Conformational adaptability of the terminal regions of flagellin

Secondary structure formation in the disordered terminal regions of flagellin were studied by circular dichroic (CD) spectroscopy, Fourier transform infrared spectroscopy, and x-ray diffraction. The terminal regions of flagellin are known to form alpha-helical bundles upon polymerization into flagellar filaments. We found from comparative CD studies of flagellin and its F40 tryptic fragment that a highly alpha-helical conformation can be induced and stabilized in the terminal regions in 2,2,2-trifluoroethanol (TFE) containing solutions, which is known to promote intra-molecular hydrogen bonding. Two oligopeptides, N(37-61) and C(470-494), each corresponding to a portion of terminal regions and predicted to have a high alpha-helix forming potential, were synthesized and studied. Both peptides were disordered in an aqueous environment, but they showed a strong tendency to assume alpha-helical structure in solutions containing TFE. On the other hand, peptides were found to form transparent gels at high concentrations (> 15 mg/ml) and all three methods confirmed that the peptides become ordered into a predominantly beta structure upon gel formation. Our results show that large segments of the disordered terminal regions of flagellin can adopt alpha-helical as well as beta structure depending on the environmental conditions. This high degree of conformational adaptability may be reflecting some unique characteristics of the flagellin termini, which are involved in self-assembly and polymorphism of flagellar filament.