Maintaining proper cellular levels of metal ions is key to the survival of all organisms. The viability of bacteria, including human pathogens, has been linked to the ability to acquire or compete for transition metals. Several human diseases have been shown to result from a breakdown in cellular metal trafficking. In the case of transition metals, maintaining proper metal concentrations involves controlling a delicate balance between optimal levels required to have functional enzymes, etc., and the concentration at which the metals become toxic. To achieve this control, organisms have developed mechanisms to ensure the acquisition of specific metals necessary for growth, exclude toxic metals, and control their cellular concentration. These metal trafficking systems rely on proteins that have the ability to distinguish between metal ions that often have similar sizes and charges. The metalloproteins involved, collectively known as metal trafficking proteins, control uptake and efflux (metallotransporters), target the delivery of metals to specific enzymes (metallochaperones) and regulate the expression of the other proteins in response to metal ion concentration (metalloregulators). Although many examples of proteins that achieve these functions for various metal ions have been characterized, the mechanisms that allow for metal recognition and metal-specific biological responses are not well known. The overall objective of the proposed research is to understand the structural parameters that that allow trafficking proteins to distinguish between metals, and the related protein structural changes that drive metal specific biological responses. Metal recognition is essential in many trafficking proteins in order to generate biological responses geared to one, or a small group of, metals and avoid crosstalk with other metal ions. Toward this goal, we plan to examine structural parameters that are involved in metal-recognition in a metallotransporter (NikA), a metallochaperone (HypA) and metalloregulators (NikR and RcnR) in E. coli and Helicobacter pylori. The approach involves cloning and expressing trafficking proteins, characterizing their metal ion affinities, and elucidating the structure of the metal sites, employing X-ray absorption spectroscopy (XAS) as a probe of metal site structure and crystallography and NMR to examine protein structure. Alterations in the metal binding site and other critical protein features can be produced by site-directed mutagenesis, and the effect on metal site structure and/or protein structure addressed. Strategies involving both in vitro and in vivo assays are employed to assess the effect on function in mutant trafficking proteins. In addition to the understanding of the basic biochemistry, a detailed understanding of the molecular mechanisms involved in metal trafficking may lead to the development of therapies for the treatment of patients with metal overloads (including poisoning) or deficiencies resulting from defects in metal metabolism, to the design of new antibiotics that interfere with bacterial metal metabolism.
The research proposed seeks to provide a detailed understanding of the molecular mechanisms involved in cellular metal trafficking. This information will be useful in the development of therapies for the treatment of patients with metal overloads (including poisoning) or deficiencies resulting from defects in metal metabolism, and in the design of new antibiotics that interfere with bacterial metal metabolism.
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