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Quantitative Modeling of Chloride Conductance in Yeast TRK Potassium Transporters

Alberto Rivetta ^*, Clifford Slayman ^* and Teruo Kuroda 

^* Department of Cellular and Molecular Physiology, Yale School of Medicine, New Haven, Connecticut; and Department of Genomics and Applied Microbiology, Graduate School of Medicine, Dentistry and Pharmaceutical Sciences, Okayama University, Okayama, Japan

Correspondence: Address reprint requests to C. L. Slayman, Tel.: 203-785-4478; Fax: 203-785-5535; E-mail: clifford.slayman{at}yale.edu.

So-called TRK proteins are responsible for active accumulation of potassium in plants, fungi, and bacteria. A pair of these proteins in the plasma membrane of Saccharomyces cerevisiae, ScTrk1p and ScTrk2p, also admit large, adventitious, chloride currents during patch-recording (Cl^– efflux). Resulting steady-state current-voltage curves can be described by two simple kinetic models, most interestingly, voltage-driven channeling of ions through a pair of activation-energy barriers that lie within the membrane dielectric, near the inner () and outer (ß) surfaces. Two barrier heights (E and E_ß) and two relative distances (a₁ and b₂) from the surfaces specify the model. Measured current amplitude parallels intracellular chloride concentration and is strongly enhanced by acidic extracellular pH. The former implies an exponential variation of a₁, between 0.2 and 0.4 of the membrane thickness, whereas the latter implies a linear variation of E_ß, by 0.69 Kcal mol^–1/pH. The model requires membrane slope conductance to rise exponentially with increasingly large negative membrane voltage, as verified by data from a few yeast spheroplasts that tolerated voltage clamping at –200 to –300 mV. The behaviors of E_ß and a₁ accord qualitatively with a hypothetical structural model for fungal TRK proteins, suggesting that chloride ions flow through a central pore formed by symmetric aggregation of four TRK monomers.

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