title: The chaperone action of bovine milk αS1- and αS2-caseins and their associated form αS-casein.
abstract: α(S)-Casein, the major milk protein, comprises α(S1)- and α(S2)-casein and acts as a molecular chaperone, stabilizing an array of stressed target proteins against precipitation. Here, we report that α(S)-casein acts in a similar manner to the unrelated small heat-shock proteins (sHsps) and clusterin in that it does not preserve the activity of stressed target enzymes. However, in contrast to sHsps and clusterin, α-casein does not bind target proteins in a state that facilitates refolding by Hsp70. α(S)-Casein was also separated into α- and α-casein, and the chaperone abilities of each of these proteins were assessed with amorphously aggregating and fibril-forming target proteins. Under reduction stress, all α-casein species exhibited similar chaperone ability, whereas under heat stress, α-casein was a poorer chaperone. Conversely, α(S2)-casein was less effective at preventing fibril formation by modified κ-casein, whereas α- and α(S1)-casein were comparably potent inhibitors. In the presence of added salt and heat stress, α(S1)-, α- and α(S)-casein were all significantly less effective. We conclude that α(S1)- and α-casein stabilise each other to facilitate optimal chaperone activity of α(S)-casein. This work highlights the interdependency of casein proteins for their structural stability.
authors: Graduate School of Medicine, University of Wollongong, New South Wales, Australia. treweek@uow.edu.au
authors: Teresa M
authors: TM
authors: Treweek
authors: David C
authors: DC
authors: Thorn
authors: William E
authors: WE
authors: Price
authors: John A
authors: JA
authors: Carver
mesh_terms: D000818:Animals; D002364:Caseins / Q000302:isolation & purification / Q000378:metabolism*; D002417:Cattle; D051152:Clusterin / Q000378:metabolism*; D018840:HSP70 Heat-Shock Proteins / Q000378:metabolism; D050888:Heat-Shock Proteins, Small / Q000378:metabolism*; D006358:Hot Temperature; D008892:Milk / Q000378:metabolism*; D010084:Oxidation-Reduction
chemical_list: D002364:Caseins; D051152:Clusterin; D018840:HSP70 Heat-Shock Proteins; D050888:Heat-Shock Proteins, Small