Identification of polysialic acid-containing glycoprotein in the jelly coat of sea urchin eggs. Occurrence of a novel type of polysialic acid structure

Sea urchin eggs are surrounded by a gelatinous layer (called the jelly coat) that consists of mixture of fucose-rich polysaccharides and sialic acid-rich glycoproteins. Chemical and 500 MHz ¹H NMR spectroscopic studies revealed for the first time the presence of a novel polysialic acid (polySia) structure in the jelly coat glycoproteins isolated from Hemicentrotus pulcherrimus (designated polySia-gp(H)). The structure of the polySia chains was thoroughly characterized as (→5-O(glycolyl)-Neu5Gcα2→)(n), where n ranges from 4 to more than 40 Neu5Gc residues. The polyNeu5Gc chains were attached to core oligosaccharides that were O-glycosidically linked to threonine residues on a core polypeptide. Each polypeptide contained about 17 O-linked polysialylglycan chains. The apparent molecular weight of polySia- gp(H) was 180,000. The expression of this new polySia structure in place of α2→8-linked polySia is the main structural feature that distinguishes polySia-gp from other known polysialylated glycoproteins. The (→5- O(glycolyl)-Neu5Gcα2→)(n) chains were resistant to exo- and endosialidases from Arthrobacter ureafaciens and bacteriophage K1F, respectively. Discovery of these (→5-O(glycolyl)-Neu5Gcα2→)(n) chains adds a new class of naturally occurring polySia to the structurally diverse family of polysialylated glycoproteins. The structure of a polySia-gp from a different sea urchin species, Strongylocentrotus purpuratus (designated polySia- gp(S)), was also determined to ascertain if there were any species-specific differences. The 500 MHz ¹H NMR spectra of the two polySia-gps were identical, indicating that at this level of molecular detail the structures were the same. The molecular weight of polySia-gp(S) was larger, however (250,000), and it contained about 25 polySia chains O-glycosidically linked to both threonine (two-thirds) and serine (one-third) residues.