A scissors mechanism for stimulation of SNARE-mediated lipid mixing by cholesterol.

Neurotransmitter release at the synapse requires membrane fusion. The SNARE complex, composed of the plasma membrane t-SNAREs syntaxin 1A and SNAP-25 and the vesicle v-SNARE synaptobrevin, mediates the fusion of 2 membranes. Synaptic vesicles contain unusually high cholesterol, but the exact role of cholesterol in fusion is not known. In this study, cholesterol was found to stimulate SNARE-mediated lipid mixing of proteoliposomes by a factor of 5 at a physiological concentration. Surprisingly, however, the stimulatory effect was more pronounced when cholesterol was on the v-SNARE side than when it was on the t-SNARE side. Site-directed spin labeling and both continuous wave (CW) and pulsed EPR revealed that cholesterol induces a conformational change of the v-SNARE transmembrane domain (TMD) from an open scissors-like dimer to a parallel dimer. When the TMD was forced to form a parallel dimer by the disulfide bond, the rate was stimulated 2.3-fold even without cholesterol, supporting the relevance of the open-to-closed conformational change to the fusion activity. The open scissors-like conformation may be unfavorable for fusion and cholesterol may relieve this inhibitory factor.