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Ferrata Storti Foundation
Haematologica 2019 Volume 104(9):1756-1767
Iron Metabolism & its Disorders
Dimeric ferrochelatase bridges ABCB7 and ABCB10 homodimers in an architecturally defined molecular complex required for heme biosynthesis
Nunziata Maio, Ki Soon Kim, Gregory Holmes-Hampton, Anamika Singh and Tracey A. Rouault
Molecular Medicine Branch, ‘Eunice Kennedy Shriver’ National Institute of Child Health and Human Development, Bethesda, MD, USA
ABSTRACT
Loss-of-function mutations in the ATP-binding cassette (ABC) trans- porter of the inner mitochondrial membrane, ABCB7, cause X-linked sideroblastic anemia with ataxia, a phenotype that remains largely unexplained by the proposed role of ABCB7 in exporting a special sulfur species for use in cytosolic iron-sulfur (Fe-S) cluster biogenesis. Here, we generated inducible ABCB7-knockdown cell lines to examine the time- dependent consequences of loss of ABCB7. We found that knockdown of ABCB7 led to significant loss of mitochondrial Fe-S proteins, which preced- ed the development of milder defects in cytosolic Fe-S enzymes. In ery- throid cells, loss of ABCB7 altered cellular iron distribution and caused mitochondrial iron overload due to activation of iron regulatory proteins 1 and 2 in the cytosol and to upregulation of the mitochondrial iron importer, mitoferrin-1. Despite the exceptionally large amount of iron imported into mitochondria, erythroid cells lacking ABCB7 showed a profound hemoglo- binization defect and underwent apoptosis triggered by oxidative stress. In ABCB7-depleted cells, defective heme biosynthesis resulted from transla- tional repression of ALAS2 by iron regulatory proteins and from decreased stability of the terminal enzyme ferrochelatase. By combining chemical crosslinking, tandem mass spectrometry and mutational analyses, we char- acterized a complex formed of ferrochelatase, ABCB7 and ABCB10, and mapped the interfaces of interactions of its components. A dimeric fer- rochelatase physically bridged ABCB7 and ABCB10 homodimers by bind- ing near the nucleotide-binding domains of each ABC transporter. Our studies not only underscore the importance of ABCB7 for mitochondrial Fe- S biogenesis and iron homeostasis, but also provide the biochemical char- acterization of a multiprotein complex required for heme biosynthesis.
Introduction
ATP-binding cassette (ABC) transporters belong to one of the most abundant families of integral membrane proteins found in all kingdoms of life1,2 and play major roles in several biological processes by mediating the active transport of a variety of molecules across cellular membranes. Three members of the ABC fam- ily have thus far been localized to the inner mitochondrial membrane, where they are predicted to act as exporters, since their nucleotide binding domains face the matrix.3,4 These members are ABCB7 (the human ortholog of yeast Atm1), ABCB10 and ABCB8. ABCB6 has been reported to reside either in the outer mito- chondrial membrane,5,6 and/or in the Golgi,7 lysosomal,8 and plasma membranes.9
ABCB7 maps to the X-chromosome in mice and humans10 and shows a ubiqui- tous expression pattern. Knockout studies in mice revealed that expression of ABCB7 was essential for early gestation.11 Mutations in ABCB7 cause X-linked sideroblastic anemia with ataxia (XLSA/A; 301310), which is a recessive disorder characterized by the onset of non- or slowly-progressive cerebellar ataxia and
Correspondence:
TRACEY A. ROUAULT
rouault@mail.nih.gov
Received: December 12, 2018. Accepted: February 7, 2019. Pre-published: February 14, 2019.
doi:10.3324/haematol.2018.214320
Check the online version for the most updated information on this article, online supplements, and information on authorship & disclosures: www.haematologica.org/content/104/9/1756
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