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 microM. 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.