Approaches to determining membrane protein structures to high resolution: do selections of subpopulations occur?

Three different methods are currently used for the study of high-resolution structures of membrane proteins: X-ray crystallography, electron crystallo graphy, and nuclear magnetic resonance (NMR) spectroscopy. Thus far, all me thods combined have yielded a rather modest number of crystal structures th at have been solved at the atomic level. It is hypothesized here that diffe rent methods may select different populations of proteins on the basis of v arious properties. Thus, protein stability may be a significant factor in t he formation of three-dimensional (3D) crystals from detergent solutions, s ince exposure of hydrophobic protein zones to water may cause structural pe rturbation or denaturation in conformationally labile proteins. This is dif ferent in the formation of two-dimensional (2D) crystals where a protein re mains protected in its native membrane environment. A biological selection mechanism may therefore be operative in that highly ordered lattices may fo rm only if strong protein-protein interactions are relevant in vivo, thereb y limiting the number of proteins that are amenable to electron crystallogr aphy. Keeping a protein in a bilayer environment throughout 3D crystallizat ion maintains the lateral pressure existing in native membranes. This can b e accomplished by using lipidic cubic phases. Alternatively, the hydrophobi c interface of a membrane protein may be spared from contact with water by crystallization from organic solvents where the polar caps are protected in reverse micelles by using appropriate detergents. Some of the criteria tha t are useful in optimizing the various approaches are given. While the usef ulness of complementary methods seems obvious, the results presented may be particularly critical in recognizing key problems in other structural appr oaches. (C) 2000 Elsevier Science Ltd. All rights reserved.