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Abstract
The action potential is the elementary unit for biological excitability in the animal kingdom. At the molecular level, voltage-gated ion channels underlie the action potential. Voltage-gated sodium (Nav) channels allow the influx of Na+ ions, initiating the depolarizing phase of the action potential and subsequently, voltage-gated potassium (Kv) channels carry the outward K+ current, restoring the membrane potential to the resting state. The precisely coordinated transitions among the conductive and nonconductive states of these ion channels give rise to the action potentials. From the broad hundreds-millisecond-long cardiac action potential to the fast action potentials in Purkinje neurons that subside within 10 milliseconds, the gating of the ion channels shape the action potentials and shape the physiology. All Nav and Kv channels possess an activation gate controlled by voltage. Upon depolarization, the voltage sensors in the channel activate and subsequently open the activation gate. However, the activation gate is not the only gate. In Nav channels, milliseconds after the opening of the activation gate, the fast inactivation gate closes, driving the channels into the nonconductive fast inactivated state. Contrary to the fast-acting fast inactivation gate in Nav channels, the slow inactivation gate in Kv channels closes over seconds or even tens of seconds. The various gates in the ion channels allow for the diverse behaviors of the ion channels and play fundamental roles in countless processes. This thesis aims to elucidate the molecular basis of the diverse gating mechanisms in Nav and Kv channels with a special emphasis on Nav channel fast inactivation. Utilizing electrophysiology, molecular dynamics, voltage clamp fluorimetry and single-particle cryo-EM, we provide mechanistic insights into fast inactivation mechanism in Nav channels and the activation/slow inactivation mechanisms in Kv channels.