Multiple phosphorylation sites in protein I and their differential regulation by cyclic AMP and calcium.

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Abstract
The phosphorylation of protein I, a specific neuronal protein, was regulated both by cyclic AMP (cAMP) and by Ca, in intact as well as in lysed synaptosome preparations from rat brain. To determine the phosphorylation site(s) of protein I that were regulated by cAMP and Ca, protein I was purified after it was phosphorylated under various conditions. This purified protein I was then subjected either to peptide mapping after limited proteolysis in sodium dodecyl sulfate/polyacrylamide gels or to tryptic fingerprinting. 8-Br-cAMP selectively increased the phosphorylation of the same protein I peptide fragment in both intact and lysed synaptosomes. Depolarization-induced Ca influx into intact synaptosomes, or the addition of Ca to lysed synaptosomes, caused a stimulation of the phosphorylation not only of this peptide but also of other distinct peptides. Differential regulation by cAMP and Ca of the phosphorylation of multiple sites on the same neuronal protein may provide a molecular basis for interactions between these 2 second-messenger systems in certain nerve terminal functions.

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