Vesicle fusion is an integral part of the life processes of a cell. Calcium-triggered exocytosis of synaptic vesicles is the major form of communication between neurons, and underlies information processing, learning and memory formation within the nervous system. Understanding the mechanism and regulation of vesicle fusion is therefore crucial to a more complete understanding of neuronal function. In the past decade, a large number of proteins involved in this process have been discovered, including the SNARE proteins. These proteins are present on vesicles and target membranes, and interact to form a tight complex, the SNARE complex, linking the two membranes. The dominant framework for understanding exocytosis has been the SNARE hypothesis, which arose out of biochemical investigations and holds that specific vesicle SNAREs bind cognate target SNAREs to match vesicles with appropriate membranes. This interaction is then thought to drive the fusion reaction. We use a combination of Drosophila genetics, mutagenesis and molecular biology to probe the function of SNAP-25, a plasma membrane SNARE protein thought to be critical for vesicle fusion and synaptic transmission. Functional analysis of a mutant form of SNAP-25, as well as the closely related isoform SNAP-24, shows that the differences between physical conformations of SNARE complexes generated with these proteins changes vesicle release probability. In addition, we find that SNAP-24 can functionally substitute for SNAP-25, uncovering context dependent redundancy between the two proteins. These results challenge several tenets of the SNARE hypothesis, and join a growing body of work that seeks to determine the function of SNARE proteins within the nervous system, and organisms in general.
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