Isolation, characterization, and localization of the inositol 1,4,5-trisphosphate receptor protein in Xenopus laevis oocytes

Inositol 1,4,5-trisphosphate (Ins(1,4,5)P₃) induces Ca²⁺ oscillations and waves in Xenopus laevis oocytes. Microsomes from oocytes exhibit high-affinity binding for Ins(1,4,5)P₃, and demonstrate Ins(1,4,5)P₃-induced Ca²⁺ release. The Ins(1,4,5)P₃ receptor (InsP₃R) was purified from oocyte microsomes as a large tetrameric complex and shown to have a monomer molecular mass of 256 kDa, compared with 273 kDa for the brain InsP₃R. Binding to the oocyte receptor is highly specific for Ins(1,4,5)P₃ and is inhibited by heparin (IC₅₀, 2 μg/ml). Immunoblot analysis revealed that an antibody against the C-terminal sequence of the brain receptor recognized the oocyte receptor. These results, in addition to the difference in pattern obtained after limited proteolysis, suggest that the oocyte InsP₃R is a new shorter isoform of the mammalian brain type I InsP₃R. Immunofluorescence experiments indicated the presence of the InsP₃R in the cortical layer and the perinuclear endoplasmic reticulum of the oocyte. However, immunological and biochemical experiments did not reveal the presence of the ryanodine receptor. The presence of an InsP₃R and the absence of a ryanodine receptor support the importance of Ins(1,4,5)P₃ in Ca²⁺ handling by oocytes and particularly in the induction of Ca²⁺ oscillations and waves.