Phe71 is essential for chaperone-like function in alpha A-crystallin

Association of Alpha-Crystallin with Fiber Cell Plasma Membrane of the Eye Lens Accompanied by Light Scattering and Cataract Formation

α-crystallin is a major protein found in the mammalian eye lens that works as a molecular chaperone by preventing the aggregation of proteins and providing tolerance to stress in the eye lens. These functions of α-crystallin are significant for maintaining lens transparency. However, with age and cataract formation, the concentration of α-crystallin in the eye lens cytoplasm decreases with a corresponding increase in the membrane-bound α-crystallin, accompanied by increased light scattering. The purpose of this review is to summarize previous and recent findings of the role of the: (1) lens membrane components, i.e., the major phospholipids (PLs) and sphingolipids, cholesterol (Chol), cholesterol bilayer domains (CBDs), and the integral membrane proteins aquaporin-0 (AQP0; formally MIP26) and connexins, and (2) α-crystallin mutations and post-translational modifications (PTMs) in the association of α-crystallin to the eye lens's fiber cell plasma membrane, providing thorough insights into a molecular basis of such an association. Furthermore, this review highlights the current knowledge and need for further studies to understand the fundamental molecular processes involved in the association of α-crystallin to the lens membrane, potentially leading to new avenues for preventing cataract formation and progression.

α-Crystallins in the Vertebrate Eye Lens: Complex Oligomers and Molecular Chaperones

α-Crystallins are small heat-shock proteins that act as holdase chaperones. In humans, αA-crystallin is expressed only in the eye lens, while αB-crystallin is found in many tissues. α-Crystallins have a central domain flanked by flexible extensions and form dynamic, heterogeneous oligomers. Structural models show that both the C- and N-terminal extensions are important for controlling oligomerization through domain swapping. α-Crystallin prevents aggregation of damaged β- and γ-crystallins by binding to the client protein using a variety of binding modes. α-Crystallin chaperone activity can be compromised by mutation or posttranslational modifications, leading to protein aggregation and cataract. Because of their high solubility and their ability to form large, functional oligomers, α-crystallins are particularly amenable to structure determination by solid-state nuclear magnetic resonance (NMR) and solution NMR, as well as cryo-electron microscopy.

The Role of the Arginine in the Conserved N-Terminal Domain RLFDQxFG Motif of Human Small Heat Shock Proteins HspB1, HspB4, HspB5, HspB6, and HspB8

Although the N-terminal domain of vertebrate small heat shock proteins (sHsp) is poorly conserved, it contains a core motif preserved in many members of the sHsp family. The role of this RLFDQxFG motif remains elusive. We analyzed the specific role of the first arginine residue of this conserved octet sequence in five human sHsps (HspB1, HspB4, HspB5, HspB6, and HspB8). Substitution of this arginine with an alanine induced changes in thermal stability and/or intrinsic fluorescence of the related HspB1 and HspB8, but yielded only modest changes in the same biophysical properties of HspB4, HspB5, and HspB6 which together belong to another clade of vertebrate sHsps. Removal of the positively charged Arg side chain resulted in destabilization of the large oligomers of HspB1 and formation of smaller size oligomers of HspB5. The mutation induced only minor changes in the structure of HspB4 and HspB6. In contrast, the mutation in HspB8 was accompanied by shifting the equilibrium from dimers towards the formation of larger oligomers. We conclude that the RLFDQxFG motif plays distinct roles in the structure of several sHsp orthologs. This role correlates with the evolutionary relationship of the respective sHsps, but ultimately, it reflects the sequence context of this motif.

Structural and functional consequences of chaperone site deletion in αA-crystallin

The chaperone-like activity of αA-crystallin has an important role in maintaining lens transparency. Previously we identified residues 70-88 as a chaperone site in αA-crystallin. In this study, we deleted the chaperone site residues to generate αAΔ70-76 and αAΔ70-88 mutants and investigated if there are additional substrate-binding sites in αA-crystallin. Both mutant proteins when expressed in E. coli formed inclusion bodies, and on solubilizing and refolding, they exhibited similar structural properties, with a 2- to 3-fold increase in molar mass compared to the molar mass of wild-type protein. The deletion mutants were less stable than the wild-type αA-crystallin. Functionally αAΔ70-88 was completely inactive as a chaperone, while αAΔ70-76 demonstrated a 40-50% reduction in anti-aggregation activity against alcohol dehydrogenase (ADH). Deletion of residues 70-88 abolished the ADH binding sites in αA-crystallin at physiological temperature. At 45°C, cryptic ADH binding site(s) became exposed, which contributed subtly to the chaperone-like activity of αAΔ70-88. Both of the deletion mutants were completely inactive in suppressing aggregation of βL-crystallin at 53°C. The mutants completely lost the anti-apoptotic property that αA-crystallin exhibits while they protected ARPE-19 (a human retinal pigment epithelial cell line) and primary human primary lens epithelial (HLE) cells from oxidative stress. Our studies demonstrate that residues 70-88 in αA-crystallin act as a primary substrate binding site and account for the bulk of the total chaperone activity. The β3 and β4 strands in αA-crystallin comprising 70-88 residues play an important role in maintenance of the structure and in preventing aggregation of denaturing proteins.

A small molecule mitigates hearing loss in a mouse model of Usher syndrome III

Usher syndrome type III (USH3), characterized by progressive deafness, variable balance disorder and blindness, is caused by destabilizing mutations in the gene encoding the clarin-1 (CLRN1) protein. Here we report a new strategy to mitigate hearing loss associated with a common USH3 mutation CLRN1(N48K) that involves cell-based high-throughput screening of small molecules capable of stabilizing CLRN1(N48K), followed by a secondary screening to eliminate general proteasome inhibitors, and finally an iterative process to optimize structure-activity relationships. This resulted in the identification of BioFocus 844 (BF844). To test the efficacy of BF844, we developed a mouse model that mimicked the progressive hearing loss associated with USH3. BF844 effectively attenuated progressive hearing loss and prevented deafness in this model. Because the CLRN1(N48K) mutation causes both hearing and vision loss, BF844 could in principle prevent both sensory deficiencies in patients with USH3. Moreover, the strategy described here could help identify drugs for other protein-destabilizing monogenic disorders.

Interaction of insulin with methyl tert-butyl ether promotes molten globule-like state and production of reactive oxygen species

Interaction of methyl tert-butyl ether (MTBE) with proteins is a new look at its potential adverse biological effects. When MTBE is released to the environment it enters the blood stream through inhalation, and could affect the properties of various proteins. Here we investigated the interaction of MTBE with insulin and its effect on insulin structural changes. Our results showed that insulin formed a molten globule (MG)-like structure in the presence of 8 μM MTBE under physiological pH. The insulin structural changes were studied using spectroscopy methods, viscosity calculation, dynamic light scattering and differential scanning calorimetry. To delineate the mechanisms involved in MTBE-protein interactions, the formation of reactive oxygen specious (ROS) and formation of protein aggregates were measured. The chemiluminscence experiments revealed an increase in ROS production in the presence of MTBE especially in the MG-like state. These results were further confirmed by the aggregation tests, which indicated more aggregation of insulin at 40 μM MTBE compared with 8 μM. Thus, the formation of initial aggregates and exposure of the hydrophobic patches upon formation of the MG-like state in the presence of MTBE drives protein oxidation and ROS generation.

The barley powdery mildew candidate secreted effector protein CSEP0105 inhibits the chaperone activity of a small heat shock protein

Pathogens secrete effector proteins to establish a successful interaction with their host. Here, we describe two barley (Hordeum vulgare) powdery mildew candidate secreted effector proteins, CSEP0105 and CSEP0162, which contribute to pathogen success and appear to be required during or after haustorial formation. Silencing of either CSEP using host-induced gene silencing significantly reduced the fungal haustorial formation rate. Interestingly, both CSEPs interact with the barley small heat shock proteins, Hsp16.9 and Hsp17.5, in a yeast two-hybrid assay. Small heat shock proteins are known to stabilize several intracellular proteins, including defense-related signaling components, through their chaperone activity. CSEP0105 and CSEP0162 localized to the cytosol and the nucleus of barley epidermal cells, whereas Hsp16.9 and Hsp17.5 are cytosolic. Intriguingly, only those specific CSEPs changed localization and became restricted to the cytosol when coexpressed with Hsp16.9 and Hsp17.5, confirming the CSEP-small heat shock protein interaction. As predicted, Hsp16.9 showed chaperone activity, as it could prevent the aggregation of Escherichia coli proteins during thermal stress. Remarkably, CSEP0105 compromised this activity. These data suggest that CSEP0105 promotes virulence by interfering with the chaperone activity of a barley small heat shock protein essential for defense and stress responses.

Novel roles for α-crystallins in retinal function and disease

α-Crystallins are key members of the superfamily of small heat shock proteins that have been studied in detail in the ocular lens. Recently, novel functions for α-crystallins have been identified in the retina and in the retinal pigmented epithelium (RPE). αB-Crystallin has been localized to multiple compartments and organelles including mitochondria, golgi apparatus, endoplasmic reticulum and nucleus. α-Crystallins are regulated by oxidative and endoplasmic reticulum stress, and inhibit apoptosis-induced cell death. α-Crystallins interact with a large number of proteins that include other crystallins, and apoptotic, cytoskeletal, inflammatory, signaling, angiogenic, and growth factor molecules. Studies with RPE from αB-crystallin deficient mice have shown that αB-crystallin supports retinal and choroidal angiogenesis through its interaction with vascular endothelial growth factor. αB-Crystallin has also been shown to have novel functions in the extracellular space. In RPE, αB-crystallin is released from the apical surface in exosomes where it accumulates in the interphotoreceptor matrix and may function to protect neighboring cells. In other systems administration of exogenous recombinant αB-crystallin has been shown to be anti-inflammatory. Another newly described function of αB-crystallin is its ability to inhibit β-amyloid fibril formation. α-Crystallin minichaperone peptides have been identified that elicit anti-apoptotic function in addition to being efficient chaperones. Generation of liposomal particles and other modes of nanoencapsulation of these minipeptides could offer great therapeutic advantage in ocular delivery for a wide variety of retinal degenerative, inflammatory and vascular diseases including age-related macular degeneration and diabetic retinopathy.

Temperature-dependent structural and functional properties of a mutant (F71L) αA-crystallin: molecular basis for early onset of age-related cataract

Previously we identified a novel mutation (F71L) in the αA-crystallin gene associated with early onset of age-related cataract. However, it is not known how the missense substitution translates into reduced chaperone-like activity (CLA), and how the structural and functional changes lead to early onset of the disease. Herein, we show that under native conditions the F71L-mutant is not significantly different from wild-type with regard to secondary and tertiary structural organization, hydrophobicity and the apparent molecular mass of oligomer but has substantial differences in structural and functional properties following a heat treatment. Wild-type αA-crystallin demonstrated increased CLA, whereas the F71L-mutant substantially lost its CLA upon heat treatment. Further, unlike the wild-type αA-subunit, F71L-subunit did not protect the αB-subunit in hetero-oligomeric complex from heat-induced aggregation. Moreover, hetero-oligomer containing F71L and αB in 3:1 ratio had significantly lower CLA upon thermal treatment compared to its unheated control. These results indicate that α-crystallin complexes containing F71L-αA subunits are less stable and have reduced CLA. Therefore, F71L may lead to earlier onset of cataract due to interaction with several environmental factors (e.g., temperature in this case) along with the aging process.

Identification and characterization of a copper-binding site in αA-crystallin

Previous studies have shown that both αA- and αB-crystallins bind Cu2+, suppress the formation of Cu2+-mediated active oxygen species, and protect ascorbic acid from oxidation by Cu2+. αA- and αB-crystallins are small heat shock proteins with molecular chaperone activity. In this study we show that the mini-αA-crystallin, a peptide consisting of residues 71-88 of αA-crystallin, prevents copper-induced oxidation of ascorbic acid. Evaluation of binding of copper to mini-αA-crystallin showed that each molecule of mini-αA-crystallin binds one copper molecule. Isothermal titration calorimetry and nanospray mass spectrometry revealed dissociation constants of 10.72 and 9.9 μM, respectively. 1,1'-Bis(4-anilino)naphthalene-5,5'-disulfonic acid interaction with mini-αA-crystallin was reduced after binding of Cu2+, suggesting that the same amino acids interact with these two ligands. Circular dichroism spectrometry showed that copper binding to mini-αA-crystallin peptide affects its secondary structure. Substitution of the His residue in mini-αA-crystallin with Ala abolished the redox-suppression activity of the peptide. During the Cu2+-induced ascorbic acid oxidation assay, a deletion mutant, αAΔ70-77, showed about 75% loss of ascorbic acid protection compared to the wild-type αA-crystallin. This difference indicates that the 70-77 region is the primary Cu2+-binding site(s) in human native full-size αA-crystallin. The role of the chaperone site in Cu2+ binding in native αA-crystallin was confirmed by the significant loss of chaperone activity by the peptide after Cu2+ binding.

Truncation attenuates molecular chaperoning and apoptosis inhibition by p26, a small heat shock protein from Artemia franciscana

The small heat shock proteins (sHSPs), which prevent irreversible protein denaturation and inhibit apoptosis, consist of an amino-terminus, the canonical α-crystallin domain, and a carboxy-terminal extension. It remains difficult, however, to define sHSP structure-function relationships and with this in mind p26, an sHSP from the crustacean Artemia franciscana, was truncated by deletion mutagenesis. Wild-type p26 cDNA and three truncated variants inserted into the eukaryotic expression vector pcDNA3.1/HisC were used to generate stably transfected 293H cells. p26 shielded transfected cells against death upon exposure to heat and oxidative stress. Truncation reduced chaperone activity, with cells synthesizing the p26 α-crystallin domain being the least resistant. Wild-type p26 inhibited apoptosis in transfected cells, with protection against oxidation-generated apoptosis being more effective than that against heat-induced apoptosis. Truncation reduced p26 apoptotic inhibitory activity, with the α-crystallin domain again being the least effective. The results show that a crustacean sHSP functions effectively in mammalian cells, demonstrating interchangeability of these proteins between distantly related organisms and indicating similarities in their mechanisms of action. Moreover, maximal activity was observed for full-length p26, indicating that structural elements required for chaperone activity and apoptosis inhibition reside throughout the protein.

Hydroimidazolone modification of human alphaA-crystallin: Effect on the chaperone function and protein refolding ability

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.

Deletion of (54)FLRAPSWF(61) residues decreases the oligomeric size and enhances the chaperone function of alphaB-crystallin

AlphaB-crystallin is a member of the small heat shock protein family and is known to have chaperone activity. Using a peptide scan approach, we previously determined that regions 42-57, 60-71, and 88-123 in alphaB-crystallin interact with alphaA-crystallin during heterooligomer formation. To further characterize the significance of the N-terminal domain of alphaB-crystallin, we prepared a deletion mutant that lacks residues (54)FLRAPSWF(61) (alphaBDelta54-61) and found that the absence of residues 54-61 in alphaB-crystallin significantly decreased the homooligomeric mass of alphaB-crystallin. The average oligomeric mass of wild-type alphaB-crystallin and of alphaBDelta54-61, calculated using multiangle light scattering, was 624 and 382 kDa, respectively. The mutant subunits aggregate to form smaller, less-compact oligomers with a 4-fold increase in subunit exchange rate. Deletion of the 54-61 region resulted in a 50% decrease in intrinsic tryptophan fluorescence. The alphaBDelta54-61 mutant showed a 2-fold increase in 1,1'-bi(4-anilino)naphthalene-5,5'-disulfonic acid (bis-ANS) binding as compared to the wild-type protein, suggesting increased hydrophobicity of the mutant protein. Accompanying the evidence of increased hydrophobicity in the deletion mutant was a 10-fold increase in antiaggregation activity. Homooligomers of 6HalphaA (750 kDa) readily exchanged subunits with alphaBDelta54-61 homooligomers at 37 degrees C, forming heterooligomers with an intermediate mass of 625 kDa. Our data suggest that residues (54)FLRAPSWF(61) contribute to the higher order assembly of alphaB-crystallin oligomers. Residues (54)FLRAPSWF(61) in alphaB-crystallin are not essential for target protein binding during chaperone action, but this region apparently has a role in the chaperone activity of native alphaB-crystallin.

A novel mutation (F71L) in alphaA-crystallin with defective chaperone-like function associated with age-related cataract

Age-related cataract (ARC) is a multifactorial disease and the leading cause of blindness worldwide. Genetic predisposition in association with other etiological factors may contribute to ARC. However, gene mutation studies on ARC are scanty. In the present work, we identified a genetic variation (F71L) in the exon-2 of CRYAA (alphaA-crystallin) gene in three unrelated female sporadic cases among 711 ARC patients but not in 265 normal non-cataractous controls by SSCP and RFLP analysis. By comparing human recombinant wild-type and F71L-alphaA-crystallin, we characterized the functional significance of this missense mutation. Chromatography, fluorescence and far- and near-UV CD studies indicated that F71L missense mutation did not significantly affect the apparent molecular mass, secondary and tertiary structures and hydrophobicity of alphaA-crystallin. While the mutant alphaA-crystallin displayed significant (35-90%) loss of chaperone-like activity (CLA) in thermal aggregation of carbonic anhydrase, betaL- and gamma-crystallins, it showed moderate (10-50%) loss in CLA in DTT-induced aggregation of insulin and lysozyme. This is the first report of an alphaA-F71L mutation being associated with ARC and suggests that ARC in individuals carrying this mutation (F71L) might be due to the overall loss of in vivo chaperone activity due to interaction with other environmental factors.

Lens aging: effects of crystallins

The primary function of the eye lens is to focus light on the retina. The major proteins in the lens--alpha, beta, and gamma-crystallins--are constantly subjected to age-related changes such as oxidation, deamidation, truncation, glycation, and methylation. Such age-related modifications are cumulative and affect crystallin structure and function. With time, the modified crystallins aggregate, causing the lens to increasingly scatter light on the retina instead of focusing light on it and causing the lens to lose its transparency gradually and become opaque. Age-related lens opacity, or cataract, is the major cause of blindness worldwide. We review deamidation, and glycation that occur in the lenses during aging keeping in mind the structural and functional changes that these modifications bring about in the proteins. In addition, we review proteolysis and discuss recent observations on how crystallin fragments generated in vivo, through their anti-chaperone activity may cause crystallin aggregation in aging lenses. We also review hyperbaric oxygen treatment induced guinea pig and 'humanized' ascorbate transporting mouse models as suitable options for studies on age-related changes in lens proteins.

Chemical modulation of the chaperone function of human alphaA-crystallin

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.

Amyloid fibril formation and chaperone-like activity of peptides from alphaA-crystallin

AlphaA-crystallin (alphaAC), a major component of eye lens, exhibits chaperone-like activity and is responsible for maintaining eye lens transparency. Synthetic peptides which corresponded to the putative substrate-binding site of alphaAC have been reported to prevent aggregation of proteins [Sharma, K. K., et al. (2000) J. Biol. Chem. 275, 3767-3771]. In this study, we found that these peptides, alphaAC(70-88), the peptide corresponding to amino acids 70-88 of alphaAC (KFVIFLDVKHFSPEDLTVK), and alphaAC(71-88), suppressed the amyloid fibril formation of amyloid beta protein (Abeta). On the other hand, while alphaAC(71-88) exhibited chaperone-like activity toward insulin, alphaAC(70-88) and alphaAC(70-88)K70D promoted rapid growth of aggregates consisting of insulin and these peptides in their solution mixtures. Interestingly, we found that alphaAC(71-88) itself can also form amyloid fibrils. It is possible that the chaperone-like activity of the alphaAC peptides is potentially related to their propensity for amyloid fibril formation. Analysis of variants of the alphaAC peptides suggested that F71 is important for amyloid formation, and interestingly, this same residue has previously been found to be essential for chaperone-like activity. Amyloid fibril formation was also observed with the shorter peptide, alphaAC(70-76)K70D, showing that the ability to form amyloid fibrils is maintained even with significant deletion of the C-terminal sequence. The formation of amyloid fibril was suppressed in alphaAC(70-88), suggesting that the K70 in the substrate binding site may play a role in suppressing the amyloid fibril formation of alphaAC, which agreed with recent proposals about the presence of an aggregation suppressor in the region flanking aggregation-prone hydrophobic sequences.

Paradoxical effects of substitution and deletion mutation of Arg56 on the structure and chaperone function of human alphaB-crystallin

Human alphaB-crystallin is a small heat-shock protein that functions as a molecular chaperone. Recent studies indicate that deletion of a peptide (54FLRAPSWF61) from its N-terminus makes it a better chaperone, and this particular sequence is thought to participate in substrate interaction and subunit exchange with alphaA-crystallin. To determine whether the positive charge on arginine 56 (R56) influences these functions, we prepared human alphaB-crystallin mutants in which R56 was deleted (DeltaR56) or replaced by alanine (R56A). To determine if the effects are specific to R56, we generated two additional mutant proteins in which the two neighboring amino acids were deleted (DeltaL55 and DeltaA57). Dynamic light scattering studies suggested that none of the mutations affected the oligomeric mass of the protein. Far-ultraviolet circular dichroism (UV CD) spectra revealed greater helicity in the secondary structures of R56A and DeltaR56 compared to that of the wild-type (Wt) protein. Near-UV CD spectra showed that the tertiary structure is perturbed in all mutants. Insulin and citrate synthase aggregation assays showed 38 and 30% improvement of chaperone function in DeltaR56 compared to that of the Wt. In contrast, the R56A mutant lost most of its chaperone function. Deletion mutants, DeltaL55 and DeltaA57, showed no significant changes in the chaperone function compared to that of the Wt. The DeltaR56 mutant had a higher surface hydrophobicity than the Wt, but the R56A mutant had a lower hydrophobicity. Our data show paradoxical effects of the deletion and substitution of R56 and imply that the chaperone function of human alphaB-crystallin is dictated not only by the positive charge on R56 but also by the conformational change that it bestows on the protein.

Conserved F84 and P86 residues in alphaB-crystallin are essential to effectively prevent the aggregation of substrate proteins

Previously, we have shown that residues 73-92 (sequence DRFSVNLDVKHFSPEELKVK) in alphaB-crystallin are involved in preventing the formation of light scattering aggregates by substrate proteins. In this study, we made single substitutions of three conserved amino acid residues (H83 --> A, F84 --> G, and P86 --> A) and a nonconserved amino acid residue (K90 --> C) in the functional region of alphaB-crystallin and evaluated their role in anti-aggregation activity. Mutation of conserved residues led to changes in intrinsic tryptophan intensity, bis-ANS binding, and in the secondary and tertiary structures. The H83A mutation led to a twofold increase in molar mass, while the other mutants did not produce significant changes in the molar mass when compared to that of wild-type protein. The chaperone-like activity of the H83A mutant was enhanced by 15%-20%, and the chaperone-like activity of F84G and P86A mutants was reduced by 50%-65% when compared to the chaperone-like activity of wild-type alphaB-crystallin. The substitution of the nonconserved residue (K90 --> C) did not induce an appreciable change in the structure and function of the mutant protein. Fluorescence resonance energy transfer (FRET) assay demonstrated that destabilized ADH interacted near the K90 region in alphaB-crystallin. The data show that F84 and P86 residues are essential for alphaB-crystallin to effectively prevent the aggregation of substrate proteins. This study further supports the involvement of the residues in the 73-92 region of alphaB-crystallin in substrate protein binding and chaperone-like action.

The N-terminal domain of αB-crystallin is protected from proteolysis by bound substrate

Alpha-crystallin, a major structural protein of the lens can also function as a molecular chaperone by binding to unfolding substrate proteins. We have used a combination of limited proteolysis at low temperature, and mass spectrometry to identify the regions of alpha-crystallin directly involved in binding to the structurally compromised substrate, reduced alpha-lactalbumin. In the presence of trypsin, alpha-crystallin which had been pre-incubated with substrate showed markedly reduced proteolysis at the C-terminus compared with a control, indicating that the bound substrate restricted access of trypsin to R157, the main cleavage site. Chymotrypsin was able to cleave at residues in both the N- and C-terminal domains. In the presence of substrate, alpha-crystallin showed markedly reduced proteolysis at four sites in the N-terminal domain when compared with the control. Minor differences in cleavage were observed within the C-terminal domain suggesting that the N-terminal region of alpha-crystallin contains the major substrate interaction sites.

The N-terminal arm of small heat shock proteins is important for both chaperone activity and substrate specificity

Small heat shock proteins (sHSPs) are a ubiquitous class of molecular chaperones that interacts with substrates to prevent their irreversible insolubilization during denaturation. How sHSPs interact with substrates remains poorly defined. To investigate the role of the conserved C-terminal alpha-crystallin domain versus the variable N-terminal arm in substrate interactions, we compared two closely related dodecameric plant sHSPs, Hsp18.1 and Hsp16.9, and four chimeras of these two sHSPs, in which all or part of the N-terminal arm was switched. The efficiency of substrate protection and formation of sHSP-substrate complexes by these sHSPs with three different model substrates, firefly luciferase, citrate synthase, and malate dehydrogenase (MDH) provide new insights into sHSP/substrate interactions. Results indicate that different substrates have varying affinities for different domains of the sHSP. For luciferase and citrate synthase, the efficiency of substrate protection was determined by the identity of the N-terminal arm in the chimeric proteins. In contrast, for MDH, efficient protection clearly required interactions with the alpha-crystallin domain in addition to the N-terminal arm. Furthermore, we show that sHSP-substrate complexes with varying stability and composition can protect substrate equally, and substrate protection is not correlated with sHSP oligomeric stability for all substrates. Protection of MDH by the dimeric chimera composed of the Hsp16.9 N-terminal arm and Hsp18.1 alpha-crystallin domain supports the model that a dimeric form of the sHSP can bind and protect substrate. In total, results demonstrate that sHSP-substrate interactions are complex, likely involve multiple sites on the sHSP, and vary depending on substrate.

The effect of dextran on subunit exchange of the molecular chaperone alphaA-crystallin

Alpha-crystallin, a member of small heat shock protein (sHsp) family, is comprised of alphaA and alphaB subunits and acts as a molecular chaperone by interacting with unfolding proteins to prevent their aggregation. The alphaA-crystallin homopolymer consists of 30-40 subunits that are undergoing dynamic exchange. In vivo, alpha-crystallin elicits its chaperone action in a crowded cellular environment (e.g. in the lens). In vitro, inert molecular crowding agents (e.g. dextran) are often used to mimic crowded conditions. In this study, it was found that alpha-crystallin and alphaA-crystallin are poorer chaperones in the presence of dextran. Using fluorescence resonance energy transfer, it is shown that the alphaA-crystallin subunit exchange rate strongly increases with temperature. Binding of reduced ovotransferrin to alphaA-crystallin markedly decreases the rate of subunit exchange, as does the presence of dextran. In addition, in the presence of dextran the effect of reduced ovotransferrin on decreasing the rate of subunit exchange of alphaA-crystallin is greater than in the absence of dextran. Under the conditions of molecular crowding, the alphaA-crystallin subunit exchange rate is not temperature-dependent. In the absence of dextran, the exchange rate of alphaA-crystallin subunits correlates with its chaperone efficiency, i.e. the chaperone ability of alphaA-crystallin increases with temperature. However in the presence of dextran, the temperature dependence of the chaperone ability of alphaA-crystallin is eliminated.

Structure-Based Analysis of the β8 Interactive Sequence of Human αB Crystallin†

The functional importance of the beta8 sequence ((131)LTITSSLS(138)), which is on the surface of the alpha crystallin core domain of human alphaB crystallin, was evaluated using site-directed mutagenesis. Ultraviolet circular dichroism determined that mutating the surface-exposed, nonconserved residues, Leu-131, Thr-132, Thr-134, Ser-135, Ser-136, and Ser-138 individually or in combination (alphaAbeta8 and CEbeta8), had no measurable effect on secondary and tertiary structure. Size exclusion chromatography determined the size of the complexes formed by the beta8 mutants to be 6-8 subunits larger than wt alphaB crystallin. In chaperone assays, the protective effect of the L131S, T132A, and S135C mutants of the beta8 sequence was similar to wt alphaB crystallin when beta(L) crystallin and alcohol dehydrogenase were the chaperone substrates and decreased to 66% when citrate synthase was the chaperone substrate. In contrast, the chaperone activity for all three substrates was dramatically reduced for the T134K, S138A, S136H, and CEbeta8 mutants. The prominent location of Thr-134, Ser-136, and Ser-138 on the exposed surface of the alpha crystallin core domain could account for the effect on complex assembly and chaperone activity. Modulation of chaperone activity by the exposed residues of the beta8 sequence in the alpha crystallin core domain was independent of complex size. The results established the beta3-beta8-beta9 surface of the alpha crystallin core domain as an interface for complex assembly and chaperone activity.

Effect of site-directed mutagenesis of methylglyoxal-modifiable arginine residues on the structure and chaperone function of human alphaA-crystallin

We reported previously that chemical modification of human alphaA-crystallin by a metabolic dicarbonyl compound, methylglyoxal (MGO), enhances its chaperone-like function, a phenomenon which we attributed to formation of argpyrimidine at arginine residues (R) 21, 49, and 103. This structural change removes the positive charge on the arginine residues. To explore this mechanism further, we replaced these three R residues with a neutral alanine (A) residue one at a time or in combination and examined the impact on the structure and chaperone function. Measurement of intrinsic tryptophan fluorescence and near-UV CD spectra revealed alteration of the microenvironment of aromatic amino acid residues in mutant proteins. When compared to wild-type (wt) alphaA-crystallin, the chaperone function of R21A and R103A mutants increased 20% and 18% as measured by the insulin aggregation assay and increased it as much as 39% and 28% when measured by the citrate synthase (CS) aggregation assay. While the R49A mutant lost most of its chaperone function, R21A/R103A and R21A/R49A/R103A mutants had slightly better function (6-14% and 10-14%) than the wt protein in these assays. R21A and R103A mutants had higher surface hydrophobicity than wt alphaA-crystallin, but the R49A mutant had lower hydrophobicity. R21A and R103A mutants, but not the R49A mutant, were more efficient than wt protein in refolding guanidine hydrochloride-treated malate dehydrogenase to its native state. Our findings indicate that the positive charges on R21, R49, and R103 are important determinants of the chaperone function of alphaA-crystallin and suggest that chemical modification of arginine residues may play a role in protein aggregation during lens aging and cataract formation.

Mini-alphaB-crystallin: a functional element of alphaB-crystallin with chaperone-like activity

Alpha-crystallin is a member of the family of small heat-shock proteins (sHSP) and is composed of two subunits, alphaA-crystallin and alphaB-crystallin, which exhibit molecular chaperone-like properties. In a previous study, we found that residues 70-88 in alphaA-crystallin can function like a molecular chaperone by preventing the aggregation and precipitation of denaturing substrate proteins [Sharma, K. K., et al. (2000) J. Biol. Chem. 275, 3767-3771]. In this study, we show that the complementary sequence in alphaB-crystallin, residues 73-92 (DRFSVNLDVKHFSPEELKVK), is the functional chaperone site of alphaB-crystallin. Like the mini-alphaA-crystallin chaperone, the mini-alphaB-crystallin chaperone interacts with 1,1'-bi(4-anilino) naphthalene-5,5'-disulphonic acid (bis-ANS) and also possesses significant beta-sheet and random coil structure. Deletion of four residues (DRFS) from the N-terminus or deletion of C-terminus LKVK residues from the 73-92 peptide abolishes the chaperone-like activity against denaturing alcohol dehydrogenase. However, removal of DRFS or HFSPEELKVK is necessary to completely abolish the antiaggregation property of the peptide in insulin reduction assay. Substitution of Asp at a site corresponding to D80 in alphaB-crystallin with d-Asp or beta-Asp results in a significant loss of chaperone-like activity. Kynurenine modification of His in the peptide abolishes the antiaggregation property of the mini-chaperone. These data suggest that the 73-92 region in alphaB-crystallin is one of the substrate binding sites during chaperone activity.

Accumulation of the hydroxyl free radical markers meta-, ortho-tyrosine and DOPA in cataractous lenses is accompanied by a lower protein and phenylalanine content of the water-soluble phase

Post-translational modifications of lens proteins play a crucial role in the formation of cataract during ageing. The aim of our study was to analyze protein composition of the cataractous lenses by electrophoretic and high-performance liquid chromatographic (HPLC) methods. Samples were obtained after extracapsular cataract surgery performed by phacoemulsification technique from cataract patients with type 2 diabetes mellitus (DM CAT, n = 22) and cataract patients without diabetes (non-DM CAT, n = 20), while non-diabetic non-cataractous lenses obtained from cadaver eyes served as controls (CONTR, n = 17). Lens fragments were derived from the surgical medium by centrifugation. Samples were homogenized in a buffered medium containing protease inhibitor. Soluble and insoluble protein fractions were separated by centrifugation. The electrophoretic studies were performed according to Laemmli on equal amounts of proteins and were followed by silver intensification. Oxidized amino acid and Phe content of the samples were also analyzed by HPLC following acid hydrolysis of proteins. Our results showed that soluble proteins represented a significantly lower portion of the total protein content in cataractous lenses in comparison with the control group (CONTR, 71.25%; non-DM CAT, 32.00%; DM CAT, 33.15%; p < 0.05 vs CONTR for both). Among the proteins, the crystallin-like proteins with low-molecular weight can be found both in the soluble and insoluble fractions, and high-molecular weight aggregates were found mainly in the total homogenates. In our HPLC analysis, oxidatively modified derivatives of phenylalanine were detected in cataractous samples. We found higher levels of m-Tyr, o-Tyr and DOPA in the total homogenates of cataractous samples compared to the supernatants. In all three groups, the median Phe/protein ratio of the total homogenates was also higher than that of the supernatants (total homogenates vs supernatants, in the CONTR group 1102 vs 633 micromol/g, in the DM CAT group 1187 vs 382 micromol/g and in the non-DM CAT group 967 vs 252 micromol/g; p < 0.05 for all). In our study we found that oxidized amino acids accumulate in cataractous lenses, regardless of the origin of the cataract. The accumulation of the oxidized amino acids probably results from oxidation of Phe residues of the non-water soluble lens proteins. We found the presence of high-molecular weight protein aggregates in cataractous total homogenates, and a decrease of protein concentration in the water-soluble phase of cataractous lenses. The oxidation of lens proteins and the oxidative modification of Phe residues in key positions may lead to an altered interaction between protein and water molecules and thus contribute to lens opacification.

Recognition sequence 2 (residues 60-71) plays a role in oligomerization and exchange dynamics of alphaB-crystallin

Previously, using the peptide scan method, we have determined that residues 42-57 and 60-71 in alphaB-crystallin (TSLSPFYLRPPSFLRA, named recognition sequence 1 or RS-1, and WFDTGLSEMRLE, named recognition sequence 2 or RS-2) are involved in interaction with alphaA-crystallin. To understand the significance of the RS-2 region in interactions between alphaA- and alphaB-crystallins, W60R, F61N, and S66G mutants of alphaB-crystallin were made and tested for their ability to interact with alphaA-crystallin. W60R and S66G mutations increased the oligomeric size of alphaB-crystallin by 1.6- and 2.7-fold respectively, whereas the F61N mutation had no effect. The tryptophan fluorescence intensity of alphaBS66G was 1.5-fold higher than that for the wild type. The intrinsic fluorescence of alphaBF61N was marginally lower than that of alphaB, whereas the fluorescence intensity of alphaBW60R decreased by 40% compared with that of alphaB. The relative availability of hydrophobic sites in the mutants was in the following order: alphaBS66G >> alphaB = alphaBF61N = alphaBW60R. The far-UV CD profiles for the wild type and alphaB-crystallin mutants indicated no significant changes in their secondary structures, except for alphaBS66G, which showed an increase in alpha-helical content. The near-UV CD profiles of alphaBW60R and alphaBF61N were nearly similar to that of wild type alphaB. On the other hand, alphaBS66G beyond 270 nm exhibited a signature completely different from that of wild type alphaB. Mutations did not alter the chaperone-like activity of these proteins. The W60R mutation did not affect the rate of subunit exchange between alphaB- and alphaA-crystallins. On the other hand, the S66G mutation increased the subunit exchange rate by 100%, whereas the F61N mutation decreased the rate of subunit exchange between alphaB- and alphaA-crystallins by 36%. Our results establish the importance of residues 60-71 in oligomerization of alphaB-crystallin and subunit interaction between alphaB- and alphaA-crystallins.

Chaperone-Like Activity of Mycobacterium tuberculosis Hsp16.3 Does Not Require Its Intact (Native) Structures

Small heat shock proteins (sHsps) were found to exhibit efficient chaperone-like activities under stress conditions although their native structures are severely disturbed. Here, using an alternative approach (site-directed mutagenesis), we obtained two structurally and functionally distinct Mycobacterium tuberculosis Hsp16.3 single-site mutant proteins. The G59W mutant protein (with Gly59 substituted by Trp) is capable of exhibiting efficient chaperone-like activity even under non-stress conditions although its secondary, tertiary, and quaternary structures are very different from that of the wild type protein. By contrast, the G59A mutant protein (with Gly59 substituted by Ala) resembles with the wild type protein in structure and function. These observations suggest that the Gly59 of the Hsp16.3 protein is critical for its folding and assembly. In particular, we propose that the exhibition of chaperone-like activity for Hsp16.3 does not require its intact (native) structures but requires the disturbance of its native structures (i.e., the native structure-disturbed Hsp16.3 retains its chaperone-like activity or even becomes more active). In addition, the behavior of such an active mutant protein (G59W) also strongly supports our previous suggestion that Hsp16.3 exhibits chaperone-like activity via oligomeric dissociation.

Insights into Hydrophobicity and the Chaperone-like Function of αA- and αB-crystallins

Alpha-crystallin, composed of two subunits, alphaA and alphaB, has been shown to function as a molecular chaperone that prevents aggregation of other proteins under stress conditions. The exposed hydrophobic surfaces of alpha-crystallins have been implicated in this process, but their exact role has not been elucidated. In this study, we quantify the hydrophobic surfaces of alphaA- and alphaB-crystallins by isothermal titration calorimetry using 8-anilino-1-napthalenesulfonic acid (ANS) as a hydrophobic probe and analyze its correlation to the chaperone potential of alphaA- and alphaB-crystallins under various conditions. Two ANS binding sites, one with low and another with high affinity, were clearly detected, with alphaB showing a higher number of sites than alphaA at 30 degrees C. In agreement with the higher number of hydrophobic sites, alphaB-crystallin demonstrated higher chaperone activity than alphaA at this temperature. Thermodynamic analysis of ANS binding to alphaA- and alphaB-crystallins indicates that high affinity binding is driven by both enthalpy and entropy changes, with entropy dominating the low affinity binding. Interestingly, although the number of ANS binding sites was similar for alphaA and alphaB at 15 degrees C, alphaA was more potent than alphaB in preventing aggregation of the insulin B-chain. Although there was no change in the number of high affinity binding sites of alphaA and alphaB for ANS upon preheating, there was an increase in the number of low affinity sites of alphaA and alphaB. Preheated alphaA, in contrast to alphaB, exhibited remarkably enhanced chaperone activity. Our results indicate that although hydrophobicity appears to be a factor in determining the chaperone-like activity of alpha-crystallins, it does not quantitatively correlate with the chaperone function of alpha-crystallins.

AlphaA-crystallin interacting regions in the small heat shock protein, alphaB-crystallin

Amino acid sequences of alphaB-crystallin, involved in interaction with alphaA-crystallin, were determined by using peptide scans. Positionally addressable 20-mer overlapping peptides, representing the entire sequence of alphaB-crystallin, were synthesized on a PVDF membrane. The membrane was blocked with albumin and incubated with purified alphaA-crystallin. Probing the membrane with alphaA-crystallin-specific antibodies revealed residues 42-57, 60-71, and 88-123 in alphaB-crystallin to interact with alphaA-crystallin. Residues 42-57 and 60-71 interacted more strongly with alphaA-crystallin than the 88-123 sequence of alphaB-crystallin. Binding of one of the alphaB peptides (42-57) to alphaA-crystallin was also confirmed by gel filtration studies and HPLC analysis. The alphaB-crystallin sequences involved in interaction with alphaA-crystallin were distinct from the chaperone sites reported earlier as binding of the alphaB sequence from residues 42-57 does not alter the chaperone-like function of alphaA-crystallin. To identify the critical residues involved in interaction with alphaA-crystallin, R50G and P51A mutants of alphaB-crystallin were made and tested for their ability to interact with alphaA-crystallin. The oligomeric size and hydrophobicity of the mutants were similar. Circular dichroism studies showed that the P51A mutation increased the alpha-helical content of the protein. While the alphaBR50G mutant showed chaperone-like activity similar to wild-type alphaB, alphaBP51A showed reduced chaperone function. Fluorescence resonance energy transfer studies showed that the P51A mutation decreased the rate of subunit exchange with alphaA by 63%, whereas the R50G mutation reduced the exchange rate by 23%. Similar to wild-type alphaB, alphaB-crystallin peptide (42-57) effectively competed with alphaBP51A and alphaBR50G for interaction with alphaA. Thus, our studies showed that the alphaB-crystallin sequence (42-57) is one of the interacting regions in alphaB and alphaA oligomer formation.

Deamidation affects structural and functional properties of human alphaA-crystallin and its oligomerization with alphaB-crystallin

To determine the effects of deamidation on structural and functional properties of alphaA-crystallin, three mutants (N101D, N123D, and N101D/N123D) were generated. Deamidated alphaB-crystallin mutants (N78D, N146D, and N78D/N146D), characterized in a previous study (Gupta, R., and Srivastava, O. P. (2004) Invest. Ophthalmol. Vis. Sci. 45, 206-214) were also used. The biophysical and chaperone properties were determined in (a) homoaggregates of alphaA mutants (N101D, N123D, and N101D/N123D) and (b) reconstituted heteroaggregates of alpha-crystallin containing (i) wild type alphaA (WT-alphaA): WT-alphaB crystallins, (ii) individual alphaA-deamidated mutants:WT-alphaB crystallins, and (iii) WT-alphaA:individual alphaB-deamidated mutant crystallins. Compared with the WT-alphaA, the three alphaA-deamidated mutants showed reduced levels of chaperone activity, alterations in secondary and tertiary structures, and larger aggregates. These altered properties were relatively more pronounced in the mutant N101D compared with the mutant N123D. Further, compared with heteroaggregates of WT-alphaA and WT-alphaB, the heteroaggregates containing deamidated subunits of either alphaA- or alphaB-crystallins and their counterpart WT proteins showed higher molecular mass, altered tertiary structures, lower exposed hydrophobic surfaces, and reduced chaperone activity. However, the heteroaggregate containing WT-alphaA and deamidated alphaB subunit showed lower chaperone activity, smaller oligomers, and 3-fold lower subunit exchange rate than heteroaggregate containing deamidated alphaA- and WT-alphaB subunits. Together, the results suggested that (a) both Asn residues (Asn-101 and Asn-123) are required for the structural integrity and chaperone function of alphaA-crystallin and (b) the presence of WT-alphaB in the alpha-crystallin heteroaggregate leads to packing-induced structural changes which influences the oligomerization and modulate chaperone activity.

Three-dimensional models corresponding to the C-terminal domain of human αA- and αB-crystallins based on the crystal structure of the small heat-shock protein HSP16.9 from wheat

We propose three-dimensional models corresponding to the C-terminal domain of human alphaA- and alphaB-crystallins by using the comparative modeling program Modeler and the more closely related crystal structure of the small heat-shock protein (sHSP) belonging to the eukaryotic species from wheat HSP16.9 as template structure. The sequence alignments differ slightly from alignments that were used previously to construct alpha-crystallin models based on homology and the crystal structure of the more distantly related small heat-shock protein from archaeal species; Methanococcus jannaschii Mj HSP16.5, the only related structure then available as a template. The alpha-crystallin models based on HSP16.9 show better 3-D profile scores and reflect the relative shifts in the beta-strands corresponding to the beta-sandwich associated with the core C-terminal domain that is common to small heat-shock proteins and the alpha-crystallins. The loop between the equivalent beta5-beta7 strands corresponds to a region of seven amino acid residues deletion in alpha-crystallins and defines the new set of amino acid residues likely to be associated with a dimer interface. The models may be useful to examine sites of mutations that are known to affect chaperone-like activity and provide the structural basis for dimerization in alpha-crystallins.

Enhancement of chaperone function of alpha-crystallin by methylglyoxal modification

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.

Structural and functional defects caused by point mutations in the alpha-crystallin domain of a bacterial alpha-heat shock protein

The diverse family of alpha-crystallin-type small heat shock proteins (alpha-Hsps or sHsps) is characterised by a central, moderately conserved alpha-crystallin domain. Oligomerisation followed by dissociation of subparticles is thought to be a prerequisite for chaperone function. We demonstrate that HspH, a bacterial alpha-Hsp from the soybean-symbiont Bradyrhizobium japonicum, assembles into dynamic complexes freely exchanging subunits with homologous and heterologous complexes. The importance of the alpha-crystallin domain for oligomerisation and chaperone activity was tested by site-directed mutagenesis of 12 different residues. In contrast to mammalian alpha-Hsps, the majority of these mutations elicited severe structural and functional defects in HspH. The individual exchange of five amino acid residues throughout the alpha-crystallin domain was found to compromise oligomerisation to various degrees. Assembly defects resulting in complexes of reduced size correlated with greatly decreased or abolished chaperone activity, reinforcing that complete oligomerisation is required for functionality. Mutation of a highly conserved glycine (G114) at the C-terminal end of the alpha-crystallin domain specifically impaired chaperone activity without interfering with oligomerisation properties, indicating that this residue is critical for substrate interaction. The structural and functional importance of this and other residues is discussed in the context of a modeled three-dimensional structure of HspH.

Alpha-crystallin

Alpha A and alpha B-crystallins are a major protein component of the mammalian eye lens. Being a member of the small heat-shock protein family they possess chaperone-like function. The alpha-crystallins and especially alpha B is also found outside the lens having an extensive tissue distribution. Alpha B-crystallin is found to be over-expressed in many neurological diseases, and mutations in alpha A or B-crystallin can cause cataract and myopathy. This review deals with some of the unique properties of the alpha-crystallins emphasizing especially what we don't know about its function and structure.

Alpha-crystallin regions affected by adenosine 5'-triphosphate identified by hydrogen-deuterium exchange

ATP interaction with lens alpha-crystallins leading to enhanced chaperone activity is not yet well understood. One model for chaperone activity of small heat shock proteins proposes that ATP causes small heat shock proteins to release substrates, which are then renatured by other larger heat shock proteins. A similar role has been proposed for ATP in alpha-crystallin chaperone activity. To evaluate this model, ATP-induced structural changes of native human alpha-crystallin assemblies were determined by hydrogen-deuterium exchange. In these experiments, hydrogen-deuterium exchange, measured by mass spectrometry, gave direct evidence that ATP decreases the accessibility of amide hydrogens in multiple regions of both alphaA and alphaB. The surface encompassed by these regions is much larger than would be shielded by a single ATP, implying that multiple ATP molecules bind to each subunit and/or ATP causes a more compact alpha-crystallin structure. Such a conformational change could release a bound substrate. The regions most affected by ATP are near putative substrate binding regions of alphaA and alphaB and in the C-terminal extension of alphaB. The widespread decrease in hydrogen-deuterium exchange with particularly large decreases near substrate binding regions suggests that ATP releases substrates via both direct displacement and a global conformational change.

Structural and functional implications of C-terminal regions of alpha-synuclein

Aggregation of alpha-synuclein is thought to play a major role in the pathogenesis of Parkinson's disease (PD), which is characterized by the presence of intracytoplasmic Lewy bodies (LB) in the brain. alpha-Synuclein and its deletion mutants are largely unfolded proteins with random coil structures as revealed by CD spectra, fluorescence spectra, gel filtration chromatography, and ultracentrifugation. On the basis of its highly unfolded and flexible conformation, we have investigated the chaperone-like activity of alpha-synuclein in vitro. In our experiments, alpha-synuclein inhibited the aggregation of model substrates and protected the catalytic activity of alcohol dehydrogenase and rhodanese during heat stress. In addition, alpha-synuclein inhibited the initial aggregation of reduced/denatured lysozyme on the refolding pathway. Interestingly, deletion of the C-terminal regions led to the abolishment of chaperone activity, although largely unstructured conformations are maintained. Moreover, alpha-synuclein could inhibit the aggregation of various Escherichia coli cellular proteins during heat stress, and C-terminal deletion mutants could not provide any protection to these cellular proteins. Results with synthetic C-terminal peptides and C-terminal deletion mutants suggest that the second acidic repeat, (125)YEMPSEEGYQDYEPEA(140), is important for the chaperone activity of alpha-synuclein, and C-terminal deletion leads to the facilitated aggregation with the elimination of chaperone activity.

Alpha-crystallin-type heat shock proteins: socializing minichaperones in the context of a multichaperone network

Alpha-crystallins were originally recognized as proteins contributing to the transparency of the mammalian eye lens. Subsequently, they have been found in many, but not all, members of the Archaea, Bacteria, and Eucarya. Most members of the diverse alpha-crystallin family have four common structural and functional features: (i) a small monomeric molecular mass between 12 and 43 kDa; (ii) the formation of large oligomeric complexes; (iii) the presence of a moderately conserved central region, the so-called alpha-crystallin domain; and (iv) molecular chaperone activity. Since alpha-crystallins are induced by a temperature upshift in many organisms, they are often referred to as small heat shock proteins (sHsps) or, more accurately, alpha-Hsps. Alpha-crystallins are integrated into a highly flexible and synergistic multichaperone network evolved to secure protein quality control in the cell. Their chaperone activity is limited to the binding of unfolding intermediates in order to protect them from irreversible aggregation. Productive release and refolding of captured proteins into the native state requires close cooperation with other cellular chaperones. In addition, alpha-Hsps seem to play an important role in membrane stabilization. The review compiles information on the abundance, sequence conservation, regulation, structure, and function of alpha-Hsps with an emphasis on the microbial members of this chaperone family.

Evaluation of hydrophobicity versus chaperonelike activity of bovine alphaA- and alphaB-crystallin

Calf lens alphaA-crystallin isolated by reversed-phase HPLC demonstrates a slightly more hydrophobic profile than alphaB-crystallin. Fluorescent probes in addition to bis-ANS, like cis-parinaric acid (PA) and pyrene, show higher quantum yields or Ham ratios when bound to alphaA-crystallin than to alphaB-crystallin at room temperature. Bis-ANS binding to both alphaA- and alphaB-crystallin decreases with increase in temperature. At room temperature, the chaperone-like activity of alphaA-crystallin is lower than that of alphaB-crystallin whereas at higher temperatures, alphaA-crystallin shows significantly higher protection against aggregation of substrate proteins compared to alphaB-crystallin. Therefore, calf lens alphaA-crystallin is more hydrophobic than alphaB-crystallin and chaperone-like activity of alpha-crystallin subunits is not quantitatively related to their hydrophobicity.