The SNARE Vti1a-beta is localized to small synaptic vesicles and participates in a novel SNARE complex

Specific soluble N-ethylmaleimide-sensitive factor attachment protein (SNAP ) receptor (SNARE) proteins are required for different membrane transport s teps. The SNARE Vti1a has been colocalized with Golgi markers and Vti1b wit h Golgi and the trans-Golgi network or endosomal markers in fibroblast cell lines. Here we study the distribution of Vti1a and Vti1b in brain. Vti1b w as found in synaptic vesicles but was not enriched in this organelle. A bra in-specific splice variant of Vti1a was identified that had an insertion of seven amino acid residues next to the putative SNARE-interacting helix. Th is Vti1a-beta was enriched in small synaptic vesicles and clathrin-coated v esicles isolated from nerve terminals. Vti1a-beta also copurified with the synaptic vesicle R-SNARE synaptobrevin during immunoisolation of synaptic v esicles and endosomes. Therefore, both synaptobrevin and Vti1a-beta are int egral parts of synaptic vesicles throughout their life cycle. Vti1a-beta wa s part of a SNARE complex in nerve terminals, which bound N-ethylmaleimide- sensitive factor and alpha-SNAP. This SNARE complex was different from the exocytic SNARE complex because Vti1a-beta was not coimmunoprecipitated with syntaxin 1 or SNAP-25. These data suggest that Vti1a-beta does not functio n in exocytosis but in a separate SNARE complex in a membrane fusion step d uring recycling or biogenesis of synaptic vesicles.