Amphipols as a Strange but Efficient Medium in Which to Fold Membrane Proteins: Applications and Implications

Abstract

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 MP is, in general, more stable than in detergent solution. In MP/APol complexes, the polymer covers the hydrophobic transmembrane surface of the protein, leaving extramembrane surfaces free to interact with water-soluble ligands. Functional perturbations appear to be rare.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 MP from a denatured state, and MP cell-free synthesis. Various other applications, e.g. in proteomics, are currently being developed.The use of APols to fold full-length MPs that have been either denatured or obtained in an inactive form as inclusion bodies is particularly promising, as it opens a novel route to producing functional MPs whose overexpression is otherwise intractable. Beyond its practical usefulness, it also raises a more fundamental problem. Neither the chemical structure of APols nor their supramolecular organization bear any similarity, beyond their general amphipathy, to that of membrane lipids. Yet, APols constitute a remarkably efficient medium for MPs to fold, even in the complete absence of lipids. Why?Reading:Dahmane et al. (2009). Amphipol-assisted in vitro folding of G protein-coupled receptors. Biochemistry 48, 6516-6521.Pocanschi et al. (2006). Amphipathic polymers: tools to fold integral membrane proteins to their active form. Biochemistry 45, 13954-13961.Popot, J.-L. (2010). Amphipols, nanodiscs, and fluorinated surfactants: three non-conventional approaches to studying membrane proteins in aqueous solutions. Annu. Rev. Biochem. 79:737-775.Web site: