Mass spectrometry of membrane transporters reveals subunit stoichiometry and interactions.

We describe a general mass spectrometry approach to determine subunit stoichiometry and lipid binding in intact membrane protein complexes. By exploring conditions for preserving interactions during transmission into the gas phase and for optimally stripping away detergent, by subjecting the complex to multiple collisions, we released the intact complex largely devoid of detergent. This enabled us to characterize both subunit stoichiometry and lipid binding in 4 membrane protein complexes.