Coordination of enzymatic activities in the course of base excision repair (BER) is essential to ensure complete repair of damaged bases. Two major mechanisms underlying the coordination of BER are known today: the "passing the baton" model and a model of preassembled stable multiprotein repair complexes called "repairosomes." In this work, we aimed to elucidate the coordination between human apurinic/apyrimidinic (AP) endonuclease APE1 and DNA polymerase Polβ in BER through studying an impact of APE1 on Polβ-catalyzed nucleotide incorporation into different model substrates that mimic different single-strand break (SSB) intermediates arising along the BER pathway. It was found that APE1's impact on separate stages of Polβ's catalysis depends on the nature of a DNA substrate. In this complex, APE1 removed 3' blocking groups and corrected Polβ-catalyzed DNA synthesis in a coordinated manner. Our findings support the hypothesis that Polβ not only can displace APE1 from damaged DNA within the "passing the baton" model but also performs the gap-filling reaction in the ternary complex with APE1 according to the "repairosome" model. Taken together, our results provide new insights into coordination between APE1 and Polβ during the BER process.
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