Two calcium/calmodulin-dependent protein kinases, which are highly concentrated in brain, phosphorylate protein I at distinct sites.

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Abstract
Two calcium-stimulated protein kinase activities (ATP:protein phosphotransferase, EC 2.7.1.37) that phosphorylate protein I, a specific synaptic protein, were identified in homogenates of rat brain. One of these is found in the particulate and cytosolic fractions and phosphorylates a region of protein I that is phosphorylated in intact synaptosomes in response to Ca but not to cAMP. The stimulation by Ca of the particulate enzyme and of the partially purified cytosolic enzyme requires the addition of calmodulin. It is not yet known whether the particulate and cytosolic enzymes are related. A 2nd calcium-stimulated protein I kinase is found only in the cytosol and phosphorylates a region of protein I that is phosphorylated in intact synaptosomes in response to either Ca or cAMP. The Ca stimulation of this latter kinase is probably mediated by calmodulin, judging from its inhibition by low concentrations of trifluoperazine. Both of the Ca-stimulated protein I kinases are more highly concentrated in brain than in other tissues. The 2 cytosolic kinases are distinguishable from each other and from myosin light chain kinase and phosphorylase b kinase by their substrate specificities and their chromatographic behavior on DEAE-cellulose.