Potassium uptake through the TOK1 K+ channel in the budding yeast

The current through TOK1 (YKC1), the outward-rectifying K+ channel in Saccharomyces cerevisiae, was amplified by expressing TOK1 from a plasmid driven by a strong constitutive promoter. TOK1 so hyper-expressed could overcome the K+ auxotrophy of a mutant missing the two K+ transporters, TRK1 and TRK2. This trk1Delta trk2Delta double mutant hyperexpressing the TOK1 transgene had a higher internal K+ content than one expressing the empty plasmid. We examined protoplasts of these TOK1-hyperexpressing cells under a patch clamp. Besides the expected K+ outward current activating at membrane potential (Vm) above the K+ equilibrium potential (EK+), a small inward current was consistently observed when the Vm was slightly below EK+. The inward and the outward currents are similar in their activation rates, deactivation rates, ion specificities and Ba2+ inhibition, indicating that they flow through the same channel. Thus, the yeast outwardly rectifying K+ channel can take up K+ into yeast cells, at least under certain conditions.