Protein arginine methyltransferase was recently identified to be associated with some proteins in signal transduction pathways. N-Arginine methylation in RNA binding proteins with arginine- and glycine-rich RGG motifs is known to be the major protein methylation in cells. Considering that arginine methylation might be involved in certain human disorders, we used human lymphoblastoid cells that can be easily prepared from lymphocytes as a model system to study the methylation. Lymphoblastoid cells grown in the presence of 20 microM indirect methyltransferase inhibitor adenosine dialdehyde (AdOx) for 72 h appeared to accumulate high levels of hypomethylated proteins for the endogenous protein methyltransferase or recombinant glutathion S-transferase-fused yeast arginine methyltransferase (RMT1). Analysis of methyl-accepting polypeptides in AdOx-treated lymphoblastoid cells by SDS-PAGE and fluorography showed that many polypeptides between 29,000 and 90,000 Da were methylated by the endogenous methyltransferase. A few polypeptides could be methylated to a higher extent upon the addition of yeast GST-RMT1 fusion protein. A peptide (GGRGRGGGF) could compete for the majority of the methyl-accepting protein substrates in the AdOx-treated lymphoblastoid cell extracts, whether or not exogenous yeast RMT1 was included in the reaction. When the arginine residues in the peptide were replaced by lysine, no competition was observed. The results indicated that the protein methyl acceptors in lymphoblastoid cells share similar RGG motifs and that arginine residues should be the site of methylation.