A mercuric ion uptake role for the integral inner membrane protein, MerC, involved in bacterial mercuric ion resistance
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L Sahlman1, W Wong, J Powlowski
1Department of Biochemistry, Umeâ University, S-901 87, Umeâ, Sweden. Lena@chem.umu.se
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.
Insights
Bacterial mercuric ion resistance involves MerC, a membrane protein that binds Hg2+ independently of MerP. This binding, crucial for resistance, is influenced by cysteine residues and protein topology, suggesting a transport role.
Area of Science:
- Microbiology
- Molecular Biology
- Biochemistry
Background:
- Bacterial detoxification of mercuric ion (Hg2+) relies on integral membrane proteins like MerC.
- These proteins are thought to transport Hg2+ from periplasmic proteins to cytoplasmic enzymes.
Purpose of the Study:
- To investigate the role of MerC from the Tn21 mer operon in bacterial Hg2+ resistance.
- To clarify the Hg2+ uptake and binding mechanisms mediated by MerC.
Main Methods:
- Overexpression of MerC and study in vesicles and purified form.
- Analysis of Hg2+ uptake in MerC-containing vesicles under varying conditions (osmotic pressure, Triton X-100).
- Investigation of the effect of thiol-specific reagents on Hg2+ binding.
Main Results:
- MerC-containing vesicles exhibited Hg2+ uptake independent of MerP.
- Uptake correlated with MerC expression and was largely unaffected by osmotic pressure.
- Binding was inhibited by thiol-specific reagents, with hydrophobic reagents abolishing Hg2+ binding, consistent with protein topology.
Conclusions:
- MerC plays a significant role in Hg2+ binding and likely transport, independent of MerP.
- The accessibility of cysteine residues suggests their importance in Hg2+ interaction.
- A revised model for bacterial Hg2+ transport involving MerC is proposed.