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 Delta 7C-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.