Multiple paths for nonphysiological transport of K+ in Escherichia coli.
Mutants of Escherichia coli lacking all of the known saturable K+ transport systems, "triple mutants," require elevated K+ concentrations for growth. K+ transport activity in such mutants, called TrkF activity, has low substrate specificity and a low rate that increases with increasing external pH. Attempts to isolate mutants requiring even higher concentrations of K+ failed, implying that either TrkF is essential or is composed of multiple minor K+ transport activities. Instead, we sought mutations that allowed triple mutants to grow at lower K+ concentrations. Mutations so identified include ones altering MscL, the large mechanosensitive channel, or Opp, the oligopeptide permease. However, a possible contribution of wild-type Opp and MscL to TrkF activity was not proven. In contrast, expression of wild-type ProP, TrkG, and TrkH proteins increased uptake when encoded on multicopy plasmids. In all of these situations, the driving force for K+ appeared to be the transmembrane electric potential, and in most cases substrate specificity was low; these are characteristics of TrkF activity. These results support the view that TrkF is composed of multiple, "aberrant" K+ transport activities, i.e., paths that, regardless of their physiological function, allow K+ to cross the cell membrane by a uniport process.
Buurman
Ed T
ET
Department of Molecular Genetics and Cell Biology, University of Chicago, Illinois 60637, USA. Ed.Buurman@astrazeneca.com
McLaggan
Debbie
D
Naprstek
Josef
J
Epstein
Wolfgang
W
D002352:Carrier Proteins / Q000502:physiology; D004926:Escherichia coli / Q000378:metabolism*; D029968:Escherichia coli Proteins / Q000502:physiology; D006863:Hydrogen-Ion Concentration; D017136:Ion Transport; D008565:Membrane Proteins / Q000502:physiology; D011188:Potassium / Q000378:metabolism*; D020917:Receptor, trkA
D002352:Carrier Proteins; D029968:Escherichia coli Proteins; D008565:Membrane Proteins; C068653:TrkG protein, E coli; D020917:Receptor, trkA; D011188:Potassium