Figure 1. Domain organization of InsP₃ receptor and InsP₃ receptor intermolecular interactions.

A, Domain organization of the InsP₃ receptor monomer, depicting membrane topology and cytoplasmic and lumenal protein orientation. Inositol 1,4,5-trisphosphate (InsP₃; oval) activates Ca²⁺ release via binding to an N-terminal region. Arrow denotes location of previously proposed ankyrin-binding motif in C-terminal domain of InsP₃ receptor (see Figure 1B). Note that these residues (rat 2546–2558) are located in the ER/SR lumen, inconsistent with an interaction with cytosolic ankyrin. B, Ankyrin-binding site in InsP₃ receptor identified by sequence similarity with CD44 ankyrin-binding motif. Minimal ankyrin-binding residues on CD44 [Bourguignon and Jin, 1995]. Amino acid sequences below CD44 denote sequence homology of residues in InsP₃ receptor C-terminus to the CD44 sequence.