Unusual sites of arginine methylation in poly(A)-binding protein II and invitro methylation by protein arginine methyltransferases PRMT1 and PRMT3

Arginine methylation is a post-translational modification found mostly in R NA-binding proteins. Poly(A) binding protein II from calf thymus was shown by mass spectrometry and sequencing to contain N-G-N-G-dimethylarginine at 13 positions in its amino acid sequence. Two additional arginine residues w ere 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 di stinct 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 coil wer e in vitro substrates for two mammalian protein arginine methyltransferases , PRMT1 and PRMT3, with S-adenosyl-L-methionine as the methyl group donor. Both PRMT1 and PRMT3 specifically methylated arginines in the C-terminal do main corresponding to the naturally modified sites.