Role of phosphatidylserine and diacylglycerol in the fusion of chromaffin granules with target membranes.

The role of phosphatidylserine and diacylglycerol in the fusion of chromaffin granules with target membranes was investigated in vitro, monitoring the mixing of membrane lipids with the octadecyl rhodamine B fluorescence dequenching assay. Polylysine was able to induce fusion of chromaffin granule ghosts with plasma membrane vesicles in the absence of Ca2+. This fusion was maximal at 1.6 microM polylysine and the kinetics were dependent on the amount of plasma membrane added. Polylysine lowered the Ca2+ threshold concentration for inducing fusion, increased the extent of fusion at a given Ca2+ concentration and acted synergistically with Ca2+ when added prior to the cation. Removal of membrane proteins by trypsinization of chromaffin granule ghost and/or plasma membranes increased the extent of polylysine induced fusion. Incorporation of diacylglycerol into the plasma membrane promoted Ca(2+)-induced fusion. Chromaffin granule ghosts were induced to fuse with model membranes of different complexities from one resembling the inner monolayer of the erythrocyte membrane to one composed solely of pure phosphatidylserine. The characteristics of the fusion with model membranes were qualitatively similar to that observed with target plasma membrane, although differences in kinetics and stoichiometries were found. Interestingly, considerably low (microM) Ca2+ concentrations were able to induce fusion of chromaffin granule ghosts with diacylglycerol containing phosphatidylserine vesicles in the presence of polylysine. Our results indicate that acidic phospholipid-like phosphatidylserine may have an important role in the process of fusion and suggest that diacylglycerol, as a destabilizing lipid, may have a role by itself in promoting exocytosis in chromaffin cells.