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, PR MT5, 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-tagge d PRMT5 complex purified from human HeLa cells catalyzes the S-adenosyl-L-[ methyl-H-3]methionine-dependent in vitro methylation of myelin basic protei n. When the radiolabeled myelin basic protein was acid-hydrolyzed to free a mino acids, and the products were separated by high-resolution cation excha nge chromatography, we were able to detect two tritiated species. One speci es co-migrated with a omega -N-G-monomethylarginine standard, and the other cochromatographed with a symmetric omega -N-G,N-G '-dimethylarginine stand ard. Upon base treatment, this second species formed methylamine, a breakdo wn product characteristic of symmetric omega -N-G,N-G '-dimethylarginine. F urther 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 glu tathione S-transferase-fibrillarin (amino acids 1-148) fusion protein (glut athione S-transferase-GAR). A mutation introduced into the S-adenosyl-L-met hionine-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-methyltransfe rase that can result in the formation of symmetric dimethylarginine residue s as observed previously in myelin basic protein, Sm small nuclear ribonucl eoproteins, and other polypeptides.