Characteristics of Zinc Transport by Two Bacterial Cation Diffusion Facilitators from Ralstonia metallidurans CH34 and Escherichia coli

Abstract

ABSTRACT CzcD from Ralstonia metallidurans and ZitB from Escherichia coli are prototypes of bacterial members of the cation diffusion facilitator (CDF) protein family. Expression of the czcD gene in an E. coli mutant strain devoid of zitB and the gene for the zinc-transporting P-type ATPase zntA rendered this strain more zinc resistant and caused decreased accumulation of zinc. CzcD, purified as an amino-terminal streptavidin-tagged protein, bound Zn 2+ , Co 2+ , Cu 2+ , and Ni 2+ but not Mg 2+ , Mn 2+ , or Cd 2+ , as shown by metal affinity chromatography. Histidine residues were involved in the binding of 2 to 3 mol of Zn 2+ per mol of CzcD. ZitB transported 65 Zn 2+ in the presence of NADH into everted membrane vesicles with an apparent K m of 1.4 μM and a V max of 0.57 nmol of Zn 2+ min −1 mg of protein −1 . Conserved amino acyl residues that might be involved in binding and transport of zinc were mutated in CzcD and/or ZitB, and the influence on Zn 2+ resistance was studied. Charged or polar amino acyl residues that were located within or adjacent to membrane-spanning regions of the proteins were essential for the full function of the proteins. Probably, these amino acyl residues constituted a pathway required for export of the heavy metal cations or for import of counter-flowing protons.