In response to DSB the lesion

In response to DSB, the lesion recognition factor Mre11-Rad50-Nbs1 (MRN) complex helps the recruitment of ATM to the damage site and its activation by phosphorylation [29]. However, whether UV-damage recognition factors directly influence ATR and ATM recruitment and their phosphorylation is not clearly established. Jiang and Sancar showed direct binding of ATR to the damaged DNA without lesion processing, raising the possibility that ATR may activate the checkpoint signaling directly [30]. Furthermore, Vrouwe et al. reported that UV-induced photolesions results in checkpoint activation in NER-dependent and -independent pathways [31]. Recently, Oh et al. reported γH2AX foci formation after UV-irradiation in cox inhibitor lacking NER [32]. In yeast, UV-induced DNA damage results in checkpoint activation independent of NER lesion processing [33], [34]. These results support that lesion processing is not essential for γH2AX formation and checkpoint activation. However, several studies reported that lesion processing by NER factors might be an essential step in γH2AX foci formation [35], [36], [37], [38], [39]. Even though these studies support that the checkpoint activation induced by UV irradiation requires a functional NER apparatus, these studies do not show how and when ATR and ATM are recruited to the damage site and result in phosphorylation of downstream substrates. It has been shown that in response to UV irradiation, RPA-coated ssDNA recruits ATR to the UV damage site [15], [16]. This supports the possibility of ATR and ATM recruitment after incision of the UV damage. However, in case of mismatch repair, ATR is recruited to the damage site by the lesion recognition factors and also by the RPA-coated ssDNA [40]. Additionally, in DSB repair pathway, the lesion recognition factor MRN complex influences ATM recruitment [41]. Furthermore, in response to cisplatin treatment, XPC physically interacts with ATM, and is involved in ATM activation [42]. Whether the NER proteins play any direct role in ATR and ATM recruitment, however, has not been shown. To further gain insight into the mechanism of ATR and ATM recruitment and activation, we examined the roles of DDB2 and XPC in the recruitment and activation of ATR and ATM. Here, we show that XPC physically interacts with ATR and ATM. Both DDB2 and XPC facilitate ATR and ATM recruitment to the damage site, and promote their phosphorylation. This eventually affects the recruitment and phosphorylation of their substrate proteins at the damage site. We propose that DDB2 and XPC help assemble the ATR and ATM complex at the UV damage site and facilitate their activation to provoke the downstream cascade constituting the DNA damage response pathway.
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Conflict of interest
Acknowledgements We would like to thank Dr. Qianzheng Zhu and Chesequa Blevins for careful reading of the manuscript. This work was supported by the National Institute of Health grant [ES2388, ES12991 & CA93413] to AAW, and Pelotonia postdoctoral fellowship to A.B.
Introduction Two major pathways take care of repairing DSBs: non-homologous end-joining (NHEJ) and homologous recombination (HR). NHEJ directly ligates together the two broken ends with little or no processing [1] and is highly efficient, but it can lead to mutations at the joining sites, as well as inversions and translocations. HR is more accurate, because it uses undamaged homologous DNA sequences (sister chromatids or homologous chromosomes) as a template for repair in an error-free manner [2]. Making the right choice between NHEJ and HR is important to ensure genome stability. Generation of DNA DSBs elicits the activation of sophisticated surveillance mechanisms, the DNA damage checkpoints, which initiate a coordinated cellular response [3]. Activation of the DNA damage checkpoint results in cell cycle arrest and DNA repair or programmed cell death. Key players in the checkpoint response are phosphatidylinositol 3-kinase related protein kinases, such as mammalian ATM (ataxia-telangiectasia-mutated) and ATR (ATM- and Rad3-related), Saccharomyces cerevisiae Tel1 and Mec1, and Schizosaccharomyces pombe Tel1 and Rad3. In humans, ATM congenital deficiency results in ataxia-telangiectasia [4], which is a rare, autosomal recessive disorder characterized by progressive cerebellar ataxia, neuro-degeneration, radiosensitivity, checkpoint defects, genome instability and predisposition to cancer. Similarly, mutations in ATR are associated with Seckel syndrome, a clinically distinct disorder characterized by proportionate growth retardation and severe microcephaly [5]. Here we will focus on the work done in S. cerevisiae and mammals to review the early steps in DSB processing and signaling, as well as the regulation of ATM/Tel1 and ATR/Mec1 signaling activities in responding to DNA DSBs.