Fig. 4

The dynamic mechanism of ADP-ribosylation reversal. ARTDs consume NAD⁺ and transfer ADP-ribose moieties onto target substrates (blue boxes) on different amino acids side chains (green [X]). These proteins can have a variety of different ADP-ribosylation modification patterns, as described in Fig. 1. In the context of severe genotoxic insult, complex PAR polymers composed of large and branched molecules are synthesized by ARTD1 and ARTD2 (PARP-1 and PARP-2). These polymers are rapidly recognized and processed by a variety of erasers in a biphasic mode. At the same time, a variety of PAR readers can bind PAR and regulate the kinetics of the erasing process. In the first phase of the ADP-ribosylation reversal, PARG activity predominates and presumably exceeds ARH3 activity since PARG possesses high affinity for complex polymers and a very rapid and processive exoglycosidic activity toward ribose-ribose linkages. The dePARylation process is enhanced by the unique ability of PARG to cleave in-chain ribose-ribose linkages and branching points owing to its endoglycosidic activity. In addition, TARG1 can contribute to protein dePARylation by detaching entire PAR chains through cleavage of the proximal protein−ribose linkage. As PAR polymers are rapidly shortened by the combined endo- and exoglycosidic activities of PARG, the dePARylation activity drops and partially trimmed apoptogenic ADP-ribose oligomers accumulate. These small PAR fragments induce a second erasing wave in which rate and processivity of PARG is markedly decreased while ARH3 activity becomes dominant. The residual PARG activity and ARH3-catalyzed PAR hydrolysis generate MARylated proteins, which are further degraded by amino acid-specific MAR hydrolases and NUDIX phosphodiesterases. These waves of ADP-ribosylation erasing generate unmodified, phosphoribosylated, and MARylated proteins as well as free ADP-ribose. The latter might be deleterious for the cells and thus recycled by NUDT5 to quickly replenish ATP levels or converted to AMP which activates the AMP kinase (AMPK) and the mTOR signalling pathway