Identification of a family of calcium sensors as protein ligands of inositol trisphosphate receptor Ca2+ release channels

The inositol trisphosphate (InsP(3)) receptor (InsP(3)R) is a ubiquitously expressed intracellular Ca2+ channel that mediates complex cytoplasmic Ca2+ signals, regulating diverse cellular processes, including synaptic plasticity. Activation of the InsP(3)R channel is normally thought to require binding of InsP(3) derived from receptor-mediated activation of phosphatidylinositol lipid hydrolysis. Here we identify a family of neuronal Ca2+-binding proteins as high-affinity protein agonists of the InsP(3)R, which bind to the channel and activate gating in the absence of InsP(3). CaBP/caldendrin, a subfamily of the EF-hand-containing neuronal calcium sensor family of calmodulin-related proteins, bind specifically to the InsP(3)-binding region of all three InsP(3)R channel isoforms with high affinity (K-a approximate to 25 nM) in a Ca2+-dependent manner (K-a approximate to 1 muM). Binding activates single-channel gating as efficaciously as InsP3, dependent on functional EF-hands in CaBP. In contrast, calmodulin neither bound with high affinity nor activated channel gating. CaBP1 and the type 1 InsP(3)R associate in rat whole brain and cerebellum lysates, and colocalize extensively in subcellular regions in cerebellar Purkinje neurons. Thus, InsP(3)R-mediated Ca2+ signaling in cells is possible even in the absence of InsP(3) generation, a process that may be particularly important in responding to and shaping changes in intracellular Ca2+ concentration by InsP(3)-independent pathways and for localizing InsP(3)-mediated Ca2+ signals to individual synapses.