We have isolated and characterized a novel iron-regulated gene that is homologous to the divalent metal transporter 1 family of metal transporters. This gene, termed metal transporter protein (mtp1), is expressed in tissues involved in body iron homeostasis including the developing and mature reticuloendothelial system, the duodenum, and the pregnant uterus. MTP1 is also expressed in muscle and central nervous system cells in the embryo. At the subcellular level, MTP1 is localized to the basolateral membrane of the duodenal epithelial cell and a cytoplasmic compartment of reticuloendothelial system cells. Overexpression of MTP1 in tissue culture cells results in intracellular iron depletion. In the adult mouse, MTP1 expression in the liver and duodenum are reciprocally regulated. Iron deficiency induces MTP1 expression in the duodenum but down-regulates expression in the liver. These data indicate that MTP1 is an iron-regulated membrane-spanning protein that is involved in intracellular iron metabolism.The uptake of iron by the duodenum and by individual cells in the body is regulated by total body iron levels and intracellular iron levels, respectively. Individual cells take up iron bound to transferrin using the transferrin receptor. Iron not immediately utilized is stored as cytosolic ferritin. The iron-dependent regulation of ferritin and transferrin receptor is mediated by the post-transcriptional interaction of iron-responsive elements (IREs), 1 found in the untranslated regions (UTRs) of the mRNAs of these genes, with cytosolic RNAbinding proteins called iron-regulatory proteins (IRP1 and IRP2) (1). The IRE is a well conserved RNA stem-loop found in the 5Ј-UTR of iron-regulated genes such as ferritin and the 3Ј-UTR of transferrin receptor and dmt1 (2, 3).Intestinal iron acquisition requires uptake of iron at the brush border of the duodenal epithelial cell and subsequent export of the iron across the basal border. DMT1 (NRAMP2, DCT1) transports iron into the cell at the apical brush border of the duodenal epithelial cell. A mouse mutant in DMT1 in unable to take up intestinal iron (3, 4). An alternative pathway for intestinal iron absorption that involves a cell surface ␤ 3 integrin, a calreticulin-like molecule called mobilferrin, and a ferrireductase has been described (5). Iron export from the epithelial cell also requires a copper-dependent ferroxidase called hephaestin (6), but otherwise little is known about this process. Another major area of iron metabolism that is not well understood is that of the recycling of hemoglobin-derived iron by the RE system. Tissue macrophages ingest recycled iron from senescent erythrocytes. Export of heme-derived iron as ferritin and low molecular weight iron by macrophages has been described (7, 8), but details of the regulation of the process and molecular mechanisms are lacking. This paper reports the cloning and characterization of a novel iron-regulated iron transporter called MTP1. MTP1 is related to the DMT1 class of divalent metal transporters and a yeast man...

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The translation of ferritin mRNA and degradation of transferrin receptor mRNA are regulated by the interaction of an RNA-binding protein, the iron-responsive element binding protein (ERE-BP), with RNA stem-oop structures known as iron-responsive elements (IREs) contained within these transcripts. IRE-BP produced in iron-replete cells has aconitase (EC 4.2.1.3) activity. The protein shows extensive sequence homology with mitochondrial aconitase, and sequences of peptides prepared from cytosolic aconitase are identical with peptides of IRE-BP. As an active aconitase, IRE-BP is expected to have an Fe-S duster, in analogy to other aconitases. This Fe-S cluster has been implicated as the region of the protein that senses intracellular iron levels and accordingly modifies the ability of the IRE-BP to interact with IREs. Expression of the IRE-BP in cultured cells has revealed that the IRE-BP functions either as an active aconitase, when the cells are iron-replete, or as an active RNA-binding protein, when the cells are iron-depleted. We compare properties of purified authentic cytosolic aconitase from beef liver with those of IRE-BP from tissue culture cells and establish that characteristics of the physiologically relevant form of the protein from iron-depleted cells resemble those of cytosolic aconitase apoprotein. We demonstrate that loss of the labile fourth iron atom of the Fe-S cluster results in loss of aconitase activity, but that more extensive cluster alteration is required before the IRE-BP acquires the capacity to bind RNA with the affinity seen in vivo. These results are consistent with a model in which the cubane Fe-S cluster is disassembled when intracellular iron is depleted.