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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., Competing Interests: Declaration of Competing Interest R.W.S. is co-founder of Canal House Biosciences, LLC, is on the Scientific Advisory Board, and has an equity interest. Canal House Biosciences was not involved in the preparation of this study nor of this manuscript. The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results., (Copyright © 2024 The Authors. Published by Elsevier B.V. All rights reserved.)

E3 ubiquitin ligases constitute a large family of enzymes that modify specific proteins by covalently attaching ubiquitin polypeptides. This post-translational modification can serve to regulate protein function or longevity. In spite of their importance in cell physiology, the biological roles of most ubiquitin ligases remain poorly understood. Here, we analyzed the function of the HECT domain family of E3 ubiquitin ligases in stem cell biology and tissue regeneration in planarians. Using bioinformatic searches, we identified 17 HECT E3 genes that are expressed in the Schmidtea mediterranea genome. Whole-mount in situ hybridization experiments showed that HECT genes were expressed in diverse tissues and most were expressed in the stem cell population (neoblasts) or in their progeny. To investigate the function of all HECT E3 ligases, we inhibited their expression using RNA interference (RNAi) and determined that orthologs of huwe1, wwp1, and trip12 had roles in tissue regeneration. We show that huwe1 RNAi knockdown led to a significant expansion of the neoblast population and death by lysis. Further, our experiments showed that wwp1 was necessary for both neoblast and intestinal tissue homeostasis as well as uncovered an unexpected role of trip12 in posterior tissue specification. Taken together, our data provide insights into the roles of HECT E3 ligases in tissue regeneration and demonstrate that planarians will be a useful model to evaluate the functions of E3 ubiquitin ligases in stem cell regulation., (Copyright © 2015 Elsevier Inc. All rights reserved.)