Characterization of the major core structures of the alpha 2->8-linked polysialic acid-containing glycan chains present in neural cell adhesion molecule in embryonic chick brains

To gain more insight into the possible functional significance of the core glycan chain(s) on which polysialylation takes place in polysialic acid (poly-Sia) containing glycoproteins, the structure of the core glycans in the embryonic form of chick brain neural cell adhesion molecule (N-CAM) were examined using chemical and instrumental techniques. The following new structural features, which had not been reported by the early pioneering study by Finne (Finne, J. (1982) J. Biol. Chem. 257, 11966-11970), were revealed (Structure I). (i) Two distinct types of multiantennary N-linked glycans, i.e. tri- and tetra-antennary structures, are present; (ii) an alpha 1-->6-linked fucosyl residue is attached to the proximal GlcNAc residue of the di-N-acetylchitobiosyl unit; (iii) that the action of GlcNAc-transferase V, which catalyzes the attachment of the beta-(1-->6)-alpha-linked GlcNAc residue on the (1-->6)-alpha-linked mannose (Man) arm, appears to be essential for polysialylation to occur on the core glycan chain is suggested by the fact that the Man residue alpha 1-->6-linked to the beta-linked Man residue is invariably 2,6-di-O-substituted by the GlcNAc residue; (iv) both type 1 (Gal beta 1-->3GlcNAc) and type 2 (Gal beta 1-->4GlcNAc) sequences are present in the peripheral portion of the core glycan structure. An extended form of the type 2 chain, i,e. Gal beta 1-->4GlcNAc beta 1-->3Gal beta 1--4GlcNAc, is also expressed on the (1-->3)- and (1-->6)-alpha-linked Man arms; (v) on average about 1.4 mol of sulfate is attached to the type 2 N-acetyllactosamine chain(s), where in the extended form the sulfate group is probably substituted at the O-3 position of the outmost GlcNAc residue, i.e. Gal beta 1-->4(HSO3-->3)GlcNAc beta 1-->3Gal beta 1-->4GlcNAc beta 1-->Man. It is possible that the unusual structural features identified in this study might play a role in the initiation of polysialylation and our data should facilitate future research regarding the signals that control polysialylation.