Fig. 1

Surface proteome enrichment and direct detection of biotinylated peptides. A Experimental strategy. PC-3 cells were treated with a membrane-impermeant non-cleavable biotinylation reagent for preferential labeling of extracellular lysines. Proteins were harvested and digested prior to enrichment of biotinylated peptides using anti-biotin antibodies for LC–MS/MS analysis. B Annotated MS/MS spectrum of a representative biotinylated peptide mapped to the GPI-anchored cell surface protein 5′-nucleotidase (NT5E, CD73). The fragment ions in red are diagnostic of the label and localization of the site of biotinylation. C The number and overlap of biotinylated peptides identified from the three replicates