Intermediate states of the Kv1.2 voltage sensor from atomistic molecular dynamics simulations

Abstract

The response of a membrane-bound Kv1.2 ion channel to an applied transmembrane potential has been studied using molecular dynamics simulations. Channel deactivation is shown to involve three intermediate states of the voltage sensor domain (VSD), and concomitant movement of helix S4 charges 10-15 Å along the bilayer normal; the latter being enabled by zipper-like sequential pairing of S4 basic residues with neighboring VSD acidic residues and membrane-lipid head groups. During the observed sequential transitions S4 basic residues pass through the recently discovered charge transfer center with its conserved phenylalanine residue, F 233 . Analysis indicates that the local electric field within the VSD is focused near the F 233 residue and that it remains essentially unaltered during the entire process. Overall, the present computations provide an atomistic description of VSD response to hyperpolarization, add support to the sliding helix model, and capture essential features inferred from a variety of recent experiments.gating charge | S4 helix | voltage-gated channel V oltage sensor domains (VSDs) are membrane-embedded constructs, which work as electrical devices responding to changes in the transmembrane (TM) voltage. They are ubiquitous to voltage-gated channels (VGCs) in which four of these units are attached to the main pore (1). During channel activation, the displacements of the charges tethered to the VSD give rise to transient "gating" currents, the time integral of which is the "gating charge" (GQR) translocated across the membrane capacitance. Phenomenological kinetic models devised to describe the time course of such currents are very diverse but all indicate that during VGC activation, the VSD undergoes a complex conformational change that encompasses many transitions (2-5).Three main models have been proposed to rationalize the transfer of a large GQR across the low dielectric membrane in VGCs (6, 7). All are associated with the motion of S4, the conserved highly positively charged helix of the VSDs (8). In the sliding helix model (9, 10), the positively charged (basic) residues of the S4 segment form sequential ion pairs with acidic residues on neighboring TM segments and move a large distance perpendicular to the membrane plane. The transporter model derives from measurements of a focused electrical field within the membrane and suggests that during activation, the latter is reshaped. Accordingly, it is posited that S4 does not move its charges physically very far across the membrane (8). A third model was introduced following publication of the KvAP structure (11). Here, the position of the S3-S4 helical hairpin with respect to the pore domain suggested a gating mechanism in which the hairpin moves through the membrane in a paddle-like motion translocating S4 basic residues across the membrane, and reaching a TM position only in the activated state. Crystal structures of the Kv1.2 channel (12) and the Kv1.2-Kv2.1 paddle chimera (13) indicated later that the KvAP structure likely represented a non...

Tracking a complete voltage-sensor cycle with metal-ion bridges

“…The C4 model is consistent with recent experimental data showing that not only R2-R4ð¼ R365-R361Þ, but also R1ð¼ R362Þ might be able to pass inside F290 (28,31,37). The cascading motion of the arginines is similar to recent simulations (38); however, in the experimentally derived models, this is achieved through a sliding 3 10 -helix present in all states from O through C3 in the VSD. Metal-ion constraints and molecular simulation relaxation predicts each C1 through C3, and, under some conditions, C4, state to correspond to one additional arginine side chain in S4 (blue sticks) translating across the hydrophobic zone lock formed by F290 (green sticks), forming salt bridges to negatively charged residues in S1-S3 (red sticks; E247 in S1and E283 in S2 above F290, and E293 in S2 and 316D in S3 below F290.…”

Functional diversity of potassium channel voltage-sensing domains

“…This has been known from detailed kinetic analysis of the activation of several voltage-dependent potassium channels 65,66 and is also reflected in some molecular dynamics simulations 27,29 and in experiments tracking the movement of S4 with metal-ion bridges. 67 Apart of a structural scaffold, the non-S4 segments of the VSD provide specific electrostatic interactions that stabilize the charged arginine and lysine residues.…”

“…The presence of water in these vestibules has been postulated on the basis of continuum electrostatic calculations in Shaker channels, 35,36 and the observation that waters occupy these regions in molecular dynamics calculations of Kv1.2 channels. 28,29,37 Water occupancy of the vestibules of VSDs is also implied by the proposed mechanism of proton permeation via water wires in Hv1 channels 38 and the existence of proton and alkali ion currents in certain mutations of the VSD of Shaker and Na channels, specifically of the charged residues in S4 and S2. [39][40][41][42] The large degree of structural conservation suggest that the mechanism of activation of all voltage sensors is also shared among them, and important clues have been obtained from studies of all of these VSDs.…”

Coarse-grained simulations of the gating current in the voltage-activated Kv1.2 channel

“…The difficulties start with the absence of an Xray structure of the closed state. Nevertheless, intensive efforts have been made in building models of the closed state that is consistent with a vast amount of biochemical and biophysical data (9,10,12). These studies led to a reasonable model for the closed form of the K + voltage-activated ion channel (see ref.…”