sparked by the identification of multiple arginine methyl-Dana-Farber Cancer Institute transferase encoding genes. Although these genes have Boston, Massachusetts 02115 been uncovered in a variety of screens designed to identify proteins involved in different cellular processes (see below), the most striking characteristic of this Posttranslational modification of proteins allows the cell emerging family of enzymes is the presence of an S-adento expand its repertoire beyond the constraints imposed osyl methionine (AdoMet) binding motif (Figure 2A), by the twenty encoded amino acids. Methylation at argiwhich is closely related to the motif found in nucleic nines, although discovered over 30 years ago, has only acid and small molecule methyltransferases that use recently come to the attention of cell biologists through AdoMet as a methyl donor. In addition to the AdoMet a combination of genetic and molecular biology experibinding motif, four of five putative mammalian arginine ments that have implicated arginine methylation in promethyltransferases studied to date also share a less cesses from signaling and transcription activation to proconserved C-terminal domain, which is presumably intein sorting. The panoply of arginine methylated substrates volved in arginine substrate interactions. suggests that this specifically eukaryotic modification The majority of arginine methylation in eukaryotic cells may parallel phosphorylation in its level of complexity. appears to be performed by a specific methyltransferase We will summarize much of the recent information about subfamily, which includes mammalian PRMT1 and its the function of methylation and the methyltransferase functional homolog, yeast Hmt1/Rmt1. The enzymes in enzymes that modify arginines. this subfamily contain few residues outside the core Many Proteins Are Targets of Arginine Methylation region. In contrast, Carm1/PRMT4 contains both N-and Three main forms of methylarginine have been identified C-terminal extensions to the methyltransferase core rein eukaryotes: N G -monomethylarginine (MMA), N G N G gion. Other family members have N-terminal extensions, (asymmetric) dimethylarginine (aDMA), and N G NΠ G (symseveral of which contain additional motifs such as an metric) dimethylarginine (sDMA), all of which involve SH3 domain (PRMT2) and a zinc finger motif (PRMT3). modification of guanidino nitrogen atoms (Figure 1). Al-Whereas all arginine methyltransferase activities identhough early purification of mammalian protein arginine tified to date can monomethylate arginine in the context methyltransferases used methylation of histones to of a protein substrate, methyltransferases have been track activity, the majority of nuclear asymmetric diclassified as type I or type II enzymes according to methylarginine residues are found in heterogeneous nuwhether further dimethylation is asymmetric (type I) or clear ribonucleoproteins (hnRNPs), which play roles in symmetric (type II). Most PRMT genes discovered to pre-mRNA processing and nucleocytoplasmic RNA date encode type I enzymes, but recent data have retransport. Subsequent work on numerous hnRNPs and vealed that PRMT5/JBP1 (Janus kinase binding protein other RNA binding proteins has revealed that they are 1) is a type II methyltransferase (Branscombe et al., methylated on arginine residues, frequently in the con-2001). Although in vivo substrates for the type I PRMT1/ text of RGG tripeptides. Notably, all methylarginine resi-Hmt1 enzymes have been defined, the substrate specidues within RGG motifs have been shown to be MMA ficity of the majority of arginine methyltransferases reor aDMA rather than sDMA. Proteins have also been mains mysterious. In the case of PRMT3, however, its identified that are asymmetrically dimethylated at RXR N-terminal zinc finger domain has been shown to influand RG motifs. ence its substrate specificity (Frankel and Clarke, 2000). Myelin basic protein, which was one of the first argi-The three-dimensional structures of the core regions nine-methylated proteins identified, stands in contrast of yeast Hmt1 and human PRMT3 have been determined to most methylated RNA binding proteins in that it conby X-ray crystallography and a comparison of these tains symmetrically dimethylated arginine residues in structures underscores the structural similarity between addition to monomethylarginine. Recently, however, these enzymes (Figure 2B) (Weiss et al., 2000; Zhang et two RNA binding proteins, spliceosomal snRNP proteins al., 2000). Whereas the AdoMet binding domains are SmD1 and SmD3, have been shown to be symmetrically virtually superimposable, the C termini contain divergent dimethylated (Brahms et al., 2000). Intriguingly, the conloops, suggesting that this region is involved in interactions with different methyltransferase substrates and text of methylated residues in these proteins (GRG) difregulators. The N termini are also variable and not comfers from the original RGG consensus for asymmetrically pletely resolved in the structures. Indeed, genetic and dimethylated proteins, suggesting the importance of biochemical experiments in yeast, combined with the Hmt1 crystal structure, have suggested a specific bind-