Cyclin E-mediated elimination of p27 requires its interaction with the nuclear pore-associated protein mNPAP60

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Abstract
The Cdk2 inhibitor, p27Kip1, is degraded in a phosphorylation‐ and ubiquitylation‐dependent manner at the G1–S transition of the cell cycle. Degradation of p27Kip1 requires import into the nucleus for phosphorylation by Cdk2. Phosphorylated p27Kip1 is thought to be subsequently re‐exported and degraded in the cytosol. Using two‐hybrid screens, we now show that p27Kip1 interacts with a nuclear pore‐associated protein, mNPAP60, map the interaction to the 310 helix of p27 and identify a point mutant in p27Kip1 that is deficient for interaction (R90G). In vivo and in vitro, the loss‐of‐interaction mutant is poorly transported into the nucleus, while ubiquitylation of p27R90G occurs normally. In vivo, co‐expression of cyclin E and Cdk2 rescues the import defect. However, mutant p27Kip1 accumulates in a phosphorylated form in the nucleus and is not efficiently degraded, arguing that at least one step in the degradation of phosphorylated p27Kip1 requires an interaction with the nuclear pore. Our results identify a novel component involved in p27Kip1 degradation and suggest that degradation of p27Kip1 is tightly linked to its intracellular transport.