Mass photometry and its expanding role in membrane protein research: The Torpedo marmorata nicotinic acetylcholine receptor as case study.

Integral membrane proteins (IMPs) are challenging, yet highly relevant targets for structural biology. Selection of the appropriate membrane mimic environment necessitates extensive optimization using a combination of biochemical and biophysical techniques. We report here on the expanding role of mass photometry (MP) as an in-solution technique to enable single-particle analysis of IMPs. To showcase MP versatility, we present here a selection of examples of MP applications for IMP analysis, using nicotinic acetylcholine receptor (nAChR) extracted from Torpedo marmorata as case study, together with additional literature examples. We further illustrate how MP supports the screening of purification conditions, and the quality control of samples prior to cryo-electron microscopy grid preparation or native mass spectrometry. The benefits of MP for IMP characterization are illustrated through its ability to quantify single-molecule populations, study membrane biomolecular assemblies, and characterize membrane-associated phenomena from very low amounts of starting material. MP therefore stands out as a unique biophysical tool that combines rapid analysis with broad adaptability to most solubilizing environments, enabling simultaneous relative quantification of all detected species in a highly user-friendly manner.