PAR recognition by multiple reader domains of PARP1 allosterically regulates the DNA-dependent activities and independently stimulates the catalytic activity of PARP1

Poly(ADP-ribosyl)ation is a post translational modification, predominantly catalyzed by Poly(ADP-ribose) polymerase 1 (PARP1) in response to DNA damage, mediating the DNA repair process to maintain genomic integrity. Single strand (SSB) and double strand (DSB) DNA breaks are bonafide stimulators of PARP1 activity. We identified that, in addition, single strand (ss) DNA also binds and stimulates the PARP1 activity. Poly(ADP-ribose) (PAR) is chemically similar to ssDNA. However, PAR mediated PARP1 regulation remains unexplored. Here, we report ZnF3, BRCT and WGR, hitherto uncharacterized, as PAR-specific reader domains of PARP1. Surprisingly, these domains recognize PARylated protein with a higher affinity compared to PAR, but do not bind to DNA. Conversely, N-terminal domains, ZnF1 and ZnF2, of PARP1 recognize DNA but not PAR. Further competition binding studies suggest that PAR binding, allosterically releases DNA from PARP1. Unexpectedly, PAR showed catalytic stimulation of PARP1 but hampers the DNA dependent stimulation. Altogether, our work discovers dedicated PAR and DNA reader domains of the PARP1, and uncovers a novel mechanism of allosteric stimulation of the catalytic activity of PARP1 but retardation of DNA-dependent activities of PARP1 by its catalytic product PAR.