Membrane fusion correlates with surface charge in exocytic vesicles

Stimulation of gastric parietal cells results in exocytic recruitment of the proton pump (H(+),K(+)-ATPase) from a pool of intracellular membranes (tubulovesicles) to the apical plasma membrane. We have previously reconstituted a step in this process, the homotypic fusion of tubulovesicles, and shown that they also fuse with liposomes in a protein-dependent manner Duman, J. G., Singh, G., Lee, G. Y., Machen, T. E., and Forte, J. G. (2002) Traffic 3, 203-17]. Further, the lipid composition of the liposomes affects their ability to undergo fusion with tubulovesicles. In the present study, we investigated the lipid requirements for tubulovesicular membrane fusion using a fluorescent probe relaxation assay as well as transfer of protein between tubulovesicles and liposomes of defined composition. Initially, we tested the ability of tubulovesicles to undergo fusion with a panel of synthetic phosphatidylcholine-based liposomes containing a variety of common membrane lipids of various shapes and charges. We found that anionic lipids such as phosphatidylserine, phosphatidic acid, and phosphoinositides were best able to enhance tubulovesicle-liposome fusion and that they did it in a dose-dependent, apparently saturable manner. Next, we altered the lipid compositions of actual tubulovesicles and observed that addition of anionic lipids was able to enhance tubulovesicle-tubulovesicle fusion in vitro; thus, we hypothesized that the charge imparted by the lipids, per se, was responsible for the enhancement of membrane fusion. Accordingly, addition of negative charges to one of two pools of tubulovesicles in a fusion assay using anionic detergents increased membrane fusion; whereas, addition of positively charged cationic detergent decreased membrane fusion and could be used to back-titrate the anionic effects. Surprisingly, when both pools of fusing membranes were loaded with anionic detergents, fusion was markedly increased. The ability of anionic charges to enhance fusion was diminished as the ionic strength of the fusion medium was increased, suggesting that the mechanism of fusion enhancement depends on the surface charge of the membranes. Finally, the fusion reaction was highly dependent on temperature, and anionic charge appears to lower the activation energy of the fusion reaction. Taken together, these data suggest that (1) tubulovesicular fusion is enhanced by an increase in membrane surface negative charge associated with a lower activation energy and (2) neutralization or reversal of the surface charge prevents tubulovesicular fusion.