On the energy-dependence of Hoechst 33342 transport by the ABC transporter LmrA.

LmrA is an ATP-binding cassette (ABC) multidrug transporter from Lactococcus lactis, and is a structural homologue of the human multidrug resistance P-glycoprotein (ABCB1), the overexpression of which is associated with multidrug resistance in tumours. We recently observed that a truncated version of LmrA lacking the nucleotide-binding domain mediates a proton motive force-dependent ethidium transport reaction by catalyzing proton-ethidium symport. This finding raised the question whether proton motive force-dependent transport can also be observed for other drugs, and whether this reaction is also relevant for full-length LmrA. Furthermore, the observations on LmrA-MD raised the question whether ATP-dependent transport by LmrA in intact cells could be due to the activity of independent ABC transporters that might become upregulated in the lactococcal cells due to the overexpression of LmrA; the recently identified ABC multidrug transporter LmrCD was put forward as a possible candidate. Here, we investigated the energy coupling to the transport of the amphiphilic dye Hoechst 33342 in proteoliposomes containing purified LmrA. For this purpose, LmrA was obtained from lactococcal cells lacking the genomic lmrA and lmrCD genes, in which LmrA was expressed from a plasmid. To separate ATP-dependence from proton motive force-dependence, we also used mutant LmrA proteins, which were affected in their ability to hydrolyse ATP. Our studies in proteoliposomes demonstrate that LmrA can catalyze Hoechst 33342 transport independent of auxiliary proteins, in an ATP-dependent fashion and a transmembrane chemical proton gradient (interior acidic)-dependent fashion.