Figure 6.

Cdk inhibitors from the CIP/KIP family block phosphorylation of cdks by p40^MO15 but not by Cak1p. Cdk2 (A, C, and D) or GST-cdk6 (B) was phosphorylated directly by GST-Cak1p (upper panels) or by p40^MO15/cyclin H (lower panels) in the presence of various concentrations of His₆-p27 (A and B), p21 (C), or p57 (D), respectively. Lanes 1 contained no inhibitors. Mass ratios of inhibitor:cdk are 10:1 (lanes 2), 5:1 (lanes 3), 2.5:1 (lanes 4), 1.2:1 (lanes 5), 0.3:1 (lanes 6), 0.09:1 (lanes 7), and 0.03:1 (lanes 8). In all experiments the cdk:cyclin A_173–432 ratio was chosen to be 1:0.75. In panel A, the position of His₆-p27 that has been phosphorylated by cdk2/cyclin A_173–432 is indicated.