Structural basis for pore blockade of the human cardiac sodium channel Nav1.5 by tetrodotoxin and quinidine

Among the nine subtypes of human voltage-gated Na+ (Nav) channels, tetrodotoxin (TTX)-resistant Nav1.5 is the primary one in heart. More than 400 point mutations have been identified in Nav1.5 that are associated with cardiac disorders exemplified by type 3 long QT syndrome and Brugada syndrome, making Nav1.5 a common target for class I antiarrythmic drugs. Here we present the cryo-EM structures of Nav1.5 bound to quinidine and TTX with resolutions at 3.2 Å and 3.3 Å, respectively, for the pore domain. The structures allow mapping of 278 disease-related mutations in the resolved region, establishing the framework for mechanistic investigation of these pathogenic mutations. Quinidine is positioned right below the selectivity filter and coordinated by residues from repeats I, III, and IV. Pore blockade is achieved through both direct obstruction of the ion permeation path and induced rotation of a Tyr residue that leads to intracellular gate closure. Structural comparison of Nav1.5 with our previously reported Nav1.2/1.4/1.7 reveals the molecular determinant for the different susceptibility to TTX by TTX-resistant and -sensitive Nav subtypes, and highlights the functional significance of the underexplored and least conserved extracellular loops.