The Mre11/Rad50/Nbs1 complex: recent insights into catalytic activities and ATP-driven conformational changes.

DNA double-strand breaks (DSBs) can arise from internal or external sources of damage, and the rapid detection, processing, and repair of this damage is important for cell viability. Failure to repair DNA damage can result in genomic instability, ultimately increasing the frequency of lymphoid disorders, neurodegeneration, and cancer. The Mre11-Rad50-Nbs1 (Xrs2) complex plays a central and critical role in detection and repair of DSBs and is conserved in all kingdoms of life, as Mre11/Rad50 (MR) in prokaryotes and as MRN/X in eukaryotes (Lamarche et al., 2010; Stracker and Petrini, 2011). The importance of this complex is emphasized by the fact that deletion of any of the three components results in embryonic lethality in mice and loss of proliferative activity in embryonic stem cells (Buis et al., 2008; Luo et al., 1999; Xiao and Weaver, 1997; Zhu et al., 2001) which is likely related to the role of MRN/X in homologous recombination. Repair of DSBs by homologous recombination involves replication of the broken region using an undamaged template, usually a sister chromatid. Deletions of other genes important for homologous forms of repair also exhibit early embryonic lethality, including Rad51, BRCA1, BRCA2, and CtBP-interacting protein (CtIP)(Chen et al., 2005b; Gowen et al., 1996; Lim and Hasty, 1996; Sharan et al., 1997). Hypomorphic mutations in MRN components result into developmental and neurodegenerative disorders in humans, including Ataxia-Telangiectasia-Like Disorder (ATLD), Nijmegen Breakage Syndrome (NBS), and NBS-like syndrome (Matsumoto et al., 2011; Stewart et al., 1999; Varon et al., 1998; Waltes et al., 2009), which are related, at least in part, to the role of MRN/X in the activation of cell-cycle checkpoints through the Ataxia-Telangiectasia-Mutated (ATM) protein kinase (Lee and Paull, 2007; Shiloh and Ziv, 2013). The roles of MRN/X also extend to the processing of DSBs during meiosis, for which it is essential, and to telomere maintenance (Borde, 2007; Lamarche et al., 2010). Repair of DSBs is achieved through two broadly-defined groups of pathways: nonhomologous end joining (NHEJ) and homologous recombination (HR) (Krogh and Symington, 2004). The choice between these pathways primarily depends on the cell-cycle phase and the complexity of the damage generated at the break site (Chapman et al., 2012; Schipler and Iliakis, 2013). In the classical NHEJ pathway, ends are bound by the Ku70–Ku80 heterodimer/DNA-dependent protein kinase catalytic subunit (DNA-PKcs) complex which recruits additional factors involved in end modifications and gap filling. DNA ends are ultimately ligated by the NHEJ-specific DNA ligase IV complex (Deriano and Roth, 2013). In mammalian cells, the C-NHEJ pathway is not dependent on the MRN complex, although in budding yeast MRX contributes to NHEJ pathway through interactions with Ku70-Ku80 and DNA Lig4 complexes (Lewis and Resnick, 2000). The MRN complex, in conjunction with CtIP/Sae2, also regulates the alternative NHEJ (A-NHEJ or MMEJ), which utilizes short microhomologies and can result in large deletions (Lee and Lee, 2007; Yun and Hiom, 2009). In mammalian cells MRN was also shown to interact with DNA ligaseIIIα/Xrcc1, the ligase complex implicated in alternative NHEJ, stimulating intermolecular ligation (Della-Maria et al., 2011). In contrast to NHEJ, HR requires the 5′–3′ resection of dsDNA to generate single-stranded DNA tails, a process that is initiated by the MRN complex and CtIP (You and Bailis, 2010). Extensive resection is perfomed by exonuclease 1 (Exo1), and Dna2 (Symington and Gautier, 2011), whose activities are also promoted by MRN (Cejka et al., 2010; Nicolette et al., 2010; Niu et al., 2010; Yang et al., 2013; Zhou et al., 2014; Zhou and Paull, 2013). 3′ ssDNA tails thus generated are bound by replication protein A (RPA), which activates ATM- and Rad3-Related (ATR), promoting replication checkpoint arrest and stabilization of replication forks (Zeman and Cimprich, 2014). RPA on these 3′ ssDNA tails is then exchanged for Rad51 to create Rad51 filaments that catalyze homology search and strand invasion, ultimately priming DNA synthesis and resolution of repair intermediates. The MRN complex plays important and diverse roles in DNA double-strand break repair and signaling. Here we review recent evidence elucidating the structures and regulation of the Mre11/Rad50 complex, focusing primarily on the enzymatic activities of MRN and the role of ATP-driven conformational changes in Rad50.