Synapsin I: A review of its distribution and biological regulation

This chapter reviews the biochemical properties, distribution, and biological regulation of Synapsin I. The available evidence strongly supports the hypothesis that Synapsin I is a part of the molecular machinery in nerve terminals that regulates neurotransmitter release. Synapsin I was first identified in 1972 in the studies designed to search for phosphoproteins that might play a role in the regulation of synaptic transmission. For many years, chemical and electrical stimuli have been known to regulate neurotransmitter release from nerve terminals. Cyclic adenosine monophosphate and calcium have been implicated as second messengers in these regulatory processes. However, virtually no information has been available concerning the molecular mechanisms through which such regulation is achieved. It is now apparent that the investigation of protein phosphorylation systems represents a powerful approach to study these mechanisms. This approach has made possible the discovery of Synapsin I, a major phosphoprotein of synaptic vesicles. The distribution of Synapsin I has been studied by a variety of techniques, including radioimmunoassay and other immunochemical techniques, sub-cellular fractionation, and light and electron microscope immunocytochemistry.