Truncated type II receptor for BMP-4 induces secondary axial structures in Xenopus embryos

BRK-3 is a vertebrate type II receptor for BMP-4 distantly related to invertebrate type II receptors for BMP-2/BMP-4/dpp, such as daf-4 and punt. BRK-3 has a long carboxy-terminal sequence following intracellular kinase domain and is capable of forming a high-affinity complex with a type I receptor, BRK-2. To examine the role of BRK-2 + BRK-3 receptor complex in BMP signaling during early embryogenesis, the dominant-negative form of BRK-3 was ectopically expressed in the Xenopus embryos. A secondary body axis expressing the Sonic hedgehog and N-CAM genes is induced by injecting mRNA encoding truncated form of BRK-3 into ventral marginal region, implicating the BMP signaling in axial mesoderm induction. Formation of the secondary axis depends on whether the deletion extends into the kinase domain, not into the carboxy-terminal tail, suggesting that the kinase domain, but not the tail region, is essential for BMP signaling.