Small multidrug resistance protein emre reduces host ph and osmotic tolerance to metabolic quaternary cation osmoprotectants

Abstract

The smallmultidrug resistance (SMR) transporter protein EmrE in Escherichia coli is known to confer resistance to toxic anti-septics classied as quaternary cation compounds (QCCs). Naturally derived QCCs synthesized duringmetabolic activities often act as osmoprotectants, such as betaine and choline, and participate in osmotic homoestasis. The goal of this study was to deter-mine if EmrE proteins transport biological QCC-based osmoprotectants. Plasmid-encoded copies of E. coli emrE and the inac-tive variant emrE-E14C (emrE with the E→;C change at position 14) were expressed in various E. coli strains grown in either rich orminimalmedia at various pHs (5 to 9) and under hypersaline (0.5 to 1.0MNaCl and KCl) conditions to identify changes in growth phenotypes induced by osmoprotectant transport. The results demonstrated that emrE expression reduced pH tolerance of E. coli strains at or above neutral pH and when grown in hypersalinemedia at or above NaCl or KCl concentrations of 0.75M. Hypersaline growth conditions were used to screen QCC osmoprotectants betaine, choline, L-carnitine, L-lysine, L-proline, and L-arginine. The study identied that betaine and choline are natural QCC substrates of EmrE. © 2012, American Society for Microbiology.