Fig. 4.

Inhibition of KSPXXXK kinase activity by PD 98059 before (•—•) and after (▴—▴) the preincubation of fraction 34 with ATP. An aliquot of fraction 34 was preincubated with 500 mm ATP in kinase buffer for 2 hr at 30°C. An equal aliquot of the same fraction similarly was preincubated with kinase assay buffer without ATP. Aliquots of each of these samples were assayed for kinase activity in the presence of varying concentrations (0.0–50 μm) of a drug (PD 98059) known specifically to inhibit MEK. The percentage of kinase activity was compared with the control at 100%. Note the inhibition of the kinase activity by the drug without preincubation of the active fraction with ATP and the absence of inhibition with the previous incubation with ATP.