PcoE — A metal sponge expressed to the periplasm of copper resistance Escherichia coli. Implication of its function role in copper resistance

Expression of the periplasmic protein PcoE of Escherichia coli is induced strongly by cupric salts under the control of the chromosomal copper tolerance system cusRS. Its isolation and study were complicated by de-amidation of Asn 54 and 103 at alkaline pH. Its apo form is essentially unstructured in solution and can be likened to a large unstructured multidentate ligand carrying multiple metal binding sites (15 Met; 10 His; 13 Asp, Glu; 10 Asn; 6 Lys). As expected, it binds multiple soft metal ions Cu+ and Ag+ non-cooperatively with the highest affinity for CuI in the picomolar range (KD ~ 10− 12 M). Binding of multiple soft ions induced dimerization and formation of some α-helical structure. PcoE also binds the harder metal ions Cu2+ or Zn2+ but with lower affinities and in smaller numbers. CuII bound in PcoE is reduced readily to more tightly bound CuI. Overall, these properties mean that it is difficult to characterize individual species of defined metal content. Similar properties and difficulties have been reported for the homologous silver-binding protein SilE from Salmonella. However, the properties are consistent with a role for PcoE as a ‘metal sponge’ acting as a first line of defence against metal toxicity (under the control of the copper tolerance system cusRS) until the copper resistance operon pcoABCD is expressed.

PcoE was expressed and characterized. This protein is an unstructured ‘metal sponge’ and sequesters multiple soft metal ions with partial structural folding. It can transfer the sequestered CuI to putative partner protein PcoC for further detoxification. The rapid response of PcoE expression to metal stress may provide a first line of defence against metal toxicity.Figure optionsDownload as PowerPoint slideHighlights
► PcoE is characterized as an unstructured 'metal sponge' for soft metal ions.
► Binding of multiple metal ions induces partial structural folding and dimerization.
► The highest affinity site for CuI is shown to be in the picomolar range.
► A copper resistance model involving all Pco proteins is proposed and discussed.