KAT1 inactivates at sub-millimolar concentrations of external potassium

Structural analysis of K+ channel pores suggests that the selectivity filter of the pore is an inherent sensor for extracellular K+ \((Kₒ^{{+}});\) channels seem to be inactivated at low \(Kₒ^{{+}}\) because of a destabilization of the conducting state and a collapse of the pore. In the present study, the effect of depleting \(Kₒ^{{+}}\) on the activity of a plant K+ channel, KAT1, from Arabidopsis thaliana was investigated. This channel is thought to be insensitive to \(Kₒ^{{+}}.\) The channel was therefore expressed in mammalian HEK293 cells and measured with patch clamp technology in the whole cell configuration. The effect of \(Kₒ^{{+}}\) depletion on channel activity was monitored from the tail currents before, during, and after washing \(Kₒ^{{+}}\) from the medium. The data show that a depletion of \(Kₒ^{{+}}\) results in a decrease in channel conductance, irrespective of whether K+ is simply removed or replaced by either Na+ or Li+. Quantitative analysis suggests that the channel has two binding sites for K+ with the dissociation constant in the order of 20 μM. This high sensitivity of the channel to \(Kₒ^{{+}}\) could serve as a safety mechanism, which inactivates the channel at low \(Kₒ^{{+}}\) and, in this way, prevents leakage of K+ from the cells via this type of channel.