IDENTIFICATION OF A K+ CHANNEL FROM POTATO LEAVES BY FUNCTIONAL EXPRESSION IN XENOPUS OOCYTES

Poly(A)(+) mRNA was isolated from leaves of potato plants (Solanum tub erosum L. cv. Desiree) according to standard protocols. This poly(A)() mRNA was injected via glass microcapillaries into oocytes that were surgically removed from the African clawed toad Xenopus laevis. As a c ontrol, oocytes were either injected with H2O or remained untreated. T hree days after injection the oocytes were analyzed by two electrode v oltage clamping. Current voltage analysis revealed that a K+ channel f rom potato was functionally expressed in injected oocytes. The identit y of this K+ channel was confirmed by its substrate specificity and a shift in the reversal potential. In particular, when the outside K+ co ncentration was increased the reversal potential of poly(A)(+) injecte d oocytes shifted to more positive values. Furthermore, KC outward cur rents declined when the outside K+ concentration was raised from 0.1 t o 100 mill. Inward currents increased with an elevation of the K+ conc entration. Several pharmaceuticals were tested for their potential to block this K+ channel. As a result, the channel was completely blocked by BaCl2. A three state reaction kinetic model was used to simulate t he currents through the Kf transport protein as function of the extrac ellular K+ concentration. In particular, the simulation revealed curre nt voltage relations that exactly matched the measured ones. Saturatio n of current voltage curves emerged from the simulation as a consequen ce of high extracellular potassium concentration.