Snares partner with Sec1/Munc18 proteins to release neurotransmitter at synapses

Lipid membranes organize eukaryotic cells into functional compartments called organelles. Material is delivered to and from organelles in a regulated fashion. Vesicles bud from a source compartment, move across the cell and fuse with a target membrane. SNARE proteins, with Sec1/Munc18 (SM) proteins, drive the fusion of vesicles with their target by bridging the apposing membranes and forcing them together. The SNARE/SM fusion complex is essential for all vesicle fusion. Each trafficking pathway utilizes a different set of SNARE/SM family members. In the nervous system the secretory pathway is responsible for the release of neurotransmitters, which pass signals between neurons. The neuronal SNAREs include synaptobrevin, syntaxin, and SNAP-25. However, it is not clear that these are the only SNAREs responsible for neurotransmitter release. In fact countless studies have reported residual neurotransmission in the absence of each of these proteins, raising the question what is the mechanism responsible for residual fusion in neuronal SNARE knockouts? In Chapter 2, I explore this question by focusing on the neuronal SNARE SNAP-25. We characterize the <italic>snap-25</italic> genetic locus in <italic>C. elegans</italic> and examine the physiology of neurons lacking the SNAP-25 protein. We find that SNAP-25 plays an important role in docking and fusing synaptic vesicles but is not strictly essential for either one. We reveal that the conserved SNARE protein, SNAP-29 is capable of substituting for SNAP-25 in synaptic vesicle fusion. We demonstrate that the SNAP-29 protein is natively expressed in neurons and localized at synapses. Our observations suggest that the canonical neuronal SNAREs may not act alone in releasing neurotransmitters. Finally, I explore the mechanism by which the neuronal SM protein (Unc18) facilitates fusion. Unc18 binds SNAREs in three configurations. A binary complex with syntaxin is important for trafficking. At nerve terminals, UNC-18 interacts with an N-terminal peptide on syntaxin and with the SNARE four-helix bundle. Our experiments demonstrate that the N-peptide of syntaxin is a passive tether facilitating Unc18's transition from the binary syntaxin interaction to a direct interaction with the ternary SNARE complex. Future work is required to elucidate the fusogenic properties of Unc18's interaction with the ternary complex.