Structure and physiological function of the human KCNQ1 channel voltage sensor intermediate state

Abstract

Voltage-gated ion channels feature voltage sensor domains (VSDs) that exist in three distinct conformations during activation: resting, intermediate, and activated. Experimental determination of the structure of a potassium channel VSD in the intermediate state has previously proven elusive. Here, we report and validate the experimental three-dimensional structure of the human KCNQ1 voltage-gated potassium channel VSD in the intermediate state. We also used mutagenesis and electrophysiology in Xenopus laevisoocytes to functionally map the determinants of S4 helix motion during voltage-dependent transition from the intermediate to the activated state. Finally, the physiological relevance of the intermediate state KCNQ1 conductance is demonstrated using voltage-clamp fluorometry. This work illuminates the structure of the VSD intermediate state and demonstrates that intermediate state conductivity contributes to the unusual versatility of KCNQ1, which can function either as the slow delayed rectifier current (IKs) of the cardiac action potential or as a constitutively active epithelial leak current.

Calmodulin acts as a state-dependent switch to control a cardiac potassium channel opening

“…1C). Previous work showed that KCNQ1 VSD can trigger pore opening when activated (8,18,21,23,24). The IO and AO states have been shown to give rise to bi-exponential current activation kinetics (18,23,24), with the fast and slow components approximating the IO and AO states, respectively (Fig.…”

“…Structurally, KCNQ1 channels comprise four identical subunits, each with six transmembrane (S1-S6) and four cytoplasmic (helix A to D; HA-HD) α-helices, with S1-S4 forming the voltage-sensing domain (VSD) and S5-S6 the pore domain (14,15) (Fig. 1A -B). Upon membrane depolarization, the KCNQ1 VSDs activate with two experimentally resolvable steps: first from the resting state to an intermediate state, and then to the fully-activated state (8,(16)(17)(18)(19)(20)(21)(22). The VSD conformational changes are communicated to the pore domain via a series of residue-residue interactions, leading to pore opening (16,18).…”

“…1C). The IO and AO states exhibit different functional properties important for KCNQ1 physiological function, with regulation of IO and AO by different auxiliary subunits leading to tissue specific current phenotypes (21). For example, although KCNQ1 conducts predominantly at the IO-state as seen by the large fraction of the fast-current component (Fig.…”

“…For example, although KCNQ1 conducts predominantly at the IO-state as seen by the large fraction of the fast-current component (Fig. 1C), the auxiliary subunit KCNE1 eliminates the kinetically fast IO state and enhances the slow AO state, contributing to the kinetically slow IKs suitable for cardiac tissue needs (8,18,21).…”

“…1A-B). The VSDs in both conformations adopt the fully-activated state (14,15,21). The CTD-bent conformation features a closed pore, while the CTD-straight conformation exhibits a dilated pore (14,15).…”