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 RMT1, the gene encoding the protein ω-NG-arginine methyltransferase, were labeled with the methyl donorS-adenosyl-l-[methyl-3H]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 °C. In the absence of the ω-NG,NG-[3H]dimethylarginine product of the RMT1 methyltransferase, we were able to detect a previously obscured 3H-methylated species that migrated in the region of methylated arginine derivatives. The [3H]methyl group(s) of this unknown species were not volatilized by treatment with 2 m NaOH at 55 °C for up to 48 h, suggesting that they were not modifications of the terminal ω-guanidino nitrogen atoms. However, this base treatment did result in the formation of a new 3H-methylated derivative that co-chromatographed with δ-N-methylornithine on high resolution cation exchange chromatography, on reverse phase high pressure liquid chromatography, and on thin layer chromatography. From these data, we suggest that the identity of the original unknown methylated residue is δ-N-monomethylarginine. The presence of this methylated residue in yeast cells defines a novel type of protein modification reaction in eukaryotes.