Forced amyloidogenic cooperativity of structurally incompatible peptide segments: Fibrillization behavior of highly aggregation-prone A-chain fragment of insulin coupled to all-L, and alternating L/D octaglutamates

Self-aggregation of individual polypeptide chains into amyloid fibrils is driven by interactions between amyloidogenic segments of whole proteins. The interplay between aggregation-prone and aggregation-resistant fragments within a single polypeptide chain is not well understood. Here, we examine fibrillization behavior of two designed chimeric peptides, ACC1-13E8 and ACC1-13E8(L/D), in which the highly amyloidogenic fragment of insulin’s A-chain (ACC1-13) is extended by an octaglutamate segment composed of all-L (E8), or alternating L/D residues (E8(L/D)). As separate entities, ACC1-13 readily forms fibrils with the infrared features of parallel β-sheet structure while acidified E8 forms so-called β2-aggregates consisting of antiparallel β-sheets and manifesting distinctly in the amide I band infrared region. This contrasts with profoundly aggregation-resistant behavior of E8(L/D) peptide although the alternating L/D motif has been hypothesized as compatible with aggregated α-sheets. ACC1-13E8 and ACC1-13E8(L/D) peptides are equally prone to fibrillization when the electrostatic repulsion between dissolved monomers is prevented either by lowering pH, or in the presence of Ca2+ ions. In the aggregated states, both ACC1-13E8 and ACC1-13E8(L/D) reveal the infrared characteristics of ordered parallel β-sheet structure with no spectral features attributable to β2-aggregates (ACC1-13E8) or α-sheets (ACC1-13E8(L/D)). Hence, the preferred structural pattern of ACC1-13 segment not only overrides the tendency of E8 to form the antiparallel β2-structure but also enforces formation of β-sheet structure within the E8(L/D) segment which on its own is entirely refractory to aggregation. We demonstrate how an alternating L/D sequence can be effectively forced to become a part of highly ordered amyloid structure scaffolded by an all-L amyloidogenic segment. Our study shows how the overall amyloidogenic characteristics of a larger hybrid sequence may be impacted and controlled by the properties of its most aggregation-prone part.