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 alpha(S)-, beta-, and kappa-casein with reduced insulin and the milk whey proteins, alpha-lactalbumin and beta-lactoglobulin, and compared it with that of the mammalian small heat shock protein (sHsp), alpha-crystallin, and clusterin. alpha(S)-Casein exhibited different chaperone behavior under reduction and heat stresses, i.e., chaperone activity increased with increasing temperature (as observed with alpha-crystallin), but under reduction stress, its chaperone activity increased at lower temperatures. beta- and kappa-casein had comparable chaperone ability with each other but were less effective than alpha(S)-casein. Under molecular crowding conditions, precipitation of stressed protein was accelerated, and alpha(S)-casein was a poorer chaperone. Furthermore, at slightly alkaline pH values, alpha(S)-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.

Small heat-shock proteins (sHsps) are a diverse family of intra-cellular molecular chaperone proteins that play a critical role in mitigating and preventing protein aggregation under stress conditions such as elevated temperature, oxidation and infection. In doing so, they assist in the maintenance of protein homeostasis (proteostasis) thereby avoiding the deleterious effects that result from loss of protein function and/or protein aggregation. The chaperone properties of sHsps are therefore employed extensively in many tissues to prevent the development of diseases associated with protein aggregation. Significant progress has been made of late in understanding the structure and chaperone mechanism of sHsps. In this review, we discuss some of these advances, with a focus on mammalian sHsp hetero-oligomerisation, the mechanism by which sHsps act as molecular chaperones to prevent both amorphous and fibrillar protein aggregation, and the role of posttranslational modifications in sHsp chaperone function, particularly in the context of disease. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 2 Abstract (word count = 159)Small heat-shock proteins (sHsps) are a diverse family of intracellular molecular chaperone proteins that play a critical role in preventing protein unfolding, misfolding and aggregation, particularly under stress conditions such as elevated temperature, oxidation and infection. In doing so, they assist in the maintenance of protein homeostasis (proteostasis) and thereby avoid the deleterious effects that result from loss of protein function and/or protein aggregation. The chaperone properties of sHsps are therefore employed extensively in many tissues to prevent the development of diseases associated with protein aggregation. There has been much research into the structure and mechanism of chaperone action of sHsps over approximately the past 30 years, and significant progress has been made of late, however, there are still many unanswered questions relating to these aspects of sHsps. In this review, we outline some of the recent advances in understanding the structure and function of mammalian sHsps, particularly in the context of their many and varied roles in disease.