Title: The Mre11-Rad50-Xrs2 Complex in the DNA Damage Response
Description: DNA is continuously subjected to various types of damage during normal cellular metabolism. Among these, a DNA double-strand break (DSB) is one of the most cytotoxic lesions, and can lead to genomic instability or cell death if misrepaired or left unrepaired. The Mre11-Rad50-Xrs2/Nbs1 (MRX/N) complex orchestrates the cellular response to DNA damage through its structural, enzymatic, and signaling roles. It senses DSBs and is essential for both of the two major repair mechanisms: non-homologous end joining (NHEJ) and homologous recombination (HR). In addition, the complex tethers DNA ends, activates Tel1/ATM kinase, resolves hairpin capped DNA ends and maintains telomere homeostasis. Although significant progress has been made in characterizing the complex, many questions regarding the precise mechanism of how this highly conserved, multifunctional complex manages its various activities in chromosome metabolism remain to be solved. The overarching focus of this thesis is to further expand our understanding of the molecular mechanism and regulation of the MRX complex.

Specifically, the contributions of Xrs2, Tel1, and Mre11 3’-5’ dsDNA exonuclease in the multiple roles of the MRX complex are examined. Xrs2/Nbs1, the eukaryotic-specific component of the complex, is required for the nuclear transport of Mre11 and Rad50 and harbors several protein-interacting domains. In order to define the role of Xrs2 as a component of the MRX complex once inside the nucleus, we fused a nuclear localization signal (NLS) to the C terminus of Mre11 and assayed for complementation of xrs2Δ defects. We found that nuclear localization of Mre11 (Mre11-NLS) is able to bypass several functions of Xrs2, including DNA end resection, meiosis, hairpin resolution, and cellular resistance to clastogens. Using purified components, we showed that the MR complex has the equivalent activity to MRX in cleavage of protein-blocked DNA ends. Although Xrs2 physically interacts with Sae2, end resection in its absence remained Sae2 dependent in vivo and in vitro. MRE11-NLS was unable to rescue the xrs2Δ defects in Tel1 kinase signaling and NHEJ, consistent with the role of Xrs2 as a chaperone and adaptor protein coordinating interactions between the MR and other repair proteins.

To further characterize the role of Xrs2 in Tel1 activation, we fused the Tel1 interaction domain of Xrs2 to Mre11-NLS (Mre11-NLS-TID). Mre11-NLS-TID was sufficient to restore telomere elongation and Tel1 signaling to Xrs2-deficient cells, indicating that Tel1 recruitment and activation are separate functions of the MRX complex. Unexpectedly, we found a role for Tel1 in stabilizing Mre11-DNA association independently of its kinase activity. This stabilization function becomes important for DNA damage resistance in the absence of Xrs2. Moreover, while nuclear-localized MR complex is sufficient for HR without Xrs2, MR is insufficient for DNA tethering, stalled replication fork stability, and suppression of chromosomal rearrangements. Enforcing Tel1 recruitment to the MR complex fully rescued these defects, highlighting the important roles for Xrs2 and Tel1 in stabilizing the MR complex to prevent replication fork collapse and genomic instability. Lastly, in order to decipher the functional significance of the Mre11 3’-5’ dsDNA exonuclease activity in DSB repair, mre11 mutant alleles reported to be proficient endonuclease and deficient exonuclease were analyzed in vivo and in vitro.

Although we did not observe a clear separation of the nuclease activities in vitro, our genetic analysis of the mutant allele is consistent with the current two-stepped, bidirectional model of end resection.