Elucidation of the Aggregation Pathways of Helix–Turn–Helix Peptides: Stabilization at the Turn Region Is Critical for Fibril Formation

Aggregation of proteins to fiberlike aggregates often involves a transformation of native monomers to β-sheet-rich oligomers. This general observation underestimates the importance of α-helical segments in the aggregation cascade. Here, using a combination of experimental techniques and accelerated molecular dynamics simulations, we investigate the aggregation of a 43-residue, apolipoprotein A-I mimetic peptide and its E21Q and D26N mutants. Our study indicates a strong propensity of helical segments not to adopt cross-β-fibrils. The helix–turn–helix monomeric conformation of the peptides is preserved in the mature fibrils. Furthermore, we reveal opposite effects of mutations on and near the turn region in the self-assembly of these peptides. We show that the E21–R24 salt bridge is a major contributor to helix–turn–helix folding, subsequently leading to abundant fibril formation. On the other hand, the K19–D26 interaction is not required to fold the native helix–turn–helix peptide. However, removal of the charged D26 residue decreases the stability of the helix–turn–helix monomer and consequently reduces the level of aggregation. Finally, we provide a more refined assembly model for the helix–turn–helix peptides from apolipoprotein A-I based on the parallel stacking of helix–turn–helix dimers.