Auto-inhibition of Ca²⁺/calmodulin-dependent protein kinase II by its ATP-binding domain

Ca²⁺/calmodulin dependent protein kinase (CaMPK) II is a key enzyme in many physiological processes. The enzyme is inactive unless Ca²⁺/CaM binds to it. In this inactive form CaMPK-II does not bind ATP suggesting that the ATP-binding domain is involved in an intramolecular interaction. We show here that F12, a 12 amino acid long peptide fragment of the ATP-binding domain (CaMPK-II_23-34, GAFSVVRRCVKV) can inhibit the Ca²⁺/CaM-dependent activity (IC₅₀ of 3 μM) but has no effect on the Ca²⁺/CaM-independent activity of CaMPK-II. Kinetic analysis exhibited mixed inhibition with respect to autocamtide-2 and ATP. The inhibition by F12 showed specificity towards CaMPK-II, but also inhibited CaMPK-I (IC₅₀ = 12.5 μM), while CaMPK-IV (IC₅₀ = 85 μM) was inhibited poorly and cAMP-dependent protein kinase (PKA) was not inhibited. Substitution of phenylalanine at position 25 to alanine (A12), had little effect on the inhibition of different Ca²⁺/CaM-dependent protein kinases, suggesting that phenylalanine 25 does not play a crucial role in the interactions involving F12. Thus the molecular interactions involving the ATP-binding domain appears to play a role in the regulation of nonphosphorylated CaMPK-II activity.