Alternatives to Detergents for Handling Membrane Proteins in Aqueous Solutions

Abstract

Attempts at substituting detergents with other surfactants for handling membrane proteins (MPs) in aqueous solutions have a long history. They are based on three main incentives: (i) trying to improve the stability of solubilized MPs; (ii) providing them with an environment that, in its physical characteristics and/or its chemical composition, is closer to the natural environment; and (iii) making them accessible to technologies that are difficult or impossible to implement in the presence of detergents. A first route is to reinsert the protein in a lipid bilayer, most often closed upon itself in the form of lipid vesicles, sometimes forming a planar “black lipid membrane.” This approach is obligatory when functional assays require the protein to have access to two distinct aqueous compartments, but the objects formed are large, if not macroscopic, and do not lend themselves well to most biophysical investigations. A second route is to substitute totally or partially the detergent with other surfactants while forming water-soluble particles of nanometric dimensions. The use of specially developed amphipathic polymers called amphipols is one such approach, which will be described in detail in Chaps. 4 and 5 , but it is far from being the only one. In order to provide a broader view of which systems are available to the experimenter, the present chapter reviews the four principal alternatives to detergents and amphipols: (i) bicelles, which are mixtures of lipids and detergents or short-chain lipids that, under appropriate conditions, form disc-shaped bilayer fragments into which MPs can integrate; (ii) nanodiscs, whose basic concept is similar to that of bicelles, but in which the rim of the bilayer disc is stabilized by specially engineered proteins; (iii) peptides or lipopeptides, which can either interact directly with the MP to be solubilized or stabilize MP/lipid complexes; and (iv) fluorinated surfactants, which resemble detergents in their chemical structure but whose hydrophobic chains contain fluorine atoms, which make them lyophobic (poorly miscible with hydrocarbons); this renders them less disruptive of the protein/protein and protein/lipid interactions that stabilize MPs.