PRMT1 Is the Predominant Type I Protein Arginine Methyltransferase in Mammalian Cells*

Type I protein arginine methyltransferases catalyze the formation of asymmetric ω-N G,N G-dimethylarginine residues by transferring methyl groups fromS-adenosyl-l-methionine to guanidino groups of arginine residues in a variety of eucaryotic proteins. The predominant type I enzyme activity is found in mammalian cells as a high molecular weight complex (300–400 kDa). In a previous study, this protein arginine methyltransferase activity was identified as an additional activity of 10-formyltetrahydrofolate dehydrogenase (FDH) protein. However, immunodepletion of FDH activity in RAT1 cells and in murine tissue extracts with antibody to FDH does not diminish type I methyltransferase activity toward the methyl-accepting substrates glutathione S-transferase fibrillarin glycine arginine domain fusion protein or heterogeneous nuclear ribonucleoprotein A1. Similarly, immunodepletion with anti-FDH antibody does not remove the endogenous methylating activity for hypomethylated proteins present in extracts from adenosine dialdehyde-treated RAT1 cells. In contrast, anti-PRMT1 antibody can remove PRMT1 activity from RAT1 extracts, murine tissue extracts, and purified rat liver FDH preparations. Tissue extracts from FDH(+/+), FDH(+/−), and FDH(−/−) mice have similar protein arginine methyltransferase activities but high, intermediate, and undetectable FDH activities, respectively. Recombinant glutathioneS-transferase-PRMT1, but not purified FDH, can be cross-linked to the methyl-donor substrateS-adenosyl-l-methionine. We conclude that PRMT1 contributes the major type I protein arginine methyltransferase enzyme activity present in mammalian cells and tissues.