An investigation into the stability of alpha-crystallin by NMR spectroscopy; evidence for a two-domain structure

Quantitative NMR study of heat-induced aggregation of eye-lens crystallin proteins under crowding conditions

The eye lens contains a highly concentrated, polydisperse mixture of crystallins, and a loss in transparency during cataract formation is attributed to the aggregation of these proteins. Most biochemical and biophysical studies of crystallins have been performed in diluted samples because of various physical limitations of the respective method at physiological concentrations of up to 200-400 mg/mL. We introduce a straightforward proton NMR transverse relaxometry method to quantify simultaneously proteins in the dissolved and aggregated states at these elevated concentrations, because these states significantly differ in their transverse relaxation properties. The key feature of this method is a direct observation of the protein signal in a wide range of relaxation delays, from few microseconds up to few hundred milliseconds. We applied this method to follow heat-induced aggregation of bovine α- and γB-crystallin between 60 and 200 mg/mL. We find that at 60 °C, a temperature where both crystallins still comprise a native tertiary structure, γB-crystallin aggregated at these high protein concentrations with a time constant of about 30-40 h. α-crystallin remained soluble at 60 mg/mL but formed a transparent gel at 200 mg/mL. This quantitative NMR method can be applied to investigations of other proteins and their mixtures under various aggregation conditions.

The functional roles of the unstructured N- and C-terminal regions in αB-crystallin and other mammalian small heat-shock proteins

Small heat-shock proteins (sHsps), such as αB-crystallin, are one of the major classes of molecular chaperone proteins. In vivo, under conditions of cellular stress, sHsps are the principal defence proteins that prevent large-scale protein aggregation. Progress in determining the structure of sHsps has been significant recently, particularly in relation to the conserved, central and β-sheet structured α-crystallin domain (ACD). However, an understanding of the structure and functional roles of the N- and C-terminal flanking regions has proved elusive mainly because of their unstructured and dynamic nature. In this paper, we propose functional roles for both flanking regions, based around three properties: (i) they act in a localised crowding manner to regulate interactions with target proteins during chaperone action, (ii) they protect the ACD from deleterious amyloid fibril formation and (iii) the flexibility of these regions, particularly at the extreme C-terminus in mammalian sHsps, provides solubility for sHsps under chaperone and non-chaperone conditions. In the eye lens, these properties are highly relevant as the crystallin proteins, in particular the two sHsps αA- and αB-crystallin, are present at very high concentrations.

Multidimensional significance of crystallin protein-protein interactions and their implications in various human diseases

BACKGROUND

Crystallins are the important structural and functional proteins in the eye lens responsible for refractive index. Post-translational modifications (PTMs) and mutations are major causative factors that affect crystallin structural conformation and functional characteristics thus playing a vital role in the etiology of cataractogenesis.

SCOPE OF REVIEW

The significance of crystallin protein-protein interactions (PPIs) in the lens and non-lenticular tissues is summarized.

MAJOR CONCLUSIONS

Aberrancy of PPIs between crystallin, its associated protein and metal ions has been accomplished in various human diseases including cataract. A detailed account on multidimensional structural and functional significance of crystallin PPI in humans must be brought into limelight, in order to understand the biochemical and molecular basis augmenting the aberrancies of such interaction. In this scenario, the present review is focused to shed light on studies which will aid to expand our present understanding on disease pathogenesis related to loss of PPI thereby paving the way for putative future therapeutic targets to curb such diseases.

GENERAL SIGNIFICANCE

The interactions with α-crystallins always aid to protect their structural and functional characteristics. The up-regulation of αB-crystallin in the non-lenticular tissues always decodes as biomarker for various stress related disorders. For better understanding and treatment of various diseases, PPI studies provide overall outline about the structural and functional characteristics of the proteins. This information not only helps to find out the route of cataractogenesis but also aid to identify potential molecules to inhibit/prevent the further development of such complicated phenomenon. This article is part of a Special Issue entitled Crystallin Biochemistry in Health and Disease.

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.

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.

Ageing and vision: structure, stability and function of lens crystallins

The alpha-, beta- and gamma-crystallins are the major protein components of the vertebrate eye lens, alpha-crystallin as a molecular chaperone as well as a structural protein, beta- and gamma-crystallins as structural proteins. For the lens to be able to retain life-long transparency in the absence of protein turnover, the crystallins must meet not only the requirement of solubility associated with high cellular concentration but that of longevity as well. For proteins, longevity is commonly assumed to be correlated with long-term retention of native structure, which in turn can be due to inherent thermodynamic stability, efficient capture and refolding of non-native protein by chaperones, or a combination of both. Understanding how the specific interactions that confer intrinsic stability of the protein fold are combined with the stabilizing effect of protein assembly, and how the non-specific interactions and associations of the assemblies enable the generation of highly concentrated solutions, is thus of importance to understand the loss of transparency of the lens with age. Post-translational modification can have a major effect on protein stability but an emerging theme of the few studies of the effect of post-translational modification of the crystallins is one of solubility and assembly. Here we review the structure, assembly, interactions, stability and post-translational modifications of the crystallins, not only in isolation but also as part of a multi-component system. The available data are discussed in the context of the establishment, the maintenance and finally, with age, the loss of transparency of the lens. Understanding the structural basis of protein stability and interactions in the healthy eye lens is the route to solve the enormous medical and economical problem of cataract.

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.

The structural differences between bovine lens alphaA- and alphaB-crystallin

Lens alphaA- and alphaB-crystallin have been reported to act differently in their protection against nonthermal destabilization of proteins. The nature of this difference, however, is not completely understood. Therefore we used a combination of thermally and solvent-induced structural changes to investigate the difference in the secondary, tertiary and quaternary structures of alphaA- and alphaB-crystallin. We demonstrate the relationship between the changes in the tertiary and quaternary structures for both polypeptides. Far-ultraviolet circular dichroism revealed that the secondary structure of alphaB-crystallin is more stable than that of alphaA-crystallin, and the temperature-induced secondary structure changes of both polypeptides are partially reversible. Tryptophan fluorescence revealed two distinct transitions for both alphaA- and alphaB-crystallin. Compared to alphaB-crystallin, both transitions of alphaA-crystallin occurred at higher temperature. The changes in the hydrophobicity are accompanied by changes in the quaternary structure and are biphasic, as shown by bis-1-anilino-8-naphthalenesulfonate fluorescence and sedimentation velocity. These phenomena explain the difference in the chaperone capacity of alphaA- and alphaB-crystallin carried out at different temperatures. The quaternary structure of alpha-crystallin is more stable than that of alphaA- and alphaB-crystallin. The latter has a strong tendency to dissociate under thermal or solvent destabilization. This phenomenon is related to the difference in subunit organization of alphaA- and alphaB-crystallin where both hydrophobic and ionic interactions are involved. We find that an important subunit rearrangement of alphaA-crystallin takes place once the molecule is destabilized. This subunit rearrangement is a requisite phenomenon for maintaining alpha-crystallin in its globular form and as a stable complex. On the base of our results, we suggest a four-state model describing the folding and dissociation of alphaA- and alphaB-crystallin better than a three-state model [Sun et al. (1999) J. Biol. Chem. 274, 34067-34071].

A small-angle X-ray solution scattering study of bovine alpha-crystallin

The native high molecular mass form of alpha-crystallin, the most important soluble protein in the eye lens, and its low molecular mass form obtained at 37 degrees C in dilute solutions were investigated by synchrotron radiation small-angle X-ray scattering. The alpha-crystallin solutions are polydisperse and good fits to the experimental data can be obtained using distributions of spheres with radii varying between about 5 and 10 nm. In spite of the polydispersity, two different ab initio methods were used to retrieve low resolution shapes from the scattering data. These shapes correspond to the z-average structure of the oligomers. In the absence of any symmetry constraints, the scattering curves of the two forms of alpha-crystallin yield bean-like shapes. The shape corresponding to the low molecular mass form has about 20% less mass at the periphery. Imposing tetrahedral symmetry on the average structures worsens the fit to the experimental data. We emphasized the apparent contradiction between hydrodynamic and molecular properties of alpha-crystallin. An explanation was put forward based on the presence of solvent-exposed flexible C-terminal extensions. We present two bead models ('hollow globule with tentacles' and 'bean with tentacles') based on NMR and cryo-electron microscopy studies and discuss how well they correspond with our data from X-ray scattering, light scattering and analytical ultracentrifugation.

Dissociation of human alphaB-crystallin aggregates by thiocyanate is structurally and functionally reversible

Conformational modifications and changes in the aggregation state of human alphaB-crystallin were investigated at different concentrations of SDS, KBr, urea, and NH4SCN and at different temperatures. Intrinsic fluorescence measurements indicated complete and reversible unfolding of the protein at 2 M NH4SCN, whereas the concentration of urea required for complete and irreversible unfolding was 6 M. Gel permeation chromatography indicated almost complete dissociation of the micelle-like aggregate of alphaB-crystallin in 2 M NH4SCN, but only partial dissociation into large-sized aggregates in 6 M urea. Thiocyanate-treated alphaB-crystallin recovered its chaperone-like activity upon dilution of the dissociating agent, whereas the urea-treated protein did not.

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.

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.

Studies of the denaturation patterns of bovine alpha-crystallin using an ionic denaturant, guanidine hydrochloride and a non-ionic denaturant, urea

The effects of non-ionic and ionic denaturation and denaturation/renaturation on the native structure of alpha-crystallin at room temperature were examined. Native alpha-crystallin, at concentrations above and below the previously reported critical micelle concentration (CMC) range, was denatured by varying concentrations of urea and guanidine hydrochloride. The resulting denatured samples were examined by gel filtration fast performance liquid chromatography (FPLC), circular dichroism spectropolarimetry (CD), and transmission electron microscopy. Elution peak samples from gel filtration chromatography with sufficiently high concentrations were examined for subunit composition by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). The studies presented herein demonstrate that the denaturation and renaturation of alpha-crystallin via non-ionic urea denaturation results in different renaturation species, depending upon the initial concentration of alpha-crystallin which is denatured and the concentration of urea, including certain species which, by gel filtration FPLC, have an apparent molecular weight greater than the native 800 kD aggregate. Transmission electron microscopy has also demonstrated the existence of a high molecular weight aggregate form for denatured samples. Ionic dissociation, in contrast, proceeds much in the same manner above and below the CMC range, the major difference occurring at 2 M guanidine hydrochloride. alpha B-crystallin is preferentially removed from the native alpha-crystallin aggregate upon treatment with 2 M guanidine hydrochloride indicating, once again, differences between the two subunits. Above and below the CMC range, dissociation with guanidine hydrochloride appears to plateau after 4 M guanidine hydrochloride as indicated by the presence of two apparent homotetrameric species and no further dissociation of these species with increasing guanidine hydrochloride concentrations. CD demonstrates that some secondary structure, which is lost with lower concentrations of alpha-crystallin, is still present when concentrations of alpha-crystallin, well above the critical micelle concentration range, are treated with high concentrations of urea at room temperature. In contrast, concentrations both above and below the CMC range demonstrate a significant loss of secondary structure upon treatment with 2 M guanidine hydrochloride. Finally, ionic denaturation and subsequent renaturation results in the formation of a species which is functionally incapable of protecting gamma-crystallin from heat-induced aggregation.

alpha-Crystallin polymers and polymerization: the view from down under

Several models have been proposed for the quaternary structure of alpha-crystallin. Some suggest the subunits are arranged in concentric shells. Others propose that the subunits are in a micelle-like arrangement. However, none is able to satisfactorily account for all observations on the protein and the quaternary structure of alpha-crystallin remains to be established. In this review, factors contributing to the assembly and polymerization are examined in order to evaluate the different models. Consideration of the variations in particle size and molecular weight under different conditions leads to the conclusion that alpha-crystallin cannot be a micelle or a layered structure. Instead, it is suggested that the protein may be assembled from a 'monomeric' unit comprising eight subunits arranged in two tetramers with cyclic symmetry. The octameric unit is proposed to be disc-like particle with a diameter of 9.5 nm and a height of 3 nm. The larger particles, chains and sheet-like structures commonly observed are assembled from the octamers. Structural predictions indicate that the polypeptide may be folded into three independent domains which have different roles in the structural organization and functions of the protein. It is suggested that the tetramers are stabilized through interactions involving the second domain (residues 64-104) while assembly into the octamers and higher polymers requires hydrophobic interactions involving the N-terminal domain. Deletion of parts of this domain by site directed mutagenesis revealed that residues 46-63 play a critical role in the assembly. Current research aims to identify the specific amino acids involved.

NMR spectroscopy of alpha-crystallin. Insights into the structure, interactions and chaperone action of small heat-shock proteins

The subunit molecular mass of alpha-crystallin, like many small heat-shock proteins (sHsps), is around 20 kDa although the protein exists as a large aggregate of average mass around 800 kDa. Despite this large size, a well-resolved 1H NMR spectrum is observed for alpha-crystallin which arises from short, polar, highly-flexible and solvent-exposed C-terminal extensions in each of the subunits, alpha A- and alpha B-crystallin. These extensions are not involved in interactions with other proteins (e.g. beta- and gamma-crystallins) under non-chaperone conditions. As determined by NMR studies on mutants of alpha A-crystallin with alterations in its C-terminal extension, the extensions have an important role in acting as solubilising agents for the relatively-hydrophobic alpha-crystallin molecule and the high-molecular-weight (HMW) complex that forms during the chaperone action. The related sHsp, Hsp25, also exhibits a flexible C-terminal extension. Under chaperone conditions, and in the HMW complex isolated from old lenses, the C-terminal extension of the alpha A-crystallin subunit maintains its flexibility whereas the alpha B-crystallin subunit loses, at least partially, its flexibility, implying that it is involved in interaction with the 'substrate' protein. The conformation of 'substrate' proteins when they interact with alpha-crystallin has been probed by 1H NMR spectroscopy and it is concluded that alpha-crystallin interacts with 'substrate' proteins that are in a disordered molten globule state, but only when this state is on its way to large-scale aggregation and precipitation. By monitoring the 1H and 31P NMR spectra of alpha-crystallin in the presence of increasing concentrations of urea, it is proposed that alpha-crystallin adopts a two-domain structure with the larger C-terminal domain unfolding first in the presence of denaturant. All these data have been combined into a model for the quaternary structure of alpha-crystallin. The model has two layers each of approximately 40 subunits arranged in an annulus or toroid. A large central cavity is present whose entrance is ringed by the flexible C-terminal extensions. A large hydrophobic region in the aggregate is exposed to solution and is available for interaction with 'substrate' proteins during the chaperone action.

Mutations and modifications support a 'pitted-flexiball' model for alpha-crystallin

alpha-Crystallin is renown for resisting crystallization and electron microscopic image analysis. The spatial conformation thus remaining elusive, the authors explored the structure and chaperone functioning by analyzing the effects of site-directed mutagenesis, the properties of naturally occurring aberrant forms of alpha-crystallin and the influence of chemical modifications. The authors observed that the globular multimeric structure, as well as the chaperoning capacity are remarkably tolerant towards changes and modifications in the primary structure. The essential features of the quaternary structure--globular shape, flexibility, highly polar exterior and accessible hydrophobic surface pockets--support a 'pitted-flexiball' model, which combines tetrameric subunit building blocks in an open micelle-like arrangement.

Refinement of 3D structure of bovine lens alpha A-crystallin

In absence of 3D structures for alpha-crystallin subunits, alpha A and alpha B, we utilized a number of experimental and molecular modeling techniques to generate working 3D models of these polypeptides (Farnsworth et al., 1994. In Molecular Modeling: From Virtual Tools to Real Problems (Eds. Kumosinski, T.F. and Liebman, M.N.) ACS Symposium Series 576, Ch. 9:123-134, 1994, ACS Books, Washington DC). The refinement of the initial bovine alpha A model was achieved using a more accurate estimation of secondary structure by new/updated methods for analyzing the far UV-CD spectra and by neural network secondary structure predictions in combination with database searches. The spectroscopic study reveals that alpha-crystallin is not an all beta-sheet protein but contains approximately 17% alpha-helices, approximately 33% beta-structures and approximately 50% turns and coils. The refinement of the alpha A structure results in an elongate, asymmetric amphipathic molecule. The hydrophobic N-terminal domain imparts the driving force for subunit aggregation while the more flexible, polar C-terminal domain imparts aggregate solubility. In our quaternary structure of the aggregate, the monomer is the minimal cooperative subunit. In bovine alpha A, the highly negatively charged C-terminal domain has three small positive areas which may participate in dimer or tetramer formation of independently expressed C-terminal domains. The electrostatic potential of positive areas is modulated and become more negative with phosphorylation and ATP binding. The refined bovine alpha A model was used to construct alpha A models for the human, chick and dogfish shark. A high degree of conservation of the three dimensional structure and the electrostatic potential was observed. Our proposed open micellar quaternary structure correlates well with experimental data accumulated over the past several decades. The structure is also predictive of the more recent data.

X-ray structures of three interface mutants of gammaB-crystallin from bovine eye lens

GammaB-crystallin consists of two domains each comprising two "Greek key" motifs. Both domains fold independently, and domain interactions contribute significantly to the stability of the C-terminal domain. In a previous study (Palme S et al., 1996, Protein Sci 6:1529-1636) it was shown that Phe56 from the N-terminal domain, a residue involved in forming a hydrophobic core at the domain interface, effects the interaction of the two domains, and therefore, the stability of the C-terminal domain. Ala or Asp at position 56 drastically decreased the stability of the C-terminal domain, whereas Trp had a more moderate effect. In this article we present the X-ray structures of these interface mutants and correlate them with the stability data. The mutations do not effect the overall structure of the molecule. No structural changes are observed in the vicinity of the replaced residue, suggesting that the local structure is too rigid to allow compensations for the amino acid replacements. In the mutants gammaB-F56A and -F56D, a solvent-filled groove accessible to the bulk solvent is created by the replacement of the bulky Phe side chain. In gammaB-F56W, the pyrrole moiety of the indole ring replaces the phenyl side chain of the wild type. With the exception of gammaB-F56W, there is a good correlation between the hydrophobicity of the amino acid at position 56 according to the octanol scale and the stability of the C-terminal domain. In gammaB-F56W, the C-terminal domain is less stable than estimated from the hydrophobicity, presumably because the ring nitrogen (Nepsilon1) has no partner to form hydrogen bonds. The data suggest that the packing of hydrophobic residues in the interface core is important for domain interactions and the stability of gammaB-crystallin. Apparently, for protein stability, the same principles apply for hydrophobic cores within domains and at domain interfaces.

Structure-function studies on small heat shock protein oligomeric assembly and interaction with unfolded polypeptides

The small heat shock protein (smHSP) and alpha-crystallin genes encode a family of 12-43-kDa proteins which assemble into large multimeric structures, function as chaperones by preventing protein aggregation, and contain a conserved region termed the alpha-crystallin domain. Here we report on the structural and functional characterization of Caenorhabditis elegans HSP16-2, a 16-kDa smHSP produced only under stress conditions. A combination of sedimentation velocity, size exclusion chromatography, and cross-linking analyses on wild-type HSP16-2 and five derivatives demonstrate that the N-terminal domain but not most of the the C-terminal extension which follows the alpha-crystallin domain is essential for the oligomerization of the smHSP into high molecular weight complexes. The N terminus of HSP16-2 is found to be buried within complexes which can accommodate at least an additional 4-kDa of heterologous sequence per subunit. Studies on the interaction of HSP16-2 with fluorescently-labeled and radiolabeled actin and tubulin reveal that this smHSP possesses a high affinity for unfolded intermediates which form early on the aggregation pathway, but has no apparent substrate specificity. Furthermore, both wild-type and C-terminally-truncated HSP16-2 can function as molecular chaperones by suppressing the thermally-induced aggregation of citrate synthase. Taken together, our data on HSP16-2 and a unique 12.6-kDa smHSP we have recently characterized demonstrate that multimerization is a prerequisite for the interaction of smHSPs with unfolded protein as well as for chaperone activity.

Structure and function of the conserved domain in alphaA-crystallin. Site-directed spin labeling identifies a beta-strand located near a subunit interface

Twelve sequential single cysteine mutants of alphaA-crystallin extending between amino acids Y109 and L120 were prepared and reacted with a sulfhydryl specific spin label in order to investigate the role of this sequence in the assembly of the alphaA-crystallin quaternary structure and its chaperone-like function. The sequence is located in the region of highest homology in the alpha-crystallin domain, a stretch of 100 amino acids conserved among lens alpha-crystallins and small heat-shock proteins (sHSPs). Analysis of the solvent accessibility and mobility of the attached nitroxides reveals that the sequence, as a whole, is relatively sequestered from the aqueous solvent. Furthermore, as teh nitroxide is scanned across the sequence, both mobility and accessibility vary with a periodicity of 2, demonstrating that the backbone conformation is that of a beta-strand. Once face of the strand, containing the highly conserved residues R112 and R116, is buried with virtually no accessibility to the aqueous solvent. Equivalent strands from different subunits are in close spatial proximity, as inferred from spin-spin interactions between identical residues along the strand. Taken together, our results are consistent with the hypothesis that the alpha-crystallin domain is a building block of the alpha-crystallins quaternary structure and suggest that the charge conservation observed in the alpha-crystallins evolution might be important for the assembly of the oligomer. This work reports the first use of SDSL to identify a beta-strand in an unknown structure and demonstrates the feasibility of using this technique to investigate the oligomeric structure of the alpha-crystallins and sHSPs.

Effect of heat-induced structural perturbation of secondary and tertiary structures on the chaperone activity of alpha-crystallin

alpha-Crystallin, a major protein of the lens, is known to have chaperone activity to protect other proteins against thermal aggregation. Heat-induced structural change of alpha-crystallin was previously shown to increase its chaperone activity. In this report, we studied the thermal reversibility of alpha-crystallin and the effect of change in secondary structure on its chaperone function in vitro. The heat-induced conformational changes in the aromatic region of near-UV CD spectra showed only a small degree of reversibility. The structural transitions from 50 to 70 degrees C were largely reversible if the incubation time was short. However, the protective ability to inhibit thermal aggregation of alcohol dehydrogenase by alpha-crystallin was essentially similar at 48 and 70 degrees C. Under long-term heating at high temperatures, there was a time-dependent irreversibility of structural change in alpha-crystallin as revealed by CD spectroscopy. Such denatured alpha-crystallin by long-term heating can still preserve its ability to prevent UV-induced aggregation of gamma-crystallin at room temperature, indicating relatively little effect of heat-induced changes in secondary structure on the chaperone activity of alpha-crystallin.

Preliminary studies on the aggregation process of alpha-crystallin

The mechanism by which alpha-crystallin subunits form the native 800 kD aggregate is currently unknown. Experiments were performed to investigate the mechanism for this process. Gel-filtration Fast Performance Liquid Chromatography (FPLC) and Sodium Dodecyl Sulfate Polyacrylamide Gel Electrophoresis (SDS-PAGE), with and without cross-linking with glutaraldehyde, indicate that alpha-crystallin undergoes a concentration-dependent aggregation process. The denaturation of alpha-crystallin, and its subsequent renaturation and reaggregation, lead to the formation of several different species. At very low concentrations (< 0.5 microM), only monomeric and/or dimeric species exist. With a ten-fold increase in alpha-crystallin concentration from 0.05 microM to 0.5 microM, the amount of the monomeric/dimeric species increases to a plateau coincident with the appearance of a tetrameric species at 0.5 microM. With an additional ten-fold increase in concentration from 0.5 microM to 5 microM, the amount of the tetrameric species increases and levels off to its own plateau coincident with the appearance of the native 800 kD alpha-crystallin aggregate at 5 microM. The amount of the native species is extremely small at this concentration, but increases sharply and linearly with increasing concentration, while the concentrations of monomeric/dimeric and tetrameric species remain constant. The concentration at which the relative amount of the native species begins to increase sharply is within the range of the critical micelle concentration previously characterized.

Detection and characterization of alpha-crystallin intermediate with maximal chaperone-like activity

Lens alpha-crystallin has been reported to act like a chaperone molecule, with the chaperone-like activity enhanced by partial unfolding. The nature of the partial unfolding, however, is not fully understood. In this project, the unfolding and refolding process of alpha-crystallin was studied with guanidine hydrochloride (GdnHCl). Trp fluorescence (tertiary structure) and far-ultraviolet circular dichroism (UVCD) (secondary structure) demonstrated the presence of an intermediate in the unfolding pathway. ANS (1-anilino-8-naphthalenesulfonate) fluorescence clearly indicated a two-step transition in the unfolding-refolding process and showed that maximum hydrophobicity of the alpha-crystallin occurred at 0.8-1.0 M GdnHCl. This alpha-crystallin intermediate appears to be in a molten globule state; conformational study by near- and far-UVCD measurements indicated that alpha-crystallin intermediate exhibited tertiary structure which was significantly altered from that of the native protein, but had nearly the same secondary structure. Quaternary structure (size of aggregate) of the intermediate also remained unchanged from that of the native protein, as shown by FPLC size exclusion chromatography. The maximal hydrophobicity of the alpha-crystallin intermediate in the unfolding-refolding pathway was accompanied by maximal protection of betaH-crystallin from aggregation. However, an adverse effect of partial unfolding is that the alpha-crystallin intermediate aggregates at high concentrations. Together, these results clearly demonstrated the biological significance of the alpha-crystallin intermediate: it is a more effective chaperone than native alpha-crystallin.

Effects of temperature and concentration on bovine lens alpha-crystallin secondary structure: a circular dichroism spectroscopic study

Elucidation of the structure of alpha-crystallin, the major protein in all vertebrate lenses, is important for understanding its role in maintaining transparency and its function in other tissues under both normal and pathological conditions. Progress toward a unified consensus concerning the tertiary and quaternary structures of alpha-crystallin depends, in part, on an accurate estimation of its secondary structure. For the first time, three algorithms, SELCON, K2D and CONTIN were used to analyze far ultra-violet circular dichroism (UV-CD) spectra of bovine lens alpha-crystallin to estimate the secondary structure and to determine the effects of temperature and concentration. Under all experimental conditions tested, the analyses show that alpha-crystallin contains 14% alpha-helix, 35% beta-sheet and the remainder, random coil and turns. The results suggest that alpha-crystallin is best classified as a mixed protein. In addition, increased temperature and concentration of alpha-crystallin result in increased alpha-helices with a compensatory decrease in beta-sheets. Such structural alterations in alpha-crystallin may be functionally important during terminal differentiation of the lens fiber cells that is accompanied by increased protein concentrations and its role as a chaperone-like protein.

The hydrophobic probe 4,4'-bis(1-anilino-8-naphthalene sulfonic acid) is specifically photoincorporated into the N-terminal domain of alpha B-crystallin

Photoincorporation of the fluorescent probe 4,4'-bis(1-anilino-8-naphthalene sulfonic acid) (bis-ANS) can be used to locate solvent-exposed hydrophobic regions in proteins. We show that bis-ANS is specifically incorporated into the putative N-terminal domain of alpha B-crystallin. This incorporation diminishes the chaperone-like activity of alpha B-crystallin, suggesting that hydrophobic surfaces in the N-terminal domain are involved in the binding of unfolding proteins.

Unique structural features of a novel class of small heat shock proteins

Small heat shock proteins (smHSPs) and alpha-crystallins constitute a family of related molecular chaperones that exhibit striking variability in size, ranging from 16 to 43 kDa. Structural studies on these proteins have been hampered by their tendency to form large, often dynamic and heterogeneous oligomeric complexes. Here we describe the structure and expression of HSP12.6, a member of a novel class of smHSPs from the nematode Caenorhabditis elegans. Like other members of its class, HSP12.6 possesses a conserved alpha-crystallin domain but has the shortest N- and C-terminal regions of any known smHSP. Expression of HSP12.6 is limited to the first larval stage of C. elegans and is not significantly up-regulated by a wide range of stressors. Unlike other smHSPs, HSP12.6 does not form large oligomeric complexes in vivo. HSP12.6 was produced in Escherichia coli as a soluble protein and purified. Cross-linking and sedimentation velocity analyses indicate that the recombinant HSP12.6 is monomeric, making it an ideal candidate for structure determination. Interestingly, HSP12.6 does not function as a molecular chaperone in vitro, since it is unable to prevent the thermally induced aggregation of a test substrate. The structural and functional implications of these findings are discussed.

Modulation of the chaperone-like activity of bovine alpha-crystallin

The effects of pantethine, glutathione, and selected chemical reagents on the anti-aggregation activity of alpha-crystallin was evaluated. Protein aggregation was monitored by light scattering of solutions of denatured beta L-crystallin or alcohol dehydrogenase (ADH). The ratios of beta L-crystallin/alpha-crystallin and ADH/alpha-crystallin were adjusted so that partial inhibition of protein aggregation at 60 degrees C or 37 degrees C, respectively, was observed and modulation of the chaperone action of alpha-crystallin could be evaluated easily with selected endogenous metabolites. Enhancement of the anti-aggregation activity in the beta L-crystallin assay was strongest with pantethine, which appeared to interact with alpha-crystallin. Enhancement of the anti-aggregation activity in the ADH assay was strongest with glutathione which appeared to interact with ADH. The results indicated that the products of common metabolic pathways can modulate the chaperone-like effects of alpha-crystallin on protein aggregation.

Immobilization of the C-terminal extension of bovine alphaA-crystallin reduces chaperone-like activity

alpha-Crystallins occur as multimeric complexes, which are able to suppress precipitation of unfolding proteins. Although the mechanism of this chaperone-like activity is unknown, the affinity of alpha-crystallin for aggregation-prone proteins is probably based on hydrophobic interactions. alpha-Crystallins expose a considerable hydrophobic surface to solution, but nevertheless they are very stable and highly soluble. An explanation for this paradox may be that alpha-crystallin subunits have a polar and unstructured C-terminal extension that functions as a sort of solubilizer. In this paper we have described five alphaA-crystallins in which charged and hydrophobic residues were inserted in the C-terminal extension. Introduction of lysine, arginine, and aspartate does not substantially influence chaperone-like activity. In contrast, introduction of a hydrophobic tryptophan greatly diminishes functional activity. CD experiments indicate that this mutant has a normal secondary structure and fluorescence measurements show that the inserted tryptophan is located in a polar environment. However, NMR spectroscopy clearly demonstrates that the presence of the tryptophan residue dramatically reduces the flexibility of the C-terminal extension. Furthermore, the introduction of this tryptophan results in a considerably decreased thermostability of the protein. We conclude that changing the polarity of the C-terminal extension of alphaA-crystallin by insertion of a highly hydrophobic residue can seriously disturb structural and functional integrity.

The conformational stability of alpha-crystallin is rather low: calorimetric results

The eye lens protein and chaperonin, alpha-crystallin, was studied by differential scanning microcalorimetry, spectroscopy and size exclusion chromatography. The thermal transition of alpha-crystallin proceeds at Ttrs = 59.8 +/- 0.6 degrees C with an enthalpy change of delta H = 336 +/- 9 kJ per mol subunit. Disagreement between previous delta H values could be attributed to a side reaction that leads, depending on the scan rate, to the formation of a non-productive folding form. The conformational stability of alpha-crystallin is rather low (delta G = 24 +/- 5 kJ/mol of subunit). The minimal cooperative unit of alpha-crystallin is the monomeric subunit.

On the interaction of alpha-crystallin with unfolded proteins

alpha-Crystallin, a major protein component of the lens, has chaperone-like properties whereby it prevents destabilised proteins from precipitating out of solution. It does so by forming a soluble high-molecular-weight (HMW) complex. A spectroscopic investigation of the HMW complex formed between a variety of unfolded proteins and bovine alpha-crystallin is presented in this paper. As monitored by fluorescence spectroscopy, a large amount of the hydrophobic probe, 8-anilino-1-naphthalene sulfonate (ANS) binds to the HMW complex implying that the complexed proteins (alcohol dehydrogenase (ADH), gamma-crystallin and rhodanese) are bound in an unfolded, possibly molten-globule state. The interaction between the anionic surfactant, sodium dodecyl sulfate (SDS) and ADH at high temperatures gives rise to a similar large increase in ANS fluorescence to that for the complex between alpha-crystallin and ADH. SDS, like alpha-crystallin, therefore complexes to proteins in their unfolded state leaving a large hydrophobic surface exposed to solvent. Unlike other chaperones (e.g., GroEL, DnaK and SecB), alpha-crystallin does not interact with unfolded, hydrophobic but stable proteins (e.g., reduced and carboxymethylated alpha-lactalbumin and alpha-casein). It is concluded that alpha-crystallin will only complex with proteins that are about to precipitate out of solution, i.e., ones that are severely compromised. 1H-NMR spectroscopy of the HMW complex formed between alpha-crystallin and gamma-crystallin indicates that the short C-terminal extension of alpha B-crystallin, but not that of alpha A-crystallin, has lost its flexibility in the complex implying that the former is involved in interactions with the unfolded gamma-crystallin molecule, possibly electrostatically via its two C-terminal lysine residues.

alpha-Crystallin quaternary structure: molecular basis for its chaperone activity

alpha-Crystallin, the major protein in all vertebrate lenses, functions as a chaperone. In the present analysis, an 'open' micellar structure composed of alpha A subunits is used to simulate chaperoning of partially heat denatured soluble gamma-crystallin. The interaction is both electrostatic and hydrophobic and satisfies experimental evidence for a 1:1 alpha/gamma molar ratio, a doubling of molecular mass and a minimal increase in the dimensions of the complex [J. Biol. Chem. (1994) 269, 13601-13608; Invest. Opthalmol. Vis. Sci. (1995) 36, 311-21]. These data are also in accord with Farahbaksh et al. [Biochemistry (1995) 34, 509-16]; i.e. the bound gamma-crystallin monomers are not in a central cavity, but are separated by alpha A subunits.

The mutation Asp69-->Ser affects the chaperone-like activity of alpha A-crystallin

alpha-Crystallins are members of the family of small heat-shock proteins. The conformation and mode of action of these 'junior chaperones' are unknown. To investigate the structure and chaperone-like activity, four mutants of bovine alpha A-crystallin were generated by site-directed mutagenesis. In comparison with wild-type alpha A-crystallin, the D69S mutant, in which a highly conserved charged residue has been replaced, forms larger multimers and displays a threefold reduced heat-protection capacity. The conformation and thermal stability of this mutant are not noticeably affected. Three other mutations, replacing hydrophobic by uncharged hydrophilic residues, were aimed at disturbing hydrophobic intersubunit interactions. None of these mutations resulted in major structural perturbations and only minor differences in heat-protective capacity were observed. Although it is assumed that small heat-shock proteins interact with denaturing proteins via their hydrophobic surfaces, this study clearly shows that charged residues in alpha-crystallin can also influence the efficiency of substrate binding.

Reduced chaperone-like activity of alpha A(ins)-crystallin, an alternative splicing product containing a large insert peptide

alpha-Crystallin is a multimeric protein complex which is constitutively expressed at high levels in the vertebrate eye lens, where it serves a structural role, and at low levels in several non-lenticular tissues. Like other members of the small heat shock protein family, alpha-crystallin has a chaperone-like activity in suppressing nonspecific aggregation of denaturing proteins in vitro. Apart from the major alpha A- and alpha B-subunits, alpha-crystallin of rodents contains an additional minor subunit resulting from alternative splicing, alpha A(ins)-crystallin. This polypeptide is identical to normal alpha A-crystallin except for an insert peptide of 23 residues. To explore the structural and functional consequences of this insertion, we have expressed rat alpha A- and alpha A(ins)-crystallin in Escherichia coli. The multimeric particles formed by alpha A(ins) are larger and more disperse than those of alpha A, but they are native-like and display a similar thermostability and morphology, as revealed by gel permeation chromatography, tryptophan fluorescence measurements, and electron microscopy. However, as compared with alpha A, the alpha A(ins)-particles display a diminished chaperone-like activity in the protection of heat-induced aggregation of beta low-crystallin. Our experiments indicate that alpha A(ins)-multimers have a 3-4-fold reduced substrate binding capacity, which might be correlated to their increased particle size and to a shielding of binding sites by the insert peptides. The structure-function relationship of the natural mutant alpha A(ins)-crystallin may shed light on the mechanism of chaperone-like activity displayed by all small heat shock proteins.

Structure and modifications of the junior chaperone alpha-crystallin. From lens transparency to molecular pathology

alpha-Crystallin is a high-molecular-mass protein that for many decades was thought to be one of the rare real organ-specific proteins. This protein exists as an aggregate of about 800 kDa, but its composition is simple. Only two closely related subunits termed alpha A- and alpha B-crystallin, with molecular masses of approximately 20 kDa, form the building blocks of the aggregate. The idea of organ-specificity had to be abandoned when it was discovered that alpha-crystallin occurs in a great variety of nonlenticular tissues, notably heart, kidney, striated muscle and several tumors. Moreover alpha B-crystallin is a major component of ubiquinated inclusion bodies in human degenerative diseases. An earlier excitement arose when it was found that alpha B-crystallin, due to its very similar structural and functional properties, belongs to the heat-shock protein family. Eventually the chaperone nature of alpha-crystallin could be demonstrated unequivocally. All these unexpected findings make alpha-crystallin a subject of great interest far beyond the lens research field. A survey of structural data about alpha-crystallin is presented here. Since alpha-crystallin has resisted crystallization, only theoretical models of its three-dimensional structure are available. Due to its long life in the eye lens, alpha-crystallin is one of the best studied proteins with respect to post-translational modifications, including age-induced alterations. Because of its similarities with the small heat-shock proteins, the findings about alpha-crystallin are illuminative for the latter proteins as well. This review deals with: structural aspects, post-translational modifications (including deamidation, racemization, phosphorylation, acetylation, glycation, age-dependent truncation), the occurrence outside of the eye lens, the heat-shock relation and the chaperone activity of alpha-crystallin.

Alpha-crystallin: molecular chaperone and protein surfactant

Bovine lens alpha-crystallin has recently been shown to function as a molecular chaperone by stabilizing proteins against heat denaturation (Horwitz, J. (1992) Proc. Natl. Acad. Sci. USA, 89, 10449-10453). An investigation, using a variety of physico-chemical methods, is presented into the mechanism of stabilization. alpha-Crystallin exhibits properties of a surfactant. Firstly, a plot of conductivity of alpha-crystallin versus concentration shows a distinct inflection in its profile, i.e., a critical micelle concentration (cmc), over a concentration range from 0.15 to 0.17 mM. Gel chromatographic and 1H-NMR spectroscopic studies spanning the cmc indicate no change in the aggregated state of alpha-crystallin implying that a change in conformation of the aggregate occurs at the cmc. Secondly, spectrophotometric studies of the rate of heat-induced aggregation and precipitation of alcohol dehydrogenase (ADH), beta L- and gamma-crystallin in the presence of alpha-crystallin and a variety of synthetic surfactants show that stabilization against precipitation results from hydrophobic interactions with alpha-crystallin and monomeric anionic surfactants. Per mole of subunit or monomer, alpha-crystallin is the most efficient at stabilization. alpha-Crystallin, however, does not preserve the activity of ADH after heating. After heat inactivation, gel permeation HPLC indicates that ADH and alpha-crystallin form a high molecular weight aggregate. Similar results are obtained following incubation of beta L- and gamma-crystallin with alpha-crystallin. 1H-NMR spectroscopy of mixtures of alpha- and beta L-crystallin, in their native states, reveals that the C-terminus of beta B2-crystallin is involved in interaction with alpha-crystallin. In the case of gamma- and alpha-crystallin mixtures, a specific interaction occurs between alpha-crystallin and the C-terminal region of gamma B-crystallin, an area which is known from the crystal structure to be relatively hydrophobic and to be involved in intermolecular interactions. The short, flexible C-terminal extensions of alpha-crystallin are not involved in specific interactions with these proteins. It is concluded that alpha-crystallin interacts with native proteins in a weak manner. Once a protein has become denatured, however, the soluble complex with alpha-crystallin cannot be readily dissociated. In the aging lens this finding may have relevance to the formation of high molecular weight crystallin aggregates.