Ca~2+-independent inhibition of inositol trisphosphate receptors by calmodulin: Redistribution of calmodulin as a possible means of regulating Ca~2+ mobilization

摘要

The interactions between calmodulin, inosi- tol 1,4,5-trisphosphate (InsP_3), and pure cerebellar InsP_3 receptors were characterized by using a scintillation proximity assay. In the absence of Ca~2+, ~125I-labeled calmodulin revers- ibly bound to multiple sites on InsP_3 receptors and Ca~2+ increased the binding by 190/100 + - 10/100; the half-maximal effect occurred when the Ca~2+ concentration was 184 + - 14 nM. In the absence of Ca~2+, calmodulin caused a reversible, concen- tration-dependent (IC_50 = 3.1 + - 0.2 μM) inhibition of [~3H]InsP_3 binding by decreasing the affinity of the receptor for InsP_3. This effect was similar at all Ca~2+ concentrations, indicating that the site through which calmodulin inhibits InsP_3 binding has similar affinities for calmodulin and Ca~2+- calmodulin.