Fig. 7. Selectivity of ERK inhibitors against upstream MAPK regulators. (A) MEK1/2 are kinases that phosphorylate ERK1 and ERK2. SILAC analyses in A549 proteomes confirmed that VX-11e and BVD-523 are not inhibitors of endogenous MEK1/2. (B–D) Activity of VX-11e and BVD-523 was tested against additional MEK kinases and found to be inactive. Sensitivity of MEK kinase active-site peptides to ATP competition (1 mM) confirmed active site-dependent probe labeling. Lack of inhibitory activity of VX-11e and BVD-523 against endogenous MEKs supports on-target activity of compounds against native ERK1 and ERK2.