ZraP, the most prominent zinc protein under zinc stress conditions has no direct role in in-vivo zinc tolerance in Escherichia coli

Abstract

Escherichia coli ZraP (zinc resistance associated protein) is the major Zn containing soluble protein under Zn stress conditions. ZraP is the accessory protein of a bacterial two-component, Zn²⁺ sensitive signal transduction system ZraSR. ZraP has also been reported to act as a Zn²⁺ dependent molecular chaperone. An explanation why ZraP is the major Zn protein under the stress condition of Zn²⁺ overload (0.2 mM) has remained elusive. We have recombinantly produced E. coli ZraP and measured Zn²⁺ and Cu²⁺ affinity in-vitro using Isothermal Titration Calorimetry. ZraP has a significantly higher affinity for Cu²⁺ than for Zn²⁺. Mutation of the conserved Cys¹⁰² to Ala or Ser resulted in a change of the oligomeric state of the protein. Mutation of the conserved His¹⁰⁷ to Ala did not affect the zinc binding affinity or the oligomeric state of the protein. Deletion of the ZraP coding gene from the E. coli genome resulted in a phenotype with tolerance to very high zinc concentrations (up to 2.5 mM) that were lethal to wild type E. coli. These results exclude a direct role for ZraP in Zn²⁺ tolerance in E. coli.