Synapsin I, a phosphoprotein associated with synaptic vesicles: possible role in regulation of neurotransmitter release.

The data presented here provide evidence that the study of neuronal phosphoproteins can lead to the identification of previously unknown proteins and that these proteins may play important roles in neuronal communication. Specifically, in the case of synapsin I, direct evidence has been obtained that this phosphoprotein is involved in regulating neurotransmitter release. A tentative explanation of the results obtained in the micro-injection studies is as follows: synapsin I, in the dephosphostate, is bound to the cytoplasmic surface of synaptic vesicles and inhibits the ability of the vesicle to interact with the plasma membrane; increases in intracellular calcium activate calmodulin kinase II which in turn phosphorylates synapsin I and the phosphorylated synapsin I dissociates from the synaptic vesicle thus removing a constraint on the release of neurotransmitter. Clearly, more studies need to be done to rigorously test this hypothesis. Nevertheless these studies of synapsin I suggest that the study of previously unknown phosphoproteins will lead to the elucidation of previously unknown regulatory processes in neurons.