The Hydrophobic Surfactant Proteins Induce Cubic Phases Without Altering Spontaneous Curvature

Abstract

Prior evidence suggests that the hydrophobic surfactant proteins, SP-B and SP-C, promote adsorption of the surfactant lipids to the alveolar air/water interface by facilitating formation of a rate-limiting negatively curved stalk between the vesicular bilayer and the interface. In support of the proposed model, the physiological mixture of the surfactant proteins (SP), in amounts as low as 0.03% (w:w), induce 1-palmitoyl-2-oleoyl phosphatidylethanolamine (POPE) to form inverse bicontinuous cubic (QII) phases, in which each leaflet has the saddle-shaped net-negative curvature predicted for the hypothetical stalk. One mechanism by which the proteins might promote formation of the QII phases is by altering the spontaneous curvature of the lipid leaflets. If the lipid-protein mixtures form the inverse hexagonal (HII) phase, then a shift in spontaneous curvature would change the dimensions of the unit cell. POPE forms HII structures only above 71°C, and only for 0-0.03% SP. To obtain HII structures with a wider range of protein contents, we substituted lipids that form HII structures at lower temperatures. X-ray diffraction showed that 1,2-dioleoyl phosphatidylethanolamine and its stereoisomer 1,2-dielaidoyl phosphatidylethanolamine form QII phases with SP at or above 0.03%. During heating from 10 to 95°C, both lipids form HII structures over the full range of protein concentrations from 0-3% SP. The dimensions of the HII unit cell were unaffected by the content of protein. The lack of any effect of the surfactant proteins on the size of the HII phase indicates that the proteins facilitate formation of the QII phases, and suggests that they promote adsorption, by a mechanism other than changing spontaneous curvature. (Studies conducted at the Stanford Synchrotron Radiation Lightsource).