New structure and function in plant K + channels: KCO1, an outward rectifier with a steep Ca 2+ dependency

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 Ca 2+ [Ca 2+ ] cyt . No K + currents were detected when [Ca 2+ ] cyt was adjusted to <150 nM. However, KCO1 strongly activated at increasing [Ca 2+ ] cyt , with a saturating activity observed at ∼300 nM [Ca 2+ ] cyt . KCO1 single channel analysis on excised membrane patches, resulting in a single channel conductance of 64 pS, confirmed outward rectification as well as Ca 2+ ‐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.