A magnetic bead-based workflow for sensitive and streamlined cell surface proteomics

Cell-surface proteins represent an important class of molecules for therapeutic targeting and defining cellular phenotypes. However, their enrichment and detection via mass spectrometry-based proteomics remains challenging due to low abundance, posttranslational modifications, hydrophobic regions, and processing requirements. To improve the identification of cell-surface proteins via their corresponding N-linked glycopeptides (N-glycopeptides), we optimized a Cell-Surface Capture (CSC) workflow which incorporates magnetic bead-based processing. Using this approach, we evaluated labeling conditions (biotin tags and catalysts), enrichment specificity (streptavidin beads), missed cleavages (lysis buffers), non-enzymatic deamidation (digestion and de-glycosylation buffers), and data acquisition methods. Our findings support the use of alkoxyamine-PEG4-biotin plus 5-methoxy-anthranilic acid and streptavidin magnetic beads for maximal N-glycopeptide detection. Furthermore, single-pot solid-phased-enhanced sample-preparation (SP3) circumvented the need to isolate cell membranes by affording the use of strong detergents and chaotropes for protein extraction. Notably, with semi-automated processing, sample handling was simplified and between ~600-900 N-glycoproteins were identified from only 25-200µg of HeLa protein. Overall, the improved efficiency of the magnetic-based CSC workflow allowed us to identify both previously reported and novel N-glycosites with less material and high reproducibility, and should help advance the field of surfaceomics by providing insight in cellular phenotypes not previously documented.