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Correspondence to Michael L. Jennings: JenningsMichaelL{at}uams.edu
Chloride homeostasis in Saccharomyces cerevisiae has been characterized with the goal of identifying new Cl– transport and regulatory pathways. Steady-state cellular Cl– contents (
0.2 mEq/liter cell water) differ by less than threefold in yeast grown in media containing 0.003–5 mM Cl–. Therefore, yeast have a potent mechanism for maintaining constant cellular Cl– over a wide range of extracellular Cl–. The cell water:medium [Cl–] ratio is >20 in media containing 0.01 mM Cl– and results in part from sequestration of Cl– in organelles, as shown by the effect of deleting genes involved in vacuolar acidification. Organellar sequestration cannot account entirely for the Cl– accumulation, however, because the cell water:medium [Cl–] ratio in low Cl– medium is 10 at extracellular pH 4.0 even in vma1 yeast, which lack the vacuolar H+-ATPase. Cellular Cl– accumulation is ATP dependent in both wild type and vma1 strains. The initial 36Cl– influx is a saturable function of extracellular [36Cl–] with K1/2 of 0.02 mM at pH 4.0 and >0.2 mM at pH 7, indicating the presence of a high affinity Cl– transporter in the plasma membrane. The transporter can exchange 36Cl– for either Cl– or Br– far more rapidly than SO4=, phosphate, formate, HCO3–, or NO3–. High affinity Cl– influx is not affected by deletion of any of several genes for possible Cl– transporters. The high affinity Cl– transporter is activated over a period of 45 min after shifting cells from high-Cl– to low-Cl– media. Deletion of ORF YHL008c (formate-nitrite transporter family) strongly reduces the rate of activation of the flux. Therefore, Yhl008cp may be part of a Cl–-sensing mechanism that activates the high affinity transporter in a low Cl– medium. This is the first example of a biological system that can regulate cellular Cl– at concentrations far below 1 mM.
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