Enhancement of Chaperone Function of α-Crystallin by Methylglyoxal Modification

Abstract

The molecular chaperone function of alpha-crystallin in the lens prevents the aggregation and insolubilization of lens proteins that occur during the process of aging. We found that chemical modification of alpha-crystallin by a physiological alpha-dicarbonyl compound, methylglyoxal (MG), enhances its chaperone function. Protein-modifying sugars and ascorbate have no such effect and actually reduce chaperone function. Chaperone assay after immunoprecipitation or with immunoaffinity-purified argpyrimidine-alpha-crystallin indicates that 50-60% of the increased chaperone function is due to argpyrimidine-modified protein. Incubation of alpha-crystallin with DL-glyceraldehyde and arginine-modifying agents also enhances chaperone function, and we believe that the increased chaperone activity depends on the extent of arginine modification. Far- and near-UV circular dichroism spectra indicate modest changes in secondary and tertiary structure of MG-modified alpha-crystallin. LC MS/MS analysis of MG-modified alpha-crystallin following chymotryptic digestion revealed that R21, R49, and R103 in alphaA-crystallin were converted to argpyrimidine. 1,1'-Bis(4-anilino)naphthalene-5,5'-disulfonic acid binding, an indicator of hydrophobicity of proteins, increased in alpha-crystallin modified by low concentrations of MG (2-100 microM). MG similarly enhances chaperone function of another small heat shock protein, Hsp27. Our results show that posttranslational modification by a metabolic product can enhance the chaperone function of alpha-crystallin and Hsp27 and suggest that such modification may be a protective mechanism against environmental and metabolic stresses. Augmentation of the chaperone function of alpha-crystallin might have evolved to protect the lens from deleterious protein modifications associated with aging.

Crystallin proteins and amyloid fibrils

“…However, to date, the impact of these modifications on the chaperone ability of sHsps to prevent amyloid fibril formation has not been investigated. This area of study is significant since these post-translational modifications occur to a significant extent to the crystallin proteins in the eye lens [12, [127][128][129][130][131][132][133]. There is increasing evidence that cataract may be an amyloid fibril based disease (see below) and, as such, the impact of such modifications on the chaperone ability of α-crystallin to prevent this process is highly relevant.…”

Impact of diabetes on alpha-crystallins and other heat shock proteins in the eye

“…In the retina of experimental autoimmune uveitis-induced mice, several proteins including alphaA-crystallin and mitochondrial Hsp70 were targeted by posttranslational modifications [63]. Chemical posttranslational modification of alpha-crystallins and Hsp27 by a metabolic product, methylglyoxal (MG), has been shown to enhance their chaperone function [64]. In the retinal epithelial cells, it has been concluded that S-glutathionylation of Hsc70 enhances its chaperone function, allowing it to prevent protein aggregation in ATP-free conditions [65].…”

Preconditioning with Maillard reaction products improves antioxidant defence leading to increased stress tolerance in cardiac cells

“…Besides the reducing sugars, α-dicarbonyl compounds like methylglyoxal or glyoxal which arise during glycolysis (Nagaraj et al 2003;Ghosh et al 2006), fatty acid degradation and autoxidation of glucose (Fu et al 1998) can react with free amino acid groups of endogenous proteins and contribute to AGE-formation. Food contains large quantities of Maillard reaction products (MRPs) or AGEs which arise during heating and processing and contribute to the characteristic flavour and colour.…”