It is of extreme importance to investigate the DNA repair pathways that will lead to a better understanding to develop cancer treatments since the primary cause of cancer is the inability of the cell to restore the damaged DNA. Upon DNA damage, the histone tails of the nucleosomes near the DNA lesions undergo a group of extensive post-translational modifications that remodels the chromatin structures in order to repair the damage. DNA double-strand break (DSB) induces the phosphorylation of histone variant H2A.X at Serine 139 residue, that is recognized by the mediator of DNA damage checkpoint, MDC1. This interaction is imperative for the recruitment of the other downstream DNA damage response (DDR) proteins th... More
It is of extreme importance to investigate the DNA repair pathways that will lead to a better understanding to develop cancer treatments since the primary cause of cancer is the inability of the cell to restore the damaged DNA. Upon DNA damage, the histone tails of the nucleosomes near the DNA lesions undergo a group of extensive post-translational modifications that remodels the chromatin structures in order to repair the damage. DNA double-strand break (DSB) induces the phosphorylation of histone variant H2A.X at Serine 139 residue, that is recognized by the mediator of DNA damage checkpoint, MDC1. This interaction is imperative for the recruitment of the other downstream DNA damage response (DDR) proteins that enact in selecting the repair pathways. Structural basis of interaction of the MDC1 BRCT domain with the phospho-peptide tail of H2A.X was previously revealed. However, the impacts of this interaction on the overall chromatin structure, and the successive nucleosome modifications are still elusive. We hypothesize that MDC1 BRCT domain not only binds to the phosphorylated H2A.X tail, but this interaction might position MDC1 in a way that its basic patch might interact with the nucleosomal DNA, or with the conserved acidic patch of H2A.X/H2B. To investigate these concepts in vitro, a successful purification of the nucleosome containing γH2A.X is the prerequisite. For decades, histones have been extracted from the cell lysate using urea denaturant with subsequent extensive dialysis to refold the heteromeric histone complexes. This method is time and cost intensive. Here, we have developed two rapid purification protocols using the co-expression systems for the recombinant human histone oligomers. Furthermore, we have established a system for the in vitro phosphorylation of H2A.X that will facilitate the structural analysis of MDC1 BRCT bound γH2A.X nucleosome complex in the future.
