A mammalian in vitro centriole duplication system: evidence for involvement of CDK2/cyclin E and nucleophosmin/B23 in centrosome duplication.

Centrosome duplication in mammalian cells is a highly regulated process, occurs in coordination of other cell cycle events. However, molecular exploration of this important cellular process had been difficult due to unavailability of a simple assay system. Here, using centrosomes loosely associated with nuclei isolated from cultured cells, we developed a cell-free centriole (duplication unit of the centrosome) duplication system: unduplicated centrosomes bound to the nuclei are able to undergo duplication in the presence of G1/S extracts. We show that the ability of G1/S extracts to induce centriole duplication in vitro depends on the presence of active CDK2/cyclin E. It has been shown that dissociation of centro-somal nucleophosmin (NPM)/B23 triggered by CDK2/cyclin E-mediated phosphorylation is required for initiation of centrosome duplication. We show that centriole duplication is blocked when nuclei were preincubated with the anti-NPM/B23 antibody that prevents phosphorylation of NPM/B23 by CDK2/cyclin E. These studies provide not only direct evidence for the requirement of CDK2/cyclin E and phosphorylation of NPM/B23 for centrosomes to initiate duplication, but a valuable experimental system for further exploration of the molecular regulation of centrosome duplication in somatic cells of higher animals.