Structures of Gating Intermediates in a K+ channel

•Ion flux through a K+ channel is regulated by channel activation and inactivation.•W67F in KcsA prevents inactivation and alters pH activation.•Structures of KcsA W67F determined in the pre-open and the pre-inactivated state.•Ion binding at second ion site in the selectivity filter required for inactivation.•Insights into the allosteric pathway coupling activation and inactivation in KcsA.