Molecular Mechanisms Controlling Synaptic Vesicle Fusion

SNARE proteins are the engines that drive membrane fusion throughout the cell. They provide this energy by zippering up into a parallel four helix bundle in a thermodynamically favored process. Because the zippering of SNAREs is spontaneous, fusion events occur immediately upon a vesicle interacting with its target membrane. But, in certain circumstances, such as in synaptic vesicles, spontaneous fusion is not desired, so a clamp protein is necessary to prevent this fusion until signaled to do otherwise. In synapses, this protein is called Complexin and a second protein, called Synaptotagmin, releases the clamp upon a rapid influx of calcium, the hallmark of an action potential. How Complexin clamps is a subject of great interest in the field, and an area of active research. What is known is that a so-called Accessory helix (residues 28-47) is responsible for clamping, while another, Central Helix (reisudes 48-70) is responsible for physically binding to the helix. A recently solved crystal structure revealed how CPX might behave before the SNAREs fully zipper, namely that the accessory helix extends away from the SNAREs at a 45° angle. But, because of the packing of the crystal, it is entirely possible that the crystal is an artifact of packing, and/or truncationIn this thesis, my work first validates the crystal structure, using a FRET pair I developed for this purpose. I establish that the angled-out positioning of the accessory helix does, in fact, occur in solution, and is not due to crystal packing or the truncation of the VAMP2 (the neuronal vesicle-associated SNARE), but rather is due to the fact that its C-terminus is not present. I describe a mechanism by which Complexin can clamp. Further, I demonstrate that the residues in VAMP2 which are responsible for the switch from the "open" to the "closed" conformation are a patch of asparatates in VAMP2 (residues 64, 65, an 68). I also establish that these three aspartates are responsible for the release of the clamp and that without them, Complexin cannot be brought into the angled-in configuration. I propose a model for how the clamp might be released by Synaptotagmin.