Unusual Sites of Arginine Methylation in Poly(A)-binding Protein II and in Vitro Methylation by Protein Arginine Methyltransferases PRMT1 and PRMT3

Arginine methylation is a post-translational modification found mostly in RNA-binding proteins. Poly(A)-binding protein II from calf thymus was shown by mass spectrometry and sequencing to containNG,NG-dimethylarginine at 13 positions in its amino acid sequence. Two additional arginine residues were partially methylated. Almost all of the modified residues were found in Arg-Xaa-Arg clusters in the C terminus of the protein. These motifs are distinct from Arg-Gly-Gly motifs that have been previously described as sites and specificity determinants for asymmetric arginine dimethylation. Poly(A)-binding protein II and deletion mutants expressed in Escherichia coli werein vitro substrates for two mammalian protein arginine methyltransferases, PRMT1 and PRMT3, withS-adenosyl-l-methionine as the methyl group donor. Both PRMT1 and PRMT3 specifically methylated arginines in the C-terminal domain corresponding to the naturally modified sites.