The SNARE Vti1a-β Is Localized to Small Synaptic Vesicles and Participates in a Novel SNARE Complex

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Abstract
Specific solubleN-ethylmaleimide-sensitive factor attachment protein (SNAP) receptor (SNARE) proteins are required for different membrane transport steps. The SNARE Vti1a has been colocalized with Golgi markers and Vti1b with Golgi and thetrans-Golgi network or endosomal markers in fibroblast cell lines. Here we study the distribution of Vti1a and Vti1b in brain. Vti1b was found in synaptic vesicles but was not enriched in this organelle. A brain-specific splice variant of Vti1a was identified that had an insertion of seven amino acid residues next to the putative SNARE-interacting helix. This Vti1a-β was enriched in small synaptic vesicles and clathrin-coated vesicles isolated from nerve terminals. Vti1a-β also copurified with the synaptic vesicle R-SNARE synaptobrevin during immunoisolation of synaptic vesicles and endosomes. Therefore, both synaptobrevin and Vti1a-β are integral parts of synaptic vesicles throughout their life cycle. Vti1a-β was part of a SNARE complex in nerve terminals, which boundN-ethylmaleimide-sensitive factor and α-SNAP. This SNARE complex was different from the exocytic SNARE complex because Vti1a-β was not coimmunoprecipitated with syntaxin 1 or SNAP-25. These data suggest that Vti1a-β does not function in exocytosis but in a separate SNARE complex in a membrane fusion step during recycling or biogenesis of synaptic vesicles.