A specific subunit of vitellogenin that mediates receptor binding.

Vitellogenin, an estrogen-induced serum protein synthesized in the liver, is composed of two Mr 250K polypeptides. It is specifically transported by a receptor-mediated endocytic process into the developing oocytes of virtually all oviparious animals. Following endocytosis, in the chicken, vitellogenin is specifically processed to yield several smaller products including the phosvitins (PV) and the lipovitellins (LV). These products are then stored within the oocyte until they are degraded during embryogenesis to provide nutrients for the developing embryo. Direct binding studies using iodinated vitellogenin demonstrate that vitellogenin binds to isolated oocyte membranes with a KD of 2.5 microM. Competition studies indicate that PV is a competitive inhibitor of vitellogenin binding. This leads us to propose that the PV portion of the circulating vitellogenin molecule mediates binding and uptake. Direct binding studies using iodinated PV show that PV binds to isolated oocyte membranes with a KD of 2.4 microM. Competition studies also demonstrate that 3.1 microM vitellogenin inhibits 50% of control 125I-PV binding, but IgG and bovine serum albumin at concentrations up to 10 microM have no effect on 125I-PV binding. Another series of competition experiments using a constant amount of vitellogenin and increasing amounts of 125I-PV indicate that vitellogenin acts as a competitive inhibitor of PV binding and has a KI of 2-3 microM. These results support our hypothesis that the receptor which mediates vitellogenin binding and uptake recognizes determinants on the PV portion of the native vitellogenin molecule.