Role of Ape1 acetylation in fine tuning its activity on telomeric regions

One of the hallmarks of cancer is loss of telomeres stability. Unprotected telomeres are prone to aberrant end-joining reactions that lead to chromosomal fusions and translocations. Human telomeres are formed by the repeated TTAGGG sequence, in which the 3’ exposed strand may adopt a G-quadruplex (G4) structure. The guanine-rich regions of telomeres are hotspots for oxidation forming 8-oxoguanine, a lesion that is handled by the base excision repair (BER) pathway. One key player of this pathway is Ape1, the main human endonuclease processing abasic sites. Recent evidences showed an important role for Ape1 in telomeric physiology, but the molecular details regulating Ape1 enzymatic activities on G4- telomeric sequences are lacking. Through a combination of in vitro assays, we demonstrate that Ape1 can bind and process different G4 structures containing abasic site analog (THF) in two different locations and that the interaction involves specific acetylatable lysine residues (i.e. K27/K31/K32/K35) present in the unstructured N-terminal domain of the protein. The cleavage of an abasic site located in a G4 structure by Ape1 depends on the DNA conformation or the position of the lesion and on electrostatic interactions between the protein and the nucleic acids. Moreover, Ape1 mutants mimicking the acetylated protein display increased cleavage activity for abasic sites. We found that nucleophosmin (NPM1), which binds the N-terminal domain of Ape1, plays a role in modulating telomere length and Ape1 activity at abasic G4 structures. Thus, the Ape1 N-domain is an important relay site for regulating the enzyme’s activity on G4-telomeric sequences, and specific acetylatable lysine residues constitute key regulatory sites of Ape1 enzymatic activity dynamics at telomeres