A MUTATION IN S6 OF SHAKER POTASSIUM CHANNELS DECREASES THE K-BINDINGSITE REVEALING ION-ION INTERACTIONS IN THE PORE( AFFINITY OF AN ION)

Under physiological conditions, potassium channels are extraordinarily selective for potassium over other ions. However, in the absence of p otassium, certain potassium channels can conduct sodium. Sodium flux i s blocked by the addition of low concentrations of potassium. Potassiu m affinity, and therefore the ability to block sodium current, varies among potassium channel subtypes (Korn, S.J., and S.R. Ikeda. 1995. Sc ience. 269:410-412; Starkus, J.G., L. Kuschel, M.D. Rayner, and S.H. H einemann. 1997. J. Gen. Physiol. 110:539-550). The Shaker potassium ch annel 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 a pparent internal potassium affinity of the Shaker channel pore from th e micromolar to the millimolar range, as determined from the ability o f potassium to block the sodium currents. Independent evidence that A4 63C decreases the apparent affinity of a binding site in the pore come s from a study of barium block of potassium currents. The A463C mutati on decreases the internal barium affinity of the channel, as expected if barium blocks current try binding to a potassium site in the pore. The decrease in the apparent potassium affinity in A463C channels allo ws further study of possible ion interactions in die pore. Our results indicate that sodium and potassium can occupy the pore simultaneously and that multiple occupancy results in interactions between ions in t he channel pore.