In eukaryotes, the two major cell cycle events of DNA replication and chromosome segregation are directed by Cyclin Dependent Kinases (CDKs) in combination with G1/S cyclins (S-CDK) and G2/M cyclins (M-CDK), respectively. It is still unclear what differentiates S-CDK activity from M-CDK activity, and how much functional redundancy exists between cyclin-CDK complexes.

Here the fission yeast G1/S cyclin-CDK complex, Cig2-Cdc2, is used to study S-CDK. It is shown that S-CDK is capable of achieving high levels of CDK activity, but is incapable of executing mitosis. However, deletion of Protein Phosphatase 1 (PP1) allows for mitotic progression, and is therefore responsible for restricting S-CDK from executing mitosis. Substrates at the yeast centrosome equivalent, the spindle pole body (SPB), are phosphorylated poorly by S-CDK. Consistent with this idea, removal of PP1 at the SPB alone allows S-CDK to drive mitosis. Phosphoproteomic analysis then shows that S-CDK and M-CDK have subtly different in vivo CDK substrate preferences, but that the majority of substrates experience S-CDK and M-CDK activity equally.

Using a CDK mutant that does not need cyclin binding for activity, evidence is presented suggesting that generic CDK activity without cyclins is sufficient for S-phase, but that cyclin-associated CDK activity is required for mitosis. In pursuit of why cyclins are essential for mitosis, the substrate docking region of the mitotic cyclin Cdc13 is mutated. The mutation of this region does not affect Cdc13-dependent S-phase progression, but cells fail to execute mitosis. Mutant Cdc13 is unable to localise correctly to the interphase SPB, and phosphoproteomic analysis suggests that this defect causes reduced CDK substrate phosphorylation at the SPB, preventing mitosis. These data are consistent with the idea that cyclins regulate CDK localisation at mitosis to influence CDK substrate selection, and suggests that correctly regulated CDK activity across cellular compartments is needed to execute mitosis.