THE OCCURRENCE OF NOVEL 9-O-SULFATED N-GLYCOLYLNEURAMINIC ACID-CAPPEDALPHA-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/po lysialic acid-containing glycopeptide fraction (designated ESP-Sia) pr epared from the egg cell surface complex of the sea urchin, Hemicentro tus pulcherrimus, by exhaustive pronase treatment, The carbohydrate ch ains isolated from ESP-Sia were shown 60 consist of O-linked oligo/pol ysialic 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) glyco sidically Linked in a novel fashion through the glycolyl group, (-->5- O(glycolyl)Neu5Gc alpha 2-->)(n). However, they differ from the egg je lly coat in two key respects. First, the average degree of polymerizat ion 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 degr ee of polymerization was about 20). Second, ESP-Sia is uniquely charac terized by the presence of 9-O-sulfated N-glycolylneuraminic acid (Neu 5Gc9HSO(3)) residues at the nonreducing termini of the (-->5-(Oglycoly l)Neu5Gc alpha 2-->)(n) chains. The terminal sialyl residues in the Ne u5Gc9HSO(3) alpha 2-->(-->5-(Oglycolyl)Neu5Gc alpha 2-->)(n) chain s w ere totally resistant to exosialidases. The discovery of Neu5Gc9HSO(3) , as the nonreducing terminal residue of oligo/poly(-->5-O(glycolyl)Ne u5Gc alpha 2-->)(n) group is especially noteworthy in that Neu5Gc9HSO( 3) appears to be of limited distribution among glycoconjugates. Follow ing the earlier discovery of oligo/polysialic acid chains capped with KDN, i.e. KDN alpha 2-->(-->8Neu5Gc alpha 2-->)(n), found in rainbow t rout egg polysialoglycoproteins, it now appears that the sulfated Neu5 Gc can serve a similar capping function.