Structural basis for gating charge movement in the voltage sensor of a sodium channel

Abstract

Voltage-dependent gating of ion channels is essential for electrical signaling in excitable cells, but the structural basis for voltage sensor function is unknown. We constructed high-resolution structural models of resting, intermediate, and activated states of the voltage-sensing domain of the bacterial sodium channel NaChBac using the Rosetta modeling method, crystal structures of related channels, and experimental data showing state-dependent interactions between the gating charge-carrying arginines in the S4 segment and negatively charged residues in neighboring transmembrane segments. The resulting structural models illustrate a network of ionic and hydrogen-bonding interactions that are made sequentially by the gating charges as they move out under the influence of the electric field. The S4 segment slides 6-8 Å outward through a narrow groove formed by the S1, S2, and S3 segments, rotates ∼30°, and tilts sideways at a pivot point formed by a highly conserved hydrophobic region near the middle of the voltage sensor. The S4 segment has a 3 10 -helical conformation in the narrow inner gating pore, which allows linear movement of the gating charges across the inner one-half of the membrane. Conformational changes of the intracellular one-half of S4 during activation are rigidly coupled to lateral movement of the S4-S5 linker, which could induce movement of the S5 and S6 segments and open the intracellular gate of the pore. We confirmed the validity of these structural models by comparing with a high-resolution structure of a NaChBac homolog and showing predicted molecular interactions of hydrophobic residues in the S4 segment in disulfide-locking studies.V oltage-gated sodium (Na V ) channels are responsible for initiation and propagation of action potentials in nerve, muscle, and endocrine cells (1, 2). They are members of the structurally homologous superfamily of voltage-gated ion channel proteins that also includes voltage-gated potassium (K V ), voltage-gated calcium (Ca V ), and cyclic nucleotide-gated (CNG) channels (3). Mammalian Na V and Ca V channels consist of four homologous domains (I through IV), each containing six transmembrane segments (S1 through S6) and a membrane-reentrant pore loop between the S5 and S6 segments (1, 3). Segments S1-S4 of the channel form the voltage-sensing domain (VSD), and segments S5 and S6 and the membrane-reentrant pore loop form the pore. The bacterial Na V channel NaChBac and its relatives consist of tetramers of four identical subunits, which closely resemble one domain of vertebrate Na V and Ca V channels, but provide much simpler structures for studying the mechanism of voltage sensing (4, 5). The hallmark feature of the voltage-gated ion channels is the steep voltage dependence of activation, which derives from the voltage-driven outward movement of gating charges in response to the membrane depolarization (6, 7). The S4 transmembrane segment in the VSD has four to seven arginine residues spaced at 3-aa intervals, which serve as gating charges in the voltage-s...

A Limited 4 Å Radial Displacement of the S4-S5 Linker Is Sufficient for Internal Gate Closing in Kv Channels

“…Values for the vertical displacement of S4 vary between 6 and 10 Å (44, 45) and 15 Å (34). The angle between the S4-S5 linker was proposed to change by 40°from 60°to 100° (45) in accordance with our experimental data.…”

“…2D displays the displacement of the S4-S5 linker during gating. The rotation is consistent with current models postulating a rotation of the S4 helix (44,45). According to our model, the S4 rotation seems to translate into a rotation of the S4-S5 linker.…”

“…The extent of the 3 10 -helix coincided with those observed in the crystal structure and other closed state models (48), despite the proposed transition from a 3 10 -to an ␣-helix, or an extension of the 3 10 -helix in the resting state (43,45,49,50). The extension of the 3 10 -region, however, might occur upon longer simulation times, as suggested by Bjelkmar et al (50).…”

Pathogenic mechanism of recurrent mutations of SCN8A in epileptic encephalopathy

“…Arginine 1617 is the outermost charged residue in the S4 voltage sensor segment of DIV, which is thought to initiate channel inactivation through a conformational change 36. In the crystal structure of the single domain bacterial channel NaChBac, the outermost arginine of S4 interacts with the negatively charged glutamate residue in segment S1 in the open state, and the negatively charged aspartate in segment S2 in the closed state 37. Loss of the positively charged arginine by p.Arg1617Gln would disrupt these electrostatic interactions, consistent with the observed defect in coupling of activation and inactivation (Fig.…”

Role of putative voltage-sensor countercharge D4 in regulating gating properties of CaV1.2 and CaV1.3 calcium channels

“…Membrane depolarization moves the positively charged S4 helices outward, causing conformational change in the VSDs that activates channel opening. The energetically unfavorable transition of the positive S4 gating charges across the plane of the membrane is facilitated by sequential exchange of ion pair partners with negatively charged residues (countercharges) in the other helices of the VSDs [3–6]. Particularly an inner negative cluster functions as charge transfer center through which the S4 charges (R1, R2, R3, K4) transit upon activation and deactivation [7].…”