COLOCALIZATION ON THE SAME SYNAPTIC VESICLES OF SYNTAXIN AND SNAP-25 WITH SYNAPTIC VESICLE PROTEINS - A REEVALUATION OF FUNCTIONAL MODELS REQUIRED

Synaptic vesicle docking and calcium dependent exocytosis are thought to require the specific interaction of proteins of the synaptic vesicl e membrane (such as VAMP/synaptobrevin and synaptotagmin) and their pl asma membrane-located counterparts (such as syntaxin and SNAP-25). Whe n isolating synaptic vesicles by glycerol velocity gradient centrifuga tion we found cosedimentation of the presumptive presynaptic plasma me mbrane proteins syntaxin and SNAP-25 with synaptic vesicle membrane pr oteins. In order to further identify the antibody binding organelles w e performed an immunoelectron microscopical analysis of synaptosomal p rofiles. Syntaxin and SNAP-25 were not only associated with the plasma membrane but to a large extent also with synaptic vesicle profiles. I n order to answer the question whether the syntaxin and SNAP-25 contai ning vesicular compartment would also carry classical synaptic vesicle membrane markers we performed double labeling experiments using poly- and monoclonal antibodies. We found colocalization on the same vesicl e not only of SNAP-25 and syntaxin but also of SNAP-25 with the synapt ic vesicle membrane proteins SV2 and synaptotagmin and of syntaxin wit h the vesicular membrane protein synaptophysin. Our results demonstrat e that syntaxin and SNAP-25 are colocalized with classical vesicle mem brane proteins on the same vesicle and suggest that the functional mod els for the interaction of presynaptic proteins need to be re-evaluate d.