HPLC and HPAEC of oligosaccharides and glycopeptides

Glycosylation and epitope mapping of the 5T4 glycoprotein oncofoetal antigen

The human 5T4 oncofoetal antigen is a focus for development of several antibody-directed therapies on the basis of the murine monoclonal antibody against 5T4 (mAb5T4), which recognizes a conformational epitope. 5T4 molecules are highly N-glycosylated transmembrane glycoproteins whose extracellular domain contains two regions of leucine-rich repeats (LRRs) and associated flanking regions, separated by an intervening hydrophilic sequence. Using a series of deletion and mutated cDNA constructs as well as chimaeras with the murine homologue, we have mapped the mAb5T4 epitope to the more membrane-proximal LRR2 or its flanking region. Analysis of the glycosylation of the seven consensus Asp-Xaa-Ser/Thr sites was consistent with all of the sites being glycosylated. A combination of two high-mannose chains (predominantly octasaccharide) and five mostly sialylated bi-, tri- and tetra-antennary complex chains with minor quantities of core fucose were detected. The two glycosylation sites, which are the most likely to have predominantly high-mannose chains, are in the only two regions that show significant differences between the human and the 81% identical mouse sequence. A site-directed mutation, which abolished glycosylation at one of these sites (position 192), did not alter antigenicity. The other, which is nearest to the N-terminus in the human, has an Asn-Leu-Thr to Asn-Leu-Leu conversion in the mouse, so cannot be glycosylated in the latter species. The large complex glycosylation at the other sites is likely to influence the antigenicity and tertiary structure generating the 5T4 epitope.

Microassay analyses of protein glycosylation

To accurately characterize the carbohydrate moieties of oligosaccharide chains in glycosylated proteins, it is necessary to distinguish exactly which types of oligosaccharides are present at which site. We describe lectin overlay assays, which take advantage of the ability of lectins to distinguish between different types of glycoproteins via recognition of terminal sugars, thus allowing the chain type and peripheral antigenic components to be determined. Three microassays involving lectins are reported in this paper: non-protease-treated intact glycoproteins; glycopeptides released by prior digestion of the glycoprotein and then separated by HPLC; and release of sugars from glycoproteins by hydrazinolysis and then coupling them to a multivalent support.