Degradation of p57Kip2Mediated by SCFSkp2-Dependent Ubiquitylation

The abundance of the cyclin-dependent kinase (CDK) inhibitor p57Kip2, an important regulator of cell cycle progression, is thought to be controlled by the ubiquitin-proteasome pathway. The Skp1/Cul1/F-box (SCF)-type E3 ubiquitin ligase complex SCFSkp2has now been shown to be responsible for regulating the cellular level of p57Kip2by targeting it for ubiquitylation and proteolysis. The elimination of p57Kip2was impaired in Skp2-/-cells, resulting in abnormal accumulation of the protein. Coimmunoprecipitation analysis also revealed that Skp2 interacts with p57Kip2in vivo. Overexpression of WT Skp2 promoted degradation of p57Kip2, whereas expression of a dominant negative mutant of Skp2 prolonged the half-life of p57Kip2. Mutation of the threonine residue (Thr-310) of human p57Kip2that is conserved between the COOH-terminal QT domains of p57Kip2and p27Kip1prevented the effect of Skp2 on the stability of p57Kip2, suggesting that phosphorylation at this site is required for SCFSkp2-mediated ubiquitylation. Finally, the purified recombinant SCFSkp2complex mediated p57Kip2ubiquitylation in vitro in a manner dependent on the presence of the cyclin E-CDK2 complex. These observations thus demonstrate that the SCFSkp2complex plays an important role in cell-cycle progression by determining the abundance of p57Kip2and that of the related CDK inhibitor p27Kip1.