The National Institute of Diabetes & Digestive & Kidney Diseases

Iron is an essential nutrient for almost every organism. It is required by every cell in the human body, yet it can also be a potent cellular toxin. Iron is essential because enzymes that require iron cofactors (namely, heme, iron-sulfur clusters, mononuclear and diiron centers) are involved in virtually every major metabolic process in the cell. Iron deficiency continues to be the most common nutritional deficiency in the world, especially among children and women of childbearing age, where it causes anemia and impairs neurological development and function. Although the pathogenesis of anemia in iron deficiency is well understood, other manifestations of iron deficiency are not understood at the cellular or metabolic level. Iron overload is a feature of an increasing number of human diseases, including genetic disorders such as hereditary hemochromatosis, thalassemias, and Friedreich ataxia, as well as chronic inflammatory diseases of the liver, such as hepatitis C. Our laboratory focuses on the genetics and cell biology of iron uptake and utilization in eukaryotes. Previously, we have identified and characterized systems of iron transport in baker’s yeast, Saccharomyces cerevisiae. More recently, we have used the genetic tractability of yeast to focus on the intracellular trafficking and distribution of iron cofactors in yeast and mammalian cells.

Mammalian cells express hundreds of metalloproteins. Most contain the abundant metals iron and zinc, while others contain various trace metals such as copper, manganese, molybdenum and cobalt. Although the incorporation of the appropriate metal ion(s) into cellular metalloproteins is a critical, essential process, the mechanism by which most metalloproteins receive their specific cofactor is unknown. Some proteins rely on metallochaperones: proteins that specifically bind metal ions and deliver them to target enzymes and transporters through direct protein-protein interactions.

We identified poly (rC) binding protein 1 (PCBP1) as a cytosolic iron chaperone that delivers iron to ferritin. In mammals, ferritin is an iron storage protein consisting of 24 subunits of heavy (H) and light (L) peptides that assemble into a hollow sphere into which iron is deposited. PCBP1 binds both Fe(II) and ferritin and facilitates the incorporation of iron into ferritin. Studies are underway to identify other iron enzymes that require PCBP1 for the insertion of iron cofactors and to further characterize the cell biology and biochemistry of iron chaperones in mammals.

The final step of heme biosynthesis occurs within the mitochondria, yet heme proteins are located in virtually every compartment of the cell. Thus heme, a hydrophobic molecule, must be transferred to the cytosol and to other membrane-bound organelles for insertion into newly synthesized heme proteins. Soluble heme-binding proteins have been identified in plants and mammals, but their roles in cellular heme metabolism or transport are unclear. We are conducting studies to identify and characterize the proteins involved in intracellular heme trafficking.

Our research program couples the power of yeast genetics, mammalian cell biology, and murine models to understand the biology of iron utilization in human health and disease.

Iron regulated genes in yeast

PCBP iron chaperone function in cells

Guo Y, Au WC, Shakoury-Elizeh M, Protchenko O, Basrai M, Prinz WA, Philpott CC. Phosphatidylserine is involved in the ferrichrome-induced plasma membrane trafficking of Arn1 in Saccharomyces cerevisiae. J Biol Chem. 2010 Dec10; 285(50):39564-73. [Full Text/Abstract]

Shakoury-Elizeh M, Protchenko O, Berger A, Cox J, Gable K, Dunn TM, Prinz WA, Bard M, Philpott CC. (2010) The metabolic response to iron deficiency in Saccharomyces cerevisiae. J Biol Chem. 2010 May 7;285(19):14823-33. [Full Text/Abstract]

Deng Y, Woodham H, Guo Y, Au WC, Shakoury-Elizeh M, Basrai MA, Bonifacino JS, Philpott CC. (2009) Gga2 mediates ubiquitin-dependent and -independent trafficking of Arn1 from the trans-Golgi network to the vacuole. J Biol Chem. 284(35):23830-41. [Full Text/Abstract]

Shi H, Bencze KZ, Stemmler TL, Philpott CC. (2008) A cytosolic iron chaperone that delivers iron to ferritin. Science;320(5880):1207-10. [Full Text/Abstract]

Protchenko O, Shakoury-Elizeh M, Keane P, Storey J, Androphy R, Philpott CC. (2008) Role of PUG1 in inducible porphyrin and heme transport in Saccharomyces cerevisiae. Eukaryot Cell;7(5):859-71. [Full Text/Abstract]

Philpott CC, Protchenko O. (2007) The Response to Iron Deprivation in Saccharomyces cerevisiae. Eukaryot Cell. 2008;7(1):20-7. [Full Text/Abstract]

Kim Y, Deng Y, and Philpott C.C. (2007) GGA2- and ubiquitin-dependent trafficking of Arn1, the ferrichrome transporter of Saccharomyces cerevisiae. Mol Biol Cell, May;18(5):1790-802. [Full Text/Abstract]

Protchenko O, Rodriguez-Suarez R, Androphy R, Bussey H, Philpott C.C. (2006) A screen for genes of heme uptake identifies the FLC family required for import of FAD into the endoplasmic reticulum. J Biol Chem. 281(30):21445-57. [Full Text/Abstract]

Philpott, C.C. (2006) Iron uptake in fungi: a system for every source. Biochim Biophys Acta. 1763(7):636-45. [Full Text/Abstract]

Shakoury-Elizeh M Tiedeman J Rashford J Ferea T Demeter J Garcia E Rolfes R Brown PO Botstein D Philpott CC Transcriptional remodeling in response to iron deprivation in Saccharomyces cerevisiae. Mol Biol Cell (15): 1233-43, 2004. [Full Text/Abstract]

Pelletier B Beaudoin J Philpott CC Labbe S Fep1 represses expression of the fission yeast Schizosaccharomyces pombe siderophore-iron transport system. Nucleic Acids Res (31): 4332-44, 2003. [Full Text/Abstract]

Protchenko O Philpott CC Regulation of intracellular heme levels by HMX1, a homologue of heme oxygenase, in Saccharomyces cerevisiae. J Biol Chem (278): 36582-7, 2003. [Full Text/Abstract]

Moore RE Kim Y Philpott CC The mechanism of ferrichrome transport through Arn1p and its metabolism in Saccharomyces cerevisiae. Proc Natl Acad Sci U S A (100): 5664-9, 2003. [Full Text/Abstract]

Kim Y Yun CW Philpott CC Ferrichrome induces endosome to plasma membrane cycling of the ferrichrome transporter, Arn1p, in Saccharomyces cerevisiae. EMBO J (21): 3632-42, 2002. [Full Text/Abstract]

Philpott CC Molecular aspects of iron absorption: Insights into the role of HFE in hemochromatosis. Hepatology (35): 993-1001, 2002. [Full Text/Abstract]

Philpott CC Protchenko O Kim YW Boretsky Y Shakoury-Elizeh M The response to iron deprivation in Saccharomyces cerevisiae: expression of siderophore-based systems of iron uptake. Biochem Soc Trans (30): 698-702, 2002. [Full Text/Abstract]

Yun CW Bauler M Moore RE Klebba PE Philpott CC The role of the FRE family of plasma membrane reductases in the uptake of siderophore-iron in Saccharomyces cerevisiae. J Biol Chem (276): 10218-23, 2001. [Full Text/Abstract]

Protchenko O Ferea T Rashford J Tiedeman J Brown PO Botstein D Philpott CC Three cell wall mannoproteins facilitate the uptake of iron in Saccharomyces cerevisiae. J Biol Chem (276): 49244-50, 2001. [Full Text/Abstract]

Yun CW Ferea T Rashford J Ardon O Brown PO Botstein D Kaplan J Philpott CC Desferrioxamine-mediated iron uptake in Saccharomyces cerevisiae. Evidence for two pathways of iron uptake. J Biol Chem (275): 10709-15, 2000. [Full Text/Abstract]

Yun CW Tiedeman JS Moore RE Philpott CC Siderophore-iron uptake in saccharomyces cerevisiae. Identification of ferrichrome and fusarinine transporters. J Biol Chem (275): 16354-9, 2000. [Full Text/Abstract]