Résumé
Potassium (K+) channels mediating important physiological functions are characterized by a common pore–forming (P) domain. We report the cloning and functional analysis of the first higher plant outward rectifying K+ channel (KCO1) from Arabidopsis thaliana. KCO1 belongs to a new class of ‘two‐pore’ K+ channels recently described in human and yeast. KCO1 has four putative transmembrane segments and tandem calcium‐binding EF‐hand motifs. Heterologous expression of KCO1 in baculovirus‐infected insect (Spodoptera frugiperda) cells resulted in outwardly rectifying, K+‐selective currents elicited by depolarizing voltage pulses in whole‐cell measurements. Activation of KCO1 was strongly dependent on the presence of nanomolar concentrations of cytosolic free Ca2+ [Ca2+]cyt. No K+ currents were detected when [Ca2+]cyt was adjusted to <150 nM. However, KCO1 strongly activated at increasing [Ca2+]cyt, with a saturating activity observed at ∼300 nM [Ca2+]cyt. KCO1 single channel analysis on excised membrane patches, resulting in a single channel conductance of 64 pS, confirmed outward rectification as well as Ca2+‐dependent activation. These data suggest a direct link between calcium‐mediated signaling processes and K+ ion transport in higher plants. The identification of KCO1 as the first plant K+ outward channel opens a new field of structure–function studies in plant ion channels.