Tying Up a Loose End: On the Role of the C-Terminal CCHHRAG Fragment of the Metalloregulator CueR

The transcriptional regulator CueR is activated by the binding of Cu^I, Ag^I, or Au^I to two cysteinates in a near-linear fashion. The C-terminal CCHHRAG sequence in Escherichia coli CueR present potential additional metal binding ligands and here we explore the effect of deleting this fragment on the binding of Ag^I to CueR. CD spectroscopic and ESI-MS data indicate that the high Ag^I-binding affinity of WT-CueR is significantly reduced in Δ7C-CueR.^[111 Ag PAC spectroscopy demonstrates that the WT-CueR metal site structure (AgS₂) is conserved, but less populated in the truncated variant. Thus, the function of the C-terminal fragment may be to stabilize the two-coordinate metal site for cognate monovalent metal ions. In a broader perspective this is an example of residues beyond the second coordination sphere affecting metal site physicochemical properties while leaving the structure unperturbed.