Separation and characterization of the two Asn-linked glycosylation sites of chicken serum riboflavin-binding protein. Glycosylation differences despite similarity of primary structure.

Serum riboflavin-binding protein, a phosphoglycoprotein from the blood of laying hens, contains two Asn-Xaa-(Thr)Ser sequons in very similar but well-separated regions of amino acid sequence. In order to evaluate the effect of local amino acid sequence on the structure of the attached oligosaccharides, serum riboflavin-binding protein was purified to homogeneity, reduced and alkylated, digested with trypsin, and the two glycopeptides were separated by reversed-phase chromatography. After digestion with peptide-N-glycosidase F the released oligosaccharides were separated by high-pH anion-exchange chromatography and the oligosaccharide profiles of the two glycopeptides were compared. Although the two asparagine residues that are glycosylated are contained in pentapeptide segments in which four out of five amino acids are identical, the array of oligosaccharides present at each site show differences in both type and distribution. This suggests that local secondary or tertiary structure, or the order of glycosylation, influences the oligosaccharide structure more than does the primary structure flanking the attachment site.