The roles of turn formation and cross-strand interactions in fibrillization of peptides derived from the OspA single-layer beta-sheet

We previously demonstrated that a beta -hairpin peptide, termed BH9-10, der ived from a single-layer beta -sheet of Borrelia OspA protein, formed a nat ive-like beta -turn in trifluoroethanol (TFE) solution, and it assembled in to amyloid-like fibrils at higher TFE concentrations. This peptide is highl y charged, and fibrillization of such a hydrophilic peptide is quite unusua l. In this study, we designed a circularly permutated peptide of BH9-10, te rmed BH10-9. When folded into their respective beta -hairpin structures fou nd in OspA, these peptides would have identical cross-strand interactions b ut different turns connecting the strands. NMR study revealed that BH10-9 h ad little propensity to form a turn structure both in aqueous and TFE solut ions. At higher TFE concentration, BH10-9 precipitated with a concomitant a lpha -to-beta conformational conversion, in a similar manner to the BH9-10 fibrillization. However, the BH10-9 precipitates were nonfibrillar aggregat ion. The precipitation kinetics of BH10-9 was exponential, consistent with a first-order molecular assembly reaction, while the fibrillization of BH9- 10 showed sigmoidal kinetics, indicative of a two-step reaction consisting of nucleation and molecular assembly. The correlation between native-like t urn formation and fibrillization of our peptide system strongly suggests th at BH9-10 adopts a native-like beta -hairpin conformation in the fibrils. R emarkably, seeding with the preformed BH10-9 precipitates changed the two-s tep BH9-10 fibrillization to a one-step molecular assembly reaction, and di srupted the BH9-10 fibril structure, indicating interactions between the BH 10-9 aggregates and the BH9-10 peptide. Our results suggest that, in these peptides, cross-strand interactions are the driving force for molecular ass embly, and turn formation limits modes of peptide assembly.