Movement of Voltage Sensor S4 in Domain 4 Is Tightly Coupled to Sodium Channel Fast Inactivation and Gating Charge Immobilization

Abstract

The highly charged transmembrane segments in each of the four homologous domains (S4D1–S4D4) represent the principal voltage sensors for sodium channel gating. Hitherto, the existence of a functional specialization of the four voltage sensors with regard to the control of the different gating modes, i.e., activation, deactivation, and inactivation, is problematic, most likely due to a functional coupling between the different domains. However, recent experimental data indicate that the voltage sensor in domain 4 (S4D4) plays a unique role in sodium channel fast inactivation. The correlation of fast inactivation and the movement of the S4D4 voltage sensor in rat brain IIA sodium channels was examined by site-directed mutagenesis of the central arginine residues to histidine and by analysis of both ionic and gating currents using a high expression system in Xenopus oocytes and an optimized two-electrode voltage clamp. Mutation R1635H shifts the steady state inactivation to more hyperpolarizing potentials and drastically increases the recovery time constant, thereby indicating a stabilized inactivated state. In contrast, R1638H shifts the steady state inactivation to more depolarizing potentials and strongly increases the inactivation time constant, thereby suggesting a preferred open state occupancy. The double mutant R1635/1638H shows intermediate effects on inactivation. In contrast, the activation kinetics are not significantly influenced by any of the mutations. Gating current immobilization is markedly decreased in R1635H and R1635/1638H but only moderately in R1638H. The time courses of recovery from inactivation and immobilization correlate well in wild-type and mutant channels, suggesting an intimate coupling of these two processes that is maintained in the mutations. These results demonstrate that S4D4 is one of the immobilized voltage sensors during the manifestation of the inactivated state. Moreover, the presented data strongly suggest that S4D4 is involved in the control of fast inactivation.

Pharmacology of the Na v 1.1 domain IV voltage sensor reveals coupling between inactivation gating processes

“…6A). The first step initiates fast inactivation and corresponds to the transition of VSDIV from the resting state to the activated state, a step that accounts for the majority of gating charge movement (15,(32)(33)(34)(35). The second, more weakly voltage-dependent, transition moves the activated VSDIV to an immobilized state that is coupled to selectivity filter collapse and slow inactivation, as has been proposed for other voltage-gated channels (14,17,36).…”

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

“…Although there are fewer studies on the discrete transitions in the activation processes of Na channels, the S4's from domains I, II, and III are thought to be principally involved in channel activation (2) while inactivation has been associated with the S4 in DIV (8,10,11,21,22,37,43,44). Studies of individual florescent-tagged voltage sensors in rat skeletal muscle Na channels (rSkM1, Na V 1.4) showed that the S4's in domains I-III all translocated rapidly during a step depolarization consistent with their role in the activation process (10).…”

“…I Na decay in WT channels is critically dependent upon inactivation from the open state (8,11,14,21,22,37,43,44); consequently the more rapid decay of I Na following stabilization of the DII-S4 suggests that inactivation has also become faster. Furthermore, the large negative shift in the V ½ of the steady-state Na channel availability curve after DII-S4 was stabilized also suggests it has a role in closed state inactivation, presumably by helping form or helping control Table 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). Furthermore, the movement of the DIV-S4 during a depolarization is slower than the other S4's (10,37), and it contributes a large component (nearly 32%) to the maximal gating charge (Q max ) of the Na channel (10,39) with the outermost extracellular arginine (DIV-R1) estimated to contribute the most charge, ϳ20%, to Q max , while DIV-R2 and R3 contribute the remaining 12% (39).…”

Impaired Inactivation Gate Stabilization Predicts Increased Persistent Current for an Epilepsy-AssociatedSCN1AMutation

“…For instance, although sodium channels are constructed of four homologous domains (DI-DIV), each domain does not contribute identically to channel gating (Hille, 2001). In addition, inactivation and activation are coupled because channel inactivation preferentially immobilizes the DIII-and DIV-associated activation gating movements (Cha et al, 1999;Kuhn and Greeff, 1999). Additional discrepancies between HH-predicted and experimentally observed sodium channel behavior have also been described including gating currents, current fluctuation analysis, singlechannel activity, and activation kinetics (Patlak, 1991;Hille, 2001; Baranauskas and Martina, 2006;Naundorf et al, 2006).…”