p27 cytoplasmic localization is regulated by phosphorylation on Ser10 and is not a prerequisite for its proteolysis

The activity of the cyclin-dependent kinase inhibitor p27 is controlled by its concentration and subcellular localization. However, the mechanisms tha t regulate its intracellular transport are poorly understood. Here we show that p27 is phosphorylated on Ser10 in vivo and that mutation of Ser10 to A la inhibits p27 cytoplasmic relocalization in response to mitogenic stimula tion. In contrast, a fraction of wild-type p27 and a p27(S10D)-phospho-mime tic mutant translocates to the cytoplasm in the presence of mitogens. G(1) nuclear export of p27 and its Ser10 phosphorylation precede cyclin-dependen t kinase 2 (Cdk2) activation and degradation of the bulk of p27. Interestin gly, leptomycin B-mediated nuclear accumulation accelerates the turnover of endogenous p27; the p27(S10A) mutant, which is trapped in the nucleus, has a shorter half-life than wild-type p27 and the p27(S10D) mutant. In summar y, p27 is efficiently degraded in the nucleus and phosphorylation of Ser10 is necessary for the nuclear to cytoplasmic redistribution of a fraction of p27 in response to mitogenic stimulation. This cytoplasmic localization ma y serve to decrease the abundance of p27 in the nucleus below a certain thr eshold required for activation of cyclin-Cdk2 complexes.