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 Mw ∼10 kD), with different number-average molecular weight (Mn) 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-Mn SMA polymers. Electron microscopy and dynamic light scattering size analyses confirmed the presence of nanodisks independent of the Mn 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-Mn SMA polymers were more stable than those bounded by low-Mn 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 Mn from commercial SMAnh blends, which allowed us to obtain insights into the importance of SMA length for polymer-lipid interactions.

Solubilization of lipids and lipid phases by the styrene-maleic acid copolymer.

A promising tool in membrane research is the use of the styrene-maleic acid (SMA) copolymer to solubilize membranes in the form of nanodiscs. Since membranes are heterogeneous in composition, it is important to know whether SMA thereby has a preference for solubilization of either specific types of lipids or specific bilayer phases. Here, we investigated this by performing partial solubilization of model membranes and analyzing the lipid composition of the solubilized fraction. We found that SMA displays no significant lipid preference in homogeneous binary lipid mixtures in the fluid phase, even when using lipids that by themselves show very different solubilization kinetics. By contrast, in heterogeneous phase-separated bilayers, SMA was found to have a strong preference for solubilization of lipids in the fluid phase as compared to those in either a gel phase or a liquid-ordered phase. Together the results suggest that (1) SMA is a reliable tool to characterize native interactions between membrane constituents, (2) 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 and (3) exploiting SMA resistance rather than detergent resistance may be an attractive approach for the isolation of ordered domains from biological membranes.