Multidrug resistance protein (Mdr)-alkaline phosphatase hybrids in Escherichia coli suggest a major revision in the topology of the C-terminal half of Mdr

Abstract

Recent studies reveal that the organization of the multidrug resistance protein (Mdr) in the membrane is probably not exactly as predicted from hydropathy profiling. When expressed in Escherichia coli, phoA gene fusions can be utilized to study the membrane topology of Mdr. Using this approach, it was proposed recently that the N-terminal hydrophobic domain of Mdr spans the membrane six times, in a different fashion from that predicted by hydropathy analysis (Bibi, E. and Beja, O. (1994) J. Biol. Chem. 269, 19910- 19915). In this study, we analyze mdr-phoA fusions constructed in the C- terminal haft of Mdr. Overall, the results presented here lead to a significant revision in the membrane topology model of the C-terminal half of Mdr. The new topology is discussed with regard to the hydropathy profiles of the well characterized ABC proteins Ma1G and Ma1F, which are strikingly similar to those of the N- and C-terminal halves of Mdr, respectively.