It has been observed that the activity of Ca2+-
calmodulin (CaM)-dependent
protein kinase I is enhanced up to 50-fold by its phosphorylation in vitro by a distinct
CaM kinase I kinase (Lee, J. C., and Edelman, A. M. (1994) J. Biol. Chem. 269, 2158-2164). It has, however, been unclear whether this event represents an acute form of cellular regulation. We demonstrate here the phosphorylation and activation of
CaM kinase I in PC12
pheochromocytoma cells in response to elevation of intracellular Ca2+. Treatment of PC12 cells with the Ca2+-
ionophore,
ionomycin, or with a depolarizing concentration of KCl, led to rapid, biphasic phosphorylation of
CaM kinase I and to increases in
CaM kinase I activity of 5.1- and 7. 3-fold, respectively. Depolarization-induced activation of
CaM kinase I was reduced by approximately 80% by blockade of Ca2+ influx through L-type voltage-dependent Ca2+ channels and completely abolished by removal of extracellular Ca2+. The ability of PC12 cell
CaM kinase I to be phosphorylated and activated by purified
CaM kinase I kinase in vitro was markedly reduced by prior depolarization of the cells, consistent with intracellular phosphorylation and activation of
CaM kinase I by
CaM kinase I kinase. These results demonstrate the existence in PC12 cells of a
CaM kinase I cascade, the function of which may be to sensitize cells to signal-induced elevations of intracellular Ca2+.