Heavy metals, like mercury, are nearly universally toxic because of their indiscriminant reactivity with multiple sites on proteins. Some bacteria thrive in the presence of high concentrations of Hg(II). This is possible only because these bacteria possess an efficient mechanism for the detoxification of mercury. It functions by transporting toxic Hg(II) into the cell where it is converted to relatively nontoxic metallic Hg(0) which is volatile and can be passively eliminated. The merP (periplasm) protein binds mercury in the periplasm and transfers it to the merT (transport) protein responsible for transporting mercury through the membrane into the cytoplasm. MerF is a variant of merT. The structures of the two membrane transport proteins are under investigation, merT and merF. We have expressed and labeled both of them in substantial quantities. Their structures will give important insights into the organization of membrane proteins with apparently four and three trans-membrane helices, respectively, and the processes responsible for the transport of mercury (and other heavy metals) across biological membranes. The solid-state NMR spectra of each of these proteins incorporated into lipid bilayers, oriented on glass plates indicate that the proteins are immobilized in the bilayer with helical domains both transmembrane and in-plane relative to the bilayer. These results are consistent with the topologies predicted by hydropathy sequence analysis of the proteins. Studies are continuing to produce a complete three-dimensional structure of the proteins.
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