THE TRANSFORMING GROWTH-FACTOR-BETA TYPE-II RECEPTOR CAN REPLACE THE ACTIVIN TYPE-II RECEPTOR IN INDUCING MESODERM

The type II receptors for the polypeptide growth factors transforming growth factor beta (TGF-beta) and activin belong to a new family of pr edicted serine/threonine protein kinases. In Xenopus embryos, the biol ogical effects of activin and TGF-beta 1 are strikingly different; act ivin induces a full range of mesodermal cell types in the animal cap a ssay, while TGF-beta 1 has no effects, presumably because of the lack of functional TGF-beta receptors. In order to assess the biological ac tivities of exogenously added TGF-beta 1, RNA encoding the TGF-beta ty pe II receptor was introduced into Xenopus embryos. In animal caps fro m these embryos, TGF-beta 1 and activin show similar potencies for ind uction of mesoderm-specific mRNAs, and both elicit the same types of m esodermal tissues. In addition, the response of animal caps to TGF-bet a 1, as well as to activin, is blocked by a dominant inhibitory ras mu tant, p21(Asn-17)(Ha-ras). These results indicate that the activin and TGF-beta type II receptors can couple to similar signalling pathways and that the biological specificities of these growth factors lie in t heir different ligand-binding domains and in different competences of the responding cells.