Mutational analysis of designed peptides that undergo structural transition from α helix to β sheet and amyloid fibril formation

Background: Conformational alteration and fibril formation of proteins have a key role in a variety of amyloid diseases. A simplified model peptide would lead to a better understanding of underlying mechanisms whereby protein misfolding and aggregation occur. Recently, we reported the design of peptides that undergo a self-initiated structural transition from an or helix to a beta sheet and form amyloid fibrils. In this study, we focus on two glutamine residues in the peptide, and report a mutational analysis of these residues. Results: A coiled-coil ct-helix structure bearing a hydrophobic adamantanecarbonyl (Ad) group at the N terminus was designed (parent peptide Ad-CC). in neutral aqueous solution, the double Gln->Ala mutant (Ad-AA) underwent the alpha->beta structural transition within four hours, which was similar to the case of Ad-CC. In contrast, two kinds of single Gln->Ala mutant (Ad-QA and Ad-AQ) required three days for the transition. Furthermore, Ad-CC and Ad-AA formed amyloid fibrils, whereas Ad-QA and Ad-AQ did not. Interestingly, however, Ad-QA and Ad-AQ complementarily assembled into the fibrils when they were mixed. Conclusions: The Gln->Ala substitution in the peptide significantly alters the alpha->beta transitional properties and the ability to form amyloid fibrils. A heterogeneous assembly of two peptide species into the fibrils is also presented. These results suggest that the secondary structural transition and self-assembly into the well-organized fibril may depend strictly on the primary structure, which determines the beta-sheet packing. The results might provide insights into misfolding and fibril formation of disease-associated mutant proteins.