Nerve growth factor (NGF) mediates the phosphorylation of tyrosine hydroxylase in PC12 cells on two distinct peptide fragments, separable by two-dimensional tryptic phosphopeptide mapping (phosphopeptides T1 and T3). Phorbol diester derivatives capable of activating Ca+2/phospholipid-dependent protein kinase (C-kinase) cause a specific phosphorylation of peptide T3 in a dose-dependent, saturable manner. Derivatives of the endogenous C-kinase activator diacylglycerol, also cause the phosphorylation of tyrosine hydroxylase on peptide T3. The C-kinase inhibitors chlorpromazine and trifluoperazine inhibit the phorbol diester stimulated phosphorylation of site T3 in a dose-dependent manner. These agents inhibit the phosphorylation of T3 in response to NGF, but have no effect on NGF's ability to cause T1 phosphorylation. In a PC12 mutant deficient in cAMP-dependent protein kinase activity, NGF mediates the phosphorylation of tyrosine hydroxylase on peptide T3 but not on T1. We conclude that NGF mediates the activation of both the cAMP-dependent protein kinase and the C-kinase to phosphorylate substrate proteins. These kinases can act independently to phosphorylate tyrosine hydroxylase, each at a different site, and each of which results in the enzyme activation. A molecular framework is thus provided for events underlying NGF action.

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