Fbw7α and Fbw7γ Collaborate To Shuttle Cyclin E1 into the Nucleolus for Multiubiquitylation

Cyclin E1, an activator of cyclin-dependent kinase 2 (Cdk2) that promotes replicative functions, is normally expressed periodically within the mammalian cell cycle, peaking at the G(1)-S-phase transition. This periodicity is achieved by E2F-dependent transcription in late G(1) and early S phases and by ubiquitin-mediated proteolysis. The ubiquitin ligase that targets phosphorylated cyclin E is SCFFbw7 (also known as SCFCdc4), a member of the cullin ring ligase (CRL) family. Fbw7, a substrate adaptor subunit, is expressed as three splice-variant isoforms with different subcellular distributions: Fbw7 alpha is nucleoplasmic but excluded from the nucleolus, Fbw7 beta is cytoplasmic, and Fbw7 gamma is nucleolar. Degradation of cyclin E in vivo requires SCF complexes containing Fbw7 alpha and Fbw7 gamma, respectively. In vitro reconstitution showed that the role of SCFFbw7 alpha in cyclin E degradation, rather than ubiquitylation, is to serve as a cofactor of the prolyl cis-trans isomerase Pin1 in the isomerization of a noncanonical proline-proline bond in the cyclin E phosphodegron. This isomerization is required for subsequent binding and ubiquitylation by SCFFbw7 gamma. Here we show that Pin1-mediated isomerization of the cyclin E phosphodegron and subsequent binding to Fbw7 gamma drive nucleolar localization of cyclin E, where it is ubiquitylated by SCFFbw7 gamma prior to its degradation by the proteasome. It is possible that this constitutes a mechanism for rapid inactivation of phosphorylated cyclin E by nucleolar sequestration prior to its multiubiquitylation and degradation.