Hydroimidazolone Modification of the Conserved Arg12 in Small Heat Shock Proteins: Studies on the Structure and Chaperone Function Using Mutant Mimics

Abstract

Methylglyoxal (MGO) is an α-dicarbonyl compound present ubiquitously in the human body. MGO reacts with arginine residues in proteins and forms adducts such as hydroimidazolone and argpyrimidine in vivo. Previously, we showed that MGO-mediated modification of αA-crystallin increased its chaperone function. We identified MGO-modified arginine residues in αA-crystallin and found that replacing such arginine residues with alanine residues mimicked the effects of MGO on the chaperone function. Arginine 12 (R12) is a conserved amino acid residue in Hsp27 as well as αA- and αB-crystallin. When treated with MGO at or near physiological concentrations (2–10 µM), R12 was modified to hydroimidazolone in all three small heat shock proteins. In this study, we determined the effect of arginine substitution with alanine at position 12 (R12A to mimic MGO modification) on the structure and chaperone function of these proteins. Among the three proteins, the R12A mutation improved the chaperone function of only αA-crystallin. This enhancement in the chaperone function was accompanied by subtle changes in the tertiary structure, which increased the thermodynamic stability of αA-crystallin. This mutation induced the exposure of additional client protein binding sites on αA-crystallin. Altogether, our data suggest that MGO-modification of the conserved R12 in αA-crystallin to hydroimidazolone may play an important role in reducing protein aggregation in the lens during aging and cataract formation.

A S52P mutation in the ‘α‐crystallin domain’ of Mycobacterium leprae HSP18 reduces its oligomeric size and chaperone function

“…Therefore, we cloned and purified these two HSP18 variants. We found that HSP18S 52 is also a molecular chaperone and an oligomeric protein. Intrinsic tryptophan fluorescence and far-UV CD measurements revealed that the S52P mutation altered the tertiary and secondary structure of HSP18.…”

“…One of the variants has serine at position 52, whereas the other one has proline at the same position. We have also reported that HSP18 having proline at position 52 (HSP18P 52 ) is a nonameric protein and exhibits chaperone function. However, the structural and functional characterization of wild-type HSP18 having serine at position 52 (HSP18S 52 ) is yet to be explored.…”

“…We have also reported that HSP18 having proline at position 52 (HSP18P 52 ) is a nonameric protein and exhibits chaperone function. However, the structural and functional characterization of wild-type HSP18 having serine at position 52 (HSP18S 52 ) is yet to be explored. Furthermore, the implications of the S52P mutation on the structure and chaperone function of HSP18 are not well understood.…”

Characterization of rice small heat shock proteins targeted to different cellular organelles

“…Both constructs show differences in their thermo-protection activity towards NdeI. Previous studies on two mutants of αA-crystallin have shown that mutations that increase the hydrodynamic radius of the assembly tend to increase chaperone activity as well (Nagaraj et al 2012). This effect was attributed to increased accessibility of chaperone sites in the mutants (Pasta et al 2004).…”

Small heat shock proteins in ageing and age-related diseases

“…Direct association of methylglyoxal modifications of sHSPs and ageing remains unascertained. An in vitro study suggests that methylglyoxal modification of α-crystallin can enhance its chaperone function and protect the lens against environmental and metabolic stress by preventing nascent polypeptide accumulation associated with ageing Nagaraj et al 2012b). Similarly, HSPB4 acetylation in human lens alters the chaperone's function, a process that is essential for impeding the age-associated accumulation of insoluble lens proteins (Nagaraj et al 2012a).…”