The variable surface glycoprotein (VSG) is the predominant component of the surface coat of the African trypanosome1. The expression of antigenically distinct VSGs on minor populations during infection allows the parasite to escape the host immune response2,3. Purification of the protein is facilitated by the enzymatic release of a soluble form of VSG (sVSG) which occurs on cell lysis4. The soluble form is a dimer with an approximate molecular weight of 120,000-130,0005. Partial proteolysis of sVSG6 reveals a protease-sensitive link between an amino-terminal domain which comprises about two-thirds of the molecule, and a C-terminal domain which contains the membrane attachment site7. We have obtained crystals suitable for high-resolution structural analysis from preparations of three sVSG: MITat 1.2, ILTat 1.25 and ILTat 1.22. The crystal structure of the dimer of the MITat 1.2 amino-terminal domain has been solved to 6 resolution. We report here that the dimer is an unusual 90 rod-like molecule composed of a helical bundle of at least four 80 -long α-helices.
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