A structural overview of the ion channels of the TRPM family

Abstract

The TRPM (transient receptor potential melastatin) family belongs to the superfamily of TRP cation channels. The TRPM subfamily is composed of eight members that are involved in diverse biological functions such as temperature sensing, inflammation, insulin secretion, and redox sensing. Since the first cloning of TRPM1 in 1998, tremendous progress has been made uncovering the function, structure, and pharmacology of this family. Complete structures of TRPM2, TRPM4, and TRPM8, as well as a partial structure of TRPM7, have been determined by cryo-EM, providing insights into their channel assembly, ion permeation, gating mechanisms, and structural pharmacology. Here we summarize the current knowledge about channel structure, emphasizing general features and principles of the structure of TRPM channels discovered since 2017. We also discuss some of the key unresolved issues in the field, including the molecular mechanisms underlying voltage and temperature dependence, as well as the functions of the TRPM channels' C-terminal domains.

Keywords: Gating mechanism; Structures; TRPM channels.

Figure 1:. Family tree and domain organization of TRPM family.

a, Domain organization of a monomer of the human TRPM family; the C-terminal domain (CTD) differs among family members. The colors of TMD refer to Figure 6. b, The relatedness of the human TRPM family members.

Figure 2:. Comparison of the tetrameric architecture and single subunits of representative TRPM channels.

a-d, Overall structures of (a) hsTRPM2, (b) hsTRPM4, (c) mmTRPM7, and (d) faTRPM8 viewed parallel to the membrane. The mmTRPM7 structure is not full length; the kinase domain is truncated. (e-h) Single subunits viewed parallel to the membrane plane, with secondary structure elements labeled in panel e. Ligands are shown as spheres: ADPR1 in green; ADPR2 in blue; Ca²⁺ in black; DVT1 in orange; DVT2 in green; icilin in orange; and PIP₂ in yellow.

Figure 3:. Ligand-binding sites of hsTRPM2, hsTRPM4, and faTRPM8.

The locations of available ligand-binding sites are boxed. Ca²⁺ is shown as black spheres, while other ligands and key residues involved in ligand binding are shown as sticks. The binding site of Ca²⁺ in (d) hsTRPM2, (e) hsTRPM4 and (f) faTRPM8. The hsTRPM2 binding sites of (g) ADPR1 in the MHR1/2 domain, (j) 8-Br-cADPR in the MHR1/2 domain, and (m) ADPR2 in the NUDT9-H domain. The binding sites in hsTRPM4 of (h) DVT1 at the kink of the rib helix and coiled-coil pole and of (k) DVT2 and (n) ATP at the interface of MHR1/2 and MHR3/4. The faTRPM8 binding sites of (i) icilin, (l) the menthol analog WS-12, and (o) PIP₂ in the TMD. Please see the report by Diver et al. for the AMTB and TC-I 2014 binding sites; the figures in this review were prepared before that report was out.

Figure 4:. Comparison of ion-conducting pores.

The shape and size of the ion-conducting pore of (a) EDTA-drTRPM2, (b) ADPR/Ca²⁺-drTRPM2, (c) Ca²⁺-nvTRPM2, (d) Ca²⁺/DVT-hsTRPM4, and (e) mmTRPM7 (not full length). The side chains of restriction residues are shown as sticks. Purple, green, and red spheres define pore radii of > 2.3 Å, 1.2–2.3 Å, and < 1.2 Å, respectively. (f), Pore radius as a function of distance along the pore axis.

Figure 5:. Gating mechanism of the voltage-independent TRPM2 channel.

Structures of the (a) EDTA-drTRPM2 and (b) ADPR/Ca²⁺-drTRPM2. Comparison of the (c) NUDT9-H domains and the (d) MHR1/2 domains of EDTA-hsTRPM2 and ADPR/Ca²⁺-hsTRPM2 by superimposition of the cap regions or the MHR1 domains. ADPR1 and ADPR2 are shown as green and blue spheres. The domain closure induced by ADPR binding in NUDT9-H and MHR1/2 is indicated. (e, f), Superimposition of the linker layers of EDTA-hsTRPM2 (blue) and ADPR/Ca²⁺-hsTRPM2 (red) by aligning the CTD coiled-coil poles. (g, h), Detailed view of the interaction between MHR4 and the TRP helix. The movement of the S4-S5 linker is indicated and the Trp1078 of TRP helix is shown in sticks. (i, j), Superimposition of EDTA-drTRPM2 (blue) and ADPR/Ca²⁺-drTRPM2 (red) structures by aligning the S1-S4 domains. (i), The conformational changes of S2-S3 linker and S3 upon Ca²⁺ binding are indicated. (j), The flipping of S4-S5 linker upon channel opening, viewed from the intracellular side, is indicated.

Figure 6:. Schematic of ligand sensing and the activation mechanism of TRPM2.

Conformational changes of TRPM2 upon ligand binding are shown by arrows. 8-Br-cADPR binds only to the MHR1/2 domain and inhibits the TRPM2 channel by stabilizing the MHR1/2 domain in apo-like conformation. Channel activation requires both Ca²⁺ and ADPR; the binding of Ca²⁺ or ADPRs lone is not sufficient to open the channel. The simultaneous binding of two ADPRs and Ca²⁺ in three indispensable binding sites open the TRPM2 channel. ADPR1 bound in the MHR1/2 domain shows a “U” shape, while ADPR2 bound in the NUDT9-H domain is in an extended shape.