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 (M(r)20,000), 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 alpha 1 --> 3-linked galactose residues which accounted for their affinity toward Bandeiraea simplicifolia I 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: Gal alpha 1 --> 3Gal beta 1 --> 6(Gal beta 1 --> 3)GalNAcH(2). An unsubstituted N-acetylgalactosamine unit and a Gal beta 1 --> 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, Gal beta 1 --> 3(GlcNAc beta 1 --> 6)GalNAcH(2) was shared by 4 of the 10 saccharides, the most complete of which was assigned the sequence NeuAc alpha 2 --> 3Gal beta l --> 3(Gal alpha l --> 3Gal beta 1 -->4GlcNAc beta 1 -->6)GalNAcH(2). (C) 1997 Academic Press