title: Multidrug resistance protein MdtM adds to the repertoire of antiporters involved in alkaline pH homeostasis in Escherichia coli.
abstract: BACKGROUND
  In neutralophilic bacteria, monovalent metal cation/H+ antiporters play a key role in pH homeostasis. In Escherichia coli, only four antiporters (NhaA, NhaB, MdfA and ChaA) are identified to function in maintenance of a stable cytoplasmic pH under conditions of alkaline stress. We hypothesised that the multidrug resistance protein MdtM, a recently characterised homologue of MdfA and a member of the major facilitator superfamily, also functions in alkaline pH homeostasis.
  
  
  RESULTS
  Assays that compared the growth of an E. coli ΔmdtM deletion mutant transformed with a plasmid encoding wild-type MdtM or the dysfunctional MdtM D22A mutant at different external alkaline pH values (ranging from pH 8.5 to 10) revealed a potential contribution by MdtM to alkaline pH tolerance, but only when millimolar concentrations of sodium or potassium was present in the growth medium. Fluorescence-based activity assays using inverted vesicles generated from transformants of antiporter-deficient (ΔnhaA, ΔnhaB, ΔchaA) E. coli TO114 cells defined MdtM as a low-affinity antiporter that catalysed electrogenic exchange of Na+, K+, Rb+ or Li+ for H+. The K+/H+ antiport reaction had a pH optimum at 9.0, whereas the Na+/H+ exchange activity was optimum at pH 9.25. Measurement of internal cellular pH confirmed MdtM as contributing to maintenance of a stable cytoplasmic pH, acid relative to the external pH, under conditions of alkaline stress.
  
  
  CONCLUSIONS
  Taken together, the results support a role for MdtM in alkaline pH tolerance. MdtM can therefore be added to the currently limited list of antiporters known to function in pH homeostasis in the model organism E. coli.
authors.affiliation: Institute for Global Food Security, School of Biological Sciences, Medical Biology Centre, Queen's University Belfast, 97 Lisburn Road, Belfast BT9 7BL, UK.
authors.forename: Scarlett R
authors.initials: SR
authors.lastname: Holdsworth
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Law
mesh_terms: D000468:Alkalies / Q000378:metabolism*; D017920:Antiporters / Q000235:genetics / Q000378:metabolism*; D002412:Cations / Q000378:metabolism; D004926:Escherichia coli / Q000235:genetics / Q000254:growth & development / Q000378:metabolism / Q000502:physiology*; D029968:Escherichia coli Proteins / Q000235:genetics / Q000378:metabolism*; D017353:Gene Deletion; D005816:Genetic Complementation Test; D006706:Homeostasis; D006863:Hydrogen-Ion Concentration; D008670:Metals / Q000378:metabolism
chemical_list: D000468:Alkalies; D017920:Antiporters; D002412:Cations; D029968:Escherichia coli Proteins; C574623:MdtM protein, E coli; D008670:Metals