Single-Molecule Studies of the Neuronal SNARE Fusion Machinery

SNAREs are essential components of the machinery for Ca2+-triggered fusion of synaptic vesicles with the plasma membrane, resulting in neurotransmitter release into the synaptic cleft. Although much is known about their biophysical and structural properties and their interactions with accessory proteins such as the Ca2+ sensor synaptotagmin, their precise role in membrane fusion remains an enigma. Ensemble studies of liposomes with reconstituted SNAREs hive demonstrated that SNAREs and accessory proteins can trigger lipid mixing/fusion, but the inability to study individual fusion events has precluded molecular insights into the fusion process. Thus, this field is ripe for studies with single-molecule methodology. In this review, we discuss applications of single-niolecule approaches to observe reconstituted SNAREs, their complexes, associated proteins, and their effect on biological membranes. Some of the findings are provocative, Such as the possibility of parallel and antiparallel SNARE complexes or of vesicle clocking with only syntaxin and synaptobrevin, but have been confirmed by other experiments.