Antigenic variation in the African trypanosome is mediated through changes in the composition of the surface coat1,2. By controlling expression of the major surface protein, the variant surface glycoprotein (VSG), from a repertoire of perhaps 1,000 different genes3 the organisms exhibit a series of antigenically distinct coats and evade the host's immune system. We have determined the 6 Å resolution structure of a T. brucei variant surface glycoprotein, ILTat 1.24, using X-ray crystallography. The crystallized protein consists of the N-terminal two-thirds of the intact VSG which has a relative molecular mass (Mr) of 60,000 (60K). The structure, which includes a 90 Å long α-helical bundle, is strikingly similar to that of the N-terminal fragment of VSG MFTat 1.2 (ref. 4). Although most known VSG sequences show little similarity of primary sequence in the N-terminal domain, the similarity between the structure of a Class I (ILTat 1.24) and a Class II (MITat 1.2) VSG antigen suggests that VSGs may share a common tertiary structure.
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