EVIDENCE FOR A MULTIION PORE BEHAVIOR IN THE PLANT POTASSIUM CHANNEL KAT1

KAT1 is a cloned voltage-gated K+ channel from the plant Arabidopsis t haliana L., which displays an inward rectification reminiscent of 'ano malous' rectification of the i(f) pacemaker current recorded in animal cells. Macroscopic conductance of KAT1 expressed in Xenopus oocytes w as 5-fold less in pure Rb+ solution than in pure K+ solution, and negl igible in pure Naf solution. Experiments in different K+/Na+ or K+/Rb mixtures revealed deviations from the principle of independence and n otably two anomalous effects of the K+/Rb+ Rb+ mole fraction (i.e., th e ratio [K+]/([K+]+[Rb+])). First, the KAT1 deactivation time constant was both voltage- and mole fraction-dependent (a so-called 'foot in t he door' effect was thus observed in KAT1 channel). Second, when plott ed against the K+/Rb+ mole fraction, KAT1 conductance values passed th rough a minimum. This minimum is more important for two pore mutants o f KAT1 (T259S and T260S) that displayed an increase in P-Rb/P-K. These results are consistent with the idea that KAT1 conduction requires se veral ions to be present simultaneously within the pore. Therefore, th is atypical 'green' member of the Shaker superfamily of K+ channels fu rther shows itself to be an interesting model as well for permeation a s for gating mechanism studies.