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• W2883825889 abstract "The Mre11 nuclease has been the subject of intensive investigation for the past 20 years because of the central role that Mre11/Rad50 complexes play in genome maintenance. The last two decades of work on this complex has led to a much deeper understanding of the structure, biochemical activities, and regulation of Mre11/Rad50 complexes from archaea, bacteria, and eukaryotic cells. This review will discuss some of the important findings over recent years that have illuminated roles for the Mre11 nuclease in these different contexts as well as the insights from structural biology that have helped us to understand its mechanisms of action. The Mre11 nuclease has been the subject of intensive investigation for the past 20 years because of the central role that Mre11/Rad50 complexes play in genome maintenance. The last two decades of work on this complex has led to a much deeper understanding of the structure, biochemical activities, and regulation of Mre11/Rad50 complexes from archaea, bacteria, and eukaryotic cells. This review will discuss some of the important findings over recent years that have illuminated roles for the Mre11 nuclease in these different contexts as well as the insights from structural biology that have helped us to understand its mechanisms of action. Double-strand breaks in DNA constitute a unique type of DNA lesion that is potentially lethal to eukaryotic cells if essential genetic material is lost at the break site, yet double-strand breaks are also important in many biological processes. Severing of both strands of DNA generates an essential intermediate that is required for homologous recombination preceding sexual reproduction in eukaryotic organisms and for programmed gene rearrangements in the vertebrate immune system. Naturally occurring double-stranded ends of chromosomes also comprise the telomere structure that affects many aspects of genome integrity and aging, and signaling induced at double-strand break sites is an important component of DNA-damage-induced checkpoint control of growth. The importance of double-strand breaks is highlighted by the evolution of protein complexes that specifically recognize this lesion within seconds of its appearance in cells. One of the primary complexes responsible for the recognition, repair, and signaling of double-strand breaks in eukaryotes is composed of the Mre11, Rad50, and Nbs1/Xrs2 proteins (MRN/X), of which Mre11 and Rad50 constitute the catalytic components. Mre11, the nuclease component of MRN/X, is conserved in all organisms, but the importance of this protein in processes related to DNA double-strand break repair and recombination was initially most evident in genetic studies in S. cerevisiae. Early studies in budding yeast showed an absolute requirement for Mre11 and its nuclease activity in the processing of covalent Spo11-bound breaks that initiate homologous recombination during meiosis (Ajimura et al., 1993Ajimura M. Leem S.H. Ogawa H. Identification of new genes required for meiotic recombination in Saccharomyces cerevisiae.Genetics. 1993; 133: 51-66Crossref PubMed Google Scholar, Moreau et al., 1999Moreau S. Ferguson J.R. Symington L.S. The nuclease activity of Mre11 is required for meiosis but not for mating type switching, end joining, or telomere maintenance.Mol. Cell. Biol. 1999; 19: 556-566Crossref PubMed Google Scholar, Nairz and Klein, 1997Nairz K. Klein F. mre11S--a yeast mutation that blocks double-strand-break processing and permits nonhomologous synapsis in meiosis.Genes Dev. 1997; 11: 2272-2290Crossref PubMed Google Scholar, Ogawa et al., 1995Ogawa H. Johzuka K. Nakagawa T. Leem S.H. Hagihara A.H. Functions of the yeast meiotic recombination genes, MRE11 and MRE2.Adv. Biophys. 1995; 31: 67-76Crossref PubMed Scopus (56) Google Scholar, Usui et al., 1998Usui T. Ohta T. Oshiumi H. Tomizawa J. Ogawa H. Ogawa T. Complex formation and functional versatility of Mre11 of budding yeast in recombination.Cell. 1998; 95: 705-716Abstract Full Text Full Text PDF PubMed Google Scholar). We now know that Mre11 complexes initiate the processing of DNA double-strand breaks in many cellular contexts outside of meiosis, that Mre11 can remove a variety of nucleic acid and protein blocks on DNA ends, and that Mre11 nuclease activity can also have deleterious effects on DNA replication intermediates in certain pathological situations. Archaea Mre11 from P. furiosus was first glimpsed at atomic resolution in 2001, which showed a dimeric Mre11 catalytic domain with an L-shaped DNA-binding groove leading to an active site with two bound metal ions (Hopfner et al., 2001Hopfner K.P. Karcher A. Craig L. Woo T.T. Carney J.P. Tainer J.A. Structural biochemistry and interaction architecture of the DNA double-strand break repair Mre11 nuclease and Rad50-ATPase.Cell. 2001; 105: 473-485Abstract Full Text Full Text PDF PubMed Scopus (353) Google Scholar), consistent with other members of the protein phosphatase 2B family of phosphoesterases (Fernandez et al., 2011Fernandez F.J. Lopez-Estepa M. Vega M.C. Nucleases of metallo-beta-lactamase and protein phosphatase families in DNA repair.in: Storici F. DNA Repair: On the Pathways to Fixing DNA Damage and Errors. Intech, 2011Google Scholar). The binding groove appeared large enough to accommodate single-stranded DNA, but not a double-stranded DNA duplex, an observation borne out in a later co-crystal structure of Mre11 bound to synapsed DNA ends and separately to a partially unwound DNA duplex (Williams et al., 2008Williams R.S. Moncalian G. Williams J.S. Yamada Y. Limbo O. Shin D.S. Groocock L.M. Cahill D. Hitomi C. Guenther G. et al.Mre11 dimers coordinate DNA end bridging and nuclease processing in double-strand-break repair.Cell. 2008; 135: 97-109Abstract Full Text Full Text PDF PubMed Scopus (303) Google Scholar). Several structures of Mre11 in complex with Rad50 core domains from bacteria and archaea have also illuminated the interactions between the nuclease and its invariant cofactor, Rad50 (Lammens et al., 2011Lammens K. Bemeleit D.J. Möckel C. Clausing E. Schele A. Hartung S. Schiller C.B. Lucas M. Angermüller C. Söding J. et al.The Mre11:Rad50 structure shows an ATP-dependent molecular clamp in DNA double-strand break repair.Cell. 2011; 145: 54-66Abstract Full Text Full Text PDF PubMed Scopus (120) Google Scholar, Lim et al., 2011Lim H.S. Kim J.S. Park Y.B. Gwon G.H. Cho Y. Crystal structure of the Mre11-Rad50-ATPγS complex: understanding the interplay between Mre11 and Rad50.Genes Dev. 2011; 25: 1091-1104Crossref PubMed Scopus (83) Google Scholar, Möckel et al., 2012Möckel C. Lammens K. Schele A. Hopfner K.P. ATP driven structural changes of the bacterial Mre11:Rad50 catalytic head complex.Nucleic Acids Res. 2012; 40: 914-927Crossref PubMed Scopus (66) Google Scholar, Williams et al., 2011Williams G.J. Williams R.S. Williams J.S. Moncalian G. Arvai A.S. Limbo O. Guenther G. SilDas S. Hammel M. Russell P. Tainer J.A. ABC ATPase signature helices in Rad50 link nucleotide state to Mre11 interface for DNA repair.Nat. Struct. Mol. Biol. 2011; 18: 423-431Crossref PubMed Scopus (102) Google Scholar). Interestingly, these studies have indicated that when the SMC-like Rad50 core domains are bound to ATP, the catalytic domains of Mre11 are completely occluded (Figure 1). Later studies have shown that Mre11 is also physically separated from the DNA duplex, which is bound in a groove on the opposite, top surface of the Rad50 catalytic head domains (Liu et al., 2016Liu Y. Sung S. Kim Y. Li F. Gwon G. Jo A. Kim A.K. Kim T. Song O.K. Lee S.E. Cho Y. ATP-dependent DNA binding, unwinding, and resection by the Mre11/Rad50 complex.EMBO J. 2016; 35: 743-758Crossref PubMed Scopus (33) Google Scholar, Seifert et al., 2016Seifert F.U. Lammens K. Stoehr G. Kessler B. Hopfner K.P. Structural mechanism of ATP-dependent DNA binding and DNA end bridging by eukaryotic Rad50.EMBO J. 2016; 35: 759-772Crossref PubMed Google Scholar). For the Mre11 catalytic domains to contact the DNA, the core Walker A/Walker B ATP-binding domains of Rad50 must presumably detach from the ATP-binding domains of the other Rad50 monomer coincident with ATP hydrolysis. Currently, however, we do not know the mechanism for DNA end recognition by Mre11 within an MRN or Mre11/Rad50 (MR) complex, and we do not know how Mre11 engages a DNA end within its active site. Biochemical analysis of Mre11 nuclease activity confirms that ATP hydrolysis by Rad50 is a prerequisite for Mre11/Rad50-mediated nuclease activity on double-stranded DNA (Connelly et al., 1997Connelly J.C. de Leau E.S. Okely E.A. Leach D.R. Overexpression, purification, and characterization of the SbcCD protein from Escherichia coli.J. Biol. Chem. 1997; 272: 19819-19826Crossref PubMed Scopus (0) Google Scholar, Deshpande et al., 2017Deshpande R.A. Lee J.H. Paull T.T. Rad50 ATPase activity is regulated by DNA ends and requires coordination of both active sites.Nucleic Acids Res. 2017; 45: 5255-5268Crossref PubMed Scopus (11) Google Scholar, Herdendorf et al., 2011Herdendorf T.J. Albrecht D.W. Benkovic S.J. Nelson S.W. Biochemical characterization of bacteriophage T4 Mre11-Rad50 complex.J. Biol. Chem. 2011; 286: 2382-2392Crossref PubMed Scopus (36) Google Scholar, Hopfner et al., 2000aHopfner K.P. Karcher A. Shin D. Fairley C. Tainer J.A. Carney J.P. Mre11 and Rad50 from Pyrococcus furiosus: cloning and biochemical characterization reveal an evolutionarily conserved multiprotein machine.J. Bacteriol. 2000; 182: 6036-6041Crossref PubMed Scopus (94) Google Scholar, Paull and Gellert, 1999Paull T.T. Gellert M. Nbs1 potentiates ATP-driven DNA unwinding and endonuclease cleavage by the Mre11/Rad50 complex.Genes Dev. 1999; 13: 1276-1288Crossref PubMed Google Scholar, Trujillo and Sung, 2001Trujillo K.M. Sung P. DNA structure-specific nuclease activities in the Saccharomyces cerevisiae Rad50∗Mre11 complex.J. Biol. Chem. 2001; 276: 35458-35464Crossref PubMed Scopus (193) Google Scholar). However, single-stranded DNA with secondary structure, such as M13 phage DNA, can be cleaved by Mre11/Rad50 complexes in the absence of ATP (Connelly and Leach, 1996Connelly J.C. Leach D.R. The sbcC and sbcD genes of Escherichia coli encode a nuclease involved in palindrome inviability and genetic recombination.Genes Cells. 1996; 1: 285-291Crossref PubMed Scopus (123) Google Scholar, Herdendorf et al., 2011Herdendorf T.J. Albrecht D.W. Benkovic S.J. Nelson S.W. Biochemical characterization of bacteriophage T4 Mre11-Rad50 complex.J. Biol. Chem. 2011; 286: 2382-2392Crossref PubMed Scopus (36) Google Scholar, Hopfner et al., 2000aHopfner K.P. Karcher A. Shin D. Fairley C. Tainer J.A. Carney J.P. Mre11 and Rad50 from Pyrococcus furiosus: cloning and biochemical characterization reveal an evolutionarily conserved multiprotein machine.J. Bacteriol. 2000; 182: 6036-6041Crossref PubMed Scopus (94) Google Scholar), and eukaryotic Mre11 exhibits 3′ to 5′ exonuclease activity in the absence of Rad50 (Paull and Gellert, 1998Paull T.T. Gellert M. The 3′ to 5′ exonuclease activity of Mre 11 facilitates repair of DNA double-strand breaks.Mol. Cell. 1998; 1: 969-979Abstract Full Text Full Text PDF PubMed Google Scholar, Trujillo et al., 1998Trujillo K.M. Yuan S.S. Lee E.Y. Sung P. Nuclease activities in a complex of human recombination and DNA repair factors Rad50, Mre11, and p95.J. Biol. Chem. 1998; 273: 21447-21450Crossref PubMed Scopus (303) Google Scholar), indicating that Rad50-catalyzed ATP hydrolysis is not essential for all Mre11 functions. T4 bacteriophage Mre11 (gp47) also has been shown to require ATP hydrolysis by Rad50 (gp46) for exonuclease activity, but not for the removal of the first nucleotide (Herdendorf et al., 2011Herdendorf T.J. Albrecht D.W. Benkovic S.J. Nelson S.W. Biochemical characterization of bacteriophage T4 Mre11-Rad50 complex.J. Biol. Chem. 2011; 286: 2382-2392Crossref PubMed Scopus (36) Google Scholar), also suggesting that the requirement of ATP hydrolysis for Mre11 nuclease activity is likely related to the conformation of the active site on DNA and how it is restrained in some circumstances by association with Rad50. In eukaryotes, the third component of the complex, Nbs1(Nibrin) in mammals and fission yeast and Xrs2 in budding yeast, regulates the catalytic activities of Mre11/Rad50. Nbs1 binds to Mre11 through an Mre11-interacting region in the C terminus, which is crystallized with S. pombe Mre11 on the surface of the phosphodiesterase domain and at the Mre11 dimer interface (Schiller et al., 2012Schiller C.B. Lammens K. Guerini I. Coordes B. Feldmann H. Schlauderer F. Möckel C. Schele A. Strässer K. Jackson S.P. Hopfner K.P. Structure of Mre11-Nbs1 complex yields insights into ataxia-telangiectasia-like disease mutations and DNA damage signaling.Nat. Struct. Mol. Biol. 2012; 19: 693-700Crossref PubMed Scopus (71) Google Scholar). Human Nbs1 is required to promote Mre11 endonuclease activity on blocked DNA ends and hairpin substrates and is important for the stimulatory effect of DNA ends on the rate of Rad50 ATP hydrolysis (Deshpande et al., 2016Deshpande R.A. Lee J.H. Arora S. Paull T.T. Nbs1 converts the human Mre11/Rad50 nuclease complex into an endo/exonuclease machine specific for protein-DNA adducts.Mol. Cell. 2016; 64: 593-606Abstract Full Text Full Text PDF PubMed Scopus (51) Google Scholar, Deshpande et al., 2017Deshpande R.A. Lee J.H. Paull T.T. Rad50 ATPase activity is regulated by DNA ends and requires coordination of both active sites.Nucleic Acids Res. 2017; 45: 5255-5268Crossref PubMed Scopus (11) Google Scholar, Paull and Gellert, 1999Paull T.T. Gellert M. Nbs1 potentiates ATP-driven DNA unwinding and endonuclease cleavage by the Mre11/Rad50 complex.Genes Dev. 1999; 13: 1276-1288Crossref PubMed Google Scholar). Nbs1 also restrains MR-catalyzed 3′ to 5′ exonuclease activity on open DNA ends while promoting 3′ to 5′ exonuclease activity at protein-blocked ends (Deshpande et al., 2016Deshpande R.A. Lee J.H. Arora S. Paull T.T. Nbs1 converts the human Mre11/Rad50 nuclease complex into an endo/exonuclease machine specific for protein-DNA adducts.Mol. Cell. 2016; 64: 593-606Abstract Full Text Full Text PDF PubMed Scopus (51) Google Scholar); thus, Nbs1 is a key determinant of the outcomes of Mre11 activity. While the N-terminal forkhead-associated (FHA) and BRCA1 C-terminal (BRCT) domains of Nbs1/Xrs2 are important for association with Mdc1, CtBP-interacting protein (CtIP)/Ctp1, and other factors in the DNA damage response (Chapman and Jackson, 2008Chapman J.R. Jackson S.P. Phospho-dependent interactions between NBS1 and MDC1 mediate chromatin retention of the MRN complex at sites of DNA damage.EMBO Rep. 2008; 9: 795-801Crossref PubMed Scopus (192) Google Scholar, Dodson et al., 2010Dodson G.E. Limbo O. Nieto D. Russell P. Phosphorylation-regulated binding of Ctp1 to Nbs1 is critical for repair of DNA double-strand breaks.Cell Cycle. 2010; 9: 1516-1522Crossref PubMed Google Scholar, Lloyd et al., 2009Lloyd J. Chapman J.R. Clapperton J.A. Haire L.F. Hartsuiker E. Li J. Carr A.M. Jackson S.P. Smerdon S.J. A supramodular FHA/BRCT-repeat architecture mediates Nbs1 adaptor function in response to DNA damage.Cell. 2009; 139: 100-111Abstract Full Text Full Text PDF PubMed Scopus (111) Google Scholar, Melander et al., 2008Melander F. Bekker-Jensen S. Falck J. Bartek J. Mailand N. Lukas J. Phosphorylation of SDT repeats in the MDC1 N terminus triggers retention of NBS1 at the DNA damage-modified chromatin.J. Cell Biol. 2008; 181: 213-226Crossref PubMed Scopus (156) Google Scholar, Spycher et al., 2008Spycher C. Miller E.S. Townsend K. Pavic L. Morrice N.A. Janscak P. Stewart G.S. Stucki M. Constitutive phosphorylation of MDC1 physically links the MRE11-RAD50-NBS1 complex to damaged chromatin.J. Cell Biol. 2008; 181: 227-240Crossref PubMed Scopus (165) Google Scholar, Wang et al., 2013Wang H. Shi L.Z. Wong C.C. Han X. Hwang P.Y. Truong L.N. Zhu Q. Shao Z. Chen D.J. Berns M.W. et al.The interaction of CtIP and Nbs1 connects CDK and ATM to regulate HR-mediated double-strand break repair.PLoS Genet. 2013; 9: e1003277Crossref PubMed Scopus (98) Google Scholar, Williams et al., 2009Williams R.S. Dodson G.E. Limbo O. Yamada Y. Williams J.S. Guenther G. Classen S. Glover J.N. Iwasaki H. Russell P. Tainer J.A. Nbs1 flexibly tethers Ctp1 and Mre11-Rad50 to coordinate DNA double-strand break processing and repair.Cell. 2009; 139: 87-99Abstract Full Text Full Text PDF PubMed Scopus (207) Google Scholar), the Mre11-interacting region of Nbs1 is necessary and sufficient for cell viability in the absence of the rest of the Nbs1 polypeptide (Kim et al., 2017Kim J.H. Grosbart M. Anand R. Wyman C. Cejka P. Petrini J.H.J. The Mre11-Nbs1 interface is essential for viability and tumor suppression.Cell Rep. 2017; 18: 496-507Abstract Full Text Full Text PDF PubMed Scopus (13) Google Scholar). In contrast, the yeast Xrs2 protein does not play as essential a role in budding yeast as Nbs1 does in human cells; fusion of a nuclear localization signal to Mre11 (normally provided through the Xrs2 protein) bypasses the requirement for Xrs2 in MRX-mediated end resection, DNA damage survival, hairpin resolution, and meiosis (Oh et al., 2016Oh J. Al-Zain A. Cannavo E. Cejka P. Symington L.S. Xrs2 dependent and independent functions of the Mre11-Rad50 complex.Mol. Cell. 2016; 64: 405-415Abstract Full Text Full Text PDF PubMed Google Scholar). Melting of the DNA duplex, as seen in structures of Mre11 bound to DNA ends (Williams et al., 2008Williams R.S. Moncalian G. Williams J.S. Yamada Y. Limbo O. Shin D.S. Groocock L.M. Cahill D. Hitomi C. Guenther G. et al.Mre11 dimers coordinate DNA end bridging and nuclease processing in double-strand-break repair.Cell. 2008; 135: 97-109Abstract Full Text Full Text PDF PubMed Scopus (303) Google Scholar), allows the enzyme to accommodate a single-stranded DNA or a 3′ end of a DNA strand. Human Mre11, when bound to Rad50, Nbs1, and ATP, was later shown to induce an unwinding of ∼15 bp at the ends of duplex DNA (Cannon et al., 2013Cannon B. Kuhnlein J. Yang S.H. Cheng A. Schindler D. Stark J.M. Russell R. Paull T.T. Visualization of local DNA unwinding by Mre11/Rad50/Nbs1 using single-molecule FRET.Proc. Natl. Acad. Sci. USA. 2013; 110: 18868-18873Crossref PubMed Scopus (29) Google Scholar), consistent with the idea that partial unwinding of the DNA duplex would be essential for access of DNA into the Mre11 active site. Evidence for this was also seen with the M. jannaschii enzyme, where structural analysis of the core of Mre11 bound to the core catalytic head domains of Rad50 combined with biochemical analysis of mutants supported a model in which rotation of the Rad50 nucleotide-binding domains generates unwinding of the helix and access to the Mre11 active sites (Liu et al., 2016Liu Y. Sung S. Kim Y. Li F. Gwon G. Jo A. Kim A.K. Kim T. Song O.K. Lee S.E. Cho Y. ATP-dependent DNA binding, unwinding, and resection by the Mre11/Rad50 complex.EMBO J. 2016; 35: 743-758Crossref PubMed Scopus (33) Google Scholar). Although we have many structures of the catalytic cores of Mre11 and Rad50 (Hopfner et al., 2000bHopfner K.P. Karcher A. Shin D.S. Craig L. Arthur L.M. Carney J.P. Tainer J.A. Structural biology of Rad50 ATPase: ATP-driven conformational control in DNA double-strand break repair and the ABC-ATPase superfamily.Cell. 2000; 101: 789-800Abstract Full Text Full Text PDF PubMed Google Scholar, Hopfner et al., 2001Hopfner K.P. Karcher A. Craig L. Woo T.T. Carney J.P. Tainer J.A. Structural biochemistry and interaction architecture of the DNA double-strand break repair Mre11 nuclease and Rad50-ATPase.Cell. 2001; 105: 473-485Abstract Full Text Full Text PDF PubMed Scopus (353) Google Scholar, Lammens et al., 2011Lammens K. Bemeleit D.J. Möckel C. Clausing E. Schele A. Hartung S. Schiller C.B. Lucas M. Angermüller C. Söding J. et al.The Mre11:Rad50 structure shows an ATP-dependent molecular clamp in DNA double-strand break repair.Cell. 2011; 145: 54-66Abstract Full Text Full Text PDF PubMed Scopus (120) Google Scholar, Lim et al., 2011Lim H.S. Kim J.S. Park Y.B. Gwon G.H. Cho Y. Crystal structure of the Mre11-Rad50-ATPγS complex: understanding the interplay between Mre11 and Rad50.Genes Dev. 2011; 25: 1091-1104Crossref PubMed Scopus (83) Google Scholar, Liu et al., 2016Liu Y. Sung S. Kim Y. Li F. Gwon G. Jo A. Kim A.K. Kim T. Song O.K. Lee S.E. Cho Y. ATP-dependent DNA binding, unwinding, and resection by the Mre11/Rad50 complex.EMBO J. 2016; 35: 743-758Crossref PubMed Scopus (33) Google Scholar, Möckel et al., 2012Möckel C. Lammens K. Schele A. Hopfner K.P. ATP driven structural changes of the bacterial Mre11:Rad50 catalytic head complex.Nucleic Acids Res. 2012; 40: 914-927Crossref PubMed Scopus (66) Google Scholar, Park et al., 2011Park Y.B. Chae J. Kim Y.C. Cho Y. Crystal structure of human Mre11: understanding tumorigenic mutations.Structure. 2011; 19: 1591-1602Abstract Full Text Full Text PDF PubMed Scopus (45) Google Scholar, Seifert et al., 2016Seifert F.U. Lammens K. Stoehr G. Kessler B. Hopfner K.P. Structural mechanism of ATP-dependent DNA binding and DNA end bridging by eukaryotic Rad50.EMBO J. 2016; 35: 759-772Crossref PubMed Google Scholar, Williams et al., 2008Williams R.S. Moncalian G. Williams J.S. Yamada Y. Limbo O. Shin D.S. Groocock L.M. Cahill D. Hitomi C. Guenther G. et al.Mre11 dimers coordinate DNA end bridging and nuclease processing in double-strand-break repair.Cell. 2008; 135: 97-109Abstract Full Text Full Text PDF PubMed Scopus (303) Google Scholar, Williams et al., 2011Williams G.J. Williams R.S. Williams J.S. Moncalian G. Arvai A.S. Limbo O. Guenther G. SilDas S. Hammel M. Russell P. Tainer J.A. ABC ATPase signature helices in Rad50 link nucleotide state to Mre11 interface for DNA repair.Nat. Struct. Mol. Biol. 2011; 18: 423-431Crossref PubMed Scopus (102) Google Scholar), we currently do not have structural detail showing end recognition of DNA in the context of the complete Mre11/Rad50 complex or the intermediate in which Mre11/Rad50 accesses DNA after ATP hydrolysis. Furthermore, a recent structure of the core catalytic domain of human Mre11 revealed that there are significant differences between bacterial and archaea Mre11 and mammalian Mre11 (Park et al., 2011Park Y.B. Chae J. Kim Y.C. Cho Y. Crystal structure of human Mre11: understanding tumorigenic mutations.Structure. 2011; 19: 1591-1602Abstract Full Text Full Text PDF PubMed Scopus (45) Google Scholar), and we do not have structural detail of the interaction between human Mre11 and Nbs1. Future structural analysis of the complete MRN complex is necessary to understand the functional regulation of its activities at a mechanistic level and decipher how it recognizes DNA ends. Purified Mre11/Rad50 complexes from all organisms exhibit exonuclease activity in vitro on blunt or recessed 3′ ends in the 3′ to 5′ direction (Connelly et al., 1999Connelly J.C. de Leau E.S. Leach D.R.F. DNA cleavage and degradation by the SbcCD protein complex from Escherichia coli.Nucleic Acids Res. 1999; 27: 1039-1046Crossref PubMed Scopus (0) Google Scholar, Herdendorf et al., 2011Herdendorf T.J. Albrecht D.W. Benkovic S.J. Nelson S.W. Biochemical characterization of bacteriophage T4 Mre11-Rad50 complex.J. Biol. Chem. 2011; 286: 2382-2392Crossref PubMed Scopus (36) Google Scholar, Hopfner et al., 2000aHopfner K.P. Karcher A. Shin D. Fairley C. Tainer J.A. Carney J.P. Mre11 and Rad50 from Pyrococcus furiosus: cloning and biochemical characterization reveal an evolutionarily conserved multiprotein machine.J. Bacteriol. 2000; 182: 6036-6041Crossref PubMed Scopus (94) Google Scholar, Paull and Gellert, 1998Paull T.T. Gellert M. The 3′ to 5′ exonuclease activity of Mre 11 facilitates repair of DNA double-strand breaks.Mol. Cell. 1998; 1: 969-979Abstract Full Text Full Text PDF PubMed Google Scholar, Trujillo and Sung, 2001Trujillo K.M. Sung P. DNA structure-specific nuclease activities in the Saccharomyces cerevisiae Rad50∗Mre11 complex.J. Biol. Chem. 2001; 276: 35458-35464Crossref PubMed Scopus (193) Google Scholar, Trujillo et al., 1998Trujillo K.M. Yuan S.S. Lee E.Y. Sung P. Nuclease activities in a complex of human recombination and DNA repair factors Rad50, Mre11, and p95.J. Biol. Chem. 1998; 273: 21447-21450Crossref PubMed Scopus (303) Google Scholar). For many years, the polarity of the exonuclease activity was difficult to reconcile with its known biological importance in promoting the 5′ to 3′ resection of DNA double-strand breaks. A critical step in unraveling this paradox was the observation that the processing of Spo11-induced breaks in S. cerevisiae occurs through an endonucleolytic cleavage of DNA adjacent to the Spo11 covalent conjugate and also subsequent 3′ to 5′ exonucleolytic degradation toward the conjugate (Garcia et al., 2011Garcia V. Phelps S.E. Gray S. Neale M.J. Bidirectional resection of DNA double-strand breaks by Mre11 and Exo1.Nature. 2011; 479: 241-244Crossref PubMed Scopus (193) Google Scholar). This endo-then-exo model of DNA end resection (Figure 2) explained the existence of the 3′ to 5′ exonuclease activity and was also demonstrated in vitro with protein-blocked ends (Anand et al., 2016Anand R. Ranjha L. Cannavo E. Cejka P. Phosphorylated CtIP functions as a co-factor of the MRE11-RAD50-NBS1 endonuclease in DNA end resection.Mol. Cell. 2016; 64: 940-950Abstract Full Text Full Text PDF PubMed Scopus (75) Google Scholar, Connelly et al., 2003Connelly J.C. de Leau E.S. Leach D.R. Nucleolytic processing of a protein-bound DNA end by the E. coli SbcCD (MR) complex.DNA Repair (Amst.). 2003; 2: 795-807Crossref PubMed Scopus (62) Google Scholar, Deshpande et al., 2016Deshpande R.A. Lee J.H. Arora S. Paull T.T. Nbs1 converts the human Mre11/Rad50 nuclease complex into an endo/exonuclease machine specific for protein-DNA adducts.Mol. Cell. 2016; 64: 593-606Abstract Full Text Full Text PDF PubMed Scopus (51) Google Scholar, Reginato et al., 2017Reginato G. Cannavo E. Cejka P. Physiological protein blocks direct the Mre11-Rad50-Xrs2 and Sae2 nuclease complex to initiate DNA end resection.Genes Dev. 2017; 31: 2325-2330Crossref PubMed Scopus (29) Google Scholar, Wang et al., 2017Wang W. Daley J.M. Kwon Y. Krasner D.S. Sung P. Plasticity of the Mre11-Rad50-Xrs2-Sae2 nuclease ensemble in the processing of DNA-bound obstacles.Genes Dev. 2017; 31: 2331-2336Crossref PubMed Scopus (27) Google Scholar). Remarkably, endonucleolytic incision of DNA ends followed by exonucleolytic degradation is observed in vitro only on DNA substrates containing a blocked end. The blocks used in vitro have generally consisted of a tightly bound or covalently bound protein, as first demonstrated by Leach and colleagues, who showed that E. coli Mre11/Rad50 (suppressor of RecBC [Sbc]CD) generates an endonucleolytic cleavage on an avidin-bound DNA (Connelly et al., 2003Connelly J.C. de Leau E.S. Leach D.R. Nucleolytic processing of a protein-bound DNA end by the E. coli SbcCD (MR) complex.DNA Repair (Amst.). 2003; 2: 795-807Crossref PubMed Scopus (62) Google Scholar). Similar endonucleolytic cutting was shown with S. cerevisiae Mre11/Rad50/Xrs2 (MRX) and with human MRN complexes adjacent to DNA ends containing biotin-streptavidin attachments, and in these cases, MRN(X) showed block-dependent endonuclease incision followed by 3′ to 5′ exonuclease degradation away from the nick site (Anand et al., 2016Anand R. Ranjha L. Cannavo E. Cejka P. Phosphorylated CtIP functions as a co-factor of the MRE11-RAD50-NBS1 endonuclease in DNA end resection.Mol. Cell. 2016; 64: 940-950Abstract Full Text Full Text PDF PubMed Scopus (75) Google Scholar, Cannavo and Cejka, 2014Cannavo E. Cejka P. Sae2 promotes dsDNA endonuclease activity within Mre11-Rad50-Xrs2 to resect DNA breaks.Nature. 2014; 514: 122-125Crossref PubMed Scopus (185) Google Scholar, Deshpande et al., 2016Deshpande R.A. Lee J.H. Arora S. Paull T.T. Nbs1 converts the human Mre11/Rad50 nuclease complex into an endo/exonuclease machine specific for protein-DNA adducts.Mol. Cell. 2016; 64: 593-606Abstract Full Text Full Text PDF PubMed Scopus (51) Google Scholar, Reginato et al., 2017Reginato G. Cannavo E. Cejka P. Physiological protein blocks direct the Mre11-Rad50-Xrs2 and Sae2 nuclease complex to initiate DNA end resection.Genes Dev. 2017; 31: 2325-2330Crossref PubMed Scopus (29) Google Scholar, Wang et al., 2017Wang W. Daley J.M. Kwon Y. Krasner D.S. Sung P. Plasticity of the Mre11-Rad50-Xrs2-Sae2 nuclease ensemble in the processing of DNA-bound obstacles.Genes Dev. 2017; 31: 2331-2336Crossref PubMed Scopus (27) Google Scholar). Nucleases are generally specific for a particular DNA lesion or structure, but it is unusual for a nuclease to require a protein to be bound adjacent to a cleavage site. The identity of this protein seems to be irrelevant since streptavidin (bound to biotin moieties on the DNA) can promote endonucleolytic activity of Mre11 (Anand et al., 2016Anand R. Ranjha L. Cannavo E. Cejka P. Phosphorylated CtIP functions as a co-factor of the MRE11-RAD50-NBS1 endonuclease in DNA end resection.Mol. Cell. 2016; 64: 940-950Abstract Full Text Full Text PDF PubMed Scopus (75) Google Scholar, Deshpande et al., 2016Deshpande R.A. Lee J.H. Arora S. Paull T.T. Nbs1 converts the human Mre11/Rad50 nuclease complex into an endo/exonuclease machine specific for protein-DNA adducts.Mol. Cell. 2016; 64: 593-606Abstract Full Text Full Text PDF PubMed Scopus (51) Google Scholar). Although the endo-then-exo catalytic activities were initially demonstrated with heterologous blocked ends, eukaryotic MRN(X) complexes were also shown to recognize naturally occurring, non-covalent DNA-bound proteins as end blocks, most notably the non-homologous end joining (NHEJ) factor Ku (Myler et al., 2017Myler L.R. Gallardo I.F. Soniat M.M. Deshpande R.A. Gonzalez X.B. Kim Y. Paull T.T. Finkelstein I.J. Single-m" @default.
• W2883825889 created "2018-08-03" @default.
• W2883825889 creator A5024968631 @default.
• W2883825889 date "2018-08-01" @default.
• W2883825889 modified "2023-10-16" @default.
• W2883825889 title "20 Years of Mre11 Biology: No End in Sight" @default.
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