Protein Arginine Methylation in Mammals: Who, What, and WhyOutline placeholderChemistry and Enzymology of Protein Arginine Methylation and DemethylationThe Mammalian PRMTsThe Major Type I Enzyme, PRMT1-Broad Specificity with Multiple Interacting PartnersTwo Minor Type I Enzymes with More Defined Cellular Localization-PRMT 3 and PRMT8Type I Enzymes with Restricted Substrate Specificities-CARM1/PRMT4 and PRMT6A Well-Characterized Type II Enzyme-PRMT5Additional Members of the Seven-beta Strand PRMT Family with No or Poorly Characterized activities-PRMT2, PRMT7, and PRMT9(4q31)Are F Box-Only Family Members Protein Arginine Methyltransferases?Biological Roles of Arginine MethylationTranscriptional CoactivatorsTranscriptional CorepressorsmRNA SplicingNuclear/Cytoplasmic ShuttlingDNA RepairSignal TransductionThe Regulation of Arginine MethylationMethylarginine-Regulated Protein-Protein InteractionsArginine Methylation and DiseaseCancerCardiovascular DiseaseViral PathogenesisSpinal Muscular AtrophyChallenges in Analyzing PRMTsEmerging ThemesAcknowledgmentsReferencesMolecular CellReviewProtein Arginine Methylation in Mammals:Who, What, and WhyMark T. Bedford1,*and Steven G. Clarke2,*1The University of Texas M.D. Anderson Cancer Center, Science Park-Research Division, P.O. Box 389, Smithville, TX 78957, USA2The Department of Chemistry and Biochemistry and the Molecular Biology Institute, University of California, Los Angeles, Los Angeles,CA 90095-1569, USA*Correspondence: [email protected] (M.T.B.), [email protected] (S.G.C.)DOI 10.1016/j.molcel.2008.12.013The covalent marking of proteins by methyl group addition to arginine residues can promote their recognitionby binding partners or can modulate their biological activity. A small family of gene products that catalyzesuch methylation reactions in eukaryotes (PRMTs) wo rks in conjunction with a changing cast of associatedsubunits to recognize distinct cellular substrates. These reactions display many of the attributes of reversiblecovalent modifications such as protein phosphorylation or protein lysine methylation; however, it is unclear towhat extent protein arginine demethylation occurs. Physiological roles for protein arginine methylation havebeen established in signal transduction, mRNA splicing, transcriptional control, DNA repair, and proteintranslocation.Chemistry and Enzymology of Protein ArginineMethylation and DemethylationBiology relies upon the enlarged repertoire of interactions thatoccur when proteins are posttranslationally modified. In recentyears, it has become clear that methyl groups stand beside phos-phate groups as major controlling elements in protein function. Awide variety of methylation (and in some cases demethylation)reactions occur at the side chains of a number of amino acid resi-dues and at protein N and C termini. These modificationsgenerate distinct sets of chemical interactions that play roles ina multitude of regulatory pathways (Clarke and Tamanoi, 2006).The modification of arginine side chain guanidino groups isquantitatively one of the most extensive protein methylationreactions in mammalian cells (Paik and Kim, 1980; Najbaueret al., 1993). The number of distinct modified proteins is alsolarge (Pahlich et al., 2006). Arginine is unique among amino acidsas its guanidino group contains five potential hydrogen bonddonors that are positioned for favorable interactions with biolog-ical hydrogen bond acceptors. In protein-DNA complexes, argi-nine residues are the most frequent hydrogen bond donors tobackbone phosphate groups and to thymine, adenine, andguanine bases (Luscombe et al., 2001). Specific networks ofhydrogen bonds can form with arginine residues and adjacentphosphate groups in RNA loops (Calnan et al., 1991), and thearginine-aspartate two H-bond interaction is especially stablein proteins (Mitchell et al., 1992). Each addition of a methyl groupto an arginine residue not only changes its shape, but also re-moves a potential hydrogen bond donor. Such chemistry couldpromote the preferential inhibition by methylation of some, butnot all, binding partners. For example, arginine methylation ofthe Sam68 proline-rich motifs can inhibit its binding to SH3,but not WW domains (Bedford et al., 2000). Methylation of argi-nine residues might also increase their affinity to aromatic rings incation-pi interactions (Hughes and Waters, 2006). Such interac-tions are seen in the aromatic cage of the SMN tudor domain thatlikely interacts with the methylated tail of the SmD splicing factor(Sprangers et al., 2003). Thus, modification of arginine residuesin proteins can readily modulate their binding interactions and,thus, can regulate their physiological functions.Three distinct types of methylated arginine residues occur inmammalian cells. The most prevalent is omega-NG,NG-dimethy-larginine (Paik and Kim, 1980). Here, two methyl groups areplaced on one of the terminal nitrogen atoms of the guanidinogroup; this derivative is commonly referred to as asymmetric di-methylarginine (ADMA) (Figure 1). Two other derivatives occur atlevels of about 20% to 50% that of ADMA (Paik and Kim, 1980).These include the symmetric dimethylated derivative, where onemethyl group is placed on each of the terminal guanidino nitro-gens (omega-NG,N0G-dimethylarginine; SDMA) and the mono-methylated derivative with a single methyl group on the terminalnitrogen atom (omega-NG-monomethylarginine; MMA). Thesethree derivatives are present on a multitude of distinct proteinspecies in the cytoplasm, nucleus, and organelles of mammaliancells (Bedford and Richard, 2005 ). Methylated arginine residuesin proteins are often flanked by one or more glycine residues(Gary and Clarke, 1998), but there are many exceptions to thisgeneral rule.The formation of MMA, ADMA, and SDMA in mammalian cellsis performed by a sequence-related family of catalytic subunitsof protein arginine methyltransferases termed PRMTs (Figure 2).The exact number of genes encoding these catalytic subunits isunder current investigation: six genes are known to encodeenzymes with well-characterized activities (PRMT1, -3, -4[CARM1], -5, -6, and -8) and another three genes encodesequence-related proteins with possible or probable methyl-transferase activities (PRMT2, -7, -9 [4q31]) (Bedford, 2007).Each PRMT species harbors the characteristic motifs of seven-beta strand methyltransferases (Katz et al., 2003), as well asadditional ‘‘double E’’ and ‘‘THW’’ sequence motifs