Fig. (2) Study of fertilization signaling focusing on structure and function of egg plasma membrane microdomains. (A) Schematic drawing of preparation of membrane microdomains from Xenopus eggs, which are unfertilized, fertilized, or artificially activated (parthenogenetically activated), and its utility in functional analysis. (B) Schematic diagram showing a series of events that operates during early phase of fertili-zation of Xenopus eggs. Sperm-induced egg activation may involve proteolytic cleavage of the egg membrane microdomain-associated uro-plakin III, catalyzed by sperm-derived protease. Uroplakin III is in association with uroplakin Ib, a tetraspanin molecule that supports the localization of uroplakin III to membrane microdomains. The uroplakin III-uroplakin Ib complex is involved in negative regulation of cyto-plasmic tyrosine kinase Src, and proteolytic cleavage of uroplakin III may lead to up-regulation of Src by unknown mechanism. Activated Src phosphorylates and activates PLC,γ which in turn promotes IP3 production and Ca²⁺transient.Src may also phosphorylate intact uro-plakin III, although its physiological importance has not yet been demonstrated