Molecular Basis of Activation and Inactivation Gating in hERG Cardiac K+ Channel

Voltage-gated potassium (Kv) channels are essential determinants of membrane excitability, coupling voltage sensor activation to selective potassium permeation through coordinated conformational rearrangements of the voltage sensors and the pore. By converting membrane depolarization into outward potassium conductance, Kv channels mediate the repolarization phase of the action potential, thereby restoring the resting membrane potential and shaping firing frequency and refractory behavior. Structurally, Kv channels share a conserved architecture with a voltage-sensing domain formed by the S1-S4 helices and a pore domain comprising the S5 and S6 helices and the intervening selectivity filter, in which voltage-driven conformational changes in the S4 helix are transmitted to the intracellular S6 bundle-crossing gate through electromechanical coupling interfaces such as the S4-S5 linker. Notwithstanding this conserved framework, individual Kv subfamilies exhibit distinct gating kinetics and regulatory mechanisms that confer specialized physiological functions. Among these, the human ether-à-go-go–related gene (hERG, Kv11.1) channel plays an essential role in cardiac electrophysiology by conducting the rapid delayed rectifier potassium current (IKr), a major contributor to phase 3 repolarization of the ventricular action potential. Dysfunction of hERG, whether due to inherited mutations or drug off-target inhibition, leads to prolonged cardiac action potentials and life-threatening arrythmias. Despite its clinical importance and recent advances in structural biology, the molecular basis by which voltage sensor activation is coupled to pore-gating in hERG remains incompletely understood, particularly with respect to the structural determinants that underlie its atypical gating kinetics and sensitivity to pharmacological modulation. Motivated by these unresolved questions, this thesis seeks to define the structural basis of ion channel gating, with a primary focus on hERG and an extension toward the evolutionary origins of mechanotransduction in sensory systems. Chapters 2 and 3 integrate cryo-electron microscopy (cryo-EM), mutagenesis, and functional analyses to delineate how the principal structural modules of hERG—the voltage-sensing domain, pore domain, and selectivity filter—coordinate to regulate channel activation, deactivation, and inactivation. Chapter 4 broadens this perspective by shifting from voltage-dependent gating to mechanically activated ion channels, thereby placing the mechanistic principles uncovered in hERG within a wider physiological and evolutionary context.