Structural biology relies on detailed descriptions of the three-dimensional structures of peptides, proteins, and other biopolymers to explain the form and function of biological systems ranging in complexity from individual molecules to entire organisms. NMR spectroscopy and X-ray crystallography, in combination with several types of calculations, provide the required structural information. In recent years, the structures of several hundred proteins have been determined by one or both of these experimental methods. However, since the protein molecules must either reorient rapidly in samples for multidimensional solution NMR spectroscopy or form high quality single crystals in samples for X-ray crystallography, nearly all of the structures determined up to now have been of the soluble, globular proteins that are found in the cytoplasm and periplasmof cells and fortuitously have these favorable properties. Since only a minority of biological properties are expressed by globular proteins, and proteins, in general, have evolved in order to express specific functions rather than act as samples for experimental studies, there are other classes of proteins whose structures are currently unknown but are of keen interest in structural biology. More than half of all proteins appear to be associated with membranes, and many cellular functions are expressed by proteins in other types of supramolecular complexes with nucleic acids, carbohydrates, or other proteins. The interest in the structures of membrane proteins, structural proteins, and proteins in complexes provides many opportunities for the further development and application of NMR spectroscopy. Our understanding of polypeptides associated with lipids in membranes, in particular, is primitive, especially compared to that for globular proteins. This is largely a consequence of the experimental difficulties encountered in their study by conventional NMR and X-ray approaches. Fortunately, the principal features of two major classes of membrane proteins have been identified from studies of several tractable examples. Bacteriorhodopsin (Henderson et al., 1990), the subunits of the photosynthetic reaction center (Deisenhofer et al., 1985), and filamentous bacteriophage coat proteins (Shon et al., 1991; McDonnell et al., 1993) have all been shown to have long transmembrane hydrophobic helices, shorter amphipathic bridging helices in the plane of the bilayers, both structured and mobile loops connecting the helices, and mobile N- and C-terminal regions.
Towards an integrative structural biology approach: combining Cryo-TEM, X-ray crystallography, and NMR