Disparate binding of three types of vitellogenin to multiple forms of vitellogenin receptor in white perch

Three types of white perch (Morone americana) vitellogenin (VtgAa, VtgAb, and VtgC) were purified, labeled with digoxigenin (DIG), and subjected to Vtg receptor (Vtgr) binding assays in 96-well plates coated with perch ovarian membrane proteins or to ligand blotting procedures. Binding specificity was evaluated by incubating membrane protein preparations with constant amounts of DIG-Vtg tracer (VtgAa, VtgAb, VtgC, or a mixture of VtgAa and VtgAb [VtgAa/b]) alone or in the presence of unlabeled Vtg ligands. At 250-fold excess molar concentration relative to the tracer, VtgAa and VtgAb were each able to displace only approximately 50% of bound DIG-VtgAa/b, but VtgAa/b could fully displace DIG-VtgAa and DIG-VtgAb under the same conditions. Over a broad range of excess molar ratios, unlabeled VtgAa and VtgAb each displaced their respective DIG-Vtg tracer much more effectively than each did the heterologous tracer (DIG-VtgAb and DIG-VtgAa, respectively). Ligand blotting revealed three forms of Vtgr, a large receptor (>212 kDa) that bound only to VtgAa and two smaller receptors (∼ 116 and ∼ 110.5 kDa) that bound preferentially to VtgAb. The VtgC did not specifically bind to ovarian membrane proteins in either assay. Collectively, these results indicate the presence of a system of multiple ovarian Vtgrs with disparate binding to the three types of Vtg present in higher-order teleosts (Acanthomorpha). To our knowledge, this is the first report on binding of multiple types of Vtg to multiple forms of Vtgr in any vertebrate.