A New Biomimetic Phase of Surfactant Bilayers Maintains Membrane Protein Activity

For several years lipidic cubic (Q) mesophases have been used to crystallize membrane proteins. Because they have the rheology of a thick paste, working with Q-phases remain a challenge. We will present a new fluid L3 phase in which transmembrane proteins such as Bacteriodopsin, SERCA1a, and Cytochrome oxidase maintain their activity. Macroscopically a L3 phase can be viewed as a sponge made of a surfactant whereas the holes in the sponge are filled with solvent. Topologically it consists of a single bilayer surrounded on either side by a solvent forming a continuous network of channels. It is comparable to a molten cubic phase, but it possesses water-like viscosity. Locally, it is similar to a lamellar phase but it is isotropic and optically transparent and thus suitable for spectroscopic studies. The L3 phases presented here were characterized by polarized light microscopy, diffusion of a fluorescent probe by fluorescence recovery after pattern photobleaching (FRAPP) and freeze fracture electron microscopy (FFEM).Tuning the distance between adjacent bilayers from 3 to 40 nm is an asset for the study of interactions between proteins. This is obtained by varying the water content of the phase. Characteristic distances (db) of the phase were obtained from small angle scattering spectra (SAXS/SANS) as well as from FFEM, which yielded similar db values: the L3 phase preserves its structure when a transmembrane protein is incorporated into the bilayers, when the non-ionic co-surfactant is replaced by another one and when the temperature varies from 6°C to 30°C. These findings illustrate that a biomimetic surfactant sponge phase can be obtained in the presence of detergents widely used to solubilize membrane proteins and thus make it a versatile medium for membrane protein studies.