Regulation of the Fast Vacuolar Channel by Cytosolic and Vacuolar Potassium

Abstract

At resting cytosolic Ca 2+, passive K + conductance of a higher plant tonoplast is likely dominated by fast vacuolar (FV) channels. This patch-clamp study describes K +-sensing behavior of FV channels in Beta vulgaris taproot vacuoles. Variation of K + between 10 and 400 mM had little effect on the FV channel conductance, but a pronounced one on the open probability. Shift of the voltage dependence by cytosolic K + could be explained by screening of the negative surface charge with a density σ = 0.25 e −/nm 2. Vacuolar K + had a specific effect on the FV channel gating at negative potentials without significant effect on closed-open transitions at positive ones. Due to K + effects at either membrane side, the potential at which the FV channel has minimal activity was always situated at ∼50 mV below the potassium equilibrium potential, E K +. At tonoplast potentials below or equal to E K +, the FV channel open probability was almost independent on the cytosolic K + but varied in a proportion to the vacuolar K +. Therefore, the release of K + from the vacuole via FV channels could be controlled by the vacuolar K + in a feedback manner; the more K + is lost the lower will be the transport rate.