Upon exocytosis, synaptic vesicle proteins are released into the plasma membrane and have to be retrieved by compensatory endocytosis. When green fluorescent protein–labeled versions of the vesicle proteins synaptobrevin-2 and synaptotagmin-1 are overexpressed in rat hippocampal neurons, up to 30% are found on axonal membranes under resting conditions. To test whether and to what extent these plasma membrane–stranded proteins participate in exo-endocytic cycling, a new proteolytic approach was used to visualize the fate of newly exocytosed proteins separately from that of the plasma membrane–stranded ones. We found that both pools were mixed and that endocytosed vesicles were largely composed of previously stranded molecules. The degree of nonidentity of vesicular proteins exo- and endocytosed depended on stimulus duration. By using an antibody to the external domain of synaptotagmin-1, we estimated that under physiological conditions a few percent of vesicular proteins were located near the active zone, from where they were preferentially recycled upon stimulation.
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