Fertilization (activation)-induced 200- to 9-kDa depolymerization of polysialoglycoprotein, a distinct component of cortical alveoli of rainbow trout eggs

Polysialoglycoprotein, a novel type of glycoprotein found in the eggs of rainbow trout has been shown to undergo dramatic depolymerization (200- to 9-kDa) upon fertilization of the eggs. Molecular mechanism of this depolymerization has been elucidated to be the result of proteolysis catalyzed by a highly specific protease induced at fertilization. The low molecular weight polysialoglycoprotein obtained from the fertilized eggs accounted for about 85% of total polysialoglycoprotein and comprised glycotridecapeptides with a uniform peptide sequence which was determined to be Asp-Asp-Ala-Thr*-Ser*-Glu-Ala-Ala-Thr*-Gly-Pro-Ser-Gly, where * indicates the site of glycosylation. This glycotridecapeptide constitutes a repeating unit of the 200-kDa polysialoglycoprotein in the unfertilized eggs: (Asp) 0-2-Ala-Thr*-Ser*-Glu-Ala-Ala-Thr*-Gly-Pro-Ser-Gly-(Asp-Asp-Ala-Thr*-Ser *-Glu- Ala-Ala-Thr*-Gly-Pro-Ser-Gly)n (n = 25) (Kitajima, K., Inoue, Y., and Inoue, S. (1986) J. Biol. Chem. 261, 5262-5269). The fertilization-induced depolymerization of polysialoglycoprotein appeared to be completed within 5 min postfertilization. The same reaction was also induced by parthenogenetic activation of the eggs by immersing in fresh water or nonelectrolyte solutions. Thus the phenomenon is closely associated with the exocytosis of cortical vesicles (alveoli) of the eggs.