Effects of lung surfactant specific protein SP-B and model SP-B peptide on lipid monolayers at the air-water interface

Lung surfactant, a mixture of dipalmitoylphosphatidylcholine, phosphatidylglycerols, fatty acids and four lung surfactant specific proteins, forms tightly packed monolayers at the alveolar interface which are capable of lowering the normal air-water surface tension to almost zero. Isolating the roles of the individual components of lung surfactant is essential to understanding the biophysics of surfactant functioning and to developing new low-cost replacement surfactants. We have used fluorescence microscopy to show that both the full length lung surfactant specific protein SP-B_1-78 and a shorter model peptide SP-B_1-25 alter the phase behavior and surface morphology of palmitic acid (PA) monolayers. PA is an important component of both natural and replacement lung surfactants. Both the protein and the peptide inhibit the formation of condensed phase in monolayers of PA, resulting in a new fluid PA-protein phase. This fluid phase forms a network that separates condensed phase domains at coexistence. The network persists to high surface pressures, altering the nucleation, growth and morphology of monolayer collapse structures, leading to lower surface tensions on compression and more reversible respreading on expansion, factors essential to the in vivo performance of both natural and replacement lung surfactants. The network is stabilized by the low line tension between the fluid phase and the condensed phase as confirmed by the formation of extended linear domains or 'stripe' phases. Similar stripes are found in monolayers of fluorescein-labeled SP-B_1-25, suggesting that the reduction in line tension is due to the protein. The peptide retains many of the essential functions of the full length protein and may be a lower cost substitute in replacement surfactants.