Mutations in the adenomatous polyposis coli (APC) gene are associated with an early onset of colorectal carcinogenesis. Previously, we described a novel role for the APC polypeptide in base excision repair (BER). The single-nucleotide (SN) and long-patch (LP) BER pathways act to repair the abasic sites in DNA that are induced by stressors, such as spontaneous oxidation/reduction, alkylation, and hyperthermia. We have shown that APC interacts with DNA polymerase β (Pol-β) and flap endonuclease 1 (Fen-1) and blocks Pol-β-directed strand-displacement synthesis. In this study, we have mapped the APC interaction site in Pol-β and have found that Thr79, Lys81, and Arg83 of Pol-β were critical for its interaction with APC. The Pol-β protein (T79A/K81A/R83A) blocked strand-displacement DNA synthesis in which tetrahydrofuran was used as DNA substrate. We further showed that the APC-mediated blockage of LP-BER was due to inhibition of Fen-1 activity. Analysis of the APC-mediated blockage of SN-BER indicated that the interaction of APC with Pol-β blocked SN-BER activity by inhibiting Pol-β-directed deoxyribose phosphate lyase activity. Collectively, our findings indicate that APC blocked both Pol-β-directed SN- and LP-BER pathways and increased sensitivity of cells to alkylation induced DNA damage.
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