Correlation of Cdk1 activity and changes in microtubule stability at metaphase to anaphase transition. (A) CSF-arrested egg extracts were incubated with calcium and cyclinBΔ90 (Ana) or CGP74514A (CGP). Ran-GTP–mediated microtubule assemblies were visualized by direct fluorescence after the addition of Cy3-labeled tubulin (top), and Cdk1 activities were determined by a histone H1 kinase activity assay (bottom). (B–D) CSF-arrested egg extracts (Meta or M) were triggered to go into anaphase by calcium addition in the presence of increasing cyclinBΔ90 concentrations. Ran-GTP–induced metaphase microtubule assemblies were visualized (B) and quantified relative to metaphase as indicated (C, black bars). In parallel, the Cdk1 kinase activity was measured using histone H1 phosphorylation (C, gray bars). (D) Anaphase was first induced with calcium in the presence of 80 nM cyclinBΔ90, increasing amounts of cyclinBΔ90 were added, and the capacity of Ran-GTP to still induce microtubule assembly was tested. These assemblies were quantified relative to metaphase (black bars), and the Cdk1 kinase activity was measured using histone H1 phosphorylation (gray bars). Error bars represent SD from three independent experiments. Ana, anaphase; Meta, metaphase. Bars, 5 μm.