Studies on the Phosphorylation of Protein Kinase B by Ca²⁺/Calmodulin-dependent Protein Kinases

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Protein kinase B (PKB) was recently reported to be activated on the phosphorylation of Thr³⁰⁸ by Ca²⁺/calmodulin-dependent protein kinase kinase α (CaM-kinase kinase α), suggesting that PKB was regulated through not only the phosphoinositide 3-kinase pathway but also the Ca²⁺/cahnodulin protein kinase pathway. The activation of PKB by CaM-kinase kinase α was as high as 300-fold after incubation for 30min under the phosphorylation conditions, and still increased thereafter, suggesting that the maximal activation of PKB on phosphorylation of the Thr³⁰⁸ residue is several hundred fold. On the other hand, the V_max value of CaM-kinase kinase α for the phosphorylation of PKB was more than two orders of magnitude lower than that for CaM-kinase IV, although the K_m values for PKB and CaM-kinase IV were not significantly different, raising the question of whether or not PKB is a physiological substrate of CaM-kinase kinase α. Besides CaM-kinase kinase α, CaM-kinase H also remarkably activated PKB. However, the specific activities of CaM-kinase kinase α and CaM-kinase II as to the activation of PEB were more than three orders of magnitude lower than that of 3-phosphoinositide-dependent protein kinase 1 (PDK1).