Casein Proteins as Molecular Chaperones

Publisher Website
Google Scholar
Abstract
Under conditions of stress, such as elevated temperature, molecular chaperones stabilize proteins from unfolding, aggregating, and precipitating. We have investigated the chaperone activity of the major milk proteins αS-, β-, and κ-casein with reduced insulin and the milk whey proteins, α-lactalbumin and β-lactoglobulin, and compared it with that of the mammalian small heat shock protein (sHsp), α-crystallin, and clusterin. αS-Casein exhibited different chaperone behavior under reduction and heat stresses, i.e., chaperone activity increased with increasing temperature (as observed with α-crystallin), but under reduction stress, its chaperone activity increased at lower temperatures. β- and κ-casein had comparable chaperone ability with each other but were less effective than αS-casein. Under molecular crowding conditions, precipitation of stressed protein was accelerated, and αS-casein was a poorer chaperone. Furthermore, at slightly alkaline pH values, αS-casein was a less effective chaperone than at neutral pH. Detailed fluorescence, size exclusion chromatography, and real-time NMR studies studies indicated that the casein proteins underwent conformational changes and stabilized the partially unfolded whey proteins prior to formation of high molecular weight soluble complexes. These results are consistent with casein proteins acting as molecular chaperones in a manner similar to sHsps and clusterin. Keywords: Milk proteins; αS-casein; β-casein; κ-casein; molecular chaperones; stability of milk proteins