Cyclin-dependent kinase control of centrosome duplication

Centrosomes nucleate microtubules and duplicate once per cell cycle. This d uplication and subsequent segregation in mitosis results in maintenance of the one centrosome/cell ratio. Centrosome duplication occurs during the G(1 )/S transition in somatic cells and must be coupled to the events of the nu clear cell cycle; failure to coordinate duplication and mitosis results in abnormal numbers of centrosomes and aberrant mitoses. Using both in vivo an d in vitro assays, we show that centrosome duplication in Xenopus laevis em bryos requires cyclin/cdk2 kinase activity. Injection of the cdk (cyclin-de pendent kinase) inhibitor p21 into one blastomere of a dividing embryo bloc ks centrosome duplication in that blastomere; the related cdk inhibitor p27 has a similar effect. An in vitro system using Xenopus extracts carries ou t separation of the paired centrioles within the centrosome. This centriole separation activity is dependent on cyclin/cdk2 activity; depletion of eit her cdk2 or of the two activating cyclins, cyclin A and cyclin E, eliminate s centriole separation activity. In addition, centriole separation is inhib ited by the mitotic state, suggesting a mechanism of linking the cell cycle to periodic duplication of the centrosome.