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 (AFM), infrared spectroscopy, circular dichroism (CD), and molecular dynamics (MD) to study fibrillization patterns of two chimeric peptides, ACC1-13E8 and ACC1-13K8, in which potent amyloidogenic stretch derived from the N-terminal segment of insulin A-chain (ACC1-13) is coupled to octaglutamate or octalysine segments, respectively. While the large electric charges on monomers of either peptide prevent aggregation at neutral pH, stoichiometric mixing of ACC1-13E8 and ACC1-13K8 triggers rapid self-assembly of two-component fibrils driven by favorable Coulombic interactions. The role of low-symmetry non-polar ACC1-13 pilot sequence is crucial in enforcing the amyloidal parallel -sheet motif as self-assembly of free poly-E and poly-K chains under similar conditions results in amorphous antiparallel -sheet conformation. Interestingly, the pathway to highly ordered fibrils is accessible to ACC1-13E8 also when paired with non-polypeptide polycationic amines such as branched poly-ethylenimine, PEI, instead of ACC1-13K8. Remarkably, such synthetic polycations are more effective in triggering fibrillization of ACC1-13E8 than poly-K (or poly-E in the case of ACC1-13K8). 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 heterogenous disease-related amyloidogenesis.