The occurrence of novel 9-O-sulfated N-glycolylneuraminic acid-capped alpha 2->5-O-glycolyl-linked oligo/polyNeu5Gc chains in sea urchin egg cell surface glycoprotein - Identification of a new chain termination signal for polysialyltransferase

We report the isolation and structural characterization of an oligo/polysialic acid-containing glycopeptide fraction (designated ESP-Sia) prepared from the egg cell surface complex of the sea urchin, Hemicentrotus pulcherrimus, by exhaustive pronase treatment, The carbohydrate chains isolated from ESP-Sia were shown 60 consist of O-linked oligo/polysialic acid-containing glycan units and N-linked carbohydrate chains. The present studies have revealed that the O-linked oligo/polysialic acid-containing glycan chains derived from the ESP-Sia were similar to those present in egg jelly coat polysialylated glycoprotein in being composed of tandem repeats of N-glycolylneuraminic acid (Neu5Gc) glycosidically Linked in a novel fashion through the glycolyl group, (-->5-O(glycolyl)Neu5Gc alpha 2-->)(n). However, they differ from the egg jelly coat in two key respects. First, the average degree of polymerization of the oligo/polysialic acid chains of ESP-Sia is only 3; a value far lower than that found in the jelly coat glycoprotein (average degree of polymerization was about 20). Second, ESP-Sia is uniquely characterized by the presence of 9-O-sulfated N-glycolylneuraminic acid (Neu5Gc9HSO(3)) residues at the nonreducing termini of the (-->5-(Oglycolyl)Neu5Gc alpha 2-->)(n) chains. The terminal sialyl residues in the Neu5Gc9HSO(3) alpha 2-->(-->5-(Oglycolyl)Neu5Gc alpha 2-->)(n) chain s were totally resistant to exosialidases. The discovery of Neu5Gc9HSO(3), as the nonreducing terminal residue of oligo/poly(-->5-O(glycolyl)Neu5Gc alpha 2-->)(n) group is especially noteworthy in that Neu5Gc9HSO(3) appears to be of limited distribution among glycoconjugates. Following the earlier discovery of oligo/polysialic acid chains capped with KDN, i.e. KDN alpha 2-->(-->8Neu5Gc alpha 2-->)(n), found in rainbow trout egg polysialoglycoproteins, it now appears that the sulfated Neu5Gc can serve a similar capping function.