Purification and 3D Structural Analysis of Oligomeric Human Multidrug Transporter ABCG2

ABCG2 is a multidrug efflux pump associated with resistance of cancer cells to a plethora of unrelated drugs. ABCG2 is a "half-transporter," and previous studies have indicated that it forms homodimers and higher oligomeric species. In this manuscript, electron microscopic structural analysis directly addressed this issue. An N-terminal hexahistidine-tagged ABCG2R482G isoform was expressed to high levels in insect cells. An extensive detergent screen was employed to effect extraction of ABCG2R482G from membranes and identified only the fos-choline detergents as efficient. Soluble protein was purified to >95% homogeneity by a three-step procedure while retaining the ability to bind substrates. Cryonegative stain electron microscopy of purified ABCG2R482G provided 3D structural data at a resolution of ∼18 Å. Single-particle analysis revealed that the complex forms a tetrameric complex (∼180 Å in diameter × ∼140 Å high) with an aqueous central region. We interpret the tetrameric structure as comprising four homodimeric ABCG2R482G complexes.