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J. Biol. Chem., Vol. 280, Issue 44, 36926-36934, November 4, 2005

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Identification of Intra- and Intermolecular Disulfide Bridges in the Multidrug Resistance Transporter ABCG2^*

Ulla Henriksen, Jacob U. Fog, Thomas Litman, and Ulrik Gether1

From the Molecular Neuropharmacology Group, Department of Pharmacology, Panum Institute, Building 18.6, University of Copenhagen, Blegdamsvej 3, DK-2200 Copenhagen N, Denmark; Bioinformatics Centre, University of Copenhagen, DK-2100 Copenhagen Ø, Denmark

ABCG2 is an ATP binding cassette (ABC) half-transporter that plays a key role in multidrug resistance to chemotherapy. ABCG2 is believed to be a functional homodimer that has been proposed to be linked by disulfide bridges. We have investigated the structural and functional role of the only three cysteines predicted to be on the extracellular face of ABCG2. Upon mutation of Cys-592 or Cys-608 to alanine (C592A and C608A), ABCG2 migrated as a dimer in SDS-PAGE under non-reducing conditions; however, mutation of Cys-603 to Ala (C603A) caused the transporter to migrate as a single monomeric band. Despite this change, C603A displayed efficient membrane targeting and preserved transport function. Because the transporter migrated as a dimer in SDS-PAGE, when only Cys-603 was present (C592A-C608A), the data suggest that Cys-603 forms a symmetrical intermolecular disulfide bridge in the ABCG2 homodimer that is not essential for protein expression and function. In contrast to C603A, both C592A and C608A displayed impaired membrane targeting and function. Moreover, when only Cys-592 or Cys-608 were present (C592A/C603A and C603A/C608A), the transporter displayed impaired plasma membrane expression and function. The combined mutation (C592A/C608A) partially restored plasma membrane expression; however, although transport of mitoxantrone was almost normal, we observed impairment of BODIPY-prazosin transport. This supports the conclusion that Cys-592 and Cys-608 form an intramolecular disulfide bridge in ABCG2 that is critical for substrate specificity. Finally, mutation of all three cysteines simultaneously resulted in low expression and no measurable function. Altogether, our data are consistent with a scenario in which an inter- and an intramolecular disulfide bridge together are of fundamental importance for the structural and functional integrity of ABCG2.

Received for publication, March 17, 2005 , and in revised form, August 16, 2005.

^* This work was supported by the Danish Cancer Society. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ To whom correspondence should be addressed: Molecular Neuropharmacology Group, Dept. of Pharmacology, Panum Institute, University of Copenhagen, DK-2200 Copenhagen N, Denmark. Tel.: 45-3532-7548; Fax: 45-3532-7610; E-mail: gether{at}neuropharm.ku.dk.

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