Ca++-induced fusion of proteoliposomes: dependence on transmembrane osmotic gradient.

The fusion of cytochrome oxidase liposomes with liposomes reconstituted with mitochondrial hydrophobic protein is dependent on the presence of an acidic phospholipid in the liposomes and on the addition of Ca++ions. Liposomes which have grown, by fusion, to diameters in excess of 1000 A lose the ability to fuse further, unless an osmotic gradient across the liposome membrane is established, with the internal osmotic pressure higher than the external. At a given Ca++ concentration, the extent to which this second fusion step takes place is determined by the ratio of internal to external osmolarity. Single-walled liposomes with diameters exceeding 1 mumM have been produced by this technique. The data suggest that the thermodynamic driving force for the Ca++-induced fusion is an excess surface free energy which can be supplied by membrane curvature or transmembrane osmotic gradients.