Global versus local mechanisms of temperature sensing in ion channels

Abstract

Ion channels turn diverse types of inputs, ranging from neurotransmitters to physical forces, into electrical signals. Channel responses to ligands generally rely on binding to discrete sensor domains that are coupled to the portion of the channel responsible for ion permeation. By contrast, sensing physical cues such as voltage, pressure, and temperature arises from more varied mechanisms. Voltage is commonly sensed by a local, domain-based strategy, whereas the predominant paradigm for pressure sensing employs a global response in channel structure to membrane tension changes. Temperature sensing has been the most challenging response to understand and whether discrete sensor domains exist for pressure and temperature has been the subject of much investigation and debate. Recent exciting advances have uncovered discrete sensor modules for pressure and temperature in force-sensitive and thermal-sensitive ion channels, respectively. In particular, characterization of bacterial voltage-gated sodium channel (BacNa_V) thermal responses has identified a coiled-coil thermosensor that controls channel function through a temperature-dependent unfolding event. This coiled-coil thermosensor blueprint recurs in other temperature sensitive ion channels and thermosensitive proteins. Together with the identification of ion channel pressure sensing domains, these examples demonstrate that "local" domain-based solutions for sensing force and temperature exist and highlight the diversity of both global and local strategies that channels use to sense physical inputs. The modular nature of these newly discovered physical signal sensors provides opportunities to engineer novel pressure-sensitive and thermosensitive proteins and raises new questions about how such modular sensors may have evolved and empowered ion channel pores with new sensibilities.

Keywords: BacNav; Bacterial voltage gated sodium channel; Coiled-coil; Heat capacity; Ion channel; TRP channels; Temperature sensing; ΔCp.

Figure 1. Ion channel sensor design for ligands and physical forces

A, Examples of ligand sensors. Left, structural organization of a neurotransmitter ion channel (5KXI) [70]. Ligand sensor domain is blue. Channel domain is orange. The ligand, acetylcholine, and binding site are indicated. Right, structural organization of a protein-gated ion channel (4KFM)[105]. Sensor domain is red. Channel domain is orange. The ligand, the G protein Gβγ subunits, is shown (sand and lime green). Schematics show the general arrangement between the sensor and channel domains. B, Examples of force sensing ion channels. Top, Composite model of a BacNa_V voltage gated ion channel (4LTO[93], 3RVY[78]) [77]. Two of four voltage-sensor domains (yellow) are shown. S4 voltage sensor is purple. Lower left, Structure of the closed (2OAR)[96] and model of the open state [97] of the mechanosensitive channel MscL. In ‘A’ and ‘B’ modular sensor domains are indicated by the grey ovals. Lower right, The issue of how thermosensitive ion channels work is highlighted.

Figure 2. Exemplar protein stability curves

ΔG of unfolding is plotted as a function of temperature using the Gibbs-Helmholtz equation for exemplar proteins. Orange and magenta curves are for two mutants of the small domain protein GB1 using thermodynamic values from [68], ΔCp = 624 cal mol⁻¹ K⁻¹. Blue curve shows a protein having a ΔCp four times larger than GB1, ΔCp = 2400 cal mol⁻¹ K⁻¹. Temperatures of cold and heat induced unfolding are indicated. Regions of the plot favoring folded and unfolded forms are indicated.

Figure 3. The BacNa_V modular temperature sensor domain

A, Temperature-dependence of the activation of the BacNa_V Na_VSp1. B, V_1/2 temperature dependence of Na_VSp1, neck mutant Na_VSp1_GGG, and Na_VSp1_ΔCTD. Dashed boxes shown the VSD, pore domain (PD), Neck, and coiled-coil domain. C, BacNa_V CTD chimeras tune voltage-dependent gating of the Na_VSp1 transmembrane core. Inset shows the effects on Na_VSp1 and chimeras bearing CTDs from Na_VBh1 (NaChBac), Bacillus halodurans [88], Na_VSp1_BhCTD; Na_VMs, Magnetococcus sp. [65], Na_VSp1MsCTD; Na_VAb1, Alcanivorax borkumensis [94], Na_VSp1_Ab1CTD; and Na_VPz, Paracoccus zeaxanthinifaciens [55], Na_VSp1_PzCTD. Temperature dependence of V_1/2 and effects on neck disruption using a triple glycine mutant (GGG) for each channel are shown. All data are from [9]. Temperature dependence in ‘B’ and ‘C’ are shown as van’t Hoff style plots.

Figure 4. Coiled-coils in thermal sensitive ion channels

A, Structure of the Alkalilimnicola ehrlichii BacNa_V, Na_VAe1 CTD (5HK7) [9]. Sequence of the Neck and Coiled-coil domain is shown highlighting the “a”–“d” core residues. Cartoon shows the structure of the Na_VAe1 CTD highlighting the polar and hydrophobic cores of the Neck (sand) and Coiled-coil (orange) domains, respectively. Arrows indicate connection to the pore domain S6 helix. B, BacNa_V composite model (4LTO[93], 3RVY[78]) [77] highlighting two of four voltage-sensor domains (yellow) and the Neck thermal sensing domain (orange) along with a channel schematic indicating the placement of the modular sensor domains. C, Left, Interactions of the TRPA1 Helix-turn-helix (HTH1) (green) and Ankyrin repeats (AR)(brown) with the C-terminal coiled-coil domain (red)(3J9P) [75]. Positions of the buried hydrophilic residues in the coiled-coil are shown in space filling. Dashed lines indicate HTH1 and ANK repeat sites of contact with the coiled-coil. Connection to TRP domain and pore is indicated. Right, TRPA1 structure (3J9P) [75]. Transmembrane domains are blue. Front Ankryin repeat domain is not shown. Non-highlighted Ankryin repeat domains are grey. Coiled-coil is red. D, Left, Structure of the Hv1 coiled-coil (green) (3VMX) [41]. Buried asparagines are shown in space filling. Connection to VSD is indicated. Right, Hv1 structure based on a composite model from 3VMX and 3WKV[99]. Coiled-coil domain is boxed. VSD and S4 segment of the VSD are colored blue and magenta, respectively. Parallel grey lines in B–D indicate the membrane.