The Lipid Cubic Phase as a Medium for the Growth of Membrane Protein Microcrystals

The lipid-based bicontinuous cubic mesophase is a nanoporous membrane mimetic with applications in areas that include medicine, personal care products, foods, and the basic sciences. An application of particular note is as a medium in which to grow crystals of membrane proteins for structure determination by X-ray crystallography. At least two variations of the mesophase exist. One is the highly viscous cubic phase, known as the lipid cubic phase (LCP), which has well-developed long-range order. The other, the so-called sponge phase, is considerably more fluid and lacks long-range order. Both phase types have been shown to be amenable for growing microcrystals of membrane proteins and for use as a delivery medium to shuttle protein crystals to an X-ray free-electron laser beam for serial femtosecond crystallography. Here, we provide background on the physicochemical properties of these mesophases and how they function to grow microcrystals of membrane proteins. Protocols implemented for the generation and use of nanoliter volumes of mesophase of suitably high microcrystal density required for serial femtosecond crystallography are described. Prospects for future uses of lipid mesophases in the serial femtosecond crystallography arena are summarized.