Tyrosine kinase-dependent activation of phospholipase Cγ is required for calcium transient in Xenopus egg fertilization

In a previous study (K.-I. Sate et al., 1999, Dev. Biol. 209, 308-320), we presented evidence that a Src-related protein-tyrosine kinase (PTK), named Xyk, may act upstream of the calcium release in fertilization of the Xenopus egg. In the present study, we examined whether PTK activation of phospholipase C gamma (PLC gamma) plays a role in the fertilization-induced calcium signaling. Immunoprecipitation studies show that Xenopus egg PLC gamma is tyrosine phosphorylated and activated within a few minutes after fertilization but not after A23187-induced egg activation. Consistently, we observed a fertilization-induced association of PLC gamma with Xyk activity that was not seen in A23187-activated eggs. A Src-specific PTK inhibitor, PP1, blocked effectively the fertilization-induced association of PLC gamma with Xyk activity and up-regulation of PLC gamma, when microinjected into the egg. In addition, a PLC inhibitor, U-73122, inhibited sperm-induced inositol 1,4,5-trisphosphate production and the calcium transient and subsequent calcium-dependent events such as cortical contraction, elevation of fertilization envelope, and tyrosine dephosphorylation of p42 MAP kinase, all of which were also inhibited by PP1. On the other hand, A23187 could cause the calcium response and calcium-dependent events in eggs injected with PP1 or U-73122. These results support the idea that Xenopus egg fertilization requires Src-family PTK-dependent PLC gamma activity that acts upstream of the calcium-dependent signaling pathway. (C) 2000 Academic Press.