All aboard the cyclin train: subcellular trafficking of cyclins and their CDK partners.

Progression through the cell cycle is governed by the periodic activation and inactivation of cyclin-dependent kinase complexes (CDK-cyclins). Although the enzymatic activity of these complexes is regulated tightly, it has recently been demonstrated that an additional facet of cell-cycle control involves the modulation of CDK-cyclin subcellular localization. Recent discoveries include the identification of nuclear transport factors responsible for ferrying some of the CDK-cyclins in and out of the nucleus, the demonstration that phosphorylation can regulate these transport processes and the establishment of potential links between cell-cycle checkpoints and the control of CDK-cyclin subcellular localization.