A conserved membrane-spanning amino acid motif drives homomeric and supports heteromeric assembly of presynaptic SNARE proteins

Assembly of the SNARE proteins synaptobrevin/VAMP, syntaxin, and SNAP-25 to binary and ternary complexes is important for docking and/or fusion of pre synaptic vesicles to the neuronal plasma membrane prior to regulated neurot ransmitter release. Despite the well characterized structure of their cytop lasmic assembly domains, little is known about the role of the transmembran e segments in SNARE protein assembly and function, Here, we identified cons erved amino acid motifs within the transmembrane segments that are required for homodimerization of synaptobrevin II and syntaxin 1A Minimal motifs of 6-8 residues grafted onto an otherwise monomeric oligoalanine host sequenc e were sufficient for self-interaction of both transmembrane segments in de tergent solution or membranes, These motifs constitute contiguous areas of interfacial residues assuming alpha-helical secondary structures. Since the motifs are conserved, they also contributed to heterodimerization of synap tobrevin II and syntaxin 1A and therefore appear to constitute interaction domains independent of the cytoplasmic coiled coil regions, Interactions be tween the transmembrane segments may stabilize the SNARE complex, cause its multimerization to previously observed multimeric superstructures, and/or be required for the fusogenic activity of SNARE proteins.