Phosphorylation of Skp2 regulated by CDK2 and Cdc14B protects it from degradation by APCCdh1

The p27Kip1 ubiquitin ligase receptor Skp2 is often over-expressed in human tumours and displays oncogenic properties. The activity of SCFSkp2 is regulated by the APCCdh1, which targets Skp2 for degradation. Here we show that Skp2 phosphorylation on Ser64/Ser72 positively regulates its function in vivo. Phosphorylation of Ser64, and to a lesser extent Ser72, stabilizes Skp2 by interfering with its association with Cdh1, without affecting intrinsic ligase activity. Cyclin-dependent kinase (CDK)2-mediated phosphorylation of Skp2 on Ser64 allows its expression in mid-G1 phase, even in the presence of active APCCdh1. Reciprocally, dephosphorylation of Skp2 by the mitotic phosphatase Cdc14B at the M-G1 transition promotes its degradation by APCCdh1. Importantly, lowering the levels of Cdc14B accelerates cell cycle progression from mitosis to S phase in an Skp2-dependent manner, demon-strating epistatic relationship of Cdc14B and Skp2 in the regulation of G1 length. Thus, our results reveal that reversible phosphorylation plays a key role in the timing of Skp2 expression in the cell cycle