Mirroring the charged termini; ETD/HCD fragmentation characteristics of LysargiNase and tryptic peptides and their benefits for peptide sequencing in proteomics

A key step in proteomics is the digestion of proteins into peptides, so far largely done by using trypsin. Tryptic digestion leads to peptides that in ESI-MS attain predominantly two charges, via protonation at the free N-terminus and at the C-terminal basic residue Arginine or Lysine. These peptides can be readily sequenced and identified by collision-induced dissociation (CID) or higher-energy collisional dissociation (HCD), as the fragmentation rules are well understood. Here we explore the trypsin mirror protease, LysargiNase, which cleaves equally specifically at Arg and Lys, albeit at the N-terminal end. The resulting peptides are therefore practically tryptic-alike in length and sequence, except that the two charges are now both positioned at the N-terminus. We compare the chromatographic separation properties, gas phase fragmentation behavior and (phospho)proteome sequence coverage of tryptic and LysargiNase peptides using electron-transfer dissociation (ETD), and for comparison HCD. We find that tryptic and LysargiNase peptides fragment nearly as mirror images. For LysargiNase predominantly N-terminal peptide ions (c-ions/ETD, b-ions/HCD) are formed, whereas for trypsin C-terminal fragment ions dominate (z-ions/ETD, y-ions/HCD). Especially during ETD LysargiNase peptides fragment into low-complexity, but information rich sequence ladders. We observe that trypsin and LysargiNase chart distinct parts of the (phospho)proteome. Therefore, we conclude that the collective use LysargiNase and Trypsin will benefit a more in-depth and reliable analysis of (phospho)proteomes.