ADP-ribosylation of mitochondrial proteins is mediated by Neuralized-like protein 4 (NEURL4)

Abstract

ADP-ribosylation is a reversible post-translational modification where an ADP-ribose moiety is covalently attached to amino acid side-chains of target proteins either as mono-ADP-ribose (MARylation or MAR) or poly-ADP-ribose chains (PARylation or PAR) by a class of enzymes called ADP-ribosyltransferases (ARTs). Although ADP-ribosylation is best known for its nuclear roles, ADP-ribosylation of extra nuclear proteins is increasingly recognized as a key regulatory strategy across cellular compartments. ADP-ribosylation of mitochondrial proteins, in particular, has been widely reported, even though the extent to which ADP-ribosylation of specific proteins regulates mitochondrial functions is unclear and the exact nature of mitochondrial ART enzymes is debated. Here, we have identified Neuralized-like protein 4 (NEURL4) as a mitochondrial ART enzyme and profiled the NEURL4-dependent ADP-ribosylome in mitochondrial extracts from Hela cells by LC-MS/MS, using isobaric tandem mass tag (TMT) labeling for relative quantification. Comparison of WT and NEURL4-KO cells generated by CRISPR/Cas9 genome editing revealed that most ART activity associated with mitochondria is lost in absence of NEURL4. Putative NEURL4 targets include numerous mitochondrial proteins previously shown to be ADP-ribosylated. In particular, we show that NEURL4 enzymatic activity is required for the regulation of mtDNA integrity via poly-ADP-ribosylation of mitochondrial specific Ligase III (mtLIG3), the rate-limiting enzyme for mitochondrial DNA (mtDNA) Base Excision Repair (BER). Collectively, our studies reveal that NEURL4 acts as the main mitochondrial ART enzyme under physiological conditions and provide novel insights in the regulation of mitochondria homeostasis through ADP-ribosylation.