Squeezing of Oil-Swollen Surfactant Bilayers by a Membrane Protein

We have performed small angle x-ray scattering experiments on a ternary system made of a nonionic surfactant, dodecane, and water, in the absence and upon insertion of a transmembrane protein. In contrast to other proteins or polymers studied, its incorporation reduces the Bragg spacing from 200 Å to 80 Å, which scales as $c^{-0.5\mathrm{}}$, where c is the protein surface density. The macromolecule incorporated into the hydrophobic part of the lamellar phase appears on freeze-fracture electron micrographs as intramembranous particles. The data are fitted by a simple model of thermally undulating lamellae decorated with protein molecules.