Structure and Function of Calmodulin-dependent Protein Kinase II and Its Physiological Significance

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Four different polypeptides (isoforms) of calmodulin-dependent protein kinase II (CaM-kinase II), alpha, beta, gamma, and delta, which derived from four different genes, were revealed by cDNA sequence analysis of the rat brain extract. RNA blot bybridization analysis of the extracts from various tissues with probes specific for the respective mRNAs showed that gamma and delta mRNAs were expressed in various tissues, while alpha and beta mRNAs were primarily, if not exclusively, expressed in brain.Autophosphorylation of CaM-kinase II is thought to be an important selfregulatory mechanism for controlling its protein kinase activity. The enzyme that had not undergone autophosphorylation exhibited only 15% of the maximum activity even in the presence of Ca^<2+>/calmodulin, but exposure of the enzyme to Ca^<2+>/calmodulin caused rapid autophosphorylation of the enzyme on Thr^<286>, thereby converting the enzyme to the most active form. The active form had the protein kinase activity even in … More the absence of Ca^<2+>/calmodulin, although the affinity for the protein substrates were much lower in the absence of Ca^<2+> than in its presence.Tyrosine hydroxylase catalyzing the rate-limiting step in the biosynthesis of catecholamines is known to be phosphorylated by three second messenger-responsive multifunctional protein kinases, cAMP-dependent protein kinase (A-kinase), protein kinase C (C-kinase), and CaM-kinase II. However, the enzyme was activated via phosphorylation by A-kinase, not acivated by C-kinase, and activated only in the presence of activator protein by CaM-kinase II. The possible regulatory mechanism of the enzyme activity by these three multifunctional protein kinases were suggested.Another calmodulin-dependent protein kinase occurring almost exclusively in brain, calmodulin-dependent protein kinase IV (CaM-kinase IV), was found to show a broad substrate specificity, suggesting its physiological significance in the regulation of the brain function by Ca^<2+>.Interestingly, CaM-kinase IV was activated by autophosphorylation but inactivated via phosphorylation by A-kinase. Less