Gating of the yeast K+ channel encoded by the Saccharomyces cerevisiae gene TOK1, unlike other outward-rectifying K+ channels that have been cloned, is promoted by membrane voltage (inside positive-going) and repressed by extracellular K+. When expressed in Xenopus laevis oocytes, the TOK1p current rectified strongly outward, its activation shifting in parallel with the K+ equilibrium potential when the external K+ concentration ([K+](0)) was increased above 3 mM. Analysis of the TOK1p current indicated that two kinetic components contributed to the conductance and the voltage sensitivity of the conductance. By contrast, the [K+](0) sensitivity of the current was accommodated entirely within the slow-relaxing component; it was diminished near 1 mM [K+](0), and at submillimolar concentrations the voltage dependence of the TOK1p conductance was insensitive to [K+](0). External Rb+, the K+ channel blockers Cs+ and Ba2+ - but not Na+, Ca2+ or Mg2+ - substituted for K+ in control of TOK1p activation, indicating a specificity in cation interaction with the TOK1p gate. These and additional results indicate that external K+ acts as a ligand to inactivate the TOK1p channel, and they implicate a gating process mediated by a single cation binding site within the membrane electric field, but distinct from the permeation pathway. (C) 1997 Federation of European Biochemical Societies
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