Two variant surface glycoproteins of Trypanosoma brucei have a conserved C-terminus

Abstract

Gene duplication and transposition are known to be involved in the mechanism of antigenic variation in Trypanosoma brucei^1–3. However, the structure of the antigens in question—the variant surface glycoproteins (VSGs)—is poorly defined. Limited sequencing data show extensive variation between the N-terminal amino acid sequences of different VSGs^4,5, although serological studies indicate common antigenic determinants in the C-terminal portion (refs 6, 7 and N. Van Meirvenne, personal communication). More recently, the existence of a C-terminal hydrophobic tail, which is absent from the isolated glycoprotein, was reported⁸. We have now compared the messenger RNA sequences corresponding to the C-terminal 115 amino acids of two serologically different variants belonging to the same serodeme and report a 35% lack of homology (52% of amino acid positions). However, there is a large homologous area of 33 amino acids making up the C-terminus, which includes the 23-amino acid hydrophobic tail. Our data indicate that this extension is a universal feature in T. brucei VSGs. In addition, comparison of two isotypes (serologically similar VSGs) belonging to different serodemes showed almost complete nucleotide homology in the coding sequence of the C-terminal 115 amino acids.