Protein Arginine Methyltransferase 7 (PRMT7): A Human Enzyme Often Overexpressed in Cancer is Most Active Under Non‐physiological Conditions

Abstract

Arginine methylation is a ubiquitous eukaryotic posttranslational modification that can modulate transcriptional activation and suppression. Protein arginine methyltransferase 7 (PRMT7), one of the nine mammalian PRMT family members, catalyzes only monomethylation of a terminal guanidino nitrogen atom on its substrates. In an effort to understand the role of PRMT7 in the cell, we characterized a recombinant human PRMT7 expressed as a GST‐fusion protein in bacterial cells. Using S‐adenosyl‐L‐[methyl‐3H]methionine (3H‐AdoMet) as a substrate, we measured the rate of methylation of a synthetic peptide containing residues 23–37 of human histone H2B. Although the physiological methyl‐accepting substrates of PRMT7 are not known, this peptide contains major in vitro monomethylation sites at arginine residues 29, 31, and 33. A P81 phosphocellulose binding assay was performed that separates the 3H‐methylated product from unused 3H‐AdoMet. We confirmed the unusual temperature dependence of PRMT7, where optimal activity was found at about 20 °C with only about 30% of this activity at 37 °C. We found that the Km of PRMT7 for the H2B‐derived peptide was about 10 μM at 20 °C and about 20 μM at 37 °C. The Km for AdoMet was about 0.5 μM at both temperatures. Additionally we showed that physiological salt conditions can significantly decrease the activity. At 150 mM concentrations, we found only about 10% activity with NaCl or KCl, but about 80–90% activity with potassium acetate or ammonium acetate. We also detected a loss of activity with high ionic strength. These results suggest that there may be a specific inhibition by chloride ion as well as by ionic strength. Finally we show that PRMT7 activity is optimal in alkaline environments. The activity of PRMT7 at the near‐physiological pH of 7.5 is only about 20% that at pH 8.5. Taken together, these results open the possibility that PRMT7 can respond to cells undergoing temperature, salt, or pH stress. These findings demonstrate a potential mechanism for the in vivo regulation of arginine methylation by PRMT7 and provide clues to why PRMT7 has pleiotropic effects involved with transcriptional regulation and translation in the cell.Support or Funding InformationThis work was supported by National Science Foundation Grant MCB‐1714569 (to S.G.C), by a predoctoral Ruth L. Kirschstein National Service Award GM007185 (to T.L.L), and by an Amgen Scholars Award (to A.V.).