The cyclin-dependent kinase inhibitor p27Kip1 is stabilized in G0 by Mirk/dyrk1B Kinase

Abstract

Elevated levels of the cyclin-dependent kinase (CDK) inhibitor p27 block the cell in G0/G1 until mitogenic signals activate G 1 cyclins and initiate proliferation. Post-translational regulation of p27 by different phosphorylation events is critical in allowing cells to proceed through the cell cycle. We now demonstrate that the arginine-directed kinase, Mirk/dyrk1B, is maximally active in G0 in NIH3T3 cells, when it stabilizes p27 by phosphorylating it at Ser-10. The phospho-mimetic mutant p27-S10D was more stable, and the non-phosphorylatable mutant p27-S10A was less stable than wild-type when expressed in G0-arrested cells. Following phosphorylation by Mirk, p27 remains a functional CDK inhibitor, capable of binding to CDK2. Mirk did not induce the translocation of p27 from the nucleus in G0, but instead co-localized with nuclear p27. Depletion of Mirk by RNA interference decreased the phosphorylation of p27 at Ser-10 and the stability of endogenous p27. RNAi to Mirk increased cell entry from G0 into G1 as shown by increased expression of proliferating cell nuclear antigen and decreased expression of p27. These data suggest a model in which Mirk increases the amount of nuclear p27 by stabilizing it during G0 when Mirk is most abundant. Mitogen stimulation then causes cells to enter G1, reduces Mirk levels (Deng, X., Ewton, D., Pawlikowski, B., Maimone, M., and Friedman, E. (2003) J. Biol. Chem. 278, 41347-41354), and initiates the translocation of p27 to the cytoplasm. In addition, depletion of Mirk by RNAi in postmitotic C2C12 myoblasts decreased protein but not MRNA levels of p27, suggesting that stabilization of p27 by Mirk also occurs during differentiation.