Membrane Solubilization by Amphipathic Copolymers: Expanding the Toolbox for the Study of Membrane Proteins

Abstract

Amphipathic copolymers have recently gained interest due to their ability to solubilize membrane proteins with a layer of surrounding lipids in the form of nanodiscs. The proteins thus retain their native membrane environment, thereby keeping them optimally stable. The nanodiscs can easily be purified and characterized by biochemical and biophysical techniques, circumventing the drawbacks of potential misfolding and aggregation that are likely to occur when conventional, detergent extraction-based methods are used of purifying and characterizing membrane proteins. This opened up new research lines in the field of development and characterization of the efficiency and mode of action of amphipathic copolymers. The dissertation comprises 4 experimental chapters. In the first chapter methods are described to hydrolyze styrene-maleic anhydrides to the active form, the styrene-maleic acid (SMA) and recommendations are given on how to do this efficiently. In the second chapter the influence of environmental factors on the efficiency of solubilization is investigated, including effects of pH, salt concentration, temperature and incubation time. An unexpected pH dependence was found and a model is proposed to explain this effect. The third and fourth chapters zoom in on the importance of the properties of the polymers themselves. Conventionally, SMA polymers have a wide variation in length and composition within a given batch. In the third chapter the importance is investigated of having a well-defined polymer length and composition. The polymers were synthesized according to a new method and tested in model membrane systems by various techniques. In the fourth chapter, the synthesis is described of a library of well-defined polymers, in which the hydrophobicity, flexibility, size and position of substituents was varied. The solubilization efficiency of the polymers was tested on both biological and model membranes. The library provides important new insights into the role of different polymer parameters. The summarizing discussion provides some new insights into the mode of action of the polymers and a future outlook for the further development of polymers and their potential significance for membrane protein studies.