Poster: Membrane Transport

Abs # 1209: High intracellular chloride triggers inward currents through the yeast K⁺ transporters which can be quantitatively described as voltage-gated chloride currents

Presenter:	Rivetta, Alberto , alberto.rivetta@cmp.yale.edu
Authors	Rivetta, Alberto  (A)   Bashi, Esther  (A)   Slayman, Clifford  (A)   Kuroda, Teruo  (B)
Affiliations:	(A): Cellular and Molecular Physiology, Yale School of Medicine (B): Gene Research Center, Okayama University, Japan

Trk1p and Trk2p, the two major K⁺-uptake proteins in Saccharomyces cerevisiae, are folded as pseudo K⁺-channels, with 4 characteristic “MPM” domains. Under normal conditions, both apparently function as two-ion transporters, coupling K⁺ at one site to K⁺, H⁺, or Na⁺ at a second site, and using both ion gradients plus membrane voltage (V_m) to drive transport. Although normal transport currents are small, < 10 pA/cell, cells stuffed with chloride (from ordinary patch pipettes) display much larger TRK-dependent currents: 50-200 pA/cell, which increase with external [H⁺] and internal [Cl^-], but are almost insensitive to [K⁺], external or internal. These inwardly rectified currents increase exponentially with (-) V_m, are nearly proportional to [Cl^-]_in at any fixed voltage, and increase by scaling as pH_out falls. Such properties are quantitatively described by a voltage-gated Cl^- conductance, for which the effective gating voltage moves about 35 mV (+) per unit fall of pH_out: from –250 mV at pH 7.5 to –145 mV at pH 4.5, for the normal [Cl^-]_in of 183 mM. The same relationship holds at fixed pH_out (e.g., 5.5), when [Cl^-]_in is varied from 0.1 mM to 260 mM, displaying a fixed limiting permeability of 1.3x10^-6 cm/sec for Cl^-. Such a gated-channel model for Cl^- currents in TRK proteins would provide a rationale for many old phenomena in plants and fungi, esp., reversible dielectric breakdown—called “punch-through”—at large negative V_m in Chara corollina (Austr. J. Biol. Sci.22:365, 1969); and short-circuiting of resting V_m in Neurospora by Cl^- (J.Membr. Biol.72:223, 1983). These deleterious effects also raise the question of how much Cl^- loading, rather than the better-known Na⁺ loading, determines the salt intolerance of crop plants and other terrestrial glycophytes.