Effects of neutral and anionic lipids on digalactosyldiacylglycerol vesicle aggregation

We have previously reported that large unilamellar liposomes made from the neutral galactolipid digalactosyldiacylglycerol (DGDG) will aggregate in the presence of monovalent or divalent cations, behavior that would not have been predicted for an uncharged lipid (Webb et al. (1988) Biochim. Biophys. Acta 938, 323-333). In this paper, the effects of including the other major thylakoid lipids on the Mg2+ concentration required for aggregation of DGDG vesicles has been examined. Addition of the neutral, hexagonal-II phase preferring lipid monogalactosyldiacylglycerol (MGDG) to DGDG up to 50 mol% had no effect, suggesting that the MGDG head group is as effective in causing aggregation as the DGDG head group. Addition of 0.5 to 5.0 mol% of either of the two anionic lipids phosphatidylglycerol (PG) or sulfoquinovosyldiacylglycerol (SQDG) inhibited the aggregation of DGDG vesicles, probably by the development of an electrostatic potential. Phosphatidylcholine (PC) in amounts up to 25 mol% did not inhibit or promote aggregation. Vesicles with a composition similar to that of thylakoids (DGDG/MGDG/SQDG/PG, 1:2:0.5:0.5) required 65 mM MgCl2 in the presence of 200 mM KCl, i.e., higher concentrations than are present in the chloroplast stroma. If MGDG made up more than 25 mol% of any combination of lipids, vesicle aggregation could not be reversed by dilution. These results are consistent with cations playing a role in mediating the close approach of bilayers via an effect on head-group hydration and head-group interaction between bilayers.