title: The impermeant ion methylammonium blocks K+ and NH4+ currents through KAT1 channel differently: evidence for ion interaction in channel permeation.
abstract: 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+, NH4+ or methylammonium (MA) in the external solution, the channel, expressed in Xenopus oocytes, appeared permeable to K+ and, to a lesser extent, to NH4+ but not to the slightly bigger, methylated analogue of NH4+, MA. Substituting NH4+ 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 NH4+ was the permeant ion (50 mM NH4+, 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.
authors.affiliation: Dipartimento di Fisiologia e Biochimica Generali, Milano, Italy.
authors.forename: A
authors.initials: A
authors.lastname: Moroni
authors.forename: L
authors.initials: L
authors.lastname: Bardella
authors.forename: G
authors.initials: G
authors.lastname: Thiel
mesh_terms: D000643:Ammonium Chloride / Q000378:metabolism*; D000818:Animals; D002463:Cell Membrane Permeability; D004305:Dose-Response Relationship, Drug; D007477:Ions; D008744:Methylamines / Q000378:metabolism*; D010940:Plant Proteins; D015221:Potassium Channels / Q000235:genetics / Q000502:physiology*; D024661:Potassium Channels, Inwardly Rectifying; D011189:Potassium Chloride / Q000378:metabolism*; D014981:Xenopus
chemical_list: D007477:Ions; D008744:Methylamines; D010940:Plant Proteins; D015221:Potassium Channels; D024661:Potassium Channels, Inwardly Rectifying; D000643:Ammonium Chloride; D011189:Potassium Chloride; C027451:methylamine