Membrane Solubilization by Styrene-Maleic Acid Copolymers: Delineating the Role of Polymer Length

Styrene-maleic acid (SMA) copolymers have attracted interest in membrane research because they allow the solubilization and purification of membrane-spanning proteins from biological membranes in the form of native-like nanodisks. However, our understanding of the underlying SMA-lipid interactions is hampered by the fact that SMA preparations are very polydisperse. Here, we obtained fractions of the two most commonly used SMA preparations: SMA 2:1 and SMA 3:1 (both with specified M(w) ∼10 kD), with different number-average molecular weight (M(n)) and styrene content. The fractionation is based on the differential solubility of styrene-maleic anhydride (SMAnh) in hexane and acetone mixtures. SMAnh fractions were hydrolyzed to SMA and added to lipid self-assemblies. It was found that SMA fractions inserted in monolayers and solubilized vesicles to a different extent, with the highest efficiency being observed for low-M(n) SMA polymers. Electron microscopy and dynamic light scattering size analyses confirmed the presence of nanodisks independent of the M(n) of the SMA polymers forming the belt, and it was shown that the nanodisks all have approximately the same size. However, nanodisks bounded by high-M(n) SMA polymers were more stable than those bounded by low-M(n) polymers, as indicated by a better retention of the native lipid thermotropic properties and by slower exchange rates of lipids between nanodisks. In conclusion, we here present a simple method to separate SMAnh molecules based on their M(n) from commercial SMAnh blends, which allowed us to obtain insights into the importance of SMA length for polymer-lipid interactions.