Correlation between membrane expansion and temperature-induced membrane fusion

Abstract

For each phospholipid membrane, there is a characteristic phase transition temperature, and for each phospholipid spherical membrane, there is a specific ‘fusion’ temperature. In order to examine the possible correlation between temperature-induced membrane fusion and membrane expansion, the relationship between the physical states of phospholipid membranes at both temperatures have been investigated by the use of the monolayer system. Monolayer expansion studies have indicated that the increase in area per lipid molecule, caused by increasing the temperature from the phase transition to the fusion temperature, is approximately the same for five different phospholipids used. With the same temperature increase, phospholipid monolayers containing cholesterol did not expand appreciably. This correlates qualitatively with the greater inhibition of membrane fusion seen in the spherical phospholipid membrane systems when cholesterol was incorporated in the membrane. The effect of pH on the expansion of phosphatidylserine monolayers was also studied in relation to membrane fusion phenomena. The shift in fusion temperature of the spherical phospholipid membranes due to the change of pH is explained by the shift in phase transition temperatures of lipid membranes. The expanded area per molecule in the monolayer caused by increasing the temperature from the phase transition to the fusion temperature was approximately the same irrespective of surface charge densities.