From an armadillo to electricity

Electricity drives more than just engines. It is primarily a fundamental feature of life. From single cell organisms to neurons, electrical gradients are used to store energy chemically (as in nucleotide triphosphates, NTPs) or to generate ion fluxes, thus allowing extracellular signals to rapidly transfer from one part of our body (or cell) to another. Ca²⁺ ions play an important role in these forms of cellular signaling and in regulating thousands of downstream targets. One of the molecules that links Ca²⁺ to the function of NTPs in the transduction of extracellular signals, such as growth factors and neurotransmitters, is the intracellular messenger inositol 1,4,5-triphosphate (InsP₃). Generated by the enzymatic action of an activated phospholipase on a precursor, InsP₃ regulates the release of intracellular Ca²⁺ stores by binding the InsP₃ receptor on the surface of the endoplasmic reticulum (ER). Despites decades of research on the ligand and its receptor, a structure had remained elusive.

In a recent issue of Nature, Bosanac et al. (Nature 420, 696–700; 2002) report the crystal structure of a fragment of the InsP₃ receptor corresponding to the InsP₃-binding domain. The 381-residue ligand-binding domain is located toward the N terminus of the protein and constitutes a small portion of the 2,749-residue InsP₃ receptor. Other structural elements in the receptor moderate the signal transduction process, form a channel for Ca²⁺ release from the ER and reduce InsP₃ binding affinity from ∼90 pM to ∼45 nM. The structure of the high-affinity, core InsP₃-binding domain in complex with InsP₃ is bipartite (pink and white surface), with an N-terminal region consisting of 12 β-strands and 2 short helices and a C-terminal domain with 8 α-helices. Structural comparisons reveal that the N-terminal region adopts the β-trefoil fold (pink), found in some interleukins and in FGF, whereas the C-terminal domain displays high homology to the armadillo repeat (white) of β-catenin and importin, among others. At the interface of the armadillo and β-trefoil folds lies an InsP₃-binding cleft lined with basic residues (blue). The conformation of the InsP₃ molecule (stick model) differs in other phosphoinositide-binding proteins, such as the pleckstrin homology and Tubby domains, suggesting that this ligand can be recognized differently.

Nine Arg/Lys residues (blue) coordinate the InsP₃ ligand and mediate critical interdomain interactions in the InsP₃-binding domain. An additional group of uncharged threonine, glycine and tyrosine residues (green) also engages in ligand recognition. Further, the structure identifies the location of two likely Ca²⁺ ion coordination sites on the β-trefoil–armadillo repeat InsP₃-binding domain. Receptor point mutations that eliminate Ca²⁺ binding map to two distinct positions on the domain. The authors hypothesize that the binding of InsP₃ to the receptor may facilitate Ca²⁺ binding by relieving the inhibitory function of its N-terminal suppression domain.

This study represents an important milestone in our understanding of these key signal transduction molecules. What it does not yet reveal is precisely how InsP₃ causes Ca²⁺ to be released and how ligand binding in the InsP₃ receptor appears to be regulated by other receptor domains. No doubt, these questions will soon be answered and increase our molecular understanding of this crucial electrical switch.