Thermally triggered self-assembly of kappa-casein amyloid nanofibrils and their nanomechanical properties

Abstract

Understanding the self-assembly process of amyloid nanofibrils is important in protein engineering for their potential uses in material sciences and medical applications. Here, we investigated the nanomechanical properties and structures of kappa-casein nanofibrils during thermally induced amyloidogenesis. After treatment with dithiothreitol to reduce disulfide bonds, kappa-casein from bovine milk was incubated at various temperatures from 37 to 95 degrees C. Twisted mature nanofibrils were only obtained at high thermal energies of 95 degrees C with fast growth kinetics. Nanomechanical studies using atomic force microscopy show the highest Young's moduli (2.6 +/- 0.4 GPa) for mature fibrils when compared to those of oligomers and protofibrils (0.6 +/- 0.1 and 2.2 +/- 0.3 GPa, respectively). These enhanced mechanical properties are mainly attributed to the twisted cores of the mature fibrils containing beta-sheet stacks. Amyloidogenesis of kappa-casein is highly dependent on the magnitude of thermal energy, determining both the types and shapes of the nanofibrils and their different mechanical properties.