GeLC-FAIMS-MS Workflow for In-Depth Middle-Down Proteomics

Middle-down proteomics is an analytical approach whereby protein samples are digested with proteases such as Glu-C to generate large peptides (>3 kDa) that are then analyzed by mass spectrometry. This method is useful for characterizing high-molecular-weight proteins that are difficult to detect by top-down proteomics, in which intact proteins are analyzed by mass spectrometry. In this study, we applied GeLC-FAIMS-MS, a multidimensional separation workflow that combines gel-based prefractionation with LC-FAIMS Orbitrap mass spectrometry, for deep middle-down proteomics. Middle-down peptides obtained under the condition of optimized limited Glu-C digestion were first size-fractionated by polyacrylamide gel electrophoresis, followed by C4 reversed-phase liquid chromatography separation and additional ion mobility fractionation, resulting in a significant increase in peptide length detectable by mass spectrometry. In addition to global analysis, the GeLC-FAIMS concept can also be applied to targeted middle-down proteomics, where only proteins in the desired molecular weight range are gel-fractionated and their Glu-C digestion products are analyzed, as demonstrated by targeted analysis of integrins in exosomes. In-depth middle-down proteomics achieved by global and targeted GeLC-FAIMS-MS allows the exploration of proteoform information not covered by conventional top-down proteomics via increase in the number of detectable proteoforms and improvement in sequence coverage.