Back to the oligomeric state: pH-induced dissolution of concanavalin A amyloid-like fibrils into non-native oligomers

Abstract

The subtle interplay between long range electrostatic forces, hydrophobic interactions and short range protein–protein interactions regulates the onset/evolution of protein aggregation processes as well as the stability of protein supramolecular structures. Using a combination of FTIR spectroscopy, light scattering and advanced imaging, we present evidence on the main role of electrostatic forces in the formation and stability of amyloid-like fibrils formed from concanavalin A (ConA), a protein showing structural homology with the human serum amyloid protein. At high protein concentration, where protein–protein interactions cannot be neglected, we highlight a thermal-induced aggregation pathway in which amyloid-like aggregates are readily formed. When dissolved in solutions at different pHs, these aggregates show either a reduced β-sheet content keeping the same morphology (3 < pH < 10) or they are promptly dissolved leaving in solution non-native oligomers (pH > 10). The latter result can be ascribed to the change of the charge state for the ConA amino acid side chain with high pK_a values. Our results support the idea of fibrils as a reservoir of oligomeric species that can be released if changes or discontinuities in the aggregate microenvironment occur.