Association behaviour of human betaB1-crystallin and its truncated forms

Potential role of βB1 crystallin in cataract formation:a systematic review

βB1 crystallin is a soluble structural protein of the lens, which plays an important role in maintaining lens transparency and cell homeostasis. βB1 crystallin has conservative dual structural domains, each of which contains two Greek key motifs. Gene mutation or post-translational modification can affect the structure and function of βB1 crystallin, leading to abnormal protein aggregation and the occurrence of cataracts. This article will review the protein structure, post-translational modification, and related gene mutations of βB1 crystallin. Understanding these molecular mechanisms of βB1crystallin mutations not only aids in clarifying the pathogenesis of cataracts but also provides potential targets for pharmacological interventions.

Kinetic Stability of Long-Lived Human Lens γ-Crystallins and Their Isolated Double Greek Key Domains

The γ-crystallins of the eye lens nucleus are among the longest-lived proteins in the human body. Synthesized in utero, they must remain folded and soluble throughout adulthood to maintain lens transparency and avoid cataracts. γD- and γS-crystallin are two major monomeric crystallins of the human lens. γD-crystallin is concentrated in the oldest lens fiber cells, the lens nucleus, whereas γS-crystallin is concentrated in the younger cells of the lens cortex. The kinetic stability parameters of these two-domain proteins and their isolated domains were determined and compared. Kinetic unfolding experiments monitored by fluorescence spectroscopy in varying concentrations of guanidinium chloride were used to extrapolate unfolding rate constants and half-lives of the crystallins in the absence of the denaturant. Consistent with their long lifespans in the lens, extrapolated half-lives for the initial unfolding step were on the timescale of years. Both proteins' isolated N-terminal domains were less kinetically stable than their respective C-terminal domains at denaturant concentrations predicted to disrupt the domain interface, but at low denaturant concentrations, the relative kinetic stabilities were reversed. Cataract-associated aggregation has been shown to proceed from partially unfolded intermediates in these proteins; their extreme kinetic stability likely evolved to protect the lens from the initiation of aggregation reactions. Our findings indicate that the domain interface is the source of significant kinetic stability. The gene duplication and fusion event that produced the modern two-domain architecture of vertebrate lens crystallins may be the origin of their high kinetic as well as thermodynamic stability.

Sequence specific 1H, 13C and 15N resonance assignments of the C-terminal domain of human γS-crystallin

The high solubility and stability of crystallins present in the human eye lens maintains its transparency and refractive index with negligible protein turnover. Monomeric γ-crystallins and oligomeric β-crystallins are made up of highly homologous double Greek key domains. These domains are symmetric and possess higher stability as a result of the complex topology of individual Greek key motifs. γS-crystallin is one of the most abundant structural βγ-crystallins present in the human eye lens. In order to understand the structural stability of individual domains of human γS-crystallin in isolation vis-à-vis full length protein, we set out to structurally characterize its C-terminal domain (abbreviated hereafter as γS-CTD) by solution NMR. In this direction, we have cloned, over-expressed, isolated and purified the γS-CTD. The 2D [¹⁵N-¹H] HSQC recorded with uniformly ¹³C/¹⁵N labeled γS-CTD showed a highly dispersed spectrum indicating the protein to adopt an ordered conformation. In this paper, we report almost complete sequence-specific ¹H, ¹³C and ¹⁵N resonance assignments of γS-CTD using a suite of heteronuclear 3D NMR experiments.

Crystal Structure of Chicken γS-Crystallin Reveals Lattice Contacts with Implications for Function in the Lens and the Evolution of the βγ-Crystallins

Previous attempts to crystallize mammalian γS-crystallin were unsuccessful. Native L16 chicken γS crystallized avidly while the Q16 mutant did not. The x-ray structure for chicken γS at 2.3Å resolution shows the canonical structure of the superfamily plus a well-ordered N-arm aligned with a β-sheet of a neighboring N-domain. L16 is also in a lattice contact, partially shielded from solvent. Unexpectedly, the major lattice contact matches a conserved interface (QR) in the multimeric β-crystallins. QR shows little conservation of residue contacts, except for one between symmetry-related tyrosines, but molecular dipoles for the proteins with QR show striking similarities while other γ-crystallins differ. In γS, QR has few hydrophobic contacts and features a thin layer of tightly bound water. The free energy of QR is slightly repulsive and AUC confirms no dimerization in solution. The lattice contacts suggest how γcrystallins allow close packing without aggregation in the crowded environment of the lens.

Human βB2-Crystallin Forms a Face-en-Face Dimer in Solution: An Integrated NMR and SAXS Study

βγ-Crystallins are long-lived eye lens proteins that are crucial for lens transparency and refractive power. Each βγ-crystallin comprises two homologous domains, which are connected by a short linker. γ-Crystallins are monomeric, while β-crystallins crystallize as dimers and multimers. In the crystal, human βB2-crystallin is a domain-swapped dimer while the N-terminally truncated βB1-crystallin forms a face-en-face dimer. Combining and integrating data from multi-angle light scattering, nuclear magnetic resonance, and small-angle X-ray scattering of full-length and terminally truncated human βB2-crystallin in solution, we show that both these βB2-crystallin proteins are dimeric, possess C2 symmetry, and are more compact than domain-swapped dimers. Importantly, no inter-molecular paramagnetic relaxation enhancement effects compatible with domain swapping were detected. Our collective experimental results unambiguously demonstrate that, in solution, human βB2-crystallin is not domain swapped and exhibits a face-en-face dimer structure similar to the crystal structure of truncated βB1-crystallin.

Modeling phase transitions in mixtures of β-γ lens crystallins

We analyze the experimentally determined phase diagram of a γD-βB1 crystallin mixture. Proteins are described as dumbbells decorated with attractive sites to allow inter-particle interaction. We use thermodynamic perturbation theory to calculate the free energy of such mixtures and, by applying equilibrium conditions, also the compositions and concentrations of the co-existing phases. Initially we fit the Tcloudversus packing fraction η measurements for a pure (x2 = 0) γD solution in 0.1 M phosphate buffer at pH = 7.0. Another piece of experimental data, used to fix the model parameters, is the isotherm x2vs. η at T = 268.5 K, at the same pH and salt content. We use the conventional Lorentz-Berthelot mixing rules to describe cross interactions. This enables us to determine: (i) model parameters for pure βB1 crystallin protein and to calculate; (ii) complete equilibrium surface (Tcloud-x2-η) for the crystallin mixtures. (iii) We present the results for several isotherms, including the tie-lines, as also the temperature-packing fraction curves. Good agreement with the available experimental data is obtained. An interesting result of these calculations is evidence of the coexistence of three phases. This domain appears for the region of temperatures just out of the experimental range studied so far. The input parameters, leading good description of experimental data, revealed a large difference between the numbers of the attractive sites for γD and βB1 proteins. This interesting result may be related to the fact that γD has a more than nine times smaller quadrupole moment than its partner in the mixture.

Cataract-causing mutation S228P promotes βB1-crystallin aggregation and degradation by separating two interacting loops in C-terminal domain

β/γ-Crystallins are predominant structural proteins in the cytoplasm of lens fiber cells and share a similar fold composing of four Greek-key motifs divided into two domains. Numerous cataract-causing mutations have been identified in various β/γ-crystallins, but the mechanisms underlying cataract caused by most mutations remains uncharacterized. The S228P mutation in βB1-crystallin has been linked to autosomal dominant congenital nuclear cataract. Here we found that the S228P mutant was prone to aggregate and degrade in both of the human and E. coli cells. The intracellular S228P aggregates could be redissolved by lanosterol. The S228P mutation modified the refolding pathway of βB1-crystallin by affecting the formation of the dimeric intermediate but not the monomeric intermediate. Compared with native βB1-crystallin, the refolded S228P protein had less packed structures, unquenched Trp fluorophores and increased hydrophobic exposure. The refolded S228P protein was prone to aggregate at the physiological temperature and decreased the protective effect of βB1-crystallin on βA3-crystallin. Molecular dynamic simulation studies indicated that the mutation decreased the subunit binding energy and modified the distribution of surface electrostatic potentials. More importantly, the mutation separated two interacting loops in the C-terminal domain, which shielded the hydrophobic core from solvent in native βB1-crystallin. These two interacting loops are highly conserved in both of the N- and C-terminal domains of all β/γ-crystallins. We propose that these two interacting loops play an important role in the folding and structural stability of β/γ-crystallin domains by protecting the hydrophobic core from solvent access.

Biophysical chemistry of the ageing eye lens

This review examines both recent and historical literature related to the biophysical chemistry of the proteins in the ageing eye, with a particular focus on cataract development. The lens is a vital component of the eye, acting as an optical focusing device to form clear images on the retina. The lens maintains the necessary high transparency and refractive index by expressing crystallin proteins in high concentration and eliminating all large cellular structures that may cause light scattering. This has the consequence of eliminating lens fibre cell metabolism and results in mature lens fibre cells having no mechanism for protein expression and a complete absence of protein recycling or turnover. As a result, the crystallins are some of the oldest proteins in the human body. Lack of protein repair or recycling means the lens tends to accumulate damage with age in the form of protein post-translational modifications. The crystallins can be subject to a wide range of age-related changes, including isomerisation, deamidation and racemisation. Many of these modification are highly correlated with cataract formation and represent a biochemical mechanism for age-related blindness.

The N-terminal extension of βB1-crystallin chaperones β-crystallin folding and cooperates with αA-crystallin

β/γ-Crystallins are the major structural proteins in mammalian lens. The N-terminal truncation of βB1-crystallin has been associated with the regulation of β-crystallin size distributions in human lens. Herein we studied the roles of βB1 N-terminal extension in protein structure and folding by constructing five N-terminal truncated forms. The truncations did not affect the secondary and tertiary structures of the main body as well as stability against denaturation. Truncations with more than 28 residues off the N-terminus promoted the dissociation of the dimeric βB1 into monomers in diluted solutions. Interestingly, the N-terminal extension facilitated βB1 to adopt the correct folding pathway, while truncated proteins were prone to undergo the misfolding/aggregation pathway during kinetic refolding. The N-terminal extension of βB1 acted as an intramolecular chaperone (IMC) to regulate the kinetic partitioning between folding and misfolding. The IMC function of the N-terminal extension was also critical to the correct refolding of β-crystallin heteromer and the action of the lens-specific molecular chaperone αA-crystallin. The cooperation between IMC and molecular chaperones produced a much stronger chaperoning effect than if they acted separately. To our knowledge, this is the first report showing the cooperation between IMC and molecular chaperones.

Comparative proteomic analysis identifies age-dependent increases in the abundance of specific proteins after deletion of the small heat shock proteins αA- and αB-crystallin

Mice with deletion of genes for small heat shock proteins αA- and αB-crystallin (αA/αB(-/-)) develop cataracts. We used proteomic analysis to identify lens proteins that change in abundance after deletion of these α-crystallin genes. Wild-type (WT) and αA/αB(-/-) knockout (DKO) mice were compared using two-dimensional difference gel electrophoresis and mass spectrometric analysis, and protein identifications were validated by Mascot proteomic software. The abundance of histones H2A, H4, and H2B fragment, and a low molecular weight β1-catenin increased 2-3-fold in postnatal day 2 lenses of DKO lenses compared with WT lenses. Additional major increases were observed in abundance of βB2-crystallin and vimentin in 30-day-old lenses of DKO animals compared with WT animals. Lenses of DKO mice were comprised of nine protein spots containing βB2-crystallin at 10-40-fold higher abundance and three protein spots containing vimentin at ≥2-fold higher abundance than in WT lenses. Gel permeation chromatography identified a unique 328 kDa protein in DKO lenses, containing β-crystallin, demonstrating aggregation of β-crystallin in the absence of α-crystallins. Together, these changes provide biochemical evidence for possible functions of specific cell adhesion proteins, cytoskeletal proteins, and crystallins in lens opacities caused by the absence of the major chaperones, αA- and αB-crystallins.

N-terminal extension of beta B1-crystallin: identification of a critical region that modulates protein interaction with beta A3-crystallin

The human lens proteins beta-crystallins are subdivided into acidic (betaA1-betaA4) and basic (betaB1-betaB3) subunit groups. These structural proteins exist at extremely high concentrations and associate into oligomers under physiological conditions. Crystallin acidic-basic pairs tend to form strong heteromolecular associations. The long N-terminal extensions of beta-crystallins may influence both homo- and heteromolecular interactions. However, identification of the critical regions of the extensions mediating protein associations has not been previously addressed. This was studied by comparing the self-association and heteromolecular associations of wild-type recombinant betaA3- and betaB1-crystallins and their N-terminally truncated counterparts (betaA3DeltaN30 and betaB1DeltaN56) using several biophysical techniques, including analytical ultracentrifugation and fluorescence spectroscopy. Removal of the N-terminal extension of betaA3 had no effect on dimerization or heteromolecular tetramer formation with betaB1. In contrast, the level of self-association of betaB1DeltaN56 increased, resulting in homotetramer formation, and heteromolecular association with betaA3 was blocked. Limited proteolysis of betaB1 produced betaB1DeltaN47, which is similar to intact protein formed dimers but in contrast showed enhanced heteromolecular tetramer formation with betaA3. The tryptic digestion was physiologically significant, corresponding to protease processing sites observed in vivo. Molecular modeling of the N-terminal betaB1 extension indicates structural features that position a mobile loop in the vicinity of these processing sites. The loop is derived from residues 48-56 which appear to be critical for mediating protein interactions with betaA3-crystallin.

Truncated human betaB1-crystallin shows altered structural properties and interaction with human betaA3-crystallin

The purpose of the study was to determine the effects of truncation of various regions of betaB1-crystallin on its structural properties and stability of heterooligomers formed by wild-type (WT) betaB1 or its deletion mutants with WT betaA3-crystallin. For these analyses, seven deletion mutants of betaB1-crystallin were generated with the following sequential deletions of either N-terminal arm [betaB1(59-252)], N-terminal arm + motif I [betaB1(99-252)], N-terminal arm + motif I + motif II [betaB1(144-252)], N-terminal arm + motif I + motif II + connecting peptide [betaB1(149-252)], C-terminal extension [betaB1(1-234)], C-terminal extension plus motif IV [betaB1(1-190)], or C-terminal extension + motif III + motif IV [betaB1(1-148)]. The betaB1-crystallin became water insoluble on the deletion of C-terminal extension and subsequent deletions of the C-terminal domain (C-terminal extension plus motifs III and IV) while it remained partially soluble on the deletion of the N-terminal domain (N-terminal arm plus motifs I and II). However, circular dichroism spectral analysis showed that the deletion of the N-terminal domain but not the C-terminal domain exhibited relatively greater structural changes in the crystallin. The deletion of the C-terminal domain resulted in a greater exposure and disturbance in the microenvironment of Trp-100, Trp-123, and Trp-126 (localized in the motif II), suggesting a relatively greater role of the C-terminal domain than the N-terminal domain in the structural stability of the crystallin. The deletion of the N-terminal extension in betaB1 resulted in maximum exposure of hydrophobic patches and compact structure and in a maximum loss of subunit exchange with WT betaA3-crystallin compared to deletion of either the C-terminal extension, the N-terminal domain, or the C-terminal domain. The thermal stability results of the heterooligomer of betaB1- plus betaA3-crystallins suggested that oligomers lose their stability on deletion of the C-terminal domain. Together, the results suggested that the N-terminal arm of betaB1-crystallin plays a major role in interaction with betaA3-crystallin during heterooligomer formation, and the solubility of betaB1-crystallin per se and that of the heterooligomer with betaA3-crystallin are dependent on the intact C-terminal domain of betaB1-crystallin.

Association properties of betaB1- and betaA3-crystallins: ability to form heterotetramers

As major constituents of the mammalian lens, beta-crystallins associate into dimers, tetramers, and higher-order complexes to maintain lens transparency and refractivity. A previous study has shown that dimerization of betaB2- and betaA3-crystallins is energetically highly favored and entropically driven. While heterodimers further associate into higher-order complexes in vivo, a significant level of reversibly associated tetrameric crystallin has not been previously observed in vitro. To enhance our understanding of the interactions between beta-crystallins, we characterized the association of betaB1-crystallin, a major component of large beta-crystallin complexes (beta-high), with itself and with betaA3-crystallin. Mouse betaB1-crystallin and human betaA3-crystallin were expressed in Escherichia coli and purified chromatographically. Their association was then characterized using size-exclusion chromatography, native gel electrophoresis, isoelectric focusing, and analytical sedimentation equilibrium centrifugation. When present alone, each beta-crystallin associates into homodimers; however, no tetramer formation is seen. Once mixing has taken place, formation of a heterocomplex between betaB1- and betaA3-crystallins is observed using size-exclusion chromatography, native gel electrophoresis, isoelectric focusing, and sedimentation equilibrium. In contrast to results previously obtained after betaB2- and betaA3-crystallins had been mixed, mixed betaB1- and betaA3-crystallins show a dimer-tetramer equilibrium with a K d of 1.1 muM, indicating that these two beta-crystallins associate predominantly into heterotetramers in vitro. Thus, while each purified beta-crystallin associates only into homodimers and under the conditions studied mixed betaB2- and betaA3-crystallins form a mixture of homo- and heterodimers, mixed betaB1- and betaA3-crystallins associate predominantly into heterotetramers in equilibrium with heterodimers. These findings suggest a unique role for betaB1-crystallin in promoting higher-order crystallin association in the lens.

Molecular diversity and genomic organisation of the alpha, beta and gamma eye lens crystallins from the Antarctic toothfish Dissostichus mawsoni

The eye lens of the Antarctic toothfish living in the -2 degrees C Southern Ocean is cold-stable. To investigate the molecular basis of this cold stability, we isolated, cloned and sequenced 22 full length crystallin cDNAs. We found two alpha crystallins (alphaA, alphaB), six beta crystallins (betaA1, betaA2, betaA4, betaB1, betaB2, betaB3) and 14 gamma crystallins (gammaN, gammaS1, gammaS2, gammaM1, gammaM3, gammaM4, gammaM5, gammaM7, gammaM8a, gammaM8b, gammaM8c, gammaM8d, gammaM8e, and gammaM9). Alignments of alpha, beta and gamma with other known crystallin sequences indicate that toothfish alpha and beta crystallins are relatively conserved orthologues of their vertebrate counterparts, but the toothfish and other fish gammaM crystallins form a distinct group that are not orthologous to mammalian gamma crystallins. A preliminary Fingerprinted Contig analysis of clones containing crystallin genes screened from a toothfish BAC library indicated alpha crystallin genes occurred in a single genomic region of ~266 kbp, beta crystallin genes in ~273 kbp, while the gamma crystallin gene family occurred in two separate regions of ~180 and ~296 kbp. In phylogenetic analysis, the gammaM isoforms of the ectothermic toothfish displayed a diversity not seen with endothermic mammalian gamma crystallins. Similar to other fishes, several toothfish gamma crystallins are methionine-rich (gammaM isoforms) which may have predisposed the toothfish lens to biochemically attenuate gamma crystallin hydrophobicity allowing for cold adaptation. In addition to high methionine content, conservation of alphabeta crystallins both in sequence and abundance suggests greater functional constraints relative to gamma crystallins. Conversely, reduced constraints upon gamma crystallins could have allowed for greater evolutionary plasticity resulting in increased polydispersity of gamma crystallins contributing to the cold-stability of the Antarctic toothfish lens.

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.

Biophysical properties of gammaC-crystallin in human and mouse eye lens: the role of molecular dipoles

The eye lens is packed with soluble crystallin proteins, providing a lifetime of transparency and light refraction. gamma-Crystallins are major components of the dense, high refractive index central regions of the lens and generally have high solubility, high stability and high levels of cysteine residues. Human gammaC belongs to a group of gamma-crystallins with a pair of cysteine residues at positions 78 and 79. Unlike other gamma-crystallins it has relatively low solubility, whereas mouse gammaC, which has the exposed C79 replaced with arginine, and a novel mouse splice variant, gammaCins, are both highly soluble. Furthermore, human gammaC is extremely stable, while the mouse orthologs are less stable. Evolutionary pressure may have favoured stability over solubility for human gammaC and the reverse for the orthologs in the mouse. Mutation of C79 to R79, in human gammaC, greatly increased solubility, however, neither form produced crystals. Remarkably, when the human gammaD R36S crystallization cataract mutation was mimicked in human gammaC-crystallin, the solubility of gammaC was dramatically increased, although it still did not crystallize. The highly soluble mouse gammaC-crystallin did crystallize. Its X-ray structure was solved and used in homology modelling of human gammaC, and its mutants C79R and R36S. The human gammaD R36S mutant was also modelled from human gammaD coordinates. Molecular dynamics simulation of the six molecules in the solution state showed that the human gammaCs differed from gammaDs in domain pairing, behaviour that correlates with interface sequence changes. When the fluctuations of the calculated molecular dipoles, for the six structures, over time were analysed, characteristic patterns for soluble gammaC and gammaD proteins were observed. Individual sequence changes that increase or decrease solubility correlated well with changes in the magnitude and direction of these dipoles. It is suggested that changes in surface residues have allowed adaptation for the differing needs of human and mouse lenses.

Mutation of interfaces in domain-swapped human betaB2-crystallin

The superfamily of eye lens betagamma-crystallins is highly modularized, with Greek key motifs being used to form symmetric domains. Sequences of monomeric gamma-crystallins and oligomeric beta-crystallins fold into two domains that pair about a further conserved symmetric interface. Conservation of this assembly interface by domain swapping is the device adopted by family member betaB2-crystallin to form a solution dimer. However, the betaB1-crystallin solution dimer is formed from an interface used by the domain-swapped dimer to form a tetramer in the crystal lattice. Comparison of these two structures indicated an intriguing relationship between linker conformation, interface ion pair networks, and higher assembly. Here the X-ray structure of recombinant human betaB2-crystallin showed that domain swapping was determined by the sequence and not assembly conditions. The solution characteristics of mutants that were designed to alter an ion pair network at a higher assembly interface and a mutant that changed a proline showed they remained dimeric. X-ray crystallography showed that the dimeric mutants did not reverse domain swapping. Thus, the sequence of betaB2-crystallin appears well optimized for domain swapping. However, a charge-reversal mutation to the conserved domain-pairing interface showed drastic changes to solution behavior. It appears that the higher assembly of the betagamma-crystallin domains has exploited symmetry to create diversity while avoiding aggregation. These are desirable attributes for proteins that have to exist at very high concentration for a very long time.

Eye lens proteomics

The eye lens is a fascinating organ as it is in essence living transparent matter. Lenticular transparency is achieved through the peculiarities of lens morphology, a semi-apoptotic process where cells elongate and loose their organelles and the precise molecular arrangement of the bulk of soluble lenticular proteins, the crystallins. The 16 crystallins ubiquitous in mammals and their modifications have been extensively characterized by 2-DE, liquid chromatography, mass spectrometry and other protein analysis techniques. The various solubility dependant fractions as well as subproteomes of lenticular morphological sections have also been explored in detail. Extensive post translational modification of the crystallins is encountered throughout the lens as a result of ageing and disease resulting in a vast number of protein species. Proteomics methodology is therefore ideal to further comprehensive understanding of this organ and the factors involved in cataractogenesis.

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.

γN-crystallin and the evolution of the βγ-crystallin superfamily in vertebrates

The beta and gamma crystallins are evolutionarily related families of proteins that make up a large part of the refractive structure of the vertebrate eye lens. Each family has a distinctive gene structure that reflects a history of successive gene duplications. A survey of gamma-crystallins expressed in mammal, reptile, bird and fish species (particularly in the zebrafish, Danio rerio) has led to the discovery of gammaN-crystallin, an evolutionary bridge between the beta and gamma families. In all species examined, gammaN-crystallins have a hybrid gene structure, half beta and half gamma, and thus appear to be the 'missing link' between the beta and gamma crystallin lineages. Overall, there are four major classes of gamma-crystallin: the terrestrial group (including mammalian gammaA-F); the aquatic group (the fish gammaM-crystallins); the gammaS group; and the novel gammaN group. Like the evolutionarily ancient beta-crystallins (but unlike the terrestrial gammaA-F and aquatic gammaM groups), both the gammaS and gammaN crystallins form distinct clades with members in fish, reptiles, birds and mammals. In rodents, gammaN is expressed in nuclear fibers of the lens and, perhaps hinting at an ancestral role for the gamma-crystallins, also in the retina. Although well conserved throughout vertebrate evolution, gammaN in primates has apparently undergone major changes and possible loss of functional expression.

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.

Crystal structure of truncated human betaB1-crystallin

Crystallins are long-lived proteins packed inside eye lens fiber cells that are essential in maintaining the transparency and refractive power of the eye lens. Members of the two-domain betagamma-crystallin family assemble into an array of oligomer sizes, forming intricate higher-order networks in the lens cell. Here we describe the 1.4 angstroms resolution crystal structure of a truncated version of human betaB1 that resembles an in vivo age-related truncation. The structure shows that unlike its close homolog, betaB2-crystallin, the homodimer is not domain swapped, but its domains are paired intramolecularly, as in more distantly related monomeric gamma-crystallins. However, the four-domain dimer resembles one half of the crystallographic bovine betaB2 tetramer and is similar to the engineered circular permuted rat betaB2. The crystal structure shows that the truncated betaB1 dimer is extremely well suited to form higher-order lattice interactions using its hydrophobic surface patches, linker regions, and sequence extensions.

Laser light-scattering evidence for an altered association of beta B1-crystallin deamidated in the connecting peptide

Deamidation is a prevalent modification of crystallin proteins in the vertebrate lens. The effect of specific sites of deamidation on crystallin stability in vivo is not known. Using mass spectrometry, a previously unreported deamidation in beta B1-crystallin was identified at Gln146. Another deamidation was investigated at Asn157. It was determined that whole soluble beta B1 contained 13%-17% deamidation at Gln146 and Asn157. Static and quasi-elastic laser light scattering, circular dichroism, and heat aggregation studies were used to explore the structure and associative properties of recombinantly expressed wild-type (wt) beta B1 and the deamidated beta B1 mutants, Q146E and N157D. Dimer formation occurred for wt beta B1, Q146E, and N157D in a concentration-dependent manner, but only Q146E showed formation of higher ordered oligomers at the concentrations studied. Deamidation at Gln146, but not Asn157, led to an increased tendency of beta B1 to aggregate upon heating. We conclude that deamidation creates unique effects depending upon where the deamidation is introduced in the crystallin structure.

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.

Human beta-crystallins modified by backbone cleavage, deamidation and oxidation are prone to associate

Information about beta-crystallins and their post-translational modifications has been scarce because of difficulties in isolating the individual beta-crystallins. These difficulties arise because the beta-crystallin sequences are highly homologous and because beta-crystallins undergo many age-related modifications that lead to a variety of molecular masses and a range of acidities for each crystallin. In this study, human beta-crystallins were isolated using several steps of chromatography both before and after two-dimensional gel electrophoresis. Many previously unidentified in vivo modifications, including deamidations among all beta-crystallins except betaB3, truncation of betaA3, betaB1 and betaA4, and oxidation of some methionines and tryptophans were located among the isolated beta-crystallins. Many modifications occurred before age 20 with modest increases in modification for beta-crystallins from lenses 20-87 years old. The tendency of the modified beta-crystallins to form non-covalent complexes was evident from their chromatographic behaviour. The presence in these complexes of betaB2-crystallin, the least modified and most soluble of the beta-crystallins, points to a possible role for betaB2 in solubilizing the more heavily modified beta-crystallins. The greater solubility of beta-crystallins compared with alpha- and gamma-crystallins in aging lenses may be due to beta-crystallin modifications and their non-covalent associations.

A nonsense mutation in CRYBB1 associated with autosomal dominant cataract linked to human chromosome 22q

Autosomal dominant cataract is a clinically and genetically heterogeneous lens disorder that usually presents as a sight-threatening trait in childhood. Here we have mapped dominant pulverulent cataract to the beta-crystallin gene cluster on chromosome 22q11.2. Suggestive evidence of linkage was detected at markers D22S1167 (LOD score [Z] 2.09 at recombination fraction [theta] 0) and D22S1154 (Z=1.39 at theta=0), which closely flank the genes for betaB1-crystallin (CRYBB1) and betaA4-crystallin (CRYBA4). Sequencing failed to detect any nucleotide changes in CRYBA4; however, a G-->T transversion in exon 6 of CRYBB1 was found to cosegregate with cataract in the family. This single-nucleotide change was predicted to introduce a translation stop codon at glycine 220 (G220X). Expression of recombinant human betaB1-crystallin in bacteria showed that the truncated G220X mutant was significantly less soluble than wild type. This study has identified the first CRYBB1 mutation associated with autosomal dominant cataract in humans.