The transcription factor p53 has been shown to mediate cellular responses to diverse stresses such as DNA damage. However, the function of p53 in cellular differentiation in response to growth factor stimulations has remained obscure. We present evidence that p53 regulates cellular differentiation by modulating signaling of the TGFβ family of growth factors during early Xenopus embryogenesis. We show that p53 functionally and physically interacts with the activin and bone morphogenetic protein pathways to directly induce the expression of the homeobox genes Xhox3 and Mix.1/2. Furthermore, functional knockdown of p53 in embryos by an antisense morpholino oligonucleotide reveals that p53 is required for the development of dorsal and ventral mesoderm. Our data illustrate a pivotal role of interplay between the p53 and TGFβ pathways in cell fate determination during early vertebrate embryogenesis.

M(r) 25,000 protein (pp25) is a protein kinase substrate detected recently in Xenopus laevis oocytes [Hashimoto, E. et al. (1995) J. Biochem. 118, 453-460], but the physiological role of this protein remains to be determined. In order to elucidate some characteristics of pp25, a polyclonal antibody was raised against it and the distribution and quantitative changes of this protein were examined using various tissues and biological systems. In Western blot analysis, pp25 was detected only in Xenopus oocytes and not in other frog tissues when the heat-stable cytosolic fraction from each tissue was examined. Although the amount of pp25 apparently did not change during oocyte maturation induced by progesterone, pp25 disappeared in embryos around Nieuwkoop/Faber stages 45-48 in parallel with the change of pNiXa (a kind of serpin). These results suggest that pp25 plays some specific role(s) in Xenopus oocytes and that the level of pp25 changes dramatically during embryonic development.