Characterization of the 38 kDa protein lacking in gastrula-arrested mutant Xenopus embryos

We have reported elsewhere that offspring from the No. 65 female of Xenopus laevis cleaved normally, but their development was arrested at the onset of gastrulation, like the Ambystoma ova-deficient (o) mutant, irrespective of mating with different wild-type males, and that an acidic, 38 kDa protein present in wild-type eggs was lacking in eggs of the female. In the current study, we first determined the partial amino acid sequence (VANLE) of one of the well-separated tryptic peptides from the protein, which was found in elongation factor 1 delta (Ef1delta) in Xenopus, and finally identified the protein as one of the Ef1delta isoforms, Ef1delta2, by peptide mass spectrometry. RT-PCR analyses for Ef1delta2 and its close homolog Ef1delta1 in wild-type oocytes and embryos demonstrated that both transcripts are maternal and Ef1delta1 is present more abundantly than Ef1delta2 throughout the stages examined. Importantly, the amount of the Ef1delta2 transcript per embryo decreased gradually after gastrulation, in accordance with the gradual decrease of the 38 kDa protein per embryo reported in our earlier study. Because pharmacological inhibition of translation induces gastrulation arrest in wild-type embryos, it is reasonable to conclude that the mutant embryos arrest in development due to the lack of Ef1delta2 that is indispensable for translation. Thus, the present study provides the first molecular information on the cause of the gastrulation-defective mutation in Amphibia.