There are aminoacyl transferases, such as arginyl tRNA protein transferase 1 (Ate1) in Eukaryotes and leucyl/phenylalanyl tRNA protein transferase (L/F transferase) in Bacteria. Both enzymes recognize specific N-terminal amino acids of a target poly-peptide and conjugate an amino acid from an aminoacylated tRNA onto the N-terminus of the targeted poly-peptide.Ate1 conjugates an arginyl moiety onto the N-terminus of a poly-peptide. Electrospray ionization of peptides critically depends on arginyl moieties to stabilize proton charges to the peptide. Taking both facts together, there is a huge untapped potential for Ate1 in the workflow of mass spectrometry facilities by providing an additional proton charge-stabilizing moiety to a peptide's N-terminus.With my proposed project I further aim to harvest the ability of L/F transferase to conjugate a single amino acid, including unnatural amino acids, to the N-terminus of specific target poly-peptide and covalently tag the N-terminus of proteins. The proteins of interest are proteolysis fragments, which will be positively enriched by taking advantage of the unnatural amino acid moiety and analyzed using mass spectrometry. The monitoring of proteolysis products has far reaching implications as proteolysis is an important causal or progression factor in various diseases such as neurodegenerative diseases, chronic inflammation, cancer and heart disease.During my post-doctoral studies at the University of Alberta, Canada, I characterized the kinetic parameters of both L/F transferase and Ate1 extensively. As Marie Curie International Incoming Fellow, I will build on my previous knowledge in a team of biochemists, cell biologists, computational biologists, and mass spectrometrists to establish sound methodologies for clinical and basic researchers alike and will assist in transferring newly developed methodologies in other mass spectrometry facilities in Europe.