Relative activation of AblKD variants by imidazole (Imid), guanidine (Gdn), 2-methylimidazole (2MeIm), or 5-methylimidazole (5MeIm) compared to basal activity (None). The letters in parentheses indicate the catalytic loop sequence. Activity was measured at 30 ° using 40 mM of the indicated activator in 100 mM Tris, pH 7.4, 10 mM MgCl2, 500 µM ATP, 2 mM Abltide, and 0.15 mCi/mL [γ−32P]ATP.