PARP1 and PARP2-Mediated Poly(ADP-ribosyl)ation

ADP-ribosylation (ADPRylation) is a fundamental and evolutionarily conserved post-translational modification (PTM) where the ADP-ribose (ADPr) moiety from nicotinamide adenine dinucleotide (NAD+) is transferred onto acceptor molecules, primarily proteins.1 This process, releasing nicotinamide (NAM) as a byproduct, plays critical roles in a vast array of cellular functions, including the maintenance of genome stability, transcriptional regulation, chromatin dynamics, cell cycle control, inflammation, metabolism, and cell death signaling.3 ADPRylation exists in two major forms: mono-ADPRylation (MARylation), the attachment of a single ADPr unit, and poly-ADPRylation (PARylation), the extension of the initial ADPr moiety into linear or branched chains of poly(ADP-ribose) (PAR).1

The enzymatic machinery responsible for ADPRylation comprises the Poly(ADP-ribose) Polymerase (PARP) superfamily, also known as ADP-ribosyltransferases Diphtheria toxin-like (ARTDs).3 In humans, this family encompasses approximately 17-18 members encoded by distinct genes, all sharing homology within a conserved catalytic domain.2 Despite the family name, the capacity for PAR synthesis (PARylation) is confirmed for only a subset of these enzymes: PARP1, PARP2, PARP5a (Tankyrase 1), and PARP5b (Tankyrase 2).3 Most other members catalyze MARylation or have yet undetermined enzymatic activity.3

The distinct and overlapping functions of PARP1 and PARP2 are rooted in their specific domain architectures.

PARP1: This multi-domain protein (~113 kDa) comprises several functionally distinct regions arranged linearly 2:

The PAR polymers synthesized by PARP1 and PARP2 are not uniform structures but exhibit significant heterogeneity, leading to the concept of a "PAR code."

PAR is a polymer composed of repeating ADPr units linked primarily through linear α(1′′→2′) O-glycosidic bonds, with occasional α(1′′′→2′′) bonds creating branch points.1 Each ADPr unit contains two phosphate groups, rendering the PAR polymer highly negatively charged.7 This polyanionic nature is crucial for many of its functions, including modulating chromatin structure and mediating protein interactions.

The biological functions of PARylation are largely mediated by proteins that recognize and bind to PAR chains non-covalently. These "reader" proteins utilize specialized PAR-binding motifs or domains to interpret the PAR signal and translate it into downstream actions.1 A diverse array of such modules has been identified, each with distinct structural features and recognition specificities.

Four classes of PAR-binding modules are particularly well-characterized 35:

PARP1 and PARP2 activation and subsequent PAR synthesis constitute one of the earliest cellular responses to DNA damage, particularly DNA strand breaks.10 The locally generated PAR polymers act as a potent signal and a dynamic scaffold, recruiting a diverse array of downstream factors—the PAR interactome—to the site of damage.7 This recruitment is essential for initiating and coordinating various DNA repair pathways and modulating chromatin structure to facilitate access to the lesion.

Numerous proteins involved in virtually all major DNA repair pathways utilize PAR binding for their recruitment or regulation:

The biological impact of PARylation is profoundly influenced by its precise timing and duration, which are tightly regulated through the interplay of synthesis and degradation enzymes.

A hallmark of the PARP1/2 response to DNA damage is its speed. PARP1, in particular, is recruited to DNA breaks almost instantaneously, with detection occurring within seconds of lesion formation.19 This rapid binding triggers immediate catalytic activation, leading to a burst of PAR synthesis.10 Intranuclear PAR levels can surge dramatically, potentially increasing by up to 500-fold over baseline levels within minutes, consuming a significant portion of the cellular NAD+ pool.19 This rapid accumulation of PAR serves as an urgent signal, broadcasting the presence of DNA damage and initiating the recruitment cascade of repair factors.

While PARP1 and PARP2 share core functionalities, significant differences in their structure, regulation, and catalytic output contribute to both redundant and specific roles in cellular processes, particularly the DDR. Furthermore, the accessory factor HPF1 dramatically modulates their activity, adding another layer of specificity.

Shared Roles and Redundancy:

Poly(ADP-ribosyl)ation mediated by PARP1 and PARP2 stands as a cornerstone of cellular regulation, extending far beyond its canonical role in the DNA damage response to influence transcription, chromatin architecture, metabolism, and cell fate decisions. This review has synthesized current understanding of the intricate molecular mechanisms governing PARP1/2 activity, the structural complexity of the PAR signal, the diverse ways this signal is interpreted by cellular machinery, and the critical temporal dynamics that shape the response.

The structure-function relationships of PARP1 and PARP2 reveal a blend of conserved mechanisms and specialized features. Both enzymes utilize a sophisticated DNA-dependent allosteric switch involving the HD domain to control catalytic activity, yet they differ significantly in their DNA recognition modules (Zn fingers vs. WGR), catalytic efficiency, contribution to PAR chain length versus branching, and recruitment kinetics. This suggests a coordinated interplay where PARP1 acts as a rapid initiator generating long PAR scaffolds, while PARP2 modulates these structures by adding branches and potentially sustaining the signal over longer periods.