Isolation and Characterization of Bone Morphogenetic Protein-binding Proteins from the Early Xenopus Embryo

Using a surface plasmon resonance biosensor as a sensitive and specific monitor, we have isolated two distinct bone morphogenetic protein (BMP)-binding proteins, and identified them as lipovitellin 1 and Ep45, respectively. Lipovitellin 1 is an egg yolk protein that is processed from vitellogenin. Both vitellogenin and Ep45 are synthesized under estrogen control in the liver, secreted, and taken up by developing oocytes. In this paper, we have shown that of the TGF-β family members tested, Ep45 can bind only to BMP-4, whereas lipovitellin 1 can bind to both BMP-4 and activin A. Because of this difference in specificity, we have focused on and further studied Ep45. Kinetic parameters were determined by surface plasmon resonance studies and showed that Ep45 associated rapidly with BMP-4 (ka = 1.06 × 104m−1s−1) and dissociated slowly (kd = 1.6 × 10−4s−1). In Xenopus embryos microinjected with Ep45 mRNA, Ep45 blocked the ability of follistatin to inhibit BMP activity and to induce a secondary body axis in a dose-dependent manner, whereas it had no effect on other BMP antagonists, chordin and noggin. These results support the possibility that Ep45 interacts with BMP to modulate its activities in vivo.