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Amphipols (APols) are short amphipathic polymers designed to substitute to detergents for handling membrane proteins (MPs) in aqueous solutions. Upon trapping a MP with APols, a non-covalent but stable complex forms, which is hydrosoluble and in which the protein is, in general, much more stable than in detergent solution. In MP/APol complexes, the surfactant adsorbs onto the hydrophobic transmembrane surface of the protein, leaving extramembrane surfaces free to interact with water-soluble ligands (cartoon). Functional per¬turbations appear to be rare. One of the first APols to be designed, called A8-35, has been extensively studied, as well as the complexes it makes with a number of MPs. A8-35 consists of a relatively short polyacrylate chain, part of the carboxylates of which have been randomly grafted with octylamine and isopropylamine, making it amphipathic. APols with a different chemical structure, such as sulfonated and non-ionic APols, have been recently developed. Over the years, variously labeled and functionalized versions of A8 35 have been synthesized and validated. Those include deuterated APols, particularly useful for neutron scattering and NMR studies, fluorescent APols, whose distribution can be easily followed during fractionation experiments and which can be used to carry out FRET studies, and tagged APols, which can mediate the attachment of MPs onto solid supports such as chips or beads.
The applications of APols that have been validated to date include stabilizing fragile MPs and MP complexes, solution NMR studies, electron microscopy, diagnostics and ligand binding studies, folding MPs from a denatured state, MP cell-free synthesis and vaccination. Various other applications, e. g. in proteomics, are currently being developed. An overview will be presented of those applications that appear most ready to be usefully exploited by the membrane protein community. The applications and implications of APol-assisted MP folding and cell-free synthesis will be discussed.