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 two pore–loop sequences that are thought to form the hydrophilic lining of the pore. Gating of the channel is promoted by membrane depolarization and is regulated by extracellular K + concentration ([K + ] o ) both in the yeast and when expressed in Xenopus oocytes. Analysis of the wild‐type current now shows that: (i) [K + ] o suppresses a very slowly relaxing component, accelerating activation; (ii) [K + ] o slows deactivation in a dose‐dependent fashion; 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 sensitivity. Substitution of these residues gave channels with reduced sensitivity to [K + ] o in macroscopic current kinetics and voltage dependence, but had only minor effects on selectivity among alkali cations in gating and on single‐channel conductance. In some mutants, activation was slowed sufficiently to confer a sigmoidicity to current rise at low [K + ] o . The results indicate that these residues are involved in [K + ] o sensing. Their situation close to the permeation pathway points to an interaction between gating and permeation.