Synaphin is a 19,000 mol. wt cytosolic protein we first found to co-purify with the docking/fusion complex crucial to neurotransmitter release from presynaptic terminals. Two isoforms of synaphin (synaphins 1 and 2) (also called complexins II and I, respectively) exist in the rat brain. On density gradient centrifugation of a Triton X-100 extract of brain membranes, synaphin was found to be associated with the 7S complex that contains synaptotagmin, syntaxin, synaptosomal-associated protein of 25,000 mol. wt and vesicle-associated membrane protein. A smaller complex devoid of synaphins was also identified by immunoprecipitation with a monoclonal antibody against synaptosomal-associated protein of 25,000 mol. wt. Messenger RNAs for synaphins 1 and 2 were expressed predominantly in the brain. In situ hybridization using probes specific to synaphins 1 and 2 indicated that the distribution of their mRNAs was significantly different in brain regions such as olfactory bulb, hippocampus, cerebral cortex, piriform cortex, cerebellum, thalamus and facial nuclei. These results show synaphin as a component of the 7S complex and suggest different physiological implications for the two isoforms.
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