Ligand-binding affinity of the type 1 and 2 inositol 1,4,5-trisphosphate receptors: effect of the membrane environment

The inositol 1,4,5-trisphosphate (InsP 3) receptor is essential for Ca 2+ release from intracellular stores. There are three InsP 3 receptor types which are targets for several types of regulation. Ca 2+, phosphorylation, and protein–protein interactions may contribute to the complex pattern of the Ca 2+ signal in stimulated cells. Furthermore, the 3 receptor types could have different affinities for InsP 3. We compared the affinities of the type 1 receptor from the cerebellum with the liver type 2 receptor both in their membrane environment and after isolation by immunoprecipitation. Measurements of [ 3H]InsP 3 binding in a cytosol-like medium revealed that the K d of the liver receptor (45 ± 5 nM, N = 14) was higher than the K d of the cerebellar receptor (28 ± 3 nM, N = 9). Solubilization and immunopurification of the liver InsP 3 receptor resulted in a 10-fold increase in its affinity for InsP 3. The affinity of the cerebellar receptor did not change under these conditions. Therefore, the extraction of the liver and the cerebellar receptors from their membrane environments induced an inversion of their relative affinities. Treatment of liver membranes with low concentrations of detergents also increased the affinity for InsP 3 binding. These data indicate that the type 1 and the type 2 InsP 3 receptors have different affinities for InsP 3 and that the properties of the type 2 receptor are strongly regulated by hydrophobic interactions within its membrane environment.