Fig. 3.

Regulation of cyclin A-associated CDK activity in G₁ phase. (a) CycA47 abundance and its associated H1 kinase activity were analyzed during the M-to-G₁ transition in p21^–/–, p27^–/–, and p21^–/– p27^–/– MEFs. The extracts from nocodazole-blocked (M) and early G₁-phase (G₁) cells were immunoprecipitated and analyzed as in Fig. 2. (b Upper) Scheme of the in vivo ³²P-labeling procedure (see Methods for details). (Lower) Phosphoamino acid analysis of the CDK proteins recovered from the immunoprecipitates after the in vivo ³²P labeling. Horizontal and vertical arrows represent the first and second electrophoresis, respectively.