Molecular genetics and structural biology of human MutT homolog, MTH1

The human MTH1 gene located on chromosome 7p22 consists of 5 major exons. M TH1 gene produces seven types of mRNAs and the B-type mRNAs with exon 2b-2c segments direct synthesis of three forms of MTH1 polypeptides (p22, p21, a nd p18) by alternative initiation of translation, while the others encode o nly p18. In human cells. p18, the major form is mostly localized in the cyt oplasm with some in the mitochondria. A single nucleotide polymorphism (SNP ) in exon 2, which is tightly liked to another SNP (GTG(83)/ATG(83)), creat es an additional alternative in-frame AUG in B-type MTH1 mRNAs yielding the fourth MTH1 polypeptide, p26 that possesses an additional mitochondrial ta rgeting signal. These SNPs are likely to be one of the risk factors for can cer or for neuronal degeneration. The 30 amino acid residues are identical between MTH1 and MutT, and there is a highly conserved region consisting of 23 residues (MTH1: Gly36 to Gly58), with 14 identical residues. A chimeric protein in which the 23 residue sequence of MTH1 was replaced with that of MutT, retains the capability to hydrolyze 8-oxo-dGTP, indicating that the 23 residue sequences of MTH1 and MutT are functionally and structurally equ ivalent, and constitute a functional phosphohydrolase module. Saturated mut agenesis of the module in MTH1 indicated that an amphipathic property of th e alpha -helix I consisting of 14 residues of the module (Thr44 to Gly58) i s essential to maintain the stable catalytic surface for 8-oxo-dGTPase. MTH 1 but not MutT efficiently hydrolyzes two forms of oxidized dATP, 2-hydroxy -dATP and 8-oxo-dATP, as well as 8-oxo-dGTP and 8-oxo-GTP. Thus, MTH1 is de signated as the oxidized purine nucleoside triphosphatase and has a much wi der substrate specificity than MutT. There is a significant homology betwee n MTH1 protein and the C-terminal half of human MYH protein, which may be i nvolved in the recognition of 8-oxoguanine and 2-hydroxyadenine, (C) 2001 E lsevier Science B.V. All rights reserved.