Regulation of the Function of CNS by Ca^<2+>/calmodulin-dependent Multifunctional Protein Kinases

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Ca^<2+>/calmodulin-dependent protein kinase II (CaM-kinase II) and IV (CaM-kinase IV), which exist particularly abundantly in the brain, are second messenger-responsive multifunctional protein kinases, just like cAMP-dependent protein kinase and protein kinase C,and therefore these protein kinases are thought to play important roles in signal transduction in the central nervous system (CNS). The present study focuses our attention on regulatory mechanisms of the activities of CaM-kinase II and CaM-kinase IV.The autoinhibitory domain of CaM-kinase II,which is known to keep the enzymeless active in the absence of Ca^<2+>/calmodulin, was found to strongly stabilize the enzyme. Thus, CaM-kinase II appears to be in an inactive and stable state in a cell when its activity is not necessary. Ca^<2+>/calmodulin required for the activity of CaM-kinase II,on the other hand, inactivated the enzyme. Such two opposite effects of calmodulin were also observed with CaM-kinase IV,although the inactivation of CaM-kinase IV by Ca^<2+>/calmodulin was reversible in contrast to the case of CaM-kinase II.Detailed sequence analysis of CaM-kinase II cDNA clones from the brain and liver suggested a possible mechanism for producing many CaM-kinase II isoforms from the alpha, beta, gamma, and delta genes of CaM-kinase II by alternative splicing.The most prominent result of our study is the discovery of CaM-kinase IV kinase in the brain which activates CaM-kinase IV 30- to 40-fold by phosphorylation. CaM-kinase IV kinase was purified from rat cerebral cortex to apparent homogeneity upon SDS-polyacrylamide gel electrophoresis, and its molecular and catalytic properties were examined. CaM-kinase IV kinase was a Ca^<2+>/calmodulin-dependent protein kinase which phosphorylated CaM-kinase IV specifically. Human CaM-kinase IV cDNA was cloned from a cDNA library of a Jurkat cell, a cell line of human T-cell, and its nucleotide sequence and the deduced amino acid sequence were determined.