From polymer chemistry to structural biology: The development of SMA and related amphipathic polymers for membrane protein extraction and solubilisation
Section snippets
Interactions between lipid membranes and amphipathic polymers
The amphipathic homopolymer poly(2-ethacrylic acid) (PEAA) (Fig. 1) was shown by Tirrell and colleagues in the late 1980s to disrupt liposomes of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and to release their contents in a pH-dependent manner (Borden et al., 1987). Using electron microscopy (EM), the authors were able to observe the formation of disc-like nanoparticles, of similar appearance to the lipid-containing structures formed by lipoproteins, specifically HDL (Borden et al., 1987
The development of SMA polymers for protein extraction from biological membranes
Experiments on detergent-solubilized integral membrane proteins have played a fundamental role in helping to understand structures and functional mechanisms. Nevertheless, micelles are poor membrane mimetics as their lateral pressure profile differs considerably from that of a bilayer environment (Cantor, 1999; Marsh, 2007). Moreover, detergents may remove or perturb annular lipids, that have a direct influence on protein function (Charalambous et al., 2008). The identification of detergents
Applications of SMA-lipid nanoparticles for membrane protein characterisation
In the autumn of 2005, the Malvern group began a collaborative project with Overduin’s group at the University of Birmingham (CTB-TTF project (Grant, 2005)) to investigate the ability of SMA copolymers to solubilise membrane proteins (in particular PagP, an eight-stranded beta-barrel outer membrane enzyme) and to characterise biophysical and biochemical properties of the resultant structures using high-field NMR spectroscopy. This work depended upon identification of a low-temperature technique
Mechanism of lipid membrane solubilisation by SMA polymers
Existing models for the process of formation of SMA nanoparticles envisage the polymer in the form of an extended chain conformation which interacts with a portion of lipid bilayer in a similar manner to that of a “cookie-cutter” in which rings of elongated polymer strands encircle 100–200 lipids, allowing the hydrophobic groups to cut out a discoidal section (Parmar et al., 2016).
A recent computational study has sought to simulate the initial interaction between polymer and model membrane,
Salt- and pH-tolerant polymers for membrane protein extraction
A limitation of SMA is its incompatibility with high concentrations of divalent cations including Ca2+ and Mg2+, which may be chelated by the carboxylate groups thereby causing the polymer to precipitate (Scheidelaar et al., 2015; Dörr et al., 2016; Lee et al., 2016b). Similarly, a decrease in pH below the apparent pKa values of the carboxylic acid groups may also result in nanoparticle aggregation and polymer precipitation in the case of 2:1 and 3:1 copolymers of SMA, a limitation that can be
Lipodisq nanoparticles for drug delivery
Lipodisq nanoparticles have potential for drug delivery purposes, as a consequence of their stability upon dilution (unlike conventional micelles or bicelles) and their nano-molecular dimensions, make them ideal candidates for interaction with sub-cellular organelles. Recently, Tanaka and colleagues have performed a biodistribution study of SMA nanoparticles using radiolabelled probes showing that lipid-only Lipodisq nanoparticles behave similarly to apolipoprotein-based lipid nanodiscs (Tanaka
Labelling of a thiol derivative of SMA (SMA-SH)
Biodistribution studies of nanoparticles often require the addition of dye markers and the modular structure of a Lipodisq nanoparticle enables fluorophores and other labels to be attached by covalent conjugation (either to lipids or to the polymer itself) without altering their size. For polymer labelling, SMA-SH (an SMA analogue carrying thiol groups) allows conjugation via maleimide groups for fluorescence microscopy and biophysical experiments such as FRET (Lindhoud et al., 2016). Briefly,
Conclusion
SMA-based Lipodisq nanoparticles (also known as SMALPs) are highly versatile self-assembling membrane mimetics, suitable for a wide range of structural and functional biomembrane studies. The lipid environment may be changed and designed as required, and further development will undoubtedly result in improved formulations based on the initial concepts described here. The monodisperse nature of Lipodisq nanoparticles allows them to be used in spectroscopy (NMR, ESR, UV–vis, CD), diffraction and
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