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

Progression through the cell cycle is governed by the periodic activation a nd inactivation of cyclin-dependent kinase complexes (CDK-cyclins). Althoug h the enzymatic activity of these complexes is regulated tightly, it has re cently been demonstrated that an additional facet of cell-cycle control inv olves the modulation of CDK-cyclin subcellular localization. Recent discove ries include the identification of nuclear transport factors responsible fo r ferrying some of the CDK-cyclins in and out of the nucleus, the demonstra tion that phosphorylation can regulate these transport processes and the es tablishment of potential links between cell-cycle checkpoints and the contr ol of CDK-cyclin subcellular localization.