Nuclear accumulation of cyclin E/Cdk2 triggers a concentration-dependent switch for the destruction of p27 (Xic1)

The action of cyclin-dependent kinases (CDKs) is regulated by phosphorylati on, cyclin levels, the abundance of CDK inhibitors, and, as recently has be en shown for cyclin B/cdc2, their localization. It is unclear how localizat ion regulates the action of cyclin E/Cdk2 and its inhibitors. Here. we show that the closest known Xenopus laevis homolog of mammalian Cdk2 inhibitors p27(Kip1) and p21(CIP1), Xic1, is concentrated, ubiquitinated, and destroy ed in the nucleus. Furthermore, Xic1 destruction requires nuclear import, b ut not nuclear export, and requires the formation of a transport-competent nuclear envelope, but not interactions between the lamina and chromatin. We show that (i) cyclin E/Cdk2 and Xic1 are transported into the nucleus as a complex and that Xicl destruction requires the activity of cyclin E, (ii) that phosphorylation of Xic1 by cyclin E/Cdk2 bypasses the requirement for nuclear formation, and (iii) that the phosphorylation of Xic1 by cyclin E/C dk2 is concentration dependent and likely realized through second-order int eractions between stable cyclin E/Cdk2/Xic1 ternary complexes. Based on the se results we propose a model wherein nuclear accumulation of the cyclin E/ Cdk2/Xic1 complex triggers a concentration-dependent switch that promotes t he phosphorylation of Xic1 and, consequently, its ubiquitination and destru ction, thus allowing subsequent activation of cyclin E/Cdk2.