Chaperone-like activity of α-crystallin and other small heat shock proteins

Small heat shock proteins (sHsps) are a large family of proteins with monomeric molecular weight of 12-43 kDa, present within the prokaryotic and eukariotic cell as large oligomeric complexes, ranging in size from 200-800 kDa. Unlike the high molecular weight Hsps, which are involved in protein folding in vivo, under normal conditions, sHsps play an important role in protecting organism from stress. SHsps share an evolutionarily conserved sequence of 80-100 amino acids, located in the C-terminal region, and called "alpha -crystallin domain"; its role in subunits interactions has been recently underlined by site-directed spin labeling studies and by fluorescence resonance energy transfer data. The N-terminal region, preceding the alpha -crystallin domain, is variable in length and amino acid sequence, contributing to structural diversity between different sHsps and having a role in multimerization. The alpha -Crystailin domain is followed by C-terminal extension, a polar structure, involved in protein solubility, which share no sequence homology. Expression of sHsps is induced in response to various kinds of stress including heat shock, oxidative stress, osmostress, or ischemia, but some sHsps are expressed constitutively under physiological conditions. In vitro, sHsps selectively bind and stabilize proteins and prevent their aggregation at elevated temperatures in an ATP-independent way and protect enzymes against heat-induced inactivation. Our own studies focused on the chaperone-like activity of a-crystallin, the major protein component of vertebrate lens, using another system than heat-induced aggregation. Our data demonstrated that alpha -crystallin specifically protects enzymes against inactivation by different posttranslational modifications such as glycation, carbamylation and aldehyde binding, and also reactivates GuHCl-denatured enzymes. Complex formation between alpha -crystallin and the denatured enzymes, was suggested as a mechanism of protection.