Down-State Model of the Voltage-Sensing Domain of a Potassium Channel

Abstract

Voltage-sensing domains (VSDs) of voltage-gated potassium (Kv) channels undergo a series of conformational changes upon membrane depolarization, from a down state when the channel is at rest to an up state, all of which lead to the opening of the channel pore. The crystal structures reported to date reveal the pore in an open state and the VSDs in an up state. To gain insights into the structure of the down state, we used a set of experiment-based restraints to generate a model of the down state of the KvAP VSD using molecular-dynamics simulations of the VSD in a lipid bilayer in excess water. The equilibrated VSD configuration is consistent with the biotin-avidin accessibility and internal salt-bridge data used to generate it, and with additional biotin-avidin accessibility data. In the model, both the S3b and S4 segments are displaced approximately 10 A toward the intracellular side with respect to the up-state configuration, but they do not move as a rigid body. Arginine side chains that carry the majority of the gating charge also make large excursions between the up and down states. In both states, arginines interact with water and participate in salt bridges with acidic residues and lipid phosphate groups. An important feature that emerges from the down-state model is that the N-terminal half of the S4 segment adopts a 3(10)-helical conformation, which appears to be necessary to satisfy a complex salt-bridge network.

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

“…The electrostatic network (Table S1) stabilizing the VSDs in ϵ agrees with electrophysiology experiments (25). The conformation in state ϵ bears strong similarities with the molecular models of the VSD down state of Kv1.2 that have been proposed previously (15,(26)(27)(28). However, the present model places R1 in a lower position.…”

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

“…The open-state model, O, with R1-R4ð¼ R362-R371Þ outside F290, is close to the previously determined structure of a K channel (5). Both the C1 and C2 states are compatible with some previous experimental data, and C3 is very similar to previous down-state models (30,(34)(35)(36). The C4 state, with all positive charges inside F290, has S4 even deeper intracellularly, but given the efforts required to capture this state and the fact that C3 is sufficient to explain the gating charge, C4 might only be present at significant hyperpolarization (which could remove it from the normal VSD cycle).…”

jShaw1, a low-threshold, fast-activating Kv3 from the hydrozoan jellyfish Polyorchis penicillatus

“…jShaw1 also differs from the mouse and D. melanogaster K v 3 channels in that it has five positively charged residues in the S4 helix -one less than the mouse channels and one more than D. melanogaster Shaw. In most voltage-gated channels, the basic residues in S4 can form stabilizing salt-bridges in both the open and closed states with acidic residues in S2 and S3 at positions equivalent to E283, E293 and D316 in D. melanogaster Shaker (Decaen et al, 2009;Papazian et al, 1995;Schow et al, 2010;Silverman et al, 2003). jShaw1 shares the same residues aligned with Shaker positions E283 and D316, but has a less bulky aspartate (D218) aligned with Shaker E293 (Fig.2A).…”