Stimulation of the Raf/MEK/ERK cascade is necessary and sufficient for activation and Thr-160 phosphorylation of a nuclear-targeted CDK2.

The activity of cyclin-dependent kinase 2 is required for G(1)-S-phase progression of the eukaryotic cell cycle. In this study, we examine the activation of CDK2-cyclin E by constructing a CDK2 that is constitutively targeted to the nucleus. Activation of CDK2 requires the removal of two inhibitory phosphates (Thr-14 and Tyr-15) and the addition of one activating phosphate (Thr-160) by a nuclear localized CDK-activating kinase, which is thought to be constitutively active. Surprisingly, nuclear localized CDK2-NLS and CDK2-NLS(A14,F15), which lacks the inhibitory phosphorylation sites, require serum to become active, despite complexing with expressed cyclin E. We show that inhibition of mitogen-mediated ERK activation by treatment with U0126, a selective MEK inhibitor, or expression of dominant-negative ERK markedly reduces the phosphorylation of Thr-160 and enzymatic activity of both CDK2-NLS constructs. Consistent with a role for ERK in Thr-160 phosphorylation, expression of constitutively active Raf-1 induces Thr-160 phosphorylation of CDK2-NLS in serum-arrested cells, an effect that is blocked by treatment with U0126. Taken together, these data show a new role for ERK in G1 cell cycle progression: In addition to its role in stimulating cyclin D1 expression and nuclear translocation of CDK2, ERK regulates Thr-160 phosphorylation of CDK2-cyclin E.