pH dependence of CheA autophosphorylation in Escherichia coli.
Chemotaxis by cells of Escherichia coli and Salmonella typhimurium depends upon the ability of chemoreceptors called transducers to communicate with switch components of flagellar motors to modulate swimming behavior. This communication requires an excitatory pathway composed of the cytoplasmic signal transduction proteins, CheAL, CheAS, CheW, CheY, and CheZ. Of these, the autokinase CheAL is most central. Modifications or mutations that affect the rate at which CheAL autophosphorylates result in profound chemotactic defects. Here we demonstrate that pH can affect CheAL autokinase activity in vitro. This activity exhibits a bell-shaped dependence upon pH within the range 6.5 to 10.0, consistent with the notion that two proton dissociation events affect CheAL autophosphorylation kinetics: one characterized by a pKa of about 8.1 and another exhibiting a pKa of about 8.9. These in vitro results predict a decrease in the rate of CheAL autophosphorylation in response to a reduction in intracellular pH, a decrease that should cause increased counterclockwise flagellar rotation. We observed such a response in vivo for cells containing a partially reconstituted chemotaxis system. Benzoate (10 mM, pH 7.0), a weak acid that when undissociated readily traverses the cytoplasmic membrane, causes a reduction of cytoplasmic pH from 7.6 to 7.3. In response to this reduction, cells expressing CheAL, CheAS, and CheY, but not transducers, exhibited a small but reproducible increase in the fraction of time that they spun their flagellar motors counterclockwise. The added presence of CheW and the transducers Tar and Trg resulted in a more dramatic response. The significance of our in vitro results, their relationships to regulation of swimming behavior, and the mechanisms by which transducers might affect the pH dependence of CheA autokinase activity are discussed.
Department of Cellular and Developmental Biology, Harvard University, Cambridge, Massachusetts 02138.
M P
MP
Conley
H C
HC
Berg
P
P
Tawa
R C
RC
Stewart
D D
DD
Ellefson
A J
AJ
Wolfe
D000255:Adenosine Triphosphate / Q000378:metabolism; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism; D002628:Chemoreceptor Cells; D002633:Chemotaxis / Q000502:physiology*; D004926:Escherichia coli / Q000378:metabolism*; D029968:Escherichia coli Proteins; D000071677:Histidine Kinase; D006863:Hydrogen-Ion Concentration; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism*; D000072236:Methyl-Accepting Chemotaxis Proteins; D010766:Phosphorylation; D011494:Protein Kinases / Q000235:genetics / Q000378:metabolism*; D011956:Receptors, Cell Surface; D011994:Recombinant Proteins / Q000378:metabolism; D015398:Signal Transduction
D001426:Bacterial Proteins; D029968:Escherichia coli Proteins; D008565:Membrane Proteins; D000072236:Methyl-Accepting Chemotaxis Proteins; D011956:Receptors, Cell Surface; D011994:Recombinant Proteins; C057331:Tar protein, E coli; C041476:Trg protein, E coli; C000607606:cheY protein, E coli; D000255:Adenosine Triphosphate; D011494:Protein Kinases; D000071677:Histidine Kinase; C000607608:cheA protein, E coli