Evidence for the involvement of a Src-related tyrosine kinase in Xenopus egg activation

Recently, we have purified a Src-related tyrosine kinase, named Xenopus tyr osine kinase (Xyk), from oocytes of Xenopus laevis and found that the enzym e is activated within 1 min following fertilization [Sato et al. (1996) T. Biol. Chem. 271, 13250-13257]. A concomitant translocation of a part of the activated enzyme from the membrane fraction to the cytosolic fraction was also observed. In the present study, we show that parthenogenetic egg activ ation by a synthetic RGDS peptide [Y. Iwao and T. Fujimura, T. (1996) Dev. Biol. 177, 558-567], an integrin-interacting peptide, but not by electrical shock or the calcium ionophore A23187 causes the kinase activation, tyrosi ne phosphorylation, and translocation of Xyk. A synthetic tyrosine kinase-s pecific inhibitor peptide was employed to analyze the importance of the Xyk activity in egg activation. We found that the peptide inhibits the kinase activity of purified Xyk at IC50 of 8 mu M. Further, egg activation induced by sperm or RGDS peptide but not by A23187 was inhibited by microinjection of the peptide. In the peptide-microinjected eggs, penetration of the sper m nucleus into the egg cytoplasm and meiotic resumption in the egg were blo cked. Indirect immunofluorescence study demonstrates that Xyk is exclusivel y localized to the cortex of Xenopus eggs, indicating that Xyk can function in close proximity to the sperm-egg or RGDS peptide-egg interaction site. Taken together, these data suggest that the tyrosine kinase Xyk plays an im portant role in the early events of Xenopus egg activation in a manner inde pendent or upstream of calcium signaling, (C) 1999 Academic Press.