Abstract

The Cyclin Dependent Kinases (CDKs) are a family of master regulators of the eukaryotic cell cycle. Work from the lab has shown that the fission yeast cell cycle can be driven by a single CDK -cyclin complex, and unexpectedly that the G1/S and G2/M CDKs have very similar substrate specificities. This suggests that rising total CDK activity in the cell plays a major role in bringing about the correct temporal order of cell cycle events.

The CDK -cyclin complex has a third CKS sub-unit (Suc1 in fission yeast) which we show enhances the phosphorylation of around 200 CDK phosphosites on 136 proteins, many of which are required for the different events of S-phase and mitosis. Ablation of Suc1 activity results in defects in both S-phase and mitosis onset and progression. Suc1 also controls the association of Wee1 and Cdc25 with CDK , thus regulating CDK Y15 phosphorylation and activity at mitotic onset.

Single cell CDK sensor assays have shown that CDK is activated first in the nucleus and then later in the cytoplasm. Nuclear CDK activation results in export of CDK into the cytoplasm activating the CDK located there. In vivo phase plots show that cyclin control over CDK activation is stronger in the nucleus than the cytoplasm. We conclude that nuclear CDK acts as the ‘mitotic pacemaker’ determining when cells enter mitosis. Locating the CDK pacemaker in the nucleus places it closer to DNA where it can be more readily controlled by the DNA checkpoints and ploidy controls essential for genome stability. These experiments emphasise the need to better understand spatial aspects of CDK regulation in the cell in addition to temporal aspects.