The synaptophysin-synaptobrevin complex: a hallmark of synaptic vesicle maturation

Exocytosis of synaptic vesicles requires the formation of a fusion complex consisting of the synaptic vesicle protein synaptobrevin (vesicle-associate d membrane protein, or VAMP) and the plasma membrane proteins syntaxin and soluble synaptosomal-associated protein of 25 kDa (or SNAP 25). In search o f mechanisms that regulate the assembly of the fusion complex, it was found that synaptobrevin also binds to the vesicle protein synaptophysin and tha t synaptophysin-bound synaptobrevin cannot enter the fusion complex. Using a combination of immunoprecipitation, cross-linking, and in vitro interacti on experiments, we report here that the synaptophysin-synaptobrevin complex is upregulated during neuronal development. In embryonic vat brain, the co mplex is not detectable, although synaptophysin and synaptobrevin are expre ssed and are localized to the same nerve terminals and to the same pool of vesicles. In contrast, the ability of synaptobrevin to participate in the f usion complex is detectable as early as embryonic day 14, The binding of sy naptoporin, a closely related homolog of synaptophysin, to synaptobrevin ch anges in a similar manner during development. Recombinant synaptobrevin bin ds to synaptophysin derived from adult brain extracts but not to that deriv ed from embryonic brain extracts. Furthermore, the soluble cytosol fraction of adult, but not of embryonic, synaptosomes contains a protein that induc es synaptophysin-synaptobrevin complex formation in embryonic vesicle fract ions. We conclude that complex formation is regulated during development an d is mediated by a posttranslational modification of synaptophysin. Further more, we propose that the synaptophysin-synaptobrevin complex is not essent ial for exocytosis but rather provides a reserve pool of synaptobrevin for exocytosis that can be readily recruited during periods of high synaptic ac tivity.