A primer on ankyrin repeat function in TRP channels and beyond

Abstract

Transient receptor potential (TRP) channels are rapidly gaining attention as important receptors and transducers of diverse sensory and environmental cues. Recent progress in the field has provided new insights into the structure and function of the ankyrin repeat motifs present in the N-terminal cytosolic domain of many TRP channels. The topics addressed in this Highlight include the structural features of canonical ankyrin repeats, new clues into the functions these repeats perform in cells, and how this information can be applied to develop further experiments on TRP channels and other proteins containing ankyrin repeats.

Fig. 1

Structure of ankyrin repeats. (A) Consensus ankyrin repeat sequence motif, with the resulting secondary structure above, and the repeat position number underneath. Consensus residues form the hydrophobic core as multiple repeats stack to form a domain, as well as the twists and turns through residues such as glycine and proline, leaving the protein surface highly variable. Red dots represent variable residues in DARPin libraries. (B) Ribbon diagram of the six ankyrin repeats of TRPV1 with an N- to C-terminal rainbow coloring. The inner helices and fingers form the concave palm surface, while the outer helices form a convex surface. (C) The shape of an ankyrin repeat domain is often likened to a hand, and analogously, the palm and fingers are the most commonly used interaction surfaces. (D–E) Structure of a five-repeat DARPin, with the variable residues in red, represented as a ribbon (D) or surface (E).

Fig. 2

Function of ankyrin repeats: beyond protein-protein interaction. (A) Structure of Gankyrin bound to the S6 ATPase of the 26S proteasome (grey). (B) Structure of the ankyrin repeats of RNase L bound to an oligoadenylate trimer (spheres). (C) Structure of the ankyrin repeats of TRPV1 bound to ATP (spheres). (D) Model of a 24-repeat domain, such as that of ankyrin, with the arrows indicating the direction of the possible spring (based on ref. 49). All ankyrin repeat domains have an N- to C-terminal rainbow coloring.

Fig. 3

The ankyrin repeats of TRPA1. (A) Sequence of the ankyrin repeats of human TRPA1 (accession number O75762) aligned against the ankyrin repeat consensus (top). Similar and identical residues are shaded grey and black, respectively. The EF-hand-like sequence motif is boxed. Colored cysteines are important in activation of TRPA1 by electrophiles as identified by the Patapoutian (red) or Julius group (cyan) or both (magenta). (B) The structure of four canonical ankyrin repeats is used to illustrate the position of important TRPA1 cysteines either in internal repeat 11 (red) or terminal repeat 17 (magenta and cyan). The cysteines are colored as in (B). (C) The structure of five canonical ankyrin repeats is used to illustrate the position of the EF-hand-like sequence motif in repeat 12 of TRPA1. Positions 467 and 481, which are adjacent to highly conserved ankyrin repeat residues, and therefore predicted to closely match this canonical structure, are 25.6 Å apart (dotted line). Note that the finger loop of TRPA1 repeat 12 contains three additional residues compared to the canonical finger loop illustrated here. (D) The structure of calcium-bound calmodulin (PDB ID Code 3CLN) with four calcium ions in green and the first EF-hand motif colored gold. The black dotted line represents a 9.1 Å distance between residues 20 and 34 (10.7 Å separate these residues in apo-calmodulin; PDB ID Code 1DMO). (E) The sequences of TRPA1 and calmodulin are aligned at their EF-hand sequence motifs (box), along with the corresponding EF-hand and ankyrin repeat consensus sequences and secondary structures (ss; L = loop, H = helix, S = sheet). In the EF-hand consensus, capital letters indicate conserved negatively charged residues; small letters, residues that fit the consensus; x, any residue; and Φ, hydrophobic residues. Segments in yellow are illustrated in parts C and D, and residues in green provide coordinating groups to the calcium in an EF-hand structure.