New structure and function in plant K+ channels: KCO1, an outward rectifier with a steep Ca2+ dependency. [Erratum: Nov 17, 1997, v. 16 (22), p. 6896.]

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 approximately 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.