Structural mechanisms of transient receptor potential ion channels

Abstract

Transient receptor potential (TRP) ion channels are evolutionarily ancient sensory proteins that detect and integrate a wide range of physical and chemical stimuli. TRP channels are fundamental for numerous biological processes and are therefore associated with a multitude of inherited and acquired human disorders. In contrast to many other major ion channel families, high-resolution structures of TRP channels were not available before 2013. Remarkably, however, the subsequent "resolution revolution" in cryo-EM has led to an explosion of TRP structures in the last few years. These structures have confirmed that TRP channels assemble as tetramers and resemble voltage-gated ion channels in their overall architecture. But beyond the relatively conserved transmembrane core embedded within the lipid bilayer, each TRP subtype appears to be endowed with a unique set of soluble domains that may confer diverse regulatory mechanisms. Importantly, TRP channel TR structures have revealed sites and mechanisms of action of numerous synthetic and natural compounds, as well as those for endogenous ligands such as lipids, Ca2+, and calmodulin. Here, I discuss these recent findings with a particular focus on the conserved transmembrane region and how these structures may help to rationally target this important class of ion channels for the treatment of numerous human conditions.

Figure 1.

Representative structures of each of the seven TRP subfamilies. TRP channels are shown in ribbon diagrams viewed parallel to the membrane, with each subunit color-coded differently. Note, TRP channels are decorated with unique soluble domains outside the relatively conserved transmembrane core structure embedded within the lipid bilayer delimited with two black horizontal lines. PDB accession nos. are as follows: TRPV1 (3J5P), TRPA1 (3J9P), TRPM2 (6MIX), TRPC3 (6CUD), no mechanoreceptor potential C (NOMPC; 5VKQ), PKD2 (5T4D), and TRPML1 (5WJ5).

Figure 2.

Determination of TRPV1 structures by single-particle cryo-EM. (A) The 3.4 Å map of TRPV1 determined by single-particle cryo-EM is shown in a side view (left) and a cytoplasmic view (right). (B) The “resolution revolution” in cryo-EM has led to a recent explosion of structures deposited at EMDB. The plot is available at https://www.ebi.ac.uk/pdbe/emdb/statistics_main.html/.

Figure 3.

Chemistry and size of the selectivity filter of representative TRP channels and VGICs. TRP channel selectivity filters are diverse in both size and constituent residues. The diameters measured from two narrowest diagonally opposed oxygen atoms are shown at the top. The selectivity filters of Ca²⁺-selective Ca_vAb and K⁺-selective K_v2.1 are also shown for comparison. Ions in TRPV6 (Ba²⁺), Ca_vAb (Ca²⁺), and K_v2.1 (K⁺) are shown as purple spheres. PDB accession nos. are as follows: TRPV1-Apo (3J5P), TRPV1-DkTx (3J5R), TRPM4 (6BQR), TRPV4 (6BBJ), TRPV6 (5IWR), Ca_vAb (5KLB), and K_v2.1 (2R9R).

Figure 4.

Structural elements that confer conformational flexibility to the pore lining S6 helix in K⁺ and TRPV1 channels. (A) Gating hinges in KcsA (Gly99; red) and K_v2.1 (Pro-Val-Pro; shown in sticks) allow for splaying open of their activation gates. (B) A π-helix bulge, highlighted in sticks, likely provides a flexing point around which the distal S6 helix can rotate and/or bend to open the lower gate in TRPV1. PDB accession nos. are as follows: KcsA (5VKE), Kv2.1 (2R9R), and TRPV1 (3J5R).

Figure 5.

Association of the S1–S4 domain with the pore domain in representative TRP and K_v channels. TRP channel voltage-sensor like domain (VSLD; cyan) typically associates with pore domain from an adjacent subunit (orange; red) via a larger interface and thus remains relatively static during channel gating as compared with VGICs. The potential hydrophobic interactions between the S3 and S6 helices in TRPV4 are highlighted with sticks. PDB accession nos. are as follows: Kv2.1 (2R9R), TRPM4 (6BQR), TRPML1 (5WJ5), and TRPV4 (6BBJ).

Figure 6.

Receptor sites for animal toxins targeting TRPV1 and VGICs. (A) The spider DkTx toxin latches onto the outer pore region of TRPV1 and stabilizes an open pore conformation. (B) Dc1a wedges into a cleft between the VSD and pore domains of Na_v and impedes VSD movement. (C) Charybodotoxin inserts a lysine side chain into the extracellular entrance to the selectivity filter of K_v and physically plugs the entryway of ion permeation. PDB accession nos. are as follows: TRPV1/DkTx (5IRX), NavPaS (6A90), and K_v2.1/charybodotoxin (4JTC).

Figure 7.

Structures of thermo-sensitive TRP channels. (A) Two diagonally apposed TRPV1 subunits are shown in ribbon diagram, with regions proposed to undergo heat-evoked conformational changes highlighted with red. The phosphoinositide lipid predicted to be ejected from the vanilloid pocket upon heating is shown in sticks. (B) Comparison of structures of warm-activated TRPM2 and cold-activated TRPM8. Only the transmembrane region of one subunit is shown here. Unlike other TRP channels, TRPM8 lacks the helical S4–S5 linker and a structurally defined selectivity filter in a closed state, hinting at structural flexibility of these structural elements. PDB accession nos. are as follows: TRPV1 (5IRX), TRPM2 (6MIX), and TRPM8 (6BPQ).