Electron crystallography of two-dimensional crystals of membrane proteins

Electron microscopy has become a powerful technique, along with X-ray crystallography and nuclear magnetic resonance spectroscopy, to study the three-dimensional structure of biological molecules. It has evolved into a number of methods dealing with a wide range of biological samples, with electron crystallography of two-dimensional crystals being so far the only method allowing data collection at near-atomic resolution. In this paper, we review the methodology of electron crystallography and its application to membrane proteins, starting with the pioneering work on bacteriorhodopsin, which led to the first visualization of the secondary structure of a membrane protein in 1975. Since then, improvements in instrumentation, sample preparation, and data analysis have led to atomic models for bacteriorhodopsin and light-harvesting complex II from higher plants. The structures of many more membrane proteins have been studied by electron crystallography and in this review examples are included where a resolution of better than 10 A has been achieved. Indeed, in some of the given examples an atomic model can be expected in the near future. Finally, a brief outlook is given on current and future developments of electron crystallographic methods. Copyright 1998 Academic Press.