The DNA repair enzyme MutY plays an important role in the prevention of DNA mutations resulting from the presence of the oxidatively damaged lesion 7,8-dihydro-8-oxo-2-deoxyguanosine (OG). MutY is a base excision repair (BER) glycosylase that removes misincorporated adenine residues from OG:A mispairs, as well as G:A and C:A mispairs. We have previously shown that, under conditions of low MutY concentrations relative to an OG:A or G:A substrate, the time course of the adenine glycosylase reaction exhibits biphasic kinetic behavior due to slow release of the DNA product by MutY. The dissociation of MutY from its product may require the recruitment of other proteins from the BER pathway, such as an apurinic-apyrimidinic (AP) endonuclease, as turnover-enhancing cofactors. The effect of the AP endonucleases endonuclease IV (Endo IV), exonuclease III (Exo III), and Ape1 on the reaction kinetics of MutY with G:A-and OG:A-containing substrates was investigated. The effect of the glycosylases UDG and MutM and the DNA polymerase pol I was also characterized. Endo IV and Exo III, unlike Ape1, UDG, and pol I, greatly enhance the rate of product release with a G:A substrate, whereas the rate constant for the adenine removal step remains unchanged. Furthermore, the turnover rate with a truncated form of MutY, Stop 225, which lacks 125 amino acids of the C terminus, is unaffected by the presence of Endo IV or Exo III. These results constitute the first evidence of an interaction between the MutY-product DNA complex and Endo IV or Exo III. Furthermore, they suggest a role for the C-terminal domain of MutY in mediating this interaction.
The base excision repair (BER)1 pathway is the primary cellular mechanism charged with the task of removing DNA bases modified via hydrolysis, oxidation, and alkylation (1). BER relies on the action of damage-specific DNA glycosylases that recognize various types of modified or inappropriate bases within the context of normal Watson-Crick DNA (2). The repair process is initiated by hydrolytic removal of the target base by the relevant DNA glycosylase and proceeds by incision at the abasic site, generation of a gap, reparative DNA synthesis, and ligation of the nicked DNA (1, 3). The BER pathway has been reconstituted in vitro with cell-free extracts or purified protein components, and these experiments have established the minimal requirements for restoration of the damaged DNA (4 -11). For example, repair of uracil in DNA was achieved by use of five Escherichia coli proteins: uracil-DNA glycosylase (UDG), endonuclease IV (an AP endonuclease), RecJ protein, DNA polymerase I, and DNA ligase (7). Both "short patch" and "long patch" BER pathways have been observed, which differ in the number of nucleotides and the various protein components involved (12, 13). The type of BER pathway utilized depends on the organism and the type of DNA damage; however, minimally the BER pathway requires a damage-specific glycosylase, an AP endonuclease, DNA polymerase, and DNA ligase.The DNA glycosylases of BER may ...