Abstract
Identification and sequence determination by mass spectrometry have become routine analyses for soluble proteins. Membrane proteins, however, remain challenging targets due to their hydrophobicity and poor annotation. In particular small membrane proteins often remain unnoticed as they are largely inaccessible to Bottom-Up proteomics. Recent advances in structural biology, though, have led to multiple membrane protein complex structures being determined at sufficiently high resolution to detect uncharacterized, small subunits. In this work we offer a guide for the mass spectrometric characterization of solvent extraction-based purifications of small membrane proteins isolated from protein complexes and cellular membranes. We first demonstrate our Top-Down MALDI-MS/MS approach on a Photosystem II preparation, analyzing target protein masses between 2.5 and 9 kDa with high accuracy and sensitivity. Then we apply our technique to purify and sequence the mycobacterial ATP synthase c subunit, the molecular target of the antibiotic drug bedaquiline. We show that our approach can be used to directly track and pinpoint single amino acid mutations that lead to antibiotic resistance in only 4 h. While not applicable as a high-throughput pipeline, our MALDI-MS/MS and ISD-based approach can identify and provide valuable sequence information on small membrane proteins, which are inaccessible to conventional Bottom-Up techniques. We show that our approach can be used to unambiguously identify single-point mutations leading to antibiotic resistance in mycobacteria.