development, where its deletion is embryonic-lethal or causes adult heart failure (PMID:39209248). In cancer contexts MED27 acts as a sequence-specific transcriptional regulator, occupying the IKKα, KLF4, and RAN promoters and interacting with NF-κB and the acetyltransferase p300 (PMID:26797421, PMID:33162555, PMID:36786527, PMID:40069732).

Mechanistic history

Synthesis pass · year-by-year structured walk · 10 steps
  1. 2000 High

    Established that this Mediator subunit physically links the complex to a corepressor, defining a mechanism by which Mediator activity can be inhibited rather than only activated.

    Evidence Two-hybrid, GST pulldown, affinity chromatography and genetic dosage suppression in yeast (Hrs1/Med3)

    PMID:10722672

    Open questions at the time
    • Whether human MED27 binds an equivalent corepressor is untested
    • Structural basis of the Cyc8-Tup1 contact not resolved
  2. 2006 Medium

    Showed MED27 has a specific developmental role in vertebrate CNS cell-fate determination, distinct from a generic Mediator housekeeping function.

    Evidence Zebrafish forward-genetic mutant (Crsp34/Med27 m885) with retinal cell-type marker analysis

    PMID:16582438

    Open questions at the time
    • Transcriptional targets driving the retinal phenotype not identified
    • Single organism, single lab
  3. 2012 High

    Defined MED27's role as an assembly factor required to bring a specific module (Tlo/Med2) into Mediator, beyond being a passive subunit.

    Evidence Mediator purification from med3 deletion strain and reconstituted direct interaction in E. coli (Candida albicans)

    PMID:22562472

    Open questions at the time
    • Whether human MED27 has an analogous module-recruitment role unknown
    • Stoichiometry and interface of the Med3-Tlo contact not mapped
  4. 2014 High

    Explained the substoichiometric abundance of this tail subunit through a Cdk8-phosphorylation/Grr1-ubiquitination autoregulatory degradation loop.

    Evidence In vitro kinase assay, site-specific mutagenesis, Med3-Grr1 interaction assay and genetic epistasis in yeast

    PMID:24550274

    Open questions at the time
    • Conservation of the Cdk8/Grr1 axis in human MED27 not demonstrated
    • Phosphosite identity in the human protein unknown
  5. 2016 Medium

    Connected MED27 to specific transcription factors and a chromatin modifier in mammalian cells, showing it is acetylated by p300 and influences NF-κB-dependent output.

    Evidence Co-IP, acetylation assay and siRNA knockdown with signaling readouts in melanoma cells

    PMID:26797421

    Open questions at the time
    • Functional consequence of MED27 acetylation not defined
    • Direct versus Mediator-mediated NF-κB contact not distinguished
  6. 2020 Medium

    Demonstrated promoter-level sequence-specific occupancy by MED27, here as a negative regulator of IKKα transcription.

    Evidence Luciferase deletion-construct mapping, promoter binding assay and rescue experiments in thyroid cancer cells plus xenografts

    PMID:33162555

    Open questions at the time
    • Whether MED27 binds DNA directly or via partner factors unresolved
    • Single lab
  7. 2023 Medium

    Extended MED27's promoter-targeting activity to KLF4 regulation controlling metastasis and stemness.

    Evidence RNA-seq target discovery, reporter/promoter mapping and KLF4 rescue in breast cancer models

    PMID:36786527

    Open questions at the time
    • DNA-binding mechanism not established
    • Relationship to full Mediator complex at the KLF4 promoter unclear
  8. 2024 High

    Established MED27 as essential for Mediator complex stability in a mammalian organ and for cardiac development, independent of MED30.

    Evidence Cardiomyocyte-specific conditional knockout mice, Western blot of subunit levels and negative MED30 rescue

    PMID:39209248

    Open questions at the time
    • Which subunit contacts MED27 stabilizes not defined structurally
    • Direct transcriptional targets in heart not pinpointed
  9. 2025 High

    Defined the disease mechanism: patient MED27 variants destabilize Mediator and impair chromatin occupancy, dysregulating neurogenic master transcription factors and causing cerebellar pathology.

    Evidence Patient iPSC lines, Med27 LoF mice, chromatin occupancy/interaction assays and spatial transcriptomics

    PMID:41017421

    Open questions at the time
    • Specific subunit interfaces disrupted by variants not resolved
    • Causal target genes among dysregulated TFs not isolated
  10. 2025 Medium

    Added a cancer-relevant transcriptional axis whereby MED27 activates RAN to drive RAN/CRM1-mediated nuclear export of PPARα and lipid droplet accumulation.

    Evidence Co-IP, luciferase reporter, lipid imaging, lipidomics and HCC models

    PMID:40069732

    Open questions at the time
    • Direct MED27 occupancy of the RAN promoter versus indirect effect not separated
    • Single lab

Open questions

Synthesis pass · forward-looking unresolved questions
  • How MED27 selects specific target promoters and whether the yeast Cdk8/Grr1 autoregulatory and corepressor-bridging mechanisms operate on human MED27 remains unresolved.
  • No structural model of human MED27 within Mediator
  • DNA-binding versus partner-mediated promoter targeting undefined
  • Conservation of degradation control in mammals untested

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0140110 transcription regulator activity 3 GO:0005198 structural molecule activity 2 GO:0060090 molecular adaptor activity 2
Localization
GO:0005634 nucleus 2
Pathway
R-HSA-1266738 Developmental Biology 3 R-HSA-74160 Gene expression (Transcription) 3
Complex memberships
Mediator complex

Evidence

Reading pass · 12 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2000 Yeast Hrs1/Med3 (ortholog of MED27) physically interacts with the Cyc8-Tup1 corepressor via direct binding, and this interaction allows Cyc8-Tup1 to co-purify with the Mediator complex in a Hrs1-dependent manner; increased Hrs1 dosage alleviates Cyc8-Tup1-mediated repression, suggesting that Mediator function is inhibited upon this interaction, thereby preventing RNA pol II holoenzyme recruitment to the core promoter. Two-hybrid assay, GST pulldown, affinity chromatography, genetic dosage suppression, artificial holoenzyme recruitment assay The Journal of biological chemistry High 10722672
2012 In Candida albicans, Med3 (MED27 ortholog) directly and physically interacts with Tlo proteins (Med2-module subunits); Med3 is required for Tlo incorporation into the Mediator complex, as Mediator purified from a med3Δ/Δ strain lacks Tlo subunits. Mediator complex purification, co-purification of tagged proteins, reconstitution of direct interaction by coexpression and copurification in E. coli Eukaryotic cell High 22562472
2014 Yeast Med3 (MED27 ortholog) is phosphorylated by Cdk8 (the Mediator kinase module subunit); site-specific phosphorylation triggers interaction with the Grr1 ubiquitin ligase and subsequent Med3 degradation, thereby suppressing transcription activation; this autoregulatory mechanism explains the substoichiometric levels of tail-module Med3 in purified Mediator. In vitro kinase assay, site-specific mutagenesis, Co-IP/interaction assay between Med3 and Grr1, genetic analysis of cdk8 mutants and grr1 mutants, protein stability assays Proceedings of the National Academy of Sciences of the United States of America High 24550274
2006 Loss of Crsp34/Med27 (zebrafish ortholog of MED27) decreases amacrine cell number and increases rod photoreceptor cells in the retina, and eliminates dopaminergic amacrine cells, demonstrating a specific role for MED27 in retinal cell-type determination during vertebrate CNS development distinct from other Mediator subunits. Zebrafish forward genetic mutation analysis (m885 allele disrupts Crsp34/Med27), histological and cell-type specific marker analysis of mutant retinas Genetics Medium 16582438
2016 MED27 interacts with NF-κB and with the acetyltransferase p300, and is acetylated by p300; MED27 knockdown inactivates PI3K/AKT and MAPK/ERK signaling and reduces NF-κB nuclear translocation and iNOS expression in melanoma cells. Co-immunoprecipitation (MED27 with NF-κB and p300), acetylation assay, siRNA knockdown with Western blot and xenograft tumor model Cancer letters Medium 26797421
2020 CRSP8/MED27 binds to the IKKα promoter region (positions -257 to -143) to negatively regulate IKKα transcription; knockdown of CRSP8 upregulates IKKα, and IKKα knockdown/overexpression reverses the differentiation and EMT marker changes caused by CRSP8 manipulation in thyroid cancer cells. Luciferase promoter assay with deletion constructs, ChIP or promoter binding assay, siRNA knockdown, rescue experiments, xenograft mouse model Cell death and differentiation Medium 33162555
2023 MED27 transcriptionally regulates KLF4 by binding to its promoter region (positions -156 to +177); KLF4 overexpression reverses the suppression of metastasis and stemness caused by MED27 silencing in breast cancer cells. RNA-seq to identify downstream targets, luciferase reporter assay/ChIP to map promoter binding, rescue experiments with KLF4 overexpression in vitro and in vivo Cancer science Medium 36786527
2024 Cardiomyocyte-specific deletion of MED27 in mice causes embryonic lethality (constitutive) or heart failure and mortality (inducible adult deletion); MED27 loss decreases protein levels of most Mediator subunits in cardiomyocytes, indicating MED27 is required for stability of the Mediator complex; this role is independent of MED30, as MED30 overexpression fails to rescue Mediator subunit levels in MED27-deficient cardiomyocytes. Conditional (floxed) cardiomyocyte-specific knockout mouse (cKO and icKO), Western blot for Mediator subunit protein levels, MED30 overexpression rescue experiment, RNAseq, morphological/histological/physiological analysis Life sciences High 39209248
2025 Patient-specific MED27 variants destabilize the Mediator complex and impair its chromatin occupancy and chromatin interactions; in vitro stem cell models and in vivo mouse LoF models recapitulate cerebellar atrophy and motor deficits; transcriptomic profiling identifies dysregulation of master regulatory transcription factors involved in neurogenesis and cerebellar development as downstream consequences. iPSC models with patient variants, Med27 LoF mouse models, molecular analyses of Mediator complex stability, chromatin occupancy assay (ChIP or related), chromatin interaction assay, single-cell spatial transcriptomics Advanced science (Weinheim, Baden-Wurttemberg, Germany) High 41017421
2024 A missense variant MED27 p.Arg25His causes a significant reduction in MED27 protein expression and, by molecular dynamics modeling, is predicted to weaken the interaction between MED27 and MED14 subunits. Western blot of transfected cells expressing Arg25His variant, molecular dynamics structural modeling Heliyon Low 39296199
2025 CRSP8/MED27 transcriptionally activates RAN expression; enhanced RAN/CRM1-mediated nuclear export induced by CRSP8 promotes cytoplasmic translocation of PPARα (formation of RAN/CRM1/PPARα heterotrimer), impairing nuclear PPARα-dependent lipophagy and fatty acid oxidation, thereby driving lipid droplet accumulation and HCC progression. Co-immunoprecipitation, luciferase reporter assay, live cell imaging of lipid droplets, transmission electron microscopy, targeted lipidomics, in vitro and in vivo HCC models Journal of experimental & clinical cancer research : CR Medium 40069732
2024 In Saccharomyces cerevisiae, Med3 interacts with the transcription factor Stb5 (by yeast two-hybrid) to regulate expression of gnd1 and ald6 (NADPH-generating genes) under hyperosmotic stress; Med3 deletion reduces intracellular NADPH, increases ROS, and impairs bud formation under hyperosmotic conditions. Yeast two-hybrid (Med3–Stb5 interaction), deletion strain phenotypic analysis, NADPH and ROS measurements, gene expression analysis Applied and environmental microbiology Medium 39082808

Source papers

Stage 0 corpus · 19 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2000 Hrs1/Med3 is a Cyc8-Tup1 corepressor target in the RNA polymerase II holoenzyme. The Journal of biological chemistry 87 10722672
2012 The Tlo proteins are stoichiometric components of Candida albicans mediator anchored via the Med3 subunit. Eukaryotic cell 45 22562472
2014 Suppression of Mediator is regulated by Cdk8-dependent Grr1 turnover of the Med3 coactivator. Proceedings of the National Academy of Sciences of the United States of America 41 24550274
2016 MED27 promotes melanoma growth by targeting AKT/MAPK and NF-κB/iNOS signaling pathways. Cancer letters 31 26797421
2020 CRSP8 promotes thyroid cancer progression by antagonizing IKKα-induced cell differentiation. Cell death and differentiation 22 33162555
2021 MED27 Variants Cause Developmental Delay, Dystonia, and Cerebellar Hypoplasia. Annals of neurology 21 33443317
2018 MicroRNA-18a inhibits cell growth and induces apoptosis in osteosarcoma by targeting MED27. International journal of oncology 19 29693135
2023 Biallelic MED27 variants lead to variable ponto-cerebello-lental degeneration with movement disorders. Brain : a journal of neurology 17 37517035
2006 Differential roles of transcriptional mediator complex subunits Crsp34/Med27, Crsp150/Med14 and Trap100/Med24 during zebrafish retinal development. Genetics 17 16582438
2023 MED27 plays a tumor-promoting role in breast cancer progression by targeting KLF4. Cancer science 14 36786527
2022 MED27, SLC6A7, and MPPE1 Variants in a Complex Neurodevelopmental Disorder with Severe Dystonia. Movement disorders : official journal of the Movement Disorder Society 10 35876425
2018 Silencing of MED27 inhibits adrenal cortical carcinogenesis by targeting the Wnt/β-catenin signaling pathway and the epithelial-mesenchymal transition process. Biological chemistry 9 29730647
2024 The essential role of MED27 in stabilizing the mediator complex for cardiac development and function. Life sciences 8 39209248
2025 CRSP8-driven fatty acid metabolism reprogramming enhances hepatocellular carcinoma progression by inhibiting RAN-mediated PPARα nucleus-cytoplasm shuttling. Journal of experimental & clinical cancer research : CR 7 40069732
2020 MED27 promotes malignant behavior of cells by affecting Sp1 in breast cancer. European review for medical and pharmacological sciences 7 32633372
2018 Candida glabrata Med3 Plays a Role in Altering Cell Size and Budding Index To Coordinate Cell Growth. Applied and environmental microbiology 3 29776932
2024 Med3-mediated NADPH generation to help Saccharomyces cerevisiae tolerate hyperosmotic stress. Applied and environmental microbiology 2 39082808
2025 Pathogenicity of Mediator Complex Subunit 27 (MED27) in a Neurodevelopmental Disorder with Cerebellar Atrophy. Advanced science (Weinheim, Baden-Wurttemberg, Germany) 1 41017421
2024 Novel homozygous missense variants in MED27 associated with neurodevelopmental disorder: Clinical and pathogenetic research. Heliyon 1 39296199

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