The iron transporter ferroportin is critical for delivering iron into plasma from duodenal enterocytes absorbing dietary iron, macrophages recycling old red blood cells, and hepatocytes storing iron. The interaction of ferroportin with its ligand, the hepatic hormone hepcidin, is the key event in systemic iron homeostasis. After it binds hepcidin, ferroportin is degraded, thereby limiting iron entry into plasma. Dysregulation of the hepcidin- ferroportin axis underlies most common iron disorders, including anemia of inflammation, anemia of chronic kidney disease, anemia of cancer, hereditary hemochromatosis and iron-loading anemias. Ferroportin is the only known cellular iron exporter in vertebrates and is conserved down to invertebrates and plants. Despite its obvious biological importance, very little is known about the ferroportin structure and the mechanisms by which ferroportin transports iron. In a recent breakthrough, we identified the prokaryotic ortholog of Fpn and obtained its structure by X-ray crystallography. We are now poised to make rapid progress toward complete understanding of the structural basis of Fpn function by combining X-ray crystallography of mammalian Fpn with detailed structure-guided mutational and functional analyses of metal transport and hepcidin-ferroportin interaction in mammalian cells and Xenopus oocytes.
Our Specific Aims are:
Aim 1. Determine the structure of ferroportin by X-ray crystallography. While these efforts are ongoing, we will use the structure of prokaryotic Fpn ortholog to guide further studies of the transport mechanism, specific function of conserved residues, and as a framework for functional and mutagenesis work on the higher orthologs.
Aim 2. Determine the mechanism of iron transport by ferroportin. These studies will determine the driving forces, ion coupling, and calcium gating of Fpn-mediated iron transport.
Aim 3. Discover the structural determinants of ferroportin function and malfunction. We will use structure- and human disease guided mutagenesis to probe critical residues involved in iron binding and translocation, transporter gating, pH dependence, oligomerization, and hepcidin binding. Successful completion of the proposed studies is of fundamental importance for iron biology. Its significance also extends to general biology: identifying the structure of a new class of membrane transporters and defining the mechanism of iron transport will impact studies of other metal and ion transporters. Finally, understanding Fpn iron-transporting function and its regulation by hepcidin is biomedically significant. The proposed studies will generate much more definitive mechanistic and structural insights which will guide the development of improved small and large molecule therapeutics for iron-restrictive anemias and iron overload disorders.
Ferroportin, an iron transporter, plays a critical role in iron homeostasis and development of iron disorders. We propose to define the structure of ferroportin and its mechanism of iron transport. Understanding the ferroportin structure-function could help develop better treatments for iron disorders.
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