Special Delivery: The Role of Iron Chaperones in the Distribution of Iron in Developing Red Cells

Iron is an essential nutrient for every cell in the human body, yet it can also be a potent cellular toxin. Iron is essential because enzymes that require iron co-factors (namely, heme, iron-sulfur clusters, mononuclear and diiron centers) are involved in virtually every major metabolic process in the cell. Hundreds of iron, zinc, copper, and manganese proteins are expressed in human cells, yet little is known about the mechanisms by which these metalloproteins acquire their native metal ligands and avoid mis-metallation. Significant advances have been made in understanding the delivery of iron to iron-dependent enzymes in the cytosol. Poly(rC)-binding proteins (PCBPs) are multifunctional adaptors that mediate interactions between single-stranded nucleic acids, iron cofactors, and other proteins, affecting the fate and activity of the components of these interactions. PCBP1 is an iron-binding protein that delivers iron to ferritin in human cells via a direct protein-protein interaction and can be described as an iron chaperone. PCBP2, a human paralog of PCBP1, is also involved in the delivery of iron to ferritin, both in yeast cells and in human cells, suggesting that PCBP1 and PCBP2 work together in iron delivery. PCBP1 and PCBP2 can also deliver iron to the two major families of non-heme iron enzymes: the mononuclear and dinuclear iron-dependent oxygenases. The prolyl hydroxylases (PHDs) are mononuclear iron enzymes that regulate the degradation of hypoxia inducible factor 1 (HIF1).  Misregulation of the HIF transcription factors leads to the development of a variety of cancers in humans. Cells depend on the iron chaperones PCBP1 and PCBP2 to maintain iron in the enzymatic active site of PHDs and the related enzyme, asparagyl hydroxylase and to maintain proper regulation of HIF1a, especially under conditions of iron limitation. Deoxyhypusine hydroxylase (DOHH) is a dinuclear iron enzyme that is required for the posttranslational modification of a single lysine residue on eukaryotic initiation factor 5A (eIF5A). EIF5A and the conversion of this conserved lysine to hypusine are essential in all eukaryotes, as it enables the translation of peptides containing polyproline sequences. We found that cells depleted of PCBP1 or PCBP2 exhibited reduced activity of DOHH, which was due to a loss of iron in the active site of the enzyme. Thus, PCBPs are basic components of a cytosolic iron delivery system that serves both of the major classes of non-heme iron enzymes in the cytosol. Recent work has indicated that a second type of iron delivery system in the cytosol is mediated by a monothiol glutaredoxin, Glrx3, which, in vitro, can bind and transfer iron-sulfur clusters to recipient apo-iron-sulfur proteins. We have determined that PCBP1 directly interacts with Glrx3-containing complexes and can affect the coordination of iron-sulfur clusters by Glrx3. The huge flux of iron through the developing erythroid cell represents unique challenges for the utilization of cellular iron. We have examined the role of PCBPs as iron chaperones in terminal erythroid differentiation. The role of ferritin in erythroid cell maturation is controversial, but our data indicate that ferritin, PCBPs and NCOA4 are critical factors in erythrocyte development. The flux of iron through ferritin via the lysosome appears to be critical for the transfer of iron to mitochondria for heme synthesis.