Human Protein Arginine Methyltransferase 7 (PRMT7) Is a Type III Enzyme Forming ω-NG-Monomethylated Arginine Residues

Full-length human protein arginine methyltransferase 7 (PRMT7) expressed as a fusion protein in Escherichia coli was initially found to generate only omega-N-G-monomethylated arginine residues in small peptides, suggesting that it is a type III enzyme. A later study, however, characterized fusion proteins of PRMT7 expressed in bacterial and mammalian cells as a type II/type I enzyme, capable of producing symmetrically dimethylated arginine (type II activity) as well as small amounts of asymmetric dimethylarginine (type I activity). We have sought to clarify the enzymatic activity of human PRMT7. We analyzed the in vitro methylation products of a glutathione S-transferase (GST)-PRMT7 fusion protein with robust activity using a variety of arginine-containing synthetic peptides and protein substrates, including a GST fusion with the N-terminal domain of fibrillarin (GST-GAR), myelin basic protein, and recombinant human histones H2A, H2B, H3, and H4. Regardless of the methylation reaction conditions (incubation time, reaction volume, and substrate concentration), we found that PRMT7 only produces omega-N-G-monomethylarginine with these substrates. In control experiments, we showed that mammalian GST-PRMT1 and Myc-PRMT5 were, unlike PRMT7, able to dimethylate both peptide P-SmD3 and SmB/D3 to give the expected asymmetric and symmetric products, respectively. These experiments show that PRMT7 is indeed a type III human methyltransferase capable of forming only omega-N-G-monomethylarginine, not asymmetric omega-N-G,N-G-dimethylarginine or symmetric omega-N-G,N-G'-dimethylarginine, under the conditions tested.