Regulation of oogenesis: the piscine receptor for vitellogenin

The receptor-mediated uptake of vitellogenin (VTG), a plasmatic lipophosphoglycoprotein, is crucial for oocyte growth in egg-laying animals. The plasma membrane receptor for VTG was characterized from oocytes of coho salmon, Oncorhynchus kisutch. In direct binding studies, the receptor exhibited high affinity (Kd, 180 nM) for salmonid VTG, and by ligand blotting with radiolabelled VTG it was visualized as a protein with an apparent Mr of 100,000, under non-reducing conditions. The fish VTG receptor was shown to share key structural elements with VTG receptors from chicken and Xenopus laevis. Namely, cross-reactivity at the level of ligand recognition was observed among VTG receptors from these species and immunological relatedness was demonstrated by immunoblotting with anti-chicken VTG receptor antibodies. In addition, as in chicken and Xenopus, binding of VTG to fish oocyte receptors was shown to be mediated by the lipovitellin domain of VTG. These results clearly indicate that regulation of oocyte growth at the level of yolk formation has been accomplished by the conservation of structural features of receptors required for internalization of VTG.