Structure determination of membrane proteins by NMR spectroscopy.

Abstract

Much of postgenomic biochemistry and all of structural biology are based on the premise that the starting point for both understanding specific biochemical processes, such as affinity, reactivity, or transport, and surveying proteomes is determining the three-dimensional structures of proteins. The two well-established methods for structure determination are highly effective when applied to samples of soluble globular proteins and their complexes: witness the enormous growth of the Protein Data Bank.1 However, the vast majority of biological functions are carried out by proteins associated with supramolecular assemblies, whose samples are problematic for both X-ray crystallography and solution NMR spectroscopy, since they are generally difficult to crystallize and do not reorient rapidly even when soluble. The examples of proteins in supramolecular assemblies whose structures have been determined with atomic resolution are exceptional and highlight the importance of developing new methods of experimental protein structure determination. The essential goals of modern structural biology are to have the capability to select proteins for study based on their biological functions and to perform genuinely unbiased surveys of proteomes unfettered by considerations of the solubility, aggregation state, or other physical properties of the polypeptides. NMR spectroscopy has the potential to accomplish these goals, since it can be applied to molecules in all physical states, including the liquid crystalline environments provided by the lipids associated with membrane proteins. Determining the atomic resolution structures of membrane proteins is of particular interest in contemporary structural biology.2 Helical membrane proteins constitute one-third of the expressed proteins encoded in a genome.3,4 Furthermore, many drugs have membrane-bound proteins as their receptors, and mutations in membrane proteins result in human diseases. They also provide daunting technical challenges for all methods of protein structure determination, including NMR spectroscopy.5