Structure and interaction in lipid bilayers analyzed using bicelles

Although many non-peptidic drugs target biological membrane and membrane proteins, it is still difficult to determine the membrane-bound conformation of the drugs. To solve those problems, we have utilized bicelles as a membrane model, since the bicelles, which have planar lipid bilayer portions, are thought to be a more appropriate membrane model than micelles. Small-sized bicelles allow for liquid NMR measurements due to isotropic fast tumbling in solution. We have applied small bicelles to erythromycin A, salinomycin, and amphidinol 3, and determined their membrane-bound structures as well as their positions and orientations in the membranes using coupling constants, NOEs, and paramagnetic relaxation methods. Recently, we found that sphingomyelin, a major lipid constituent of lipid rafts, also forms bicelles, and established its conformation in the bicelles. These studies show the general utility of small bicelles for detailed conformation and orientation analysis of membrane-associated drugs and lipid molecules. We are now developing a bicelle-based crystallization method for membrane proteins, which will facilitate the cocrystalization of membrane proteins and hydrophobic drugs.