KINETICS OF PHOSPHOLIPID MEMBRANE-FUSION INDUCED BY SURFACTANT APOPROTEIN-A AND APOPROTEIN-B

Surfactant apoproteins A (SP-A) and B (SP-B) interact with the lipids of surfactant and such protein-lipid interactions may be of importance in several of the steps in the surfactant cycle. We analyzed the kine tics of fusion of ipalmitoylphosphatidylcholine-phosphatidylglycerol ( DPPC:PG; 7:3, w/w) phospholipid vesicles induced by SP-B alone, in the presence of 5 mM calcium, and in the presence of calcium and SP-A. Me mbrane fusion was measured by the method of resonance energy transfer between non-exchangeable fluorophores incorporated in the membrane, Da ta were analyzed using a mass action kinetic model for membrane fusion between phospholipid vesicles. We found a SP-B dose-dependent increas e in lipid mixing within a range of phospholipid concentration of 5 to 100 mu M. Calcium caused a small additive increase in lipid mixing, b ut calcium and SP-A combined markedly increased lipid mixing induced b y SP-B. Both aggregation and fusion rate constants increased with an i ncrease in the SP-B/lipid ratio. In the presence of calcium and SP-A, the number of vesicles per fusion product markedly increased, as did t he aggregation rate constants, whereas the fusion rate constants remai ned essentially unchanged.