Ca2+-independent inhibition of inositol trisphosphate receptors by calmodulin: Redistribution of calmodulin as a possible means of regulating Ca2+ mobilization

摘要

The interactions between calmodulin, inositol 1,4,5-trisphosphate (InsP3), and pure cerebellar InsP3 receptors were characterized by using a scintillation proximity assay. In the absence of Ca²⁺, ¹²⁵I-labeled calmodulin reversibly bound to multiple sites on InsP3 receptors and Ca²⁺ increased the binding by 190% ± 10%; the half-maximal effect occurred when the Ca²⁺ concentration was 184 ± 14 nM. In the absence of Ca²⁺, calmodulin caused a reversible, concentration-dependent (IC50 = 3.1 ± 0.2 μM) inhibition of [³H]InsP3 binding by decreasing the affinity of the receptor for InsP3. This effect was similar at all Ca²⁺ concentrations, indicating that the site through which calmodulin inhibits InsP3 binding has similar affinities for calmodulin and Ca²⁺-calmodulin. Calmodulin (10 μM) inhibited the Ca²⁺ release from cerebellar microsomes evoked by submaximal, but not by maximal, concentrations of InsP3. Tonic inhibition of InsP3 receptors by the high concentrations of calmodulin within cerebellar Purkinje cells may account for their relative insensitivity to InsP3 and limit spontaneous activation of InsP3 receptors in the dendritic spines. Inhibition of InsP3 receptors by calmodulin at all cytosolic Ca²⁺ concentrations, together with the known redistribution of neuronal calmodulin evoked by protein kinases and Ca²⁺, suggests that calmodulin may also allow both feedback control of InsP3 receptors and integration of inputs from other signaling pathways.