The multidrug resistance P-glycoprotein - Oligomeric state and intramolecular interactions

The human multidrug resistance P-glycoprotein (P-gp), a member of the ATP-b inding cassette (ABC) superfamily of transporters, is frequently responsibl e for the failure of chemotherapy by virtue of its ability to export hydrop hobic cytotoxic drugs from cells. Elucidating the inter- and intramolecular interactions of this protein is critical to understanding its cellular fun ction and mechanism of action. Toward this end, we have used both biochemic al and genetic techniques to probe potential oligomerization interactions o f P-gp. Differentially epitope-tagged P-gp molecules did not co-immunopreci pitate when co-expressed in HEK293 cells or when co-translated in vitro, de monstrating that P-gp is monomeric in both the presence and absence of dete rgents. The two cytoplasmic domains of P-gp did not interact with each othe r in vivo when coexpressed as gene fusions in yeast. In contrast, the homol ogous domains of the transporter associated with antigen processing (TAP), which reside on separate polypeptides and must form a heterodimeric transpo rter (TAP1/TAP2), did interact in this system, suggesting a role for these domains in TAP dimerization. Implications for understanding the subunit org anization of ABC transporters are discussed.