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J. Gen. Physiol.,
Volume 112, Number 2, August 1, 1998 243-257
From the Department of Molecular and Cellular Physiology, Howard Hughes Medical Institute, Stanford University, Stanford,
California 94305
Under physiological conditions, potassium channels are extraordinarily selective for potassium over
other ions. However, in the absence of potassium, certain potassium channels can conduct sodium. Sodium flux is
blocked by the addition of low concentrations of potassium. Potassium affinity, and therefore the ability to block
sodium current, varies among potassium channel subtypes (Korn, S.J., and S.R. Ikeda. 1995. Science. 269:410-412;
Starkus, J.G., L. Kuschel, M.D. Rayner, and S.H. Heinemann. 1997. J. Gen. Physiol. 110:539-550). The Shaker potassium channel conducts sodium poorly in the presence of very low (micromolar) potassium due to its high potassium affinity (Starkus, J.G., L. Kuschel, M.D. Rayner, and S.H. Heinemann. 1997. J. Gen. Physiol. 110:539-550; Ogielska, E.M., and R.W. Aldrich. 1997. Biophys. J. 72:A233 [Abstr.]). We show that changing a single residue in
S6, A463C, decreases the apparent internal potassium affinity of the Shaker channel pore from the micromolar to
the millimolar range, as determined from the ability of potassium to block the sodium currents. Independent evidence that A463C decreases the apparent affinity of a binding site in the pore comes from a study of barium block
of potassium currents. The A463C mutation decreases the internal barium affinity of the channel, as expected if
barium blocks current by binding to a potassium site in the pore. The decrease in the apparent potassium affinity
in A463C channels allows further study of possible ion interactions in the pore. Our results indicate that sodium and potassium can occupy the pore simultaneously and that multiple occupancy results in interactions between
ions in the channel pore.
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