A variant surface glycoprotein of Trypanosoma brucei synthesized with a C-terminal hydrophobic ‘tail’ absent from purified glycoprotein

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Abstract
Sequential expression of variant surface glycoproteins (VSGs) enables the parasitic protozoan Trypanosoma brucei to evade the immune response of its mammalian hosts1,2. Studies of several VSGs, which have been isolated as soluble molecules following disruption of cells in the absence of detergent, have indicated extensive amino acid diversity3–5 and the absence of a hydrophobic segment which might serve to anchor the carboxy terminus to the membrane5. The carboxy-terminal tryptic peptides of six VSGs have recently been characterized and shown to be glycosylated6. Three of these VSGs terminated with a glycosylated aspartate or asparagine residue (Asx), suggesting that the VSG was cleaved following synthesis and glycosylation and before characterization. We present here nucleotide sequence data which suggest that the primary translation product of one VSG gene contains a hydrophobic tail at the carboxy terminus which is not found on the isolated, mature glycoprotein6. The data also predict that the glycosylated residue is aspartic acid rather than the anticipated asparagine.