Degradation of the CDK inhibitor p27^<Kip1> by a novel ubiquitin ligase.

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Cell cycle progression is controlled by cyclin-dependent kinases (CDKs). The CDK activities are negatively regulated by CDK inhibitor proteins (CKIs). In the quiescent and early G1 phase, p27^<Kip1>, one of the Cip/Kip-type CKIs, exists in abundance in the cell nucleus to suppress cell cycle progression. p27^<Kip1> is degraded in late G1 phase by the ubiquitin-proteasome pathway, allowing cells to enter S phase. In this study, we identified p27NBP1 as a p27^<Kip1>-interacting protein. p27NBP1 physically interacted with p27^<Kip1> and directly ubiquitinated p27^<Kip1> in an intact RING finger domain-dependent manner. Since we isolated p27NBP1 as a protein that interacts with the amino-terminal portion of p27^<Kip1> containing Ser10, we next examined whether interaction between these two proteins was affected by Ser10 phosphorylation status. In spite of the substitution of Ser10, p27^<Kip1> mutants associated with p27NBP1 to an extent similar to that of wild-type p27^<Kip1>. This result suggested that p27NBP1 interacts with p27^<Kip1> regardless of the status of Ser10 phosphorylation. Ablation of endogenous p27NBP1 by small interfering RNA increased the steady-state level of p27^<Kip1> and decelerated p27^<Kip1> turnover. p27^<Kip1> was decreased in synchronization with accumulation of p27NBP1 in late G1 phase. Furthermore, depletion of p27NBP1 resulted in inhibition of cell cycle progression from G1 to S phase in a p53-independent manner. Overall, the results indicate that p27NBP1 acts as a negative regulator of p27^<Kip1> function by promoting ubiquitin-dependent proteasomal degradation.