Identification and characterization in Xenopus of XsmgGDS, a RalB binding protein.

We have previously isolated a cDNA coding for a RalB protein from a Xenopus maternal library. By mutagenesis of RalB and microinjection of its mRNA in embryos we show that RalB is involved in Xenopus early development. We have used the yeast two-hybrid system to screen a Xenopus maternal cDNA library in order to isolate proteins interacting to RalB. We have identified a full-length cDNA encoding a protein homologous to mammalian smgGDS. The Xenopus gene shows 84.6% identity with the mammalian counterpart and contains one additional armadillo repeat. The XsmgGDS mRNA is expressed from oogenesis up to late embryogenesis at a higher level than that in adult tissues. Thus RalB is another protein that interacts with smgGDS which suggests that RalB may be activated by a Ras independent pathway.