Mechanism of Divalent Metal Transport by NRamp-Family Transporters

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Research Summary Metals such as iron and manganese are essential to many physiological processes including oxygen transport and energy metabolism. But overabundance of these metals is toxic, and their physiological levels are therefore tightly regulated. Nramps (natural resistance-associated macrophage proteins) are membrane transporters that import divalent metal ions into cells. Nramps are important for both divalent metal uptake from the diet and cellular import of metals into the cytosol. Nramps are therefore critical factors in maintaining homeostasis of divalent metals, particularly iron and manganese. Nramp proteins are conserved from bacteria to man, and in many bacterial species they are the principal manganese import system. Nramps are metal-proton symporters, using a pH gradient to drive the co- transport of divalent metals and protons. The overall goal is to determine the molecular mechanism of metal ion-proton symport by the Nramp family of proteins through biochemical, computational, and structural studies of bacterial Nramp proteins. We recently determined the crystal structure of a bacterial Nramp homolog, which serves as a scaffold for generating hypotheses and interpreting data. We have developed a range of in vivo and in vitro activity assays, which we will use to define how metal selectivity is encoded in the sequences of Nramp family transporters. We will also use these assays to understand how protons and protonation events influence the transport cycle. We will study the conformation changes during metal transport using biochemical approaches, engineer constructs that stabilize particular conformational states for high-resolution structure determination by x-ray crystallography, and molecular dynamic simulations in various protonation states to model dynamics produced by protonation or deprotonation events. Our overall goal is to establish an atomic- resolution model of the proton-coupled metal ion transport cycle. While Nramps are part of the well-characterized LeuT-fold superfamily of transporters, they are unusual because they are neither sodium-coupled symporters nor antiporters. Our proposed research focuses on several of Nramps’ unique features and will thus expand our knowledge of the mechanistic diversity enabled by the LeuT fold. Both bacterial and mammalian Nramp proteins have an impact on human health. Bacterial Nramps increase pathogenicity by facilitating the uptake of essential divalent metals. Human Nramps are particularly important in immunity to intracellular pathogens, liver and blood homeostasis, and brain function. Nramps have been implicated in numerous pathologic conditions including autoimmune diseases, anemia and Parkinson disease. The proposed basic research will have a major impact on the field by providing sorely needed mechanistic information on the biomedically relevant class of Nramp transporters. These structural and mechanistic insights into metal ion transport by Nramp proteins can eventually contribute to the development of drugs and therapeutic strategies to treat disorders resulting from imbalances in metal ion homeostasis.