Polysialylated Glycoproteins Found in Sea Urchin Eggs

Abstract

Polysialic acid (polySia), a polymer of sialic acid residue, is recognized as an oncodevelopmental antigen. In 1978, Salmonidae fish eggs were reported to have α2, 8-linked polysialylated glycoproteins (PSGPs) within the cortical alveoli. Subsequent studies showed the presence of another type of α2, 8-linked polysialylated glycoproteins, KDN-gps, in which deaminated neuraminic acid (KDN) is polymerized, in the vitelline envelope and ovarian fluid of salmonid fish. Even though it is expected that polySia chains of the PSGP and KDN-gp are somehow involved in cell adhesion and/or recognition events during fertilization and early embryogenesis, the precise function of these polySia chains remains largely unknown. To understand the biological significance of polySia structures during fertilization and embryonic development, research has been initiated to identify polySia-containing glycoproteins in sea urchin gametes. Sea urchins have been used for long time to study fertilization and early embryogenesis, because (i) it is relatively easy to collect their gametes and to observe their fertilization and their subsequent development and (ii) because the development of embryos can be synchronized. In fact, novel type of polySia structures in sea urchin egg jelly coat glycoprotein (polySia-gp) and the egg receptor glycoprotein for sperm were identified, respectively. Interestingly both polySia-gp and the egg receptor protein have a unique α2, 5-O_glycolyl-linked poly(Neu5Gc) chain in which each Neu5Gc residue is linked through the hydroxyl group of the glycolyl moiety of the penultimate Neu5Gc residue. The polySia chain in the egg receptor for sperm has an additional feature; a nonreducing terminal Neu5Gc is sulfated at the hydroxyl group of C9. Most interestingly, sulfated polySia chains found as a component of the receptor protein for sperm inhibited fertilization; the nonsulfated form of this polySia chains has little inhibitory activity. Recent finding regarding the structure and function of polySia chains in sea urchin eggs are mainly described in this review.