The function of the 12 positive charges in the 53-residue III/IV interdomain linker of the cardiac Na+ channel is unclear. We have identified a four-generation family, including 17 gene carriers with long QT syndrome, Brugada syndrome, and conduction system disease with deletion of lysine 1500 (ΔK1500) within the linker. Three family members died suddenly. We have examined the functional consequences of this mutation by measuring whole-cell and single-channel currents in 293-EBNA cells expressing the wild-type and ΔK1500 mutant channel. The mutation shifted the potential for half inactivation (V1/2hx) to more negative values and reduced its voltage dependence consistent with a reduction of inactivation valence of 1. The shift in inactivation was the result of an increase in closed-state inactivation rate (11-fold at -100 mV). The potential for half activation (V1/2m) was shifted to more positive potentials, and its voltage dependence reduced by 50% in the ΔK1500 mutant. To determine whether the positive charge deletion was the basis for the gating changes, we performed the mutations K1500Q and K1500E (Δ charge, -1, -2). For both mutations, V1/2hx was shifted back toward control; however, V1/2m shifted progressively to more positive potentials. The late component of Na+ current was increased in the ΔK1500 mutant channel. These changes can account for the complex phenotype in this kindred and point to an important role of the III/IV linker in channel activation.
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