C-terminal residues of DNA polymerase β and E3 ligase required for ubiquitin-linked proteolysis of oxidative DNA-protein crosslinks

Abstract

Free radicals produce in DNA a large variety of base and deoxyribose lesions that are corrected by the base excision DNA repair (BER) system. However, the C1′-oxidized abasic residue 2-deoxyribonolactone (dL) traps DNA repair lyases in covalent DNA-protein crosslinks (DPC), including the core BER enzyme DNA polymerase beta (Polβ). Polβ-DPC are rapidly processed in mammalian cells by proteasome-dependent digestion. Blocking the proteasome causes oxidative Polβ-DPC to accumulate in a ubiquitylated form, and this accumulation is toxic to human cells. In the current study, we investigated the mechanism of Polβ-DPC processing in cells exposed to the dL-inducing oxidant 1,10-copper-ortho-phenanthroline. Alanine substitution of either or both of two Polβ C-terminal residues, lysine-206 and lysine-244, enhanced the accumulation of mutant Polβ-DPC relative to the wild-type protein, and removal of the mutant DPC was diminished. Substitution of the N-terminal lysines 41, 61, and 81 did not affect Polβ-DPC processing. For Polβ with the C-terminal lysine substitutions, the amount of ubiquitin in the stabilized DPC was lowered by ∼40 % relative to wild-type Polβ. Suppression of the HECT domain-containing E3 ubiquitin ligase TRIP12 augmented the formation of oxidative Polβ-DPC and prevented Polβ-DPC removal in oxidant-treated cells. Consistent with the toxicity of accumulated oxidative Polβ-DPC, TRIP12 knockdown increased oxidant-mediated cytotoxicity. Thus, ubiquitylation of lysine-206 and lysine-244 by TRIP12 is necessary for digestion of Polβ-DPC by the proteasome as the rapid first steps of DPC repair to prevent their cytotoxic accumulation. Understanding how DPC formed with Polβ or other AP lyases are repaired in vivo is an important step in revealing how cells cope with the toxic potential of such adducts.