Identification of residues participating in the interaction between an intraluminal loop of inositol 1,4,5-trisphosphate receptor and a conserved N-terminal region of chromogranin B

The inositol 1,4,5-trisphosphate receptor (IP3R) is a membrane channel that conducts calcium ions from the intracellular calcium stores. Despite a wealth of information on the cytoplasmic regulation of the IP3R, little is known about its regulation on the luminal side of the calcium stores. Here, we report studies on the IP3R intraluminal loop L3-2 and a conserved N-terminal region of chromogranin B. The IP3R loop is an important part of the channel's pore-forming region, and the chromogranin peptide has been shown to competitively inhibit calcium signaling by IP3R. Using the NMR titration approach, we showed that a part of the L3-2 is involved in a specific interaction with the chromogranin B peptide. Further NMR resonance assignments revealed that the 14th–20th residues of L3-2 are the keys to the binding to the chromogranin B peptide. Through detailed analysis of the data, we suggest a mechanism of IP3R regulation by chromogranin B involving conformational exchanges of the L3-2 region. Our report presents the findings of the first study on the interaction between the luminal loop of the IP3 receptor and its regulator at residue-resolution. The approaches described here should help to guide further studies on the interactions between the IP3R and other luminal side regulators.