Targeting, Import, and Dimerization of a Mammalian Mitochondrial ATP Binding Cassette (ABC) Transporter, ABCB10 (ABC-me)

Abstract

ATP binding cassette (ABC) transporters are a diverse superfamily of energy-dependent membrane translocases. Although responsible for the majority of transmembrane transport in bacteria, they are relatively uncommon in eukaryotic mitochondria. Organellar trafficking and import, in addition to quaternary structure assembly, of mitochondrial ABC transporters is poorly understood and may offer explanations for the paucity of their diversity. Here we examine these processes in ABCB10 (ABC-me), a mitochondrial inner membrane erythroid transporter involved in heme biosynthesis. We report that ABCB10 possesses an unusually long 105-amino acid mitochondrial targeting presequence (mTP). The central subdomain of the mTP (amino acids (aa) 36 -70) is sufficient for mitochondrial import of enhanced green fluorescent protein. The N-terminal subdomain (aa 1-35) of the mTP, although not necessary for the trafficking of ABCB10 to mitochondria, participates in the proper import of the molecule into the inner membrane. We performed a series of amino acid mutations aimed at changing specific properties of the mTP. The mTP requires neither arginine residues nor predictable ␣-helices for efficient mitochondrial targeting. Disruption of its hydrophobic character by the mutation L46Q/ I47Q, however, greatly diminishes its efficacy. This mutation can be rescued by cryptic downstream (aa 106 -715) mitochondrial targeting signals, highlighting the redundancy of this protein's targeting qualities. Mass spectrometry analysis of chemically cross-linked, immunoprecipitated ABCB10 indicates that ABCB10 embedded in the mitochondrial inner membrane homodimerizes and homo-oligomerizes. A deletion mutant of ABCB10 that lacks its mTP efficiently targets to the endoplasmic reticulum. Quaternary structure assembly of ABCB10 in the ER appears to be similar to that in the mitochondria.ABC 1 transporters comprise a large and diverse family of membrane translocases (1). Their function ranges from peptide transport to phospholipid flipping to anion channel formation. Members of the ABC transporter superfamily have been implicated in numerous human diseases (including cystic fibrosis (CFTR/ABCC7), adrenoleukodystrophy (ALDP/ABCD1), Zellweger's syndrome (PMP70/ABCD3), progressive familial intrahepatic cholestasis (SPGP/ABCB11), and Stargardt macular dystrophy (ABCR/ABCA4)) (2). The basic structure of ABC transporters is relatively well conserved and includes a hydrophilic ATP binding cassette and a hydrophobic membranespanning domain (3). Whereas the large members of the family contain two of each domain, the "half-transporters" contain one of each and are predicted to dimerize (3-6). Mammalian ABC transporters are found predominantly in the plasma membrane but are also known to play essential roles in a number of organelles, including the endoplasmic reticulum, peroxisome, and the mitochondrion (2). Mammalian mitochondrial ABC transporters have recently gained attention for their role in heme biosynthesis and iron sulfur cluster synthesis (7-12). According...

Ferrochelatase forms an oligomeric complex with mitoferrin-1 and Abcb10 for erythroid heme biosynthesis

“…19 Because mitochondrial heme is biosynthesized by the enzyme Fech, our discovery of the Abcb10-Fech interaction raises the possibility that Abcb10 may function in heme export. An interaction between Fech and erythroid-induced Abcb10 and Abcb7 may also provide reversible inhibition of Fech to help regulate iron distribution between the synthesis of both Fe-S clusters and heme.…”

Structures and functions of mitochondrial ABC transporters

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Heme and erythropoieis: more than a structural role

“…ABCB10 is a mitochondrial inner membrane protein which homodimerizes to form a functional transporter. 20 In addition to its role in the stabilization of MFRN1,…”