Role of the N-terminal transmembrane region of the multidrug resistance protein MRP2 in routing to the apical membrane in MDCKII cells

In polarized cells, the multidrug resistance protein MRP2 is localized in the apical plasma membrane, whereas MRP1, another multidrug resistance protein (MRP) family member, is localized in the basolateral membrane. MRP1 and MRP2 are thought to contain an N-terminal region of five transmembrane segments (TMDO) coupled to 2 times six transmembrane segments via an intracellular loop (L-0). We previously demonstrated for MRP1 that a mutant lacking TMD0 but still containing L-0, called L(0)DeltaMRP1, was functional and routed to the lateral plasma membrane. To investigate the role of the TMD0L0 region of MRP2 in routing to the apical membrane, we generated mutants similar to those made for MRP1. In contrast to L(0)DeltaMRP1, L(0)DeltaMRP2 was associated with an intracellular compartment, most likely endosomes. Co-expression with TMD0, however, resulted in apical localization of L(0)DeltaMRP2 and transport activity. Uptake experiments with vesicles containing L(0)DeltaMRP2 demonstrated that the molecule is able to transport LTC4. An MRP2 mutant without TMD0L0, DeltaMRP2, was only core-glycosylated and localized intracellularly. Co-expression of DeltaMRP2 with TMD0L0 resulted in an increased protein level of DeltaMRP2, full glycosylation of the protein, routing to the apical membrane, and transport activity. Our results suggest that the TMD0 region is required for routing to or stable association with the apical membrane.