Cyclin E-associated cdk2 in PMA-treated cells. (A) Cyclin E/cdk2 complexes are present in U937 cells during PMA-induced differentiation. Cells were treated with PMA for the indicated times and cyclin E immunoprecipitated from cell extract (1.4 mg total protein). An aliquot of the identical extract was also immunoprecipitated with cdk2 antibody. The immunoprecipitates were run on an SDS-PAGE, transferred to nitrocellulose, and on a Western blot probed with a cdk2 polyclonal antibody (1:3750 dilution). (B) PMA inhibits cyclin E-associated cdk2 activity. Cyclin E or cdk2 was immunoprecipitated from PMA-treated cell lysates (500 μg and 200 μg total protein, respectively) and analyzed for histone H1 activity as described in Materials and Methods.