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.