Crystallization and Structure Analysis of Membrane Proteins

In recent years, there has been great progress in the determination of high-resolution three-dimensional (3D) structures of membrane proteins. The first major breakthrough came with the crystallization (1 ) and X-ray crystallography (2 ,3 ) of the bacterial photosynthetic reaction center (see refs. 4 and 5 for reviews). The structure of another, entirely different membrane protein, the bacterial outer membrane porin from Rhodobacter capsulatus , has now been determined by X-ray crystallography (6 ). Recent results by electron crystallography of two-dimensional (2D) crystals have been most encouraging. The high-resolution 3D structure of bacteriorhodopsin (7 ) plant light-harvesting complex (8 ) and projection maps of several other membrane proteins at similar resolutions (9 –11 ) have been obtained by this technique. Electron crystallography seems particularly appropriate for membrane proteins that are prone to form 2D crystals, and it is hoped that many more structures will be determined in this way.