Heat-induced conformational change and increased chaperone activity of lens alpha-crystallin

Conformational Changes of α-Crystallin Proteins Induced by Heat Stress

α-crystallin is a major structural protein in the eye lenses of vertebrates that is composed of two relative subunits, αA and αB crystallin, which function in maintaining lens transparency. As a member of the small heat-shock protein family (sHsp), α-crystallin exhibits chaperone-like activity to prevent the misfolding or aggregation of critical proteins in the lens, which is associated with cataract disease. In this study, high-purity αA and αB crystallin proteins were expressed from E. coli and purified by affinity and size-exclusion chromatography. The size-exclusion chromatography experiment showed that both αA and αB crystallins exhibited oligomeric complexes in solution. Here, we present the structural characteristics of α-crystallin proteins from low to high temperature by combining circular dichroism (CD) and small-angle X-ray scattering (SAXS). Not only the CD data, but also SAXS data show that α-crystallin proteins exhibit transition behavior on conformation with temperature increasing. Although their protein sequences are highly conserved, the analysis of their thermal stability showed different properties in αA and αB crystallin. In this study, taken together, the data discussed were provided to demonstrate more insights into the chaperone-like activity of α-crystallin proteins.

Modulation of the Structure and Stability of Novel Camel Lens Alpha-Crystallin by pH and Thermal Stress

Alpha-crystallin protein performs structural and chaperone functions in the lens and comprises alphaA and alphaB subunits at a molar ratio of 3:1. The highly complex alpha-crystallin structure challenges structural biologists because of its large dynamic quaternary structure (300−1000 kDa). Camel lens alpha-crystallin is a poorly characterized molecular chaperone, and the alphaB subunit possesses a novel extension at the N-terminal domain. We purified camel lens alpha-crystallin using size exclusion chromatography, and the purity was analyzed by gradient (4−12%) sodium dodecyl sulfate−polyacrylamide gel electrophoresis. Alpha-crystallin was equilibrated in the pH range of 1.0 to 7.5. Subsequently, thermal stress (20−94 °C) was applied to the alpha-crystallin samples, and changes in the conformation and stability were recorded by dynamic multimode spectroscopy and intrinsic and extrinsic fluorescence spectroscopic methods. Camel lens alpha-crystallin formed a random coil-like structure without losing its native-like beta-sheeted structure under two conditions: >50 °C at pH 7.5 and all temperatures at pH 2.0. The calculated enthalpy of denaturation, as determined by dynamic multimode spectroscopy at pH 7.5, 4.0, 2.0, and 1.0 revealed that alpha-crystallin never completely denatures under acidic conditions or thermal denaturation. Alpha-crystallin undergoes a single, reversible thermal transition at pH 7.5. The thermodynamic data (unfolding enthalpy and heat capacity change) and chaperone activities indicated that alpha-crystallin does not completely unfold above the thermal transition. Camels adapted to live in hot desert climates naturally exhibit the abovementioned unique features.

Protection of ζ-crystallin by α-crystallin under thermal stress

Cataract is one of the major causes of blindness worldwide. Several factors including post-translational modification, thermal and solar radiations promote cataractogenesis. The camel lens proteins survive very harsh desert conditions and resist cataractogenesis. The folding and aggregation mechanism of camel lens proteins are poorly characterized. The camel lens contains three ubiquitous crystallins (α-, β-, and γ-crystallin) and a novel protein (ζ-crystallin) in large amounts. In this study, a sequence similarity search of camel α-crystallin with that of other organisms showed that the camel αB-crystallin consists of an extended N-terminal domain. Our results indicate that camel α-crystallin efficiently prevented aggregation of ζ-crystallin, with or without an obligate cofactor up to 89 °C. It performed a quick and efficient holdase function irrespective of the unfolding stage or aggregation. Camel α-crystallin exhibits approximately 20% chaperone activity between 30 and 40 °C and is completely activated above 40 °C. Camel α-crystallin underwent a single reversible thermal transition without loss of β-sheet secondary structure. Intrinsic tryptophan fluorescence and ANS binding experiments revealed two transitions which corresponded to activation of its chaperone function. In contrast to earlier studies, camel α-crystallin completely protected lens proteins during thermal stress.

Characteristics of the Protein Complexes and Pores Formed by Bacillus cereus Hemolysin BL

Bacillus cereus Hemolysin BL is a tripartite toxin responsible for a diarrheal type of food poisoning. Open questions remain regarding its mode of action, including the extent to which complex formation prior to cell binding contributes to pore-forming activity, how these complexes are composed, and the properties of the pores formed in the target cell membrane. Distinct complexes of up to 600 kDa were found on native gels, whose structure and size were primarily defined by Hbl B. Hbl L1 and L2 were also identified in these complexes using Western blotting and an LC-MS approach. LC-MS also revealed that many other proteins secreted by B. cereus exist in complexes. Further, a decrease of toxic activity at temperatures ≥60 °C was shown, which was unexpectedly restored at higher temperatures. This could be attributed to a release of Hbl B monomers from tight complexation, resulting in enhanced cell binding. In contrast, Hbl L1 was rather susceptible to heat, while heat treatment of Hbl L2 seemed not to be crucial. Furthermore, Hbl-induced pores had a rather small single-channel conductance of around 200 pS and a probable channel diameter of at least 1 nm on planar lipid bilayers. These were highly instable and had a limited lifetime, and were also slightly cation-selective. Altogether, this study provides astonishing new insights into the complex mechanism of Hbl pore formation, as well as the properties of the pores.

Structure and function of α-crystallins: Traversing from in vitro to in vivo

BACKGROUND

The two α-crystallins (αA- and αB-crystallin) are major components of our eye lenses. Their key function there is to preserve lens transparency which is a challenging task as the protein turnover in the lens is low necessitating the stability and longevity of the constituent proteins. α-Crystallins are members of the small heat shock protein family. αB-crystallin is also expressed in other cell types.

SCOPE OF THE REVIEW

The review summarizes the current concepts on the polydisperse structure of the α-crystallin oligomer and its chaperone function with a focus on the inherent complexity and highlighting gaps between in vitro and in vivo studies.

MAJOR CONCLUSIONS

Both α-crystallins protect proteins from irreversible aggregation in a promiscuous manner. In maintaining eye lens transparency, they reduce the formation of light scattering particles and balance the interactions between lens crystallins. Important for these functions is their structural dynamics and heterogeneity as well as the regulation of these processes which we are beginning to understand. However, currently, it still remains elusive to which extent the in vitro observed properties of α-crystallins reflect the highly crowded situation in the lens.

GENERAL SIGNIFICANCE

Since α-crystallins play an important role in preventing cataract in the eye lens and in the development of diverse diseases, understanding their mechanism and substrate spectra is of importance. To bridge the gap between the concepts established in vitro and the in vivo function of α-crystallins, the joining of forces between different scientific disciplines and the combination of diverse techniques in hybrid approaches are necessary. This article is part of a Special Issue entitled Crystallin Biochemistry in Health and Disease.

Control of the structural stability of α-crystallin under thermal and chemical stress: the role of carnosine

The structural properties of α-crystallin, the major protein of the eye lens of mammals, in aqueous solution are investigated by means of small angle X-ray and dynamic light scattering. The research interest is devoted in particular to the effect of carnosine in protecting the protein under stress conditions, like temperature increase and presence of denaturant (guanidinium-HCl). The results suggest that carnosine interacts, through mechanisms involving hydrophobic interactions, with α-crystallin and avoids the structural changes in the quaternary structure induced by thermal and chemical stress. It is also shown that, if mediated by carnosine, the self-aggregation of α-crystallin induced by the denaturant at higher temperature can be controlled and even partially reversed. Therefore, carnosine is effective in preserving the structural integrity of the protein, suggesting the possibility of new strategies of intervention for preventing or treating pathologies related to protein aggregation, like cataracts.

The effect of Arg on the structure perturbation and chaperone activity of α-crystallin in the presence of the crowding agent, dextran

α-Crystallin is a protein that is expressed at high levels in all vertebrate eye lenses. It has a molecular weight of 20 kDa and is composed of two subunits: αA and αB. α-Crystallin is a member of the small heat shock protein (sHsps) family that has been shown to prevent protein aggregation. Small molecules are organic compounds that have low molecular weight (<800 Da). Arginin (Arg) is a small molecule and has been shown to prevent protein aggregation through interaction with partially folded intermediates. In this study, the effect of Arg on the chaperone activity of α-crystallin in the presence of dextran, as a crowding agent, against ordered and disordered aggregation of different target proteins (α-lactalbumin, ovotransferrin, and catalase) has been investigated. The experiments were done using visible absorption spectroscopy, ThT-binding assay, fluorescence spectroscopy, and CD spectroscopy. The results showed that in amorphous aggregation and amyloid fibril formation, both in the presence and absence of dextran, Arg had a positive effect on the chaperone action of α-crystallin. However, in the presence of dextran, the effect of Arg on the chaperone ability of α-crystallin was less than in its absence. Thus, our result suggests that crowding interior media decreases the positive effect of Arg on the chaperone ability of α-crystallin. This is a very important issue, since we are trying to find a mechanism to protect living cells against the toxic effect of protein aggregation.

Structural alteration of Escherichia coli Hsp31 by thermal unfolding increases chaperone activity

Escherichia coli Hsp31, encoded by hchA, is a heat-inducible molecular chaperone. We found that Hsp31 undergoes a conformational change via temperature-induced unfolding, generating a high molecular weight (HMW) form with enhanced chaperone activity. Although it has previously been reported that some subunits of the Hsp31 crystal structure show structural heterogeneity with increased hydrophobic surfaces, Hsp31 basically forms a dimer. We found that a C-terminal deletion (CΔ19) of Hsp31 exhibited structurally and functionally similar characteristics to that of the HMW form. Both the CΔ19 and HMW forms achieved a structure with considerably more β-sheets and less α-helices than the native dimeric form, exposing a portion of its hydrophobic surfaces. The structural alterations were determined from its spectral changes in circular dichroism, intrinsic fluorescence of tryptophan residues, and fluorescence of bis-ANS binding to a hydrophobic surface. Interestingly, during thermal transition, the dimeric Hsp31 undergoes a conformational change to the HMW species via the CΔ19 structure, as monitored with near-UV CD spectrum, implying that the CΔ19 resembles an intermediate state between the dimer and the HMW form. From these results, we propose that Hsp31 transforms itself into a fully functional chaperone by altering its tertiary and quaternary structures.

NEW INSIGHTS INTO STRUCTURAL CHANGES OF LENS PROTEINS

X-ray scattering techniques were used to study the effects of heating on whole eye lens and α-crystallin gels. The temperature range used was from 20 to 70°C. The position of single X-ray reflection seen in whole lens was unchanged in the temperature range 20 to 45°C, with a continuous spacing of 152 Å. However, at 50°C the spacing increased from 152 Å to 165 Å. An interpretation of these results is that in eye lens, α-crystallin is protecting other lens proteins from super-aggregation up to 50°C. In α-crystallin gels a moderate increase in both the spacing and intensity of the reflection was observed from 20 to 45°C, followed by a dramatic increase from 45 to 70°C. Over the whole temperature range the spacing changed from 138 Å at 70°C to 195 Å at 70°. After eleven hours of cooling, this effect was found to be irreversible.

Structure and mechanism of protein stability sensors: chaperone activity of small heat shock proteins

Small heat shock proteins (sHSP) make up a remarkably diverse group of molecular chaperones possessing a degree of structural plasticity unparalleled in other protein superfamilies. In the absence of chemical energy input, these stability sensors can sensitively recognize and bind destabilized proteins, even in the absence of gross misfolding. Cellular conditions regulate affinity toward client proteins, allowing tightly controlled switching and tuning of sHSP chaperone capacity. Perturbations of this regulation, through chemical modification or mutation, directly lead to a variety of disease states. This review explores the structural basis of sHSP oligomeric flexibility and the corresponding functional consequences in the context of a model describing sHSP activity with a set of three coupled thermodynamic equilibria. As current research illuminates many novel physiological roles for sHSP outside of their traditional duties as molecular chaperones, such a conceptual framework provides a sound foundation for describing these emerging functions in physiological and pathological processes.

Mechanism of suppression of protein aggregation by α-crystallin

This review summarizes experimental data illuminating the mechanism of suppression of heat-induced protein aggregation by alpha-crystallin, one of the small heat shock proteins. The dynamic light scattering data show that the initial stage of thermal aggregation of proteins is the formation of the initial aggregates involving hundreds of molecules of the denatured protein. Further sticking of the starting aggregates proceeds in a regime of diffusion-limited cluster-cluster aggregation. The protective effect of alpha-crystallin is due to transition of the aggregation process to the regime of reaction-limited cluster-cluster aggregation, wherein the sticking probability for the colliding particles becomes lower than unity.

Fluorescence study on the interaction between hypericin and lens protein "alpha-crystallin"

Hypericin has been reported as a potent photosensitizing agent exhibiting antiviral, antibacterial, antineoplastic activities. Although its photophysics and mode of action are strongly modulated by the binding protein, detailed information about its mechanism of interaction with possible cellular targets, including proteins, is still lacking. Previous in vitro studies demonstrated that hypericin can be uptaken by intact lens and is able to bind to the major lens protein "alpha-crystallin." In this study, the mechanism of interaction of this potent drug with alpha-crystallin was studied using the chemical denaturant guanidine hydrochloride (GdnHCl) and the hydrophobic surface probe, 8-anilino-1-naphthalenesulfonic acid (ANS). Fluorescence measurements showed that the increased exposure of tryptophan resulting from partial unfolding of alpha-crystallin incubated with 1.0 mol L(-1) of GdnHCl corresponds to the maximum accessibility of hydrophobic sites to ANS at the same GdnHCl concentration. Interestingly at this additional hydrophobicity of the protein, hypericin exhibited its maximum fluorescence intensity. This in vitro study implied that hydrophobic sites of alpha-crystallin play a significant role in its interaction with hypericin. The binding between alpha-crystallin and hypericin was found to be enhanced by partial perturbation of the protein.

How alpha-crystallin prevents the aggregation of insulin

Using steady-state, polarized, and phase-modulation fluorometry, the dithiothreitol-induced denaturation of insulin and formation of its complex with alpha-crystallin in solution were studied. Prevention of the aggregation of insulin by alpha-crystallin is due to formation of chaperone complexes, i.e. interaction of chains of the denatured insulin with alpha-crystallin. The conformational changes in alpha-crystallin that occur during complex formation are rather small. It is unlikely that N-termini are directly involved in the complex formation. The 8-anilino-1-naphthalenesulfonate (ANS) is not sensitive to the complex formation. ANS emits mainly from alpha-crystallin monomers, dimers, and tetramers, but not from oligomers or aggregates. The possibility of highly sensitive detection of aggregates by light scattering using a spectrofluorometer with crossed monochromators is demonstrated.

Folding and stability of the isolated Greek key domains of the long-lived human lens proteins gammaD-crystallin and gammaS-crystallin

The transparency of the eye lens depends on the high solubility and stability of the lens crystallin proteins. The monomeric gamma-crystallins and oligomeric beta-crystallins have paired homologous double Greek key domains, presumably evolved through gene duplication and fusion. Prior investigation of the refolding of human gammaD-crystallin revealed that the C-terminal domain folds first and nucleates the folding of the N-terminal domain. This result suggested that the human N-terminal domain might not be able to fold on its own. We constructed and expressed polypeptide chains corresponding to the isolated N- and C-terminal domains of human gammaD-crystallin, as well as the isolated domains of human gammaS-crystallin. Both circular dichroism and fluorescence spectroscopy indicated that the isolated domains purified from Escherichia coli were folded into native-like monomers. After denaturation, the isolated domains refolded efficiently at pH 7 and 37 degrees C into native-like structures. The in vitro refolding of all four domains revealed two kinetic phases, identifying partially folded intermediates for the Greek key motifs. When subjected to thermal denaturation, the isolated N-terminal domains were less stable than the full-length proteins and less stable than the C-terminal domains, and this was confirmed in equilibrium unfolding/refolding experiments. The decrease in stability of the N-terminal domain of human gammaD-crystallin with respect to the complete protein indicated that the interdomain interface contributes of 4.2 kcal/mol to the overall stability of this very long-lived protein.

Unfolding and Refolding of Bovine α-Crystallin in Urea and Its Chaperone Activity

We undertook an unfolding and refolding study of alpha(L)-crystallin in presence of urea to explore the breakdown and formation of various levels of structure and to find out whether the breakdown of various levels of structure occurs simultaneously or in a hierarchal manner. We used various techniques such as circular dichroism, fluorescence spectroscopy, light scattering, polarization to determine the changes in secondary, tertiary, and quaternary structure. Unfolding and refolding occurred through a number of intermediates. The results showed that all levels of structure in alpha(L)-crystallin collapsed or reformed simultaneously. The intermediates that occurred in the 2-4 M urea concentration range during unfolding and refolding differed from each other in terms of the polarity of the tryptophan environment. The ANS binding experiments revealed that refolded alpha(L)-crystallin had higher number of hydrophobic pockets compared to native one. On the other hand, polarity of these pockets remained same as that of the native protein. Both light scattering and polarization measurements showed smaller oligomeric size of refolded alpha(L)-crystallin. Thus, although the secondary structural changes were almost reversible, the tertiary and quaternary structural changes were not. The refolded alpha(L)-crystallin had more exposed hydrophobic sites with increased binding affinity. The refolded form also showed higher chaperone activity than native one. Since the refolded form was smaller in oligomeric size, some buried hydrophobic sites were available. The higher chaperone activity of lower sized oligomer of alpha(L)-crystallin again revealed that chaperone activity was dependent on hydrophobicity and not on oligomeric size.

Trehalose effects on α-crystallin aggregates

alpha-Crystallin in its native state is a large, heterogeneous, low-molecular weight (LMW) aggregate that under certain conditions may progressively became part of insoluble high-molecular weight (HMW) systems. These systems are supposed to play a relevant role in eye lens opacification and vision impairment. In this paper, we report the effects of trehalose on alpha-crystallin aggregates. The role of trehalose in alpha-crystallin stress tolerance, chaperone activity and thermal stability is studied. The results show that trehalose stabilizes the alpha-crystallin native structure, inhibits alpha-crystallin aggregation, and disaggregates preformed LMW systems not affecting its chaperone activity.

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 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.

Temperature effects on alpha-crystallin structure probed by 6-bromomethyl-2-(2-furanyl)-3-hydroxychromone, an environmentally sensitive two-wavelength fluorescent dye covalently attached to the single Cys residue

The single Cys residue in the C-terminal domain of bovine eye lens alpha-crystallin was covalently labelled with 6-bromomethyl-2-(2-furanyl)-3-hydroxychromone. This novel SH-reactive two-band ratiometric fluorescent dye is characterized by excited state intramolecular proton transfer reaction yielding two highly emissive N* and T* bands separated by more than 100 nm. Their relative intensities are known to be highly sensitive to the H-bonding ability of the environment. Properties of the environment of the dye attached to the protein were studied under native-like conditions and at a range of elevated temperatures that are known to facilitate alpha-crystallin chaperone-like activity. We observe that on heating, the environment of the dye becomes more flexible and the H-bonding of the dye with the protein vicinity decreases. The spectroscopic properties observed on heating were partially restored after cooling, but the initial values were not reached on the time scale of our experiments (up to 3 h). This suggests that the changes of the dye microenvironment are connected with the rearrangements of alpha-crystallin quaternary structure. Since there is only one Cys residue in alphaA subunit of alpha-crystallin (whereas alphaB subunit contains no Cys), we attributed the observed temperature-induced changes of the dye's microenvironment to the particular site within alpha-crystallin molecule.

Heat perturbation of bovine eye lens alpha-crystallin probed by covalently attached ratiometric fluorescent dye 4'-diethylamino-3-hydroxyflavone

Bovine eye lens alpha-crystallin was covalently labeled with 6-bromomethyl-4'-diethylamino-3-hydroxyflavone and studied under native-like conditions and at the elevated temperature (60 degrees C) that is known to facilitate alpha-crystallin chaperone-like activity. This novel SH-reactive two-band ratiometric fluorescent probe is characterized by two highly emissive N*- and T*-bands; the latter appears due to excited state intramolecular proton transfer reaction. The positions of these bands and the ratio of their intensities for the alpha-crystallin-dye conjugate are the sensitive indicators of polarity of the dye environment and its participation in intermolecular hydrogen bonding. Although we found that the dye labels both the SH and the NH2 groups in alpha-crystallin, a recently developed procedure allowed us to distinguish between the heat-induced spectral changes of the dye molecules attached to SH and NH2 groups. We observed that at elevated temperature the environment of the SH-attached dye becomes more polar and flexible. The number of H-bond acceptor groups in the vicinity of the dye decreases. Since alpha-crystallin contains a single Cys residue within the C-terminal domain of its (alpha)A subunit (the (alpha)B subunit contains none), we can attribute the observed effects to temperature-induced changes in the C-terminal domain of this protein.

Mechanism of Chaperone Function in Small Heat Shock Proteins

Mammalian small heat shock proteins (sHSP) form polydisperse and dynamic oligomers that undergo equilibrium subunit exchange. Current models of their chaperone activity hypothesize that recognition and binding of protein non-native states involve changes in the oligomeric state. The equivalent thermodynamic representation is a set of three coupled equilibria that includes the sHSP oligomeric equilibrium, the substrate folding equilibrium, and the equilibrium binding between the sHSP and the substrate non-native states. To test this hypothesis and define the binding-competent oligomeric state of human Hsp27, we have perturbed the two former equilibria and quantitatively determined the consequences on binding. The substrate is a set of T4 lysozyme (T4L) mutants that bind under conditions that favor the folded state over the unfolded state by 10(2)-10(4)-fold. The concentration-dependent oligomer equilibrium of Hsp27 was perturbed by mutations that alter the relative stability of two major oligomeric states including phosphorylation-mimicking mutations that result in the dissociation to a small multimer over a wide range of concentrations. Correlation of binding isotherms with size exclusion chromatography analysis of the Hsp27 oligomer equilibrium demonstrates that the multimer is the binding-competent state. Binding occurs through two modes, each characterized by different affinity and number of binding sites, and results in T4L.Hsp27 complexes of different hydrodynamic properties. Mutants of the Hsp27 phosphorylation mimic that reverse the reduction in oligomer size also reduce the extent of T4L binding. Taken together, these results suggest a central role for the oligomeric equilibrium in regulating the chaperone activity of sHSP. The mutants identify sequence features important for modulating this equilibrium.

The excised heat-shock domain of αB crystallin is a folded, proteolytically susceptible trimer with significant surface hydrophobicity and a tendency to self-aggregate upon heating

The lens protein, alpha-crystallin, is a molecular chaperone that prevents the thermal aggregation of other proteins. The C-terminal domain of this protein (homologous to domains present in small heat-shock proteins) is implicated in chaperone function, although the domain itself has been reported to show no chaperone activity. Here, we show that the domain can be excised out of the intact alphaB polypeptide and recovered directly in pure form through the transfer of CNBr digests of whole lens homogenates into urea-containing buffer, followed by dialysis-based refolding of digests under acidic conditions and a single gel-filtration purification step. The folded (beta sheet) domain thus obtained is found to be (a) predominantly trimeric, and to display (b) significant surface hydrophobicity, (c) a marked tendency to undergo degradation, and (d) a tendency to aggregate upon heating, and on exposure to UV light. Thus, the twin 'chaperone' features of multimericity and surface hydrophobicity are clearly seen to be insufficient for this domain to function as a chaperone. Since alpha-crystallin interacts with its substrates through hydrophobic interactions, the hydrophobicity of the excised domain indicates that separation of domains may regulate function; at the same time, the fact is also highlighted that surface hydrophobicity is a liability in a chaperone since heating strengthens hydrophobic interactions and can potentially promote self-aggregation. Thus, it would appear that the role of the N-terminal domain in alpha-crystallin is to facilitate the creation of a porous, hollow structural framework of >/=24 subunits in which solubility is effected through increase in the ratio of exposed surface area to buried volume. Trimers of interacting C-terminal domains anchored to this superstructure, and positioned within its interior, might allow hydrophobic surfaces to remain accessible to substrates without compromising solubility.

Structural Changes in α-Crystallin and Whole Eye Lens During Heating, Observed by Low-angle X-ray Diffraction

Whole eye lens and alpha-crystallin gels and solutions were investigated using X-ray scattering techniques at temperatures ranging from 20 degrees C to 70 degrees C. In whole lens isolated in phosphate-buffered saline, the spacing of the dominant X-ray reflection seen with low-angle scattering was constant from 20 degrees C to 45 degrees C but increased at 50 degrees C from 15.2 nm to 16.5 nm. At room temperature, the small-angle X-ray diffraction pattern of the intact lens was very similar to the pattern of alpha-crystallin gels at near-physiological concentration (approximately 300 mg/ml), so it is reasonable to assume that the alpha-crystallin pattern dominates the pattern of the intact lens. Our results therefore indicate that in whole lens alpha-crystallin is capable of maintaining its structural properties over a wide range of temperature. This property would be useful in providing protection for other lens proteins super-aggregating. In the alpha-crystallin gels, a moderate increase in both the spacing and intensity of the reflection was observed from 20 degrees C to 45 degrees C, followed by an accelerated increase from 45 degrees C to 70 degrees C. Upon cooling, this effect was found to be irreversible over 11 hours. Qualitatively similar results were observed for alpha-crystallin solutions at a variety of lower concentrations.

Thermal stability of human alpha-crystallins sensed by amide hydrogen exchange

The alpha-crystallins, alphaA and alphaB, are major lens structural proteins with chaperone-like activity and sequence homology to small heat-shock proteins. As yet, their crystal structures have not been determined because of the large size and heterogeneity of the assemblies they form in solution. Because alpha-crystallin chaperone activity increases with temperature, understanding structural changes of alpha-crystallin as it is heated may help elucidate the mechanism of chaperone activity. Although a variety of techniques have been used to probe changes in heat-stressed alpha-crystallin, the results have not yet yielded a clear understanding of chaperone activity. We report examination of native assemblies of human lens alpha-crystallin using hydrogen/deuterium exchange in conjunction with enzymatic digestion and analysis by mass spectrometry. This technique has the advantage of sensing structural changes along much of the protein backbone and being able to detect changes specific to alphaA and alphaB in the native assembly. The reactivity of the amide linkages to hydrogen/deuterium exchange was determined for 92% of the sequence of alphaA and 99% of alphaB. The behavior of alphaA and alphaB is remarkably similar. At low temperatures, there are regions at the beginning of the alpha-crystallin domains in both alphaA and alphaB that have high protection to isotope exchange, whereas the C termini offer little protection. The N terminus of alphaA also has low protection. With increasing temperatures, both proteins show gradual unfolding. The maximum percent change in exposure with increasing temperatures was found in alphaA 72-75 and alphaB 76-79, two regions considered critical for chaperone activity.

Mechanism of chaperone function in small heat-shock proteins. Fluorescence studies of the conformations of T4 lysozyme bound to alphaB-crystallin

To further develop the mechanistic understanding of small heat-shock protein (sHSP) chaperone activity, we investigate the nature of the intermediate states recognized by alpha-crystallin and the conformations that are stably bound. The model substrates consist of a set of well characterized, destabilized T4 Lysozyme (T4L) mutants that have been shown to differentially bind alpha-crystallin in a manner that reflects their free-energy of unfolding. A new approach for the detection of complex formation is introduced based on the conformational sensitivity of the fluorescent probe bimane, site-specifically introduced in T4L. Emission spectra of bimane-labeled T4L reveal two distinct patterns of intensity changes upon binding that depend on the molar ratio of alpha-crystallin to T4L. This directly demonstrates the two-mode nature of the binding process by the alpha-crystallins. Biphasic binding isotherms, obtained and analyzed over a wide range of T4L concentrations, demonstrate a substantially quenched bimane fluorescence in the low affinity-bound T4L that is similar to the quenching level observed due to denaturant unfolding. Furthermore, the pattern of intensity changes that occur upon binding of a T4L variant, bimane-labeled at an alternative solvent-exposed site, establishes a direct correlation between the quenching level observed in binding and unfolding. The results can be interpreted in terms of a model where alpha-crystallin binds at least two conformationally distinct non-native states of T4L, one of which is substantially unfolded and is bound with low affinity. A high affinity binding mode to compact states may be relevant to chaperone function in the lens, where protein damage is unlikely to cause global unfolding.

Mechanism of chaperone function in small heat-shock proteins. Phosphorylation-induced activation of two-mode binding in alphaB-crystallin

The consequences of alphaB-crystallin phosphorylation on its chaperone activity were investigated using a detailed analysis of the recognition and binding of destabilized T4 lysozyme (T4L) mutants by alphaB-crystallin phosphorylation mimics containing combinations of serine to aspartate substitutions. The T4L site-directed mutants were selected to constitute an energetic ladder of progressively destabilized proteins having similar structures in the folded state. alphaB-crystallin and its variants differentially recognize the T4L mutants, binding the more destabilized ones to a larger extent. Furthermore, the aspartate substitutions result in an increase in the extent of binding to the same T4L mutant and in the appearance of biphasic binding isotherms. The latter indicates the presence of two modes of binding characterized by different affinities and different numbers of binding sites. The transition to two-mode binding can also be induced by temperature or pH activation of the second mode. The similarity between the phosphorylation, pH, and temperature effects suggests a common structural origin. The location of the phosphorylation sites in the N-terminal domain and the hypothesized burial of this domain in the core of the oligomeric structure are consistent with a critical role for the destabilization of the quaternary structure in the process of recognition and binding by small heat-shock proteins.

Calcium-induced decrease of the thermal stability and chaperone activity of alpha-crystallin

Alpha-crystallin, one of the major proteins in the vertebrate eye lens, acts as a molecular chaperone, like the small heat-shock proteins, by protecting other proteins from denaturing under stress or high temperature conditions. alpha-Crystallin aggregation is involved in lens opacification, and high [Ca(2+)] has been associated with cataract formation, suggesting a role for this cation in the pathological process. We have investigated the effect of Ca(2+) on the thermal stability of alpha-crystallin by UV and Fourier-transform infrared (FTIR) spectroscopies. In both cases, a Ca(2+)-induced decrease in the midpoint of the thermal transition is detected. The presence of high [Ca(2+)] results also in a marked decrease of its chaperone activity in an insulin-aggregation assay. Furthermore, high Ca(2+) concentration decreases Cys reactivity towards a sulfhydryl reagent. The results obtained from the spectroscopic analysis, and confirmed by circular dichroism (CD) measurements, indicate that Ca(2+) decreases both secondary and tertiary-quaternary structure stability of alpha-crystallin. This process is accompanied by partial unfolding of the protein and a clear decrease in its chaperone activity. It is concluded that Ca(2+) alters the structural stability of alpha-crystallin, resulting in impaired chaperone function and a lower protective ability towards other lens proteins. Thus, alpha-crystallin aggregation facilitated by Ca(2+) would play a role in the progressive loss of transparency of the eye lens in the cataractogenic process.

Mechanism of chaperone function in small heat shock proteins. Two-mode binding of the excited states of T4 lysozyme mutants by alphaA-crystallin

To elucidate the mechanism of alphaA-crystallin chaperone function, a detailed thermodynamic analysis of its binding to destabilized, site-directed mutants of T4 lysozyme was carried out. The selected mutants form a ladder of stabilities spanning the 5-10 kcal/mol range of free energy of unfolding. The crystal structures of the majority of the mutants have been previously determined and found to be similar to that of the wild type with no evidence of static local unfolding. Complex formation between alphaA-crystallin and T4 lysozyme was observed directly via the changes in the electron paramagnetic resonance lineshape of a nitroxide introduced at a non-destabilizing, solvent exposed site in T4 lysozyme. AlphaA-crystallin differentially interacts with the mutants, binding the more destabilized ones to a larger extent despite the similar structure of their native states. Our results suggest that the states recognized by alphaA-crystallin are non-native excited states distinct from the unfolded state. Stable complexes are formed when the free energy of binding to alphaA-crystallin is on the order of the free energy associated with the transition from the excited state to the native state. Biphasic binding isotherms reveal two modes of interactions with distinct affinities and stoichiometries. Highly destabilized mutants preferentially bind to the high capacity mode, suggesting conformational preference in the use of each mode. Furthermore, binding can be enhanced by increased temperature and pH, which may be reflecting conformational changes in alphaA-crystallin oligomeric structure.

Decreased subunit exchange of heat-treated lens alpha A-crystallin

alpha A-Crystallin high-molecular-weight (HMW) aggregates were prepared by preheating at 80-90 degrees C and studied using spectroscopic measurements. Conformational differences were suggested based on data of increased bis-ANS (4,4(')-dianilino-1,1(')-binaphthalene-5,5(')-disulfonic acid) and ThT (thioflavin T) fluorescence as well as increased far-UV and decreased near-UV circular dichroism (CD). These results indicated that HMW aggregated alpha-crystallin was more hydrophobic than the native alpha-crystallin, possibly resulting from partial unfolding of alpha-crystallin. The two cysteines in alpha A-crystallin were mostly oxidized in HMW aggregates. The effects of HMW aggregation on the dynamic structure were studied with fluorescence resonance energy transfer; subunit exchange became slower. These results strongly suggest that HMW alpha A-crystallin aggregates result from exposure of buried beta-pleated sheets and increased hydrophobic interaction.

Effect of trifluoroethanol on the structural and functional properties of alpha-crystallin

Alpha crystallin is an eye lens protein with a molecular weight of approximately 800 kDa. It belongs to the class of small heat shock proteins. Besides its structural role, it is known to prevent the aggregation of beta- and gamma-crystallins and several other proteins under denaturing conditions and is thus believed to play an important role in maintaining lens transparency. In this communication, we have investigated the effect of 2,2,2-trifluoroethanol (TFE) on the structural and functional features of the native alpha-crystallin and its two constituent subunits. A conformational change occurs from the characteristic beta-sheet to the alpha-helix structure in both native alpha-crystallin and its subunits with the increase in TFE levels. Among the two subunits, alphaA-crystallin is relatively stable and upon preincubation prevents the characteristic aggregation of alphaB-crystallin at 20% and 30% (v/v) TFE. The hydrophobicity and chaperone-like activity of the crystallin subunits decrease on TFE treatment. The ability of alphaA-crystallin to bind and prevent the aggregation of alphaB-crystallin, despite a conformational change, could be important in protecting the lens from external stress. The loss in chaperone activity of alphaA-crystallin exposed to TFE and the inability of peptide chaperone--the functional site of alphaA-crystallin--to stabilize alphaB-crystallin at 20-30% TFE suggest that the site(s) involved in subunit interaction and chaperone-like function are quite distinct.

Oxidation of human lens recombinant alphaA-crystallin and cysteine-deficient mutants

Disulfide cross-linking, one of the results of oxidative stress, has been thought to play an important role in cataractogenesis. High molecular mass (HMM) protein aggregation also contributes to cataract development, and a prevailing speculation is that disulfide cross-linking induces HMM aggregation. However, there is no direct evidence to support this speculation. Dimerization is an effect of disulfide cross-linking but cannot explain the size of HMM aggregates observed in the lens. alphaA-crystallin has two cysteine residues (Cys131 and Cys142) and we have prepared three Cys-deficient mutants, two single mutants (C131I and C142I) and one double mutant (C131I/C142I). They were subjected to H202 oxidation in an ascorbate-FeCl(3)-EDTA-H202 system. The effects of oxidation on the mutants, including changes in aggregate size and conformation, were compared with those of the wild-type alphaA-crystallin by FPLC gel filtration, absorption, fluorescence, and circular dichroism measurements. The results indicated that other amino acid residues besides Cys, such as Trp and Tyr, were also oxidized by H202. Disulfide dimerization alone seems to play a less important role in HMM aggregation than does the secondary conformational change resulting from the combined effect of the oxidation of Trp and Tyr as well as Cys.

Human lens high-molecular-weight alpha-crystallin aggregates

Alpha-crystallin high-molecular-weight (HMW) aggregates can be formed in vitro by many mechanisms, but the mechanism of in vivo aggregation has not been clearly established. HMW and LMW (low-molecular-weight) alpha-crystallins were isolated from human lenses 50-60 years of age and some spectroscopic measurements were performed. Conformational differences were suggested based on data of increased bis-ANS (4,4'-dianilino-1,1'-binaphthalene-5, 5'-disulfonic acid) and ThT (thioflavin T) fluorescence as well as increased far-UV and decreased near-UV circular dichroism (CD). These results indicated that HMW alpha-crystallin was more hydrophobic than LMW alpha-crystallin, possibly resulting from partial unfolding of alpha-crystallin. On the other hand, the increased ThT fluorescence and far-UV CD intensities indicate that an increased amount of beta-sheet conformation was involved in aggregation. These data, along with little difference in chaperone-like activity between the LMW and HMW alpha-crystallins, strongly suggest that HMW alpha-crystallin aggregates resulted from partial unfolding and disassembling-reassembling of LMW alpha-crystallin caused by posttranslational modification rather than chaperone complex formation.

Functional and structural studies of alpha-crystallin from galactosemic rat lenses

Chaperone-like activity and structural changes of lens alpha-crystallin from rats fed with galactose at various time intervals have been studied using high-performance liquid chromatograph (HPLC), circular dichroism (CD), and 1-anilinonaphthalene-8-sulfonic acid (ANS) fluorescence emission. It was found that chaperone-like activity of alpha-crystallin from galactose-fed rats toward dithiothreitol (DTT)-induced insulin B aggregation started to decrease after 3 weeks and decreased significantly after 5 weeks. Consistent results were observed in lens morphology, and lens opacity slightly developed after 3 weeks and became obvious after 5 weeks. HPLC analysis for chaperone function showed that the formation of high molecular weight aggregates (HMWA) of alpha-/gamma-crystallins decreases with the increase of galactose-feeding time, revealing that chaperone-like activity is concomitant with the formation of HMWA. Circular dichroism results showed the reduction of beta-sheet structure and loss of microenvironment of aromatic-type amino acids for opaque lenses, indicating alpha-crystallin's secondary and tertiary structure changed with the development of the lens opacity. ANS binding site estimated by Klotz equation showed it is 1.5 times higher at room temperature and is 2.4 times higher at 58 degrees C for age-matched normal alpha-crystallin than for 5-week galactose-fed lens alpha-crystallin, indicating opaque lens alpha-crystallin loses the ability to assemble into an appropriately placed hydrophobic regions. The overall results accordingly indicated that galactose-induced cataractous alpha-crystallin has disordered structure, leading to the loss of its chaperone-like activity.

Characterization of alpha-crystallin-plasma membrane binding

Alpha-crystallin, a large lenticular protein complex made up of two related subunits (alphaA- and alphaB-crystallin), is known to associate increasingly with fiber cell plasma membranes with age and/or the onset of cataract. To understand better the binding mechanism, we developed a sensitive membrane binding assay using lens plasma membranes and recombinant human alphaA- and alphaB-crystallins conjugated to a small fluorescent tag (Alexa350). Both alphaA and alphaB homopolymer complexes, as well as a reconstituted 3:1 heteromeric complex, bind to lens membranes in a specific, saturable, and partially irreversible manner that is sensitive to both time and temperature. The amount of alpha-crystallin that binds to the membrane increases under acidic pH conditions and upon removal of exposed intrinsic membrane protein domains but is not affected at high ionic strength, suggesting that alpha-crystallin binds to the fiber cell plasma membranes mainly through hydrophobic interactions. The binding capacity and affinity for the reconstituted 3:1 heteromeric complex were measured to be 3. 45 +/- 0.11 ng/microg of membrane and 4.57 +/- 0.50 x 10(-4) microg(-1) of membrane, respectively. The present membrane binding data support the hypothesis that the physical properties of a mixed alpha-crystallin complex may hold particular relevance for the function of alpha-crystallin within the lens.

Correlation between the chaperone-like activity and aggregate size of alpha-crystallin with increasing temperature

alpha-Crystallin, the major protein of the mammalian eye lens, is also found in the major tissues of the body, where one or the other of its two isoforms is characteristically expressed. Both isoform sequences are highly related to others of the small heat shock protein superfamily, leading to speculation about their functions in vivo outside of the lens. Tests of chaperone-like activity at 37 and 66 degrees C indicate that the protein can act to prevent the superaggregation of partially denatured proteins, but both alpha-crystallin aggregate size and shape are significantly altered with increasing temperature. Characterization of these changes indicates that secondary, tertiary, and quaternary structure are modified, with the latter effect especially striking above 50 degrees C. Furthermore, these changes appear to be irreversible when the temperature is returned to 25 or 37 degrees C. Functionally, the protein is effective in chaperone-like activity at all temperatures, but exhibits a somewhat increased capability after a cycle of heating and cooling. The results presented here indicate the heat-induced formation of high-molecular-weight aggregates of alpha-crystallin is a slow progressive process. The increased activity of these aggregates suggests that chaperone-like activity depends in part on the packing parameters of the aggregate and on conformation of the subunit within that aggregate.

Thermodynamic stability of human lens recombinant alphaA- and alphaB-crystallins

Lens alpha-crystallin is a 600-800-kDa heterogeneous oligomer protein consisting of two subunits, alphaA and alphaB. The homogeneous oligomers (alphaA- and alphaB-crystallins) have been prepared by recombinant DNA technology and shown to differ in the following biophysical/biochemical properties: hydrophobicity, chaperone-like activity, subunit exchange rate, and thermal stability. In this study, we studied their thermodynamic stability by unfolding in guanidine hydrochloride. The unfolding was probed by three spectroscopic parameters: absorbance at 235 nm, Trp fluorescence intensity at 320 nm, and far-UV circular dichroism at 223 nm. Global analysis indicated that a three-state model better describes the unfolding behavior than a two-state model, an indication that there are stable intermediates for both alphaA- and alphaB-crystallins. In terms of standard free energy (DeltaG(NU)(H(2)(O))), alphaA-crystallin is slightly more stable than alphaB-crystallin. The significance of the intermediates may be related to the functioning of alpha-crystallins as chaperone-like molecules.

Eye lens alphaA- and alphaB-crystallin: complex stability versus chaperone-like activity

The major lens protein alpha-crystallin is composed of two related types of subunits, alphaA- and alphaB-crystallin, of which the former is essentially lens-restricted, while the latter also occurs in various other tissues. With regard to their respective chaperone capacities, it has been reported that homomultimeric alphaA-crystallin complexes perform better in preventing thermal aggregation of proteins, while alphaB-crystallin complexes protect more efficiently against reduction-induced aggregation of proteins. Here, we demonstrate that this seeming discrepancy is solved when the reduction assay is performed at increasing temperatures: above 50 degrees C alphaA- performs better than alphaB-crystallin also in this assay. This inversion in protective capacity might relate to the greater resistance of alphaA-crystallin to heat denaturation. Infrared spectroscopy, however, revealed that this is not due to a higher thermostability of alphaA-crystallin's secondary structure. Also the accessible hydrophobic surfaces do not account for the chaperoning differences of alphaA- and alphaB-crystallin, since regardless of the experimental temperature alphaB-crystallin displays a higher hydrophobicity. It is argued that the greater complex stability of alphaA-crystallin, as evident upon urea denaturation, and the higher chaperone capacity of alphaB-crystallin at physiological temperatures reflect the evolutionary compromise to obtain an optimal functioning of heteromeric alpha-crystallin as a lens protein.

The formation of oxidatively induced high-molecular-weight aggregate of alpha-/gamma-crystallins

alpha-/gamma-Crystallin interactions under oxidation with ascorbate-FeCl3-EDTA-H2O2 followed by dialysis have been studied. A high-molecular-weight aggregate (HMWA) composed of alpha- and gamma-crystallin was observed for the mixture of the dialyzed alpha-crystallin and the oxidized gamma-crystallin through gel-filtration chromatography. This illustrates an interaction between alpha-crystallin and partially denatured gamma-crystallin induced by oxidation. No HMWA formation was observed under the condition without dialysis and/or with the addition of catalase to the oxidized gamma-crystallin prior to the addition of alpha-crystallin. More HMWA was formed by oxidized gamma-crystallin followed by the addition of alpha-crystallin than by simultaneous oxidation of both alpha- and gamma-crystallins. Conformational changes of alpha-crystallin during oxidation analyzed by circular dichroism spectra showed that oxidized alpha-crystallin can gradually be restored to an ordered structure through dialysis. The overall results imply that structural changes of both alpha- and gamma-crystallins and dialysis are required to form HMWA. The observation of this oxidatively induced chaperone/substrate complex suggests that an efficient chaperone-like protective action against oxidative insults may exist in vivo.

Temperature induced structural changes of beta-crystallin and sphingomyelin binding

The study of the binding of alpha-crystallin to membranes is potentially important for understanding the function of alpha-crystallin in the ocular lens and the formation of cataracts. Using fluorescence probes, N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-1,2-dihexadecanoyl-sn-glycero-3 -phosphoethanolamine, triethylammonium salt (NBD-PE) and (1,1'-bi(4-anilino)naphthalene-5,5'-disulfonic acid, dipotassium salt (bis-ANS), the temperature dependence of the binding of alpha-crystallin to sphingomyelin liposomes, and the structural changes of alpha-crystallin and sphingomyelin induced by temperature were studied. The influence of the binding of alpha-crystallin on the mobility of the head group region of liposomes of sphingomyelin was dependent on the thermal history of alpha-crystallin. Binding of alpha-crystallin to sphingomyelin caused a decrease in the anisotropy of the fluorophore NBD-PE at or below 37 degrees C. However, when alpha-crystallin or the mixture of alpha-crystallin/sphingomyelin were preincubated near the secondary structure phase transition temperature of 60 degrees C, an increase of the anisotropy of NBD-PE (decrease of lipid head group mobility) was observed when measured at 22 degrees C or 37 degrees C. An inflection near 47 degrees C in the curve of fluorescence anisotropy of bis-ANS pre-incorporated into the alpha-crystallin corresponded to a 3 degrees or 4 degrees structural change of alpha-crystallin. alpha-Crystallin either increases or decreases the flexibility of the head group of sphingomyelin liposomes depending on its structure.

Interaction of 1,1'-bi(4-anilino)naphthalene-5,5'-disulfonic acid with alpha-crystallin

The hydrophobic sites in alpha-crystallin were evaluated using a fluorescent probe 1,1'-bi(4-anilino)naphthalenesulfonic acid (bis-ANS). Approximately one binding site/subunit of alpha-crystallin at 25 degrees C was estimated by equilibrium binding and Scatchard analysis (Kd = 1.1 microM). Based on fluorescence titration, the dissociation constant was 0.95 microM. The number of bis-ANS binding sites nearly doubled upon heat treatment of the protein at 60 degrees C. Likewise, the exposure of alpha-crystallin to 2-3 M urea resulted in increased binding of bis-ANS. Above 3 M urea there was a rapid loss in the fluorescence indicating the loss of interaction between bis-ANS and protein. The alpha-crystallin refolded from 6 M urea showed tryptophan fluorescence emission similar to the native alpha-crystallin. However, the refolded alpha-crystallin showed a 60% increase in bis-ANS binding, suggesting distinct changes on the protein surface resulting from exposure to urea similar to the changes occurring due to heat treatment. The fluorescence of tryptophan in native alpha-crystallin was quenched by the addition of bis-ANS. The quenching was inversely related to the amount of bis-ANS bound to alpha-crystallin. Additionally, the binding of bis-ANS reduced the chaperone-like activity of the protein. Photolysis of bis-ANS-alpha-crystallin complex resulted in incorporation of the probe to both A- and B-subunits, indicating that both subunits in native alpha-crystallin contribute to the surface hydrophobicity of the protein.

Physiological role of the association complexes of alpha-crystallin and its substrates on the chaperone activity

Previous reports on the chaperone activity of alpha-crystallin to prevent protein denaturation and thermal aggregation have suggested that partially denatured proteins can bind alpha-crystallin in its central region. Likewise, beta- and gamma-crystallin can also be localized to the central cavity of alpha-crystallin particle in vivo, which provides indirect evidence that alpha-crystallin can function as a chaperone in the intact lens. In this study, we have further demonstrated that the binding of the substrate proteins to alpha-crystallin by short-term preincubation may mimic the in vivo conditions of crystallin association. Preheating of alpha-crystallin with its substrate proteins at 60 degrees C for 20 min resulted in the formation of stable complexes between alpha-crystallin and its substrates (8.0% of insulin or 5.3% of gamma-crystallin was involved in complex formation). Under such conditions, the chaperone activity of alpha-crystallin to inhibit dithiothreitol-, ultraviolet-, or oxidation-induced protein aggregation can be greatly enhanced. Since UV-irradiation and oxidative stress are common insults to eye lenses under normal physiological conditions, the presence of alpha/gamma and alpha/beta complex in vivo may play an important role to maintain the lens in a transparent state.

Intermolecular exchange and stabilization of recombinant human alphaA- and alphaB-crystallin

Lens alpha-crystallin subunits alphaA and alphaB are differentially expressed and have a 3-to-1 ratio in most mammalian lenses by intermolecular exchange. The biological significance of this composition and the mechanism of exchange are not clear. Preparations of human recombinant alphaA- and alphaB-crystallins provide a good system in which to study this phenomenon. Both recombinant alphaA- and alphaB-crystallins are folded and aggregated to the size of the native alpha-crystallin. During incubation together, they undergo an intermolecular exchange as shown by native isoelectric focusing. Circular dichroism measurements indicate that the protein with a 3-to-1 ratio of alphaA- and alphaB-crystallins has the same secondary structure but somewhat different tertiary structures after exchange: the near-UV CD increases after exchange. The resulting hybrid aggregate is more stable than the individual homogeneous aggregates: at 62 degrees C, alphaB-crystallin is more susceptible to aggregation and displays a greater light scattering than alphaA-crystallin. This heat-induced aggregation of alphaB-crystallin, however, was suppressed by intermolecular exchange with alphaA-crystallin. These phenomena are also observed by fast performance liquid chromatography gel filtration patterns. The protein structure of alphaB-crystallin is stabilized by intermolecular exchange with alphaA-crystallin.