Effect of aging on the chaperone-like function of human alpha-crystallin assessed by three methods.
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ABSTRACT:
SUBMITTER: Derham BK
PROVIDER: S-EPMC1218984 | biostudies-other | 1997 Dec
REPOSITORIES: biostudies-other
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Project description:The chaperone-like activity of alpha-crystallin is considered to play an important role in the maintenance of the transparency of the eye lens. However, in the case of aging and in diabetes, the chaperone function of alpha-crystallin is compromized, resulting in cataract formation. Several post-translational modifications, including non-enzymatic glycation, have been shown to affect the chaperone function of alpha-crystallin in aging and in diabetes. A variety of agents have been identified as the predominant sources for the formation of AGEs (advanced glycation end-products) in various tissues, including the lens. Nevertheless, glycation of alpha-crystallin with various sugars has resulted in divergent results. In the present in vitro study, we have investigated the effect of glucose, fructose, G6P (glucose 6-phosphate) and MGO (methylglyoxal), which represent the major classes of glycating agents, on the structure and chaperone function of alpha-crystallin. Modification of alpha-crystallin with all four agents resulted in the formation of glycated protein, increased AGE fluorescence, protein cross-linking and HMM (high-molecular-mass) aggregation. Interestingly, these glycation-related profiles were found to vary with different glycating agents. For instance, CML [N(epsilon)-(carboxymethyl)lysine] was the predominant AGE formed upon glycation of alpha-crystallin with these agents. Although fructose and MGO caused significant conformational changes, there were no significant structural perturbations with glucose and G6P. With the exception of MGO modification, glycation with other sugars resulted in decreased chaperone activity in aggregation assays. However, modification with all four sugars led to the loss of chaperone activity as assessed using an enzyme inactivation assay. Glycation-induced loss of alpha-crystallin chaperone activity was associated with decreased hydrophobicity. Furthermore, alpha-crystallin isolated from glycated TSP (total lens soluble protein) had also increased AGE fluorescence, CML formation and diminished chaperone activity. These results indicate the susceptibility of alpha-crystallin to non-enzymatic glycation by various sugars and their derivatives, whose levels are elevated in diabetes. We also describe the effects of glycation on the structure and chaperone-like activity of alpha-crystallin.
| S-EPMC2267351 | biostudies-other
Project description:Aging of the lens is accompanied by extensive deamidation of the lens specific proteins, the crystallins. Deamidated crystallins are increased in the insoluble proteins and may contribute to cataracts. Deamidation has been shown in vitro to alter the structure and decrease the stability of human lens betaB1, betaB2 and betaA3-crystallin. Of particular interest, betaB2 mutants were constructed to mimic the effect of in vivo deamidations at the interacting interface between domains, at Q70 in the N terminal domain and at Q162, its C-terminal homologue. The double mutant was also constructed. We previously reported that deamidation at the critical interface sites decreased stability, while preserving the dimeric 3D structure. In the present study, dynamic light scattering, differential scanning calorimetry and small angle X-ray scattering were used to investigate the effect of deamidation on stability, thermal unfolding and aggregation. The bovine betaLb fraction was used for comparative analysis. The chaperone requirements of the various samples were determined using bovine alpha-crystallins as the chaperone. Deamidation at both interface Gln residues or at Q70, but not Q162, significantly lowered the temperature for unfolding and aggregation, which was rapidly followed by precipitation. This deamidation-induced aggregation and precipitation was not completely prevented by alpha-crystallin chaperone. A potential mechanism for cataract formation in vivo involving accumulation of deamidated beta-crystallin aggregates is discussed.
| S-EPMC2904749 | biostudies-literature
Project description:AlphaA-crystallin is a molecular chaperone; it prevents aggregation of denaturing proteins. We have previously demonstrated that upon modification by a metabolic alpha-dicarbonyl compound, methylglyoxal (MGO), alphaA-crystallin becomes a better chaperone. AlphaA-crystallin also assists in refolding of denatured proteins. Here, we have investigated the effect of mild modification of alphaA-crystallin by MGO (with 20-500 microM) on the chaperone function and its ability to refold denatured proteins. Under the conditions used, mildly modified protein contained mostly hydroimidazolone modifications. The modified protein exhibited an increase in chaperone function against thermal aggregation of beta(L)- and gamma-crystallins, citrate synthase (CS), malate dehydrogenase (MDH) and lactate dehydrogenase (LDH) and chemical aggregation of insulin. The ability of the protein to assist in refolding of chemically denatured beta(L)- and gamma-crystallins, MDH and LDH, and to prevent thermal inactivation of CS were unchanged after mild modification by MGO. Prior binding of catalytically inactive, thermally denatured MDH or the hydrophobic probe, 2-p-toluidonaphthalene-6-sulfonate (TNS) abolished the ability of alphaA-crystallin to assist in the refolding of denatured MDH. However, MGO modification of chaperone-null TNS-bound alphaA-crystallin resulted in partial regain of the chaperone function. Taken together, these results demonstrate that: 1) hydroimidazolone modifications are sufficient to enhance the chaperone function of alphaA-crystallin but such modifications do not change its ability to assist in refolding of denatured proteins, 2) the sites on the alphaA-crystallin responsible for the chaperone function and refolding are the same in the native alphaA-crystallin and 3) additional hydrophobic sites exposed upon MGO modification, which are responsible for the enhanced chaperone function, do not enhance alphaA-crystallin's ability to refold denatured proteins.
| S-EPMC2826572 | biostudies-literature
Project description:alphaA-crystallin is abundant in the lens of the eye and acts as a molecular chaperone by preventing aggregation of denaturing proteins. We previously found that chemical modification of the guanidino group of selected arginine residues by a metabolic alpha-dicarbonyl compound, methylglyoxal (MGO), makes human alphaA-crystallin a better chaperone. Here, we examined how the introduction of additional guanidino groups and modification by MGO influence the structure and chaperone function of alphaA-crystallin. alphaA-crystallin lysine residues were converted to homoarginine by guanidination with o-methylisourea (OMIU) and then modified with MGO. LC-ESI-mass spectrometry identified homoargpyrimidine and homohydroimidazolone adducts after OMIU and MGO treatment. Treatment with 0.25 M OMIU abolished most of the chaperone function. However, subsequent treatment with 1.0 mM MGO not only restored the chaperone function but increased it by approximately 40% and approximately 60% beyond that of unmodified alphaA-crystallin, as measured with citrate synthase and insulin aggregation assays, respectively. OMIU treatment reduced the surface hydrophobicity but after MGO treatment, it was approximately 39% higher than control. FRET analysis revealed that alphaA-crystallin subunit exchange rate was markedly retarded by OMIU modification, but was enhanced after MGO modification. These results indicate a pattern of loss and gain of chaperone function within the same protein that is associated with introduction of guanidino groups and their neutralization. These findings support our hypothesis that positively charged guanidino group on arginine residues keeps the chaperone function of alphaA-crystallin in check and that a metabolic alpha-dicarbonyl compound neutralizes this charge to restore and enhance chaperone function.
| S-EPMC3564238 | biostudies-literature
Project description:Small heat-shock proteins (sHSPs) are a widely expressed family of ATP-independent molecular chaperones that are among the first responders to cellular stress. Mechanisms by which sHSPs delay aggregation of client proteins remain undefined. sHSPs have high intrinsic disorder content of up to ~60% and assemble into large, polydisperse homo- and hetero-oligomers, making them challenging structural and biochemical targets. Two sHSPs, HSPB4 and HSPB5, are present at millimolar concentrations in eye lens, where they are responsible for maintaining lens transparency over the lifetime of an organism. Together, HSPB4 and HSPB5 compose the hetero-oligomeric chaperone known as α-crystallin. To identify the determinants of sHSP function, we compared the effectiveness of HSPB4 and HSPB5 homo-oligomers and HSPB4/HSPB5 hetero-oligomers in delaying the aggregation of the lens protein γD-crystallin. In chimeric versions of HSPB4 and HSPB5, chaperone activity tracked with the identity of the 60-residue disordered N-terminal regions (NTR). A short 10-residue stretch in the middle of the NTR ("Critical sequence") contains three residues that are responsible for high HSPB5 chaperone activity toward γD-crystallin. These residues affect structure and dynamics throughout the NTR. Abundant interactions involving the NTR Critical sequence reveal it to be a hub for a network of interactions within oligomers. We propose a model whereby the NTR critical sequence influences local structure and NTR dynamics that modulate accessibility of the NTR, which in turn modulates chaperone activity.
| S-EPMC9963673 | biostudies-literature
Project description:?-Crystallin is the archetypical chaperone of the small heat-shock protein family, all members of which contain the so-called "?-crystallin domain" (ACD). This domain and the N- and C-terminal extensions are considered the main functional units in its chaperone function. Previous studies have shown that a 19-residue fragment of the ACD of human ?A-crystallin called mini-?A-crystallin (MAC) shows chaperone properties similar to those of the parent protein. Subsequent studies have confirmed the function of this peptide, but no studies have addressed the mechanistic basis for the chaperone function of MAC. Using human ?D-crystallin (HGD), a key substrate protein for parent ?-crystallin in the ocular lens, we show here that MAC not only protects HGD from aggregation during thermal and chemical unfolding but also binds weakly and reversibly to HGD (Kd ? 200-700 ?M) even when HGD is in the native state. However, at temperatures favoring the unfolding of HGD, MAC forms a stable complex with HGD similar to parent ?-crystallin. Using nuclear magnetic resonance spectroscopy, we identify the residues in HGD that are involved in these two modes of binding and show that MAC protects HGD from aggregation by binding to Phe 56 and Val 132 at the domain interface of the target protein, and residues Val 164 to Leu 167 in the core of the C-terminal domain. Furthermore, we suggest that the low-affinity, reversible binding of MAC on the surface of HGD in the native state is involved in facilitating its binding to both the domain interface and core regions during the early stages of the unfolding of HGD. This work highlights some structural features of MAC and MAC-like peptides that affect their chaperone activity and can potentially be manipulated for translational studies.
| S-EPMC4303307 | biostudies-literature
Project description:Γ-Crystallins play a major role in age-related lens transparency. Their destabilization by mutations and physical chemical insults are associated with cataract formation. Therefore, drugs that increase their stability should have anticataract properties. To this end, we screened 2560 Federal Drug Agency-approved drugs and natural compounds for their ability to suppress or worsen H2O2 and/or heat-mediated aggregation of bovine γ-crystallins. The top two drugs, closantel (C), an antihelminthic drug, and gambogic acid (G), a xanthonoid, attenuated thermal-induced protein unfolding and aggregation as shown by turbidimetry fluorescence spectroscopy dynamic light scattering and electron microscopy of human or mouse recombinant crystallins. Furthermore, binding studies using fluorescence inhibition and hydrophobic pocket-binding molecule bis-8-anilino-1-naphthalene sulfonic acid revealed static binding of C and G to hydrophobic sites with medium-to-low affinity. Molecular docking to HγD and other γ-crystallins revealed two binding sites, one in the "NC pocket" (residues 50-150) of HγD and one spanning the "NC tail" (residues 56-61 to 168-174 in the C-terminal domain). Multiple binding sites overlap with those of the protective mini αA-crystallin chaperone MAC peptide. Mechanistic studies using bis-8-anilino-1-naphthalene sulfonic acid as a proxy drug showed that it bound to MAC sites, improved Tm of both H2O2 oxidized and native human gamma D, and suppressed turbidity of oxidized HγD, most likely by trapping exposed hydrophobic sites. The extent to which these drugs act as α-crystallin mimetics and reduce cataract progression remains to be demonstrated. This study provides initial insights into binding properties of C and G to γ-crystallins.
| S-EPMC9525908 | biostudies-literature
Project description:Alpha-crystallins are molecular chaperones that protect vertebrate eye lens proteins from detrimental protein aggregation. alphaB-Crystallin, 1 of the 2 alpha-crystallin isoforms, is also associated with myopathies and neuropathological diseases. Despite the importance of alpha-crystallins in protein homeostasis, only little is known about their quaternary structures because of their seemingly polydisperse nature. Here, we analyzed the structures of recombinant alpha-crystallins using biophysical methods. In contrast to previous reports, we show that alphaB-crystallin assembles into defined oligomers consisting of 24 subunits. The 3-dimensional (3D) reconstruction of alphaB-crystallin by electron microscopy reveals a sphere-like structure with large openings to the interior of the protein. alphaA-Crystallin forms, in addition to complexes of 24 subunits, also smaller oligomers and large clusters consisting of individual oligomers. This propensity might explain the previously reported polydisperse nature of alpha-crystallin.
| S-EPMC2726422 | biostudies-literature
Project description:?-Crystallin is a major protein in the human lens that is perceived to help to maintain the transparency of the lens through its chaperone function. In this study, we demonstrate that many lens proteins including ?A-crystallin are acetylated in vivo. We found that K70 and K99 in ?A-crystallin and, K92 and K166 in ?B-crystallin are acetylated in the human lens. To determine the effect of acetylation on the chaperone function and structural changes, ?A-crystallin was acetylated using acetic anhydride. The resulting protein showed strong immunoreactivity against a N(?)-acetyllysine antibody, which was directly related to the degree of acetylation. When compared to the unmodified protein, the chaperone function of the in vitro acetylated ?A-crystallin was higher against three of the four different client proteins tested. Because a lysine (residue 70; K70) in ?A-crystallin is acetylated in vivo, we generated a protein with an acetylation mimic, replacing Lys70 with glutamine (K70Q). The K70Q mutant protein showed increased chaperone function against three client proteins compared to the Wt protein but decreased chaperone function against ?-crystallin. The acetylated protein displayed higher surface hydrophobicity and tryptophan fluorescence, had altered secondary and tertiary structures and displayed decreased thermodynamic stability. Together, our data suggest that acetylation of ?A-crystallin occurs in the human lens and that it affects the chaperone function of the protein.
| S-EPMC3249504 | biostudies-literature
Project description:BackgroundA substitution mutation in human ?A-crystallin (?AG98R) is associated with autosomal dominant cataract. The recombinant mutant ?AG98R protein exhibits altered structure, substrate-dependent chaperone activity, impaired oligomer stability and aggregation on prolonged incubation at 37 °C. Our previous studies have shown that ?A-crystallin-derived mini-chaperone (DFVIFLDVKHFSPEDLTVK) functions like a molecular chaperone by suppressing the aggregation of denaturing proteins. The present study was undertaken to determine the effect of ?A-crystallin-derived mini-chaperone on the stability and chaperone activity of ?AG98R-crystallin.Methodology/principal findingsRecombinant ?AG98R was incubated in presence and absence of mini-chaperone and analyzed by chromatographic and spectrometric methods. Transmission electron microscope was used to examine the effect of mini-chaperone on the aggregation propensity of mutant protein. Mini-chaperone containing photoactive benzoylphenylalanine was used to confirm the interaction of mini-chaperone with ?AG98R. The rescuing of chaperone activity in mutant?-crystallin (?AG98R) by mini-chaperone was confirmed by chaperone assays. We found that the addition of the mini-chaperone during incubation of ?AG98R protected the mutant crystallin from forming larger aggregates that precipitate with time. The mini-chaperone-stabilized ?AG98R displayed chaperone activity comparable to that of wild-type ?A-crystallin. The complexes formed between mini-?A-?AG98R complex and ADH were more stable than the complexes formed between ?AG98R and ADH. Western-blotting and mass spectrometry confirmed the binding of mini-chaperone to mutant crystallin.Conclusion/significanceThese results demonstrate that mini-chaperone stabilizes the mutant ?A-crystallin and modulates the chaperone activity of ?AG98R. These findings aid in our understanding of how to design peptide chaperones that can be used to stabilize mutant ?A-crystallins and preserve the chaperone function.
| S-EPMC3435407 | biostudies-literature