Factors influencing the solubilization of membrane proteins from Escherichia coli membranes by styrene–maleic acid copolymers

Styrene–maleic acid (SMA) copolymers are a promising alternative to detergents for the solubilization of membrane proteins. Here we employ Escherichia coli membranes containing KcsA as a model protein to investigate the influence of different environmental conditions on SMA solubilization efficiency. We show that SMA concentration, temperature, incubation time, ionic strength, presence of divalent cations and pH all influence the amount of protein that is extracted by SMA. The observed effects are consistent with observations from lipid-only model membrane systems, with the exception of the effect of pH. Increasing pH from 7 to 9 was found to result in an increase of the solubilization yield of E. coli membranes, whereas in lipid-only model systems it decreased over the same pH range, based on optical density (OD) measurements. Similar opposite pH-dependent effects were observed in OD experiments comparing solubilization of native yeast membranes and yeast lipid-only membranes. We propose a model in which pH-dependent electrostatic interactions affect binding of the polymers to extramembraneous parts of membrane proteins, which in turn affects the availability of polymer for membrane solubilization. This model is supported by the observations that a similar pH-dependence as for SMA is observed for the anionic detergent SDS, but not for the nonionic detergent DDM and that the pH-dependence can be largely overcome by increasing the SMA concentration. The results are useful as guidelines to derive optimal conditions for solubilization of biological membranes by SMA.