Abstract
Membrane proteins are traditionally extracted and purified in detergent for biochemical and structural characterisation. This process is often costly and laborious, and the stripping away of potentially stabilising lipids from the membrane protein of interest can have detrimental effects on protein integrity. Recently, styrene-maleic acid (SMA) co-polymers have offered a solution to this problem by extracting membrane proteins directly from their native membrane, while retaining their naturally associated lipids in the form of stable SMA lipid particles (SMALPs). However, the inherent nature and heterogeneity of the polymer renders their use challenging for some downstream applications – particularly mass spectrometry (MS). While advances in cryo-electron microscopy (cryo-EM) have enhanced our understanding of membrane protein:lipid interactions in both SMALPs and detergent, the resolution obtained with this technique is often insufficient to accurately identify closely associated lipids within the transmembrane annulus. Native-MS has the power to fill this knowledge gap, but the SMA polymer itself remains largely incompatible with this technique. To increase sample homogeneity and allow characterisation of membrane protein:lipid complexes by native-MS, we have developed a novel SMA-exchange method; whereby the membrane protein of interest is first solubilised and purified in SMA, then transferred into amphipols or detergents. This allows the membrane protein and endogenously associated lipids extracted by SMA co-polymer to be identified and examined by MS, thereby complementing results obtained by cryo-EM and creating a better understanding of how the lipid bilayer directly affects membrane protein structure and function.
Highlights
Despite their physiological importance, our structural, biochemical and biophysical understanding of membrane proteins in vivo is limited
A further advantage is that it allows native lipids to be carried through the initial protein solubilisation/purification steps, which enhances the stability of the membrane protein in solution
As detergents and amphipols are not required in the membrane protein preparation until the final exchange step, it can significantly reduce the costs commonly associated with standard detergent purifications
Summary
Our structural, biochemical and biophysical understanding of membrane proteins in vivo is limited. This is largely due to the challenges associated with their extraction and subsequent instability outside of their native lipid environment. The detergent remains present throughout all stages of the purification, but can be later exchanged for a different detergent or a more suitable detergent system/ solubilisation platform for downstream experimentation. A number of alternative reconstitution platforms have been developed to combat these detergent-associated issues, such as membrane scaffold protein nanodiscs [1,2], amphipols [3,4], peptidiscs [5], bicelles [6] and liposomes [7]; but all still require an initial detergent solubilisation step, often resulting in reduced membrane protein activity and/or detrimental structural perturbations [8,9,10,11,12]
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