Metal Exchange in the Interprotein Zn -Binding Site of the Rad50 Hook Domain: Structural Insights into Cd -Induced DNA-Repair Inhibition.

Cd is a major genotoxic agent that readily displaces Zn in a multitude of zinc proteins, abrogates redox homeostasis, and deregulates cellular metalloproteome. To date, this displacement has been described mostly for cysteine(Cys)-rich intraprotein binding sites in certain zinc finger domains and metallothioneins. To visualize how a Zn -to-Cd swap can affect the target protein's status and thus understand the molecular basis of Cd -induced genotoxicity an intermolecular Zn -binding site from the crucial DNA repair protein Rad50 and its zinc hook domain were examined. By using a length-varied peptide base, Zn -to-Cd displacement in Rad50's hook domain is demonstrated to alter it in a bimodal fashion: 1) Cd induces around a two-orders-of-magnitude stabilization effect (log  =20.8 vs. log  =22.7), which defines an extremely high affinity of a peptide towards a metal ion, and 2) the displacement disrupts the overall assembly of the domain, as shown by NMR spectroscopic and anisotropy decay data. Based on the results, a new model explaining the molecular mechanism of Cd genotoxicity that underlines Cd 's impact on Rad50's dimer stability and quaternary structure that could potentially result in abrogation of the major DNA damage response pathway is proposed.