Effect of Styrene Maleic Acid Copolymer Length on Biological Membrane Solubilisation and Properties of Native Nanodiscs

Abstract

The styrene-maleic acid copolymer (SMA) has proven to be a valuable tool in the study of membrane proteins because it can be used to isolate membrane proteins in native nanodiscs. Of the commercially available SMA copolymers, the one with the smallest average weight (Xiran 30010) was shown to be the most efficient for biological membrane solubilisation. However, Xiran 30010 still has a relatively large average size and a broad size distribution. Less polydisperse and smaller SMA fragments have been isolated and tested on phospholipid vesicles(Domínguez Pardo et al., 2018) but not on biological membranes. Here we prepared such smaller SMA fragments to assess the amount of solubilisation of E. coli membranes with over-expressed KcsA by turbidity assays and SDS-PAGE analysis. The size of the native nanodiscs was analyzed by SEC, EM and dynamic light scattering and the stability of proteins in the nanodiscs was studied using spectroscopic methods. The results show that there is an optimal size for membrane solubilisation, which is different from the situation in model systems where the smallest polymers are better solubilisers. The results provide new insights into the mode of action of SMA copolymers and will be helpful for optimising conditions to solubilise and characterise membrane proteins in their native environment.