Inositol 1,4,5-Trisphosphate Receptor

The specific type of inositol 1,4,5-trisphosphate receptor (InsP₃R) protein discussed here is the mouse type 1 InsP₃R, also called InsP₃R1. This polypeptide contains three major regions: the amino terminal inositol 1,4,5-trisphosphate (InsP₃) binding region, the central modulatory region, and the carboxy-terminus channel region.^[1] The protein forms an L-shaped structure composed of two asymmetric domains perpendicular to each other.^[1] The N-terminal domain is made up of 12 β-strands and 2 single-turn helices, which come together to form a barrel.^[1] The C-terminal end is quite different, consisting of a bundle made of eight α-helices.^[1] The interface of the two domains is lined with basic residues and forms the receptor site for InsP₃.^[1] The InsP₃R protein does not belong to a superfamily of proteins. The receptor is thought to span the membrane 6 times, leaving the C-terminus in the cytoplasm.^[2]

Domain Structure

The protein fold of the β-domain can also be called the β-trefoil. This element is present in other proteins as well, including fibroblast growth factors and mannose receptors.^[1] In the case of the InsP₃R β-trefoil, the structure was found to be very similar to the β-trefoil of the mannose receptor.^[1] In the β-domain of InsP₃R1, three of six two-stranded hairpins come together to form a barrel and the other three form a triangular cap for the barrel.^[1]

The α-domain of InsP₃R shows a high degree of homology with an element called an armidillo repeat fold found in proteins such as β-catenin and importins.^[1] In β-catenin and importins, the armadillo repeat functions as a motif for protein-protein interactions.^[1] Within the α-domain of mouse InsP₃R1, there are two large, highly conserved surfaces.^[1] Both regions are rich in aromatic residues, indicating that they may function as interaction sites for parts of the receptor or other cellular proteins.^[1] A possible option for this kind of binding domain would be the InsP₃ binding suppressor domain present at the N-terminus which reduces the binding affinity for the InsP₃ ligand.^[1]

The two domains of the inositol 1,4,5-trisphosphate receptor protein. The yellow ribbons represent the β-domain and the red helices represent the α-domain

Binding the InsP₃ Ligand: Mechanism and Structural Components

The InsP₃ sits between the two domains of the protein. Highly basic amino acid residues are present on both domains and are responsible for the binding of InsP₃ to InsP₃R.^[1] Since the InsP₃ ligand is highly charged, it is very likely to interact with the positively charged amino acids present in the N-terminus InsP₃-binding domain.^[2] In binding, water molecules are involved in hydrogen bonding between InsP₃ and its receptor as well as interactions between protein side chains and phosphorous.^[1] Coordination of phosphorous groups is mediated by residues in both the β-domain and α-domain. The hydroxyl groups of InsP₃ play a small role in binding to InsP₃.^[1] Additionally, 9 out of 12 Arg/Lys residues play a very important role in ligand binding and salt bridges to stabilize between the domain regions.^[1] The non-basic residues T266, T267, G268, and Y567 are also integral in InsP₃ coordination: if T267, G268 or Y567 residues are mutated then there will be a significant reduction in ligand binding.^[1] In all likelihood, the InsP₃-binding site has been found to be made up of multiple sequences present throughout the N-terminal area of the protein.^[2] This makes the tertiary structure of the protein and proper folding absolutely integral to the function: if the protein does not fold correctly, then the multiple sequences of the protein making up the binding region cannot come together to be at all functional in binding the InsP₃ ligand.