Structural biology of TRP channels

Abstract

Membrane proteins remain challenging targets for structural biologists, despite recent technical developments regarding sample preparation and structure determination. We review recent progress towards a structural understanding of TRP channels and the techniques used to that end. We discuss available low-resolution structures from electron microscopy (EM), X-ray crystallography, and nuclear magnetic resonance (NMR) and review the resulting insights into TRP channel function for various subfamily members. The recent high-resolution structure of TRPV1 is discussed in more detail in Chapter 11. We also consider the opportunities and challenges of using the accumulating structural information on TRPs and homologous proteins for deducing full-length structures of different TRP channel subfamilies, such as building homology models. Finally, we close by summarizing the outlook of the "holy grail" of understanding in atomic detail the diverse functions of TRP channels.

Fig. 1

Structures of representative ion channels. Structures of TRPV1 (pdb: 3j5p), and other ion channels, such as the Kv1.2/2.1 chimera (pdb: 2r9r) or the GIRK channel in complex with PIP₂ (pdb: 3sya) have been determined at high resolution. BK channels (pdb: 3naf), like TRP channels are only moderately voltage dependent and their function is modulated by large cytoplasmic domains. The TRPV1 construct used to determine the structure has been engineered to remove parts of the N- and C-termini (residues 1–109 and 765–838, respectively) and of the extracellular turret (residues 604–626).

Fig. 2

Schematic overview of TRP channel domain organization. The domain architecture of N- and C-termini, which differs widely across TRP channel subfamilies, is depicted schematically with approximate size scales. TRPML proteins contain a lipase domain between transmembrane helix S1 and S2 (LaPlante et al., 2011). Domains with available high-resolution structural data are grey.

Fig. 3

Overview of spatial organization of TRPM6/M7 channels and TRPP/PKD complexes. (A) For TRPM6/M7, transmembrane helices S1–S6 are drawn indicating possible 3D topology of the tetramer in the membrane (based on Kv channel topology). For cytoplasmic domains, the antiparallel coiled coil (pdb: 3e7k) and α-kinase domain (pdb: 1ia9) of TRPM7 are shown; no structure is available for the TRPM homology region (MHR). The coiled coil crystallized as a tetramer, and one antiparallel pair is colored. Only one dimer of the α-kinase is shown for clarity. (B) For TRPP/PKC complexes, the three TRPP2/P3 subunits are depicted in dark grey and the 11-transmembrane helix PKD1/PKD1L3 subunit in light grey. One extracellular PKD repeat of PKD1 is depicted (pdb: 1b4r). For TRPP2, the intracellular EF hand (pdb: 2kq6) as well as the parallel coiled coil trimer (pdb: 2hrn) are shown.

Fig. 4

Crystal structure of the human TRPV4 ARD shown in cartoon and surface representations from three angles (pdb: 4dx1). Locations of disease-causing mutations are mapped onto the structure (spheres). Residues described in patients suffering from skeletal dysplasia and arthropathy are predominantly found on the “front” or “palm” surface, and those causing peripheral neuropathies localize preferentially to the “back”.

Fig. 5

Crystal structure of the C-terminal fragment of TRPV1 in complex with Ca²⁺-calmodulin (pdb: 3sui). The fully Ca²⁺-loaded CaM tightly winds around the TRPV1 peptide, forming an antiparallel complex where the CaM N-lobe interacts with the TRPV1 peptide’s C-terminus and the C-lobe with the N-terminus. Each EF hand is in a different shade.