δ-N-methylarginine is a novel posttranslational modification of arginine residues in yeast proteins

We have found a novel modification of protein arginine residues in the yeast Saccharomyces cerevisiae, Intact yeast cells lacking RIMT1, the gene encoding the protein omega N-G-arginine methyltransferase, were labeled with the methyl donor S-adenosyl-L-[methyl-H-3]methionine, The protein fraction was acid-hydrolyzed to free amino acids, which were then fractionated on a high resolution sulfonated polystyrene cation exchange column at pH 5.27 and 55 degrees C, In the absence of the omega-N-G,(NL)-L-G-[H-3]dimethylarginine product of the RIMT1 methyltransferase, we were able to detect a previously obscured H-3-methylated species that migrated in the region of methylated arginine derivatives. The [H-3]methyl group(s) of this unknown species were not volatilized by treatment with 2 M NaOH at 55 degrees C for up to 48 h, suggesting that they were not modifications of the terminal omega-guanidino nitrogen atoms. However, this base treatment did result in the formation of a new H-3 methylated derivative that co-chromatographed with delta-N-methylornithhine on high resolution cation exchange chromatography, on reverse phase high pressure liquid chroma tography, and on thin layer chromatography, From these data, we suggest that the identity of the original unknown methylated residue is delta-N-monomethylarginine. The presence of this methylated residue in yeast cells defines a novel type of protein modification reaction in eukaryotes.