Functional significance of the conserved residues for the 23-residue module among MTH1 and MutT family proteins

Human MTH1 and Escherichia coli Mutt proteins hydrolyze 7,8-dihydro-8- oxo-dGTP (8-oxo-dGTP) to monophosphate, thus avoiding the incorporation of 8- oxo-7,8-dihydroguanine into nascent DNA. Although only 30 amino acid residues (23%) are identical between MTH1 and MutT, there is a highly conserved region consisting of 23 residues (MTH1, Gly³⁶-Gly⁵⁸) with 14 identical residues. A chimeric protein MTH1-Ec, in which the 23-residue sequence of MTH1 was replaced with that of MutT, retains its capability to hydrolyze 8-oxo-dGTP, thereby indicating that the 23-residue sequences of MTH1 and Mutt are functionally and structurally equivalent and constitute functional modules. By saturation mutagenesis of the module in MTH1, 14 of the 23 residues proved to be essential to exert 8-oxo-dGTPase activity. For the other 9 residues (40, 42, 44, 46, 47, 49, 50, 54, and 58), positive mutants were obtained, and Arg⁵⁰ can be replaced with hydrophobic residues (Val, Leu, or Ile), with a greater stability and higher specific activity of the enzyme. Indispensabilities of Val³⁹, Ile⁴⁵, and Leu⁵³ indicate that an amphipathic property of α-helix I consisting of 14 residues of the module (Thr⁴⁴-Gly⁵⁸) is essential to maintain the stable catalytic surface for 8-oxo-dGTPase.