Formation of three-dimensional protein-lipid aggregates in monolayer filmsinduced by surfactant protein B

This study focuses on the structural organization of surfactant protein B ( SP-B) containing lipid monolayers. The artificial system is composed of the saturated phospholipids dipalmitoylphosphatidylcholine (DPPC) and dipalmit oylphosphatidylglycerol (DPPG) in a molar ratio of 4:1 with 0.2 mol% SP-B. The different "squeeze-out" structures of SP-B were visualized by scanning probe microscopy and compared with structures formed by SP-C. Particularly, the morphology and material properties of mixed monolayers containing 0.2 mol% SP-B in a wide pressure range of 10 to 54 mN/m were investigated revea ling that filamentous domain boundaries occur at intermediate surface press ure (15-30 mN/m), while disc-like protrusions prevail at elevated pressure (50-54 mN/m). in contrast, SP-C containing lipid monolayers exhibit large f lat protrusions composed of stacked bilayers in the plateau region (app. 52 mN/m) of the pressure-area isotherm. By using different scanning probe tec hniques (lateral force microscopy, force modulation, phase imaging) it was shown that SP-B is dissolved in the liquid expanded rather than in the liqu id condensed phase of the monolayer. Although artificial, the investigation of this system contributes to further understanding of the function of lun g surfactant in the alveolus.