Multiple vitellogenins and product yolk proteins in striped bass, Morone saxatilis: molecular characterization and processing during oocyte growth and maturation

The multiple vitellogenin (Vtg) system of striped bass, a perciform species spawning nearly neutrally buoyant eggs in freshwater, was investigated. Vitellogenin cDNA cloning, Western blotting of yolk proteins (YPs) using Vtg and YP type-specific antisera, and tandem mass spectrometry (MS/MS) of the YPs revealed the complex mechanisms of yolk formation and maturation in this species. It was discovered that striped bass possesses a tripartite Vtg system (VtgAa, VtgAb, and VtgC) in which all three forms of Vtg make a substantial contribution to the yolk. The production of Vtg-derived YPs is generally similar to that described for other perciforms. However, novel amino-terminal labeling of oocyte YPs prior to MS/MS identified multiple alternative sites for cleavage of these proteins from their parent Vtg, revealing a YP mixture far more complex than reported previously. This approach also revealed that the major YP product of each form of striped bass Vtg, lipovitellin heavy chain (LvH), undergoes limited degradation to smaller polypeptides during oocyte maturation, unlike the case in marine fishes spawning buoyant eggs in which LvHAa undergoes extensive proteolysis to osmotically active free amino acids. These differences likely reflect the lesser need for hydration of pelagic eggs spawned in freshwater. The detailed characterization of Vtgs and their proteolytic fate(s) during oocyte growth and maturation establishes striped bass as a freshwater model for investigating teleost multiple Vtg systems.