10.1021/acs.biochem.5b00414.s001 Thanh D. Do Thanh D. Do Ali Chamas Ali Chamas Xueyun Zheng Xueyun Zheng Aaron Barnes Aaron Barnes Dayna Chang Dayna Chang Tjitske Veldstra Tjitske Veldstra Harmeet Takhar Harmeet Takhar Nicolette Dressler Nicolette Dressler Benjamin Trapp Benjamin Trapp Kylie Miller Kylie Miller Audrene McMahon Audrene McMahon Stephen C. Meredith Stephen C. Meredith Joan-Emma Shea Joan-Emma Shea Kristi Lazar Cantrell Kristi Lazar Cantrell Michael T. Bowers Michael T. Bowers Elucidation of the Aggregation Pathways of Helix–Turn–Helix Peptides: Stabilization at the Turn Region Is Critical for Fibril Formation American Chemical Society 2015 aggregation cascade fiberlike aggregates helix peptide helical segments fibril formation Aggregation Pathways Fibril FormationAggregation assembly model D 26N mutants D 26 residue decreases E 21Q dynamics simulations 2015-07-07 00:00:00 Dataset https://acs.figshare.com/articles/dataset/Elucidation_of_the_Aggregation_Pathways_of_Helix_Turn_Helix_Peptides_Stabilization_at_the_Turn_Region_Is_Critical_for_Fibril_Formation/2152117 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.