ABSTRACT
Tok1p is a highly specific yeast plasma membrane potassium channel with strong outward directionality. Its opening is induced by membrane depolarization. Although the biophysical properties of Tok1p are well-described, its potentially important physiological role is currently largely unexplored. To address this issue, we examined the Tok1p activity following chemically-induced depolarization by measuring changes of plasma membrane potential (ΔΨ) using the diS-C3(3) fluorescence assay in a Tok1p-expressing and a Tok1p-deficient strain. We report that Tok1p channel activity in response to chemical stress does not depend solely on the extent of depolarization, as might have been expected, but may also be negatively influenced by accompanying effects of the used compound. The stressors may interact with the plasma membrane or the channel itself, or cause cytosolic acidification. All of these effects may negatively influence the Tok1p channel opening. While ODDC-induced depolarization exhibits the cleanest Tok1p activation, restoring an astonishing 75% of lost ΔΨ, higher BAC concentrations reduce Tok1p activity, probably because of direct interactions with the channel and/or its lipid microenvironment. This is not only the first study of the physiological role of Tok1p in ΔΨ maintenance under chemical stress, but also the first estimate of the extent of depolarization the channel is able to counterbalance.