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.

Background: Protein arginine methyltransferase 7 (PRMT7) is associated with various functions and diseases, but its substrate specificity is poorly defined. Results: Insect cell-expressed PRMT7 forms -monomethylarginine residues at basic RXR sequences in peptides and histone H2B. Conclusion: PRMT7 is a type III PRMT with a unique substrate specificity. Significance: Novel post-translational modification sites generated by PRMT7 may regulate biological function.

Background: Newly discovered protein arginine methyltransferase 9 (PRMT9) modulates alternative splicing by methylation of SF3B2. Results: Biochemical probes of PRMT9 and its substrate protein revealed domains and residues required for methylation. Conclusion: PRMT9 is unique among PRMTs in its narrow range of methyl-accepting substrates. Significance: Understanding PRMT9 catalysis will help elucidate how it may control the activity of SF3B2 and other potential endogenous substrates.

Background: Mammalian PRMT7 has been implicated in multiple biological processes, but its in vivo substrates have not been identified. Results: Mutagenesis studies indicate key residues that affect the unique substrate preference of PRMT7. Conclusion: Acidic residues within the double E loop confer specificity to PRMT7. Significance: Understanding how PRMT7 recognizes its substrates will enhance our knowledge of its physiological role.

In the family of protein arginine methyltransferases (PRMTs) that predominantly generate either asymmetric or symmetric dimethylarginine (SDMA), PRMT7 is unique in producing solely monomethylarginine (MMA) products. The type of methylation on histones and other proteins dictates changes in gene expression, and numerous studies have linked altered profiles of methyl marks with disease phenotypes. Given the importance of specific inhibitor development, it is crucial to understand the mechanisms by which PRMT product specificity is conferred. We have focused our attention on active-site residues of PRMT7 from the protozoan Trypanosoma brucei. We have designed 26 single and double mutations in the active site, including residues in the Glu-Xaa 8 -Glu (double E) loop and the Met-Gln-Trp sequence of the canonical Thr-His-Trp (THW) loop known to interact with the methyl-accepting substrate arginine. Analysis of the reaction products by high resolution cation exchange chromatography combined with the knowledge of PRMT crystal structures suggests a model where the size of two distinct subregions in the active site determines PRMT7 product specificity.