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

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, ACC_1-13E₈ and ACC_1-13K₈, in which a potent amyloidogenic stretch derived from the N-terminal segment of the insulin A-chain (ACC_1-13) is coupled to octaglutamate or octalysine segments, respectively. While large electric charges prevent aggregation of either peptide at neutral pH, stoichiometric mixing of ACC_1-13E₈ and ACC_1-13K₈ triggers rapid self-assembly of two-component fibrils driven by favorable Coulombic interactions. The low-symmetry nonpolar ACC_1-13 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, ACC_1-13E₈ forms highly ordered fibrils also when paired with nonpolypeptide polycationic amines such as branched polyethylenimine, instead of ACC_1-13K₈. Such synthetic polycations are more effective in triggering the fibrillization of ACC_1-13E₈ than poly-K (or poly-E in the case of ACC_1-13K₈). 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.