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SNARE proteins mediate intracellular membrane fusion by forming a coiled-coil complex to merge opposing membranes. A âfusion-activeâ
neuronal SNARE complex is a parallel four-helix bundle containing two coiled-coil domains from SNAP-25 and one coiled-coil
domain each from syntaxin-1a and VAMP-2. âPrefusionâ assembly intermediate complexes can also form from these SNAREs. We studied
the N-terminal coiled-coil domain of SNAP-23 (SNAP-23N), a non-neuronal homologue of SNAP-25, and its interaction with other
coiled-coil domains. SNAP-23N can assemble spontaneously with the coiled-coil domains from SNAP-23C, syntaxin-4, and VAMP-3
to form a heterotetrameric complex. Unexpectedly, pure SNAP-23N crystallizes as a coiled-coil homotetrameric complex. The
four helices have a parallel orientation and are symmetrical about the long axis. The complex is stabilized through the interaction
of conserved hydrophobic residues comprising the a and d positions of the coiled-coil heptad repeats. In addition, a central, highly conserved glutamine residue (Gln-48) is buried
within the interface by hydrogen bonding between glutamine side chains derived from adjacent subunits and to solvent molecules.
A comparison of the SNAP-23N structure to other SNARE complex structures reveals how a simple coiled-coil motif can form diverse
SNARE complexes.