Signal transduction pathways leading to Ca2+ release in fertilization: focusing on egg membrane-associated protein complexes

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We have analyzed molecular mechanism of fertilization by using African clawed frog Xenopus laevis as a main model organism. By focusing on molecular identity of a 57-kDa protein-tyrosine kinase that is immunochemically related to the Src family kinases and the structure and function of the egg plasma membrane-associated protein complexes, we have found that Xenopus Src gene product (termed xSrc) and a 30-kDa transmembrane protein uroplakin III (termed xUPIII) are important factors for sperm-induced egg activation, namely transient Ca2+ release within the fertilized egg. xUPIII has been initially identified as a major tyrosine-phosphorylated protein in fertilized Xenopus eggs. Further analysis of xUPIII has demonstrated that it is a target of sperm-derived protease activity and that partial proteolysis of the extracellular domain of xUPIII at fertilization may act as a signal to promote sperm-induced xSrc activation. Because both xUPIII and Src localizes predominantly in cholesterol-enriched egg membrane microdomains or "rafts", it is assumed that protein complexes organized in the membrane microdomains serve as a functional platform for sperm-egg membrane interaction and subsequent Src-dependent calcium signaling at fertilization. We have also identified uroplakin Ib (termed xUPIb), as a binding partner for UPIII and expression studies using HEK293 cells show that co-expression of xUPIII and xUPIb is required for their proper localization to the membrane microdomain as well as maintenance of xSrc tyrosine kinase activity in an inactive state. These results highlights the scheme that sperm-induced egg activation in Xenopus involves molecular machinery that act for gamete interaction in the extracellular space and that translates the sperm signal into the intracellular space, both of which are localized in the egg membrane microdomains