Characterization of styrene-maleic acid copolymer membrane solubilization using reductionist and mitochondrial systems

Membrane-bound proteins (MBPs) have immense significance because they perform inherently refractory to isolation and to structural and functional characterization due to the instability of their hydrophobic domains outside of their native membranes. At the leading edge of membrane research are styrene-maleic acid (SMA) polymers that encapsulate membrane protein complexes into SMA lipoprotein particles (SMALPs) in their native and functionally active states. In this study, we employed an approach where we used reductionist and mitochondrial systems in tandem to elucidate conditions for optimal SMA extraction. The reductionist system was composed of large unilamellar vesicles (LUV) that were solubilized by SMA and the resulting products were analyzed by dynamic light scattering and transmission-electron microscopy. LUVs with distinct lipid compositions were used to analyze the effect of surface charge, lateral pressure, head group structure, degree of unsaturation, and chain length on SMA solubilization. We found that each of these physical parameters had an effect on SMA solubilization; however, increased lateral lipid pressure appeared to have the greatest inhibitory effect. The mitochondrial system entailed SMA being added to isolated mitochondria and probing for SMALP-encapsulated proteins in supernatants by Western blotting. To this end, mitochondria were titrated with SMA and one representative protein complex from different membrane compartments (outer membrane, inner boundary membrane, and cristae) were tested for using Western blotting. Our results on this subset of proteins suggests that SMA solubilization efficiency may be determined by submitochondrial localization of MBPs. Taken together, our work provides new insights into the mechanism by which SMA solubilizes lipid-only systems as well as cellular membranes.