CaBP1, a neuronal Ca 2 + sensor protein, inhibits inositol trisphosphate receptors by clamping intersubunit interactions

Calcium-binding protein 1 (CaBP1) is a neuron-specific member of the calmodulin superfamily that regulates several Ca(2+) channels, including inositol 1,4,5-trisphosphate receptors (InsP3Rs). CaBP1 alone does not affect InsP3R activity, but it inhibits InsP3-evoked Ca(2+) release by slowing the rate of InsP3R opening. The inhibition is enhanced by Ca(2+) binding to both the InsP3R and CaBP1. CaBP1 binds via its C lobe to the cytosolic N-terminal region (NT; residues 1-604) of InsP3R1. NMR paramagnetic relaxation enhancement analysis demonstrates that a cluster of hydrophobic residues (V101, L104, and V162) within the C lobe of CaBP1 that are exposed after Ca(2+) binding interact with a complementary cluster of hydrophobic residues (L302, I364, and L393) in the β-domain of the InsP3-binding core. These residues are essential for CaBP1 binding to the NT and for inhibition of InsP3R activity by CaBP1. Docking analyses and paramagnetic relaxation enhancement structural restraints suggest that CaBP1 forms an extended tetrameric turret attached by the tetrameric NT to the cytosolic vestibule of the InsP3R pore. InsP3 activates InsP3Rs by initiating conformational changes that lead to disruption of an intersubunit interaction between a "hot-spot" loop in the suppressor domain (residues 1-223) and the InsP3-binding core β-domain. Targeted cross-linking of residues that contribute to this interface show that InsP3 attenuates cross-linking, whereas CaBP1 promotes it. We conclude that CaBP1 inhibits InsP3R activity by restricting the intersubunit movements that initiate gating.