Functional reassembly of ATP-dependent xenobiotic transport by the N- and C-terminal domains of RLIP76 and identification of ATP binding sequences.

We have recently shown that RLIP76, a Ral-binding, GTPase-activating protein, is an ATP-dependent transporter of doxorubicin (DOX) as well as glutathione conjugates [Awasthi, S., et al. (2000) Biochemistry 39, 9327-9334]. RLIP76 overexpressed in human cells or transformed E. coli undergoes proteolysis to yield several fragments, including two prominent peptides, N-RLIP76(1-367) and C-RLIP76(410-655), from the N- and C-terminal domains, respectively. To investigate whether the fragmentation of RLIP76 has any relevance to its transport function, we have studied the characteristics of these two peptide fragments. Recombinant N-RLIP76(1-367) and C-RLIP76(410-655) were purified from overexpressing transformed E. coli. While N-RLIP76(1-367) readily underwent proteolysis, showing SDS-gel patterns similar to those of RLIP76, C-RLIP76(410-655) was resistant to such degradation. Both N-RLIP76(1-367) and C-RLIP76(410-655) had ATPase activity (K(m) for ATP, 2.5 and 2.0 mM, respectively) which was stimulated by DNP-SG, DOX, and colchicine (COL). ATP binding to both peptides was confirmed by photoaffinity labeling with 8-azido-ATP that was increased in the presence of compounds that stimulated their ATPase activity. Photoaffinity labeling was also increased in the presence of vanadate, indicating trapping of a reaction intermediate in the ATP binding site. The ATP binding sites in N-RLIP76(1-367) and C-RLIP76(410-655) were identified to be (69)GKKKGK(74) and (418)GGIKDLSK(425), respectively. Mutation of K(74) and K(425) to M residues, in N-RLIP76(1-367) and C-RLIP76(410-655), respectively, abrogated their ATPase activity as well as azido-ATP labeling. Proteoliposomes reconstituted with either N-RLIP76(1-367) or C-RLIP76(410-655) alone did not catalyze ATP-dependent transport of DOX or COL. However, proteoliposomes reconstituted with a mixture of N-RLIP76(1-367) and C-RLIP76(410-655) mediated such transport. Proteoliposomes reconstituted with the mixture of mutant peptides lacking ATPase activity did not exhibit transport activity. Present studies have identified the ATP binding sites in RLIP76, and show that DOX and COL transport can be reconstituted by two fragments of RLIP76.