Evidence for a C-terminal turn in PBAN. An NMR and distance geometry study.

The solution conformation of the pheromone biosynthesis activating neuropeptide (PBAN) of the moth Helicoverpa zea has been determined using homonuclear two-dimensional nuclear magnetic resonance techniques and distance geometry-restrained energy minimization. The insect peptide hormone showed a random distribution of conformers in aqueous solution, whereas in a less polar medium of trifluoroethanol and water, a reordering process was observed. In particular, the C-terminal region (Phe-Ser-Pro-Arg-Leu-NH2) adopts a type I' beta-turn conformation, residues 20-27 are in a helix conformation, and residues 1-19 exhibit a high degree of flexibility. Direct observation of the C-terminal beta-turn configuration of PBAN is consistent with a previous report that showed a rigid, cyclic analog of the C-terminal pentapeptide of PBAN retained pheromonotropic activity [Nachman, R.J., Kuniyoshi, H., Roberts, V.A., Holman, G.M. & Suzuki, A. (1993). Biochem. Biophys. Res. Commun. 661-666]. Furthermore, our results show no interaction between the C-terminal turn and the rest of the polypeptide chain, thus providing further evidence that the C-terminal turn is indeed the important conformation recognized by the PBAN receptor.