Structural Insights into Ca2+-dependent Regulation of Inositol 1,4,5-Trisphosphate Receptors by CaBP1

Calcium-binding protein 1 (CaBP1), a neuron-specific member of the calmodulin (CaM) superfamily, modulates Ca2+-dependent activity of inositol 1,4,5-trisphosphate receptors (InsP(3)Rs). Here we present NMR structures of CaBP1 in both Mg2+-bound and Ca2+-bound states and their structural interaction with InsP(3)Rs. CaBP1 contains four EF-hands in two separate domains. The N-domain consists of EF1 and EF2 in a closed conformation with Mg2+-bound at EF1. The C-domain binds Ca2+ at EF3 and EF4, and exhibits a Ca2+-induced closed to open transition like that of CaM. The Ca2+-bound C-domain contains exposed hydrophobic residues (Leu(132), His(134), Ile(141), Ile(144), and Val(148)) that may account for selective binding to InsP(3)Rs. Isothermal titration calorimetry analysis reveals a Ca2+-induced binding of the CaBP1 C-domain to the N-terminal region of InsP(3)R (residues 1-587), whereas CaM and the CaBP1 N-domain did not show appreciable binding. CaBP1 binding to InsP(3)Rs requires both the suppressor and ligand-binding core domains, but has no effect on InsP(3) binding to the receptor. We propose that CaBP1 may regulate Ca2+-dependent activity of InsP(3)Rs by promoting structural contacts between the suppressor and core domains.