Glycosylation and posttranslational maturation of glycoproteins in embryonal carcinomas: identification of two distinct pools of high-mannose glycans.

Embryonal carcinomas and early embryonic cells assemble a family of unusually large and complex carbohydrates. These glycans contain large amounts of the sugars galactose and N-acetylglucosamine and are decorated with fucose, sulfate, and sialic acids. We show that, by their sensitivity to inhibition by tunicamycin and by their resistance to cleavage by alkaline hydrolysis, in teratocarcinoma stem cells the expression of these glycans is on asparagine-linked cores. These glycans are part of the large spectrum of glycans that are assembled on mannose cores derived from a common, lipid-linked precursor glycan. We examined the fate of this precursor glycan after its transfer to protein and found that there are two distinct pools of protein-linked, high-mannose glycans, which can be distinguished on the basis of their rate of processing. One pool is processed rapidly to provide a wide spectrum of complex-type glycans. This processing occurs efficiently with little evidence of intermediate structures. The other, larger pool remains unprocessed, beyond glucose removal, at a time when complex-type glycans cease to accumulate. In contrast, high-mannose glycans are relatively minor components of the glycans labeled during long-term, continuous labeling, and in this situation they are processed to provide a spectrum of trimmed glycans.