Identification of protein-arginine N-methyltransferase as 10- formyltetrahydrofolate dehydrogenase

S-Adenosylmethionine:protein-arginine N-methyl-transferase (EC 2.1.1.23; protein methylase I) transfers the methyl group of S-adenosyl-L-methionine to an arginine residue of a protein substrate. The homogeneous liver protein methylase I was subjected to tryptic digestion followed by reverse phase high performance liquid chromatography (HPLC) separation and either 'on-line' mass spectrometric fragmentation or 'off-line' Edman sequencing of selected fractions. Data base searching of both the mass spectrometric and Edman sequencing data from several peptides identified the protein methylase as 10- formyltetrahydrofolate dehydrogenase (EC 1.5.1.6; Cook, R. J., Lloyd, R. S., and Wagner, C. (1991) J. Biol. Chem. 266, 4965-4973; Swiss accession number P28037). This identification was confirmed by comparative HPLC tryptic peptide mapping and affinity chromatography of the methylase on the 5- formyl-tetrahydrofolate-Sepharose affinity gel used to purify the dehydrogenase. The purified rat liver methylase had approximately 33% of the 10-formyltetrahydrofolate dehydrogenase and 36% of the aldehyde dehydrogenase activity as compared with the recombinant dehydrogenase, which also had protein methylase I activity. Polyclonal antibodies against recombinant dehydrogenase reacted with protein methylase I purified either by polyacrylamide gel electrophoresis or 5-formyltetrahydrofolate affinity chromatography. In each instance there was only a single immunoreactive band at a molecular weight of ~106,000. Together, these results confirm the co- identity of protein-arginine methyltransferase and 10-formyltetrahydrofolate dehydrogenase.