Membrane solubilization by styrene–maleic acid copolymers: towards new applications in membrane protein research

Abstract

Membrane proteins are essential molecules that are involved in numerous processes such as signal transduction in nerve cells, cell–cell communication and solute transport across membranes. They also have a high pharmacological relevance, with approximately half of all currently-available drugs being targeted to a membrane protein. For biochemical investigation, however, membrane proteins generally need to be extracted from cell membranes by solubilizing agents such as detergents. This often renders them unstable which can lead to their inactivation or aggregation, which strongly hampers the downstream study of their structure and function. In his thesis, Jonas Dörr describes the investigation of a novel approach using styrene–maleic acid (SMA) copolymers instead of conventional detergents to assess their potential for membrane protein research. He showed that SMA can be used as an alternative to detergents for efficient extraction and purification of a potassium channel protein in intact patches of biomembranes, thus conserving a native lipid environment around the protein. Since such patches are discoidal and have a diameter of about 10 nm, they are referred to as “native nanodiscs”. Dörr furthermore showed that these particles convey a higher protein stability than do commonly-used detergents and that they are readily amenable to biophysical and biochemical characterization. The fact that native lipid material is coisolated with the protein was exploited to show that the composition of the nanodiscs that contain the channel differed from the composition of the original membrane. Thus, native nanodiscs likely provide a snapshot of the immediate molecular environment of the protein inside the native membrane. This can be used as a novel approach to investigate preferential interactions of lipids and membrane proteins. Dörr also provided a proof-of-principle example that membrane proteins can be transferred from native nanodiscs back to an extended bilayer membrane. In the case of the channel protein, this enabled functional characterization by electrophysiology, that showed that the extraction method did not affect protein function. This experiment also represents the first instance where a membrane protein is transferred into a controlled artificial environment without being in contact with detergent at any step in the process. The results in this thesis open the door for a number of new applications in membrane protein research, including pharmacological characterization.