Calcium Binding to the Turret Region Controls Inactivation Gating of a Voltage-Gated K+ Channel

Inactivation is an intrinsic property of numerous voltage-gated K+ (Kv) channels and can occur by N-type or/and C-type mechanisms. While fast N-type inactivation involves the inner pore occlusion by N-terminal peptide domains of α and β subunits, C-type inactivation is suggested to involve structural rearrangements in the outer pore leading to a loss of K+ coordination sites in the selectivity filter. In Kv7.1 channels, inactivation is invisible macroscopically and does not exhibit the hallmarks of N- and C-type mechanisms. However, Kv7.1 inactivation is revealed by hooked tail currents, which reflect the recovery from an inactivation state. Here, we show that removal of external Ca2+ produces a striking voltage-dependent macroscopic inactivation of Kv7.1 channels. Increasing external Ca2+ suppresses macroscopic inactivation with an EC50 of 1.5 μM. While Sr2+ and Cd2+ mimic the effects of Ca2+, other divalent cations like Mg2+ and Mn2+ are ineffective. Elevating external K+ concentration (50 mM) does not prevent macroscopic inactivation evoked in Ca2+-free external solutions. Experimental data and kinetic modeling indicate that KCNQ1 channels exhibit two distinct inactivation states. Mutagenesis studies and molecular modeling suggest that external Ca2+ ions are coordinated, at least by two glutamate residues E295 and E284 located at the outer pore in the turret domain. Our results reveal a new mechanism whereby external Ca2+ exquisitely controls inactivation gating of a Kv channel via a discrete pore turret region.