Biological insights from SMA-extracted proteins

Abstract

In the twelve years since styrene maleic acid (SMA) was first used to extract and purify a membrane protein within a native lipid bilayer, this technological breakthrough has provided insight into the structural and functional details of protein–lipid interactions. Most recently, advances in cryo-EM have demonstrated that SMA-extracted membrane proteins are a rich-source of structural data. For example, it has been possible to resolve the details of annular lipids and protein–protein interactions within complexes, the nature of lipids within central cavities and binding pockets, regions involved in stabilising multimers, details of terminal residues that would otherwise remain unresolved and the identification of physiologically relevant states. Functionally, SMA extraction has allowed the analysis of membrane proteins that are unstable in detergents, the characterization of an ultrafast component in the kinetics of electron transfer that was not possible in detergent-solubilised samples and quantitative, real-time measurement of binding assays with low concentrations of purified protein. While the use of SMA comes with limitations such as its sensitivity to low pH and divalent cations, its major advantage is maintenance of a protein's lipid bilayer. This has enabled researchers to view and assay proteins in an environment close to their native ones, leading to new structural and mechanistic insights.

Signaling Mechanisms and Pharmacological Modulators Governing Diverse Aquaporin Functions in Human Health and Disease

“…However, the structures of the intracellular AQP termini and the quaternary structures of AQP-containing complexes are poorly understood. Recent developments in the solubilization of membrane proteins [582] and cryo-electron microscopy for structural biology [583] will likely support progress in this area.…”

Biophysical Characterization of Membrane Proteins Embedded in Nanodiscs Using Fluorescence Correlation Spectroscopy

“…Other varieties of nanodisc have been developed that use a co-polymer rather than lipid-binding proteins as the surrounding belt, as reviewed recently [34,49,50]. The most widely used polymer is styrene maleic acid (SMA), first shown to form nanodiscs on introduction to bacteriorhodopsin loaded liposomes by Knowles et al in 2009 [30].…”

The function of BK channels extracted and purified within SMALPs

“…For the recent cryo-EM structure of BK channel, the α subunit was truncated at the C-terminus to improve stability, and during purification cholesterol hemisuccinate and lipids had to be added [10]. In recent years, the development of SMA (styrene maleic acid) polymer as an alternative to detergents for membrane protein solubilisation has shown great promise for providing greater stability to membrane proteins [11][12][13][14]. Whereas detergent can disrupt protein:protein interactions, SMA often extracts complexes intact [15][16][17].…”