Membrane Protein Fragments Reveal Both Secondary and Tertiary Structure of Membrane Proteins
Structural data on membrane proteins, while crucial to understanding cellular function, are scarce due to difficulties in applying to membrane proteins the common techniques of structural biology. Fragments of membrane proteins have been shown to reflect,
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1. Introduction Membrane proteins enable and regulate a wide variety of cell functions such as cell signaling, membrane transport, biosynthesis, and cell morphology. Membrane proteins are major targets for drug development. Three-dimensional structure is as important for membrane proteins as it is for soluble proteins in understanding function, regulation, and external control by pharmaceuticals. A severe deficit of membrane protein structures at the atomic level, however, hampers our understanding of the function of these proteins in cellular membranes. The protein structure databases contain tens of thousands of atomic structures of proteins, but less than 0.5% of the structures are of membrane proteins. One database lists fewer than 200 unique membrane protein structures determined to atomic level resolution (1). This information deficit delineates the frontier of structure biology today: developing new approaches and refining existing Jean-Jacques Lacapère (ed.), Membrane Protein Structure Determination: Methods and Protocols, Methods in Molecular Biology, vol. 654, DOI 10.1007/978-1-60761-762-4_15, © Springer Science+Business Media, LLC 2010
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techniques to increase the breadth of structural knowledge of membrane proteins. It is only with such structural information that mechanistic understanding of cellular processes can be expressed at the atomic level. And it is such structural information that informs most effectively the development of new interventions to modify function, such as therapies for diseases. The major methodology behind the available structural information for membrane proteins is X-ray crystallography, just as it is for soluble proteins. Many examples will be seen in other chapters in this volume. However, X-ray crystallography has generated far fewer structures of membrane proteins than it has for soluble proteins because membrane proteins, which are not soluble in aqueous media, are not readily amenable to the commonly used technologies for crystal growth required by X-ray crystallography. Therefore, alternative methodologies of structural biology have been explored in the search for new membrane protein structural information. The major alternative methodology for determining highresolution structures of soluble proteins is nuclear magnetic resonance (NMR). Modern NMR approaches have led to an explosion of structural data derived from proteins in solution. This is particularly important because it circumvents the crystallization requirement. A number of investigators have worked to translate this approach from soluble protein structural determination to the determination of the structures of membrane proteins. Unfortunately, the insolubility of membrane proteins in aqueous media has inhibited the application of solution NMR methodology. Isolated membrane proteins reconstituted into a lipid bilayer form a complex too large to experience rotational diffusion rapid enough for solution NMR. To address this problem, membrane proteins have been solubilized in deterg
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