ABSTRACT: Cyclin C was cloned as a growth-promoting G1 cyclin1,2, and several studies postulated a role for cyclin C in driving cell proliferation3-8 . Moreover, cyclin C, together with its kinase partner, the cyclin-dependent kinase CDK8, is believed to represent an essential component of basal transcriptional machinery where it globally represses gene expression9-13. However, the function of cyclin C in vivo has never been addressed. Here we show that in the living organism cyclin C acts as a haploinsufficient tumor suppressor, through its function of controlling Notch1 oncogene levels. Cyclin C activates an “orphan” CDK19 kinase14, as well as CDK8 and CDK3. These cyclin C-CDK complexes phosphorylate Notch1 intracellular domain (ICN1), which allows binding of ICN1 to Fbw7 and triggers ICN1 polyubiquitination. Genetic ablation of cyclin C blocks ICN1 phosphorylation, disrupts Fbw7 binding, and decreases ICN1 ubiquitination in vivo, thereby strongly elevating ICN1 levels in several compartments of cyclin C knockout mice. Cyclin C was cloned as a growth-promoting G1 cyclin1,2, and several studies postulated a role for cyclin C in driving cell proliferation3-8 . Moreover, cyclin C, together with its kinase partner, the cyclin-dependent kinase CDK8, is believed to represent an essential component of basal transcriptional machinery where it globally represses gene expression9-13. However, the function of cyclin C in vivo has never been addressed. Here we show that in the living organism cyclin C acts as a haploinsufficient tumor suppressor, through its function of controlling Notch1 oncogene levels. Cyclin C activates an “orphan” CDK19 kinase14, as well as CDK8 and CDK3. These cyclin C-CDK complexes phosphorylate Notch1 intracellular domain (ICN1), which allows binding of ICN1 to Fbw7 and triggers ICN1 polyubiquitination. Genetic ablation of cyclin C blocks ICN1 phosphorylation, disrupts Fbw7 binding, and decreases ICN1 ubiquitination in vivo, thereby strongly elevating ICN1 levels in several compartments of cyclin C knockout mice.
Steven P Gygi 