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 mon itor, we have isolated two distinct bone morphogenetic protein (BMP)-bindin g proteins, and identified them as lipovitellin 1 and Ep45, respectively. L ipovitellin 1 is an egg yolk protein that is processed from vitellogenin. B oth vitellogenin and Ep45 are synthesized under estrogen control in the liv er, secreted, and taken up by developing oocytes. In this paper, we have sh own that of the TGF-beta family members tested, Ep45 can bind only to BMP-4 , whereas lipovitellin 1 can bind to both BMP-4 and activin A. Because of t his difference in specificity, we have focused on and further studied Ep45. Kinetic parameters were determined by surface plasmon resonance studies an d showed that Ep45 associated rapidly with BMP-4 (k(a) = 1.06 x 10(4) M(-1) s(-1)) and dissociated slowly (k(d) = 1.6 x 10(-4) s(-1)). In Xenopus embry os microinjected with Ep45 mRNA Ep45 blocked the ability of follistatin to inhibit BMP activity and to induce a secondary body axis in a dose-dependen t manner, whereas it had no effect on other BMP antagonists, chordin and no ggin. These results support the possibility that Ep45 interacts with BMP to modulate its activities in vivo.