Cryo-EM structure of the Mre11-Rad50-Nbs1 complex reveals the molecular mechanism of scaffolding functions

Area of Science:

• Molecular Biology
• Structural Biology
• DNA Repair

Background:

• The Mre11-Rad50-Nbs1 (MRN) complex is crucial for DNA double-strand break (DSB) repair.
• MRN's dual roles as a nuclease and scaffold protein remain incompletely understood.
• Understanding MRN's structure is key to elucidating its diverse functions in DNA repair pathways.

Purpose of the Study:

• To determine the high-resolution structure of the MRN complex.
• To elucidate the architectural basis for MRN's nuclease and DNA-tethering functions.
• To investigate the mechanism of MRN complex assembly and DNA interaction.

Main Methods:

• Cryo-electron microscopy (cryo-EM) was used to determine the structure of the Chaetomium thermophilum MRN complex.
• Structural analysis focused on the stoichiometry, domain organization, and DNA-binding interfaces.
• Biochemical assays were inferred to understand DNA-binding modes.

Main Results:

• The cryo-EM structure revealed a 2:2:1 stoichiometry of Mre11, Rad50, and Nbs1, with Nbs1 wrapping the Mre11 dimer.
• MRN exhibits two distinct DNA-binding modes: ATP-dependent loading and ATP-independent interaction via Mre11's C terminus.
• The complex forms a linear rod with two MRN complexes capable of dimerizing into larger 120-nm structures.

Conclusions:

• The study reveals the detailed architecture of the MRN complex, explaining its dual catalytic and scaffolding roles.
• The findings provide mechanistic insights into how MRN binds DNA ends and tethers broken DNA.
• The structural data offers a foundation for understanding MRN's function in DNA repair and its regulation.