Kinase experiment:

CaM-KI (2.5 μg/mL), CaM-KII (0.75 μg/mL), CaM-KIV (9 μg/mL), and mLCK (0.6 μg/mL) are incubated with 40 μM syntide-2 or 50 μM mLC peptide (for mLCK) at 30 ℃ for 5 min in a solution (25 μL) containing 50 mM HEPES (pH 7.5), 10 mM Mg(Ac)2, 1 mM DTT, 50 μM [γ-32P]ATP (4500 cpm/pmol) with various concentrations of STO-609 (0–10 μg/mL)in Me2SO at a final concentration of 4%) in the presence of 1 mM CaCl2, 2 μM CaM. Protein kinase activity is measured by the phosphocellulose filter method. Specific activities of CaM-KI, CaM-KII, CaM-KIV, and mLCK in the absence of STO-609 are calculated[1]. STO-609 is bound in the ATP-binding pocket of the CaMKKβ KD. The inhibition mechanism of STO-609 is ATP-competitive[2].

STO-609 is a selective and cell-permeable inhibitor of the Ca2+/calmodulin-dependent protein kinase kinase (CaM-KK), with Ki values of 80 and 15 ng/mL for recombinant CaM-KKα and CaM-KKβ, respectively. STO-609 inhibits AMP-activated protein kinase kinase (AMPKK) activity in HeLa cell lysates with an IC50 ~0.02 g/ml.

STO-609 inhibits the activities of recombinant CaM-KKα and CaM-KKβ isoforms, with Ki values of 80 and 15 ng/mL, respectively, and also inhibits their autophosphorylation activities. STO-609 is highly selective for CaM-KK without any significant effect on the downstream CaM kinases (CaM-KI and -IV), and the IC50 value of the compound against CaM-KII is 10 μg/mL. STO-609 inhibits constitutively active CaM-KKα as well as the wild-type enzyme. In transfected HeLa cells, STO-609 suppresses the Ca2+-induced activation of CaM-KIV in a dose-dependent manner. STO-609 significantly reduces the endogenous activity of CaM-KK in SH-SY5Y neuroblastoma cells at a concentration of 1μg/mL (80% inhibitory rate)[1].

References:
[1]. Tokumitsu H, et al. STO-609, a specific inhibitor of the Ca(2+)/calmodulin-dependent protein kinase kinase. J Biol Chem. 2002 May 3;277(18):15813-8.
[2]. Kukimoto-Niino M, et al. Crystal structure of the Ca /calmodulin-dependent protein kinase kinase in complex with the inhibitor STO-609. J Biol Chem. 2011 Jun 24;286(25):22570-9.