Two histone H1-specific protein-lysine N-methyltransferases from Euglena gracilis. Purification and characterization

Two forms of a histone H1-specific S-adenosylmethionine:protein-lysine N-methyltransferase (protein methylase III) have been purified from Euglena gracilis 48- and 214-fold, respectively, with yields of 3.4 and 4.6%. The enzymes were purified on DEAE-cellulose and histone-Sepharose affinity chromatography and found to be highly specific toward histone H1 as a substrate. However, one of the enzymes also methylates other histone subfractions to a limited extent. Of the proteins other than histones, only myosin showed measurable methyl-accepting capability. Both enzymes were found to be inhibited by S-adenosylhomocysteine (D and L forms), S-adenosyl-L-ethionine, and sinefungin. While the Ki values for S-adenosyl-L-ethionine were similar for both enzymes, the values for S-adenosyl-L-homocysteine and sinefungin were 10-fold lower for the second form. The Km values for histone H1 and S-adenosyl-L-methionine were found to be 3.1 X 10(-7) and 2.7 X 10(-5) M, respectively, for the first enzyme, and 4.4 X 10(-7) and 3.45 X 10(-5) M for the second. Peptide analysis of methyl-14C-labeled H1 revealed that the two enzymes methylate different sites within the histone H1 molecule. The two enzymes were found to have molecular weights of 55,000 and 34,000, respectively. Both enzymes have an optimum pH of 9.0, which is identical to that of other protein-lysine N-methyltransferases thus far identified.