Molecular Chaperone-like Properties of an Unfolded Protein, αs-Casein

Abstract

All molecular chaperones known to date are well organized, folded protein molecules whose three-dimensional structure are believed to play a key role in the mechanism of substrate recognition and subsequent assistance to folding. A common feature of all protein and nonprotein molecular chaperones is the propensity to form aggregates very similar to the micellar aggregates. In this paper we show that ␣ s -casein, abundant in mammalian milk, which has no well defined secondary and tertiary structure but exits in nature as a micellar aggregate, can prevent a variety of unrelated proteins/ enzymes against thermal-, chemical-, or light-induced aggregation. It also prevents aggregation of its natural substrates, the whey proteins. ␣ s -Casein interacts with partially unfolded proteins through its solvent-exposed hydrophobic surfaces. The absence of disulfide bridge or free thiol groups in its sequence plays important role in preventing thermal aggregation of whey proteins caused by thiol-disulfide interchange reactions. Our results indicate that ␣ s -casein not only prevents the formation of huge insoluble aggregates but it can also inhibit accumulation of soluble aggregates of appreciable size. Unlike other molecular chaperones, this protein can solubilize hydrophobically aggregated proteins. This protein seems to have some characteristics of cold shock protein, and its chaperone-like activity increases with decrease of temperature.

Invited review: Caseins and the casein micelle: Their biological functions, structures, and behavior in foods

“…As discussed in the section on The Chaperone Action of Caseins, the chaperone ability of α S -CN increases with decreasing temperature (Bhattacharyya and Das, 1999;Morgan et al, 2005). Potentially, this behavior could be utilized more generally in food processing to stabilize ingredients, particularly proteins, in frozen and cold foods to increase their storage and shelf life.…”

“…Like sHsp, caseins are promiscuous chaperones: they stabilize a range of unrelated target proteins from amorphous aggregation induced by heat, reduction, and UV-induced stress (Bhattacharyya and Das, 1999;Matsudomi et al, 2004;Morgan et al, 2005;Zhang et al, 2005;Hassanisadi et al, 2008;Koudelka et al, 6133 2009;Treweek et al, 2011). Moreover, α S1 -and β-CN can also prevent the formation of fibrillar structures by target proteins, including α S2 -and κ-CN (Thorn et al, 2005, ovalbumin (Khodarahmi et al, 2008), and amyloid-β peptide (Carrotta et al, 2012).…”

“…The initial report that α S -CN stabilized and prevented the aggregation and precipitation of heat-stressed proteins, including milk whey proteins (Bhattacharyya and Das 1999), has been followed by a number of in vitro studies of the chaperone action of individual caseins and casein micelles with target proteins under different stress conditions (reviewed in Yong and Foegeding, 2010). To date, most of the work has used standard light scattering or fluorescence-based assays to monitor protein aggregation.…”

“…The chaperone ability of caseins (Bhattacharyya and Das, 1999;Morgan et al, 2005) has potential widespread application in food processing; for example, in preventing undesirable precipitation of proteins and other components as a result of heat or pressure treatments. Indeed, this potential has been recognized by Yong and Foegeding (2010) in their review article summarizing the literature on casein chaperone action from 1999 to 2008.…”

Chaperone-like Activity of Tubulin

“…2 and 5). It must be remembered that aggregation measured by the 90°scattering technique used here detects relatively large particles (30), which level off before 5 min (Fig. 2).…”