Residues contributing to drug transport by ABCG2 are localised to multiple drug-binding pockets

Multidrug binding and transport by the ATP binding cassette transporter ABCG2 is a factor in the clinical resistance to chemotherapy in leukaemia, and a contributory factor to the pharmacokinetic profiles of many other prescribed drugs. Despite its importance, the structural basis of multidrug transport, i.e. the ability to transport multiple distinct chemicals, has remained elusive. Previous research has shown that at least two residues positioned towards the cytoplasmic end of transmembrane helix 3 (TM3) of the transporter play a role in drug transport. We hypothesised that other residues, either in the longitudinal span of TM3, or a perpendicular slice through the intracellular end of other TM helices would also contribute to drug binding and transport by ABCG2. Single point mutant isoforms of ABCG2 were made at approximately 30 positions and were analysed for effects on protein expression, localisation (western blotting, confocal microscopy) and function (flow cytometry) in a mammalian stable cell line expression system. Our data were interpreted in terms of recent structural data on the ABCG protein subfamily and enabled us to propose a surface binding site for the drug mitoxantrone as well as a second, buried site for the same drug. Further mutational analysis of residues that spatially separate these two sites prompt us to suggest a molecular and structural pathway for mitoxantrone binding by ABCG2.