Membranes provide a barrier to cells and organelles, and allow the selective transport of molecules between compartments. Membrane fusion is essential for organelle biogenesis as well as trafficking of molecules between cellular compartments. Membrane fusion is also required for the formation of the branched network of tubules that make up the Endoplasmic Reticulum (ER). One protein implicated in ER fusion is Atlastin, a dynamin like GTPase. Mutations in Atlastin-1, among others, cause Hereditary Spastic Paraplegias (HSP), a group of neurological disorders that cause progressive weakness of lower extremities. We have shown that the C-terminal tail of atlastin is necessary for membrane fusion. The requirement of the C-terminal tail can be partially abrogated in an unstable lipid environment. This implies that the C-terminal tail of Atlastin plays a role in perturbing the lipid bilayer to allow membrane fusion. Understanding the molecular details of how Atlastin drives membrane fusion may help elucidate the pathogenesis of HSP. Intracellular fusion at the plasma membrane is SNARE mediated and regulated by Sec1p/Munc18 (SM) proteins. Increased rate of glucose transport into fat and muscles cells by translocation of glucose transporter GLUT4 in response to insulin is a SNARE regulated fusion process. Recent reports have linked Munc18c and Syntaxin4 with obesity and Type 2 diabetes. We characterized the function of Munc18c, an SM protein, in regulating GLUT-4 containing vesicle fusion with the plasma membrane. We have shown that Munc18c directly inhibits membrane fusion by interacting with its cognate SNARE complexes. Characterization of membrane fusion in a minimal system as the in vitro liposome fusion assay offers a powerful tool with which to finely dissect the mechanistic basis of SM protein function.