K+-dependent composite gating of the yeast K+ channel, Tok1

TOK1 encodes an outwardly rectifying K+ channel in the plasma membrane of t he budding yeast Saccharomyces cerevisiae. It is capable of dwelling in two kinetically distinct impermeable states, a near-instantaneously activating R state and a set of I-elated delayed activating C states (formerly called C-2 and C-1, respectively). Dwell in the R state is dependent on membrane potential and both internal and external K+ in a manner consistent with the K+ electrochemical potential being its determinant, where dwell in the C s tates is dependent on voltage and only external Kt. Whereas activation from the C states showed high temperature dependencies, typical of gating trans itions in other Shaker-like channels, activation from the R state had a tem perature dependence nearly as low as that of simple ionic diffusion. These findings lead us to conclude that although the C states reflect the activit y of an internally oriented channel gate, the R state results from an intri nsic gating property of the channel filter region.