SummaryThe open reading frame from the Arabidopsis thaliana KAT1 cDNA was cloned in a transcription plasmid between the 3' and 5' untranslated regions of a J3-globin cDNA from Xanopus oocyte. The polyedenylated transcripts resulting from in vitro transcription gave rise to high levels of expression of KAT1 channel when injected in Xanopus oocytes. Upon hyperpolarization, a slow activating current could be recorded, inwardly-or outwardly-directed, depending on K + external concentration. Predictions of the voltage-gated channel theory were shown to fit the data well. The equivalent gating charge and the half-activation potential ranged around 2 and -145 mV, respectively. KAT1 gating characteristics did not depend on K + external concentration nor on external pH in the 5.0-7.5 range. KAT1 conductance was, however, increased (40%) when external pH was decreased from 6.5 to 5.0. The apparent affinity constant of KAT1 for K + lay in the range 15-30 raM, at external pH 7.4. As for many K + channels of animal ceils, external caesium caused a voltage-dependent blockage of inward (but not outward) KAT1 current, whereas tetraethylammonium caused a voltage-independent block of both inward and outward KAT1 currents. In conclusion, high levels of expression made it possible to carry out the first quantitative analysis of KAT1 macroscopic currents. KAT1 channel was shown to display features similar to those of as yet uncloned inward-rectifying voltage-gated channels described in both plant cells (namely guard cells) and animal cells.

The plant K+ channel KAT1 shows some similarity to animal voltage-gated channels of the Shaker superfamily. Contrary to these animal counterparts, this plant channel is inwardly rectifying, being gated upon hyperpolarization. Different levels of expression of KAT1 in Xenopus oocytes could be obtained by increasing the amount of injected cRNA. The resulting KAT1 gating and sensitivity to external caesium were significantly changed. Similar findings have been published regarding animal voltage-gated channels. The present data show that plant channels may also undergo modification of their activity upon modification of their level of expression.