Dephosphorylation of α_S- and β-caseins and its effect on chaperone activity: A structural and functional investigation

Milk casein proteins can act as molecular chaperones: under conditions of stress, such as elevated temperature, molecular chaperones stabilize proteins from unfolding, aggregating, and precipitating. In this study, α_S- and β-caseins were dephosphorylated using alkaline phosphatase. A structural and functional investigation was undertaken to determine the effect of dephosphorylation on the chaperone activity of α_S- and β-caseins against two types of protein misfolding, i.e., amorphous aggregation and amyloid fibril assembly. The dephosphorylation of α_S- and β-caseins resulted in a decrease in the chaperone efficiency against both heat- and reduction-induced amorphously aggregating target proteins. In contrast, dephosphorylation had no effect on the chaperone activity of α_S- and β-caseins against the amyloid-forming target protein κ-casein. Circular dichroism and fluorescence spectroscopic data indicated that the loss of negative charge associated with dephosphorylation led to an increase in ordered structure of α_S- and β-caseins. It is concluded that the flexible, dynamic, and relatively unstructured and amphiphatic nature of α_S- and β-caseins is important in their chaperone action.