Secondary structure of the variant surface glycoproteins of trypanosomes

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Abstract
The secondary structure of seven variant surface glycoproteins (VSGs) of trypanosomes has been determined by Raman spectroscopy. They are all predominantly α-helical, the α-helix content varying between 50 and 60%. The β-strand content varies between 20 and 25%, and the content of β-turn and nonregular structures is about 25%. For three VSGs the N-terminal domain obtained by proteolytic cleavage was found to have essentially the same secondary structure as the complete VSGs. For three VSGs a secondary structure prediction has been performed applying the rules of Chou and Fasman. In all cases, two long α-helices extending over about 50 residues or 80 Å are predicted in agreement with the X-ray diffraction data of Freymann et al. [(1984) Nature 311, 167–169] and Metcalf et al. [(1987) Nature 325, 84–86]. The region between the two α-helical segments exhibits a high potential of β-turns, suggesting that this segment may be exposed on the cell surface and carry major antigenic determinants.