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 ABCG2(R482G) isoform was expressed to high levels in insect cells. An extensive detergent screen was employed to effect extraction of ABCG2(R482G) 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 ABCG2(R482G) provided 3D structural data at a resolution of approximately 18 A. Single-particle analysis revealed that the complex forms a tetrameric complex ( approximately 180 A in diameter x approximately 140 A high) with an aqueous central region. We interpret the tetrameric structure as comprising four homodimeric ABCG2(R482G) complexes.