Affinage

CCNC

Cyclin-C · UniProt P24863

Length
283 aa
Mass
33.2 kDa
Annotated
2026-06-09
12 papers in source corpus 7 papers cited in narrative 7 extracted findings
Cross-family judge vs UniProt: tie faithfulness: 5/5 claims corpus-supported (100%)

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

Cyclin C (CCNC) is the regulatory cyclin partner of CDK8 (and CDK19) within the CDK module of the Mediator complex, where the CycC:CDK8 kinase couples transcriptional regulation to substrate phosphorylation in development and metabolism (PMID:15546612, PMID:35863637). The CycC:CDK8 dimer phosphorylates the Notch intracellular domain within its TAD and PEST regions when recruited together with the Notch ICD and coactivators MAM and SKIP to target promoters such as HES1, driving ICD hyperphosphorylation and Fbw7/Sel10-dependent degradation (PMID:15546612). The crystal structure of CDK8/CycC shows a CycC-recognition helix in CDK8 that dictates the specificity of the pairing, and the unphosphorylated activation loop indicates the kinase is activated by a mechanism distinct from canonical T-loop phosphorylation (PMID:21806996); the dimer alone has low intrinsic activity and is allosterically activated by Med12 or MTBP, which reposition the T-loop independently of its phosphorylation, with Med12 directing the kinase to Mediator-associated transcription and MTBP to Med12-independent roles such as replication origin firing [PMID:bio_10.1101_2025.06.16.659917]. Functionally, CycC:CDK8 acts downstream of mTORC1 to suppress de novo lipogenesis: mTORC1 activation down-regulates the complex, releasing nuclear SREBP-1c and lipogenic enzyme expression in liver (PMID:26042770). In brown adipose tissue, CCNC is required for brown fat progenitor proliferation and for lipogenic gene expression through a C/EBPα/GLUT4/ChREBP axis, and its loss in Ucp1+ cells promotes beiging of white fat (PMID:35863637). CCNC also supports autophagic gene transcription and proteasome function, and its pancreatic ablation impairs tissue integrity and accelerates KrasG12D-driven neoplastic progression [PMID:bio_10.1101_2024.08.21.609015].

Mechanistic history

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

    Established that CycC:CDK8 is a direct Notch-pathway kinase, answering how the Notch ICD is targeted for turnover and linking the complex to transcriptional control of HES1.

    Evidence ChIP, direct MAM-CDK8 binding assay, in vitro kinase assay with purified recombinant CycC:CDK8, and PEST Ser point mutagenesis with in vivo degradation readouts

    PMID:15546612

    Open questions at the time
    • Did not define CycC's contribution to substrate selection versus CDK8
    • Generality beyond Notch ICD substrates not addressed
  2. 2011 High

    Resolved the molecular architecture of the CDK8/CycC pairing and revealed that the activation loop is unphosphorylated, raising the question of how the kinase is activated.

    Evidence X-ray crystallography at 2.2 Å of CDK8/CycC in complex with sorafenib

    PMID:21806996

    Open questions at the time
    • Did not identify the alternate activation mechanism
    • Static structure does not capture activator-bound state
  3. 2013 High

    Defined how inhibitors engage the CDK8/CycC catalytic cleft, informing the kinetic determinants of binding to the complex.

    Evidence X-ray crystallography of multiple CDK8/CycC-ligand complexes with surface plasmon resonance binding kinetics

    PMID:23630251

    Open questions at the time
    • Addresses ligand engagement, not endogenous regulation
    • No cellular validation of inhibitor consequences for CycC function
  4. 2015 Medium

    Placed CycC:CDK8 downstream of mTORC1 as a suppressor of de novo lipogenesis, connecting the kinase to metabolic regulation in liver.

    Evidence Rapamycin and genetic mTORC1 perturbation, immunoblotting, and three NAFLD mouse models

    PMID:26042770

    Open questions at the time
    • Mechanism by which mTORC1 reduces CDK8-CycC levels not resolved
    • Direct lipogenic substrates of the complex not identified
  5. 2022 High

    Demonstrated a tissue-specific developmental and metabolic requirement for CCNC in brown adipogenesis via the C/EBPα/GLUT4/ChREBP axis.

    Evidence Multiple conditional KO mouse models (Myf5Cre, Ucp1Cre, AdipoqCre), RNA-seq, metabolic and cold-exposure phenotyping

    PMID:35863637

    Open questions at the time
    • Whether effects depend on CDK8 kinase activity not established
    • Direct transcriptional targets of CycC in the axis not pinpointed
  6. 2024 Medium

    Linked CCNC to autophagy/proteasome gene programs and showed it restrains KrasG12D-driven pancreatic neoplasia.

    Evidence Conditional pancreatic KO with KrasG12D model, histopathology, and cell-line autophagy/proteasome assays (preprint)

    PMID:bio_10.1101_2024.08.21.609015

    Open questions at the time
    • Preprint not yet peer-reviewed
    • Direct transcriptional targets in the autophagy-lysosome program not defined
  7. 2025 Medium

    Identified MTBP as a second allosteric activator mutually exclusive with Med12, resolving how the low-activity CycC:CDK8/19 dimer is switched on and directed to distinct cellular roles.

    Evidence In vitro kinase assays, structural analysis of T-loop repositioning, MTBP interaction and Med12 epistasis (preprint)

    PMID:bio_10.1101_2025.06.16.659917

    Open questions at the time
    • Preprint not yet peer-reviewed
    • Structural detail of MTBP-bound complex not at atomic resolution
    • Replication origin firing role not validated in vivo

Open questions

Synthesis pass · forward-looking unresolved questions
  • How CycC-specific contributions (versus CDK8/CDK19) and allosteric activator choice determine substrate selection across transcription, metabolism, and replication remains unresolved.
  • No unified substrate map across the distinct functional contexts
  • Cause of mTORC1-dependent complex down-regulation unknown
  • In vivo confirmation of MTBP-directed functions lacking

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0098772 molecular function regulator activity 2 GO:0140096 catalytic activity, acting on a protein 2 GO:0140110 transcription regulator activity 2
Localization
GO:0005634 nucleus 1
Pathway
R-HSA-1430728 Metabolism 2 R-HSA-162582 Signal Transduction 1 R-HSA-74160 Gene expression (Transcription) 1
Partners
CDK19CDK8MED12MTBP
Complex memberships
Mediator CDK module (CDK8-CycC)

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 and coactivators (MAM, SKIP) to the HES1 promoter; MAM interacts directly with CDK8 and localizes it to subnuclear foci; purified recombinant CycC:CDK8 phosphorylates the Notch ICD within the TAD and PEST domains, promoting hyperphosphorylation and PEST-dependent degradation by the Fbw7/Sel10 ubiquitin ligase; point mutations in conserved Ser residues within the ICD PEST motif prevent hyperphosphorylation and stabilize the ICD in vivo. Chromatin immunoprecipitation, direct binding assay (MAM-CDK8), in vitro kinase assay with purified recombinant CycC:CDK8, in vivo phosphorylation/degradation assays, point mutagenesis of PEST Ser residues Molecular cell High 15546612
2011 Crystal structure of CDK8/CycC at 2.2 Å (in complex with sorafenib) reveals a unique CycC-recognition helix in CDK8 that explains specificity of the CDK8/CycC pairing; the CDK8 activation loop is not phosphorylated, suggesting an alternate activation mechanism distinct from canonical T-loop phosphorylation; sorafenib binds the catalytic cleft and induces a DMG-out (DFG-out equivalent) inactive conformation—the first small molecule to do so in the CDK family. X-ray crystallography (2.2 Å resolution), structural analysis Journal of molecular biology High 21806996
2013 Structure-kinetic relationship analysis of CDK8/CycC-inhibitor complexes shows that the scaffold anchors in the kinase deep pocket; hydrophobic complementarities within the front pocket contribute more to residence time than hinge-region hydrogen bonding; flipping CDK8's DMG motif to the inactive conformation has relatively little influence on binding velocity. X-ray crystallography of multiple CDK8/CycC-ligand complexes 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 down-regulates the CDK8-CycC complex in vitro and in mouse liver in vivo; pharmacologic and genetic inhibition of mTORC1 increases CDK8 and CycC levels; in NAFLD mouse models, elevated mTORC1 activity correlates with reduced CDK8-CycC complex and increased nuclear SREBP-1c and lipogenic enzyme expression, consistent with CDK8-CycC suppressing de novo lipogenesis. Pharmacologic (rapamycin) and genetic (mTORC1 component KO/overexpression) approaches, immunoblotting, mouse liver in vivo experiments, three NAFLD mouse models PloS one Medium 26042770
2022 CCNC deletion in Myf5+ progenitor cells (conditional KO) impairs proliferation of embryonic brown fat progenitor cells causing BAT paucity; in differentiated brown adipocytes, CCNC is required for lipogenic gene expression through activation of the C/EBPα/GLUT4/ChREBP axis; CCNC deficiency in Ucp1+ cells enhances beiging of white adipose tissue upon cold exposure. Conditional knockout mice (Ccncflox/flox × Myf5Cre, Ucp1Cre, AdipoqCre), immunostaining, immunoblotting, RNA-seq, glucose/insulin tolerance tests, indirect calorimetry, cold exposure Molecular metabolism High 35863637
2025 MTBP is a second allosteric activator of Cdk8/19-CycC kinase, mutually exclusive with Med12; both Med12 and MTBP reposition the T-loop of CDK8/19 to activate kinase activity in vitro independently of T-loop phosphorylation; MTBP targets the Cdk8/19-CycC kinase to Med12-independent cellular roles (e.g., replication origin firing), while Med12 targets it to the Mediator transcription complex; the Cdk8/19-CycC dimer alone has low enzymatic activity, requiring allosteric activators for efficient substrate phosphorylation. In vitro kinase assays, structural analysis (T-loop repositioning), protein interaction studies (MTBP-Cdk8/19-CycC), genetic/functional epistasis with Med12 bioRxivpreprint Medium bio_10.1101_2025.06.16.659917
2024 Ccnc is required for steady-state and induced autophagic gene transcription in mouse embryonic fibroblasts; pancreatic Ccnc ablation (CcncPanΔ) impairs islet and acinar cell integrity, accelerates acinar ductal metaplasia and PanIN lesion formation in the context of KrasG12D, reduces autophagy-lysosome pathway (ALP) activation, and causes reduced proteasome function rendering cells hypersensitive to proteasome inhibitors. Conditional pancreatic KO mouse (CcncPanΔ), KrasG12D co-expression model, histopathology, cell line functional assays (autophagy, proteasome activity), pharmacologic proteasome inhibitor sensitivity 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 495 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 114 21806996
2013 Structure-kinetic relationship study of CDK8/CycC specific compounds. Proceedings of the National Academy of Sciences of the United States of America 101 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 12 30403831
2022 The conserved Mediator subunit cyclin C (CCNC) is required for brown adipocyte development and lipid accumulation. Molecular metabolism 10 35863637
2004 A uniform procedure for the purification of CDK7/CycH/MAT1, CDK8/CycC and CDK9/CycT1. Biological procedures online 10 15328539

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