04-P018 The RNA-binding protein XSeb4R binds directly to and stabilizes Sox3 mRNA required for ectoderm formation in Xenopus

Maternal mARNs are key players of the first steps of development in vertebrates. Initially silenced in oocytes, these transcripts are activated and translated after fertilization and play crucial roles in early cell lineage allocation into the germ layers. We previously reported that the RNA-binding protein XSeb4R regulates endoderm and mesoderm formation by stabilizing and stimulating the translation of VegT mRNA in Xenopus. Here we provide evidence that XSeb4R also plays a role in ectoderm specification. Depletion of XSeb4R by morpholino antisense oligonucleotide injection expands mesodermal marker genes into the ectoderm. Conversely, forced expression of XSeb4R in ectodermal cells is sufficient to activate the expression of ectodermal determinants, including Sox3, an upstream modulator of the TGFb/nodal antagonists such as Ectodermin, required for ectoderm specification. Importantly, XSeb4R specifically interacts with the 30UTR and stabilizes Sox3 transcripts in the context of ectoderm formation. Strikingly, the ability of XSeb4R to activate Sox3 but not Ectodermin is dependent on protein homodimerization, suggesting that XSeb4R, as a single RRM-type RNA-binding protein, modulates the activities of its target transcripts in a protein complex-dependent manner during embryogenesis.