Novel Function of Human RLIP76:  ATP-Dependent Transport of Glutathione Conjugates and Doxorubicin^†

Active transport of conjugated and unconjugated electrophiles out of cells is essential for cellular homeostasis. We have previously identified in human tissues a transporter, DNP-SG [S-(2,4-dinitrophenyl)glutathione] ATPase, capable of carrying out this function [Awasthi et al. (1998) Biochemistry 37, 5231−5238, 5239−5248]. We now report the cloning of DNP-SG ATPase. The sequence of the cDNA clone was identical to that of human RLIP76, a known Ral-binding protein. RLIP76 expressed in E. coli was purified by DNP-SG affinity chromatography. Purified recombinant RLIP76:  (1) had ATPase activity stimulated by DNP-SG or doxorubicin (DOX), and the K_m values of RLIP76 for ATP, DOX, and DNP-SG were similar to those reported for DNP-SG ATPase; (2) upon reconstitution with asolectin as well as with defined lipids, catalyzed ATP-dependent transport of DNP-SG and DOX with kinetic parameters similar to those of DNP-SG ATPase; (3) when transfected into K562 cells, resulted in increased resistance to DOX, and increased ATP-dependent transport of DNP-SG and DOX by inside-out membrane vesicles from transfected cells; (4) direct uptake of purified RLIP76 protein into mammalian cells from donor proteoliposomes confers DOX resistance. These results indicate that RLIP76, in addition to its role in signal transduction, can catalyze transport of glutathione conjugates and xenobiotics, and may contribute to the multidrug resistance phenomenon.