Calcium-regulated phosphorylation in synaptosomal cytosol: dependence on calmodulin

Abstract

Calcium stimulated the phosphorylation of several specific synaptosomal cytosolic proteins. The effects of calcium were both concentration and time dependent and were most apparent for proteins with molecular weights of 50,000, 55,000, and 60,000. Exogenous calcium (1.0-100 microM) enhanced the net incorporation of phosphate into protein by as much as 23-fold. In the absence of added calcium, the calcium chelator [ethylenebis(oxyethylenenitriolo)]tetraacetic acid did not lower the phosphorylation of any protein below control levels. The antipsychotic, fluphenazine (1.0-100 microM), caused a concentration-dependent decrease in calcium-stimulated protein phosphorylation. When the heat-stable calcium-binding protein, calmodulin, was removed from synaptosomal cytosol by affinity chromatography on fluphenazine-Sepharose, calcium-stimulated protein phosphorylation was abolished. Responsiveness to calcium could be restored by the addition of calmodulin to the phosphorylation assay. These results indicate that calcium-dependent protein kinases are of major importance in regulating the phosphorylation of specific cytosolic proteins in neuronal tissue. Furthermore, it would appear that one of the three substrates under investigation is specific to synaptosomal cytosol whereas the other two are present in both the cytosol and membrane fractions.