Microstructure and dynamic surface properties of surfactant protein SP-B/dipalmitoylphosphatidylcholine interfacial films spread from lipid-protein bilayers

Suspensions of dipalmitoylphosphatidylcholine (DPPC) bilayers containing 5, 10 or 20% (w/w) surfactant protein SP-B have been reconstituted and spread at air-liquid interfaces. Compression isotherms of DPPC/SP-B monolayers sp read from these preparations were qualitatively comparable to the isotherms of the corresponding DPPC/SP-B monolayers spread from solvents. SP-B was s queezed-out at higher pressures from vesicle-spread films than from solvent -spread monolayers. SP-B caused a marked decrease on the rate of relaxation of DPPC collapse phases to equilibrium pressures in all the lipid/protein films assayed. This stabilizing effect was higher in vesicle-spread than in solvent-spread monolayers. Inclusion in the films of traces of the fluores cent probe NBD-PC (1 mol%) and use of a fluorescent derivative of SP-B labe led with a rhodamine derivative. Texas Red. allowed for direct observation of protein and lipid domains at the interface by epifluorescence microscopy . Upon compression, SP-B altered the packing of phospholipids in the bilaye r-spread films, observed as a SP-B-induced reduction of the area of liquid- condensed domains, in a way similar to its effect in solvent-spread monolay ers. SP-B was not associated with condensed regions of the films. Fluoresce nce images from vesicle-spread films showed discrete fluorescent aggregates that could be consistent with the existence of lipid-protein vesicles in c lose association with the monolayer. Both the retention of SP-B at higher s urface pressures and the greater stability of collapse phases of DPPC/SP-B films prepared by spreading from liposomes in comparison to those spread fr om solvents can be interpreted as a consequence of formation of complex bil ayer-monolayer interacting systems.