A SINGLE CHICKEN OOCYTE PLASMA-MEMBRANE PROTEIN MEDIATES UPTAKE OF VERY LOW-DENSITY-LIPOPROTEIN AND VITELLOGENIN

Specific cell-surface receptors mediate the uptake of plasma proteins into growing oocytes of oviparous species, thereby forming yolk. Quantitatively the most important yolk precursors are the lipoproteins, very low density lipoprotein, and vitellogenin. We show that a single major chicken oocyte plasma membrane protein with an apparent molecular mass of 95 kDa as determined by SDS/PAGE under nonreducing conditions is the receptor for both of these ligands. Binding activities for the two ligands copurified on ligand affinity matrices and were inhibited by the same antibody preparations, and the ligands competed with each other for binding to the 95-kDa protein. In addition to these biochemical and immunological lines of evidence for the density of the vitellogenin receptor with the very low density lipoprotein receptor, genetic proof was obtained. We have previously shown that the mutant nonlaying 'restricted-ovulator' hen carries a defect in the gene responsible for functional expression of the oocyte 95-kDa protein. Here we demonstrate that this single gene defect in the restricted-ovulator hen has detrimental consequences for the binding not only of very low density lipoprotein but also of vitellogenin to the 95-kDa receptor normally present in oocytes. The intriguing bifunctionality of this chicken oocyte membrane protein possibly relates to its crucial role in receptor-mediated control of oocyte growth.