Envisioning the dynamics and flexibility of Mre11-Rad50-Nbs1 complex to decipher its roles in DNA replication and repair

Abstract

The Mre11-Rad50-Nbs1 (MRN) complex is a dynamic macromolecular machine that acts in the first steps of DNA double strand break repair, and each of its components has intrinsic dynamics and flexibility properties that are directly linked with their functions. As a result, deciphering the functional structural biology of the MRN complex is driving novel and integrated technologies to define the dynamic structural biology of protein machinery interacting with DNA. Rad50 promotes dramatic long-range allostery through its coiled-coil and zinc-hook domains. Its ATPase activity drives dynamic transitions between monomeric and dimeric forms that can be modulated with mutants modifying the ATPase rate to control end joining versus resection activities. The biological functions of Mre11’s dual endo- and exonuclease activities in repair pathway choice were enigmatic until recently, when they were unveiled by the development of specific nuclease inhibitors. Mre11 dimer flexibility, which may be regulated in cells to control MRN function, suggests new inhibitor design strategies for cancer intervention. Nbs1 has FHA and BRCT domains to bind multiple interaction partners that further regulate MRN. One of them, CtIP, modulates the Mre11 excision activity for homologous recombination repair. Overall, these combined properties suggest novel therapeutic strategies. Furthermore, they collectively help to explain how MRN regulates DNA repair pathway choice with implications for improving the design and analysis of cancer clinical trials that employ DNA damaging agents or target the DNA damage response.

Functional Analysis of the Bacteriophage T4 Rad50 Homolog (gp46) Coiled-coil Domain

“…T4 Rad50 has a relatively short coiled-coil (ϳ250 amino acids) when compared with prokaryotes (ϳ500), archaea (ϳ600), and eukaryotes (ϳ900). The longer coiled-coil in these other systems may allow Rad50 to adopt additional conformational states, including the open state seen in crystal structures and solution (12,47,48).…”

Rad51 recombinase prevents Mre11 nuclease-dependent degradation and excessive PrimPol-mediated elongation of nascent DNA after UV irradiation

“…6 C and D). We speculate that the excessive DNA elongation is inhibited by the Mre11 endonuclease activity or by the MRN complex, which is capable of promoting tethering of DNA ends with long-range allostery (40). In fact, Mre11 has been proposed to tether two DNA strands together, independent of DSB formation (29).…”

“…Such an observation emphasizes a selective effect of the exonuclease activity of Mre11 on the CldU track. We speculated that this exonuclease-independent Mre11 function relates to the tethering ability of the Mre11/Rad50/Nsb1 (MRN) complex and perhaps its capacity for long-range allostery (40). We speculate that the tethering ability of the MRN complex may possibly prevent the excessive unwinding of fork ends.…”

When two is not enough: a CtIP tetramer is required for DNA repair by Homologous Recombination

“…8,9 The critical function of CtIP/Ctp1/Sae2 at DNA ends is carried out in close functional and physical cooperation with the DNA damage sensor and repair complex MRE11-RAD50-NBS1 (MRN, where NBS1 is Xrs2 in budding yeast). [10][11][12][13][14][15] The central role of CtIP/Ctp1/Sae2 in DSB repair is further highlighted by its complex regulation, mediated by an array of post-translational modifications that include phosphorylation by cyclin-dependent and DNA-damage activated kinases, acetylation, ubiquitylation, NEDDylation and proline isomerisation. 3 Despite its importance to DSB repair, our mechanistic understanding of how CtIP promotes resection remains incomplete.…”