Lipids and membrane protein structures.

Membrane proteins do not work alone. The interaction of proteins with membrane lipids can be highly specific and is often important for full functional and structural integrity of the protein. Providing the appropriate lipid environment is of great importance for the purification and crystallisation of membrane proteins. The lipid content can be modulated by adjusting purification protocols or by adding back native or non-native lipids. Lipids can facilitate crystallisation by stabilising the protein and by providing lattice contacts. Of special interest is the crystallisation in lipidic cubic phase and with bicelles, as they appear to provide a membrane-like environment. These strategies have been instrumental for recent successful structure determinations of a human G-protein-coupled receptor, the beta(2)-adrenergic receptor. Lipid supplementation can also help to obtain membrane protein structures in a native conformation, as shown for voltage-gated potassium channels. Membrane protein structures, especially those derived from lipid-enriched preparations, contain bound lipid molecules. Specific protein-lipid interactions not only require careful evaluation and interpretation, but also permit a directed approach to elucidate the structural and/or functional role of these interactions.