Folding and self-assembly of the domains of betaB2-crystallin from rat eye lens

The βγ-crystallins: native state stability and pathways to aggregation

The βγ-crystallins are among the most stable and long-lived proteins in the human body. With increasing age, however, they transform to high molecular weight light-scattering aggregates, resulting in cataracts. This occurs despite the presence in the lens of high concentrations of the a-crystallin chaperones. Aggregation of crystallins can be induced in vitro by a variety of stresses, including acidic pH, ultraviolet light, oxidative damage, heating or freezing, and specific amino acid substitutions. Accumulating evidence points to the existence of specific biochemical pathways of protein: protein interaction and polymerization. We review the methods used for studying crystallin stability and aggregation and discuss the sometimes counterintuitive relationships between factors that favor native state stability and those that favor non-native aggregation. We discuss the behavior of βγ-crystallins in mixtures and their chaperone ability; the consequences of missense mutations and covalent damage to the side-chains; and the evolutionary strategies that have shaped these proteins. Efforts are ongoing to reveal the nature of cataractous crystallin aggregates and understand the mechanisms of aggregation in the context of key models of protein polymerization: amyloid, native-state, and domain-swapped. Such mechanistic understanding is likely to be of value for the development of therapeutic interventions and draw attention to unanswered questions about the relationship between a protein's native state stability and its transformation to an aggregated state.

Lens β-crystallins: the role of deamidation and related modifications in aging and cataract

Crystallins are the major proteins in the lens of the eye and function to maintain transparency of the lens. Of the human crystallins, α, β, and γ, the β-crystallins remain the most elusive in their structural significance due to their greater number of subunits and possible oligomer formations. The β-crystallins are also heavily modified during aging. This review focuses on the functional significance of deamidation and the related modifications of racemization and isomerization, the major modifications in β-crystallins of the aged human lens. Elucidating the role of these modifications in cataract formation has been slow, because they are analytically among the most difficult post-translational modifications to study. Recent results suggest that many amides deamidate to similar extent in normal aged and cataractous lenses, while others may undergo greater deamidation in cataract. Mimicking deamidation at critical structural regions induces structural changes that disrupt the stability of the β-crystallins and lead to their aggregation in vitro. Deamidations at the surface disrupt interactions with other crystallins. Additionally, the α-crystallin chaperone is unable to completely prevent deamidated β-crystallins from insolubilization. Therefore, deamidation of β-crystallins may enhance their precipitation and light scattering in vivo contributing to cataract formation. Future experiments are needed to quantify differences in deamidation rates at all Asn and Gln residues within crystallins from aged and cataractous lenses, as well as racemization and isomerization which potentially perturb protein structure greater than deamidation alone. Quantitative data is greatly needed to investigate the importance of these major age-related modifications in cataract formation.

Microbial βγ-crystallins

βγ-Crystallins have emerged as a superfamily of structurally homologous proteins with representatives across the domains of life. A major portion of this superfamily is constituted by members from microorganisms. This superfamily has also been recognized as a novel group of Ca(2+)-binding proteins with huge diversity. The βγ domain shows variable properties in Ca(2+) binding, stability and association with other domains. The various members present a series of evolutionary adaptations culminating in great diversity in properties and functions. Most of the predicted βγ-crystallins are yet to be characterized experimentally. In this review, we outline the distinctive features of microbial βγ-crystallins and their position in the βγ-crystallin superfamily.

The congenital cataract-linked A2V mutation impairs tetramer formation and promotes aggregation of βB2-crystallin

β/γ-Crystallins, the major structural proteins in human lens, are highly conserved in their tertiary structures but distinct in the quaternary structures. The N- and C-terminal extensions have been proposed to play a crucial role in mediating the size of β-crystallin assembly. In this research, we investigated the molecular mechanism underlying the congenital hereditary cataract caused by the recently characterized A2V mutation in βB2-crystallin. Spectroscopic experiments indicated that the mutation did not affect the secondary and tertiary structures of βB2-crystallin. The mutation did not affect the formation of βB2/βA3-crystallin heteromer as well as the stability and folding of the heteromer, suggesting that the mutation might not interfere with the protein interacting network in the lens. However, the tetramerization of βB2-crystallin at high protein concentrations was retarded by the A2V mutation. The mutation slightly decreased the thermal stability and promoted the thermal aggregation of βB2-crystallin. Although it did not influence the stability of βB2-crystallin against denaturation induced by chemical denaturants and UV irradiation, the A2V mutant was more prone to be trapped in the off-pathway aggregation process during kinetic refolding. Our results suggested that the A2V mutation might lead to injury of lens optical properties by decreasing βB2-crystallin stability against heat treatment and by impairing βB2-crystallin assembly into high-order homo-oligomers.

Aggregation-prone near-native intermediate formation during unfolding of a structurally similar nonlenticular βγ-crystallin domain

The folding and unfolding of structurally similar proteins belonging to a family have long been a focus of investigation of the structure-(un)folding relationship. Such studies are yet to reach a consensus about whether structurally similar domains follow common or different unfolding pathways. Members of the βγ-crystallin superfamily, which consists of structurally similar proteins with limited sequence similarity from diverse life forms spanning microbes to mammals, form an appropriate model system for exploring this relationship further. We selected a new member, Crybg3_D3, the third βγ-crystallin domain of non-lens vertebrate protein Crybg3 from mouse brain. The crystal structure determined at 1.86 Å demonstrates that the βγ-crystallin domain of Crybg3 resembles more closely the lens βγ-crystallins than the microbial crystallins do. However, interestingly, this structural cousin follows a quite distinct (un)folding pathway via formation of an intermediate state. The intermediate species is in a nativelike conformation with variation in flexibility and tends to form insoluble aggregates. The individual domains of lens βγ-crystallins (and microbial homologues) do not follow such an unfolding pattern. Thus, even the closest members of a subfamily within a superfamily do not necessarily follow similar unfolding paths, suggesting the divergence acquired by these domains, which could be observed only by unfolding. Additionally, this study provides insights into the modifications that this domain has undergone during its recruitment into the non-lens tissues in vertebrates.

Changes in solvent accessibility of wild-type and deamidated βB2-crystallin following complex formation with αA-crystallin

Aberrant protein interactions can lead to aggregation and insolubilization, such as occurs during cataract formation. Deamidation, a prevalent age-related modification in the lens of the eye, decreases stability of the major lens proteins, crystallins. The mechanism of deamidation altering interactions between αA-crystallin and βB2-crystallin was investigated by detecting changes in solvent accessibility upon complex formation during heating. Solvent accessibility was determined by measuring hydrogen/deuterium exchange levels of backbone amides by high-resolution mass spectrometry. Deuterium levels in wild type βB2-crystallin increased 50-60% in both domains following complex formation with αA-crystallin. This increased solvent accessibility indicated a general loosening along the backbone amides. Peptides with the greatest deuterium increases were located at the buried monomer-monomer interface, suggesting that the βB2 dimer was disrupted. The only region where the deuterium levels decreased was in βB2 peptide 123-139, containing an outside loop, and may be a potential site of interaction with αA. Mimicking deamidation at the βB2 dimer interface prevented complex formation with αA. When temperatures were lowered, an αA/βB2 Q70E/Q162E complex formed with similar solvent accessibilities as αA/WT βB2. Deamidation did not disrupt specific αA/βB2 interactions but favored aggregation before complex formation with αA. We conclude that deamidation contributes to cataract formation through destabilization of crystallins before they can be rescued by α-crystallin.

Solvent accessibility of betaB2-crystallin and local structural changes due to deamidation at the dimer interface

In the lens of the eye the ordered arrangement of the major proteins, the crystallins, contributes to lens transparency. Members of the beta/gamma-crystallin family share common beta-sheet rich domains and hydrophobic regions at the monomer-monomer or domain-domain interfaces. Disruption of these interfaces, due to post-translational modifications, such as deamidation, decreases the stability of the crystallins. Previous experiments have failed to define the structural changes associated with this decreased stability. Using hydrogen/deuterium exchange with mass spectrometry (HDMS), deamidation-induced local structural changes in betaB2-crystallin were identified. Deamidation was mimicked by replacing glutamines with glutamic acids at homologous residues 70 and 162 in the monomer-monomer interface of the betaB2-crystallin dimer. The exchange-in of deuterium was determined from 15 s to 24 h and the global and local changes in solvent accessibility were measured. In the wild type betaB2-crystallin (WT), only about 20% of the backbone amide hydrogen was exchanged, suggesting an overall low accessibility of betaB2-crystallin in solution. This is consistent with a tightly packed domain structure observed in the crystal structure. Deuterium levels were initially greater in N-terminal domain (N-td) peptides than in homologous peptides in the C-terminal domain (C-td). The more rapid incorporation suggests a greater solvent accessibility of the N-td. In the betaB2-crystallin crystal structure, interface Gln are oriented towards their opposite domain. When deamidation was mimicked at Gln70 in the N-td, deuterium levels increased at the interface peptide in the C-td. A similar effect in the N-td was not observed when deamidation was mimicked at the homologous residue, Gln162, in the C-td. This difference in the mutants can be explained by deamidation at Gln70 disrupting the more compact C-td and increasing the solvent accessibility in the C-td interface peptides. When deamidation was mimicked at both interface Gln, deuterium incorporation increased in the C-td, similar to deamidation at Gln70 alone. In addition, deuterium incorporation was decreased in the N-td in an outside loop peptide adjacent to the mutation site. This decreased accessibility may be due to newly exposed charge groups facilitating ionic interactions or to peptides becoming more buried when other regions became more exposed. The highly sensitive HDMS methods used here detected local structural changes in solution that had not been previously identified and provide a mechanism for the associated decrease in stability due to deamidation. Changes in accessibility due to deamidation at the interface led to structural perturbations elsewhere in the protein. The cumulative effects of multiple deamidation sites perturbing the structure both locally and distant from the site of deamidation may contribute to aggregation and precipitation during aging and cataractogenesis in the lens.

Aggregation of deamidated human betaB2-crystallin and incomplete rescue by alpha-crystallin chaperone

Aging of the lens is accompanied by extensive deamidation of the lens specific proteins, the crystallins. Deamidated crystallins are increased in the insoluble proteins and may contribute to cataracts. Deamidation has been shown in vitro to alter the structure and decrease the stability of human lens betaB1, betaB2 and betaA3-crystallin. Of particular interest, betaB2 mutants were constructed to mimic the effect of in vivo deamidations at the interacting interface between domains, at Q70 in the N terminal domain and at Q162, its C-terminal homologue. The double mutant was also constructed. We previously reported that deamidation at the critical interface sites decreased stability, while preserving the dimeric 3D structure. In the present study, dynamic light scattering, differential scanning calorimetry and small angle X-ray scattering were used to investigate the effect of deamidation on stability, thermal unfolding and aggregation. The bovine betaLb fraction was used for comparative analysis. The chaperone requirements of the various samples were determined using bovine alpha-crystallins as the chaperone. Deamidation at both interface Gln residues or at Q70, but not Q162, significantly lowered the temperature for unfolding and aggregation, which was rapidly followed by precipitation. This deamidation-induced aggregation and precipitation was not completely prevented by alpha-crystallin chaperone. A potential mechanism for cataract formation in vivo involving accumulation of deamidated beta-crystallin aggregates is discussed.

Association of partially folded lens betaB2-crystallins with the alpha-crystallin molecular chaperone

Age-related cataract is a result of crystallins, the predominant lens proteins, forming light-scattering aggregates. In the low protein turnover environment of the eye lens, the crystallins are susceptible to modifications that can reduce stability, increasing the probability of unfolding and aggregation events occurring. It is hypothesized that the alpha-crystallin molecular chaperone system recognizes and binds these proteins before they can form the light-scattering centres that result in cataract, thus maintaining the long-term transparency of the lens. In the present study, we investigated the unfolding and aggregation of (wild-type) human and calf betaB2-crystallins and the formation of a complex between alpha-crystallin and betaB2-crystallins under destabilizing conditions. Human and calf betaB2-crystallin unfold through a structurally similar pathway, but the increased stability of the C-terminal domain of human betaB2-crystallin relative to calf betaB2-crystallin results in the increased population of a partially folded intermediate during unfolding. This intermediate is aggregation-prone and prevents constructive refolding of human betaB2-crystallin, while calf betaB2-crystallin can refold with high efficiency. alpha-Crystallin can effectively chaperone both human and calf betaB2-crystallins from thermal aggregation, although chaperone-bound betaB2-crystallins are unable to refold once returned to native conditions. Ordered secondary structure is seen to increase in alpha-crystallin with elevated temperatures up to 60 degrees C; structure is rapidly lost at temperatures of 70 degrees C and above. Our experimental results combined with previously reported observations of alpha-crystallin quaternary structure have led us to propose a structural model of how activated alpha-crystallin chaperones unfolded betaB2-crystallin.

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.

Deamidation alters the structure and decreases the stability of human lens betaA3-crystallin

According to the World Health Organization, cataracts account for half of the blindness in the world, with the majority occurring in developing countries. A cataract is a clouding of the lens of the eye due to light scattering of precipitated lens proteins or aberrant cellular debris. The major proteins in the lens are crystallins, and they are extensively deamidated during aging and cataracts. Deamidation has been detected at the domain and monomer interfaces of several crystallins during aging. The purpose of this study was to determine the effects of two potential deamidation sites at the predicted interface of the betaA3-crystallin dimer on its structure and stability. The glutamine residues at the reported in vivo deamidation sites of Q180 in the C-terminal domain and at the homologous site Q85 in the N-terminal domain were substituted with glutamic acid residues by site-directed mutagenesis. Far-UV and near-UV circular dichroism spectroscopy indicated that there were subtle differences in the secondary structure and more notable differences in the tertiary structure of the mutant proteins compared to that of the wild type betaA3-crystallin. The Q85E/Q180E mutant also was more susceptible to enzymatic digestion, suggesting increased solvent accessibility. These structural changes in the deamidated mutants led to decreased stability during unfolding in urea and increased precipitation during heat denaturation. When simulating deamidation at both residues, there was a further decrease in stability and loss of cooperativity. However, multiangle-light scattering and quasi-elastic light scattering experiments showed that dimer formation was not disrupted, nor did higher-order oligomers form. These results suggest that introducing charges at the predicted domain interface in the betaA3 homodimer may contribute to the insolubilization of lens crystallins or favor other, more stable, crystallin subunit interactions.

Analysis of betaB1-crystallin unfolding equilibrium by spin and fluorescence labeling: evidence of a dimeric intermediate

A central step in understanding lens aging is to characterize the thermodynamic stability of its proteins and determine the consequences of changes in the primary sequence on their folding equilibria. For this purpose, destabilized mutations were introduced in betaB1-crystallin targeting the domain interface within the fold of a subunit. Global unfolding was monitored by tryptophan fluorescence while concomitant structural changes at the dimer interface were monitored by fluorescence and spin labels. Both spectral probes report explicit evidence of multi-state unfolding equilibrium. The biphasic nature of the unfolding curves was more pronounced at higher protein concentration. Distinct shifts in the midpoint of the second transition reflect the population of a dimeric intermediate. This intermediate may be a critical determinant for the life-long stability of the beta-crystallins and has important consequences on interactions with alpha-crystallin.

Truncation of motifs III and IV in human lens betaA3-crystallin destabilizes the structure

The purpose of our study was to determine the effects of specific truncations on the structural properties of human betaA3-crystallin. The following eight deletion mutants of betaA3-crystallin were generated: (i) N-terminal extension (NTE) 21 amino acids (betaA3[21] mutant), (ii) NTE 22 amino acids (betaA3[22] mutant), (iii) NTE (betaA3[N] mutant), (iv) NTE plus motif I (betaA3[N+I] mutant), (v) NTE plus motifs I and II (betaA3[N+I+II] mutant), (vi) NTE plus motifs I and II and connecting peptide (betaA3[N+I+II+CP] mutant), (vii) motifs III and IV (betaA3[III+IV] mutant), and (viii) motif IV (betaA3 [IV] mutant). The DNA sequencing and MALDI-TOF mass spectrometric methods confirmed desired specific deletions, and the purified mutant proteins exhibited a single band during SDS-PAGE analysis. When ANS bound, all the mutant proteins exhibited fluorescence quenching and a red shift, suggesting that the truncations caused changes in the exposed hydrophobic patches. The CD spectra showed that deletion of either NTE or the N-terminal domain (motifs I and II) had a relatively weaker effect on the structural stability than deletion of the C-terminal domain (motifs III and IV). Intrinsic Trp fluorescence spectral studies suggested changes in the microenvironment of the mutant proteins following truncations. HPLC multiangle light scattering analyses showed that truncation led to higher-order aggregation compared to that in the wild-type protein. Equilibrium unfolding and refolding of WT betaA3 with urea were best fit to a three-state model with transition midpoints at 2.2 and 3.1 M urea. However, the two transition midpoints of betaA3[21] and betaA3[22] and betaA3[N] mutants were similar to those of the wild type, suggesting that these truncations had a minimal effect on structural stabilization. Further, the mutant proteins containing the N-terminal domain (i.e., betaA3[III+IV] and betaA3[IV] mutants) exhibited higher transition midpoints compared to the transition midpoints of the mutant protein with the C-terminal domain (i.e., betaA3[N+I+II+CP] mutant). The results suggested that the N-terminal domain is relatively more stable than the C-terminal domain in betaA3-crystallin.

Deamidation in human lens betaB2-crystallin destabilizes the dimer

Two major determinants of the transparency of the lens are protein-protein interactions and stability of the crystallins, the structural proteins in the lens. betaB2 is the most abundant beta-crystallin in the human lens and is important in formation of the complex interactions of lens crystallins. betaB2 readily forms a homodimer in vitro, with interacting residues across the monomer-monomer interface conserved among beta-crystallins. Due to their long life spans, crystallins undergo an unusually large number of modifications, with deamidation being a major factor. In this study the effects of two potential deamidation sites at the monomer-monomer interface on dimer formation and stability were determined. Glutamic acid substitutions were constructed to mimic the effects of previously reported deamidations at Q162 in the C-terminal domain and at Q70, its N-terminal homologue. The mutants had a nativelike secondary structure similar to that of wild type betaB2 with differences in tertiary structure for the double mutant, Q70E/Q162E. Multiangle light scattering and quasi-elastic light scattering experiments showed that dimer formation was not interrupted. In contrast, equilibrium unfolding and refolding in urea showed destabilization of the mutants, with an inflection in the transition of unfolding for the double mutant suggesting a distinct intermediate. These results suggest that deamidation at critical sites destabilizes betaB2 and may disrupt the function of betaB2 in the lens.

Interdomain side-chain interactions in human gammaD crystallin influencing folding and stability

Human gammaD crystallin (HgammaD-Crys) is a two domain, beta-sheet eye lens protein that must remain soluble throughout life for lens transparency. Single amino acid substitutions of HgammaD-Crys are associated with juvenile-onset cataracts. Features of the interface between the two domains conserved among gamma-crystallins are a central six-residue hydrophobic cluster, and two pairs of interacting residues flanking the cluster. In HgammaD-Crys these pairs are Gln54/Gln143 and Arg79/Met147. We previously reported contributions of the hydrophobic cluster residues to protein stability. In this study alanine substitutions of the flanking residue pairs were constructed and analyzed. Equilibrium unfolding/refolding experiments at 37 degrees C revealed a plateau in the unfolding/refolding transitions, suggesting population of a partially folded intermediate with a folded C-terminal domain (C-td) and unfolded N-terminal domain (N-td). The N-td was destabilized by substituting residues from both domains. In contrast, the C-td was not significantly affected by substitutions of either domain. Refolding rates of the N-td were significantly decreased for mutants of either domain. In contrast, refolding rates of the C-td were similar to wild type for mutants of either domain. Therefore, domain interface residues of the folded C-td probably nucleate refolding of the N-td. We suggest that these residues stabilize the native state by shielding the central hydrophobic cluster from solvent. Glutamine and methionine side chains are among the residues covalently damaged in aged and cataractous lenses. Such damage may generate partially unfolded, aggregation- prone conformations of HgammaD-Crys that could be significant in cataract.

Unfolding crystallins: the destabilizing role of a beta-hairpin cysteine in betaB2-crystallin by simulation and experiment

The thermodynamic and kinetic stabilities of the eye lens family of betagamma-crystallins are important factors in the etiology of senile cataract. They control the chance of proteins unfolding, which can lead to aggregation and loss of transparency. betaB2-Crystallin orthologs are of low stability and comprise two typical betagamma-crystallin domains, although, uniquely, the N-terminal domain has a cysteine in one of the conserved folded beta-hairpins. Using high-temperature (500 K) molecular dynamics simulations with explicit solvent on the N-terminal domain of rodent betaB2-crystallin, we have identified in silico local flexibility in this folded beta-hairpin. We have shown in vitro using two-domain human betaB2-crystallin that replacement of this cysteine with a more usual aromatic residue (phenylalanine) results in a gain in conformational stability and a reduction in the rate of unfolding. We have used principal components analysis to visualize and cluster the coordinates from eight separate simulated unfolding trajectories of both the wild-type and the C50F mutant N-terminal domains. These data, representing fluctuations around the native well, show that although the mutant and wild-type appear to behave similarly over the early time period, the wild type appears to explore a different region of conformational space. It is proposed that the advantage of having this low-stability cysteine may be correlated with a subunit-exchange mechanism that allows betaB2-crystallin to interact with a range of other beta-crystallin subunits.

Contributions of hydrophobic domain interface interactions to the folding and stability of human gammaD-crystallin

Human gammaD-crystallin (HgammaD-Crys) is a monomeric eye lens protein composed of two highly homologous beta-sheet domains. The domains interact through interdomain side chain contacts forming two structurally distinct regions, a central hydrophobic cluster and peripheral residues. The hydrophobic cluster contains Met43, Phe56, and Ile81 from the N-terminal domain (N-td) and Val132, Leu145, and Val170 from the C-terminal domain (C-td). Equilibrium unfolding/refolding of wild-type HgammaD-Crys in guanidine hydrochloride (GuHCl) was best fit to a three-state model with transition midpoints of 2.2 and 2.8 M GuHCl. The two transitions likely corresponded to sequential unfolding/refolding of the N-td and the C-td. Previous kinetic experiments revealed that the C-td refolds more rapidly than the N-td. We constructed alanine substitutions of the hydrophobic interface residues to analyze their roles in folding and stability. After purification from E. coli, all mutant proteins adopted a native-like structure similar to wild type. The mutants F56A, I81A, V132A, and L145A had a destabilized N-td, causing greater population of the single folded domain intermediate. Compared to wild type, these mutants also had reduced rates for productive refolding of the N-td but not the C-td. These data suggest a refolding pathway where the domain interface residues of the refolded C-td act as a nucleating center for refolding of the N-td. Specificity of domain interface interactions is likely important for preventing incorrect associations in the high protein concentrations of the lens nucleus.

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.

Binding of Destabilized βB2-Crystallin Mutants to α-Crystallin

Age-related changes in protein-protein interactions in the lens play a critical role in the temporal evolution of its optical properties. In the relatively non-regenerating environment of the fiber cells, a critical determinant of these interactions is partial or global unfolding as a consequence of post-translational modifications or chemical damage to individual crystallins. One type of attractive force involves the recognition by alpha-crystallins of modified proteins prone to unfolding and aggregation. In this paper, we explore the energetic threshold and the structural determinants for the formation of a stable complex between alpha-crystallin and betaB2-crystallin as a consequence of destabilizing mutations in the latter. The mutations were designed in the framework of a folding model that proposes the equilibrium population of a monomeric intermediate. Binding to alpha-crystallin is detected through changes in the emission properties of a bimane fluorescent probe site-specifically introduced at a solvent exposed site in betaB2-crystallin. alpha-Crystallin binds the various betaB2-crystallin mutants, although with a significantly lower affinity relative to destabilized T4 lysozyme mutants. The extent of binding, while reflective of the overall destabilization, is determined by the dynamic population of a folding intermediate. The existence of the intermediate is inferred from the biphasic bimane emission unfolding curve of a mutant designed to disrupt interactions at the dimer interface. The results of this paper are consistent with a model in which the interaction of alpha-crystallins with substrates is not solely triggered by an energetic threshold but also by the population of excited states even under favorable folding conditions. The ability of alpha-crystallin to detect subtle changes in the population of betaB2-crystallin excited states supports a central role for this chaperone in delaying aggregation and scattering in the lens.

The stability of human acidic beta-crystallin oligomers and hetero-oligomers

Crystallins are bulk structural proteins of the eye lens that have to last a life time. They gradually become modified with age, denature and form light scattering centres. High thermodynamic and kinetic stability of the crystallins enables them to resist unfolding and delay cataract. Here we have made recombinant human betaA1-, betaA3-, and betaA4-crystallins. The betaA3-crystallin formed higher oligomers that lead to precipitation at ambient temperature. Heat-induced precipitation of betaA3-crystallin was compared with human and calf betaB2-crystallins, showing that the human proteins start to precipitate above 50 degrees C while the calf betaB2-crystallin stays in solution even when unfolded. The stabilities of these human acidic beta-crystallin homo-oligomers have been estimated by measuring their unfolding in urea at neutral pH. BetaA3/1/betaB1 and betaA4/betaB1-crystallin hetero-oligomers have been prepared from homo-oligomers by subunit exchange. The resolution of the methodology used was insufficient to detect a stabilization of the betaA4-crystallin subunit in the hetero-oligomer, the betaA1-crystallin subunit was clearly stabilized by its interaction with betaB1-crystallin. Circular dichroism and fluorescence spectroscopies show that homo-dimer surface tryptophans become buried in the betaA3/1/betaB1-crystallin hetero-dimer concomitant with changes in polypeptide chain conformation.

Stability, homodimerization, and calcium-binding properties of a single, variant betagamma-crystallin domain of the protein absent in melanoma 1 (AIM1)

AIM1 (absent in melanoma), a candidate suppressor of malignancy in melanoma, is a nonlens member of the betagamma-crystallin superfamily, which contains six predicted betagamma domains. The first betagamma-crystallin domain of AIM1 (AIM1-g1) diverges most in sequence from the superfamily consensus. To examine its ability to fold and behave like a normal betagamma domain, we cloned AIM1-g1 and overexpressed it in Escherichia coli as a recombinant protein. The recombinant domain was found to be a stable, soluble protein, similar to lens protein gammaBeta-crystallin in secondary structure. The tertiary structure of AIM1-g1 is dominated by the contribution of aromatic amino acids and cysteine. AIM1-g1 undergoes concentration-independent, noncovalent homodimerization with no trace of monomer, similar to a one-domain protein spherulin 3a. Since many betagamma domain proteins bind calcium, we have also investigated the calcium-binding properties of AIM1-g1 by various methods. AIM1-g1 binds the calcium-mimic dye Stains-all, the calcium probe terbium (with K(D) 170 microM), and (45)Ca when blotted on a membrane. AIM1-g1 binds calcium (K(D) 30 microM) with a comparatively higher affinity than bovine lens gamma-crystallin (90 microM). However, calcium binding does not induce significant change in the protein conformation in the near- and far-UV CD and in fluorescence. The AIM1-g1 domain is as stable as domains of betagamma-crystallins (betaB2- or gammaS-crystallins) as monitored by guanidinium chloride unfolding (midpoint of unfolding transition is 1.8 M GdmCl), and the stability of the protein is not altered upon binding calcium as evaluated by equilibrium unfolding. These results show that, despite the sequence variation, AIM1-g1 folds such as a betagamma domain, binds calcium and undergoes dimerization.

Rapid purification of recombinant betaB2-crystallin using hydrophobic interaction chromatography

BetaB2-crystallin, the major subunit of beta-crystallins, is difficult to purify either from lens homogenate or from betaH-or betaL-crystallins. It has been prepared by heterologous expression in Escherichia coli. Most often, the methods used for purifying a recombinant globular protein employ the combination of ion-exchange with gel filtration chromatography. In the case of betaB2-crystallin too, different approaches have been used to obtain the purified protein, majority of which use a combination of ion-exchange and gel filtration chromatography. We present a new approach to purify betaB2-crystallin using hydrophobic interaction chromatography. In this method, the protein is bound to the hydrophobic matrix in the presence of high concentration of a non-chaotropic salt and eluted by decreasing the salt concentration. The method that we have used for the purification of this globular protein has definite advantages over the earlier methods in its simplicity and efficiency. The most noted advantage of this procedure is the rapid purification with a relatively purified product and a comparatively high yield (>20 mg/L of culture). Over all, the present protocol provides a rapid, efficient and simplified procedure for the preparation of betaB2-crystallin in large yield, sufficient for structural and functional studies.

Deamidation, but not truncation, decreases the urea stability of a lens structural protein, betaB1-crystallin

Crystallins, the major structural proteins in the lens of the eye, are maintained with little turnover throughout the lifetime of the host. With time, lens crystallins undergo post-translational modifications that may play an important role in loss of vision during aging and cataract formation. Specific modifications include deamidation and truncation. Urea-induced denaturation was studied for recombinantly expressed wild-type betaB1 (WT), the deamidated mutant (Q204E), an N-terminally truncated mutant (betaB1(DeltaN41)), and other truncated versions of these proteins generated by calpain II digestion. Tryptophan fluorescence was used to monitor loss of global tertiary structure. Loss of secondary structure was followed by circular dichroism, and electron paramagnetic resonance site-directed spin labeling was used to monitor loss of tertiary structure selectively in the N-terminal domain. Our results indicated that the deamidated mutant was significantly destabilized relative to WT. Q204E showed a two-step denaturation curve with transitions at 4.1 and 7.2 M urea, whereas denaturation of WT occurred in a cooperative single step with a transition midpoint of 5.9 M urea. Unfolding of WT was completely reversible, whereas Q204E failed to fully refold. Prolonged incubation under denaturing conditions led to aggregation, which was also more pronounced for Q204E dimers than for WT. Truncation of 41 residues from the N-terminus or 47 and 5 residues from the N- and C-termini did not affect stability. These studies indicated that a single-site deamidation could significantly diminish the stability of lens betaB1-crystallin, supporting the idea that such modifications may play an important role in age-related cataract formation.

Unfolding of human lens recombinant betaB2- and gammaC-crystallins

betaB2- and gammaC-crystallins belong to the betagamma-crystallin superfamily and have very similar structures. Molecular spectroscopic techniques such as UV-visible absorption, circular dichroism, and fluorescence indicate they have similar biophysical properties. Their structures are characterized by the presence of two domains consisting of four Greek key motifs. The only difference is the connecting peptide of the two domains, which is flexible in gamma-crystallin but extended in beta-crystallin; thus, an intradomain association and a monomer are formed in gamma-crystallin and an interdomain association and a dimer are formed in beta-crystallin. The difference may be reflected in the thermodynamic stability. In the present study, we calculated the standard free-energy by equilibrium unfolding transition in guanidine hydrochloride using three spectroscopic parameters: absorbance at 235nm, Trp fluorescence intensity at 320nm, and far-UV circular dichroism at 223nm. Global analyses indicate that both dimeric betaB2- and monomeric gammaC-crystallins are a better fit to a three-state model than to a two-state model. In terms of standard free-energy, deltaG(0)(H(2)O,i) both betaB2-crystallin and gammaC-crystallin are stable proteins and dimeric betaB2-crystallin is more stable than the monomeric gammaC-crystallin. The significance of the thermodynamic stability for betaB2- and gammaC-crystallins may be related to their functions in the lens.

Folding properties of the nucleotide exchange factor GrpE from Thermus thermophilus: GrpE is a thermosensor that mediates heat shock response

Hsp70 proteins like DnaK bind unfolded polypeptides in a nucleotide-dependent manner. The switch from high-affinity ADP-state to low- affinity ATP-state with concomitant substrate release is accelerated significantly by GrpE proteins. GrpE thus fulfils an important role in regulation of the chaperone cycle. Here, we analysed the thermal stability of GrpE from Thermus thermophilus using differential scanning calorimetry and CD-spectroscopy. The protein exhibits unusual unfolding characteristics with two observable thermal transitions. The first transition is CD-spectroscopically silent with a transition midpoint at 90 degrees C. The second transition, mainly constituting the CD-signal, ranges between 100 and 105 degrees C depending on the GrpE(Tth) concentration, according to the model N(2) <==> I(2) <==> 2U. Using a C-terminally truncated version of GrpE(Tth) it was possible to assign the second thermal transition to the dimerisation of GrpE(Tth), while the first transition represents the completely reversible unfolding of the globular C-terminal domain. The unfolding of this domain is accompanied by a distinct decrease in nucleotide exchange rates and impaired binding to DnaK(Tth). Under heat shock conditions, the DnaK-ADP-protein-substrate complex is thus stabilised by a reversibly inactivated GrpE-protein that refolds under permissive conditions. In combination with studies on GrpE from Escherichia coli presented recently by Christen and co-workers, it thus appears that the general role of GrpE is to function as a thermosensor that modulates nucleotide exchange rates in a temperature-dependent manner to prevent substrate dissociation at non-permissive conditions.

The N-terminal domain of betaB2-crystallin resembles the putative ancestral homodimer

betagamma-crystallins from the eye lens are proteins consisting of two similar domains joined by a short linker. All three-dimensional structures of native proteins solved so far reveal similar pseudo-2-fold pairing of the domains reflecting their presumed ancient origin from a single-domain homodimer. However, studies of engineered single domains of members of the betagamma-crystallin superfamily have not revealed a prototype ancestral solution homodimer. Here we report the 2.35 A X-ray structure of the homodimer of the N-terminal domain of rat betaB2-crystallin (betaB2-N). The two identical domains pair in a symmetrical manner very similar to that observed in native betagamma-crystallins, where N and C-terminal domains (which share approximately 35% sequence identity) are related by a pseudo-2-fold axis. betaB2-N thus resembles the ancestral prototype of the betagamma-crystallin superfamily as it self-associates in solution to form a dimer with an essentially identical domain interface as that between the N and C domains in betagamma-crystallins, but without the benefit of a covalent linker. The structure provides further evidence for the role of two-domain pairing in stabilising the protomer fold. These results support the view that the betagamma-crystallin superfamily has evolved by a series of gene duplication and fusion events from a single-domain ancestor capable of forming homodimers.

Partially unfolded species populated during equilibrium denaturation of the beta-sheet protein Y74W apo-pseudoazurin

Apo-pseudoazurin is a single domain cupredoxin. We have engineered a mutant in which a unique tryptophan replaces the tyrosine residue found in the tyrosine corner of this Greek key protein, a region that has been proposed to have an important role in folding. Equilibrium denaturation of Y74W apo-pseudoazurin demonstrated multistate unfolding in urea (pH 7.0, 0.5 M Na(2)SO(4) at 15 degrees C), in which one or more partially folded species are populated in 4. 3 M urea. Using a variety of biophysical techniques, we show that these species, on average, have lost a substantial portion of the native secondary structure, lack fixed tertiary packing involving tryptophan and tyrosine residues, are less compact than the native state as determined by fluorescence lifetimes and time-resolved anisotropy, but retain significant residual structure involving the trytophan residue. Peptides ranging in length from 11 to 30 residues encompassing this region, however, did not contain detectable nonrandom structure, suggesting that long-range interactions are important for stabilizing the equilibrium partially unfolded species in the intact protein. On the basis of these results, we suggest that the equilibrium denaturation of Y74W apo-pseudoazurin generates one or more partially unfolded species that are globally collapsed and retain elements of the native structure involving the newly introduced tryptophan residue. We speculate on the role of such intermediates in the generation of the complex Greek key fold.

Calorimetric analysis of the Ca(2+)-binding betagamma-crystallin homolog protein S from Myxococcus xanthus: intrinsic stability and mutual stabilization of domains

The betagamma-crystallin superfamily consists of a class of homologous two-domain proteins with Greek-key fold. Protein S, a Ca(2+)-binding spore-coat protein from the soil bacterium Myxococcus xanthus exhibits a high degree of sequential and structural homology with gammaB-crystallin from the vertebrate eye lens. In contrast to gammaB-crystallin, which undergoes irreversible aggregation upon thermal unfolding, protein S folds reversibly and may therefore serve as a model in the investigation of the thermodynamic stability of the eye-lens crystallins. The thermal denaturation of recombinant protein S (PS) and its isolated domains was studied by differential scanning calorimetry in the absence and in the presence of Ca(2+) at varying pH. Ca(2+)-binding leads to a stabilization of PS and its domains and increases the cooperativity of their equilibrium unfolding transitions. The isolated N-terminal and C-terminal domains (NPS and CPS) obey the two-state model, independent of the pH and Ca(2+)-binding; in the case of PS, under all conditions, an equilibrium intermediate is populated. The first transition of PS may be assigned to the denaturation of the C-terminal domain and the loss of domain interactions, whereas the second one coincides with the denaturation of the isolated N-terminal domain. At pH 7.0, in the presence of Ca(2+), where PS exhibits maximal stability, the domain interactions at 20 degrees C contribute 20 kJ/mol to the overall stability of the intact protein.

Stability of a homo-dimeric Ca(2+)-binding member of the beta gamma-crystallin superfamily: DSC measurements on spherulin 3a from Physarum polycephalum

Spherulin 3a (S3a) from Physarum polycephalum represents the only known single-domain member of the superfamily of beta gamma eye-lens crystallins. It shares the typical two Greek-key motif and is stabilized by dimerization and Ca(2+)-binding. The temperature and denaturant-induced unfolding of S3a in the absence and in the presence of Ca2+ were investigated by differential scanning calorimetry and fluorescence spectroscopy. To accomplish reversibility without chemical modification of the protein during thermal denaturation, the only cysteine residue (Cys4) was substituted by serine; apart from that, the protein was destabilized by adding 0.5-1.8 M guanidinium chloride (GdmCl). The Cys4Ser mutant was found to be indistinguishable from natural S3a. The equilibrium unfolding transitions obey the two-state model according to N2-->2 U, allowing thermodynamic parameters to be determined by linear extrapolation to zero GdmCl concentration. The corresponding transition temperatures TM for the Ca(2+)-free and Ca(2+)-loaded protein were found to be 65 and 85 degrees C, the enthalpy changes delta Hcal, 800 and 1280 kJ/mol(dimer), respectively. The strong dependencies of TM and delta Hcal on the GdmCl concentration allow the molar heat capacity change delta Cp to be determined. As a result, delta Cp = 18 kJ/(K mol(dimer)) was calculated independent of Ca2+. No significant differences were obtained between the free energy delta G degree calculated from delta Hcal and TM, and extrapolated from the stability curves in the presence of different amounts of denaturant. The free energy derived from thermal unfolding was confirmed by the spectral results obtained from GdmCl-induced equilibrium transitions at different temperatures for the Ca(2+)-free or the Ca(2+)-loaded protein, respectively. Within the limits of error, the delta G degree values extrapolated from the transitions of chemical denaturation to zero denaturant concentration are identical with the calorimetric results.

Kinetic and thermodynamic stabilization of the betagamma-crystallin homolog spherulin 3a from Physarum polycephalum by calcium binding

Globular proteins may be stabilized, either intrinsically, at the various levels of the structural hierarchy, or extrinsically, by ligand binding. In the case of the dormant all-beta protein spherulin 3a (S3a) from the slime mold Physarum polycephalum, binding of calcium ions causes extreme kinetic and thermodynamic stabilization. S3a is the only known single-domain member of the two Greek key superfamily of betagamma-crystallins sharing the extreme long-term stability of its homologs in vertebrate eye lens. Spectral analysis allows two Ca2+-binding sites with KD=9 microM and 200 microM to be distinguished. Unfolding in the absence and in the presence of Ca2+gives evidence for extreme kinetic stabilization of the protein: In the absence of Ca2+, the half-time of unfolding in 2. 5 M guanidinium chloride (GdmCl) equals 8.3 minutes, whereas in the presence of Ca2+, even in 7.5 M GdmCl, it exceeds nine hours. To reach the equilibrium of unfolding in the absence and in the presence of Ca2+takes one day and eight weeks, respectively. The corresponding Gibbs free energies (based on the two-state model) are 77 and 135 kJ/mol. Saturation of S3a with Ca2+leads to an upward shift of the temperature-induced equilibrium transition by ca 20 deg. C. The in situ Ca2+concentration in the spherules is sufficient for the complete complexation of S3a in vivo.