Structural studies on the major oligosaccharides in a variant surface glycoprotein of Trypanosoma congolense

The carbohydrate moieties in the four isotypes of a variant surface glycoprotein from Trypanosoma congolense were analyzed. All variant surface glycoprotein isotypes were found to contain up to 15% by weight of D-galactose, D-mannose, and N-acetyl-D-glucosamine in molar ratios approaching 1:3.2:3.9 (isotypes I-III) or 1:2.4:2.4 (isotype IV); in addition, the presence of sialic acid could be demonstrated. After metabolic labelling with D-[6-3H]glucosamine, the four isoglycoproteins were successively digested with pronase and with endo-beta-N-acetylglucosaminidase H. Up to two thirds of the oligosaccharides were thus liberated and were separated by gel filtration, and by high performance liquid chromatography. Using methylation, gas chromatography, mass spectrometry and digestion with alpha-mannosidase, they were shown to be mainly typical oligomannosidic oligosaccharides of size classes Man5GlcNAc to Man9GlcNAc. The residual glycans were liberated by hydrazinolysis, and were fractionated by serotonin affinity chromatography. After separation by gel filtration, the neutral oligosaccharides from isotype I were subjected to methylation analysis and successive exoglycosidase digestions. They were found to be biantennary oligosaccharides of the N-acetyllactosaminic type: (GalGlcNAc)2Man3GlcNAc1-2. Only about 30% of the sialylated glycans were susceptible to neuraminidases. The T. congolense variant surface glycoprotein studied here contains mainly high mannose and biantennary 'complex' oligosaccharides as found in many other eukaryotic glycoproteins, except that they seem to carry unusually substituted/linked sialic acid residues.