One SNARE complex is sufficient for membrane fusion

In eukaryotes, most intracellular membrane fusion reactions are mediated by the interaction of SNARE proteins that are present in both fusing membranes. However, the minimal number of SNARE complexes needed for membrane fusion is not known. Here, we show unambiguously that one SNARE complex is sufficient for membrane fusion. We performed controlled in vitro Förster resonance energy transfer (FRET) experiments and found that liposomes bearing only a single SNARE molecule are still capable of fusion with other liposomes, or with purified synaptic vesicles. Furthermore, we demonstrate that multiple SNARE complexes do not act cooperatively, showing that synergy between several SNARE complexes is not needed for membrane fusion. Our findings shed new light on the mechanism of SNARE-mediated membrane fusion and ask for a revision of current views of fusion events such as the fast release of neurotransmitters.