A PROTEIN THAT BINDS AN EXOGASTRULA-INDUCING PEPTIDE, EGIP-D, IN THE HYALINE LAYER OF SEA-URCHIN EMBRYOS

Exogastrula-inducing peptides are present in eggs and embryos of the s ea urchin Anthocidaris crassispina. They induce exogastrulation when a dded exogenously to the embryos. In the present study, we investigated an EGIP-D-binding protein in the embryos. EGIP-D was incubated with h omogenates of embryos. EGIP-D was then cross-linked to the binding pro tein by use of disuccinimidyl suberate (DSS) and the complex was analy zed by western blotting with an EGIP-D-specific antibody. A 90-kDa pro tein was detected in both eggs and embryos. To examine the localizatio n of this protein, EGIP-D was added to intact embryos, cross-linked to proteins by use of DSS, and the complexes were again analyzed by west ern blotting. The EGIP-D-binding protein was detected in intact embryo s but not in embryos treated with Ca2+- and Mg2+-free seawater (CMF-SW ) that removes the hyaline layer (HL). It appeared, therefore, that th is protein was present on the outer surface of the embryo, being a con stituent of the HL. The CMF-SW extract that contained EGIP-D-binding p rotein, inhibited the induction of exogastrulation by EGIP-D. Furtherm ore, the treatment of embryos with CMF-SW prevented EGIP-D from induci ng exogastrulation. Our observations indicate that the interaction bet ween EGIP-D and the binding protein is a prerequisite for induction of exogastrulation by EGIP-D.