CCNH

CCNH (cyclin H) is the regulatory cyclin subunit of the CDK-activating kinase (CAK), forming an active trimeric complex with CDK7 and MAT1 that can be reconstituted from recombinant subunits (PMID:15328539). Within this complex CCNH/CDK7 phosphorylates CDC2 (CDK1) at threonine 161 to activate MPF and drive meiotic resumption, with gain- and loss-of-function manipulation accelerating or blocking GVBD, cyclin B synthesis, and MPF activation in oocytes (PMID:21778139). As part of TFIIH, the CDK7-cyclin H-p36 complex also phosphorylates p53 at C-terminal serines (371, 376, 378, 392), enhancing its sequence-specific DNA-binding activity (PMID:9315650). Beyond its kinase functions, CCNH/CDK7 directly binds the transcriptional corepressor CtBP2 and stabilizes it by competing with HIPK2, blocking HIPK2-mediated phosphorylation, dimerization, and proteasomal degradation of CtBP2, thereby promoting cancer cell invasion and migration (PMID:23393140).

1. 1997 High

   Established that the CDK7-cyclin H-p36 complex acts on a substrate beyond CDKs by phosphorylating p53 and modulating its activity, linking CAK/TFIIH to tumor suppressor function.

   Evidence In vitro kinase assay with purified CDK7-CycH-p36, phosphorylation site mapping, and gel mobility shift assay

   PMID:9315650

   Open questions at the time

   • In vitro reconstitution only; cellular relevance of p53 phosphorylation by this complex not tested
   • Functional consequence limited to DNA binding, not downstream p53 transcriptional output
2. 1998 Medium

   Assigned the human CCNH gene to a defined chromosomal locus, providing a genomic anchor for the cyclin H subunit.

   Evidence FISH, somatic cell hybrid analysis, and YAC contig mapping

   PMID:9465303

   Open questions at the time

   • Mapping only; no functional or regulatory information about the locus
3. 2004 Medium

   Demonstrated that CCNH assembles with CDK7 and MAT1 into a catalytically active trimeric complex, enabling biochemical study of the CAK.

   Evidence Recombinant co-expression in insect cells, affinity and ion-exchange purification, kinase activity assay

   PMID:15328539

   Open questions at the time

   • No mutagenesis or structural validation of subunit contributions
   • Substrate specificity of the reconstituted complex not characterized
4. 2011 Medium

   Defined a meiotic role for the CCNH/CDK7 CAK by showing it phosphorylates CDC2 at T161 to activate MPF and drive oocyte meiotic resumption.

   Evidence Overexpression and antisense knockdown in pig oocytes, T161 immunoblot, MPF activity and GVBD assays

   PMID:21778139

   Open questions at the time

   • Single-species (porcine oocyte) system
   • Whether CCNH is rate-limiting versus permissive for CDK1 activation not resolved
5. 2013 Medium

   Identified a non-catalytic role for CCNH/CDK7 in stabilizing the corepressor CtBP2 by competing with HIPK2, connecting CCNH to cancer cell motility.

   Evidence Reciprocal Co-IP, siRNA knockdown, proteasome inhibitor, phosphorylation/dimerization, and invasion/migration assays in breast cancer cells

   PMID:23393140

   Open questions at the time

   • Mechanism by which CCNH/CDK7 competes with HIPK2 not structurally defined
   • Single lab; in vivo tumor relevance not established
6. 2015 Low

   Extended the CCNH/CDK7-CtBP2 interaction to esophageal squamous cell carcinoma, supporting generality of the migration-promoting axis.

   Evidence Co-IP and migration assays with CCNH depletion in ESCC cells

   PMID:25820824

   Open questions at the time

   • Single Co-IP and migration assay largely recapitulating the prior breast cancer study
   • No reciprocal validation or mechanistic depth beyond confirming interaction dependence

• How CCNH's CAK kinase activity, its TFIIH transcriptional role, and its non-catalytic CtBP2-stabilizing function are integrated within a single cell remains unresolved.
• No structural model of the human CCNH-CDK7-MAT1 complex in the corpus
• Whether CtBP2 stabilization requires CDK7 catalytic activity is undefined
• No in vivo demonstration of CCNH-dependent p53 phosphorylation