Degradation of p57^Kip2 mediated by SCF^Skp2-dependent ubiquitylation

The abundance of the cyclin-dependent kinase (CDK) inhibitor p57 ^Kip2, 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 SCF^Skp2 has now been shown to be responsible for regulating the cellular level of p57^Kip2 by targeting it for ubiquitylation and proteolysis. The elimination of p57 ^Kip2 was impaired in Skp2^-/- cells, resulting in abnormal accumulation of the protein. Coimmunoprecipitation analysis also revealed that Skp2 interacts with p57^Kip2 in vivo. Overexpression of WT Skp2 promoted degradation of p57^Kip2, whereas expression of a dominant negative mutant of Skp2 prolonged the half-life of p57^Kip2. Mutation of the threonine residue (Thr-310) of human p57^Kip2 that is conserved between the COOH-terminal QT domains of p57^Kip2 and p27^Kip1 prevented the effect of Skp2 on the stability of p57 ^Kip2, suggesting that phosphorylation at this site is required for SCF^Skp2-mediated ubiquitylation. Finally, the purified recombinant SCF^Skp2 complex mediated p57^Kip2 ubiquitylation in vitro in a manner dependent on the presence of the cyclin E-CDK2 complex. These observations thus demonstrate that the SCF^Skp2 complex plays an important role in cell-cycle progression by determining the abundance of p57^Kip2 and that of the related CDK inhibitor p27^Kip1.