Amphipols: A general introduction and some protocols

Abstract

Membrane proteins (MPs) exhibit a broad range of activities, which are crucial for cell survival. They can be pumps, channels, enzymes, scaffolds, signal transmitters, or a combination of these functions. Understanding their molecular mechanisms generally requires their extraction out of membranes and their purification. Solubilization and isolation are usually carried out using detergents, which disrupt the membrane and adsorb onto the hydrophobic surface of the transmembrane domain of MPs, keeping them water soluble. Detergents, however, tend to inactivate most MPs more or less rapidly, making their biochemical and biophysical studies challenging. Specially designed amphipathic polymers called "amphipols" (APols) have been developed with the view of improving the stability of MPs in aqueous solutions. In this chapter, the properties of APols and of the complexes they form with MPs are summarized, and a brief overview of APol applications that have been validated thus far is presented. Five experimental protocols are described in detail: (1) trapping MPs in APols, (2) measuring the amount of APol bound per MP, (3) APol-assisted folding of MPs, (4) APol-assisted production of MPs by cell-free expression, and (5) immobilizing MPs onto solid surfaces for screening purposes using functionalized APols.