Gating transitions in the selectivity filter region of a sodium channel are coupled to the domain IV voltage sensor

Abstract

Voltage-dependent ion channels are crucial for generation and propagation of electrical activity in biological systems. The primary mechanism for voltage transduction in these proteins involves the movement of a voltage-sensing domain (D), which opens a gate located on the cytoplasmic side. A distinct conformational change in the selectivity filter near the extracellular side has been implicated in slow inactivation gating, which is important for spike frequency adaptation in neural circuits. However, it remains an open question whether gating transitions in the selectivity filter region are also actuated by voltage sensors. Here, we examine conformational coupling between each of the four voltage sensors and the outer pore of a eukaryotic voltage-dependent sodium channel. The voltage sensors of these sodium channels are not structurally symmetric and exhibit functional specialization. To track the conformational rearrangements of individual voltagesensing domains, we recorded domain-specific gating pore currents. Our data show that, of the four voltage sensors, only the domain IV voltage sensor is coupled to the conformation of the selectivity filter region of the sodium channel. Trapping the outer pore in a particular conformation with a high-affinity toxin or disulphide crossbridge impedes the return of this voltage sensor to its resting conformation. Our findings directly establish that, in addition to the canonical electromechanical coupling between voltage sensor and inner pore gates of a sodium channel, gating transitions in the selectivity filter region are also coupled to the movement of a voltage sensor. Furthermore, our results also imply that the voltage sensor of domain IV is unique in this linkage and in the ability to initiate slow inactivation in sodium channels. electrophysiology | Nav1.4 | outer pore conformation

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

“…Notably, the concept that slow inactivation is tied to a VSDIV transition subsequent to activation supports previous models proposing that Na v channel voltage sensor immobilization and slow inactivation are coupled (1,(15)(16)(17)(18)36). Taken together, our results provide support for a VSDIV-centric model of Na v 1.1 inactivation in which both fast and slow inactivation processes are coupled to sequential movements of this voltage sensor (Fig.…”

“…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). The effect of Hm1a (10) on Na v 1.1 (Figs.…”

“…Mammalian Na v channels are large proteins composed of four homologous domains that form a pseudo-fourfold symmetric channel comprising a central pore surrounded by four voltage sensors, one from each domain (1). Accumulating evidence suggests that activation gate opening is associated with movement of voltage sensors in domains I-III (VSDI-III), whereas fast inactivation is initiated by subsequent movement of the voltage sensor in domain IV (VSDIV) (1,15,16); however, the role of VSDIV in slow inactivation and the possibility of functional coupling between fast and slow inactivation remain matters of debate (17)(18)(19).…”

Mapping of voltage sensor positions in resting and inactivated mammalian sodium channels by LRET

“…6E, suggest a functional asymmetry of the VSDs in Nav1.4, with respect to KIIIA-Bodipy as follows: (i) For DI, the position of R(−5) in the resting state lies further from KIIIA-Bodipy than the distances of R(−6) and R(−7), whereas there are no significant differences between distances of DI R(−5), DI R(−6), and DI R(−7) with respect to KIIIA-Bodipy in the slow inactivated state, and (ii) for DIV, on the other hand, there is essentially no difference in the distances between KIIIA-Bodipy and R(−5), R(−6), or R(−7) in the resting state, whereas the R(−7) position seems to be closest to KIIIA-Bodipy in the slow inactivated state. This reciprocal change observed between DI and DIV may underlie the different functional roles of DI and DIV VSDs previously proposed based on site-directed fluorimetry (2,3,37).…”

Proton Sensors in the Pore Domain of the Cardiac Voltage-gated Sodium Channel

“…Further, mutations involving the cytoplasmic S4-S5 linkers as well as the S4, S5, and S6 helices have been shown to affect Na V channel SI (21-28). Capes et al (29) showed that prolonged depolarizations that induced slow inactivation inhibited gating pore currents through the DIV voltage-sensing domain and demonstrated that movements within the Na V channel voltagesensing domain and selectivity filter are coupled.…”