A middle-affinity estrogen-specific binding protein in livers of vitellogenic and nonvitellogenic Xenopus laevis

Administration of estradiol-17β to male Xenopus laevis evokes massive liver synthesis of the egg yolk precursor protein, vitellogenin, and its cognate mRNA. Since previous experiments had implicated only a nuclear estrogen receptor in vitellogenesis, we examined Xenopus liver cytosol for other estrogen-binding proteins. Cytoplasmic extracts from unstimulated Xenopus liver contain high levels (approximately 500,000 sites/cell) of an estrogen-specific binding protein. This protein exhibits a K_d of approximately 4 × 10^-8M for estradiol-17β, binds estrogenic steroids only, and has a sedimentation coefficient in the range 2-3 S. It is not a classical estrogen receptor, as it does not translocate into the nucleus following estrogen administration. We discuss possible functions of this protein, which include a role in the ontogeny of the vitellogenic response, and in the cytoplasmic transport and storage of estrogen.