text: Upon the identification of the early targets of arginylation by ATE1 (in vitro and in vivo), a pattern emerged. This pattern showed that ATE1 displayed a high affinity for proteins and peptides containing the acidic amino acids asparagine or glutamine which were exposed on the N-terminal side of the protein or peptide. Further studies aided by high precision mass spectrometry have revealed hundreds of proteins from different cells and tissues which have been arginylated. Several of these proteins also displayed arginylation at their N-chain termini, but contained residues other than asparagine or glutamine. As such, arginylation studies are still in the introductory stages and further research into the specificity of arginylation must be performed.   
  However, the assumption that arginylation only occurs at the N-terminus severely limited the amount of proteins which were likely to be arginylated. This is due to the fact that, if the preference of arginylation to occur only at the N-terminus assumption was true, then arginylation would never be able to happen on intact proteins due to protein sequences beginning with methionine at the N-terminus and not the preferred asparagine or glutamine. This assumption was soon proved false when a protein was discovered with an arginylated residue in the middle of its sequence.