Xenopus FRS2 is involved in early embryogenesis in cooperation with the Src family kinase Laloo

FRS2 has been identified in mammalian cells as a protein that is tyrosine p hosphorylated and binds to Grb2 and Shp2 in response to fibroblast growth f actor (FGF) or nerve growth factor (NGF) stimulation. But neither its exist ence in other vertebrate classes or invertebrates nor its function during e mbryonic development has been defined. Here we have identified and characte rized a Xenopus homolog of FRS2 (xFRS2). xFRS2 is tyrosine phosphorylated i n early embryos, and overexpression of an unphosphorylatable form of xFRS2 interferes with FGF-dependent mesoderm formation. The Src family kinase Lal oo, which was shown to function in FGF signaling during early Xenopus devel opment, binds to xFRS2 and promotes tyrosine phosphorylation of xFRS2. More over, xFRS2 and Laloo are shown to bind to Xenopus FGF receptor 1. These re sults suggest that xFRS2 plays an important role in FGF signaling in cooper ation with Laloo during embryonic development.