Identification of a family of calcium sensors as protein ligands of inositol trisphosphate receptor Ca 2+ release channels

Abstract

The inositol trisphosphate (Ins P 3 ) receptor (Ins P 3 R) is a ubiquitously expressed intracellular Ca 2+ channel that mediates complex cytoplasmic Ca 2+ signals, regulating diverse cellular processes, including synaptic plasticity. Activation of the Ins P 3 R channel is normally thought to require binding of Ins P 3 derived from receptor-mediated activation of phosphatidylinositol lipid hydrolysis. Here we identify a family of neuronal Ca 2+ -binding proteins as high-affinity protein agonists of the Ins P 3 R, which bind to the channel and activate gating in the absence of Ins P 3 . CaBP/caldendrin, a subfamily of the EF-hand-containing neuronal calcium sensor family of calmodulin-related proteins, bind specifically to the Ins P 3 -binding region of all three Ins P 3 R channel isoforms with high affinity (K a ≈ 25 nM) in a Ca 2+ -dependent manner (K a ≈ 1 μM). Binding activates single-channel gating as efficaciously as Ins P 3 , dependent on functional EF-hands in CaBP. In contrast, calmodulin neither bound with high affinity nor activated channel gating. CaBP1 and the type 1 Ins P 3 R associate in rat whole brain and cerebellum lysates, and colocalize extensively in subcellular regions in cerebellar Purkinje neurons. Thus, Ins P 3 R-mediated Ca 2+ signaling in cells is possible even in the absence of Ins P 3 generation, a process that may be particularly important in responding to and shaping changes in intracellular Ca 2+ concentration by Ins P 3 -independent pathways and for localizing Ins P 3 -mediated Ca 2+ signals to individual synapses.