Voltage-Gated Sodium and Calcium Channels at Atomic Resolution: Structure, Function, and Pharmacology

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Voltage-gated sodium channels initiate action potentials, and voltage-gated calcium channels initiate neurotransmission and contraction. Recent discovery of their bacterial ancestors, including NavAb, allowed us to determine the structural basis for voltage-dependent activation, inactivation, and selective ion conductance. Structural studies support a sliding-helix mechanism of voltage-dependent activation, in which gating charges in the S4 transmembrane helix move across the membrane through the protein structure, exchange ion-pair partners, and initiate a conformational change to open the pore. Pore-opening is mediated by subtle rotation and bending of the pore-lining S6 helices to open the activation gate at their intracellular ends to an orifice of ～10.5 Å. Slow inactivation involves partial collapse of the pore, in which two S6 segments move toward the central axis. Rapid and selective ion conductance is mediated by an ion selectivity filter that is ～4.6 Å wide and water-filled. Sodium is conducted as a partially hydrated cation. It interacts first with a square array of glutamate sidechains, which displace a variable number of waters and catalyze inward sodium movement with a dunking motion. Sodium then binds to two additional coordination sites formed by backbone carbonyls. Mutation of three negative charges to give the construct CavAb changes ion selectivity 12,000-fold to Ca:Na=400. High-resolution structures reveal the mechanism of ion conduction and selectivity through interactions of hydrated calcium ions with sites in the extracellular vestibule and selectivity filter. High-affinity calcium binding prevents monovalent cation permeation, and alternating occupancy of three calcium-binding sites generates a knock-off effect and mediates rapid and selective conductance. Sodium and calcium channels are drug targets for pain, epilepsy, arrhythmia, hypertension, and angina pectoris. We imaged CavAb with phenylalkylamine and dihydropyridine calcium-antagonist drugs bound to their receptor sites. Verapamil binds in the ion permeation pathway, just on the intracellular side of the selectivity filter. Dihydropyridines bind to an allosteric site on the lipid-facing surface of the pore, creating an asymmetric pore structure with calcium tightly bound in a blocking position. The binding sites for sodium-channel-blocking local anesthetics and antiarrhythmic drugs are now being elucidated at atomic resolution, with important implications for structure-based drug design.