The Multidrug Transporter LmrP Protein Mediates Selective Calcium Efflux

LmrP is a major facilitator superfamily multidrug transporter from Lactococcus lactis that mediates the efflux of cationic amphiphilic substrates from the cell in a proton-motive force-dependent fashion. Interestingly, motif searches and docking studies suggested the presence of a putative Ca2+-binding site close to the interface between the two halves of inward facing LmrP. Binding experiments with radioactive Ca-45(2+) demonstrated the presence of a high affinity Ca2+-binding site in purified LmrP, with an apparent K-d of 7.2 mu M, which is selective for Ca2+ and Ba2+ but not for Mn2+, Mg2+, or Co2+. Consistent with our structure model and analogous to crystal structures of EF hand Ca2+-binding proteins, two carboxylates (Asp-235 and Glu-327) were found to be critical for Ca-45(2+) binding. Using Ca-45(2+) and a fluorescent Ca2+-selective probe, calcium transport measurements in intact cells, inside-out membrane vesicles, and proteoliposomes containing functionally reconstituted purified protein provided strong evidence for active efflux of Ca2+ by LmrP with an apparent K-t of 8.6 mu M via electrogenic exchange with three or more protons. These observations demonstrate for the first time that LmrP mediates selective calcium/proton antiport and raise interesting questions about the functional and physiological links between this reaction and that of multidrug transport.