Human protein arginine methyltransferase 7 (PRMT7) is a type III enzyme forming ω-N G-monomethylated arginine residues

Abstract

Full-length human protein arginine methyltransferase 7 (PRMT7) expressed as a fusion protein in Escherichia coli was initially found to generate only ω-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).Wehave 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),wefoundthatPRMT7onlyproduces ω-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/D3to give the expected asymmetric and symmetric products, respectively. These experiments show thatPRMT7is indeed a type III human methyltransferase capable of forming only ω-N Gmonomethylarginine, not asymmetric ω-N G,N G-dimethylarginine or symmetric ω-N G,N G′-dimethylarginine, under the conditions tested. © 2012 by The American Society for Biochemistry and Molecular Biology, Inc.