PRMT5 (Janus kinase-binding protein 1) catalyzes the formation of symmetric dimethylarginine residues in proteins

We have identified a new mammalian protein arginine N-methyltransferase, PRMT5, formerly designated Janus kinase-binding protein 1, that can catalyze the formation of omega -N-G-monomethylarginine and symmetric omega -N-G,N-G '-dimethylarginine in a variety of proteins. A hemagglutinin peptide-tagged PRMT5 complex purified from human HeLa cells catalyzes the S-adenosyl-L-[methyl-H-3]methionine-dependent in vitro methylation of myelin basic protein. When the radiolabeled myelin basic protein was acid-hydrolyzed to free amino acids, and the products were separated by high-resolution cation exchange chromatography, we were able to detect two tritiated species. One species co-migrated with a omega -N-G-monomethylarginine standard, and the other cochromatographed with a symmetric omega -N-G,N-G '-dimethylarginine standard. Upon base treatment, this second species formed methylamine, a breakdown product characteristic of symmetric omega -N-G,N-G '-dimethylarginine. Further analysis of these two species by thin layer chromatography confirmed their identification as omega -N-G-monomethylarginine and symmetric omega -N-G,N-G '-dimethylarginine. The hemagglutinin-PRMT5 complex was also able to monomethylate and symmetrically dimethylate bovine histone H2A and a glutathione S-transferase-fibrillarin (amino acids 1-148) fusion protein (glutathione S-transferase-GAR). A mutation introduced into the S-adenosyl-L-methionine-binding motif I of a myc-tagged PRMT5 construct in COS-1 cells led to a near complete loss of observed enzymatic activity. PRMT5 is the first example of a catalytic chain for a type II protein arginine N-methyltransferase that can result in the formation of symmetric dimethylarginine residues as observed previously in myelin basic protein, Sm small nuclear ribonucleoproteins, and other polypeptides.