Empirically, α-helical membrane protein folding stability in surfactant micelles can be tuned by varying the mole fraction MF^SDS of anionic (sodium dodecyl sulfate (SDS)) relative to nonionic (e.g., dodecyl maltoside (DDM)) surfactant, but we lack a satisfying physical explanation of this phenomenon. Cysteine labeling (CL) has thus far only been used to study the topology of membrane proteins, not their stability or folding behavior. Here, we use CL to investigate membrane protein folding in mixed DDM-SDS micelles. Labeling kinetics of the intramembrane protease GlpG are consistent with simple two-state unfolding-and-exchange rates for seven single-Cys GlpG variants over most of the explored MF^SDS range, along with exchange from the native state at low MF^SDS (which inconveniently precludes measurement of unfolding kinetics under native conditions). However, for two mutants, labeling rates decline with MF^SDS at 0–0.2 MF^SDS (i.e., native conditions). Thus, an increase in MF^SDS seems to be a protective factor for these two positions, but not for the five others. We propose different scenarios to explain this and find the most plausible ones to involve preferential binding of SDS monomers to the site of CL (based on computational simulations) along with changes in size and shape of the mixed micelle with changing MF^SDS (based on SAXS studies). These nonlinear impacts on protein stability highlights a multifaceted role for SDS in membrane protein denaturation, involving both direct interactions of monomeric SDS and changes in micelle size and shape along with the general effects on protein stability of changes in micelle composition.

根据经验，表面活性剂胶束中的α-螺旋膜蛋白折叠稳定性可以通过改变阴离子（十二烷基硫酸钠（SDS））相对于非离子（例如，十二烷基麦芽糖苷（DDM））表面活性剂的摩尔分数MF ^SDS来调节，但我们缺乏对这一现象的令人满意的物理解释。迄今为止，半胱氨酸标记 (CL) 仅用于研究膜蛋白的拓扑结构，而不是它们的稳定性或折叠行为。在这里，我们使用 CL 研究混合 DDM-SDS 胶束中的膜蛋白折叠。^{膜内蛋白酶 GlpG 的标记动力学与在大多数已探索的 MF SDS}范围内的七个单半胱氨酸 GlpG 变体的简单二态展开和交换率一致，以及在低 MF 下从天然状态交换^SDS（​​不方便地排除在天然条件下展开动力学的测量）。然而，对于两个突变体，标记率随着 MF ^SDS在 0-0.2 MF ^SDS（即天然条件）下下降。因此，MF ^SDS的增加似乎是这两个职位的保护因素，但不是其他五个职位。我们提出了不同的方案来解释这一点，并找到最合理的方案涉及 SDS 单体优先结合到 CL 位点（基于计算模拟）以及混合胶束的大小和形状随 MF ^{SDS的变化而变化}（基于 SAXS 研究）。这些对蛋白质稳定性的非线性影响突出了 SDS 在膜蛋白变性中的多方面作用，包括单体 SDS 的直接相互作用和胶束大小和形状的变化以及胶束组成变化对蛋白质稳定性的一般影响。