DNA-induced dimerization of poly(ADP-ribose) polymerase-1 triggers its activation

In response to DNA strand breaks in the genome of higher eukaryotes, poly(ADP-ribose)polymerase 1 (PARP-1) catalyses the covalent attachment of ADP-ribose units from NAD+ to various nuclear acceptor proteins including PARP-I itself. This post-translational modification affecting proteins involved in chromatin architecture and in DNA repair plays a critical role in cell survival as well as in caspase-independent cell death. Although PARP-1 has been best-studied for its role in genome stability, several recent reports have demonstrated its role in the regulation of transcription. In this study, fluorescence spectroscopy and biochemical techniques are used to investigate the association of the amino-terminal DNA-binding domain of human PARP-1 (hPARP-1 DBD) with various DNA substrates, characterized by different DNA ends and sequence features (5- or 3'-recessed end, double strands, telomeric repeats, and the palindromic sequence of a Not I restriction site). The correlation between the binding mode of hPARP-1 DBD to the DNA oligoduplexes and the enzymatic activation of hPARP-1 is analyzed. We show that hPARP-1 DBD binds a 5'-recessed DNA end cooperatively with a stoichiometry of two proteins per DNA molecule. In contrast, a 1:1 stoichiometry is found in the presence of a 3'-recessed end and double-strand DNA. A palindromic structure like the Not I restriction site is shown to induce protein dimerization and high enzymatic activation, suggesting that it can represent a recognition element for hPARP-1 in undamaged cells. Protein dimerization is found to be a requisite for high enzymatic activity. Taken together, our data allow further characterization of the features of hPARP-1 recognition in damaged cells and bring additional evidence that hPARP-1 may also play a role in undamaged cells.