Structural Organization of the Synaptic Exocytosis Core Complex

Syntaxin, vesicle-associated membrane protein (VAMP), and synaptosome-associated protein of 25 kDa (SNAP-25) form a ternary "core complex" central to the process of synaptic vesicle docking and fusion. Several lines of evidence support the hypothesis that the proteins assemble in a coiled-coil structure, but the alignment of alpha helices in this coil and the overall conformation of the coil are unknown. We employ the technique of fluorescence resonance energy transfer (FRET) to investigate the alignment between syntaxin and VAMP. With the acceptor probe coupled to the amino-terminal end of the VAMP coiled-coil domain, the donor probe fluorescence is quenched to a greater extent when it is on the amino-terminal end of the syntaxin H3 domain than when it is on the carboxy-terminal end. The data indicate that syntaxin and VAMP bind primarily in a parallel arrangement and suggest a coiled-coil structure that is bent rather than fully extended. We propose a model in which binding of SNAP receptor (SNARE) protein coiled-coil domains helps drive vesicle fusion.