The permeation properties of KAT1, an inward rectifying potassium channel from plant cells, were investigated with different ions in the external medium. With either K+, NH+ 4 or methylammonium (MA) in the external solution, the channel, expressed in Xenopus oocytes, appeared permeable to K+ and, to a lesser extent, to NH+ 4 but not to the slightly bigger, methylated analogue of NH+ 4, MA. Substituting NH+ 4 for K+ shifted the voltage dependency of channel activation further negative and hastened activation kinetics. This suggests that channel operation depends on the transported substrate. In mixed solution (50 mm K+, 50 mm MA) MA inhibited K+ current in a voltage-independent manner. The maximum block did not exceed 50% of the K+ current. In contrast, when NH+ 4 was the permeant ion (50 mm NH+ 4, 50 mm MA) MA caused a voltage-dependent, slowly developing open channel block, achieving complete inhibition at very negative voltages. The latter block could be partially overcome by the addition of K+ in the external solution. The data support a model in which ions, after entering the channel pore, compete with different affinities for binding sites on their permeation pathway.
The Journal of Membrane Biology – Springer Journals
Published: May 1, 1998
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