Molecular and Biochemical Parasitology, 7 (1983) 331-338 331 Elsevier Biomedical Press CHARACTERISATION OF THE CROSS-REACTING CARBOHYDRATE GROUPS ON TWO VARIANT SURFACE GLYCOPROTEINS OF TR YPANOSOMA BR UCEI ANTHONY A. HOLDER Department of Molecular Biology, The Wellcome Research Laboratories, Langley Court, Beckenham, Kent BR3 3BS, England (Received 1 November 1982, accepted 23 December 1982) The nature of the cross-reacting groups on two variant surface glycoproteins of Trypanosoma brucei has been investigated after isolation of glycopeptides produced by extensive proteolytic diges- tion of the proteins. One variant yielded two glycopeptides after pronase digestion, one of which was the glycosylated C-terminal aspartic acid. In a second variant there are two carbohydrate groups close to the C-terminus. Considerable heterogeneity in the size of the sugar attached to an asparagine residue was detected whereas the C-terminal sefine was glycosylated but showed no size heterogeneity. For both variants it was shown that the C-terminal glycosylated amino acid (either aspartic acid or serine) was responsible for the immunological cross-reaction between distinct variant glycoproteins. The stability of the cross-reacting determinant was also investigated. Key words: Trypanosoma brucei; Variant surface glycoprotein; Carbohydrate heterogeneity; Glycopeptide; Cross-reaction INTRODUCTION The sequential expression of genes for different variant surface glycoproteins (VSGs) of Trypanosoma brucei is responsible for the antigenic variation displayed by this parasite (reviewed by Borst and Cross [1]). VSGs which are immunologically distinct differ ex- tensively in their primary amino acid sequence. Despite this variation there are cross- reacting determinants on VSGs which can be detected by antisera raised against the purified proteins [2, 3]. The basis of this cross-reaction was shown to be a carbohydrate moiety attached at or close to the C-terminus of the mature protein [4]. In five variants the terminal amino acid, either aspartic acid or serine, was found to be glycosylated. In at least two of these proteins the carbohydrate is attached to the c~-carboxyl group of the terminal amino acid through ethanolamine [5]. In one group of VSGs there are two glycosylation sites within 5 amino acid residues of the C-terminus and there can be considerable heterogeneity in the size of the C-terminal tryptic peptide as a result of variation in the size of the sugar side chains attached [4]. It is shown here that the size heterogeneity is due to the sugar N-linked to an asparagine 0166-6851/83/0000- 0000/$03.00 © 1983 Elsevier Science Publishers