Post-translational modifications in DNA base excision repair : The roles of CK2 and PARP-1

Abstract: Base lesions and DNA single-strand breaks (SSBs) are very common types of DNA damage. The base excision repair (BER) and single-strand break repair (SSBR) machineries both require a succession of enzymatic events in order to remove these types of endogenous lesions and to restore the DNA. Coordinated repair involves signalling between the proteins concerned and is achieved by post-translational modification. Here, we study two types of modifications in the context of BER and SSBR.Poly(ADP-ribose) polymerase-1 (PARP-1) is a known SSB sensor, which utilizes NAD+ and converts these to ADP-ribose polymers as a post-translational modification of primarily itself, to accelerate repair. However, its role in BER is not as clear. By quantification of SSBs in vivo, we find that PARP inhibition prevents the completion of BER, while siRNA knockdown of PARP-1 leaves repair unaffected. Our results indicate that PARP-1 is not required for BER to progress, but that the enzyme interferes with the SSB intermediate.Another known post-translational modification in SSBR is the phosphorylation of XRCC1 by CK2. Here, we show that the majority of the cellular XRCC1 is phosphorylated and that CK2 is the main kinase responsible for this. We find that this modification prevents degradation of XRCC1 by the proteasome, resulting in faster repair of oxidative damage in the DNA. In addition, we propose a new role for CK2 modifications of XRCC1 in BER. We demonstrate that, even though the presence of XRCC1 or the activity of PARP are not required for SSB intermediate formation, the expression of a non-phosphorylated form of XRCC1 results in reduced SSB levels. Furthermore, the affinity of XRCC1 for a nicked DNA substrate increases when the CK2 phosphorylation sites are mutated.To summarise, our findings increase the knowledge of the BER and SSBR processes and demonstrate that the impact of post-translational modifications is more complex than it originally appeared.