Outward stabilization of the voltage sensor in domain II but not domain I speeds inactivation of voltage-gated sodium channels

Abstract

Sheets MF, Chen T, Hanck DA. Outward stabilization of the voltage sensor in domain II but not domain I speeds inactivation of voltage-gated sodium channels. Am J Physiol Heart Circ Physiol 305: H1213-H1221, 2013. First published July 26, 2013; doi:10.1152/ajpheart.00225.2013.-To determine the roles of the individual S4 segments in domains I and II to activation and inactivation kinetics of sodium current (I Na ) in Na V1.5, we used a tethered biotin and avidin approach after a site-directed cysteine substitution was made in the second outermost Arg in each S4 (DI-R2C and DII-R2C). We first determined the fraction of gating charge contributed by the individual S4's to maximal gating current (Q max), and found that the outermost Arg residue in each S4 contributed ϳ19% to Q max with minimal contributions by other arginines. Stabilization of the S4's in DI-R2C and DII-R2C was confirmed by measuring the expected reduction in Q max. In DI-R2C, stabilization resulted in a decrease in peak I Na of ϳ45%, while its peak current-voltage (I-V) and voltage-dependent Na channel availability (SSI) curves were nearly unchanged from wild type (WT). In contrast, stabilization of the DII-R2C enhanced activation with a negative shift in the peak I-V relationship by Ϫ7 mV and a larger Ϫ17 mV shift in the voltage-dependent SSI curve. Furthermore, its I Na decay time constants and time-to-peak INa became more rapid than WT. An explanation for these results is that the depolarized conformation of DII-S4, but not DI-S4, affects the receptor for the inactivation particle formed by the interdomain linker between DIII and IV. In addition, the leftward shifts of both activation and inactivation and the decrease in G max after stabilization of the DII-S4 support previous studies that showed ␤-scorpion toxins trap the voltage sensor of DII in an activated conformation. sodium channel; gating charge; inactivation; voltage sensor; stabilization; ␤-scorpion toxin BECAUSE VOLTAGE-GATED sodium channels are intrinsically involved in the conduction of action potentials in the heart and the nervous system, they have become targets of both therapeutic medications and for nature's toxins. One family of Na channel toxins, ␤-scorpion toxin (a site-4 toxin), has been shown to target the voltage sensor (S4) in the second domain (DII) of the Na channel (for review see 26). Voltage-gated Na channels contain a single ␣ subunit of ϳ2,000 amino acids organized into four different domains, in contrast to the four separate, homologous domains of most voltage-gated potassium (K) channels that are thought to contribute to channel activation (2). Because all of the four domains are different from one another in the Na channel, each domain may make a unique contribution to channel kinetic transitions. For example, the outward movement of the fourth transmembrane segment in the fourth domain (DIV-S4) of the Na channel during a step depolarization has been associated with the Na channel kinetic transition from an open to a fast-inactivated state (8,11,21,22,43,44). Fur...

Direct Measurement of Cardiac Na + Channel Conformations Reveals Molecular Pathologies of Inherited Mutations

“…In our data, we observed a slow DI-VSD transition at elevated potentials that may be linked to this peak reduction through slow inactivation (Figure 2). In contrast, DII-VSD outward stabilization does affect rapid gating kinetics, shifting the G-V leftward along with SSI 28 . Our results show that this DII-VSD regulation of channel activation is through an allosteric connection, which facilitates channel opening when the DII-VSD is activated.…”

“…For example, outward stabilization of the DI-S4 strongly reduced peak Na V 1.5 current (45%) without affecting rapid channel kinetics 28 . In our data, we observed a slow DI-VSD transition at elevated potentials that may be linked to this peak reduction through slow inactivation (Figure 2).…”

Molecular Pathophysiology of Congenital Long QT Syndrome

“…The asymmetric nature of eukaryotic voltage-gated Na ϩ channels allows the voltage sensors to modulate different aspects of channel gating. Mutagenesis (66,384), cysteine accessibility (380,381,383), and voltage-clamp fluorometry (71,72) have revealed distinct properties of each voltage sensor. Movements of the DIV voltage sensor have been shown to correlate with the voltage dependence and kinetics of inactivation (71,147), and DIV-S4 activation is necessary and sufficient for inactivation gating (66).…”