Genome Maintenance

DNA double-strand breaks (DSBs) are a major threat to genome stability in all kingdoms of life. DSBs typically arise during chromosomal replication but can also be products of environmental stress in the form of ionizing radiation and genotoxic chemicals. Unrepaired or misrepaired DSBs as well as defects in the cellular response to DSBs can lead to gross chromosomal aberrations, genome instability and therefore to the development of cancer.
The Mre11 nuclease and Rad50 ABC ATPase complex (MR) is an evolutionary conserved sensor and processing factor for DNA double-strand breaks. To characterize the functional architecture and structural mechanism of this DSB recognition and processing machinery, we use a combination of methods including X-ray crystallography, small angle X-ray scattering (SAXS) as well as biochemicaland in vivo assays. To date we determined the crystal structure of the catalytic head of the bacterial MR complex and analyzed ATP-dependent conformational changes. MR adopts an open form with a central Mre11 nuclease dimer and two peripheral Rad50 molecules, a form well suited for sensing even obstructed DNA ends. The Mre11 C-terminal helix-loop-helix domain binds Rad50 and attaches flexibly to the nuclease domain, enabling large conformational changes. ATP binding to the two Rad50 subunits induces a rotation of the Mre11 helix-loop-helix and Rad50 coiled-coil domains, creating a clamp conformation with increased DNA-binding activity. The results suggest that MR is an ATP-controlled transient molecular clamp at DNA double-strand breaks.