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

Jaya Bhattacharyya,Kali P Das

doi:10.1074/jbc.274.22.15505

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 alpha(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. alpha(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 alpha(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.

Highlights

No common sequence has been identified among chaperones of different family of proteins, some common features emerges
Tubulin, reported recently by us [16] to act like a chaperone, associates to form microtubules. ␣-Crystallin, which belongs to small heat shock protein family [17], has been proposed to have a micellar architecture [11, 12]
In this paper we report for the first time that a random coil protein ␣s-casein can prevent in vitro the thermal aggregation of whey proteins from milk as well as the aggregation of a variety of unrelated proteins/enzymes caused by thermal, chemical, and light-induced stress

Summary

Introduction

We show that unlike other chaperones, ␣s-casein can solubilize hydrophobically aggregated proteins and possesses some features similar to the cold shock proteins (CSP).1 Prevention of thermal aggregation of substrate proteins by molecular chaperones is a commonly used method for in vitro assay of their activity. When solutions of substrate proteins such as alcohol dehydrogenase, carbonic anhydrase, and ␤-crystallin are heated at 40, 60, and 60 °C, respectively, the solutions get turbid because of the formation of large aggregates.

Results

Conclusion