PRMT9 is a Type II methyltransferase that methylates the splicing factor SAP145

The human genome encodes a family of nine protein arginine methyltransferases (PRMT1-9), which members can catalyze three distinct types of methylation on arginine residues. Here, we identify two spliceosome-associated proteins – SAP145 and SAP49 – as PRMT9 binding partners, linking PRMT9 to U2snRNP maturation. We show that SAP145 is methylated by PRMT9 at arginine 508, which takes the form of monomethylated arginine (MMA) and symmetrically dimethylated arginine (SDMA). PRMT9 thus joins PRMT5 as the only mammalian enzymes capable of depositing the SDMA mark. Methylation of SAP145 on Arg508 generates a binding site for the Tudor domain of the Survival of Motor Neuron (SMN) protein, and RNA-seq analysis reveals gross splicing changes when PRMT9 levels are attenuated. These results identify PRMT9 as a non-histone methyltransferase that primes the U2snRNP for interaction with SMN.