The hydrophobic surfactant proteins SP-B and SP-C promote rapid adsorption of pulmonary surfactant to an air/water interface. Previous evidence suggests that they achieve this effect by facilitating the formation of a rate-limiting negatively curved stalk between the vesicular bilayer and the interface. To determine whether the proteins can alter the curvature of lipid leaflets, we used x-ray diffraction to investigate how the physiological mixture of these proteins affects structures formed by 1-palmitoyl-2-oleoyl phosphatidylethanolamine, which by itself undergoes the lamellar-to-inverse hexagonal phase transition at 71 degrees C. In amounts as low as 0.03% (w:w) and at temperatures as low as 57 degrees C, the proteins induce formation of bicontinuous inverse cubic phases. The proteins produce a dose-related shift of diffracted intensity to the cubic phases, with minimal evidence of other structures above 0.1% and 62 degrees C, but no change in the lattice-constants of the lamellar or cubic phases. The induction of the bicontinuous cubic phases, in which the individual lipid leaflets have the same saddle-shaped curvature as the hypothetical stalk-intermediate, supports the proposed model of how the surfactant proteins promote adsorption.
peptide to membranes that also contain the channel-forming peptide gramicidin A allowed us to observe the membrane adsorption of A1AT. The protein interacts with the channel and produces transient current interruptions. The on-rate of these events depends non-linearly on the A1AT concentration and scales with the mole percentage of the charged lipid in the membrane. The measured off-rate is surface-charge independent. Thus, our results suggest that the membrane lipid composition plays an important regulatory role in the physiological activity of the A1AT and its interaction with the membrane.
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