title: Site-directed mutagenesis of the human DNA repair enzyme HAP1: identification of residues important for AP endonuclease and RNase H activity.
abstract: HAP1 protein, the major apurinic/apyrimidinic (AP) endonuclease in human cells, is a member of a homologous family of multifunctional DNA repair enzymes including the Escherichia coli exonuclease III and Drosophila Rrp1 proteins. The most extensively characterised member of this family, exonuclease III, exhibits both DNA- and RNA-specific nuclease activities. Here, we show that the RNase H activity characteristic of exonuclease III has been conserved in the human homologue, although the products resulting from RNA cleavage are dissimilar. To identify residues important for enzymatic activity, five mutant HAP1 proteins containing single amino acid substitutions were purified and analysed in vitro. The substitutions were made at sites of conserved amino acids and targeted either acidic or histidine residues because of their known participation in the active sites of hydrolytic nucleases. One of the mutant proteins (replacement of Asp-219 by alanine) showed a markedly reduced enzymatic activity, consistent with a greatly diminished capacity to bind DNA and RNA. In contrast, replacement of Asp-90, Asp-308 or Glu-96 by alanine led to a reduction in enzymatic activity without significantly compromising nucleic acid binding. Replacement of His-255 by alanine led to only a very small reduction in enzymatic activity. Our data are consistent with the presence of a single catalytic active site for the DNA- and RNA-specific nuclease activities of the HAP1 protein.
authors.affiliation: Imperial Cancer Research Fund Laboratories, University of Oxford, John Radcliffe Hospital, UK.
authors.forename: G
authors.initials: G
authors.lastname: Barzilay
authors.forename: L J
authors.initials: LJ
authors.lastname: Walker
authors.forename: C N
authors.initials: CN
authors.lastname: Robson
authors.forename: I D
authors.initials: ID
authors.lastname: Hickson
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D019757:Carbon-Oxygen Lyases; D004260:DNA Repair; D043603:DNA-(Apurinic or Apyrimidinic Site) Lyase; D043223:Deoxyribonuclease IV (Phage T4-Induced); D004330:Drosophila / Q000201:enzymology; D004789:Enzyme Activation; D004926:Escherichia coli / Q000201:enzymology; D029968:Escherichia coli Proteins; D006801:Humans; D008190:Lyases / Q000378:metabolism; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D009687:Nuclear Proteins / Q000235:genetics* / Q000378:metabolism; D012259:Ribonuclease, Pancreatic / Q000378:metabolism; D016415:Sequence Alignment
chemical_list: D029968:Escherichia coli Proteins; D009687:Nuclear Proteins; D043223:Deoxyribonuclease IV (Phage T4-Induced); C468972:endonuclease IV, E coli; D012259:Ribonuclease, Pancreatic; D008190:Lyases; D019757:Carbon-Oxygen Lyases; C085108:APEX1 protein, human; D043603:DNA-(Apurinic or Apyrimidinic Site) Lyase