Abstract
Styrene-maleic acid copolymers (SMA) have been gaining interest in the field of membrane research due to their ability to solubilize membranes into nanodics. The SMA molecules act as an amphipathic belt that surrounds the nanodiscs, whereby the hydrophobic styrene moieties can insert in between the lipid acyl chains. Here we used SMA variants with different styrene-to-maleic acid ratio (i.e. 2:1, 3:1 and 4:1) to investigate how lipid packing in the nanodiscs is affected by the presence of the polymers and how it depends on polymer composition. This was done by analyzing the thermotropic properties of a series of saturated phosphatidylcholines in nanodiscs using laurdan fluorescence and differential scanning calorimetry. In all cases it was found that the temperature of the main phase transition (Tm) of the lipids in the nanodiscs is downshifted and that its cooperativity is strongly reduced as compared to the situation in vesicles. These effects were least pronounced for lipids in nanodiscs bounded by SMA 2:1. Unexpected trends were observed for the calorimetric enthalpy of the transition, suggesting that the polymer itself contributes, possibly by rearranging around the nanodiscs when the lipids adopt the fluid phase. Finally, distinct differences in morphology were observed for nanodiscs at relatively high polymer concentrations, depending on the SMA variant used. Overall, the results suggest that the extent of preservation of native thermodynamic properties of the lipids as well as the stability of the nanodiscs at high polymer concentrations is better for SMA 2:1 than for the other SMA variants.
Highlights
Styrene-maleic acid copolymers (SMA) are rapidly gaining interest as tools in membrane protein research due to their capacity of solubilizing biological membranes into nanodiscs without the need for detergents
To allow comprehensive and systematic comparison, we studied a series of saturated phosphatidylcholine lipids in SMA-nanodiscs by laurdan fluorescence and differential scanning calorimetry (DSC) analysis
Laurdan probes are extremely sensitive to solvent relaxation and can be used to track changes in membrane fluidity based on a shift in emission maximum
Summary
Styrene-maleic acid copolymers (SMA) are rapidly gaining interest as tools in membrane protein research due to their capacity of solubilizing biological membranes into nanodiscs without the need for detergents. These so-called “native nanodiscs” enclose membrane proteins embedded in a lipid bilayer (for review see (Dörr et al, 2016)), while retaining native protein-lipid interactions (Dörr et al, 2014; Long et al, 2013; Prabudiansyah et al, 2015; Swainsbury et al, 2014). Similar results were reported for solubilization of proteins from E.coli (Morrison et al, 2016)
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