Abstract
INTRODUCTION
As recently as 10 years ago, the prospect of solving the structure of any membrane protein by X-ray crystallography seemed remote. Since then, the threedimensional (3-D) structures of two membrane protein complexes, the bacterial photosynthetic reaction centres of Rhodopseudomonas viridis (Deisenhofer et al. 1984,1985) and of Rhodobacter sphaeroides (Allen et al. 1986, 19870,6; Chang et al. 1986) have been determined at high resolution. This astonishing progress would not have been possible without the pioneering work of Michel and Garavito who first succeeded in growing 3-D crystals of the membrane proteins bacteriorhodopsin (Michel & Oesterhelt, 1980) and matrix porin (Garavito & Rosenbusch, 1980). X-ray crystallography is still the only routine method for determining the 3-D structures of biological macromolecules at high resolution and well-ordered 3-D crystals of sufficient size are the essential prerequisite. In spite of these remarkable achievements, our understanding of the detailed structure and, therefore, of the function of membrane proteins lags far behind our knowledge of soluble proteins. This is all the more frustrating since membrane proteins carry out some of the most fundamental, fascinating and important processes in biology. All cells, bacterial or eukaryotic, and compartments within them are surrounded by a lipid membrane acting as a selective permeability barrier. The proteins in this membrane are necessarily involved whenever cells interact with one another or with the outside world. Membrane proteins carry out active transport of ions and metabolites across membranes, form junctions between cells, or are responsible for the non-selective permeability of membranes to small molecules. Others act as receptors of neurotransmitters, hormones and growth factors, or respond to external stimuli. Membrane proteins include some highly antigenic viral spike proteins and histocompatibility antigens which are active in cell-cell recognition. Energy-transducing complexes, transporting electrons or protons across biological membranes, are an important and comparatively well-characterized group of membrane proteins. Mitochondria and chloroplasts, the sites of energy conversion in eukaryotic cells, contain many copies of membrane protein complexes in their extensive, highly folded internal membranes. Chlorophyllprotein complexes in the chloroplast membrane convert solar into chemical energy and drive oxygen evolution and carbon fixation. Mitochondrial membrane protein complexes carry out oxidative phosphorylation and provide animal cells with the energy for all other life functions. Using the methods developed by Michel and Garavito, about 15-20 different membrane proteins have now been crystallized. However, not many of the known membrane proteins (see e.g. Nelson & Robinson, 1983) have been isolated in a form pure enough for cyrstallization experiments to seem promising, and a very large number probably remain to be discovered. I hope this review will make the reader familiar with the general approach and some of the specific problems of growing crystals of membrane proteins, and will ultimately stimulate the crystallization of more membrane proteins. Some essential aspects of the 3-D crystallization of membrane proteins have been discussed in two brief but excellent reviews by Michel (1983) and Garavito et al. (1986). A series of articles by experts in this field may be found in a book edited by Michel (1988).
