Affinage

CCNC

Cyclin-C · UniProt P24863

Length
283 aa
Mass
33.2 kDa
Annotated
2026-04-28
12 papers in source corpus 7 papers cited in narrative 7 extracted findings

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

Cyclin C (CCNC) is the obligate activating subunit of CDK8 (and CDK19), forming a kinase complex that phosphorylates transcriptional regulators to control gene expression programs in development, metabolism, and stress responses. The CycC–CDK8 complex phosphorylates the Notch intracellular domain within its TAD and PEST domains, triggering Fbw7/Sel10-dependent ubiquitin-mediated degradation (PMID:15546612), and suppresses de novo lipogenesis by promoting turnover of nuclear SREBP-1c downstream of mTORC1 (PMID:26042770). Structural studies show that CDK8 employs a unique CycC-recognition helix and an unphosphorylated activation loop, with full kinase activation requiring allosteric T-loop repositioning by Med12 or MTBP (PMID:21806996). In vivo, CCNC is required for brown adipocyte progenitor proliferation via the C/EBPα/GLUT4/ChREBP axis (PMID:35863637) and for autophagy-lysosome pathway gene transcription in the pancreas, where its loss accelerates Kras-driven neoplasia [PMID:bio_10.1101_2024.08.21.609015].

Mechanistic history

Synthesis pass · year-by-year structured walk · 7 steps
  1. 2004 High

    Establishing that CycC–CDK8 is a Notch ICD kinase answered how the Notch transcriptional activation domain is turned over: direct phosphorylation by CycC–CDK8 primes the PEST domain for Fbw7-mediated ubiquitination and degradation, linking Mediator kinase activity to Notch signaling output.

    Evidence Reconstituted in vitro kinase assay with purified CycC–CDK8, ChIP at HES1 promoter, PEST mutagenesis, and in vivo degradation assays in mammalian cells

    PMID:15546612

    Open questions at the time
    • Physiological relevance in developmental Notch signaling contexts not tested
    • Whether CDK8 kinase activity is rate-limiting for Notch turnover in vivo is unresolved
    • Other CDK8 substrates at Notch target promoters not identified
  2. 2011 High

    The crystal structure of CDK8–CycC revealed that CDK8 uniquely lacks activation-loop phosphorylation and possesses a CycC-specific recognition helix, answering how the CycC–CDK8 pair achieves specificity and raising the question of what alternative mechanism activates the kinase.

    Evidence 2.2 Å X-ray crystal structure of CDK8–CycC in complex with sorafenib

    PMID:21806996

    Open questions at the time
    • The activation mechanism in the absence of T-loop phosphorylation was not resolved
    • Structure of the full CDK module (with Med12/Med13) was not available
    • How CycC conformational coupling activates the CDK8 catalytic cleft was unclear
  3. 2013 High

    Structure-kinetic analyses of CDK8–CycC with diverse ligands established that front-pocket hydrophobic complementarity, not DFG-out flipping, governs inhibitor residence time, providing a framework for selective CDK8 inhibitor design.

    Evidence Multiple co-crystal structures of CDK8–CycC with inhibitors plus surface plasmon resonance kinetics

    PMID:23630251

    Open questions at the time
    • Selectivity against CDK19–CycC was not addressed structurally
    • Cellular target engagement of characterized inhibitors not measured
    • Catalytic mechanism with physiological substrates not captured structurally
  4. 2015 Medium

    Placing CDK8–CycC downstream of mTORC1 and upstream of SREBP-1c degradation revealed a metabolic tumor-suppressor axis: mTORC1 activation reduces CDK8–CycC levels, de-repressing lipogenic gene expression and promoting de novo lipogenesis.

    Evidence Pharmacologic and genetic mTORC1 manipulation in cell lines and mouse liver NAFLD models, immunoblotting for CDK8–CycC and SREBP-1c

    PMID:26042770

    Open questions at the time
    • Direct phosphorylation of SREBP-1c by CDK8–CycC was not demonstrated with purified components
    • Mechanism by which mTORC1 reduces CDK8–CycC protein levels is unknown
    • Single-lab observation not independently replicated at the time
  5. 2022 High

    Conditional knockout studies established that CCNC is essential in vivo for brown adipocyte progenitor proliferation and lipogenic gene expression through the C/EBPα/GLUT4/ChREBP transcriptional axis, separating its role in proliferation from adipogenesis.

    Evidence Myf5-Cre, Ucp1-Cre, and Adipoq-Cre conditional Ccnc knockout mice with RNA-seq, metabolic phenotyping, and immunostaining

    PMID:35863637

    Open questions at the time
    • Whether CDK8 kinase activity or a scaffolding function of CycC is required was not distinguished
    • Direct transcriptional targets of the CycC–CDK8 complex in brown fat progenitors not mapped genome-wide by ChIP
    • Relevance to human brown fat biology untested
  6. 2024 Medium

    Pancreatic ablation of Ccnc revealed a requirement for CycC in autophagy-lysosome gene transcription and proteasome function, showing that CycC loss phenocopies autophagy deficiency and accelerates Kras-driven pancreatic neoplasia.

    Evidence Conditional Ccnc knockout in mouse pancreas ± KrasG12D, autophagy flux and proteasome activity assays, histopathology (preprint)

    PMID:bio_10.1101_2024.08.21.609015

    Open questions at the time
    • Preprint not yet peer-reviewed
    • Whether CycC acts through CDK8-dependent transcription of specific autophagy genes or a non-catalytic mechanism is unresolved
    • Generalizability beyond pancreatic tissue not established
  7. 2025 Medium

    Identification of MTBP as a second allosteric activator of CycC–CDK8/19, mutually exclusive with Med12, resolved the long-standing puzzle of how the unphosphorylated T-loop becomes catalytically competent: both activators reposition the T-loop independently of phosphorylation.

    Evidence In vitro kinase assays, structural studies, and mutagenesis (preprint)

    PMID:bio_10.1101_2025.06.16.659917

    Open questions at the time
    • Preprint not yet peer-reviewed
    • In vivo relevance of MTBP-mediated activation versus Med12-mediated activation not tested
    • Substrate specificity conferred by each activator is unknown

Open questions

Synthesis pass · forward-looking unresolved questions
  • The full spectrum of CDK8–CycC substrates, the structural basis of allosteric activation by Med12 and MTBP within the intact Mediator complex, and how tissue-specific requirements for CycC (brown fat, pancreas, Notch-dependent tissues) are coordinated remain undefined.
  • Comprehensive substrate identification for CDK8–CycC has not been performed
  • No high-resolution structure of CycC within the intact CDK module bound to Mediator
  • Mechanism linking mTORC1 to CDK8–CycC protein turnover is unknown

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0098772 molecular function regulator activity 3
Localization
GO:0005634 nucleus 2
Pathway
R-HSA-1430728 Metabolism 2 R-HSA-162582 Signal Transduction 2 R-HSA-9612973 Autophagy 1
Partners
CDK19CDK8FBXW7MED12MTBPNOTCH1SREBF1
Complex memberships
Mediator CDK module (CDK8/CDK19–CycC–Med12–Med13)

Evidence

Reading pass · 7 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2004 CycC:CDK8 is recruited with Notch ICD, MAM, and SKIP to the HES1 promoter; purified recombinant CycC:CDK8 directly phosphorylates the Notch ICD within the TAD and PEST domains, promoting PEST-dependent degradation by the Fbw7/Sel10 ubiquitin ligase. MAM interacts directly with CDK8 and can localize it to subnuclear foci. Chromatin immunoprecipitation, in vitro kinase assay with purified recombinant CycC:CDK8, co-immunoprecipitation, site-directed mutagenesis of PEST Ser residues, in vivo degradation assays Molecular cell High 15546612
2011 Crystal structure of CDK8/CycC at 2.2 Å resolution reveals a unique CycC-recognition helix in CDK8 that explains specificity of the CDK8/CycC pair. Unlike other CDKs, the CDK8 activation loop is not phosphorylated, suggesting an alternate activation mechanism. Sorafenib binds the catalytic cleft and induces a DFG-out (DMG-out) conformation, the first such conformation in the CDK family. X-ray crystallography (2.2 Å resolution crystal structure of CDK8/CycC:sorafenib complex) Journal of molecular biology High 21806996
2013 Structure-kinetic relationship studies of CDK8/CycC show that hydrophobic complementarities within the kinase front pocket are the primary determinant of ligand residence time; DFG-out (DMG-out) conformation flip has little influence on binding velocity, while hydrogen bonding at the hinge region contributes to residence time. Co-crystal structures of CDK8/CycC with diverse ligands combined with surface plasmon resonance binding kinetics Proceedings of the National Academy of Sciences of the United States of America High 23630251
2015 mTORC1 activation causes reduction of the CDK8-CycC complex both in vitro and in mouse liver in vivo, placing mTORC1 upstream of the CDK8-CycC complex; loss of CDK8-CycC leads to accumulation of nuclear SREBP-1c and lipogenic enzymes, establishing that CDK8-CycC suppresses de novo lipogenesis downstream of mTORC1. Pharmacologic inhibition and genetic manipulation of mTORC1 in cell lines and mouse models (NAFLD models), immunoblotting, in vivo mouse liver experiments PloS one Medium 26042770
2022 CCNC (cyclin C) in brown adipocytes is required for lipogenic gene expression through activation of the C/EBPα/GLUT4/ChREBP axis; conditional knockout of Ccnc in Myf5+ progenitor cells impairs proliferation of embryonic brown fat progenitor cells without affecting adipogenesis or cell death, establishing a role in BAT development. Conditional knockout mice (Myf5Cre, Ucp1Cre, AdipoqCre crossed with Ccncflox/flox), RNA-seq, immunostaining, immunoblotting, qRT-PCR, metabolic phenotyping Molecular metabolism High 35863637
2025 MTBP is a second allosteric activator of Cdk8/19-CycC kinase activity, mutually exclusive with Med12 targeting. Both Med12 and MTBP reposition the T-loop of CDK8 independently of T-loop phosphorylation to activate kinase activity in vitro, revealing that the Cdk8/19-CycC dimer alone has low enzymatic activity and requires accessory factors for efficient substrate phosphorylation. In vitro kinase assays, structural studies, mutagenesis (preprint) bioRxivpreprint Medium bio_10.1101_2025.06.16.659917
2024 Cyclin C (Ccnc) is required for both steady-state and induced autophagic gene transcription; pancreatic ablation of Ccnc causes phenotypes mirroring autophagy deficiency (islet atrophy, acinar cell damage) and accelerates ADM and PanIN formation in the context of oncogenic Kras. Ccnc-deficient cells show reduced autophagy-lysosome pathway activation and reduced proteasome function, rendering them hypersensitive to proteasome inhibitors. Conditional knockout mice (Ccnc pancreatic ablation ± KrasG12D), cell line studies, autophagy and proteasome activity assays, histopathology (preprint) bioRxivpreprint Medium bio_10.1101_2024.08.21.609015

Source papers

Stage 0 corpus · 12 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2004 Mastermind recruits CycC:CDK8 to phosphorylate the Notch ICD and coordinate activation with turnover. Molecular cell 494 15546612
2011 The structure of CDK8/CycC implicates specificity in the CDK/cyclin family and reveals interaction with a deep pocket binder. Journal of molecular biology 113 21806996
2013 Structure-kinetic relationship study of CDK8/CycC specific compounds. Proceedings of the National Academy of Sciences of the United States of America 100 23630251
1996 Molecular cloning and chromosomal localization of the human cyclin C (CCNC) and cyclin E (CCNE) genes: deletion of the CCNC gene in human tumors. Genomics 62 8833152
1995 Chromosomal mapping of the genes for the human cell cycle proteins cyclin C (CCNC), cyclin E (CCNE), p21 (CDKN1) and KAP (CDKN3). Cytogenetics and cell genetics 54 7698009
2015 mTORC1 Down-Regulates Cyclin-Dependent Kinase 8 (CDK8) and Cyclin C (CycC). PloS one 25 26042770
2018 A molecular dynamics investigation of CDK8/CycC and ligand binding: conformational flexibility and implication in drug discovery. Journal of computer-aided molecular design 19 29737445
2014 All-atomic molecular dynamic studies of human CDK8: insight into the A-loop, point mutations and binding with its partner CycC. Computational biology and chemistry 19 24754906
1991 Characterization of cytochromes c550 and c555 from Bradyrhizobium japonicum: cloning, mutagenesis, and sequencing of the c555 gene (cycC). Journal of bacteriology 17 1660457
2018 Discovery of CDK8/CycC Ligands with a New Virtual Screening Tool. ChemMedChem 11 30403831
2004 A uniform procedure for the purification of CDK7/CycH/MAT1, CDK8/CycC and CDK9/CycT1. Biological procedures online 10 15328539
2022 The conserved Mediator subunit cyclin C (CCNC) is required for brown adipocyte development and lipid accumulation. Molecular metabolism 9 35863637