Fractionation of Styrene and Maleic Acid Copolymers: The “Holy Grail” of Membrane Solubilization

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Styrene-and-maleic acid copolymers (SMA) are gaining interest in membrane protein research due to their ability of solubilizing lipid membranes into nanodiscs. Using different commercially available SMA preparations it was found that the average length and hydrophobicity are important parameters in determining solubilization efficiency and properties of the resulting nanodiscs. However, our understanding of the underlying SMA-lipid interactions is hampered by the fact that SMA preparations are very polydisperse. Here, we fractionated widely used SMA preparations (Mw ∼10kD) into 4 fractions of polymers with different average length, based on their solubility in hexane/acetone mixtures and we investigated their interaction with lipid self-assemblies. We found that fractions with shorter SMA polymers solubilize phosphatidylcholine vesicles more rapidly than fractions with longer polymers. On the other hand, nanodiscs bounded by long SMA polymers were notably more stable than those bounded by shorter polymers, as indicated by a better retention of the native lipid thermotropic properties and by slower exchange rates between lipids in nanodiscs. When the SMA fractions were mixed with E.coli membranes, a somewhat different behavior was observed, probably due to the complexity of these membranes. Here the fractions with relatively long polymers were found to be most efficient in solubilizing the tetrameric KcsA channel. Competitive solubilization experiments showed that short polymers even hinder solubilization by long polymers. In conclusion, we here present a simple method to purify SMA fractions from commercial SMA in order to improve solubilization of biomembranes as well as stability of the resulting nanodiscs.