|CDK kinase assay
||For CDK1/cyclin B1 kinase assay, kinase reactions consist of 100 ng of baculovirus expressed GST-CDK1/cyclin B1 (human) complex, 1 μg histone HI, 0.2 μCi [γ-33P]ATP, 25 μM ATP in 50 μL kinase buffer (50 mM Tris, pH 8.0, 10 mM MgCl2, 1 mM EGTA, 0.5 mM DTT). For CDK2/cyclin E kinase assay, kinase reactions consist of 5 ng of baculovirus expressed GST-CDK2/cyclin E (human) complex, 0.5 μg GST-RB fusion protein (amino acids 776-928 of retinoblastoma protein), 0.2 μCi [γ-33P]ATP, 25 μM ATP in 50 μL kinase buffer (50 mM Hepes, pH 8.0, 10 mM MgCl2, 1 mM EGTA, 2 mM DTT). For CDK4/cyclin D1 kinase assay, kinase reactions consist of 150 ng of baculovirus expressed GST-CDK4/cyclin D1 (human), 280 ng of Stag-cyclin D1, 0.5 μg GST-RB fusion protein (amino acids 776-928 of retinoblastoma protein), 0.2 μCi [γ-33P]ATP, 25 μM ATP in 50 μL kinase buffer (50 mM Hepes, pH 8.0, 10 mM MgCl2, 1 mM EGTA, 2 mM DTT). Reactions are incubated for 45 minutes for CDK1 and CDK2, or 1 hour for CDK4 at 30 °C and stopped by the addition of cold trichloroacetic acid (TCA) to a final concentration 15%. TCA precipitates are collected onto GF/C unifilter plates using a Filtermate universal harvester and the filters are quantitated using a TopCount 96-well liquid scintillation counter. Flavopiridol is dissolved at 10 mM in dimethylformamide (DMF) and evaluated at six concentrations, each in triplicate. The final concentration of DMF in the assay = 2%. IC50 values are derived by nonlinear regression analysis and have a coefficient of variance = 16%. To assay Flavopiridol activity on CDK6, a filter-binding assay is established. The following are combined in the reaction mixture: 2 μL of CDK6 (0.7 mg/μL), 5 μL of histone H1 (6 mg/mL), 14 μL of kinase buffer (60 mM β-glycerophosphate, 30 mM p-nitrophenyl phosphate, 25 mM MOPS (pH 7.0), 5 mM EGTA, 15 mM MgCl2, 1 mM DTT, 0.1 mM Na-vanadate), 3 μL of increasing concentrations of Flavopiridol diluted in 50% DMSO, and 6 μL of 33P-ATP (1 mCi/mL) in nonradioactive ATP at 90 μM concentration (final concentration: 15 μM). The assay is initiated by the addition of 33P-ATP. The reaction is incubated for 20 minutes at 30°C. A 25 μL aliquot of the supernatant is then spotted onto Whatman P81 phosphocellulose paper. Filters are washed 5 times with 1% phosphoric acid solution. Wet filters are counted in the presence of 1 mL of scintillation fluid. Cdk9 activity is measured using 50 nM of recombinant Cdk9/cyclin T in 50 mM HEPES pH 7.5, 10 mM MgCl2, 1 mM DTT, 3 μM Na3VO4, 150 μM RNA polymerase CDT peptide and 80 μM ATP. Cdk7 assay is performed in the same buffer using 37 nM of purified kinase in the presence of 200 μM ATP and 10 μM myelin binding protein as a substrate. The potency of Flavopiridol toward CDK9 and CDK7 is determined using either a strong anion exchanger (Dowex 1-X8 resin, formate form)-based assay or a scintillation proximity assay. IC50 values are calculated from the dose-response curves.