TRANSPORT OF SERUM TRANSTHYRETIN INTO CHICKEN OOCYTES - A RECEPTOR-MEDIATED MECHANISM

Transthyretin (TTR) is involved in the transport of thyroid hormones a nd, due to its interaction with serum retinol-binding protein, also of vitamin A. The importance of both ligands in vertebrate embryonic dev elopment has prompted us to investigate the molecular details of TTR t ransport function in a powerful germ cell system, the rapidly growing chicken oocytes. Yolk TTR is derived from the circulatory system, sinc e biotinylated TTR was recovered by immunoaffinity chromatography of y olk obtained from a hen previously infused with in vitro biotinylated chicken serum proteins. In concordance with the intraoocytic localizat ion in an endosomal compartment, ligand blotting and chemical cross-li nking experiments revealed the presence of a similar to 115-kDa TTR-bi nding oocyte membrane protein. This putative TTR receptor was not dete cted in chicken ovarian granulosa cells or embryonic fibrobfasts and w as different from the previously described oocyte-specific receptor fo r two estrogen induced chicken serum lipoproteins, vitellogenin and ve ry low density lipoprotein (Barber, D. L., Sanders, E. J., Aebersold, R., and Schneider, W. J. (1991) J. Biol. Chem. 266, 18761-18770). Furt hermore, in contrast to the serum levels of the yolk precursor lipopro teins, those of TTR were not significantly changed by estrogen; thus, TTR represents a newly defined, estrogen-independent class of yolk pre cursor proteins. These data strongly suggest that oocytic TTR is deriv ed from the circulation, where it is a constitutive component, and dep osited into yolk as a result of endocytosis mediated by a specific rec eptor.