Fig. 1. PP1 blocks SCF-stimulated MAPK activation. A, H526 cells incubated in serum-free medium overnight were either left untreated (NS), or stimulated with 100 ng/ml SCF or 20 ng/ml IGF-1 in the presence of DMSO vehicle or 10 µM PP1 and lysed in SDS sample buffer after 5 min. Duplicate samples were resolved by PAGE through a 10% gel, Western blotted, and stained with either an activation-specific or a pan-ERK antibody. SCF stimulation resulted in a 10-fold increase in MAPK activity (determined by densitometry), which was completely inhibited by PP1. IGF-1 resulted in a 2-fold increase in MAPK activity that was not affected by PP1. B, H526 cells were treated as above, lysed, and ERK 1 and 2 were immunoprecipitated with a pan-ERK antibody. The immunoprecipitate was incubated in the presence of myelin basic protein and {gamma}32P-ATP in triplicate and the labeled myelin basic protein was bound to phosphocellulose paper, was washed, and was quantitated by scintillation counting.