A Mercuric Ion Uptake Role for the Integral Inner Membrane Protein, MerC, Involved in Bacterial Mercuric Ion Resistance

Abstract

Bacterial detoxification of mercuric ion depends on the presence of one or more integral membrane proteins (MerT and/or MerC) whose postulated function is in transport of Hg2+ from a periplasmic Hg2+-binding protein (MerP) to cytoplasmic mercuric reductase. In this study, MerC from the Tn21-encoded mer operon was overexpressed and studied in vesicles and in purified form to clarify the role played by this protein in mercuric ion resistance. MerC-containing vesicles were found to take up mercuric ion independently of MerP. Since uptake correlated with the level of MerC expression was unaffected by osmotic pressure, and was only partially decreased in the presence of 0.05% Triton X-100, the observed uptake appears to represent mainly binding to MerC. Binding was inhibited by thiol-specific reagents, consistent with an essential role for cysteine residues. The essential thiol groups were inaccessible to hydrophilic thiol reagents, whereas hydrophobic reagents completely abolished Hg2+ binding. These observations are consistent with the predicted topology of the protein, wherein all 4 cysteine residues are either in the cytoplasm or the bilayer. A role for MerC in Hg2+ transport is thus also likely. Based on these results, a modified model for bacterial Hg2+ transport is proposed.