Identification of the Sea Urchin 350-kDa Sperm-binding Protein as a New Sialic Acid-binding Lectin That Belongs to the Heat Shock Protein 110 Family

Abstract

The 350-kDa sperm-binding protein (SBP), a speciesspecific sperm-binding protein, is localized in the vitelline layer of sea urchin eggs. In this study, we have shown for the first time that sperm gangliosides are ligands for the intact glycosylated SBP. Using recombinant fragments of the SBP, the N-terminal heat shock protein 110-like domain was shown to be responsible for the binding. The intact SBP could bind various gangliosides, and the binding was sialidase-sensitive and inhibited by sialyllactose, thus indicating that it is the sialic acid-binding protein. Calcium and magnesium ions were not required but they did enhance the binding activity of SBP. The observation that bacterially expressed recombinant SBP and the sialidase-treated intact glycosylated SBP lost divalent cation-dependent enhancement of binding activity suggests that the sialylated carbohydrate moieties of the SBP may be involved in this property. Furthermore, the SBP was shown to bind sperm lipid rafts, in which gangliosides are enriched, and this binding was lost upon sialidase treatment of the lipid rafts. Finally, liposomes containing the ganglioside specifically inhibited fertilization. Taken together, these results allow us to identify SBP as a member of a new class of sialic acid-binding lectin belonging to the Hsp110 family, and indicate that SBP may be involved in interaction of sperm with the vitelline layer of the egg.

Discovery, Primary, and Crystal Structures and Capacitation-related Properties of a Prostate-derived Heparin-binding Protein WGA16 from Boar Sperm

“…It has been demonstrated that lipid rafts, isolated as low density detergent-insoluble or detergent-resistant membranes from capacitated pig sperm had high affinity for the ZP (10). Involvement of lipid rafts as platforms of cell adhesion and signaling molecules has been established earlier in fertilization (15)(16)(17), medaka fish embryogenesis (18,19), and other cellular processes (20 -23). Thus, we believe that the remodeling of lipid rafts during capacitation is of particular importance.…”

Glycobiology of Polysialic Acids on Sea Urchin Gametes

“…Second, ESP-Sia is uniquely characterized by the presence of 9-O-sulfated Neu5Gc residues at the nonreducing termini of the α2,5O glycolyllinked oligoNeu5Gc chains. Sulfation of Sia residues might serve as a stop signal for the elongation of polySia chain as well as a protection mechanism for the polySia structure, because the sulfated Sia residues are resistant to bacterial sialidases (31,33,34). The sulfated oligoSia chains units attached to the O-linked glycan chains of SBP in the vitelline layer (81).…”

“…Lipid rafts are characterized by the colocalization of receptor and transducer proteins, and recognized as a hot spot for signal transduction (35)(36)(37). We have shown that lipid rafts from sperm are bound by 350-kDa spermbinding protein (SBP) in the vitelline layer, during which a Sia recognition domain of SBP recognizes the Sia residues of the sperm gangliosides in the lipid rafts (31). Because sea urchin fertilization is inhibited by liposome containing the sperm ganglioside, the binding of the sperm ganglioside to SBP may be involved in interaction of sperm with the vitelline layer of the egg (31).…”

“…We have shown that lipid rafts from sperm are bound by 350-kDa spermbinding protein (SBP) in the vitelline layer, during which a Sia recognition domain of SBP recognizes the Sia residues of the sperm gangliosides in the lipid rafts (31). Because sea urchin fertilization is inhibited by liposome containing the sperm ganglioside, the binding of the sperm ganglioside to SBP may be involved in interaction of sperm with the vitelline layer of the egg (31). In contrast to the suggested importance of the sperm ganglioside during sea urchin fertilization, little attention has been paid to the structures and functions of Sia residues on glycoproteins.…”

“…We recently demonstrated that cell surface Sia residues have an important role in sperm-egg interaction during sea urchin fertilization (29)(30)(31). Unique Sia-containing glycolipids (gangliosides) are present in sea urchin gametes (32)(33)(34).…”