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.

Aggregation of proteins into amyloid fibrils is driven by interactions between relatively small amyloidogenic segments. The interplay between aggregation-prone and aggregation-resistant fragments within a single polypeptide chain remains obscure. Here, we examine fibrillization behavior of two chimeric peptides, ACCE and ACCE, in which the highly amyloidogenic fragment of insulin (ACC) is extended by an octaglutamate segment composed of all-L (E), or alternating L/D residues (E). As separate entities, ACC readily forms fibrils with the infrared features of parallel β-sheet while E forms antiparallel β-sheets with the distinct infrared characteristics. This contrasts with the profoundly aggregation-resistant E, although L/D patterns have been hypothesized as compatible with aggregated α-sheets. ACCE and ACCE are found to be equally prone to fibrillization at low pH, or in the presence of Ca ions. Fibrillar states of both ACCE and ACCE reveal the infrared features of highly ordered parallel β-sheet without evidence of β-aggregates (ACCE) or α-sheets (ACCE). Hence, the preferred structural pattern of ACC overrides the tendency of E to form antiparallel β-sheets and enforces the fibrillar order in E. We demonstrate how the powerful amyloid stretch determines the overall amyloid structure forcing non-amyloidogenic fragments to participate in its native amyloid pattern.