The role of π-helices in TRP channel gating

Abstract

Transient Receptor Potential (TRP) channels are a large superfamily of polymodal ion channels, which perform important roles in numerous physiological processes. The architecture of their transmembrane (TM) domains closely resembles that of voltage-gated potassium channels (K_V). However, recent cryoEM and crystallographic studies of TRP channels have identified π-helices in functionally important regions, and it is increasingly recognized that they utilize a distinct mechanism of gating that relies on α-to-π secondary structure transitions. Here we review our current understanding of the role of π-helices in TRP channel function and their broader impact on different classes of ion channels.

Figure 1. Gating in tetrameric potassium and TRP ion channels.

(a) Potassium channels utilize a conserved glycine hinge to open the helix bundle gate. (b) TRP channels use a fundamentally different mechanism to open the helix bundle gate. The human TRPV3 channel possesses straight pore-lining S6 α-helices in its closed state. Upon sensitization, an α-to-π secondary structure transition occurs in S6, inducing a helical bend. This bend is utilized to achieve the open state. (c) CryoEM density maps (gray mesh) and refined models surrounding the pore of the closed (blue, left panel) and sensitized (orange, right panel) TRPV3 channel. The 3D reconstruction of the closed TRPV3 channel was resolved to 3.4 Å, and the map was sharpened using an inverse b factor of −73 Ų. The map was contoured at a level of 0.02 and radius 1.3. The 3D reconstruction of the sensitized TRPV3 channel was resolved to 3.2 Å and was sharpened using an inverse b factor of −51 Ų. The map was contoured at a level of 0.04 and radius 1.3. (d), An overlay of the coordinates and cryoEM density around the S6 helices from the closed TRPV3 (blue) and sensitized TRPV3 channels (gold). The N-terminal part of the S6 helices, which adopt an α-helical conformation, align well (black dotted line). However, the π-helical turn in the sensitized TRPV3 induces a bend in the S6 helix, causing it to diverge from the straight α-helical S6 (indicated by the blue dotted line). The map of the closed TRPV3 was contoured at level of 0.02, while the map of the sensitized TRPV3 was contoured at level of 0.04.

Figure 2. The helical structure in α and π helices.

(a) In an α helix, hydrogen bonding occurs between the backbone of residues that are set four amino acids apart in sequence (i+4). (b) A π helix possesses an additional amino acid (i+5), and results in a wider helical turn. This leaves a lone hydrogen donor and results in a helix-bending alteration of the torsion angles in the helical backbone. Close-up views in (a) and (b) are shown to the right of each figure. (c) A close-up of the S6 cryoEM density map in the closed (blue, left panel) and sensitized (gold, right panel) TRPV3 structures. The black dotted lines indicate H-bonding within the helices. The S6 of the closed TRPV3 is α-helical, while it adopts a π-helical turn in the sensitized structure. The cryoEM density map of the closed TRPV3 is contoured at a level of 0.02, and the map of the sensitized TRPV3 is contoured at a level of 0.04.

Figure 3. The involvement of π-helices in TRPV ligand binding.

(a) Top view of the tetrameric TRPV2 channel (PDB ID 5AN8); colored by chain. The channel adopts a domain-swap arrangement, with voltage-sensing like domains (VSLD) from one protomer interacting with the pore module of the neighboring protomer. (b) Side view of the transmembrane domain of a single TRPV2 protomer. (c) The resiniferatoxin (RTx) binding site in TRPV2. Left panel shows an overlay of the apo and RTx-bound TRPV2 structures. The middle and right panels show the apo (PDB ID 6BWM) and RTx-bound (PDB ID 6BWJ) structures, respectively. The S4–S5 linker π-helix is colored in red. (d) Ligand-induced, peristaltic-like movements of the S4–S5 linker π-helix along the junction between the S4–S5 linker and the S5 helix.

Figure 4. Overview of ion channels and membrane proteins that contain π-helices.

(a) Representative structures of members of TRP channel subfamilies that contain π-helices. TRPA1 (PDB 3J9P), TRPM4 (PDB 6BCJ), TRPC6 (PDB 5YX9), TRPML3 (PDB 5W3S), PKD2 (PDB 5K47) and NOMPC (PDB 5VKQ). For ease of viewing, two chains are removed, and only the transmembrane domains are shown. (b) Representative structures of non-TRP proteins that contain π-helices. IP₃R (PDB 6DRA), RyR (PBD 3J8H), Na_V1.7 (PDB 6J8H), TMEM16A (PDB 6BGI), TMEM16 scramblase (PDB 6E0H), and MurJ (PDB 6NC7). The shaded red boxes indicate the locations of the π-helices.