Aggregation Behavior of Bovine κ- and β-Casein Studied with Small Angle Neutron Scattering, Light Scattering, and Cryogenic Transmission Electron Microscopy

In the native bovine casein micelle the calcium sensitive caseins (alpha(S1)-, alpha(S2)- and beta-casein) sequester amorphous calcium phosphate in nanometer-sized clusters, whereas the calcium-insensitive K-casein limits the growth of the micelle. In this paper, we further investigate the self-association of kappa- and beta-casein, which are two of the key proteins that control the substructure of the milk casein micelle, using neutron and light scattering techniques and cryogenic transmission electron microscopy. Results demonstrate that K-casein can, apart from the known self-assembly, form amyloid-like fibrils already at temperatures of 25 degrees C when subject to agitation. This extended aggregation behavior of kappa-casein is inhibited by beta-casein, as reported by others. These findings have implications for the structure and stability of casein micelles. The neutron scattering data was used to gain information on the self-assembly structure of kappa-casein. beta-Casein shows similar self-association behavior as kappa-casein, but unlike kappa-casein, the self-association exhibits temperature dependence within the studied temperatures (6 and 25 degrees C). Here, we will discuss our extended study of the known self-assembly of casein in the context of the fibrillation of kappa-casein.