Integral membrane proteins (IMPs) are biologically highly significant but challenging to study because they require maintaining a cellular lipid-like environment. Here, we explore the application of mass photometry (MP) to IMPs and membrane-mimetic systems at the single-particle level. We apply MP to amphipathic vehicles, such as detergents and amphipols, as well as to lipid and native nanodiscs, characterizing the particle size, sample purity, and heterogeneity. Using methods established for cryogenic electron microscopy, we eliminate detergent background, enabling high-resolution studies of membrane-protein structure and interactions. We find evidence that, when extracted from native membranes using native styrene-maleic acid nanodiscs, the potassium channel KcsA is present as a dimer of tetramers—in contrast to results obtained using detergent purification. Finally, using lipid nanodiscs, we show that MP can help distinguish between functional and non-functional nanodisc assemblies, as well as determine the critical factors for lipid nanodisc formation.

整体膜蛋白（IMP）在生物学上非常重要，但由于需要维持细胞脂质样环境，因此具有挑战性。在这里，我们探索了在单粒子水平上，质谱仪（MP）在IMP和膜模拟系统中的应用。我们将MP应用于两亲性载剂（例如去污剂和两性霉素）以及脂质和天然纳米圆盘，以表征粒径，样品纯度和异质性。使用为低温电子显微镜建立的方法，我们消除了去污剂的背景，从而能够对膜-蛋白质结构和相互作用进行高分辨率研究。我们发现有证据表明，使用天然苯乙烯-马来酸纳米片从天然膜中提取时，钾通道KcsA以四聚体的二聚体形式存在-与使用洗涤剂纯化获得的结果相反。最后，使用脂质纳米光盘，我们表明MP可以帮助区分功能性和非功能性纳米光盘组件，以及确定脂质纳米光盘形成的关键因素。