TURNOVER OF CYCLIN-E BY THE UBIQUITIN-PROTEASOME PATHWAY IS REGULATEDBY CDK2 BINDING AND CYCLIN PHOSPHORYLATION

Cyclin E is a mammalian G(1) cyclin that is both required and rate lim iting for entry into S phase. The expression of cyclin E is periodic, peaking at the G(1)-S transition and then decaying as S phase progress es. To understand the mechanisms underlying cyclin E periodicity, we h ave investigated the regulation of cyclin E degradation. We find that cyclin E is degraded by the ubiquitin-proteasome system, and that this degradation is regulated by both cdk2 binding and cdk2 catalytic acti vity. Free cyclin E is readily ubiquitinated and degraded by the prote asome. Binding to cdk2 protects cyclin E from ubiquitination, and this protection is reversed by cdk2 activity in a process that involves ph osphorylation of cyclin E itself. The data are most consistent with a model in which cdk2 activity initiates cyclin E degradation by promoti ng the disassembly of cyclin E-cdk2 complexes, followed by the ubiquit ination and degradation of free cyclin E.