We have identified a new mammalian protein arginine N-methyltransferase, PRMT5, formerly designated Janus kinase-binding protein 1, that can catalyze the formation of -N G -monomethylarginine and symmetric -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-3 H]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 -N G -monomethylarginine standard, and the other cochromatographed with a symmetric -N G ,N G -dimethylarginine standard. Upon base treatment, this second species formed methylamine, a breakdown product characteristic of symmetric -N G ,N G -dimethylarginine. Further analysis of these two species by thin layer chromatography confirmed their identification as -N Gmonomethylarginine and symmetric -N G ,N G -dimethylarginine. The hemagglutinin-PRMT5 complex was also able to monomethylate and symmetrically dimethylate bovine histone H2A and a glutathione S-transferasefibrillarin (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.

Protein arginine methylation is a prevalent posttranslational modification in eukaryotic cells that has been implicated in signal transduction, the metabolism of nascent pre-RNA, and the transcriptional activation processes. In searching the human genome for protein arginine N-methyltransferase (PRMT) family members, a novel gene has been found on chromosome 1 that encodes for an apparent methyltransferase, PRMT6. The polypeptide chain of PRMT6 is 41.9 kDa consisting of a catalytic core sequence common to other PRMT enzymes. Expressed as a glutathione S-transferase fusion protein, PRMT6 demonstrates type I PRMT activity, capable of forming both -N G -monomethylarginine and asymmetric -N G ,N G -dimethylarginine derivatives on the recombinant glycine-and arginine-rich substrate in a processive manner with a specific activity of 144 pmol methyl groups transferred min ؊1 mg ؊1 enzyme. A comparison of substrate specificity reveals that PRMT6 is functionally distinct from two previously characterized type I enzymes, PRMT1 and PRMT4. In addition, PRMT6 displays automethylation activity; it is the first PRMT to do so. This novel human PRMT, which resides solely in the nucleus when fused to the green fluorescent protein, joins a family of enzymes with diverse functions within cells.