Secondary structure of the variant surface glycoproteins of trypanosomes

The secondary structure of seven variant surface glycoproteins (VSGs) of trypanosomes has been determined by Raman spectroscopy. They are all predominantly alpha-helical, the alpha-helix content varying between 50 and 60%. The beta-strand content varies between 20 and 25%, and the content of beta-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 alpha-helices extending over about 50 residues or 80 A 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 alpha-helical segments exhibits a high potential of beta-turns, suggesting that this segment may be exposed on the cell surface and carry major antigenic determinants.