A major current deficit in structural biology is the lack of high-resolution structures of eukaryotic membrane proteins, many of which are key drug targets for the treatment of disease. Numerous eukaryotic membrane proteins require specific lipids for their stability and activity, and efforts to crystallize and solve the structures of membrane proteins that do not address the issue of lipids frequently end in failure rather than success. To help address this problem, we have developed a sparse matrix crystallization screen consisting of 48 lipidic-sponge phase conditions. Sponge phases form liquid lipid bilayer environments which are suitable for conventional hanging- and sitting-drop crystallization experiments. Using the sponge phase screen, we obtained crystals of several different membrane proteins from bacterial and eukaryotic sources. We also demonstrate how the screen may be manipulated by incorporating specific lipids such as cholesterol; this modification led to crystals being recovered from a bacterial photosynthetic core complex.

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The phase behavior of 1-glyceryl monooleyl ether (GME) in mixtures of propylene glycol (PG) and water was investigated by visual inspection, polarization microscopy, small-angle X-ray diffraction, and conductance measurements. A phase diagram, based on over 200 samples of the ternary system GME-PG-water, was constructed at 20 degrees C. Without PG, GME forms a reverse micellar phase with up to 10 wt % water and a reverse hexagonal liquid-crystalline phase between 10 and 25 wt % water, a phase that can coexist with excess water. If PG is added in amounts exceeding about 10 wt %, then cubic and lamellar liquid-crystalline phases start to form. A cubic phase, belonging to space group Pn3m, can coexist with excess PG-water mixtures. If even more PG is added, then the cubic phase is transformed into a sponge phase. A lamellar phase forms at water contents between 10 and 15 wt % and with widely differing PG/GME weight ratios. We postulate that the phase behavior is caused by the fact that PG makes the interfacial region between self-assembled GME and PG-water less negatively curved, which in turn allows for the formation of the new phases. The phase behavior obtained for the GME system shows a striking similarity with the phase behavior of the corresponding system in which the GME has been replaced by the ester, 1-glycerol monooleate (GMO), differing only in one extra carbonyl oxygen. The major difference is the lower amount of water present in the GME phases, an effect that is mainly due to the more hydrophobic character of GME compared to that of GMO.

Currently, the major deficit in structural biology is a lack of highresolution structures of mammalian membrane proteins, many of which are key drug targets for the treatment of human disease. Numerous membrane proteins, particularly those of mammalian origin, require specific lipids for their stability and activity. To address this issue we have developed a sparse matrix crystallization screen consisting of 48 different lipidic-sponge phase conditions. Sponge phases consist of lipid bilayers with intersecting water channels. The water channels allow membrane proteins with large aqueous domains to be incorporated with their hydrophobic domains reconstituted into the membrane, mimicking their native environment and thus facilitating crystallization. The sponge phases are liquid at room temperature and the most obvious practical advantage of this approach is that they can be used directly in vapour diffusion experiments. This liquid property is also compatible with crystal drop dispensing using crystallization robots and greatly facilitates the mounting of protein crystals in nylon loops. Furthermore it allows optimization using additives as well as other techniques such as seeding. The sponge phase screen was designed to contain different solvents, salts and pH to accommodate the requirements of many membrane proteins. In some cases, other lipids such as cholesterol were incorporated into the phases to provide extra stability for the proteins. This approach yielded crystals of the photosynthetic core complex of Blastochloris viridis. The screen's effectiveness was further proven by crystallization experiments using protein from other bacteria as well as from higher plants. Crystals were obtained for 8 out of 12 proteins and are currently undergoing optimization.