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Volume 272, Number 47, Issue of November 21, 1997 pp. 29518-29526
©1997 by The American Society for Biochemistry and Molecular Biology, Inc.

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

(Received for publication, April 22, 1997, and in revised form, August 20, 1997)

Lena Sahlman , Wendy Wong ^¶ and Justin Powlowski ^¶

From the  Department of Biochemistry, Umeå University, S-901 87, Umeå, Sweden and the ^¶ Department of Chemistry and Biochemistry, Concordia University, Montreal, Quebec H3G 1M8, Canada

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.

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