Nonequivalence of membrane voltage and ion-gradient as driving forces for the bacterial flagellar motor at low load

Biophys J. 2007 Jul 1;93(1):294-302. doi: 10.1529/biophysj.106.095265. Epub 2007 Apr 6.

Abstract

Many bacterial species swim using flagella. The flagellar motor couples ion flow across the cytoplasmic membrane to rotation. Ion flow is driven by both a membrane potential (V(m)) and a transmembrane concentration gradient. To investigate their relation to bacterial flagellar motor function we developed a fluorescence technique to measure V(m) in single cells, using the dye tetramethyl rhodamine methyl ester. We used a convolution model to determine the relationship between fluorescence intensity in images of cells and intracellular dye concentration, and calculated V(m) using the ratio of intracellular/extracellular dye concentration. We found V(m) = -140 +/- 14 mV in Escherichia coli at external pH 7.0 (pH(ex)), decreasing to -85 +/- 10 mV at pH(ex) 5.0. We also estimated the sodium-motive force (SMF) by combining single-cell measurements of V(m) and intracellular sodium concentration. We were able to vary the SMF between -187 +/- 15 mV and -53 +/- 15 mV by varying pH(ex) in the range 7.0-5.0 and extracellular sodium concentration in the range 1-85 mM. Rotation rates for 0.35-microm- and 1-microm-diameter beads attached to Na(+)-driven chimeric flagellar motors varied linearly with V(m). For the larger beads, the two components of the SMF were equivalent, whereas for smaller beads at a given SMF, the speed increased with sodium gradient and external sodium concentration.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Computer Simulation
  • Escherichia coli / physiology*
  • Flagella / physiology*
  • Ions
  • Membrane Potentials / physiology*
  • Models, Biological*
  • Molecular Motor Proteins / physiology*
  • Motion
  • Movement / physiology
  • Sodium / metabolism*
  • Stress, Mechanical

Substances

  • Ions
  • Molecular Motor Proteins
  • Sodium