Recently, we have purified a Src-related tyrosine kinase, named Xenopus tyrosine kinase (Xyk), from oocytes of Xenopus laevis and found that the enzyme is activated within 1 min following fertilization [Sato et al. (1996) J. 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 activation 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, tyrosine phosphorylation, and translocation of Xyk. A synthetic tyrosine kinase-specific 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 microM. 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 sperm nucleus into the egg cytoplasm and meiotic resumption in the egg were blocked. Indirect immunofluorescence study demonstrates that Xyk is exclusively 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 important role in the early events of Xenopus egg activation in a manner independent or upstream of calcium signaling.

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Fertilization is accompanied by a rapid and transient calcium release in eggs

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With the use of 3H-labeled R17 amB2 phage RNA having an UAG codon at the seventh triplet of the coat cistron, release of the RNA from ribosomes at the termination codon was studied. The ribosome-releasing factor previously described was shown to stimulate the process of mRNA release at the termination codon. The stimulatory effect of the factor on this process was dependent on the termination factor (RF-1). GTP was required for this process and guanosine 5'-(P7y-methylene)triphosphate could not replace GTP. No apparent change of size of R17 RNA was observed during the release of the R17 RNA from the ribosomes. The ribosome-releasing factor is distinct from the known termination codon-specific factor such as RF-1.In the preceding publications [l -31 it was demonstrated that elongation factor G(EF-G), a previously unknown factor (tentatively named ribosome-releasing factor), and GTP were necessary for release of ribosomes from naturally occurring polysome which had been treated with puromycin. The ribosomereleasing factor has been purified to electrophoretic homogeneity and has a molecular weight of approximately 18000. It was proposed that this factor may play some role in the release of ribosomes from messenger RNA at the termination point of each cistron.In this communication, evidence is presented that ribosome-releasing factor can function for the release of ribosomes at the termination codon as well as other points in the messenger RNA. With the use of amB, R17RNA which has an amber codon at the seventh triplet of coat cistron [4-61, it was demonstrated that the factor catalyzed the release of mRNA from ribosomes at the amber codon. The released RNA was not fragmented during the release from ribosomes. Definirion. A,, unit, the quantitity of material contained in 1 ml of a solution which has an absorbance of 1 at 260 nm, when measured in a 1-cm pathlength cell. EXPERIMENTAL PROCEDURE Preparation of Various FactorsThe ribosome-releasing factor was purified to electrophoretic homogeneity as described previously [l]. The crude soluble fraction of Escherichia coli containing the termination factor RF-1, was prepared free of ribosome-releasing factor as follows. The ribosome-free supernatant fluid (1 1, 9.26 g) prepared as described previously (step one of the purification of the factor by Hirashima and Kaji [I]). The protein fraction precipitated between 45 and 75 % saturation of ammonium sulfate, collected by centrifugation at 8500 rev./min for 15 min in a Sorvall GSA rotor, dissolved in 80ml of 10mM Tris-HC1 (pH 7.8), l.OmM magnesium acetate, 20 mM ammonium chloride, 0.5 mM dithiothreitol, 5 % glycerol, and dialyzed against the same buffer overnight. Insoluble material was removed by centrifugation and the solution (80 ml) containing 5.33 g of protein was placed on a DEAEcellulose column (2.6 x 86 cm), which had been equilibrated with the same buffer. The loaded column was washed with the same buffer. The ribosome-releasing factor was eluted during this washing process. The proteins retained on the column were...