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H3TH domain of Exonuclease-1, a structure-specific, divalent-metal-ion dependent, 5' nuclease Exonuclease-1 (EXO1) is involved in multiple, eukaryotic DNA metabolic pathways, including DNA replication processes (5' flap DNA endonuclease activity and double stranded DNA 5'-exonuclease activity), DNA repair processes (DNA mismatch repair (MMR) and post-replication repair (PRR), recombination, and telomere integrity. EXO1 functions in the MMS2 error-free branch of the PRR pathway in the maintenance and repair of stalled replication forks. Studies also suggest that EXO1 plays both structural and catalytic roles during MMR-mediated mutation avoidance. Members of this subgroup include the H3TH (helix-3-turn-helix) domains of EXO1 and other similar eukaryotic 5' nucleases. These nucleases contain a PIN (PilT N terminus) domain with a helical arch/clamp region/I domain (not included here) and inserted within the PIN domain is an atypical helix-hairpin-helix-2 (HhH2)-like region. This atypical HhH2 region, the H3TH domain, has an extended loop with at least three turns between the first two helices, and only three of the four helices appear to be conserved. Both the H3TH domain and the helical arch/clamp region are involved in DNA binding. Studies suggest that a glycine-rich loop in the H3TH domain contacts the phosphate backbone of the template strand in the downstream DNA duplex. These nucleases have a carboxylate rich active site that is involved in binding essential divalent metal ion cofactors (Mg2+ or Mn2+) required for nuclease activity. The first metal binding site is composed entirely of Asp/Glu residues from the PIN domain, whereas, the second metal binding site is composed generally of two Asp residues from the PIN domain and one Asp residue from the H3TH domain. Together with the helical arch and network of amino acids interacting with metal binding ions, the H3TH region defines a positively charged active-site DNA-binding groove in structure-specific 5' nucleases. EXO1 nucleases also have C-terminal Mlh1- and Msh2-binding domains which allow interaction with MMR and PRR proteins, respectively. |
Jiyao W et al.(2023). "The conserved domain database in 2023.",