κ-Casein inhibits amorphous aggregation of β-casein

Caseins are mammalian milk proteins that are commonly utilized in food industries due to their various nutritional and functional properties. Among various caseins, viz. α_S1-, α_S2-, β- and κ-casein, β-casein acts as a natural emulsifier and foam stabilizer as well as a molecular chaperone. However, these properties get significantly altered during food processing, which raises concerns about the food quality. Recently, there has been an increasing interest in using chaperone-like molecules to restrict the aggregation of food proteins. Here we show that κ-casein, which coexists with β-casein in milk micelles, exhibits its chaperonic activity and can effectively inhibit calcium ion-induced amorphous aggregation of β-casein in a dose-dependent manner. Using turbidity assays, dynamic light scattering, and electron microscopy, we demonstrate that inter-protein complexes comprising β- and κ-caseins are formed that preclude the binding of calcium ions (Ca²⁺) to β-casein. This further impedes the multivalent interactions between β-casein polypeptides, thus inhibiting inter-casein interactions required for spontaneous Ca²⁺-induced β-casein self-assembly. We also report the disintegration of pre-formed β-casein aggregates upon the addition of κ-casein, which may have implications in devising strategies for regulating the assembly and disassembly processes of β-casein and other food proteins.