Cyclin A/cdk2 coordinates centrosomal and nuclear mitotic events

Cyclin A/cdk2 has a role in progression through S phase, and a large pool is also activated in G 2 phase. Here we report that this G 2 phase pool regulates the timing of progression into mitosis. Knock down of cyclin A by siRNA or addition of a specific cdk2 small molecule inhibitor delayed entry into mitosis by delaying cells in G 2 phase. The G 2 phase-delayed cells contained elevated levels of inactive cyclin B/cdkl. However, increased microtubule nucleation at the centrosomes was observed, and the centrosomes stained for markers of cyclin B/cdkl activity. Both microtubule nucleation at the centrosomes and the phosphoprotein markers were lost with short-term treatment of the cdkl/2 inhibitor roscovitine but not the Mekl/2 inhibitor U0126. Cyclin A/cdk2 localized at the centrosomes in late G 2 phase after separation of the centrosomes but before the start of prophase. Thus G 2 phase cyclin A/cdk2 controls the timing of entry into mitosis by controlling the subsequent activation of cyclin B/cdkl, but also has an unexpected role in coordinating the activation of cyclin B/cdkl at the centrosome and in the nucleus.