THE XENOPUS IP(3) RECEPTOR - STRUCTURE, FUNCTION, AND LOCALIZATION INOOCYTES AND EGGS

To study the role of the IP3 receptor (IP3R) upon egg activation, cDNA clones encoding IP3R expressed in the Xenopus oocytes were isolated. By analyses of the primary structure and functional expression of the cDNA, Xenopus IP3R (XIP3R) was shown to have an IP3-binding domain and a putative Ca2+ channel region. Immunocytochemical studies revealed p olarized distribution of XIP3R in the cytoplasm of the animal hemisphe re in a well-organized endoplasmic reticulum-like structure and intens ive localization in the perinuclear region of stage VI immature oocyte s. In ovulated unfertilized eggs, XIP3R was densely enriched in the co rtical region of both hemispheres in addition to its polarized localiz ation. After fertilization, XIP3R showed a drastic change in its distr ibution in the cortical region. These results imply the predominant ro le of the XIP3R in both the formation and propagation of Ca2+ waves at fertilization.