Proteolytic cleavage of vitellogenin and yolk proteins during vitellogenin uptake and oocyte maturation in barfin flounder (Verasper moseri)

Abstract

AbstractVitellogenin (Vg) and vitellogenin‐derived yolk proteins of growing oocytes and ovulated eggs were purified and characterized to evaluate structural changes of yolk proteins during oocyte growth and maturation in barfin flounder, Verasper moseri, a marine teleost which lays pelagic eggs. Native circulating vitellogenin, estimated to be a 520 kDa protein using Superose 6 column chromatography, cleaved into the 410 kDa major yolk protein, lipovitellin (Lv), in vitellogenic oocytes. An additional minor yolk protein at 19 kDa in native form, which was immunoreactive to an antiserum raised against Vg, and a highly phosphorylated 38 kDa band of phosvitin (Pv) in SDS‐PAGE with reduction were also identified in vitellogenic oocytes. Analysis of the amino acid composition of the 19 kDa yolk protein showed it to be similar to the β′‐component of egg yolk from salmonid fish. The 410 kDa Lv in vitellogenic oocytes was proteolytically cleaved into an 170 kDa major yolk protein during final oocyte maturation. The close similarity of amino acid composition between the two proteins combined with the results of lipid analysis suggested that the 410 kDa Lv cleaved into two homologous 170 kDa monomeric Lv. Both 19 kDa yolk protein (β′‐component) and Pv became undetectable after oocyte maturation. We propose that the three classes of yolk proteins (Lv, Pv, and β′‐component) are products of Vg cleavage, and that all of them undergo proteolysis again during oocyte maturation. © 1995 Wiley‐Liss, Inc.