Sensing voltage across lipid membranes

Abstract

The detection of electrical potentials across lipid bilayers by specialized membrane proteins is required for many fundamental cellular processes such as the generation and propagation of nerve impulses. These membrane proteins possess modular voltage-sensing domains, a notable example being the S1-S4 domains of voltage-activated ion channels. Ground-breaking structural studies on these domains explain how voltage sensors are designed and reveal important interactions with the surrounding lipid membrane. Although further structures are needed to understand the conformational changes that occur during voltage sensing, the available data help to frame several key concepts that are fundamental to the mechanism of voltage sensing.

Figure 1. Types of membrane proteins that contain S1-S4 voltage-sensing domains and structure of an S1-S4 domain

a) Cartoon illustration of S1-S4 voltage-sensing domains in different types of membrane proteins. S1-S4 helices are labeled with the paddle motif colored purple. In voltage-activated ion channels the S1-S4 domains couple to an ion selective pore domain (yellow); only one of four S1-S4 domains is shown for clarity. In enzymes like the voltage-sensitive phosphates (Ci-VSP) the S1-S4 domain couples to a soluble phosphatase domain (yellow). In voltage-activated proton channels, protons are thought to permeate the S1-S4 domain directly. b) Ribbon representations of the X-ray structure of the S1-S4 domain of KvAP (PDB accession code 1ORS) with the paddle motif colored purple. The outer four Arg residues in S4 are shown as stick representations with carbon colored yellow and nitrogen colored blue. Structural representations in all figures were generated using PyMOL (DeLano Scientific).

Figure 2. Structure of the paddle-chimera Kv channel

a) Ribbon representation of the X-ray structure of the paddle-chimera channel viewed from the external side of the membrane. The paddle motif is colored purple, the pore domain is colored yellow, lipids are colored teal and basic residues in S4 are shown as stick representations with carbon atoms colored yellow, oxygen atoms colored red and nitrogen atoms colored blue. PDB accession code is 2R9R. Red arrow identifies outer S4 Arg residues projecting towards the lipid membrane. b) Side view of the paddle-chimera channel focusing on the S1-S4 voltage-sensing domain and its interface with the pore domain.

Figure 3. Charged amino acids in S1-S4 voltage-sensing domains

Superposition of the structures of the S1-S4 domains of KvAP (yellow ribbon) and the paddle-chimera channel (white ribbon) shown as a stereo pair and viewed from the side. The structures were aligned to minimize deviations between the S1 and S2 helices . Basic and acidic residues are shown as stick representations with carbon colored yellow or white, oxygen colored red and nitrogen colored blue. The green arrow identifies a aqueous crevice within the S1-S4 domain that would be continuous with the external solution and the red arrow identifies outer S4 Arg residues projecting towards the lipid membrane. PDB accession code for S1-S4 of KvAP is 1ORS and that for the paddle-chimera is 2R9R. b) Superimposed structures viewed from the external side of the membrane.

Figure 4. Changes in charge interactions during movements of voltage sensors

The cartoons illustrate shifting interactions between positively charged Arg residues in S4 and either negatively charged phosphate lipid headgroups (orange) or acidic residues (red) in the external and internal acidic residue clusters. Cartoon to the left is for the activated state that occurs at positive membrane voltages and that on the right is for the resting state that occurs at negative voltages. The illustrated motions of helices are not meant to imply anything about the structural changes occurring during voltage sensor movement.

Figure 5. Inferring motions from accessibility and bridging experiments

a) Ribbon representation of the structure of the S1-S4 domain of KvAP showing accessibility of biotinylated positions to avidin with 9 and 10Å tethers . Positions marked by red spheres (α-carbon) are accessible to only external avidin, those marked by blue spheres are accessible to only internal avidin, and those marked by yellow spheres are accessible to either external or internal avidin. Black spheres mark inaccessible positions. The structure is of an activated voltage sensor, so yellow and blue positions located towards the external side of the membrane must move to within ∼10 Å of the internal side of the membrane (dashed line) in the resting state, as indicated by the black arrow. Basic and acidic residues are shown as stick representations. b) Ribbon representation of the structure of the S1-S4 domain of the paddle-chimera channel showing positions that form disulphide or metal bridges in the resting state . The structures shown in a and b are aligned as in Fig 3. In the Shaker Kv channel, Cys substituted at R362 in S4 (equivalent to R294 in the paddle-chimera; green sphere) can bridge with either I241C in S1 (I177 in the paddle-chimera; light pink sphere) or I287C in S2 (I230 in the paddle-chimera; magenta sphere) when the voltage sensor are in a resting state. c) Bridging positions from b viewed from the external side of the membrane.