Regulation of PARP1 and its apoptotic variant activity by single-stranded DNA.

PARP1 is a nuclear protein involved in the maintenance of genomic stability. It catalyzes the formation of poly(ADP-ribose) (PAR) to recruit repair proteins at the site of DNA lesions, such as double-strand and single-strand breaks. In the process of DNA replication or repair, there could occur stretch of ssDNA, usually protected by ssDNA binding proteins, but when present in abundance can turn into DNA beaks and cause cell death. PARP1 is an extremely sensitive sensor of DNA breaks, however, the interaction of PARP1 with single-stranded DNA (ssDNA) remains unexplored. Here we report that the two Zn-fingers, ZnF1 and ZnF2, of PARP1, mediate high-affinity recognition of ssDNA. Our studies suggest that though PAR and ssDNA are chemical analogues, they are recognized by a distinct set of domains of PARP1, yet PAR not only induces dislodging of ssDNA from PARP1 but also hampers the ssDNA-dependent PARP1 activity. It is noteworthy that PAR carrier apoptotic fragment PARP1 ΔZnF1-2 gets cleaved from PARP1 to facilitate apoptosis, leaving behind the DNA-bound ZnF1-ZnF2 PARP1 . Our studies demonstrate that the PARP1 ΔZnF1-2 is competent for ssDNA-dependent stimulation only in the presence of another apoptotic fragment ZnF1-ZnF2 PARP1 , suggesting the indispensability of DNA-bound ZnF1-ZnF2 PARP1 dual domains for the same.