Tyrosine kinase-dependent activation of phospholipase C gamma 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 Xenopu s egg. In the present study, we examined whether PTK activation of phosphol ipase 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 fertilizat ion 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, bloc ked 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 calciu m-dependent events such as cortical contraction, elevation of fertilization envelope, and tyrosine dephosphorylation of p42 MAP kinase, all of which w ere also inhibited by PP1. On the other hand, A23187 could cause the calciu m response and calcium-dependent events in eggs injected with PP1 or U-7312 2. These results support the idea that Xenopus egg fertilization requires S rc-family PTK-dependent PLC gamma activity that acts upstream of the calciu m-dependent signaling pathway. (C) 2000 Academic Press.