Fig. 4.
Modulation of CDK complex activity by λ-phosphatase and CDK7/cyclin H. (A) Kinase activity of different CDK-containing species toward GST-pRbCT was assayed by monitoring 32P incorporation either before (lanes 1, 3, and 5) or after (lanes 2, 4, and 6) treatment with λ-phosphatase. See Materials and Methods for further details. (B) Reactivation of repurified dephosphorylated CDK4/cyclin D3 by CDK7/cyclin H. Partially phosphorylated preparations of CDK4/cyclin D3 were assayed either as purified from insect cells (lane 1) or as repurified after treatment with λ-phosphatase (lanes 2 and 3). Repurified material was either not treated (lane 2) or treated (lane 3) with CDK7/cyclin H. Potential GST-pRbCT kinase activity in the CDK7/cyclin H is controlled against in lane 4.