Vesicle fusion to planar membranes is enhanced by cholesterol and low temperature

Lipid composition and properties play an important role in many cellular properties such as fusion of vesicles to cell membranes, an essential process for exocytosis. Using a model system composed of artificial vesicles (liposomes) and artificial membranes (planar lipid bilayers), we observed that fusion is significantly affected by the lipid phase of the planar membrane. To determine the effect of lipid phases on fusion rates, we utilized the nystatin/ergosterol fusion assay and stimulated fusion with an osmotic gradient. Phase of the planar membrane was altered by changing cholesterol or temperature while the vesicular lipids were held constant. Liquid disordered (Ld or Lα) planar membranes were formed from phosphatidylethanolamine and phosphatidylcholine with unsaturated acyl chains. Addition of cholesterol shifts these membranes to the liquid ordered (Lo) phase and increases liposome fusion. Planar membranes in the Lα phase were also made from dipalmitoylphoshatidylcholine (DPPC) above the transition temperature (Tm) of 41.5 °C. Decreasing the temperature below Tm shifts these membranes into the ripple phase (Pβ′) and also increases liposome fusion. The cholesterol and temperature data are consistent with the hypothesis that fusion is promoted in membranes that have greater exposure of their lipid tails or in membranes which can form leaflet domains with negative curvature. The data are not consistent with the hypothesis that lipid mismatch drives fusion.

► We modeled fusion with a system composed of liposomes and planar lipid bilayers.
► Increasing cholesterol in the bilayer increased fusion rates.
► Lipid phase changes could be observed in bilayers formed of DPPC.
► Lowering temperature below the main phase transition increased fusion rates.
► These data show that fusion is affected by the lipid phase of the bilayer.