Fig. 6: MdtEF-TolC has acid-activated efflux.
From: Molecular basis for multidrug efflux by an anaerobic-associated RND transporter
Steady state accumulation levels of A HT and B NPN were measured in E. coli Δ9-pore cells, harbouring an empty vector or pUC19-MdtEFWT plasmid, as a function of external substrate concentration. Steady state accumulation levels were measured comparing either: A exponential or stationary phases or B differences in pH, ranging from pH 5 to 8. This was performed in the A absence or B presence of outer membrane Pore expression, due to differences in Pore expression at exponential and stationary phases (Supplementary Fig. 27A). Individual data points represent mean values from three independent measurements, and error bars are indicative of the standard deviation (n = 3). C Schematic of MdtEF-TolC acid-activated efflux in a CFA-remodelled inner membrane. MdtEF-TolC can provide bacterial defence and toxin disposal mechanisms for a range of bacterial environments, but seems to be a less efficient system than the constitutively expressed AcrAB-TolC pump under physiological conditions. mdtEF is markedly, significantly upregulated in environmental settings that coincide with increased cyclopropanation of unsaturated inner membrane lipids, for example, under anaerobic stress and transition to stationary phase, which likely makes this its natural milieu. Both MdtF and CFA seem to have overlapped roles in maintaining bacterial fitness to acid shock. mdtEF becomes activated in response to acid to become much more efficient at substrate efflux. Where CFA lipids significantly change the membrane properties, leading to decreased permeability to protons, alongside increased membrane fluidity, rigidity, and stability. CFA cyclopropanated fatty acid, HT Hoechst 33342, NPN N-Phenyl-1-naphthylamine.
