Cyclin B knows its place

In mammalian cells the work of the cell cycle is divided between two workhorses: Cdk2/cyclin E for S phase and Cdk1/cyclin B for mitosis. Now, Jonathan Moore, Jane Kirk, and Tim Hunt (Cancer Research UK London Research Institute, London, UK) show that this apparent specificity is achieved by limiting access to substrates. By denying entrance to the nucleus, cells prevent Cdk1–cyclin B from jumpstarting S phase at inopportune times.

In frog egg extracts, S phase is induced in nuclei by Cdk2–cyclin E, but not by Cdk1–cyclin B. This difference has often been construed as specificity in cyclin substrate preferences, but the new results show that cyclin/Cdk pairs are in fact surprisingly promiscuous enzymes. Hunt's group simply replaced the nuclear export signal from cyclin B with a nuclear localization...