Sequence specificity and role of proximal amino acids of the histone H3 tail on catalysis of murine G9a lysine 9 histone H3 methyltransferase

The activity of recombinant murine G9a toward lysine 9 of histone H3 was investigated. GST fusion proteins containing various lengths of the histone H3 amino-terminal tail were used as substrates in the presence of recombinant G9a enzyme and AdoMet cosubstrate. The minimal substrate methylated by G9a contained seven amino acids (TARKSTG) of the histone H3 tail. Furthermore, mutational analysis of the minimal substrate was performed to identify the amino acids essential for G9a-mediated methylation. All amino acids except Thr-11 were indispensable for the methylation reaction. Steady-state kinetic analysis of the wild-type and histone H3 point mutants, lysine 4 changed to alanine (K4A) or lysine 27 changed to alanine (K27A), with purified G9a revealed similar catalytic efficiency but a reduction in turnover number (k(cat)) from 78 to 58 h(-1). G9a methylated synthetic peptide substrates containing the first 13 amino acids of histone H3 efficiently, although methylation, acetylation, or mutation of proximal Lys-4 amino acids reduced Lys-9 methylation. The k(cat) for wild-type peptide substrate vs Lys-4 acetyl- or trimethyl-modified peptide were 88 and 32 h-1, respectively, and the K-m for the peptides varied from 0.6 to 2.2 mu M, resulting in a large difference (15-91) in catalytic efficiency. Ser-10 or Thr-11 phosphorylation resulted in poor methylation by G9a. Immunoprecipitation of unmodified and Ser-10 and Thr-11 phosphorylated histone H3 displayed mostly Lys-4 dimethylation. Dimethylated Lys-9 was reduced in Set-10 and Thr-11 immunoprecipitated phosphorylated histones as compared to nonphosphorylated H3. In an immunocytochemical assay, GFP fusion SUV39H1 or G9a did not colocalize with phosphorylated histone H3. Thus, Set-10/Thr-11 phosphorylation impairs Lys-9 methylation. These data suggest that the sequence context of the modified residue affects G9a activity and the modification in the proximal amino acids influences methylation.