Protein arginine methyltransferase 5 catalyzes substrate dimethylation in a distributive fashion

Protein arginine methyltransferase 5 (PRMT5) is a histone-modifying enzyme whose activity is aberrantly upregulated in various cancers and thereby contributes to a progrowth phenotype. Indeed, knockdown of PRMT5 leads to growth arrest and apoptosis, suggesting that inhibitors targeting this enzyme may have therapeutic utility in oncology. To aid the development of inhibitors targeting PRMT5, we initiated mechanistic studies geared to understand how PRMT5 selectively catalyzes the symmetric dimethylation of its substrates. Toward that end, we characterized the regiospecificity and processivity of bacterially expressed Caenorhabditis elegans PRMT5 (cPRMT5), insect cell-expressed human PRMT5 (hPRMT5), and human PRMT5 complexed with methylosome protein 50 (MEP50), i.e., the PRMT5·MEP50 complex. Our studies confirm that arginine 3 is the only site of methylation in both histone H4 and H4 tail peptide analogues and that sites distal to the site of methylation promote the efficient symmetric dimethylation of PRMT5 substrates by increasing the affinity of the monomethylated substrate for the enzyme. Additionally, we show for the first time that both cPRMT5 and the hPRMT5·MEP50 complex catalyze substrate dimethylation in a distributive manner, which is assisted by long-range interactions. Finally, our data confirm that MEP50 plays a key role in substrate recognition and activates PRMT5 activity by increasing its affinity for protein substrates. In total, our results suggest that it may be possible to allosterically inhibit PRMT5 by targeting binding pockets outside the active site.