Hydrophobic Surfactant Proteins Strongly Induce Negative Curvature

Abstract

The hydrophobic surfactant proteins SP-B and SP-C greatly accelerate the adsorption of vesicles containing the surfactant lipids to form a film that lowers the surface tension of the air/water interface in the lungs. Pulmonary surfactant enters the interface by a process analogous to the fusion of two vesicles. As with fusion, several factors affect adsorption according to how they alter the curvature of lipid leaflets, suggesting that adsorption proceeds via a rate-limiting structure with negative curvature, in which the hydrophilic face of the phospholipid leaflets is concave. In the studies reported here, we tested whether the surfactant proteins might promote adsorption by inducing lipids to adopt a more negative curvature, closer to the configuration of the hypothetical intermediate. Our experiments used x-ray diffraction to determine how the proteins in their physiological ratio affect the radius of cylindrical monolayers in the negatively curved, inverse hexagonal phase. With binary mixtures of dioleoylphosphatidylethanolamine (DOPE) and dioleoylphosphatidylcholine (DOPC), the proteins produced a dose-related effect on curvature that depended on the phospholipid composition. With DOPE alone, the proteins produced no change. With an increasing mol fraction of DOPC, the response to the proteins increased, reaching a maximum 50% reduction in cylindrical radius at 5% (w/w) protein. This change represented a doubling of curvature at the outer cylindrical surface. The change in spontaneous curvature, defined at approximately the level of the glycerol group, would be greater. Analysis of the results in terms of a Langmuir model for binding to a surface suggests that the effect of the lipids is consistent with a change in the maximum binding capacity. Our findings show that surfactant proteins can promote negative curvature, and support the possibility that they facilitate adsorption by that mechanism.

The Lγ Phase of Pulmonary Surfactant

“…For mixtures of N&PL and CLSE, the L γ phase begins to disappear at X CLSE = 0.7, when the net molecular charge has fallen only by 5% to −0.095 (Table S1.B). In addition to their electrostatic effect, the proteins induce large changes in the spontaneous curvature of the lipids; 40 they may also alter flexibility of the leaflets. These effects seem more likely to explain the inhibition of the L γ phase than a reduction in net charge.…”

Spectral phasor analysis of LAURDAN fluorescence in live A549 lung cells to study the hydration and time evolution of intracellular lamellar body-like structures

“…7% PS+PI are reported in LBs [47]) can promote non-lamellar phases in conditions characterized by very low water content, low pH and high Ca 2+ concentrations [52]. Additionally, non-lamellar structures can be stabilized by the presence of SP-B and SP-C, as reported in a model system composed of DOPC:DOPE [53]. Indeed in vitro experiments using extract of broncho-alveolar lavages from sheep lungs reported by Teubner et al demonstrated lyotropicmesomorphism induced by the percentage of water in the system [54].…”

Divide & Conquer: Surfactant Protein SP-C and Cholesterol Modulate Phase Segregation in Lung Surfactant

“…Given that DPPC can be packed more tightly than unsaturated lipids, this could be relevant for reaching low surface tensions during exhalation. The change in curvature could also explain, to some extent, SP-C-mediated lipid adsorption, which has been suggested to occur through a negatively curved stalk in cooperation with SP-B (60)(61)(62). For such a structure to occur, SP-B and SP-C would need to generate membrane curvature, a condition that although already demonstrated (9,63,64), does not provide enough mechanistic insights to determine how this process is accomplished.…”