Effect of hydrophobic surfactant protein SP-C on binary phospholipid monolayers. Molecular machinery at the air/water interface Original Research Article

Fluorescent and modified dark-field microscopies were used to investigate the phase behavior of physiologically relevant lipid/protein monomolecular films containing surfactant protein C(SP-C). Synthetic human SP-C(1-34) was labeled at its N-terminus using the fluorescent probe 6-(((4(4,4-difluoro-5-(2-thienyl)-4-bora-3a,4a-diaza-s-indacene-3-yl)phenoxy)acetyl)amino)hexanoic acid (BODIPY/TR-X). Using dual fluorescent labeling (lipid and protein) in the monolayers, we have correlated (at physiologically small concentrations of the protein) the lipid phase separation and protein distribution in situ. A comparison of the lipid and protein dye fluorescent micrographs indicates that SP-C(1-34) is preferentially associated with the disordered lipid phase. Three concepts arise from our results. (1) The presence of SP-C alone does not result in the complete dissolution of condensed phase domains in a fashion that we have previously reported for the entire hydrophobic surfactant protein (SP-B/C) fraction (Biophys. J. 77 (1999) 903). Rather, the use of relatively high amounts (∼10 wt.%) of the labeled SP-C protein is needed to reproduce the fluorescence monolayer morphology previously observed for small concentrations (∼1 wt.%) of the natural SP-B/C mixture. (2) Scattered light, dark-field microscopy performed using grazing angle laser illumination reveals the presence of surface-associated, three-dimensional (3D) structures of micrometer-sized dimensions when the labeled BODIPY/TR-X:SP-C(1-34) protein is included in the monolayer, as previously observed with the naturally isolated SP-B/C mixture. The 3D structures are associated exclusively with the presence of the SP-C protein in disordered monolayer phases. (3) To explain these results, we have derived a molecular model accounting for the structure and physico-chemical properties of the SP-C protein in terms of its energetics. The molecular events involved in the SP-C-mediated production of the 3D surface particles are explained using the analogy of a simple molecular machine, namely a loaded spring. This interpretation is supported by an energetic analysis that suggests the major factor contributing to the formation of the 3D particles is the energy liberated by re-expansion of the surrounding phospholipid film into the area vacated by the SP-C protein as it re-orients away from the surface.