H2A is methylated by protein arginine methyltransferase (PRMT)1 at novel sites. (A) Biochemical purification scheme (for details see Additional file 5, supplementary materials and methods) of H2A-specific histone methyltransferase from HeLa nuclear extract. (B) (Upper panel) Silver stain of heparin-Sepharose fractions 1 to 8. PRMT1 was identified by mass spectrometry (asterisk) (see Additional file 1, Figure S 1). Autoradiography of histone methyltransferase assays of indicated heparin-Sepharose fractions using (middle panel) H2A 4-129 and (lower panel) endogenous acidic extracted histones, full-length recombinant H2A, myelin basic protein (MBP), and glutathione S-transferase (GST)-GAR (glycine- and arginine-rich) as substrates.