Gradual loss of DNA-PK activity from the cytoplasm is coincident with the nuclear translocation of its activator Ku during early development of Xenopus laevis.

DNA-dependent protein kinase is a serine/ threonine kinase consisting of a catalytic subunit, p460, and a regulatory subunit called Ku (p80/p70). DNA-dependent protein kinase plays a role in transcription, non-homologous recombination, and DNA repair. Previous data have shown the presence of DNA-dependent protein kinase in Xenopus oocytes and changes in its activity during vitellogenesis. Metabolic labelling studies have shown that the increased enzyme activity in vitellogenic oocytes correlates with increased levels of Ku protein, and compared to the pre- and early-vitellogenic oocytes, vitellogenic and post-vitellogenic oocytes show an increased level of DNA-dependent protein kinase activity. Whether DNA-dependent protein kinase activity is altered during early embryogenesis in Xenopus is not known. The present study demonstrates that DNA-dependent protein kinase activity is gradually lost from the cytoplasm in the early embryonic cells of blastulae and gastrulae. The gradual loss of DNA-dependent protein kinase activity during post-fertilization early embryogenesis in Xenopus is consistent with the reports on Arbacia punctulata. Immunohistochemistry of the oocytes at various stages and early embryos (gastrulae) shows that Ku70, a regulatory subunit of DNA-dependent protein kinase, is present both in the cytoplasm and nucleus in the pre-vitellogenic oocytes, full-grown post-vitellogenic oocytes and in the gastrula cells. However, Ku70 appears to accumulate in higher concentrations in the nuclei of gastrula cells. These results suggest that gradual loss of DNA-dependent protein kinase activity from the cytoplasm of the early embryos could be the consequence of Ku translocating to the nuclei that may be necessary for post-zygotic transcription followed by cellular differentiation.