Negative allosteric regulation of protein arginine methyltransferase 1 (PRMT1) through half-of-sites reactivity

Protein arginine methyltransferase 1 (PRMT1) activity is essential for maintaining proper eukaryotic cellular function; however, its dysregulation contributes to the progression of several human diseases and cancers, making PRMT1 an attractive therapeutic target. Efforts to inhibit PRMT1 through active-site-directed competitive strategies have faced significant challenges, highlighting the need to discover and exploit allosteric regulatory mechanisms. We hypothesized that PRMT1 dimerization enables communication between individual PRMT1 subunits, which modulates catalytic activity. Consistent with this idea, molecular dynamics simulations found that chemical reactivity was productive only when a single active site within the PRMT1 dimer was occupied by peptide substrate, suggesting that PRMT1 exhibits half-of-sites reactivity. To validate this experimentally, we engineered a model PRMT1 dimer with only a single functional active site using dimeric Caenorhabditis elegans PRMT1 (cePRMT1). In vitro kinetic characterization showed that the single active site mutant enzyme exhibits ∼90% of the maximal activity of the wild-type enzyme, supporting that PRMT1 is half-of-sites reactive. Additionally, native mass spectrometry showed that the half-of-sites reactivity in the cePRMT1 dimer is driven by negative cooperative binding for peptide substrate. Together, these findings expand our mechanistic understanding of inter-subunit communication in PRMT1 oligomers and identify half-of-sites reactivity as a previously unrecognized regulatory feature that may be leveraged for the development of PRMT1-selective allosteric inhibitors.