Mutations in the pore regions of the yeast K+ channel YKC1 affect gating by extracellular K+

The product of the Saccharomyces cerevisiae K+-channel gene YKC1 includes t wo pore-loop sequences that are thought to form the hydrophilic lining of t he pore. Gating of the channel is promoted by membrane depolarization and i s regulated by extracellular K+ concentration ([K+](o)) both in the yeast a nd when expressed in Xenopus oocytes, Analysis of the wild-type current now shows that: (i) [K+](o) suppresses a very slowly relaxing component, accel erating activation; (ii) [K+](o) slows deactivation in a dose-dependent fas hion; and (iii) Rb+, Cs+ and, to a lesser extent, Na+ substitute for K+ in its action on gating, We have identified single residues, L293 and A428, at equivalent positions within the two pore loops that affect the [K+](o) sen sitivity. Substitution of these residues gave channels with reduced sensiti vity to [K+](o) in macroscopic current kinetics and voltage dependence, but had only minor effects on selectivity among alkali cations in gating and o n single-channel conductance, In some mutants, activation was slowed suffic iently to confer a sigmoidicity to current rise at low [K+](o). The results indicate that these residues are involved in [K+](o) sensing. Their situat ion close to the permeation pathway points to an interaction between gating and permeation.