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.

中文翻译：

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影响苯乙烯-马来酸共聚物溶解大肠杆菌膜蛋白的膜蛋白的因素。

苯乙烯-马来酸（SMA）共聚物是用于膜蛋白增溶的去污剂的有前途的替代品。在这里，我们采用含有KcsA作为模型蛋白的大肠杆菌膜来研究不同环境条件对SMA增溶效率的影响。我们表明，SMA的浓度，温度，孵育时间，离子强度，二价阳离子的存在和pH值均会影响SMA提取的蛋白质量。观察到的效果与仅脂质模型膜系统的观察值一致，除了pH值的影响。根据光密度（OD）测量，发现将pH从7增加到9可导致大肠杆菌膜的增溶产率增加，而在仅脂质模型系统中，pH值在相同pH范围内降低。在OD实验中观察到相似的相反的pH依赖效应，该实验比较了天然酵母膜和仅酵母脂质膜的增溶作用。我们提出了一个模型，其中pH依赖的静电相互作用影响聚合物与膜蛋白膜外部分的结合，进而影响聚合物对膜增溶的可用性。该模型得到以下观察结果的支持：对于阴离子去污剂SDS观察到与SMA类似的pH依赖性，但对于非离子去污剂DDM则观察不到，并且通过增加SMA浓度可以大大克服pH依赖性。该结果可作为指导准则，以得出SMA溶解生物膜的最佳条件。