Protein Arginine Methylation in Mammals: Who, What, and Why

The covalent marking of proteins by methyl group addition to arginine residues can promote their recognition by binding partners or can modulate their biological activity. A small family of gene products that catalyze such methylation reactions in eukaryotes (PRMTs) works in conjunction with a changing cast of associated subunits to recognize distinct cellular substrates. These reactions display many of the attributes of reversible covalent modifications such as protein phosphorylation or protein lysine methylation; however, it is unclear to what extent protein arginine demethylation occurs. Physiological roles for protein arginine methylation have been established in signal transduction, mRNA splicing, transcriptional control, DNA repair, and protein translocation. The covalent marking of proteins by methyl group addition to arginine residues can promote their recognition by binding partners or can modulate their biological activity. A small family of gene products that catalyze such methylation reactions in eukaryotes (PRMTs) works in conjunction with a changing cast of associated subunits to recognize distinct cellular substrates. These reactions display many of the attributes of reversible covalent modifications such as protein phosphorylation or protein lysine methylation; however, it is unclear to what extent protein arginine demethylation occurs. Physiological roles for protein arginine methylation have been established in signal transduction, mRNA splicing, transcriptional control, DNA repair, and protein translocation. Biology relies upon the enlarged repertoire of interactions that occur when proteins are posttranslationally modified. In recent years, it has become clear that methyl groups stand beside phosphate groups as major controlling elements in protein function. A wide variety of methylation (and in some cases demethylation) reactions occur at the side chains of a number of amino acid residues and at protein N and C termini. These modifications generate distinct sets of chemical interactions that play roles in a multitude of regulatory pathways (Clarke and Tamanoi, 2006Clarke S.G. Tamanoi F. Protein Methyltransferases. The Enzymes, Third Edition. Volume XXIV. Academic Press, San Diego, CA2006Google Scholar). The modification of arginine side chain guanidino groups is quantitatively one of the most extensive protein methylation reactions in mammalian cells (Paik and Kim, 1980Paik W.K. Kim S. Natural occurrence of various methylated amino acid derivatives.in: Meister A. Protein Methylation. John Wiley & Sons, New York1980: 8-25Google Scholar, Najbauer et al., 1993Najbauer J. Johnson B.A. Young A.L. Aswad D.W. Peptides with sequences similar to glycine, arginine-rich motifs in proteins interacting with RNA are efficiently recognized by methyltransferase(s) modifying arginine in numerous proteins.J. Biol. Chem. 1993; 268: 10501-10509Abstract Full Text PDF PubMed Google Scholar). The number of distinct modified proteins is also large (Pahlich et al., 2006Pahlich S. Zakaryan R.P. Gehring H. Protein arginine methylation: Cellular functions and methods of analysis.Biochim. Biophys. Acta. 2006; 1764: 1890-1903Crossref PubMed Scopus (109) Google Scholar). Arginine is unique among amino acids as its guanidino group contains five potential hydrogen bond donors that are positioned for favorable interactions with biological hydrogen bond acceptors. In protein-DNA complexes, arginine residues are the most frequent hydrogen bond donors to backbone phosphate groups and to thymine, adenine, and guanine bases (Luscombe et al., 2001Luscombe N.M. Laskowski R.A. Thornton J.M. Amino acid-base interactions: a three-dimensional analysis of protein-DNA interactions at an atomic level.Nucleic Acids Res. 2001; 29: 2860-2874Crossref PubMed Scopus (755) Google Scholar). Specific networks of hydrogen bonds can form with arginine residues and adjacent phosphate groups in RNA loops (Calnan et al., 1991Calnan B.J. Tidor B. Biancalana S. Hudson D. Frankel A.D. Arginine-mediated RNA recognition: the arginine fork.Science. 1991; 252: 1167-1171Crossref PubMed Google Scholar), and the arginine-aspartate two H-bond interaction is especially stable in proteins (Mitchell et al., 1992Mitchell J.B. Thornton J.M. Singh J. Price S.L. Towards an understanding of the arginine-aspartate interaction.J. Mol. Biol. 1992; 226: 251-262Crossref PubMed Scopus (63) Google Scholar). Each addition of a methyl group to an arginine residue not only changes its shape, but also removes a potential hydrogen bond donor. Such chemistry could promote the preferential inhibition by methylation of some, but not all, binding partners. For example, arginine methylation of the Sam68 proline-rich motifs can inhibit its binding to SH3, but not WW domains (Bedford et al., 2000Bedford M.T. Frankel A. Yaffe M.B. Clarke S. Leder P. Richard S. Arginine methylation inhibits the binding of proline-rich ligands to Src homology 3, but not WW, domains.J. Biol. Chem. 2000; 275: 16030-16036Crossref PubMed Scopus (163) Google Scholar). Methylation of arginine residues might also increase their affinity to aromatic rings in cation-pi interactions (Hughes and Waters, 2006Hughes R.M. Waters M.L. Arginine methylation in a beta-hairpin peptide: implications for Arg-pi interactions, DeltaCp(o), and the cold denatured state.J. Am. Chem. Soc. 2006; 128: 12735-12742Crossref PubMed Scopus (34) Google Scholar). Such interactions are seen in the aromatic cage of the SMN tudor domain that likely interacts with the methylated tail of the SmD splicing factor (Sprangers et al., 2003Sprangers R. Groves M.R. Sinning I. Sattler M. High-resolution X-ray and NMR structures of the SMN Tudor domain: conformational variation in the binding site for symmetrically dimethylated arginine residues.J. Mol. Biol. 2003; 327: 507-520Crossref PubMed Scopus (102) Google Scholar). Thus, modification of arginine residues in proteins can readily modulate their binding interactions and, thus, can regulate their physiological functions. Three distinct types of methylated arginine residues occur in mammalian cells. The most prevalent is omega-NG,NG-dimethylarginine (Paik and Kim, 1980Paik W.K. Kim S. Natural occurrence of various methylated amino acid derivatives.in: Meister A. Protein Methylation. John Wiley & Sons, New York1980: 8-25Google Scholar). Here, two methyl groups are placed on one of the terminal nitrogen atoms of the guanidino group; this derivative is commonly referred to as asymmetric dimethylarginine (ADMA) (Figure 1). Two other derivatives occur at levels of about 20% to 50% that of ADMA (Paik and Kim, 1980Paik W.K. Kim S. Natural occurrence of various methylated amino acid derivatives.in: Meister A. Protein Methylation. John Wiley & Sons, New York1980: 8-25Google Scholar). These include the symmetric dimethylated derivative, where one methyl group is placed on each of the terminal guanidino nitrogens (omega-NG,N′G-dimethylarginine; SDMA) and the monomethylated derivative with a single methyl group on the terminal nitrogen atom (omega-NG-monomethylarginine; MMA). These three derivatives are present on a multitude of distinct protein species in the cytoplasm, nucleus, and organelles of mammalian cells (Bedford and Richard, 2005Bedford M.T. Richard S. Arginine methylation an emerging regulator of protein function.Mol. Cell. 2005; 18: 263-272Abstract Full Text Full Text PDF PubMed Scopus (489) Google Scholar). Methylated arginine residues in proteins are often flanked by one or more glycine residues (Gary and Clarke, 1998Gary J.D. Clarke S. RNA and protein interactions modulated by protein arginine methylation.Prog. Nucleic Acid Res. Mol. Biol. 1998; 61: 65-131Crossref PubMed Google Scholar), but there are many exceptions to this general rule. The formation of MMA, ADMA, and SDMA in mammalian cells is performed by a sequence-related family of catalytic subunits of protein arginine methyltransferases termed PRMTs (Figure 2). The exact number of genes encoding these catalytic subunits is under current investigation: six genes are known to encode enzymes with well-characterized activities (PRMT1, -3, -4 [CARM1], -5, -6, and -8) and another three genes encode sequence-related proteins with possible or probable methyltransferase activities (PRMT2, -7, -9 [4q31]) (Bedford, 2007Bedford M.T. Arginine methylation at a glance.J. Cell Sci. 2007; 120: 4243-4246Crossref PubMed Scopus (108) Google Scholar). Each PRMT species harbors the characteristic motifs of seven-beta strand methyltransferases (Katz et al., 2003Katz J.E. Dlakic M. Clarke S. Automated identification of putative methyltransferases from genomic open reading frames.Mol. Cell. Proteomics. 2003; 2: 525-540Crossref PubMed Scopus (128) Google Scholar), as well as additional “double E” and “THW” sequence motifs particular to the PRMT subfamily (Cheng et al., 2005Cheng X. Collins R.E. Zhang X. Structural and sequence motifs of protein (histone) methylation enzymes.Annu. Rev. Biophys. Biomol. Struct. 2005; 34: 267-294Crossref PubMed Scopus (144) Google Scholar). It has also been proposed that the FBXO11 and FBXO10 proteins, which do not harbor these signature motifs, represent a second family of protein arginine methyltransferases; however these activities require validation (Cook et al., 2006Cook J.R. Lee J.H. Yang Z.H. Krause C.D. Herth N. Hoffmann R. Pestka S. FBXO11/PRMT9, a new protein arginine methyltransferase, symmetrically dimethylates arginine residues.Biochem. Biophys. Res. Commun. 2006; 342: 472-481Crossref PubMed Scopus (85) Google Scholar, Krause et al., 2007Krause C.D. Yang Z.H. Kim Y.S. Lee J.H. Cook J.R. Pestka S. Protein arginine methyltransferases: evolution and assessment of their pharmacological and therapeutic potential.Pharmacol. Ther. 2007; 113: 50-87Crossref PubMed Scopus (141) Google Scholar). The gene products with well-characterized activities, generally purified as fusion proteins, catalyze MMA formation; PRMT1, -3, -4 (CARM1), -6 and -8 additionally catalyze ADMA formation, whereas PRMT5 additionally catalyzes SDMA formation. Enzymes that form ADMA are designated “type I”; those that form SDMA are designated “type II” (Gary and Clarke, 1998Gary J.D. Clarke S. RNA and protein interactions modulated by protein arginine methylation.Prog. Nucleic Acid Res. Mol. Biol. 1998; 61: 65-131Crossref PubMed Google Scholar). To date, no enzyme has been found that forms both ADMA and SDMA derivatives. A clear understanding of how PRMT catalytic polypeptides can be assembled into active complexes with other protein subunits in cells is needed. Although PRMT1, -3, -4 (CARM1), -5, -6, and -8 are active methyltransferases in the absence of other polypeptide species in vitro, they might also bind additional partners for their functions in vivo. It now appears that interactions between PRMTs and their binding partners (and presumed regulatory subunits) can be transient or permanent (Table 1). It is possible that the species with poorly defined activities require the presence of other subunits for catalytic activity.Table 1Interacting Protein Partners of Mammalian PRMTsPRMT SpeciesInteracting SpeciesPRMT1poly(ADP-ribose) polymerase and NF-kappaBSam68 and the MLL complexBtg1 and Tis2/Btg2intracytoplasmic domain of the type I interferon receptorhCAF1PRMT2RBPRMT8 (via SH3 domain)PRMT3DAL-1/4.1BZn-finger domain – binds 40S rpS2CARM1/PRMT4p160 family coactivatorsSWI/SNF complexFlightless-1PRMT5nucleolinaPRMT5-interacting species that may be compromised by the use of FLAG-tagged fusion proteins (Nishioka and Reinberg, 2003).hIws1 transcription elongation factorHistone-binding protein COPR5LIM protein AJUBAaPRMT5-interacting species that may be compromised by the use of FLAG-tagged fusion proteins (Nishioka and Reinberg, 2003).SWI-SNFmethylosome complex with pICln and MEP50Blimp1DAL-1/4.1B tumor suppressorRNA polymerase II FCP phosphataseSPT4/SPT5 transcription elongation factoraPRMT5-interacting species that may be compromised by the use of FLAG-tagged fusion proteins (Nishioka and Reinberg, 2003).PRMT7PRMT7CTCFL testis-specific factorPRMT5PRMT8Ewing Sarcoma proteinPRMT1SH3 domains of PRMT2, Fyn, Plcγ, and P85a PRMT5-interacting species that may be compromised by the use of FLAG-tagged fusion proteins (Nishioka and Reinberg, 2003Nishioka K. Reinberg D. Methods and tips for the purification of human histone methyltransferases.Methods. 2003; 31: 49-58Crossref PubMed Scopus (29) Google Scholar). Open table in a new tab An important question under current investigation is whether protein arginine demethylation reactions occur to reverse the effects of the modifications. Initial studies indicated that methyl groups were stable on arginine residues. The apparent absence of protein arginine demethylases suggested that the only way to reverse the effects of the modification would be to degrade the protein to its component amino acids and then make a new unmodified version by protein synthesis. However, two types of enzymes that can remove methyl groups from arginine residues in proteins were recently identified. MMA residues in proteins can be deiminated to citrulline residues by the PAD4 peptidylarginine deiminase (Thompson and Fast, 2006Thompson P.R. Fast W. Histone citrullination by protein arginine deiminase: is arginine methylation a green light or a roadblock?.ACS Chem. Biol. 2006; 1: 433-441Crossref PubMed Scopus (66) Google Scholar). It is unknown whether the citrulline residue might be converted to an arginine residue to complete the demethylation process. It has been suggested that this enzyme is unlikely to play a physiological demethylation role. Indeed, peptides containing MMA are more slowly deiminated than those containing arginine residues, and those containing ADMA are not deiminated at all. Thus, methylation might, in fact, inhibit a reaction that normally converts arginine residues to citrulline residues (Raijmakers et al., 2007Raijmakers R. Zendman A.J. Egberts W.V. Vossenaar E.R. Raats J. Soede-Huijbregts C. Rutjes F.P. van Veelen P.A. Drijfhout J.W. Pruijn G.J. Methylation of arginine residues interferes with citrullination by peptidylarginine deiminases in vitro.J. Mol. Biol. 2007; 367: 1118-1129Crossref PubMed Scopus (49) Google Scholar). Additionally, recent work has suggested that a second type of enzyme, the Jumonji domain-containing proteins, which was originally identified as a family of lysine demethylases, can also demethylate arginine residues. Indeed, JMJD6 has been reported to directly regenerate arginine residues from methylated histone species (Chang et al., 2007Chang B. Chen Y. Zhao Y. Bruick R.K. JMJD6 is a histone arginine demethylase.Science. 2007; 318: 444-447Crossref PubMed Scopus (185) Google Scholar). It will be important to determine if there are additional enzymes that catalyze similar reactions and to assess the biological significance of each of these demethylation pathways in regulating protein arginine methylation. In the section below, we describe the conserved family of mammalian PRMTs 1–9, as well as a second family of putative enzymes related to the F box-only proteins (Figure 2). There does not appear to be major redundancy between these enzymes; mouse knockouts display generally clear and dramatic phenotypes (Table 2). The diversity of these enzymes is enhanced by alternative splicing reactions that lead to amino acid sequence variants (Goulet et al., 2007Goulet I. Gauvin G. Boisvenue S. Cote J. Alternative splicing yields protein arginine methyltransferase 1 isoforms with distinct activity, substrate specificity, and subcellular localization.J. Biol. Chem. 2007; 282: 33009-33021Crossref PubMed Scopus (48) Google Scholar).Table 2Phenotypes of Mice Lacking Protein Arginine MethyltransferasesGenePhenotype(s)Prmt1embryonic lethal, but ES cells can survivetrapping mutant generates a hypomorphic allelePrmt2mice are viablemouse embryo fibroblasts have increased activity of NF-kappaB and are more resistant to apoptosis than wild type cellsPrmt3mice are viable, but mutant embryos are slightly smallertrapping mutant generates a hypomorphic alleleCarm1/Prmt4newborns small and die shortly after birthT cell development blocked in thymuslipid metabolism altered in embryos Open table in a new tab PRMT1 was the first mammalian protein arginine methyltransferase identified as a single gene product and was initially characterized as a GST-fusion protein expressed in bacterial cells (Lin et al., 1996Lin W.J. Gary J.D. Yang M.C. Clarke S. Herschman H.R. The mammalian immediate-early TIS21 protein and the leukemia-associated BTG1 protein interact with a protein-arginine N-methyltransferase.J. Biol. Chem. 1996; 271: 15034-15044Crossref PubMed Scopus (286) Google Scholar). Previous purifications of these enzymes from mammalian tissues was complicated by their low abundance and the presence of a multitude of polypeptides in the final preparations, making it difficult to discern which were contaminants and which were truly associated with the enzymatic activity (Gary and Clarke, 1998Gary J.D. Clarke S. RNA and protein interactions modulated by protein arginine methylation.Prog. Nucleic Acid Res. Mol. Biol. 1998; 61: 65-131Crossref PubMed Google Scholar). PRMT1 is responsible for the bulk (about 85%) of total protein arginine methylation activity in cultured RAT1 fibroblast cells as well as in mouse liver (Tang et al., 2000Tang J. Frankel A. Cook R.J. Kim S. Paik W.K. Williams K.R. Clarke S. Herschman H.R. PRMT1 is the predominant type I protein arginine methyltransferase in mammalian cells.J. Biol. Chem. 2000; 275: 7723-7730Crossref PubMed Scopus (150) Google Scholar). PRMT1 has a very wide substrate specificity, with a preference for methylating arginine residues that are flanked by one or more glycine residues (Gary and Clarke, 1998Gary J.D. Clarke S. RNA and protein interactions modulated by protein arginine methylation.Prog. Nucleic Acid Res. Mol. Biol. 1998; 61: 65-131Crossref PubMed Google Scholar, Lee and Bedford, 2002Lee J. Bedford M.T. PABP1 identified as an arginine methyltransferase substrate using high-density protein arrays.EMBO Rep. 2002; 3: 268-273Crossref PubMed Scopus (113) Google Scholar). Structural studies have identified three different peptide-binding channels, suggesting that different PRMT1 substrates might approach the active site from different angles (Zhang and Cheng, 2003Zhang X. Cheng X. Structure of the Predominant Protein Arginine Methyltransferase PRMT1 and Analysis of Its Binding to Substrate Peptides.Structure. 2003; 11: 509-520Abstract Full Text Full Text PDF PubMed Scopus (140) Google Scholar). This view is supported by surface-scanning mutational analysis that identified certain mutations that selectively block the binding of distinct substrates (Lee et al., 2007Lee D.Y. Ianculescu I. Purcell D. Zhang X. Cheng X. Stallcup M.R. Surface-scanning mutational analysis of protein arginine methyltransferase 1: roles of specific amino acids in methyltransferase substrate specificity, oligomerization, and coactivator function.Mol. Endocrinol. 2007; 21: 1381-1393Crossref PubMed Scopus (21) Google Scholar). Recent kinetic studies demonstrated that distal substrate residues influence arginine methylation, and the available evidence suggests a partially processive mechanism (Osborne et al., 2007Osborne T.C. Obianyo O. Zhang X. Cheng X. Thompson P.R. Protein arginine methyltransferase 1: positively charged residues in substrate peptides distal to the site of methylation are important for substrate binding and catalysis.Biochemistry. 2007; 46: 13370-13381Crossref PubMed Scopus (54) Google Scholar). The three-dimensional structure of PRMT1 suggests that it is active as a homodimer (Zhang and Cheng, 2003Zhang X. Cheng X. Structure of the Predominant Protein Arginine Methyltransferase PRMT1 and Analysis of Its Binding to Substrate Peptides.Structure. 2003; 11: 509-520Abstract Full Text Full Text PDF PubMed Scopus (140) Google Scholar). However, PRMT1 appears to be associated with a number of proteins in the cell (Table 1). Gel filtration analyses of the enzyme in mammalian cell extracts suggest sizes ranging from 275 kDa to 450 kDa; by contrast, the PRMT1 dimer is expected to be 80 kDa (Gary and Clarke, 1998Gary J.D. Clarke S. RNA and protein interactions modulated by protein arginine methylation.Prog. Nucleic Acid Res. Mol. Biol. 1998; 61: 65-131Crossref PubMed Google Scholar, Hung et al., 2007Hung C.J. Chen D.H. Shen Y.T. Li Y.C. Lin Y.W. Hsieh M. Li C. Characterization of protein arginine methyltransferases in porcine brain.J. Biochem. Mol. Biol. 2007; 40: 617-624Crossref PubMed Google Scholar). This variation could represent the range of different oligomers that are present in the cell. PRMT3 is located exclusively in the cytosol and has a zinc finger domain that appears to anchor it to its substrates (Frankel and Clarke, 2000Frankel A. Clarke S. PRMT3 is a distinct member of the protein arginine N-methyltransferase family. Conferral of substrate specificity by a zinc-finger domain.J. Biol. Chem. 2000; 275: 32974-32982Crossref PubMed Scopus (82) Google Scholar). A major methyl-accepting substrate in mammalian cells is the S2 protein of the small ribosomal subunit; much of PRMT3 is, in fact, associated with ribosomes via an interaction between its zinc finger domain and the S2 protein (Swiercz et al., 2007Swiercz R. Cheng D. Kim D. Bedford M.T. Ribosomal protein rpS2 is hypomethylated in PRMT3-deficient mice.J. Biol. Chem. 2007; 282: 16917-16923Crossref PubMed Scopus (36) Google Scholar). PRMT8 is an unusual catalytic subunit for two reasons. Although its amino acid sequence is closely related to PRMT1, it has a relatively narrow tissue distribution, being limited mainly to brain (Lee et al., 2005aLee J. Sayegh J. Daniel J. Clarke S. Bedford M.T. PRMT8, a new membrane-bound tissue-specific member of the protein arginine methyltransferase family.J. Biol. Chem. 2005; 280: 32890-32896Crossref PubMed Scopus (108) Google Scholar). It is also the only PRMT known to be membrane associated; PRMT8 is attached to the plasma membrane via N-terminal myristoylation (Lee et al., 2005aLee J. Sayegh J. Daniel J. Clarke S. Bedford M.T. PRMT8, a new membrane-bound tissue-specific member of the protein arginine methyltransferase family.J. Biol. Chem. 2005; 280: 32890-32896Crossref PubMed Scopus (108) Google Scholar). The in vitro activity of the full-length recombinant enzyme is low; however, removal of the N-terminal domain by truncation or proteolysis results in a large activation (Sayegh et al., 2007Sayegh J. Webb K. Cheng D. Bedford M.T. Clarke S.G. Regulation of protein arginine methyltransferase 8 (PRMT8) activity by its N-terminal domain.J. Biol. Chem. 2007; 282: 36444-36453Crossref PubMed Scopus (43) Google Scholar). The N-terminal region contains two proline-rich sequences that can bind a number of SH3 domains, including that of PRMT2. It is possible that binding of proteins to this N-terminal domain can result in physiological PRMT8 activation and/or changes in its cellular location. Recently, a number of PRMT8-binding proteins were identified in cultured cells of probable neuronal origin (Pahlich et al., 2008Pahlich S. Zakaryan R.P. Gehring H. Identification of proteins interacting with protein arginine methyltransferase 8: the Ewing sarcoma (EWS) protein binds independent of its methylation state.Proteins. 2008; 72: 1125-1137Crossref PubMed Scopus (22) Google Scholar). CARM1/PRMT4 can be distinguished from PRMT1 because it catalyzes the methylation of only a few distinct substrates (Lee and Bedford, 2002Lee J. Bedford M.T. PABP1 identified as an arginine methyltransferase substrate using high-density protein arrays.EMBO Rep. 2002; 3: 268-273Crossref PubMed Scopus (113) Google Scholar). It binds the steroid receptor coactivators (SRC1-3) and has clear transcriptional coactivator activity itself, thus its name—the coactivator associated arginine methyltransferase 1 (CARM1) (Chen et al., 1999Chen D. Ma H. Hong H. Koh S.S. Huang S.M. Schurter B.T. Aswad D.W. Stallcup M.R. Regulation of transcription by a protein methyltransferase.Science. 1999; 284: 2174-2177Crossref PubMed Scopus (750) Google Scholar). As it was the fourth arginine methyltransferase described, it is also referred to as PRMT4. CARM1 loss is not compatible with life (Yadav et al., 2003Yadav N. Lee J. Kim J. Shen J. Hu M.C. Aldaz C.M. Bedford M.T. Specific protein methylation defects and gene expression perturbations in coactivator-associated arginine methyltransferase 1-deficient mice.Proc. Natl. Acad. Sci. USA. 2003; 100: 6464-6468Crossref PubMed Scopus (149) Google Scholar). In cells, CARM1 forms a complex with ATP-remodeling (SWI/SNF) factors (Xu et al., 2004Xu W. Cho H. Kadam S. Banayo E.M. Anderson S. Yates III, J.R. Emerson B.M. Evans R.M. A methylation-mediator complex in hormone signaling.Genes Dev. 2004; 18: 144-156Crossref PubMed Scopus (88) Google Scholar). CARM1 recruitment to transcriptional promoters feeds into the “histone code,” resulting in elevated levels of H3R17 (histone H3 Arg-17) and H3R26 methylation, which are associated with transcriptional activation. It also methylates other transcriptional coactivators and a subset of splicing factors (Cheng et al., 2007Cheng D. Cote J. Shaaban S. Bedford M.T. The arginine methyltransferase CARM1 regulates the coupling of transcription and mRNA processing.Mol. Cell. 2007; 25: 71-83Abstract Full Text Full Text PDF PubMed Scopus (128) Google Scholar, Feng et al., 2006Feng Q. Yi P. Wong J. O'Malley B.W. Signaling within a coactivator complex: methylation of SRC-3/AIB1 is a molecular switch for complex disassembly.Mol. Cell. Biol. 2006; 26: 7846-7857Crossref PubMed Scopus (72) Google Scholar). The CARM1 crystal structure was solved by two groups, providing insight into its mechanism of action (Troffer-Charlier et al., 2007Troffer-Charlier N. Cura V. Hassenboehler P. Moras D. Cavarelli J. Functional insights from structures of coactivator-associated arginine methyltransferase 1 domains.EMBO J. 2007; 26: 4391-4401Crossref PubMed Scopus (48) Google Scholar, Yue et al., 2007Yue W.W. Hassler M. Roe S.M. Thompson-Vale V. Pearl L.H. Insights into histone code syntax from structural and biochemical studies of CARM1 methyltransferase.EMBO J. 2007; 26: 4402-4412Crossref PubMed Scopus (43) Google Scholar). A comparison of the apo and holo states of CARM1 reveals AdoHcy-induced conformational changes of the cofactor pocket, which likely generate an access channel for the target arginine (Yue et al., 2007Yue W.W. Hassler M. Roe S.M. Thompson-Vale V. Pearl L.H. Insights into histone code syntax from structural and biochemical studies of CARM1 methyltransferase.EMBO J. 2007; 26: 4402-4412Crossref PubMed Scopus (43) Google Scholar). Structural analysis reveals that the CARM1 N-terminus assumes a PH domain-like fold (Troffer-Charlier et al., 2007Troffer-Charlier N. Cura V. Hassenboehler P. Moras D. Cavarelli J. Functional insights from structures of coactivator-associated arginine methyltransferase 1 domains.EMBO J. 2007; 26: 4391-4401Crossref PubMed Scopus (48) Google Scholar). PRMT6 is a nuclear enzyme characterized by its specificity for distinct methyl-accepting substrates and by its automethylation (Frankel et al., 2002Frankel A. Yadav N. Lee J. Branscombe T.L. Clarke S. Bedford M.T. The novel human protein arginine N-methyltransferase PRMT6 is a nuclear enzyme displaying unique substrate specificity.J. Biol. Chem. 2002; 277: 3537-3543Crossref PubMed Scopus (186) Google Scholar). PRMT6-specific substrates include the nuclear scaffold protein HMGA1a/b (Miranda et al., 2005Miranda T.B. Webb K.J. Edberg D.D. Reeves R. Clarke S. Protein arginine methyltransferase 6 specifically methylates the nonhistone chromatin protein HMGA1a.Biochem. Biophys. Res. Commun. 2005; 336: 831-835Crossref PubMed Scopus (34) Google Scholar, Sgarra et al., 2006Sgarra R. Lee J. Tessari M.A. Altamura S. Spolaore B. Giancotti V. Bedford M.T. Manfioletti G. The AT-hook of the chromatin architectural transcription factor high mobility group A1a is arginine-methylated by protein arginine methyltransferase 6.J. Biol. Chem. 2006; 281: 3764-3772Crossref PubMed Scopus (45) Google Scholar), DNA polymerase beta (El-Andaloussi et al., 2007El-Andaloussi N. Valovka T. Toueille M. Hassa P.O. Gehrig P. Covic M. Hubscher U. Hottiger M.O. Methylation of DNA polymerase beta by protein arginine methyltransferase 1 regulates its binding to proliferating cell nuclear antigen.FASEB J. 2007; 21: 26-34Crossref PubMed Scopus (24) Google Scholar), the HIV Tat protein (Boulanger et al., 2005Boulanger M.C. Liang C. Russell R.S. Lin R. Bedford M.T. Wainberg M.A. Richard S. Methylation of Tat by PRMT6 regulates human immunodeficiency virus type 1 gene expression.J. Virol. 2005; 79: 124-131Crossref PubMed Scopus (97) Google Scholar), and histone H3 (Guccione et al., 2007Guccione E. Bassi C. Casadio F. Martinato F. Cesaroni M. Schuchlautz H. Luscher B. Amati B. Methylation of histone H3R2 by PRMT6 and H3K4 by an MLL complex are mutually exclusive.Nature. 2007; 449: 933-937Crossref PubMed Scopus (169) Google Scholar, Hyllus et al., 2007Hyllus D. Stein C. Schnabel K. Schiltz E. Imhof A. Dou Y. Hsieh J. Bauer U.M. PRMT6-mediated methylation of R2 in histone H3 antagonizes H3 K4 trimethylation.Genes Dev. 2007; 21: 3369-3380Crossref PubMed Scopus (93) Google Scholar, Iberg et al., 2008Iberg A.N. Espejo A. Cheng D. Kim D. Michaud-Levesque J. Richard S. Bedford M.T. Arginine methylation of the histone h3 tail impedes effector binding.J. Biol. Chem. 2008; 283: 3006-3010Crossref PubMed Scopus (72) Google Scholar). PRMT6 is the major PRMT responsible for histone H3R2 methylation, and it has a clearly defined role in antagonizing the MLL-complex-de