Biochemical control of CARM1 enzymatic activity by phosphorylation

Coactivator-associated arginine methyltransferase 1 (CARM1) is a dual functional coregulator that facilitates transcription initiation by methylation of Arg¹⁷ and Arg²⁶ of histone H3 and also dictates the subsequent coactivator complex disassembly by methylation of the steroid receptor coactivator family coactivators and p300/cAMP-response element-binding protein-binding protein. However, the regulation of CARM1 enzymatic activity and substrate specificity remains largely unknown. In this study, we report that CARM1 function is regulated by phosphorylation at Ser²¹⁷, a residue completely conserved in the type I protein arginine methyltransferase (PRMT) family of enzymes. Comparative analysis of the published CARM1 crystal structures reveals that the hydroxyl group of Ser²¹⁷ forms a strong hydrogen bond with the carbonyl oxygen atom of Tyr¹⁵⁴ to lock the cofactor S-adenosylmethionine inside the binding cavity. Phosphorylation of Ser²¹⁷ disrupts this hydrogen bond and subsequently abolishes S-adenosylmethionine binding and its methyltransferase activity. Importantly, Tyr¹⁵⁴ is also conserved in the type I PRMT family of enzymes, suggesting a general role of this hydrogen bond in maintaining the holo structure of the type I PRMT catalytic domain. Moreover, we found that phosphorylation at Ser²¹⁷ also promoted CARM1 cytoplasmic localization and that this translocation occurred mainly during mitosis. We propose that phosphorylation at Ser²¹⁷ serves as a molecular switch for controlling CARM1 enzymatic activity during the cell cycle.