The Effect of the Hydrophobic Surfactant Proteins on HII-Curvature Depends on the Cylindrical Radius

Abstract

Several small hydrophobic peptides can promote the fusion of lipid bilayers. The stalk model proposes that fusion proceeds via a negatively curved intermediate, and suggests that the peptides might achieve their fusogenic effect by enhancing negative spontaneous curvature of the individual lipid leaflets. Direct measurements on the unpaired monolayers of the inverse hexagonal (HII) phase have shown that the peptides can induce more negative curvature, but to a limited extent. The hydrophobic surfactant proteins, SP-B and SP-C, show similar behavior. They promote the adsorption of vesicles to an air/water interface by a process analogous to fusion. They enhance the negative spontaneous curvature of HII monolayers formed by dioleoyl phosphatidylethanolamine (DOPE):dioleoyl phosphatidylglycerol (9:1, mol:mol), but only by 8%. The studies here tested whether the limited response might reflect the size of the HII cylinder and the restricted space available for incorporation of the proteins. Small angle x-ray scattering showed that different ratios of DOPE with dioleoyl phosphatidylcholine (DOPC) produced HII cylinders with different radii. The effects of the proteins depended on the cylindrical size. Up to 3% (w:w) of the proteins produced no change in the smallest cylinders, formed by DOPE alone. With larger cylinders, the proteins reduced the radius. The magnitude of the change increased with cylindrical size. At 80% (mol:mol) DOPC, 5% (wt:wt) protein reduced the outer cylindrical radius by 78%. This change represented a 90% increase in the estimated magnitude of negative spontaneous curvature. These results show that demonstrating the ability of the hydrophobic surfactant proteins to promote negative curvature requires HII cylinders above a minimum size. Our findings raise the possibility that the effects on HII-curvature of other fusogenic peptides might also depend on the cylindrical radius. Acknowledgements: NIH; SSRL.