Expression and purification of membrane proteins for structural studies

Despite the fact that over 200 unique membrane-protein structures have now been solved, significant challenges remain at all stages of protein production for both structural and functional studies. First, recombinant expression levels of membrane proteins are typically very low. Second, target proteins must be solubilized from their native environment, the cell membrane, and purified. This requires the use of detergents that can also cause the protein to denature or aggregate. Indeed, the repertoire of detergents that have been used successfully for membrane-protein structural studies is surprisingly limited. Third, obtaining well ordered crystals is often very difficult, even if milligramme quantities of high-quality membrane protein can be obtained. Despite these and other barriers, significant progress has been made over the last several years in determining the structures of members of many challenging membrane-protein families, including transporters, channels, intramembrane proteases and G-protein-coupled receptors. Based largely on these past successes, and analyses of recurrent trends, reasonable ‘first-pass’ strategies can now be proposed for most membrane-protein targets. Here, the current state of producing membrane proteins for structural studies is reviewed. An evidence-guided approach is detailed that should provide reasonable starting points for the production of many membrane-protein targets. Keywords: membrane proteins; expression; purification; affinity tags