Characterization of the in vitro reconstituted cyclin A or B1-dependent cdk2 and cdc2 kinase activities.

Human cyclins A and B1 were assembled with the cdk2 or cdc2 protein to reconstitute their respective kinase activities in vitro. Both cyclins complemented either cdk2 or cdc2, yielding kinase activities that supported the phosphorylation of histone H1. Activation of cdk2-catalyzed H1 kinase activity by cyclin A required a 10-min preincubation of the two components, whereas cdc2 kinase supported phosphate incorporation without a detectable time lag upon the addition of cyclin B1, suggesting a slower association rate of cdk2 with cyclin A compared with cdc2 and cyclin B1. Both cdk2 and cyclin A, as well as cdc2 and cyclin B1, formed stable complexes in the absence of ATP and substrate that could be isolated after glycerol gradient centrifugation. Incubation of the isolated complexes with ATP and histone H1 supported the phosphorylation of the substrate. Cyclin A-activated cdk2 or cdc2 phosphorylated p107, a pRB-related cellular protein, 10 times more effectively than the cyclin B1-complexed kinases. This was most likely due to a direct association of cyclin A with p107 (Ewen, M. E., Faha, B., Harlow, E., and Livingston, D. (1992) Science 255, 85-87; Faha, B., Ewen, M. E., Tsai, L.-H., Livingston, D., and Harlow, E. (1992) Science 255, 87-90). The reconstituted cdc2-cyclin B1 complex incorporated 4-5-fold more phosphate into the p34 subunit of the three-subunit (p70, p34, and p14) human single-stranded DNA-binding protein (also called RP-A), a DNA replication and DNA repair factor, than cdc2-cyclin A. No detectable phosphorylation of the p34 protein was observed with cdk2 complexed with either cyclin B1 or A. These data indicate that both cyclins as well as the catalytic subunits are important factors in controlling the rate of phosphorylation of a given substrate. The cyclin-activated cdc2 family kinases may target their cellular substrates through cyclin-mediated protein-protein interactions.