Figure 3.

Effect of cyclins on phosphorylation of cdks by Cak1p and p40^MO15/cyclin H. Phosphorylation (A) and activation (B, measured as the phosphorylation of histone H1 by cdk2) of wild-type cdk2 or phosphorylation of cdk2^N132A (E) by GST-Cak1p (triangles) and by p40^MO15/cyclin H (squares) were determined in the presence of increasing concentrations of cyclin A_173–432. Phosphorylation of cdk6 expressed in insect cells was analyzed in a similar manner in the presence of various concentrations of cyclin A_173–432 (C), which binds cdk6 very weakly, or of His₆-cyclin D3 (D). Note that His₆-cyclin D3 binds less tightly to cdk6 in vitro (Kato et al., 1994a) than does cyclin A_173–432 to cdk2, resulting in less pronounced effects on cdk phosphorylation. Data were analyzed by phosphorimaging and are expressed in arbitrary units. The cyclin:cdk ratios are mass ratios of the indicated proteins.