The Hydrophobic Surfactant Proteins Induce Cubic-Phase Formation in a Hii Forming Phospholipid

Abstract

The hydrophobic surfactant proteins, SP-B and SP-C, which represent ∼1.5% (w:w) of pulmonary surfactant, promote the rapid adsorption of the surfactant phospholipids into the alveolar air/water interface. Available evidence suggests that they facilitate adsorption by stabilizing a tightly curved rate-limiting structure that bridges the gap between the adsorbing vesicle and the nascent interfacial film. To determine if the surfactant proteins can affect the curvature of phospholipid leaflets, we used small-angle X-ray diffraction to determine how the physiological mixture of SP-B and SP-C affected the structure of 1-palmitoyl-2-oleoyl phosphatidylethanolamine, which forms the inverted hexagonal (Hii) phase at high temperatures. Measurements from 5-95°C at 5-10°C intervals on the phospholipid alone were consistent with previously-described transitions between two lamellar phases at 24°C, with formation of the Hii phase at ∼72°C. Addition of 0.1-2.0% (w:w) protein had no effect on the lamellar phases, but induced formation of two cubic phases, Pm3n and Im3m, beginning at temperatures as low as 50°C. The intensity of cubic relative to lamellar peaks increased with larger amounts of the proteins and temperature. At 55°C, for instance, progressive increases in protein shifted the samples from strictly lamellar structures to lamellar-cubic coexistence to strictly cubic phases. Similarly, in samples with 0.1% protein, increasing temperature shifted lamellar structures through a coexistence region (60-70°C) to exclusively cubic diffraction at higher temperatures. These results indicate that with a phospholipid that can form curved structures, the hydrophobic surfactant proteins can significantly alter its polymorphisms. These findings support the model in which the proteins achieve their kinetic effects by stabilizing a curved structure.Portions of this research were carried out at the Stanford Synchrotron Radiation Laboratory, a national user facility operated by Stanford University on behalf of the U.S. Department of Energy.