Loss of p21 results in cyclical Cdc2 kinase activity during MTI-induced endoreduplication. (A) Western analysis of cyclin B1 and Cdc2 proteins. Asynchronous cells were treated with nocodazole (83 nM) for the indicated times, and protein was harvested. (B) Cyclin B1-associated kinase activity in HCT116 p21 +/+ and p21 −/− cells. Anti-cyclin B1 antibody was used to immunoprecipitate kinase complexes; histone H1 was used as a substrate. Quantification of the autoradiogram signals is presented in the histograms. Results are representative of three independent experiments. (C) Whole-cell lysates from HCT116 p21 +/+ cells were immunoprecipitated with anti-cyclin B1, separated by SDS-PAGE, and analyzed for coimmunoprecipitation of p21 by Western blot analysis. Con, control representing p21 protein coimmunoprecipitated with anti-cyclin B1 from a whole-cell extract of WI-38 fibroblasts.