Structural biology of TRP channels

Abstract

Structural studies on TRP channels, while limited, are poised for a quickened pace and rapid expansion. As of yet, no high-resolution structure of a full length TRP channel exists, but low-resolution electron cryomicroscopy structures have been obtained for 4 TRP channels, and high-resolution NMR and X-ray crystal structures have been obtained for the cytoplasmic domains, including an atypical protein kinase domain, ankyrin repeats, coiled coil domains and a Ca(2+)-binding domain, of 6 TRP channels. These structures enhance our understanding of TRP channel assembly and regulation. Continued technical advances in structural approaches promise a bright outlook for TRP channel structural biology.

Fig. 1.1

TRP channel subfamilies and the transmembrane topology and domain organization of their subunits. Only commonly present and readily identifiable domains or motifs in the cytoplasmic N and C termini are indicated. Examples of high-resolution structures of some domains or motifs are presented

Fig. 1.2

Examples of membrane protein structures at different resolutions. (a) Side view (left) and top view (right) of a cryo-EM structure of the Drosophila Shaker K⁺ channel at 25 Å resolution, revealing a fourfold symmetry and a two-layered architecture [76]. (b) Side view of the structure of a monomer of aquaporin 1 obtained by 2D cryo-EM at 6 Å resolution, revealing 6 distinct tilted rods that correspond to membrane-spanning α helices [77]. (c) X-ray crystal structure of the rat K_v1.2 channel at 2.9 Å resolution (left, PDB code 2A79) [78] and (d) The electron density map and side chain assignment of the ion selectivity filter of a rat K_v1.2–K_v2.1 chimeric channel at 2.4 Å resolution (right, PDB code 2R9R) [79]

Fig. 1.3

TRP channel EM structures. (a) Cryo-EM structure of TRPV1 [21], superimposed with the crystal structure of the K_v1.2 transmembrane domains (maroon; PDB code 2A79) and of the ankyrin repeat domain of TRPV1 (green; PDB code 2PNN). (b) Cryo-EM structure of TRPV4 [22], superimposed with the crystal structure of Mlotik1 (top; PDB code 3BEH) and of the ankyrin repeat domain of TRPV1 (bottom). (c) EM structure of TRPM2 with negative staining [19]. (d) Cryo-EM structure of TRPC3 [20]. All structures are side-views. The white lines mark putative transmembrane regions, so do the blue lines, as presented in [20]. The resolutions of all four structures are based on the 0.5 cutoff criterion in the Fourier shell correlation

Fig. 1.4

Crystal structure of the TRPM7 α-kinase domain. (a) The dimeric structure of the TRPM7 α-kinase domain (PDB code 1IA9). (b) and (c) Comparison of the TRPM7 α-kinase domain (b, PDB code 1IA9) and PKA kinase domain (c, PDB code 1CDK). Regions and residues critical for catalysis are shown; they include the P-loop and the GXA(G)XXG motif in the TRPM7 α-kinase domain (b), and the P-loop, activation loop and catalytic loop in the PKA kinase domain (c) A bound zinc ion is present in the TRPM7 α-kinase domain, and a bound AMP-PNP, an ATP analog, is present in both structures

Fig. 1.5

Crystal structures of TRPV ankyrin repeat domains (ARDs). (a) Superposition of the structures of the ARD of TRPV1 (PDB code 2PNN), TRPV2(PDB code 2ETB), TRPV4 (PDB code 3JXI), and TRPV6 (PDB code 2RFA). AR, ankyrin repeat. (b) Structure of the TRPV1-ARD with a bound ATP molecule (red). Side-chains of residues involved in ATP binding are marked in blue. (c) Structure of the TRPV4-ARD, showing the position of the residues that cause diseases when mutated. Residues numbers correspond to human TRPV4

Fig. 1.6

Comparison of the crystal structure of a C-terminal coiled coil domain of TRPM7 (a, PDB code 3E7K) and TRPP2 (b, PDB code 3HRN). Side-chains of residues at the a and d positions of the heptad repeats are shown. Three cross sections, taken at the indicated positions, shows examples of the extensive van der Waals interactions among these residues

Fig. 1.7

NMR structure of an E-F hand in the TRPP2 C terminus. Side-chains of residues involved in Ca²⁺ binding are indicated. PDB code 2KQ6