Insights into membrane solubilization by styrene-maleic acid copolymers

Abstract

Styrene-and-maleic acid copolymers (SMA) are gaining interest in membrane protein research due to their ability of solubilizing lipid membranes into nanodiscs. Using commercially available SMA preparations it was found that SMA blends are fully promiscuous when mixed with membranes and that any solubilization preference of SMA is not due to properties of individual lipids but rather due to properties of the membrane or membrane domains in which these lipids reside. Furthermore, we demonstrate that the average length and hydrophobicity are important parameters in determining solubilization efficiency and properties of the resulting nanodiscs. Low Mn polymers were found to insert to a higher extent into lipid monolayers and solubilized most efficiently lipid vesicles. On the other hand, nanodiscs bounded by high Mn polymers were found to be more stable, as indicated by a better retention of the native lipid thermotropic properties and by slower exchange rates between lipids in nanodiscs. Stability was further improved by using polymer blends with a relatively low styrene content. Finally, it was found that SMA-bounded nanodiscs “breathe”, allowing expansion of the enclosed lipids. Hence, these nanodiscs most likely allow conformational changes of membrane proteins embedded in them. Overall, this thesis contains systematic studies that contribute to understanding, optimization and further development of a new procedure to reconstitute membrane proteins into nanodiscs: the SMA solubilization approach.