Studies of the physiological role of specific neuronal phosphoproteins.

Studies in the last five years have provided conclusive evidence that protein phosphorylation is involved in the regulation of neuronal function. Direct evidence from microinjection experiments has shown that four distinct classes of protein kinases modulate physiological processes in neurons. In addition, a large number of substrates for these proteins have been identified in neurons. Three of these phosphoproteins have been discussed here: first, synapsin I, a substrate protein present in nerve terminals, the phosphorylation of which appears to regulate neurotransmitter release from those nerve terminals; second, the acetylcholine receptor, the phosphorylation of which regulates its rate of desensitization in the presence of acetylcholine; and, finally, DARPP-32, the phosphorylation of which converts it into a very potent phosphatase inhibitor that may be involved in the regulation by the neuromodulator dopamine of the effects of the neurotransmitter glutamate. These studies of specific phosphoproteins suggest that the identification and characterization of additional neuronal phosphoproteins should lead to the clarification of additional molecular mechanisms by which signal transduction is carried out in nerve cells.