The addition to proteins of the negatively charged polymer of ADP-ribose (PAR), which is synthesized by PAR polymerases (PARPs) from NAD(+), is a unique post-translational modification. It regulates not only cell survival and cell-death programmes, but also an increasing number of other biological functions with which novel members of the PARP family have been associated. These functions include transcriptional regulation, telomere cohesion and mitotic spindle formation during cell division, intracellular trafficking and energy metabolism.

SummaryPoly(ADP-ribosyl)ation is an immediate DNA-damagedependent post-translational modification of histones and other nuclear proteins that contributes to the survival of injured proliferating cells. Poly(ADP-ribose) polymerases (PARPs) now constitute a large family of 18 proteins, encoded by different genes and displaying a conserved catalytic domain in which PARP-1 (113 kDa), the founding member, and PARP-2 (62 kDa) are so far the sole enzymes whose catalytic activity has been shown to be immediately stimulated by DNA strand breaks. A large repertoire of sequences encoding novel PARPs now extends considerably the field of poly(ADP-ribosyl)ation reactions to various aspects of the cell biology including cell proliferation and cell death. Some of these new members interact with each other, share common partners and common subcellular localizations suggesting possible fine tuning in the regulation of this posttranslational modification of proteins. This review summarizes our present knowledge of this emerging superfamily, which might ultimately improve pharmacological strategies to enhance both antitumor efficacy and the treatment of a number of inflammatory and neurodegenerative disorders. A provisional nomenclature is proposed.

The DNA damage dependence of poly(ADP-ribose) polymerase-2 (PARP-2) activity is suggestive of its implication in genome surveillance and protection. Here we show that the PARP-2 gene, mainly expressed in actively dividing tissues follows, but to a smaller extent, that of PARP-1 during mouse development. We found that PARP-2 and PARP-1 homo-and heterodimerize; the interacting interfaces, sites of reciprocal modification, have been mapped. PARP-2 was also found to interact with three other proteins involved in the base excision repair pathway: x-ray cross complementing factor 1 (XRCC1), DNA polymerase ␤, and DNA ligase III, already known as partners of PARP-1. XRCC1 negatively regulates PARP-2 activity, as it does for PARP-1, while being a polymer acceptor for both PARP-1 and PARP-2. To gain insight into the physiological role of PARP-2 in response to genotoxic stress, we developed by gene disruption mice deficient in PARP-2. Following treatment by the alkylating agent N-nitroso-N-methylurea (MNU), PARP-2-deficient cells displayed an important delay in DNA strand breaks resealing, similar to that observed in PARP-1 deficient cells, thus confirming that PARP-2 is also an active player in base excision repair despite its low capacity to synthesize ADP-ribose polymers.In response to DNA interruptions, PARP-1, 1 the founding member of the poly(ADP-ribose) polymerase superfamily, catalyzes the successive covalent addition of ADP-ribose units from NAD to a limited number of nuclear acceptors to form a branched anionic polymer. PARP-1 is a nuclear enzyme involved in the detection and signaling of DNA strand breaks introduced either directly by ionizing radiation or indirectly following enzymatic incision of a DNA lesion (abasic sites or oxidized or alkylated bases) repaired by the base excision repair (BER) pathway (see for review Ref. 1). The discovery of numerous PARP-1 protein partners and/or poly(ADP-ribose) acceptors involved in DNA architecture (histones H1 and H2B, lamin B, and high mobility group proteins) or in DNA metabolism (DNA replication factors, DNA repair proteins, i.e. XRCC1, transcription factors, topoisomerases, and PARP-1 itself) has shed light onto the implication of PARP-1 in these processes (see for review Ref. 1).The function of PARP-1 in BER has long been assumed, until direct evidence demonstrated the presence of PARP-1 in the BER complex, associated to XRCC1 (2, 3) and DNA polymerase (pol) ␤ (4). The polymer produced by PARP-1 upon activation by DNA breaks triggers the recruitment of XRCC1, which shows high affinity for oligo(ADP-ribosyl)ated PARP-1 (3-5). The requirement of PARP-1 in BER was established in vivo, because PARP-1 knock-out cells displayed a severe defect in strand breaks resealing following genotoxic treatment (6, 7). The preferential role of PARP-1 in long patch BER was observed using extracts from these PARP-1 knock-out cells (4). Photoaffinity labeling experiments revealed that PARP-1 binds to BER intermediates (8). In reconstituted in vitro systems containing purified human B...

Poly(ADP-ribosylation) is a post-translational modification of nuclear proteins in response to DNA damage that activates the base excision repair machinery. Poly-(ADP-ribose) polymerase which we will now call PARP-1, has been the only known enzyme of this type for over 30 years. Here, we describe a cDNA encoding a 62-kDa protein that shares considerable homology with the catalytic domain of PARP-1 and also contains a basic DNA-binding domain. We propose to call this enzyme poly(ADP-ribose) polymerase 2 (PARP-2). The PARP-2 gene maps to chromosome 14C1 and 14q11.2 in mouse and human, respectively. Purified recombinant mouse PARP-2 is a damaged DNA-binding protein in vitro and catalyzes the formation of poly(ADP-ribose) polymers in a DNA-dependent manner. PARP-2 displays automodification properties similar to PARP-1. The protein is localized in the nucleus in vivo and may account for the residual poly(ADP-ribose) synthesis observed in PARP-1-deficient cells, treated with alkylating agents or hydrogen peroxide.In response to DNA-strand breaks introduced either directly by ionizing radiation or indirectly following enzymatic incision at a DNA lesion, the immediate poly(ADP-ribosylation) of nuclear proteins converts the DNA ends into intracellular signals that modulate DNA repair and cell survival programs. At the sites of DNA breakage, poly(ADP-ribose) polymerase (PARP) 1 (EC 2.4.2.30) catalyzes the transfer of the ADP-ribose moiety from its substrate NAD ϩ , to a limited number of proteins involved in chromatin architecture, DNA repair, or in DNA metabolism including PARP itself (1-4). Recently, the generation of PARP-deficient mice by homologous recombination (5, 6) has clearly demonstrated the involvement of PARP in the maintenance of the genomic integrity due to its role during base excision repair (7-9). An substantial delay in DNA strandbreak repair was observed following treatment of PARP-deficient cells with monofunctional alkylating agents (10). This severe DNA repair defect appears to be the primary cause for the observed cytotoxicity of N-methyl-N-nitrosourea, methylmethanesulfonate (MMS), or ␥-rays leading to cell death occurring after a G 2 /M block (10).It was assumed for many years that PARP activity was associated with a single protein displaying unique DNA damage detection and signaling properties. This assumption was challenged by the recent discovery in Arabidopsis thaliana of a gene coding for a PARP-related polypeptide of a calculated molecular mass of 72 kDa (11). It then became evident that two structurally different PARP proteins, both possessing DNA-dependent poly(ADP-ribose) activities, were present in both A. thaliana as well as in maize (12)(13)(14). 2 Furthermore, it has been reported recently that mouse embryonic fibroblasts derived from PARP knockout are capable of synthesizing ADP-ribose polymers in response to DNA damage (15), suggesting that in mammals, like in plants, at least one additional member of the PARP family may exist in addition to the classical zinc finger containing PAR...

Genome integrity is constantly threatened by DNA lesions arising from numerous exogenous and endogenous sources. Survival depends on immediate recognition of these lesions and rapid recruitment of repair factors. Using laser microirradiation and live cell microscopy we found that the DNA-damage dependent poly(ADP-ribose) polymerases (PARP) PARP-1 and PARP-2 are recruited to DNA damage sites, however, with different kinetics and roles. With specific PARP inhibitors and mutations, we could show that the initial recruitment of PARP-1 is mediated by the DNA-binding domain. PARP-1 activation and localized poly(ADP-ribose) synthesis then generates binding sites for a second wave of PARP-1 recruitment and for the rapid accumulation of the loading platform XRCC1 at repair sites. Further PARP-1 poly(ADP-ribosyl)ation eventually initiates the release of PARP-1. We conclude that feedback regulated recruitment of PARP-1 and concomitant local poly(ADP-ribosyl)ation at DNA lesions amplifies a signal for rapid recruitment of repair factors enabling efficient restoration of genome integrity.