Poly(ADP-ribose) polymerase (PARP) proteins comprise a group of ADP-ribosyl transferase enzymes, which transfer negatively charged ADP-ribose groups from donor NAD+ molecules onto their target proteins post-translationally. Through poly(ADP-ribosyl)ation (PARylation) of target proteins, PARPs control a wide array of cellular processes, such as DNA repair, transcriptional regulation, RNA interference, mitochondrial function, formation of subnuclear bodies and cell division. PARylation is particularly prevalent during stress responses that require rapid adaptation. The functions and mechanisms of PARylation have been best characterized for a few PARPs, including PARP1, PARP2 and tankyrases. PARylation is a posttranslational modification in which ADP-ribose units are added to Glu, Asp and Lys residues of target (or acceptor) proteins by members of the PARP family. The PARP catalytic domain binds NAD+ via a protein fold that shares homology with mono(ADP-ribosyl) ating bacterial exotoxins, such as diphtheria toxin and exotoxin A. 
PARP proteins are responsible for the synthesis of PAR and, as might be expected, a number of proteins with PAR-degrading activities promote the rapid catabolic destruction of PAR almost immediately after synthesis. PARP1 association with histones or structured DNA can directly stimulate its activity. Post-translational modification of PARPs can also alter PARP activation. PARP family members, and their targets, are subjected to a wide range of post-translational modifications, including phosphorylation, acetylation, methylation, SUMOylation and ubiquitylation. PARPs regulate a wide array of cellular processes, including transcription, DNA repair, mitochondrial function and the formation of cytoplasmic and nuclear suborganellar bodies.


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