Driving Forces underlying Selectivity Filter Gating in the MthK Potassium Channel

K+ channel activity can be limited by C-type inactivation, which is likely initiated in part by dissociation of K+ ions from the selectivity filter, and modulated by side chains surrounding the selectivity filter. Whereas crystallographic and computational studies have linked inactivation to a “collapsed” selectivity filter conformation in the KcsA channel, the structural basis for selectivity filter gating in other K+ channels has been less clear. Here, we combined electrophysiological recordings with molecular dynamics based, in silico electrophysiology simulations to study selectivity filter gating in the model potassium channel MthK and the MthK mutant V55E (analogous to KcsA E71) in the pore-helix. We find that MthK V55E has lower open probability than the WT channel, due to decreased stability of the open state. Simulations account for aspects of these observations on the atomistic scale, showing that ion permeation in V55E is differentially altered in two distinct orientations of the E55 side chain. In the ‘vertical’ orientation of E55, in which E55 forms a hydrogen bond with D64 (as observed with KcsA WT channels), the filter displays reduced conductance compared to MthK WT. In contrast, with ‘horizontal’ orientation, K+ conductance is similar to MthK WT; however the filter is less stable in the conductive conformation. Surprisingly, transitions of MthK WT and V55E channels to the non-conducting (inactivated) state observed in simulations are associated with a widening selectivity filter, unlike the narrowing selectivity filter observed with KcsA, and similar to recent structures of stably-inactivated Shaker W434F and Kv1.2 W362F mutants.