The effect of Arg and Gly on the chaperone propensity of bovine milk protein, β-casein, in preventing protein aggregation

Abstract

Protein aggregation can proceed via disordered (amorphous) or ordered (amyloid fibril) mechanisms. Conditions such as low pH or high temperature cause a conformation change in proteins, leading to aggregation and precipitation. β-Casein, a member of the casein family, has been demonstrated to exhibit chaperone-like activity. In the present study, we have examined the effect of small molecules like Arg and Gly on the chaperone activity of β-casein in preventing the aggregation of target proteins (insulin, α-lactalbumin, and ovotransferrin) using visible absorption spectroscopy, intrinsic fluorescence spectroscopy, 1-anilino-8-naphthalene sulfonic acid binding assay and circular dichroism spectroscopy. Arg decreased the rate of aggregation in all target proteins, while Gly increased it in the insulin and α-lactalbumin and slightly decreased the rate of aggregation in ovotransferrin. Importantly, our results show that the chaperone action of β-casein against the aggregation of reduced proteins is decreased in the presence of Arg and Gly. The effect of these additives, however, is dependent on the target protein undergoing aggregation. Apart from this, we found that Arg and Gly have a structural effect on β-casein, thus decreasing its chaperone activity. Thus, our results suggest that both the target protein and the structural change in β-casein in the presence of Arg and Gly play a critical role in decreasing the protective activity of β-casein.