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: PRMT7 is a poorly characterized member of the family of protein arginine methyltransferases (PRMTs) that catalyze the modification of a wide group of proteins. Results: A bacterially expressed fusion protein of human PRMT7 produces only -monomethylated arginine (-MMA) derivatives. Conclusion: PRMT7 is a type III PRMT distinct from type I and type II PRMTs characterized to date. Significance: -MMA may be a distinct post-translational modification.

We have shown that Rpl3, a protein of the large ribosomal subunit from baker's yeast (Saccharomyces cerevisiae), is stoichiometrically monomethylated at position 243, producing a 3-methylhistidine residue. This conclusion is supported by topdown and bottom-up mass spectrometry of Rpl3, as well as by biochemical analysis of Rpl3 radiolabeled in vivo with S-adenosyl-L-[methyl-3 H]methionine. The results show that a ؉14-Da modification occurs within the GTKKLPRKTHRGLRKVAC sequence of Rpl3. Using high-resolution cation-exchange chromatography and thin layer chromatography, we demonstrate that neither lysine nor arginine residues are methylated and that a 3-methylhistidine residue is present. Analysis of 37 deletion strains of known and putative methyltransferases revealed that only the deletion of the YIL110W gene, encoding a seven ␤-strand methyltransferase, results in the loss of the ؉14-Da modification of Rpl3. We suggest that YIL110W encodes a protein histidine methyltransferase responsible for the modification of Rpl3 and potentially other yeast proteins, and now designate it Hpm1 (Histidine protein methyltransferase 1). Deletion of the YIL110W/HPM1 gene results in numerous phenotypes including some that may result from abnormal interactions between Rpl3 and the 25 S ribosomal RNA. This is the first report of a methylated histidine residue in yeast cells, and the first example of a gene required for protein histidine methylation in nature.The addition of methyl groups to proteins from the methyl donor S-adenosylmethionine is one of the most common posttranslational modifications, resulting in an expansion of the physico-chemical characteristics of amino acids and the potential to modulate protein function (1). Major sites of protein methylation are at lysine and arginine residues (2, 3), and less major sites include glutamate, glutamine, and histidine residues, as well as N-terminal amino and C-terminal carboxyl groups (4 -6). The extensive role of histone methylation in transcriptional control highlights the biological significance of this modification (7-10). Protein methylation is also important in the translational machinery. Indeed, many proteins involved in translation, including ribosomal proteins and various elongation and release factors, are subject to methylation in both prokaryotes and eukaryotes (11).Saccharomyces cerevisiae is an ideal organism to investigate the methylation of ribosomal proteins; its genome is well annotated and single open reading frame gene deletion mutants are available. High-resolution intact mass spectrometry suggested that six proteins of the large ribosomal subunit may be methylated: Rpl1ab, Rpl3, Rpl12ab, Rpl23ab, Rpl42ab, and Rpl43ab (12).2 This study, however, did not identify the sites of methylation in these proteins nor did it identify the corresponding methyltransferases. In our laboratory, we have been interested in characterizing these modifications and identifying the methyltransferases involved in an effort to understand their physiological significance in trans...

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