Fast, SNARE-induced Single-Vesicle Fusion

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The list of neurological diseases is extensive: Alzheimer's, Parkinson's, ALS, and MS, to name just a few. The goal of this work is to understand how neurons work at the molecular level. Neurons communicate with each other by releasing neurotransmitters through the plasma membrane (PM) into the synapse in a process called exocytosis. The neurotransmitter is stored in small secretory vesicles (~50 nm diameter) docked at the PM. When a neuron depolarizes, an influx of Ca2+ to the cytosolic side of the PM causes Ca2+- triggered exocytosis in less than 1 ms. Release involves fusion of the lipid bilayer of the vesicle with that of the PM. The identity of many key components of this vesicle fusion machinery is established. However, little is known at the molecular level about how the components work together to carry out exocytosis. A central component is the frans-SNARE complex, comprising the v-SNARE protein synaptobrevin (Syb) anchored in the vesicle and the t-SNARE proteins syntaxin (Syx) and SNAP-25anchored in the PM. Synaptotagmin (Syt), also anchored in the vesicle, is probably the Ca2+ sensor that evidently triggers fusion by altering binding relationships among proteins and lipids. The fusion machinery is so complex that mechanistic inferences drawn from in vivo studies are necessarily indirect. We and others have been working to develop a reconstituted model systemthat faithfully captures the Ca2+-triggered fusion observed in neurons. Such a system would enable very direct study of many key mechanistic questions by adding or subtracting components one by one. We now have a model that allows direct observation of single v-SNARE vesicle docking and fusion on a planar t-SNARE/lipid bilayer in real time by widefield fluorescence microscopy. Ours is the only in vitro system thus far that exhibits SNARE-dependent fusion on a 25-ms time scale, approaching that in nature. The goals of this work are: to determine the intrinsic SNARE-driven rate of fusion by optimizing protein and lipid components; to learn about the nature of the fusion pore itself by measuring the contents release and lipid mixing time scales with 1-3 ms resolution; and to recapture Ca2+ triggering by introducing Syt into the assay. Along theway, these experiments are poised to answer a wide variety of mechanistic questions in an unusually incisive manner.