Fusion of phosphatidic acid-phosphatidylcholine mixed lipid vesicles

Ca²⁺-induced transformation of phosphatidylcholine-phosphatidic acid vesicles to larger bilayer structures has been examined using nuclear magnetic resonance, electron microscopy, gel permeation and radioisotope tracer techniques. For concentrated vesicle preparations where phosphatidic acid content remains less than 50% of total lipid, transformation to larger well defined unilamellar structures can be induced. The size of the product formed is dependent on phosphatidic acid content and on Ca²⁺ content when Ca²⁺ levels are between 0.3 and 1.0 mol ratios with respect to phosphatidic acid. During transformation bilayer composition remains unchanged and internal contents are retained in the final structure. These properties are indicative of concerted two vesicle and multiple vesicle fusions. The controllable and concerted fusions make the phosphatidic acid system suitable for further mechanistic studies.