Styrene maleic-acid lipid particles (SMALPs) into detergent or amphipols: An exchange protocol for membrane protein characterisation

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 started to address this issue, as they can extract membrane protein complexes directly from the membrane alongside their native lipid environment (SMALPs). However, their use is not without limitations. The inherent nature of the polymer renders SMALPs challenging for some downstream applications – such as mass spectrometry (MS), which is a powerful tool for examining membrane protein:lipid interactions. While great progress has been made in the field of cryo-electron microscopy (EM), the resolution obtained is often insufficient to accurately identify closely associated lipids within the annulus. Native-MS has the potential 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 SMALP-exchange method; whereby the MP of interest is first solubilised and purified in SMALPs, then transferred into amphipols or detergents. This potentially allows endogenously associated lipids extracted by the 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.