Modulation of C-Type Inactivation by K + at the Potassium Channel Selectivity Filter

Abstract

With prolonged or repetitive activation, voltage-gated K + channels undergo a slow (C-type) inactivation mechanism, which decreases current flow through the channel. Previous observations suggest that C-type inactivation results from a localized constriction in the outer mouth of the channel pore and that the rate of inactivation is controlled by the rate at which K + leaves an unidentified binding site in the pore. We have functionally identified two K + binding sites in the conduction pathway of a chimeric K + channel that conducts Na + in the absence of K +. One site has a high affinity for K + and contributes to the selectivity filter mechanism for K + over Na +. Another site, external to the high-affinity site, has a lower affinity for K + and is not involved in channel selectivity. Binding of K + to the high-affinity binding site slowed inactivation. Binding of cations to the external low-affinity site did not slow inactivation directly but could slow it indirectly, apparently by trapping K + at the high-affinity site. These data support a model whereby C-type inactivation involves a constriction at the selectivity filter, and the constriction cannot proceed when the selectivity filter is occupied by K +.