Structure of the O-Linked Oligosaccharides from a Major Thyroid Cell Surface Glycoprotein

A major glycoprotein at the surface of calf thyroid cells, GP-3 (Mr20,000), contains the I-antigenic activity of calf thyroid which has been attributed to its poly-N-acetyllactosamine N-linked saccharide chains (Edge, A. S. B., and Spiro, R. G.,J. Biol. Chem.260, 15332–15338, 1985). The present study demonstrated that alkaline borohydride treatment of GP-3 results in the release of five neutral and five acidic saccharides that were found to represent over 30% of the saccharides of this carbohydrate-rich glycoprotein. Three of the oligosaccharides contained terminal α1 → 3-linked galactose residues which accounted for their affinity towardBandeiraea simplicifoliaI lectin. The saccharides could be grouped into several distinct categories on the basis of their internal sequence. A novel tetrasaccharide in GP-3 was shown to have the structure: Galα1 → 3Galβ1 → 6(Galβ1 → 3)GalNAcH2. An unsubstitutedN-acetylgalactosamine unit and a Galβ1 → 3GalNAc disaccharide were prominent O-linked constituents, with the disaccharide serving as a core unit for the attachment of sialic acid residues to form tetra- and trisaccharides. A branched core structure, Galβ1 → 3(GlcNAcβ1 → 6)GalNAcH2was shared by 4 of the 10 saccharides, the most complete of which was assigned the sequence NeuAcα2 → 3Galβ1 → 3(Galα1 → 3Galβ1 → 4GlcNAcβ1 → 6)GalNAcH2.