One SNARE complex is sufficient for membrane fusion

Abstract

Most intracellular membrane fusion reactions in eukaryotes are mediated by SNARE proteins present in both fusing membranes. It has been unclear how many SNARE complexes are needed for fusion. FRET studies now show that liposomes with a single SNARE molecule are able to fuse with other liposomes or with purified synaptic vesicles. 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 demonstrated 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 call for a revision of current views of fusion events such as the fast release of neurotransmitters.