The human multidrug resistance protein (MRP1) contributes to drug resistance in cancer cells. In addition to an MDR1-like core, MRP1 contains an N-terminal membrane-bound (TMD(0)) region and a cytoplasmic linker (L(0)), both characteristic of several members of the MRP family. In order to study the role of the TMD(0) and L(0) regions, we constructed various truncated and mutated MRP1, and chimeric MRP1-MDR1 molecules, which were expressed in insect (Sf9) and polarized mammalian (MDCKII) cells. The function of the various proteins was examined in isolated membrane vesicles by measuring the transport of leukotriene C(4) and other glutathione conjugates, and by vanadate-dependent nucleotide occlusion. Cellular localization, and glutathione-conjugate and drug transport, were also studied in MDCKII cells. We found that chimeric proteins consisting of N-terminal fragments of MRP1 fused to the N terminus of MDR1 preserved the transport, nucleotide occlusion and apical membrane routing of wild-type MDR1. As shown before, MRP1 without TMD(0)L(0) (Delta MRP1), was non-functional and localized intracellularly, so we investigated the coexpression of Delta MRP1 with the isolated L(0) region. Coexpression yielded a functional MRP1 molecule in Sf9 cells and routing to the lateral membrane in MDCKII cells. Interestingly, the L(0) peptide was found to be associated with membranes in Sf9 cells and could only be solubilized by urea or detergent. A 10-amino-acid deletion in a predicted amphipathic region of L(0) abolished its attachment to the membrane and eliminated MRP1 transport function, but did not affect membrane routing. Taken together, these experiments suggest that the L(0) region forms a distinct domain within MRP1, which interacts with hydrophobic membrane regions and with the core region of MRP1.
Role of the N-terminal Transmembrane Region of the Multidrug Resistance Protein MRP2 in Routing to the Apical Membrane in MDCKII Cells
