Immunoisolation of vesicles using synaptophysin and syntaxin specific antibodies revealed that most SVs contain syntaxin 1. Thus, both proteins are significant components of SVs although less abundant than synaptobrevin (8.7% of the total protein). Using recombinant proteins as standards for quantitation, we found that syntaxin 1 and SNAP-25 each comprise approximately 3% of the total protein in highly purified SVs. Both proteins cofractionate with SVs and clathrin-coated vesicles upon subcellular fractionation. Here we demonstrate that major pools of syntaxin 1 and SNAP-25 recycle with SVs. According to the current concept, syntaxin 1 and SNAP-25 are members of larger protein families, collectively designated as target-SNAP receptors (t-SNAREs), whose specific localization to subcellular membranes define where transport vesicles bind and fuse. This binding is thought to be responsible for vesicle docking and apparently precedes membrane fusion. Both proteins form a complex with synaptobrevin, an intrinsic membrane protein of SVs. Syntaxin 1 and synaptosome-associated protein of 25 kD (SNAP-25) are neuronal plasmalemma proteins that appear to be essential for exocytosis of synaptic vesicles (SVs).
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