Sn1 and Sn2). The interacting amino acid residues that zip the SNARE complex can be grouped into layers. Each layer has 4 amino acid residues – one residue per each of the 4 -helices. In the center of the complex is the zero ionic layer composed of one arginine (R) and three glutamine (Q) residues, and it is flanked by leucine zippering. Layers '-1', '+1' and '+2' at the centre of the complex most closely follow ideal leucine-zipper geometry and aminoacid composition. The zero ionic layer is composed of R56 from VAMP-2, Q226 from syntaxin-1A, Q53 from Sn1 and Q174 from Sn2, and is completely buried within the leucine-zipper layers. The positively charged guanidino group of the arginine (R) residue interact with the carboxyl groups of each of the three glutamine (Q) residues. The flanking leucine-zipper layers act as a water-tight seal to shield the ionic interactions from the surrounding solvent. Exposure of the zero ionic layer to the water solvent by breaking the flanking leucine zipper leads to instability of the SNARE complex and is the putative mechanism by which -SNAP and NSF recycle the SNARE complexes after the completion of synaptic vesicle exocytosis. Mechanism of membrane fusion Assembly SNARE proteins must assemble into trans-SNARE complexes to provide the force that is necessary for vesicle fusion. The four α-helix domains (1 each from synaptobrevin and syntaxin, and 2 from SNAP-25) come together to form a coiled-coil motif. The rate-limiting step in the assembly process is the association of the syntaxin SNARE domain, since it is usually found in a "closed" state where it is incapable of interacting with other SNARE proteins. When syntaxin is in an open state, trans-SNARE complex formation begins with the association of the four SNARE domains at their N-termini. The SNARE domains proceed in forming a coiled-coil motif in the direction of the C-termini of their respective domains. SNAP and NSF also associate with the complex formed by SNAREs during this step and participate in the later events of priming and disassembly. The SM protein Munc18 is thought to play a role in assembly of the SNARE complex, although the exact mechanism by which it acts is still under debate. It is known that the clasp of Munc18 locks syntaxin in a closed conformation by binding to its α-helical SNARE domains, which inhibits syntaxin from entering SNARE complexes (thereby inhibiting fusion). The clasp is also