Title

Role of the selectivity filter in inactivation of voltage-gated K+ channels

Date of Completion

January 1998

Keywords

Biology, Neuroscience|Biology, Animal Physiology

Degree

Ph.D.

Abstract

Voltage-gated K+ channels are a family of multi-ion, single file pores that display a high degree of selectivity for K+ over other ions. This selectivity is achieved by means of a high affinity K+ binding site associated with a narrow constriction along the permeation pathway of the pore. With prolonged activation, voltage-gated K+ channels undergo a slow (C-type) inactivation mechanism, which is thought to result from a localized constriction in the outer mouth of the pore. The conformational change that results from inactivation limits the access of K+ ions to the pore and inhibits current flow through the channel. We identified two functionally distinct K+ binding sites in the K+ channel pore. One of the binding sites has a high affinity for K+ and contributes to both the selectivity filter mechanism and the inactivation mechanism. K+ occupancy of this site slows the rate at which the channel enters the inactivated state. Occupancy of a lower affinity site, localized external to the high affinity site, does not influence inactivation rate but can slow inactivation by trapping K+ at the high affinity site. We also find that the conformational change associated with inactivation produces a change in the functional properties of the selectivity filter. During inactivation, voltage-gated K+ channels undergo a multi-step change in selectivity. Initially, the ability of K+ to conduct decreases and the permeability of Na + relative to K+ increases. As inactivation proceeds, the channel enters a state through which neither ion conducts. The change in selectivity was observed at all internal [K+] ≤ 80 mM, displayed both voltage- and time-dependent properties of slow inactivation and was observed with K+ on either side of the channel. These results were observed in K+ channels from two different molecular families, which suggests that this type of change in selectivity filter function may be a common element of C-type inactivation in most voltage-gated K + channels. Taken together, these findings indicate that the selectivity filter is an integral component of the inactivation mechanism, and suggest that slow inactivation occurs due to a constriction of the narrow region of the pore that constitutes the selectivity filter. ^