title: ABCG2 transports and transfers heme to albumin through its large extracellular loop.
abstract: ABCG2 is an ATP-binding cassette (ABC) transporter preferentially expressed by immature human hematopoietic progenitors. Due to its role in drug resistance, its expression has been correlated with a protection role against protoporhyrin IX (PPIX) accumulation in stem cells under hypoxic conditions. We show here that zinc mesoporphyrin, a validated fluorescent heme analog, is transported by ABCG2. We also show that the ABCG2 large extracellular loop ECL3 constitutes a porphyrin-binding domain, which strongly interacts with heme, hemin, PPIX, ZnPPIX, CoPPIX, and much less efficiently with pheophorbide a, but not with vitamin B12. K(d) values are in the range 0.5-3.5 μm, with heme displaying the highest affinity. Nonporphyrin substrates of ABCG2, such as mitoxantrone, doxo/daunorubicin, and riboflavin, do not bind to ECL3. Single-point mutations H583A and C603A inside ECL3 prevent the binding of hemin but hardly affect that of iron-free PPIX. The extracellular location of ECL3 downstream from the transport sites suggests that, after membrane translocation, hemin is transferred to ECL3, which is strategically positioned to release the bound porphyrin to extracellular partners. We show here that human serum albumin could be one of these possible partners as it removes hemin bound to ECL3 and interacts with ABCG2, with a K(d) of about 3 μm.
authors: From the ABC Transporters and Multidrug Resistance Laboratory, "Equipe Labellisée Ligue 2009," Institute of Protein Biology and Chemistry, Unité Mixte de Recherche 5086 CNRS-Université Lyon 1, IFR 128 Lyon, France.
authors: Elodie
authors: E
authors: Desuzinges-Mandon
authors: From the ABC Transporters and Multidrug Resistance Laboratory, "Equipe Labellisée Ligue 2009," Institute of Protein Biology and Chemistry, Unité Mixte de Recherche 5086 CNRS-Université Lyon 1, IFR 128 Lyon, France.
authors: Ophélie
authors: O
authors: Arnaud
authors: From the ABC Transporters and Multidrug Resistance Laboratory, "Equipe Labellisée Ligue 2009," Institute of Protein Biology and Chemistry, Unité Mixte de Recherche 5086 CNRS-Université Lyon 1, IFR 128 Lyon, France.
authors: Lorena
authors: L
authors: Martinez
authors: From the ABC Transporters and Multidrug Resistance Laboratory, "Equipe Labellisée Ligue 2009," Institute of Protein Biology and Chemistry, Unité Mixte de Recherche 5086 CNRS-Université Lyon 1, IFR 128 Lyon, France.
authors: Frédéric
authors: F
authors: Huché
authors: From the ABC Transporters and Multidrug Resistance Laboratory, "Equipe Labellisée Ligue 2009," Institute of Protein Biology and Chemistry, Unité Mixte de Recherche 5086 CNRS-Université Lyon 1, IFR 128 Lyon, France.
authors: Attilio
authors: A
authors: Di Pietro
authors: From the ABC Transporters and Multidrug Resistance Laboratory, "Equipe Labellisée Ligue 2009," Institute of Protein Biology and Chemistry, Unité Mixte de Recherche 5086 CNRS-Université Lyon 1, IFR 128 Lyon, France. Electronic address: p.falson@ibcp.fr.
authors: Pierre
authors: P
authors: Falson
mesh_terms: D000070997:ATP Binding Cassette Transporter, Subfamily G, Member 2; D018528:ATP-Binding Cassette Transporters / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D000970:Antineoplastic Agents / Q000737:chemistry / Q000378:metabolism / Q000494:pharmacology; D015687:Cell Hypoxia / Q000502:physiology; D002734:Chlorophyll / Q000031:analogs & derivatives / Q000737:chemistry / Q000378:metabolism / Q000494:pharmacology; D006412:Hematopoietic Stem Cells / Q000378:metabolism; D006418:Heme / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D020014:K562 Cells; D008652:Mesoporphyrins / Q000737:chemistry / Q000378:metabolism / Q000494:pharmacology; D009363:Neoplasm Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D017319:Photosensitizing Agents / Q000737:chemistry / Q000378:metabolism / Q000494:pharmacology; D017354:Point Mutation; D017433:Protein Structure, Secondary; D011524:Protoporphyrins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism / Q000494:pharmacology; D012709:Serum Albumin / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D014805:Vitamin B 12 / Q000737:chemistry / Q000378:metabolism / Q000494:pharmacology; D014803:Vitamin B Complex / Q000737:chemistry / Q000378:metabolism / Q000494:pharmacology
chemical_list: C116614:ABCG2 protein, human; D000070997:ATP Binding Cassette Transporter, Subfamily G, Member 2; D018528:ATP-Binding Cassette Transporters; D000970:Antineoplastic Agents; D008652:Mesoporphyrins; D009363:Neoplasm Proteins; D017319:Photosensitizing Agents; D011524:Protoporphyrins; D012709:Serum Albumin; D014803:Vitamin B Complex; D002734:Chlorophyll; D006418:Heme; C028025:protoporphyrin IX; C032623:pheophorbide a; D014805:Vitamin B 12