Structural insights into the gating mechanisms of TRPV channels

Abstract

Transient Receptor Potential channels from the vanilloid subfamily (TRPV) are a group of cation channels modulated by a variety of endogenous stimuli as well as a range of natural and synthetic compounds. Their roles in human health make them of keen interest, particularly from a pharmacological perspective. However, despite this interest, the complexity of these channels has made it difficult to obtain high resolution structures until recently. With the cryo-EM resolution revolution, TRPV channel structural biology has blossomed to produce dozens of structures, covering every TRPV family member and a variety of approaches to examining channel modulation. Here, we review all currently available TRPV structures and the mechanistic insights into gating that they reveal.

Keywords: Cryo-EM; Ion channels; TRP channels; TRPV; X-ray crystallography.

Figure 1.

Domain architecture and 3-dimensional organization of TRPV channels. (a) Cartoon representation of a TRPV monomer. The N-terminal domain (NTD) is depicted in grey, the ankyrin repeat domain (ARD) in yellow, the N-linker in orange, the pre-S1 helix in pink, the S1-S4 helices in magenta, the S4-S5 linker in purple, S5-PH-S6 in purple, the TRP helix in blue, and the C-terminal domain (CTD) in green. (b) The same domains colored on a monomer of a representative TRPV channel (TRPV5, PDB 6DMR). (c) The tetrameric assembly of a representative TRPV channel (TRPV5, PDB 6DMR) from the side (left) and from the extracellular face (right), with helices depicted as tubes.

Figure 2.

Pore diagrams of TRPV1-4. (a) rTRPV1_mi closed (PDB 5IRZ). (b) rTRPV1_mi open (PDB 5IRX). (c) rbTRPV2_mi closed (PDB 5AN8). (d) rTRPV2 semi-open (PDB 6U86). (e) mTRPV3 closed (PDB 6DVW). (f) mTRPV3_Y564A open with 2-APB (PDB 6DVZ). (g) hTRPV3_T96A sensitized (PDB 6MHS). (h) xTRPV4_miN647Q closed (PDB 6BBJ). Residues of interest depicted as sticks.

Figure 3.

Pore diagrams of TRPV5-6. (a) rbTRPV5 closed (PDB 6DMR). (b) rbTRPV5 open (PDB 6DMU). (c) rTRPV6* closed (PDB 5WO7). (d) rTRPV6 open (PDB 6BOB). Residues of interest depicted as sticks.

Figure 4.

TRPV mechanisms revealed by structural biology. (a) All open TRPV structures determined so far have a π-helix (purple) in S6 (green). Some of these channels already have a π-helix in their apo states, which acts as a hinge allowing the lower portion of S6 to open. Other channels have a totally α-helical S6 in their apo states (grey), which requires the lower portion of S6 to rotate by ~100° to form the π-helix. TRPV3 has been captured in a sensitized state (yellow) where the channel has transitioned from an α- to π-helix in S6, but remains closed, (b) The CTD/NTD switch model for proposed by the Lee group and seen in TRPV2 and TRPV3. The ‘off’ state sees the CTD on one monomer (pink) wrapped around the beta-sheet region and making contacts with the ARD of an adjacent monomer (grey). The ‘on’ state has the CTD form a short helix on the interior of the ARD skirt (dark pink), while the NTD of the adjacent monomer (grey) occupies the CTD vacated. The interaction of the helical CTD (dark pink) with the interior of the adjacent ARD (grey) may contribute to channel opening. (c) At low Ca²⁺ levels, only the C-Lobe of CaM (orange) is Ca²⁺ (yellow) bound and thought to be persistently bound to the distal CTD (blue) of TRPV5 and TRPV6. When Ca²⁺ levels increase, the N-Lobe (orange) binds and is activated by Ca²⁺ and can then in turn bind to the proximal CTD (light blue), bringing the C-Lobe in close enough to block the bottom of the pore.