SP-B and SP-C alter diffusion in bilayers of pulmonary surfactant

The hydrophobic proteins SP-B and SP-C promote rapid adsorption of pulmonary surfactant to an air/water interface by an unknown mechanism. We tested the hypothesis that these proteins accelerate adsorption by disrupting the structure of the lipid bilayer, either by a generalized increase in fluidity or by a focal induction of interfacial boundaries within the bilayer. We used fluorescence recovery after photobleaching to measure diffusion of nitrobenzoxadiazolyl-dimyristoyl-phosphatidylethanolamine between 11 and 54°C in multilayers containing the complete set of lipids and proteins in calf lung surfactant extract (CLSE), or the complete set of neutral and phospholipids without the proteins. Above 35°C, Arrhenius plots of diffusion were parallel for CLSE and neutral and phospholipids, but shifted to lower values for CLSE, suggesting that the proteins rigidity the lipid bilayer rather than producing the proposed increase in membrane fluidity. The slopes of the Arrhenius plots for CLSE were steeper below 35°C, suggesting that the proteins induce phase separation at that temperature. The mobile fraction fell below 27°C, consistent with a percolation threshold of coexisting gel and liquid-crystal phases. The induction of lateral phase separation in CLSE, however, does not correlate with apparent changes in adsorption kinetics at this temperature. Our results suggest that SP-B and SP-C accelerate adsorption through a mechanism other than the disruption of surfactant bilayers, possibly by stabilizing a high-energy, highly curved adsorption intermediate.