Self-Assembly of Insulin-Derived Chimeric Peptides into Two-Component Amyloid Fibrils: The Role of Coulombic Interactions

Canonical amyloid fibrils are composed of covalently identical polypeptide chains. Here, we employ kinetic assays, atomic force microscopy, infrared spectroscopy, circular dichroism, and molecular dynamics simulations to study fibrillization patterns of two chimeric peptides, ACCE and ACCK, in which a potent amyloidogenic stretch derived from the N-terminal segment of the insulin A-chain (ACC) is coupled to octaglutamate or octalysine segments, respectively. While large electric charges prevent aggregation of either peptide at neutral pH, stoichiometric mixing of ACCE and ACCK triggers rapid self-assembly of two-component fibrils driven by favorable Coulombic interactions. The low-symmetry nonpolar ACC pilot sequence is crucial in enforcing the fibrillar structure consisting of parallel β-sheets as the self-assembly of free poly-E and poly-K chains under similar conditions results in amorphous antiparallel β-sheets. Interestingly, ACCE forms highly ordered fibrils also when paired with nonpolypeptide polycationic amines such as branched polyethylenimine, instead of ACCK. Such synthetic polycations are more effective in triggering the fibrillization of ACCE than poly-K (or poly-E in the case of ACCK). The high conformational flexibility of these polyamines makes up for the apparent mismatch in periodicity of charged groups. The results are discussed in the context of mechanisms of heterogeneous disease-related amyloidogenesis.