New structure and function in plant K+ channels: KCO1, an outward rectifier with a steep Ca2+ 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 Ca2+ [Ca2+]cyt. No K+ currents were detected when [Ca2+]cyt was adjusted to