Fusion Step-Specific Influence of Cholesterol on SNARE-Mediated Membrane Fusion

Cholesterol is a major component of biological membranes and is known to affect vesicle fusion. However, the mechanism by which cholesterol modulates SNARE-dependent intracellular fusion is not well understood. Using the fluorescence assay and dye-labeled SNAREs and the fluorescent lipids, we dissected cholesterol effects on individual fusion steps including SNARE complex formation, hemifusion, pore formation, and pore dilation. At physiological high concentrations, cholesterol stimulated hemifusion as much as 30-fold, but its stimulatory effect diminished to 10-fold and three-fold for subsequent pore formation and pore expansion at 40 mol %, respectively. The results show that cholesterol serves as a strong stimulator for hemifusion but acts as mild stimulators for pore opening and expansion. Strong stimulation of hemifusion and mild stimulation of pore formation are consistent with the fusion model based on the intrinsic negative curvature of cholesterol. However, even a milder effect of cholesterol on pore expansion is contradictory to such a simple curvature-based prediction. Thus, we speculate that cholesterol also affects the conformation of the transmembrane domains of SNAREs, which modulates the fusion kinetics.