Nature Cell Biology3, 715 - 722 (2001)
Published online: 10 July 2001; | doi:10.1038/35087026
Triggering ubiquitination of a CDK inhibitor at origins of DNA replication
Laura Furstenthal1, 2, Craig Swanson1, 2, Brett K. Kaiser1, Adam G. Eldridge1
& Peter K. Jackson1
1
Departments of Pathology and Microbiology & Immunology, Stanford University School of Medicine, 300 Pasteur Drive, MC 5324, Palo Alto, California 94305-5324, USA
To ensure proper timing of the G1−S transition in the cell cycle, the cyclin E−Cdk2 complex, which is responsible for the initiation of DNA replication, is restrained by the p21Cip1/p27Kip1/p57Kip2 family of CDK (cyclin-dependent kinase) inhibitors in humans and by the related p27Xic1 protein in Xenopus. Activation of cyclin E−Cdk2 is linked to the ubiquitination of human p27Kip1 or Xenopus p27Xic1 by SCF (for Skp1−Cullin−F-box protein) ubiquitin ligases. For human p27Kip1, ubiquitination requires direct phosphorylation by cyclin E−Cdk2. We show here that Xic1 ubiquitination does not require phosphorylation by cyclin E−Cdk2, but it does require nuclear accumulation of the Xic1−cyclin E−Cdk2 complex and recruitment of this complex to chromatin by the origin-recognition complex together with Cdc6 replication preinitiation factors; it also requires an activation step necessitating cyclin E−Cdk2-kinase and SCF ubiquitin-ligase activity, and additional factors associated with mini-chromosome maintenance proteins, including the inactivation of geminin. Components of the SCF ubiquitin-ligase complex, including Skp1 and Cul1, are also recruited to chromatin through cyclin E−Cdk2 and the preinitiation complex. Thus, activation of the cyclin E−Cdk2 kinase and ubiquitin-dependent destruction of its inhibitor are spatially constrained to the site of a properly assembled preinitiation complex.
