Identification of N-terminal protein processing sites by chemical labeling mass spectrometry

Abstract

Rationale Proteins undergo post‐translational modifications and proteolytic processing that can affect their biological function. Processing often involves the loss of single residues. Cleavage of signal peptides from the N‐terminus is commonly associated with translocation. Recent reports have suggested that other processing sites also exist. Methods The secreted proteins from S. aureus N315 were precipitated with trichloroacetic acid (TCA) and amidinated with S‐methyl thioacetimidate (SMTA). Amidinated proteins were digested with trypsin and analyzed with a high‐resolution orbitrap mass spectrometer. Results Sixteen examples of Staphylococcus aureus secretory proteins that lose an N‐terminal signal peptide during their export were identified using this amidination approach. The N‐termini of proteins with and without methionine were identified. Unanticipated protein cleavages due to sortase and an unknown protease were also uncovered. Conclusions A simple N‐terminal amidination based mass spectrometry approach is described that facilitates identification of the N‐terminus of a mature protein and the discovery of unexpected processing sites.