Competing processes of micellization and fibrillization in native and reduced casein proteins.

Kappa-casein (κCN) and beta-casein (ßCN) are disordered proteins present in mammalian milk. In vitro, ßCN self-assembles into core-shell micelles. κCN self assembles into similar micelles, as well as into amyloid-like fibrils. Recent studies indicate that fibrillization can be suppressed by mixing ßCN and κCN, but the mechanism of fibril inhibition has not been identified. Examining the interactions of native and reduced kappa-caseins (N-κCN and R-κCN) with ßCN, we expose a competition between two different self-assembly processes: micellization and fibrillization. Quite surprisingly, however, we find significant qualitative and quantitative differences in the self-assembly between the native and reduced κCN forms. Specifically, thermodynamic analysis reveals exothermic demicellization for ßCN and its mixtures with R-κCN, as opposed to endothermic demicellization of N-κCN and its mixtures with ßCN at the same temperature. Furthermore, with time, R-κCN/ßCN mixtures undergo phase separation into pure ßCN micelles and R-κCN fibrils, while in the N-κCN/ßCN mixtures fibril formation is considerably delayed and mixed micelles persist for longer periods of time. Fibrils formed in N-κCN/ßCN mixtures are shorter and more flexible than those formed in R-κCN/ßCN systems. Interestingly, in the N-κCN/ßCN mixtures, the sugar moieties of N-κCN oligomers seem to organize on the mixed micelles surface in a manner similar to the organization of κCN in milk casein micelles.