Multiple vitellogenin-derived yolk proteins in gray mullet (Mugil cephalus): disparate proteolytic patterns associated with ovarian follicle maturation.

ABSTRACT

Disparate proteolytic patterns of yolk proteins, derived from three types of vitellogenin (VgA, VgB, and VgC), were observed in gray mullet. Immuno-biochemical analyses of extracts obtained from vitellogenic ovaries (VO) and ovulated eggs (OE) confirmed that a large proportion of VgA-derived lipovitellin (LvA) was degraded into free amino acids (FAAs) during ovarian follicle maturation. The maturation-associated alteration of VgB-derived Lv (LvB) involved only limited proteolysis; the heavy and light LvB chains were dissociated into at least three and one polypeptide fragments, respectively. The native mass of VgC-derived Lv (LvC) exhibited little difference between VO and OE, although it was apparent that the LvC was 'nicked' during maturation, resulting in the appearance of several bands in OE. Similar analyses confirmed that VgA-derived beta'-component (beta'-cA) and VgB-derived beta'-c (beta'-cB) decreased during maturation in both quantity and native mass, while phosvitin derived from either VgA (PvA) or VgB (PvB) appeared to be degraded into FAAs. The pattern of maturation-associated proteolysis of mullet yolk proteins is similar to that reported for other marine teleosts spawning pelagic eggs. However, the depository ratio of the three distinct types of Lv in the mullet VO appeared to be different from that estimated for another marine pelagophil, the barfin flounder. These results support a recent paradigm regarding the significance of Vg multiplicity upon successive physiological events in this group of fishes including the hydration of maturing oocytes, the acquisition of proper egg buoyancy, and the generation of requisite nutrient stocks for each stage of embryogenesis and larval development.