The membrane form of variant surface glycoproteins of Trypanosoma brucei

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Abstract
African trypanosomes are parasitic protozoa which are enveloped by a surface coat consisting of a matrix of identical glycoprotein molecules1. Variations in the composition of these variant surface glycoproteins (VSGs) allow the parasite to escape the host's immune system and render effective immunoprophylaxis improbable2–5. However, underlying the surface coat, all variant antigen types contain common membrane components6, some of which can activate complement by the alternative pathway7, leading to lysis of uncoated trypanosomes. Hence, stimulation of VSG release in vivo should be a potential form of chemotherapy, and we have therefore investigated the mode of attachment of VSG to the plasma membrane. Biochemical characterization of VSGs from several species has been performed on material purified after release from the cell surface following rupture of the trypanosome8–11. We demonstrate here that VSGs of Trypanosoma brucei when bound to the membrane exist in a form which differs both biochemically and immunochemically from VSGs purified in the conventional manner. After rupture of the cell, membrane-form VSG (mfVSG) is enzymatically transformed into the commonly isolated water-soluble released form (sVSG). In conditions in which this modification does not take place, purified VSGs have amphiphilic properties and behave as integral membrane proteins by the criterion of charge-shift electrophoresis. The difference between the two forms lies in the C-terminal domain, which is phosphorylated in both forms. This domain in sVSGs contains an immunogenic oligosaccharide known as the cross-reacting determinant (CRD), attached to the C-terminal amino acid12. Recognition of this determinant by anti-CRD antibodies is impaired in the membrane form.