Cryo-electron microscopy for structure analyses of membrane proteins in the lipid bilayer

Asymmetric distributions of materials, such as ions, solutes, chemicals, and information across the bio-membrane, which are generated by membrane proteins, are crucial hallmarks of all living organisms. Since the appropriate asymmetric distribution depends on the precise function of membrane proteins in the lipid bilayer, structural and functional analyses of the membrane-embedded forms of these proteins are crucial toward elucidating their mechanisms. In this review, we provide remarkable examples of structural analyses of membrane proteins in lipid bilayers based on electron crystallography of two-dimensional crystals. We also discuss the recent development of single-particle cryo-electron microscopy and propose a conceptual strategy for structural analysis of membrane proteins in the lipid bilayer.