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Selective inhibitors of protein lysine methyltransferases, including SET domain-containing protein 8 (SETD8), are highly desired, as only a fraction of these enzymes are associated with high-quality inhibitors. From our previously discovered SETD8 inhibitor, we developed a more potent analog and solved a cocrystal structure, which is the first crystal structure of SETD8 in complex with a small-molecule inhibitor. This cocrystal structure allowed the design of a covalent inhibitor of SETD8 (MS453), which specifically modifies a cysteine residue near the inhibitor binding site, has an IC₅₀ value of 804 nM, reacts with SETD8 with near-quantitative yield, and is selective for SETD8more » against 28 other methyltransferases. We also solved the crystal structure of the covalent inhibitor in complex with SETD8. In conclusiont, this work provides atomic-level perspective on the inhibition of SETD8 by small molecules and will help identify high-quality chemical probes of SETD8.« less

Protein arginine methyltransferases (PRMTs) play a crucial role in a variety of biological processes. Overexpression of PRMTs has been implicated in various human diseases including cancer. Consequently, selective small-molecule inhibitors of PRMTs have been pursued by both academia and the pharmaceutical industry as chemical tools for testing biological and therapeutic hypotheses. PRMTs are divided into three categories: type I PRMTs which catalyze mono- and asymmetric dimethylation of arginine residues, type II PRMTs which catalyze mono- and symmetric dimethylation of arginine residues, and type III PRMT which catalyzes only monomethylation of arginine residues. Here, we report the discovery of a potent,more » selective, and cell-active inhibitor of human type I PRMTs, MS023, and characterization of this inhibitor in a battery of biochemical, biophysical, and cellular assays. MS023 displayed high potency for type I PRMTs including PRMT1, -3, -4, -6, and -8 but was completely inactive against type II and type III PRMTs, protein lysine methyltransferases and DNA methyltransferases. A crystal structure of PRMT6 in complex with MS023 revealed that MS023 binds the substrate binding site. MS023 potently decreased cellular levels of histone arginine asymmetric dimethylation. It also reduced global levels of arginine asymmetric dimethylation and concurrently increased levels of arginine monomethylation and symmetric dimethylation in cells. We also developed MS094, a close analog of MS023, which was inactive in biochemical and cellular assays, as a negative control for chemical biology studies. In conclusion, MS023 and MS094 are useful chemical tools for investigating the role of type I PRMTs in health and disease.« less