Type III protein-arginine methyltransferase from the yeast Saccharomyces cerevisiae (RMT2) was expressed in Escherichia coli and purified to apparent homogeneity. The cytosolic, ribosomal, and ribosome salt wash fractions from yeast cells lacking RMT2 were used as substrates for the recombinant RMT2. Using S-adenosyl-L-methionine as co-substrate, RMT2 methylated a protein in the ribosome salt wash fraction. The same protein in the ribosomal fraction was also methylated by RMT2 after pretreating the sample with endonuclease. Amino acid analysis affirmed that the labeling products were ␦-N-monomethylarginines. The methylated protein from the ribosomal or the ribosome salt wash fraction was isolated by two-dimensional gel electrophoresis and identified as ribosomal protein L12 by mass spectrometry. Using synthetic peptides, recombinant L12, and its mutant as substrates, we pinpointed Arg 67 on ribosomal protein L12 as the methyl acceptor. At least five complete genes have been reported for the type I PRMTs. They are the PRMT1 from rat and human, the protein-arginine methyltransferase 1 from Saccharomyces cerevisiae (RMT1), the PRMT3 from rat, and the mouse coactivator-associated arginine methyltransferase 1/PRMT4. The genes of human PRMT1 (4), rat PRMT1 (5), PRMT3 (6), and coactivator-associated arginine methyltransferase 1/PRMT4 (7) were isolated in two-hybrid screening experiments by interacting with type 1 interferon receptor, the immediate early protein, rat PRMT1, and the hormone receptor coactivator, respectively. The yeast RMT1 was identified independently by homology searches of the yeast genomic data base (1) or genetic screening of proteins that interact with Npl3p, a poly(A) ϩ -RNA-binding protein (8). These enzymes methylate proteins with an Arg-Gly-Gly-rich (9) or Arg-Xaa-Arg-rich (10) region.In vitro experiments have shown that heterogeneous nuclear ribonucleoprotein A1 (5), fibrillarin (6), histone H3 (7), Npl3p (8), and poly(A)-binding protein II (10) can be methylated by these type I PRMTs. However, only Npl3p has been shown to be the in vivo substrate of RMT1 (8).A putative human type II PRMT, Jak-binding protein 1/PRMT5 (11, 12), and its yeast homologue (Hsl7p) (13) were recently identified. Jak-binding protein 1/PRMT5 was found interacting with Janus kinase 2 (Jak2) (11) and the nonstructural protein 3 (NS3) of hepatitis C virus (12). It has been shown that Jak-binding protein 1/PRMT5 can utilize myelin basic protein, histones H2A and H4, and interestingly, a glutathione S-transferase fibrillarin glycine-and arginine-rich domain fusion protein (GST-GAR) as substrates. GST-GAR has been routinely used in the assay of type I PRMTs (2, 6).In addition to the PRMTs mentioned above, there are two other clones that code for arginine methyltransferases. The human PRMT2 (HRMT1L1) has 60% amino acid sequence similarity with the yeast RMT1 protein (14). However, the recombinant GST-PRMT2 fusion protein does not show any enzymatic activity with GST-GAR, myelin basic protein, or cytosolic extracts from a yeas...