FIG. 2.

ERK2-Δ19-25 mutants are phosphorylated in response to mitogens but have little kinase activity. COS-1 cells were transfected with either empty vector, WT-ERK2, or an ERK2 mutant. The following day the cells were serum starved for 4 h before stimulation for 5 min with either 10 ng of EGF per ml or 10% serum. FLAG-ERK immunoprecipitates were used for an in vitro kinase assay using MBP as the substrate. The reaction mixtures were immunoblotted with antibodies for FLAG and phospho-ERK to determine protein expression and phosphorylation. SF, serum-free; P-ERK, phospho-ERK.