| Every eukaryotic cell maintains a complex system of membrane trafficking pathways to transport cellular components to distinct organelles or membranes, or to secrete compounds into the extracellular space. All cells utilize a constitutive secretory pathway for cell maintenance, whereas only specialized cells utilize both a constitutive and regulated pathway. During regulated secretion, contents are packaged into granules, transported to the plasma membrane where they first dock, or attach to the membrane, and undergo a priming process to become fusion competent. Then, upon a rise in intracellular calcium, the granule fuses with the plasma membrane to release its contents. Granule priming dictates the number of vesicles that fuse with the plasma membrane and yet, very little is known about this process.;The SNARE proteins are necessary for membrane fusion to occur. The vesicle-SNARE, VAMP-2, forms a four helix bundled-bundle, termed the "SNARE complex" with the SNAREs found on the target membrane, syntaxin-1 and SNAP-25. The formation of the SNARE complex brings the vesicle within close proximity to the plasma membrane and may provide the driving force for fusion to occur. During vesicle priming, the SNARE complex assembles and forms intermediate complexes. Several proteins are thought to regulate SNARE complex assembly, and these include proteins that contain an alpha helical region termed the "MUN domain". MUN domain containing proteins include the Munc13 protein family and the calcium-dependent activator protein for secretion (CAPS) family of proteins, which are known to function during vesicle priming. Here, we used liposome co-flotation assays and liposome fusion assays to determine that Munc13-1, CAPS-1 and Munc13-4 bind to SNARE proteins and promote SNARE-dependent liposome fusion. |
