<P> The core SNARE complex is a 4 - α (\ displaystyle \ alpha) - helix bundle . Synaptobrevin and syntaxin contribute one α (\ displaystyle \ alpha) - helix each, while SNAP - 25 participates with two α (\ displaystyle \ alpha) - helices (abbreviated as 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 α (\ displaystyle \ alpha) - 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 . </P> <P> 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 . </P> <P> 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 α (\ displaystyle \ alpha) - SNAP and NSF recycle the SNARE complexes after the completion of synaptic vesicle exocytosis . </P> <P> 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 . </P>

Which of the following proteins is likely the calcium sensor of the snare complex