The effectiveness of styrene-maleic acid (SMA) copolymers for solubilisation of integral membrane proteins from SMA-accessible and SMA-resistant membranes

David J.K Swainsbury,Stefan Scheidelaar,Nicholas Foster,Rienk Van Grondelle,J Antoinette Killian,Michael R Jones

doi:10.1016/j.bbamem.2017.07.011

David J.K Swainsbury, Stefan Scheidelaar + Show 4 more

Open Access

https://doi.org/10.1016/j.bbamem.2017.07.011

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Abstract

Solubilisation of biological lipid bilayer membranes for analysis of their protein complement has traditionally been carried out using detergents, but there is increasing interest in the use of amphiphilic copolymers such as styrene maleic acid (SMA) for the solubilisation, purification and characterisation of integral membrane proteins in the form of protein/lipid nanodiscs. Here we survey the effectiveness of various commercially-available formulations of the SMA copolymer in solubilising Rhodobacter sphaeroides reaction centres (RCs) from photosynthetic membranes. We find that formulations of SMA with a 2:1 or 3:1 ratio of styrene to maleic acid are almost as effective as detergent in solubilising RCs, with the best solubilisation by short chain variants (<30kDa weight average molecular weight). The effectiveness of 10kDa 2:1 and 3:1 formulations of SMA to solubilise RCs gradually declined when genetically-encoded coiled-coil bundles were used to artificially tether normally monomeric RCs into dimeric, trimeric and tetrameric multimers. The ability of SMA to solubilise reaction centre-light harvesting 1 (RC-LH1) complexes from densely packed and highly ordered photosynthetic membranes was uniformly low, but could be increased through a variety of treatments to increase the lipid:protein ratio. However, proteins isolated from such membranes comprised clusters of complexes in small membrane patches rather than individual proteins. We conclude that short-chain 2:1 and 3:1 formulations of SMA are the most effective in solubilising integral membrane proteins, but that solubilisation efficiencies are strongly influenced by the size of the target protein and the density of packing of proteins in the membrane.

Highlights

Styrene—maleic acid (SMA) is a copolymer of styrene and maleic acid moieties that shows great promise as an alternative to detergents for the solubilisation, purification and characterisation of integral membrane proteins [1,2,3]
In previous work [8] we have shown that SMA can be used to extract and purify the reaction centre (RC) pigment-protein from membranes from a strain of the purple photosynthetic bacterium Rhodobacter (R.) sphaeroides lacking both types of light harvesting “antenna” complex [22]
As described in Materials and Methods, all eight copolymers listed in Table 1 were tested for their abilities to solubilise His-tagged wild-type RC complexes from membranes prepared from a strain of R. sphaeroides that lacks the LH1 and LH2 light harvesting proteins

Summary

Introduction

Styrene—maleic acid (SMA) is a copolymer of styrene and maleic acid moieties that shows great promise as an alternative to detergents for the solubilisation, purification and characterisation of integral membrane proteins [1,2,3]. Unlike detergents, which tend to strip away most or all of the lipids in the immediate environment of a membrane protein, SMA extracts proteins in the form of a lipid/protein nanodisc [4]. These typically range in size from 10 to 15 nm, and estimates of the number of lipids they contain have ranged from 11 to 150 (see [2] for a review). SMA has been used to prepare multicomponent membrane protein complexes that are not stable in detergent [15,16], and to investigate transient associations of membrane proteins during the formation of metabolons in plant cell membranes [17]

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