Two Forms of Vitellogenin, Yielding Two Distinct Lipovitellins, Play Different Roles during Oocyte Maturation and Early Development of Barfin Flounder, Verasper moseri, a Marine Teleost that Spawns Pelagic Eggs
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Two forms of vitellogenin (Vg), Vg A and Vg B, were identified in serum from estrogen-treated barfin flounder (Verasper moseri). Structural changes of lipovitellins (Lvs) derived from the two Vgs were examined during vitellogenesis and oocyte maturation. Two Lvs, vLv A and vLv B, were identified electrophoretically and immunologically in postvitellogenic oocytes. Each appeared to be composed of distinct heavy chains (vLvH A, Mr 107,000, and vLvH B, Mr 94,000) and light chains (vLvL A, Mr 30,000, and vLvL B, Mr 28,000) when analyzed by SDS–PAGE. Results from N-terminal amino acid sequencing and Western blotting using antisera to vLvH A and vLvH B verified that there are two Vg polypeptides in serum from estrogen-treated fish, Vg A (Mr 168,000) and Vg B (Mr 175,000), which give rise to vLvH A–vLvL A and vLvH B–vLvL B, respectively. N-terminal sequencing revealed two sequences for both phosvitin and β′-component, supporting the concept of duality for all three classes of Vg-derived yolk proteins. During oocyte maturation, native dimeric vLv B was dissociated into a native Mr 170,000 monomer (oLv B). Meanwhile, vLv A was extensively cleaved including complete degradation of vLvH A into free amino acids. We propose that the quantitative ratio of vLv A to vLv B in postvitellogenic oocytes regulates the buoyancy of the spawned pelagic eggs by controlling availability of free amino acids which function as osmotic effectors during oocyte hydration. The vLv A/vLv B ratio likely also controls the proportional availability of different types of nutrients, free amino acids versus Lv, for use during embryonic development.