The receptor for yolk lipoprotein deposition in the chicken oocyte

Abstract

The final rapid growth phase of the chicken oocyte is characterized by massive uptake of hepatically synthesized yolk precursor proteins from the plasma. The two major yolk-forming components, very low density lipoprotein (VLDL) and vitellogenin (VTG), have been shown to interact with a 95-kDa protein present in detergent extracts of ovarian membranes; this protein is absent in hens of a mutant nonlaying chicken strain (Nimpf, J., Radosavljevic, M., and Schneider, W. J. (1989) J. Biol. Chem. 264, 1393-1398). Here, we have purified the 95-kDa protein by ligand and immunoaffinity chromatography and demonstrated its role in receptor-mediated endocytosis by ultrastructural immunolocalization, structural, and functional studies. The receptor was visualized exclusively in the oocyte proper and was absent from somatic cells, in agreement with the previously reported expression of two different lipoprotein receptors in somatic cells and oocytes, respectively, of laying hens (Hayashi, K., Nimpf, J., and Schneider, W. J. (1989) J. Biol. Chem. 264, 3131-3139). Amino acid sequences of tryptic fragments of the oocyte receptor were obtained, and its kinship to somatic low density lipoprotein receptors was confirmed through the demonstration of sequence conservation in three characteristic domains. In particular, the chicken receptor's internalization sequence, Phe-Asp-Asn-Pro-Val-Tyr, is identical with that in low density lipoprotein receptors from mammals as well as Xenopus laevis. The ligand-binding properties, specificity, and kinetic parameters of the oocyte receptor were characterized in filtration assays employing pure ligands and receptor. In conjunction with ligand-blotting experiments following limited protease digestion of the receptor, the binding assay data suggest that VTG recognizes a substructure of the VLDL-binding site. These studies establish that a cell-specific receptor mediates the endocytosis of VTG and VLDL into growing chicken oocytes and thus possibly plays a key role in control of oocyte growth.