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Depolarization‐Dependent Protein Phosphorylation in Rat Cortical Synaptosomes: Characterization of Active Protein Kinases by Phosphopeptide Analysis of Substrates

Depolarization of synaptosomes is known to cause a calcium‐dependent increase in the phosphorylation of a number of proteins. It was the aim of this study to determine which protein kinases are activated on depolarization by analyzing the incorporation of 32P1 into synaptosomal phosphoproteins and phosphopeptides. The following well‐characterized phosphoproteins were chosen for study: phosphoprotein “87K,” synapsin Ia and Ib, phosphoproteins IIIa and IIIb, the catalytic subunits of calmodulin kinase II, and the B‐50 protein. Each was initially identified as a phosphoprotein in lysed synaptosomes after incubation with [γ‐32P]ATP. Mobility on two‐dimensional polyacrylamide gels and phosphorylation by specific protein kinases were the primary criteria used for identification. A technique was developed that allowed simultaneous analysis of the phosphopeptides derived from all of these proteins. Phosphopeptides were characterized in lysed synaptosomes after activating cyclic AMP‐, calmodulin‐, and phospholipid‐stimulated protein kinases in the presence of [γ‐32P]ATP. Phosphoproteins labelled in intact synaptosomes after incubation with 32Pi were then compared with those seen after ATP‐labelling of lysed synaptosomes. As expected from previous work, phosphoprotein “87K,” and synapsin Ia and Ib were‐labelled, but for the first time, phosphoproteins IIIa, IIIb, and the B‐50 protein were identified as being labelled in intact synaptosomes; the calmodulin kinase II subunits were hardly phosphorylated. From a comparison of the phosphopeptide profiles it was found that cyclic AMP‐, calmodulin‐, and phospholipidstimulated protein kinases are all active in intact synaptosomes and their activity is dependent on extrasynaptosomal calcium. The activation of cyclic AMP‐stimulated protein kinases in intact synaptosomes was confirmed by the addition of dibutryl cyclic AMP and theophylline which specifically increased the labelling of phosphopeptides in synapsin Ia and Ib and in phosphoproteins IIIa and IIIb. On depolarization of intact synaptosomes, a number of phosphopeptides showed increased labelling and the pattern suggested that cyclic AMP‐, calmodulin‐, and phospholipid‐stimulated protein kinases were all activated. No new peptides were phosphorylated, suggesting that depolarization simply increased the activity of already active protein kinases and that there was no depolarization‐specific increase in protein phosphorylation.