Structures of the oligosaccharides present at the three asparagine-linked glycosylation sites of human IgD.

The complete amino acid sequence of the human myeloma IgD:WAH has been determined and the sites of asparagine glycosylation identified as residues 354, 445, and 496 (Takahashi, N., Tetaert, D., Debuiere, B., Lin, L.-C., and Putnam, F. W. (1982) Proc. Natl. Acad. Sci. U. S. A. 79, 2850-2854). We have determined the structures of the oligosaccharides at each of these positions. Asn 354 bears oligosaccharides exclusively of the high mannose type containing from 5 to 9 mannose residues. Twenty per cent of the oligosaccharides at this site contain 1 glucose residue at the terminus of the branch emanating from the alpha 1 leads to 3-linked core mannose which is believed to reflect incomplete processing of the triglucosyl-high mannose oligosaccharide intermediate following transfer from dolichol to nascent peptide. Asn 445 and Asn 496 bear exclusively dibranched complex oligosaccharide structures; 30-40% of these molecules contain a bisecting GlcNAc-linked beta 1 leads to 4 to the innermost core mannose residue. At Asn 445, 40% of both the bisected and nonbisected oligosaccharides contain 1 residue of fucose on the Asn-linked GlcNAc and 50% bear a single N-acetylneuraminic acid residue. The oligosaccharides at Asn 496 are devoid of sialic acid and fucose. Thus, IgD:WAH is notable for the presence of virtually unprocessed oligosaccharide structures (glucosylated high mannose) on the same peptide backbone as extensively processed complex type molecules. The finding that each of the 3 Asn glycosylation sites of IgD:WAH bears either exclusively a complex or a high mannose type oligosaccharide indicates that there is considerable specificity in the glycosylation process. These oligosaccharides, nonetheless, display extensive microheterogeneity at each location.