Shifting the Paradigm: The Putative Mitochondrial Protein ABCB6 Resides in the Lysosomes of Cells and in the Plasma Membrane of Erythrocytes

Katalin Kiss,Michel Vidal,Alexandra Toth,Henri Vial,Laurence Berry,Gergely Szakacs,Alice Vallentin,Anna Brozik,Nora Kucsma,Melinda Gera

doi:10.1371/journal.pone.0037378

Abstract

ABCB6, a member of the adenosine triphosphate–binding cassette (ABC) transporter family, has been proposed to be responsible for the mitochondrial uptake of porphyrins. Here we show that ABCB6 is a glycoprotein present in the membrane of mature erythrocytes and in exosomes released from reticulocytes during the final steps of erythroid maturation. Consistent with its presence in exosomes, endogenous ABCB6 is localized to the endo/lysosomal compartment, and is absent from the mitochondria of cells. Knock-down studies demonstrate that ABCB6 function is not required for de novo heme biosynthesis in differentiating K562 cells, excluding this ABC transporter as a key regulator of porphyrin synthesis. We confirm the mitochondrial localization of ABCB7, ABCB8 and ABCB10, suggesting that only three ABC transporters should be classified as mitochondrial proteins. Taken together, our results challenge the current paradigm linking the expression and function of ABCB6 to mitochondria.

Highlights

ATP-binding cassette (ABC) transporters comprise a superfamily of membrane spanning multidomain proteins
We show that ABCB6, a protein currently assigned to mitochondria in the major protein databases [37], is abundantly expressed in the erythrocyte membrane
We show that ABCB6 is expressed in its full length, glycosylated form, suggesting that the plasma membrane localization in red blood cell does not rely on erythrocyte-specific posttranslational modifications

Summary

Introduction

ATP-binding cassette (ABC) transporters comprise a superfamily of membrane spanning multidomain proteins. ABC proteins are located in the membrane compartment of the cells, where they mediate translocation of various molecules across these barriers. 48 different ABC proteins (grouped into seven subfamilies ranging from A to G) have been defined in the human genome. Most human ABC proteins function as efflux pumps, translocating their substrates into the extracellular space or intracellular organelles. Human ABC transporters may be expressed in the plasma membranes of the cells, and in intracellular membrane compartments including the endoplasmic reticulum (ER), lysosomes, peroxisomes and mitochondria. The B subfamily consists of 11 members, with functions ranging from the translocation of phosphatidylcholine (ABCB4-MDR3) to secretion of bile acids (ABCB11) or the protection of cells against xenobiotics (ABCB1-MDR1/Pgp). ABCB2 and ABCB3 (TAP1-2) act as heterodimers mediating the translocation of immunogenic peptides from the cytosol into the endoplasmic reticulum, ABCB9 has been localized to the lysosomes [1,2], and the cellular localization of ABCB5 remains to be clarified

Methods

Results

Conclusion