Alpha-casein as a molecular chaperone

Alpha-casein, more specifically known as αS-casein, is a predominant milk protein with important nutritional properties. αS-Casein, composed of two individual gene products, αS1and αS2-casein, has been described in the past few decades as having molecular chaperone properties. In performing as molecular chaperones, αScasein and its purified constituent proteins, αS1and αS2-casein, stabilise a wide range of proteins from milk and non-milk sources against aggregation and precipitation under conditions of stress (e.g. heat, reduction). Investigations into the chaperone action of αS-casein have revealed that it stabilises its partially unfolded ‘target’ proteins by interacting with them and forming a soluble, high molecular weight complex similar to that formed by the small heat-shock proteins (sHsps) and another unrelated chaperone, clusterin. In addition, it has been shown that αS-casein is able to protect target proteins from aggregation when forming either amorphous or fibrillar aggregates, and that its chaperone activity is dependent on the target protein present, the stress conditions applied, the mode of protein aggregation (i.e. amorphous versus fibrillar), the speed of aggregation, and the presence of competing ions. Like the sHsps and clusterin, αS-casein is ATP-independent in its action, is unable to refold partially unfolded proteins and cannot prevent loss of enzyme activity under heat stress. Unlike the sHsps and clusterin, however, αS-casein binds its target proteins in a way that does not facilitate interaction with Hsp70 which in the presence of ATP can refold partially unfolded target proteins. The exploration of αS-casein’s relatively new role as a molecular chaperone in milk is of great interest to the food industry as it opens up new avenues for the stabilisation of milk and milk products under a range of environmental conditions (e.g. elevated temperature) and provides new possibilities for the development of dairy foods with unique properties and textures.