Protein phosphorylation and neuronal function.

Studies in the past several years have provided direct evidence that protein phosphorylation is involved in the regulation of neuronal function. Electrophysiological experiments have demonstrated that three distinct classes of protein kinases, i.e., cyclic AMP-dependent protein kinase, protein kinase C, and CaM kinase II, modulate physiological processes in neurons. Cyclic AMP-dependent protein kinase and kinase C have been shown to modify potassium and calcium channels, and CaM kinase II has been shown to enhance neurotransmitter release. A large number of substrates for these protein kinases have been found in neurons. In some cases (e.g., tyrosine hydroxylase, acetylcholine receptor, sodium channel) these proteins have a known function, whereas most of these proteins (e.g., synapsin I) had no known function when they were first identified as phosphoproteins. In the case of synapsin I, evidence now suggests that it regulates neurotransmitter release. These studies of synapsin I suggest that the characterization of previously unknown neuronal phosphoproteins will lead to the elucidation of previously unknown regulatory processes in neurons.