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J. Gen. Physiol.,
Volume 111, Number 2, February 1, 1998 195-206
From the Department of Physiology and Neurobiology, University of Connecticut, Storrs, Connecticut 06269
Voltage-gated potassium (K+) channels are multi-ion pores. Recent studies suggest that, similar to
calcium channels, competition between ionic species for intrapore binding sites may contribute to ionic selectivity
in at least some K+ channels. Molecular studies suggest that a putative constricted region of the pore, which is presumably the site of selectivity, may be as short as one ionic diameter in length. Taken together, these results suggest that selectivity may occur at just a single binding site in the pore. We are studying a chimeric K+ channel that
is highly selective for K+ over Na+ in physiological solutions, but conducts Na+ in the absence of K+. Na+ and K+
currents both display slow (C-type) inactivation, but had markedly different inactivation and deactivation kinetics;
Na+ currents inactivated more rapidly and deactivated more slowly than K+ currents. Currents carried by 160 mM
Na+ were inhibited by external K+ with an apparent IC50 <30 µM. K+ also altered both inactivation and deactivation kinetics of Na+ currents at these low concentrations. In the complementary experiment, currents carried by 3 mM K+ were inhibited by external Na+, with an apparent IC50 of ~100 mM. In contrast to the effects of low [K+]
on Na+ current kinetics, Na+ did not affect K+ current kinetics, even at concentrations that inhibited K+ currents
by 40-50%. These data suggest that Na+ block of K+ currents did not involve displacement of K+ from the high affinity site involved in gating kinetics. We present a model that describes the permeation pathway as a single high affinity, cation-selective binding site, flanked by low affinity, nonselective sites. This model quantitatively predicts
the anomalous mole fraction behavior observed in two different K+ channels, differential K+ and Na+ conductance, and the concentration dependence of K+ block of Na+ currents and Na+ block of K+ currents. Based on
our results, we hypothesize that the permeation pathway contains a single high affinity binding site, where selectivity and ionic modulation of gating occur.
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