Suppressors of variegation 3-9 homolog 1 (SUV39H1), the enzyme responsible for establishing histone H3 lysine 9 trimethylation (H3K9me3) marks in heterochromatin, is frequently dysregulated in cancers. However, the mechanisms underlying SUV39H1 dysregulation in breast cancer remain largely unclear. Here, we report that protein arginine methyltransferase 1 (PRMT1) directly interacts with SUV39H1 and dimethylates it at arginine 378 (R378). PKC signaling-mediated phosphorylation of SUV39H1 at S391 ...