A Detergent-Free Approach to Membrane Protein Research: Polymer-Bounded “Native” Nanodiscs

Abstract

Styrene-Maleic acid (SMA) copolymers have emerged as a powerful alternative to detergents for applications in membrane research [1]. Most notably, these amphipathic polymers can be used to directly extract and purify membrane proteins from intact cells of different organisms in the form of “native nanodiscs”. These particles stabilize the protein in a near native environment comprising conserved native lipids as well as other membrane components and they readily allow for structural and functional characterization of the protein [2,3].To evaluate the general applicability of SMA-mediated membrane protein solubilization, we employed a combined imaging and biochemistry approach using HeLa cells as a model. The results indicate that SMA solubilization of (human) cells is an all-or-none process that is not specific for any (sub)cellular membrane, as seen by the solubilization of all intracellular organelles that were tested. These findings suggest that SMA isolation is applicable to any membrane protein irrespective of which cellular membrane it resides in.Since lipid properties strongly influence the solubilization process [4], we then tested whether SMA exhibits selectivity for certain lipids within a given membrane. To this end, we studied the effect of the polymer on model membranes with different lipid compositions. The results revealed a promiscuity of SMA with respect to lipid headgroups in homogeneous lipid mixtures. However, in phase-separating systems of fluid phases with either gel-phase or liquid-ordered phases it showed a distinct preference for lipids in the fluid phase. Implications for the solubilization of proteins from such membranes will be discussed.[1] Dorr JM, et al., 2015, Eur Biophys J, submitted.[2] Swainsbury DJK, et al., 2014, Angew Chem Int Ed Engl 53(44):11803-11807.[3] Dorr JM, et al., 2014, PNAS 111(52):18607-18612.[4] Scheidelaar S, et al., 2015, Biophys J 108(2):279-290.