Alpha-crystallin: molecular chaperone and protein surfactant

Mutation screening in autosomal dominant congenital cataract families from North India

Congenital cataract an opacity of the eye lens is present at birth and results in visual impairment during early childhood. If left untreated, it can lead to permanent blindness. Its prevalence is ten times higher in developing countries like India. Thus, we aimed to investigate the underlying genetic defects in three autosomal dominant congenital cataract (ADCC) families from North India. Detailed family histories were collected, pedigrees drawn followed by slit-lamp examination and lens photography. Mutation screening was performed in the candidate genes for crystallins, connexins, and membrane proteins by Sanger sequencing. Pathogenicity of novel variant was assessed bioinformatically. In an ADCC (CC-3006) family with bilateral membranous cataract and microcornea, a novel change (c.1114C>T;p.P372S) in GJA3 has been detected. In other two ADCC families affected with subcapsular (CC-286) and shrunken membranous hypermature cataract (CC-3014), a nonsense mutation (c.463C>T;p.Q155X) in CRYβB2 and a frameshift deletion (c.590_591delAG;p.E197VfsX22) in CRYβA1/A3 respectively, are observed. These variants segregated completely with the phenotypes in respective families and were absent in their unaffected family members and unrelated controls (tested for novel variant in GJA3). Earlier p.Q155X (CRYβB2) and p.E197VfsX22 (CRYβA1/A3) are reported with entirely different phenotypes. Thus, findings in present study expand the mutation spectrum and phenotypic heterogeneity linked with GJA3, CRYβB2, and CRYβA1/A3 for congenital cataracts. Identifying underlying genetic defects is essential for disease management and appropriate genetic counseling.

Crystallins and Their Complexes

The crystallins (α, β and γ), major constituent proteins of eye lens fiber cells play their critical role in maintaining the transparency and refractive index of the lens. Under different stress factors and with aging, β- and γ-crystallins start to unfold partially leading to their aggregation. Protein aggregation in lens basically enhances light scattering and causes the vision problem, commonly known as cataract. α-crystallin as a molecular chaperone forms complexes with its substrates (β- and γ-crystallins) to prevent such aggregation. In this chapter, the structural features of β- and γ-crystallins have been discussed. Detailed structural information linked with the high stability of γC-, γD- and γS-crystallins have been incorporated. The nature of homologous and heterologous interactions among crystallins has been deciphered, which are involved in their molecular association and complex formation.

Differential Glycosylation and Modulation of Camel and Human HSP Isoforms in Response to Thermal and Hypoxic Stresses

Increased expression of heat shock proteins (HSPs) following heat stress or other stress conditions is a common physiological response in almost all living organisms. Modification of cytosolic proteins including HSPs by O-GlcNAc has been shown to enhance their capabilities for counteracting lethal levels of cellular stress. Since HSPs are key players in stress resistance and protein homeostasis, we aimed to analyze their forms at the cellular and molecular level using camel and human HSPs as models for efficient and moderate thermotolerant mammals, respectively. In this study, we cloned the cDNA encoding two inducible HSP members, HSPA6 and CRYAB from both camel (Camelus dromedarius) and human in a Myc-tagged mammalian expression vector. Expression of these chaperones in COS-1 cells revealed protein bands of approximately 25-kDa for both camel and human CRYAB and 70-kDa for camel HSPA6 and its human homologue. While localization and trafficking of the camel and human HSPs revealed similar cytosolic localization, we could demonstrate altered glycan structure between camel and human HSPA6. Interestingly, the glycoform of camel HSPA6 was rapidly formed and stabilized under normal and stress culture conditions whereas human HSPA6 reacted differently under similar thermal and hypoxic stress conditions. Our data suggest that efficient glycosylation of camel HSPA6 is among the mechanisms that provide camelids with a superior capability for alleviating stressful environmental circumstances.

Foveolar Müller Cells of the Pied Flycatcher: Morphology and Distribution of Intermediate Filaments Regarding Cell Transparency

Specialized intermediate filaments (IFs) have critical importance for the clearness and uncommon transparency of vertebrate lens fiber cells, although the physical mechanisms involved are poorly understood. Recently, an unusual low-scattering light transport was also described in retinal Müller cells. Exploring the function of IFs in Müller cells, we have studied the morphology and distribution pattern of IFs and other cytoskeletal filaments inside the Müller cell main processes in the foveolar part of the avian (pied flycatcher) retina. We found that some IFs surrounded by globular nanoparticles (that we suggest are crystallines) are present in almost every part of the Müller cells that span the retina, including the microvilli. Unlike IFs implicated in the mechanical architecture of the cell, these IFs are not connected to any specific cellular membranes. Instead, they are organized into bundles, passing inside the cell from the endfeet to the photoreceptor, following the geometry of the processes, and repeatedly circumventing numerous obstacles. We believe that the presently reported data effectively confirm that the model of nanooptical channels built of the IFs may provide a viable explanation of Müller cell transparency.

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

BACKGROUND

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

SCOPE OF REVIEW

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

MAJOR CONCLUSIONS

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

GENERAL SIGNIFICANCE

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

Interaction of α-crystallin with some small molecules and its effect on its structure and function

BACKGROUND

α-Crystallin acts like a molecular chaperone by interacting with its substrate proteins and thus prevents their aggregation. It also interacts with various kinds of small molecules that affect its structure and function.

SCOPE OF REVIEW

In this article we will present a review of work done with respect to the interaction of ATP, peptide generated from lens crystallin and other proteins and some bivalent metal ions with α-crystallin and discuss the role of these interactions on its structure and function and cataract formation. We will also discuss the interaction of some hydrophobic fluorescence probes and surface active agents with α-crystallin.

MAJOR CONCLUSIONS

Small molecule interaction controls the structure and function of α-crystallin. ATP and Zn+2 stabilize its structure and enhance chaperone function. Therefore the depletion of these small molecules can be detrimental to maintenance of lens transparency. However, the accumulation of small peptides due to protease activity in the lens can also be harmful as the interaction of these peptides with α-crystallin and other crystallin proteins in the lens promotes aggregation and loss of lens transparency. The use of hydrophobic probe has led to a wealth of information regarding the location of substrate binding site and nature of chaperone-substrate interaction. Interaction of surface active agents with α-crystallin has helped us to understand the structural stability and oligomeric dissociation in α-crystallin.

GENERAL SIGNIFICANCE

These interactions are very helpful in understanding the mechanistic details of the structural changes and chaperone function of α-crystallin. This article is part of a Special Issue entitled Crystallin Biochemistry in Health and Disease.

A missense mutation in CRYBB2 leads to progressive congenital membranous cataract by impacting the solubility and function of βB2-crystallin

Congenital cataract is a major cause of visual impairment and childhood blindness. The solubility and stability of crystallin proteins play critical roles in maintaining the optical transparency of the lens during the life span. Previous studies have shown that approximately 8.3%∼25% of congenital cataracts are inherited, and mutations in crystallins are the most common. In this study, we attempted to identify the genetic defect in a four-generation family affected with congenital cataracts. The congenital cataract phenotype of this four-generation family was identified as membranous cataract by slit-lamp photography. Mutation screening of the candidate genes detected a heterozygous c.465G→C change in the exon6 of the βB2-crystallin gene (CRYBB2) in all family members affected with cataracts, resulting in the substitution of a highly conserved Tryptophan to Cystine (p.W151C). The mutation was confirmed by restriction fragment length polymorphism (RFLP) analysis and found that the transition resulted in the absence of a BslI restriction site in the affected members of the pedigree. The outcome of PolyPhen-2 and SIFT analysis predicted that this W151C mutation would probably damage to the structure and function of βB2-crystallin. Wild type (wt) and W151C mutant βB2-crystallin were expressed in human lens epithelial cells (HLECs), and the fluorescence results showed that Wt-βB2-crystallin was evenly distributed throughout the cells, whereas approximately 34.7% of cells transfected with the W151C mutant βB2-crystallin formed intracellular aggregates. Taken together, these data suggest that the missense mutation in CRYBB2 gene leads to progressive congenital membranous cataract by impacting the solubility and function of βB2-crystallin.

Serum albumin prevents protein aggregation and amyloid formation and retains chaperone-like activity in the presence of physiological ligands

Although serum albumin has an established function as a transport protein, evidence is emerging that serum albumin may also have a role as a molecular chaperone. Using established techniques to characterize chaperone interactions, this study demonstrates that bovine serum albumin: 1) preferentially binds stressed over unstressed client proteins; 2) forms stable, soluble, high molecular weight complexes with stressed client proteins; 3) reduces the aggregation of client proteins when it is present at physiological levels; and 4) inhibits amyloid formation by both WT and L55P transthyretin. Although the antiaggregatory effect of serum albumin is maintained in the presence of physiological levels of Ca(2+) and Cu(2+), the presence of free fatty acids significantly alters this activity: stabilizing serum albumin at normal levels but diminishing chaperone-like activity at high concentrations. Moreover, here it is shown that depletion of albumin from human plasma leads to a significant increase in aggregation under physiologically relevant heat and shear stresses. This study demonstrates that serum albumin possesses chaperone-like properties and that this activity is maintained under a number of physiologically relevant conditions.

Enhanced molecular chaperone activity of the small heat-shock protein alphaB-cystallin following covalent immobilization onto a solid-phase support

The well-characterized small heat-shock protein, alphaB-crystallin, acts as a molecular chaperone by interacting with unfolding proteins to prevent their aggregation and precipitation. Structural perturbation (e.g., partial unfolding) enhances the in vitro chaperone activity of alphaB-crystallin. Proteins often undergo structural perturbations at the surface of a synthetic material, which may alter their biological activity. This study investigated the activity of alphaB-crystallin when covalently bound to a support surface; alphaB-crystallin was immobilized onto a range of solid material surfaces, and its characteristics and chaperone activity were assessed. Immobilization was achieved via a plasma-deposited thin polymeric interlayer containing aldehyde surface groups and reductive amination, leading to the covalent binding of alphaB-crystallin lysine residues to the surface aldehyde groups via Schiff-base linkages. Immobilized alphaB-crystallin was characterized by X-ray photoelectron spectroscopy, atomic force microscopy, and quartz crystal microgravimetry, which showed that 300 ng cm(-2) (dry mass) of oligomeric alphaB-crystallin was bound to the surface. Immobilized alphaB-crystallin exhibited a significant enhancement (up to 5000-fold, when compared with the equivalent activity of alphaB-crystallin in solution) of its chaperone activity against various proteins undergoing both amorphous and amyloid fibril forms of aggregation. The enhanced molecular chaperone activity of immobilized alphaB-crystallin has potential applications in preventing protein misfolding, including against amyloid disease processes, such as dialysis-related amyloidosis, and for biodiagnostic detection of misfolded proteins.

Mechanism of aggregation of UV-irradiated β(L)-crystallin

Thermal denaturation and aggregation of UV-irradiated β(L)-crystallin from eye lenses of steers have been studied. The data on size-exclusion chromatography and SDS-PAGE indicated that UV irradiation of β(L)-crystallin at 10 °С resulted in fragmentation of the protein molecule and formation of cross-linked aggregates. Fluorescence data showed that tryptophan fluorescence in the irradiated protein decreased exponentially with the UV dose. Decrease in tryptophan fluorescence is a result of photochemical destruction, but not of conformational changes of protein, because there is no red shift in the fluorescence maximum. The differential scanning calorimetry (DSC) profiles of the samples of UV-irradiated and wild type β(L)-crystallin were registered. The area under curves, which is proportional to the amount of the native protein, decreased exponentially with increasing the irradiation dose. The shape of the DSC profiles for the samples of UV-irradiated β(L)-crystallin was identical to that for wild type β(L)-crystallin. The DSC data allowed estimating the portion of UV-denatured β(L)-crystallin, which is not registered by DSC, and the portion of the combined fraction consisting of native and UV-damaged molecules retaining the native structure. A conclusion has been made that UV-induced denaturation of β(L)-crystallin follows the one-hit model. The study of the kinetics of thermal aggregation of UV-irradiated β(L)-crystallin at 37 °С using dynamic light scattering showed that the initial stage of aggregation was that of formation of the start aggregates with the hydrodynamic radius of 20 nm. Further sticking of the start aggregates proceeded in the regime of reaction-limited cluster-cluster aggregation. Splitting of the aggregate population into two components occurred above a definite point in time.

Chaperone-like activities of different molecular forms of beta-casein. Importance of polarity of N-terminal hydrophilic domain

As a member of intrinsically unstructured protein family, beta-casein (beta-CN) contains relatively high amount of prolyl residues, adopts noncompact and flexible structure and exhibits chaperone-like activity in vitro. Like many chaperones, native beta-CN does not contain cysteinyl residues and exhibits strong tendencies for self-association. The chaperone-like activities of three recombinant beta-CNs wild type (WT) beta-CN, C4 beta-CN (with cysteinyl residue in position 4) and C208 beta-CN (with cysteinyl residue in position 208), expressed and purified from E. coli, which, consequently, lack the phosphorylated residues, were examined and compared with that of native beta-CN using insulin and alcohol dehydrogenase as target/substrate proteins. The dimers (beta-CND) of C4-beta-CN and C208 beta-CN were also studied and their chaperone-like activities were compared with those of their monomeric forms. Lacking phosphorylation, WT beta-CN, C208 beta-CN, C4 beta-CN and C4 beta-CND exhibited significantly lower chaperone-like activities than native beta-CN. Dimerization of C208 beta-CN with two distal hydrophilic domains considerably improved its chaperone-like activity in comparison with its monomeric form. The obtained results demonstrate the significant role played by the polar contributions of phosphorylated residues and N-terminal hydrophilic domain as important functional elements in enhancing the chaperone-like activity of native beta-CN. (c) 2009 Wiley Periodicals, Inc. Biopolymers 91: 623-632, 2009.This article was originally published online as an accepted preprint. The "Published Online" date corresponds to the preprint version. You can request a copy of the preprint by emailing the Biopolymers editorial office at biopolymers@wiley.com.

Mechanism of suppression of protein aggregation by α-crystallin

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

Protein-protein interactions and lens transparency

Past studies have identified posttranslational modifications of human lens proteins occurring during cataract formation, and have also demonstrated that protein-protein interactions exist between different lens crystallins. Based upon current theories of lens transparency, these posttranslational modifications and their possible effects upon crystallin interactions may be the key to understanding why the lens is able to transmit light, and why transmission is decreased during cataractogenesis. This review will summarize current knowledge of posttranslational modifications during human cataractogenesis, and will propose their possible role in protein-protein interactions that are thought to be necessary for lens transparency. Based upon this premise, model systems will be described that will test the validity of the theory.

Multi-crystallin complexes exist in the water-soluble high molecular weight protein fractions of aging normal and cataractous human lenses

The purpose of the study was to identify non-covalently held complexes that exist in the water-soluble high molecular weight (WS-HMW) protein fractions of normal human lenses of 20-year-old and 60- to 70-year-old, and in the age-matched 60- to 70-year-old cataractous lenses. The WS protein fractions were prepared from five pooled normal lenses of 20-year-old donors or five pooled lenses of 60- to 70-year-old donors or four pooled cataractous lenses (with nuclear opacity) of 60- to 70-year-old donors. Each WS protein fraction was subjected to size-exclusion chromatography using an Agarose A 5m column to recover the void volume WS-HMW protein fraction. A method known as blue-native polyacrylamide gel electrophoresis (BN-PAGE), which allows the isolation of large multi-protein complexes (MPCs) in their native state for further characterization, was used to separate such complexes from individual WS-HMW protein fractions. The protein species that existed as a complex were excised from a gel and trypsin-digested, and the amino acid sequences of the tryptic fragments analyzed by electrospray tandem mass spectrometry (ES-MS/MS). After the second-dimensional sodium dodecyl sulfate-PAGE during BN-PAGE, protein complexes containing a total of 16, 12, and 24 species with M(r) between 10 and 90 kDa were identified in the HMW protein fractions of normal lenses of 20-year-old, 60- to 70-year-old and cataractous lenses of 60- to 70-year-old donors, respectively. Based on the amino acid sequences of tryptic peptides of individual protein species in the complexes by the ES-MS/MS method, the presence of alpha-, beta-, and gamma-crystallin species along with beaded filament proteins (filensin and phakinin) was observed in the 20-year-old normal lenses. The 60- to 70-year-old normal lenses contained filensin and aldehyde dehydrogenase in addition to the above crystallins. Similarly, the age-matched cataractous lenses also contained the above crystallins and aldehyde dehydrogenase but lacked beaded filament proteins. Protein complexes, held mostly via non-covalent bonding, were seen in the WS-HMW proteins of 20-year-old normal, 60- to 70-year-old normal, and 60- to 70-year-old cataractous lenses. The complexes in the normal lenses were made of alpha-, beta-, and gamma-crystallin species, beaded filament proteins (filensin and/or phakinin), and aldehyde dehydrogenase. The complexes in the age-matched cataractous lenses also contained these crystallins, and aldehyde dehydrogenase, but not the beaded filament proteins. Further, the crystallin fragments were greater in number in the cataractous lenses compared to the age-matched normal lenses. During multi-angle light scattering (MALS), the HMW proteins from cataractous lenses exhibited species with lower molecular weight range than age-matched normal lenses. The HMW protein preparations from both normal and cataractous lenses showed spherical structures on electron microscopic analysis.

Differential recognition of natural and nonnatural substrate by molecular chaperone α-crystallin—A subunit exchange study

alpha-Crystallin is a molecular chaperone that recognizes proteins substrates in stress. It binds to the unstable conformer of a large variety of related or unrelated substrates and thus prevents them aggregating and holds them in a folding competent state. In this article, we have tried to critically analyze, from experimental point of view, whether alpha-crystallin has any preference for its natural substrates compared to the nonnatural one. Our results clearly show that alpha-crystallin is exceptionally active and sensitive in preventing aggregation of its natural substrates and can fully prevent such an aggregation in a substoichiometric ratio, but nonnatural substrates require a considerably higher amount of alpha-crystallin. Using suitable fluorescent-labeled alpha-crystallins and performing fluorescence resonance energy transfer experiments, we were able to determine the subunit exchange kinetics between the alpha-crystallin oligomers. It was found that while alpha-crystallin was bound to its natural substrate, the rate of subunit exchange was slightly decreased. But, when a nonnatural substrate carbonic anhydrase remained bound to the chaperone, further loss in subunit exchange rate was observed. Nonnatural substrate was found to create higher activation energy barrier for the subunit exchange reaction compared to the native substrates. Similarities in major beta-sheet structure of both alpha-crystallin and its natural substrates may be the reason for the preference in molecular recognition in comparison with the nonnatural substrate.

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

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

Decreased association of aged alpha crystallins with gamma crystallins

Previous studies have demonstrated non-covalent interactions of alpha crystallins with gamma crystallins, under true equilibrium conditions. These interactions could affect short-range interactions of lens crystallins that are necessary for the transparent properties of the lens. Since the transparent properties of the lens decrease during aging, it is possible that there are corresponding changes in the ability of aged alpha crystallins to interact with gamma crystallins. In the following study, alpha crystallins were prepared from fetal and aged bovine lenses, then tested for binding to gamma crystallins using microequilibrium dialysis. The results demonstrate that during aging of the normal bovine lens, there is a decrease in the ability of alpha crystallins to bind to gamma crystallins, consistent with the involvement of this interaction in the transparent properties of the lens.

Recombinant human alphaA-crystallin can protect the enzymatic activity of CpUDG against thermal inactivation

alpha-Crystallin is one of the major protein components in mammalian lens fiber cells. It is composed of alphaA and alphaB subunits that have structural homology to the family of mammalian small heat shock proteins. Horwitz firstly characterized native alpha-crystallin as a molecular chaperone in vitro based on its ability to prevent heat-induced aggregation of lens proteins and enzymes. Andley et al. cloned and expressed human alphaA-crystallin in Escherichia coli and confirmed its chaperone activity by suppression of thermal aggregation and singlet oxygen-induced opacification. Although alphaA-crystallin acts as a chaperone protein, there is no report showing on its ability to protect enzymes against thermal inactivation. Here, we present data showing that alphaA-crystallin can prevent thermal inactivation of CpUDG that catalyzes uracil removal from DNAs.

Casein proteins as molecular chaperones

Under conditions of stress, such as elevated temperature, molecular chaperones stabilize proteins from unfolding, aggregating, and precipitating. We have investigated the chaperone activity of the major milk proteins alpha(S)-, beta-, and kappa-casein with reduced insulin and the milk whey proteins, alpha-lactalbumin and beta-lactoglobulin, and compared it with that of the mammalian small heat shock protein (sHsp), alpha-crystallin, and clusterin. alpha(S)-Casein exhibited different chaperone behavior under reduction and heat stresses, i.e., chaperone activity increased with increasing temperature (as observed with alpha-crystallin), but under reduction stress, its chaperone activity increased at lower temperatures. beta- and kappa-casein had comparable chaperone ability with each other but were less effective than alpha(S)-casein. Under molecular crowding conditions, precipitation of stressed protein was accelerated, and alpha(S)-casein was a poorer chaperone. Furthermore, at slightly alkaline pH values, alpha(S)-casein was a less effective chaperone than at neutral pH. Detailed fluorescence, size exclusion chromatography, and real-time NMR studies studies indicated that the casein proteins underwent conformational changes and stabilized the partially unfolded whey proteins prior to formation of high molecular weight soluble complexes. These results are consistent with casein proteins acting as molecular chaperones in a manner similar to sHsps and clusterin.

Long-term lens organ culture system to determine age-related effects of UV irradiation on the eye lens

Aging of the eye lens represents the life-long accumulation of damage. Factors responsible for age-related cataract are unknown because medical evaluations of aged populations demonstrate a wide range of systemic diseases and medical disorders. There are some main suspected factors, which may contribute to accumulated age-related damage in the eye lens. (1) Diseases, such as diabetes, substantially increase the probability of cataract formation in the age group from 40 to 49, and double or triple this probability for ages 50 to 69. (2) Drugs, including systemic medications such as steroids. (3) Environmental factors, such as UV radiation, heat and electromagnetic radiation. Our study represents an effort to determine the effects of suspected cataractogenic factors on the eye lens. The experiments are performed using a unique long-term lens organ culture system of bovine lenses. In our system it is possible to give controlled amounts of insult and monitor changes in lens optical quality throughout the culture period of 8-15 days. The optical properties, monitored in association with biochemical analysis of lens epithelium, cortex and nuclear samples, help in determining the mechanisms of cataract formation. The present study investigates mechanisms by which UV-A radiation at 365 nm causes damage to the lens. It is believed that solar radiation is one of the major environmental factors involved in lens cataractogenesis. Bovine lenses were placed in our special culture cells for pre-incubation of 24 hr followed by irradiation of 29 or 33 J cm(-2). The lenses were maintained in the cells during irradiation. After irradiation, lens optical quality was monitored throughout the culture period and lens epithelium was taken for enzyme analysis. Using the culture system we learned that: (a) young lenses (less than one-year-old) are less sensitive to UV radiation than 3-year-old lenses; (b) the lenses have the ability to recover in organ culture conditions; (c) applying the insult in one step results in less damage than dividing the same insult in 4 steps with 24 hr interval between each one; and (d) the damage from UV is greater if the intervals between each irradiation stage are insufficient to permit full recovery.

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.

Enhancement of chaperone function of alpha-crystallin by methylglyoxal modification

The molecular chaperone function of alpha-crystallin in the lens prevents the aggregation and insolubilization of lens proteins that occur during the process of aging. We found that chemical modification of alpha-crystallin by a physiological alpha-dicarbonyl compound, methylglyoxal (MG), enhances its chaperone function. Protein-modifying sugars and ascorbate have no such effect and actually reduce chaperone function. Chaperone assay after immunoprecipitation or with immunoaffinity-purified argpyrimidine-alpha-crystallin indicates that 50-60% of the increased chaperone function is due to argpyrimidine-modified protein. Incubation of alpha-crystallin with DL-glyceraldehyde and arginine-modifying agents also enhances chaperone function, and we believe that the increased chaperone activity depends on the extent of arginine modification. Far- and near-UV circular dichroism spectra indicate modest changes in secondary and tertiary structure of MG-modified alpha-crystallin. LC MS/MS analysis of MG-modified alpha-crystallin following chymotryptic digestion revealed that R21, R49, and R103 in alphaA-crystallin were converted to argpyrimidine. 1,1'-Bis(4-anilino)naphthalene-5,5'-disulfonic acid binding, an indicator of hydrophobicity of proteins, increased in alpha-crystallin modified by low concentrations of MG (2-100 microM). MG similarly enhances chaperone function of another small heat shock protein, Hsp27. Our results show that posttranslational modification by a metabolic product can enhance the chaperone function of alpha-crystallin and Hsp27 and suggest that such modification may be a protective mechanism against environmental and metabolic stresses. Augmentation of the chaperone function of alpha-crystallin might have evolved to protect the lens from deleterious protein modifications associated with aging.

Identification of a region in alcohol dehydrogenase that binds to alpha-crystallin during chaperone action

alpha-Crystallin, the major eye lens protein and a member of the small heat-shock protein family, has been shown to protect the aggregation of several proteins and enzymes under denaturing conditions. The region(s) in the denaturing proteins that interact with alpha-crystallin during chaperone action has not been identified. Determination of these sites would explain the wide chaperoning action (promiscuity) of alpha-crystallin. In the present study, using two different methods, we have identified a sequence in yeast alcohol dehydrogenase (ADH) that binds to alpha-crystallin during chaperone-like action. The first method involved the incubation of alpha-crystallin with ADH peptides at 48 degrees C for 1 h followed by separation and analysis of bound peptides. In the second method, alpha-crystallin was first derivatized with a photoactive trifunctional cross-linker, sulfosuccinimidyl-2[6-(biotinamido)-2-(p-azidobenzamido)-hexanoamido]ethyl-1,3di-thiopropionate (sulfo-SBED), and then complexed with ADH at 48 degrees C for 1 h in the dark. The complex was photolyzed and digested with protease, and the biotinylated peptide fragments were isolated using an avidin column and then analyzed. The amino acid sequencing and mass spectral analysis revealed the sequence YSGVCHTDLHAWHGDWPLPVK (yeast ADH(40-60)) as the alpha-crystallin binding site in ADH. The interaction was further confirmed by demonstrating complex formation between alpha-crystallin and a synthetic peptide representing the binding site of ADH.

Subunit organization in cytoplasmic dynein subcomplexes

Because cytoplasmic dynein plays numerous critical roles in eukaryotic cells, determining the subunit composition and the organization and functions of the subunits within dynein are important goals. This has been difficult partly because of accessory polypeptide heterogeneity of dynein populations. The motor domain containing heavy chains of cytoplasmic dynein are associated with multiple intermediate, light intermediate, and light chain accessory polypeptides. We examined the organization of these subunits within cytoplasmic dynein by separating the molecule into two distinct subcomplexes. These subcomplexes were competent to reassemble into a molecule with dynein-like properties. One subcomplex was composed of the dynein heavy and light intermediate chains whereas the other subcomplex was composed of the intermediate and light chains. The intermediate and light chain subcomplex could be further separated into two pools, only one of which contained dynein light chains. The two pools had distinct intermediate chain compositions, suggesting that intermediate chain isoforms have different light chain-binding properties. When the two intermediate chain pools were characterized by analytical velocity sedimentation, at least four molecular components were seen: intermediate chain monomers, intermediate chain dimers, intermediate chain monomers with bound light chains, and a mixture of intermediate chain dimers with assorted bound light chains. These data provide new insights into the compositional heterogeneity and assembly of the cytoplasmic dynein complex and suggest that individual dynein molecules have distinct molecular compositions in vivo.

Effect of calpain on hereditary cataractous rat, ICR/f

The crystallins in the lenses of ICR/f mutation rat, a known hereditary cataract model, were analyzed during cataractogenesis. Opacification of the mutant lenses was found to be accompanied by changes in crystallin structure and composition, including several deletions of the N-terminals of beta-crystallins and low molecular weight alpha- crystallins. Because similar deletions were observed when the soluble fraction of normal lens protein was incubated with calpain, we considered that calpain could be related to the deletions in mutant lenses. Although measurement of the content of calpain protein by the ELISA method revealed no significant difference between mutant and normal lenses, it was found that the concentrations of Ca2+ and K+ were different between the two lenses and that calpain activity was dependent on both ion concentrations. Endogenous m-calpain in the soluble fraction from normal lenses was activated by addition of 1 mm calcium chloride in the presence of 50 mm KCl (the same concentration as in mutant lenses), and insoluble protein was found in the fraction 1 d after calpain activation. On the other hand, the presence of 120 mm KCl (the concentration in normal lenses) inhibited calpain activity and prevented this insolubilization. These results suggest that calpain in mutant lenses is involved in the proteolysis of crystallins and the progression of cataract formation.

Structural changes in alpha-synuclein affect its chaperone-like activity in vitro

Alpha-synuclein, a major constituent of Lewy bodies (LBs) in Parkinson's disease (PD), has been implicated to play a critical role in synaptic events, such as neuronal plasticity during development, learning, and degeneration under pathological conditions, although the physiological function of alpha-synuclein has not yet been established. We here present biochemical evidence that recombinant alpha-synuclein has a chaperone-like function against thermal and chemical stress in vitro. In our experiments, alpha-synuclein protected glutathione S-transferase (GST) and aldolase from heat-induced precipitation, and alpha-lactalbumin and bovine serum albumin from dithiothreitol (DTT)-induced precipitation like other molecular chaperones. Moreover, preheating of alpha-synuclein, which is believed to reorganize the molecular surface of alpha-synuclein, increased the chaperone-like activity. Interestingly, in organic solvents, which promotes the formation of secondary structure, alpha-synuclein aggregated more easily than in its native condition, which eventually might abrogate the chaperone-like function of the protein. In addition, alpha-synuclein was also rapidly and significantly precipitated by heat in the presence of Zn2+ in vitro, whereas it was not affected by the presence of Ca2+ or Mg2+. Circular dichroism spectra confirmed that alpha-synuclein underwent conformational change in the presence of Zn2+. Taken together, our data suggest that alpha-synuclein could act as a molecular chaperone, and that the conformational change of the alpha-synuclein could explain the aggregation kinetics of alpha-synuclein, which may be related to the abolishment of the chaperonic-like activity.

Complete protection by alpha-crystallin of lens sorbitol dehydrogenase undergoing thermal stress

Sorbitol dehydrogenase (l-iditol:NAD(+) 2-oxidoreductase, E.C. 1.1.1. 14) (SDH) was significantly protected from thermally induced inactivation and aggregation by bovine lens alpha-crystallin. An alpha-crystallin/SDH ratio as low as 1:2 in weight was sufficient to preserve the transparency of the enzyme solution kept for at least 2 h at 55 degrees C. Moreover, an alpha-crystallin/SDH ratio of 5:1 (w/w) was sufficient to preserve the enzyme activity fully at 55 degrees C for at least 40 min. The protection by alpha-crystallin of SDH activity was essentially unaffected by high ionic strength (i.e. 0.5 m NaCl). On the other hand, the transparency of the protein solution was lost at a high salt concentration because of the precipitation of the alpha-crystallin/SDH adduct. Magnesium and calcium ions present at millimolar concentrations antagonized the protective action exerted by alpha-crystallin against the thermally induced inactivation and aggregation of SDH. The lack of protection of alpha-crystallin against the inactivation of SDH induced at 55 degrees C by thiol blocking agents or EDTA together with the additive effect of NADH in stabilizing the enzyme in the presence of alpha-crystallin suggest that functional groups involved in catalysis are freely accessible in SDH while interacting with alpha-crystallin. Two different adducts between alpha-crystallin and SDH were isolated by gel filtration chromatography. One adduct was characterized by a high M(r) of approximately 800,000 and carried exclusively inactive SDH. A second adduct, carrying active SDH, had a size consistent with an interaction of the enzyme with monomers or low M(r) aggregates of alpha-crystallin. Even though it had a reduced efficiency with respect to alpha-crystallin, bovine serum albumin was shown to mimic the chaperone-like activity of alpha-crystallin in protecting SDH from thermal denaturation. These findings suggest that the multimeric structural organization of alpha-crystallin may not be a necessary requirement for the stabilization of the enzyme activity.

The small heat-shock chaperone protein, alpha-crystallin, does not recognize stable molten globule states of cytosolic proteins

The small heat-shock protein (sHsp), alpha-crystallin, acts as a molecular chaperone by interacting with destabilized 'substrate' proteins to prevent their precipitation from solution under conditions of stress. alpha-Crystallin and all sHsps are intracellular proteins. Similarly to other chaperones, the 'substrate' protein is in an intermediately folded, partly structured molten globule state when it interacts and complexes with alpha-crystallin. In this study, stable molten globule states of the cytosolic proteins, gamma-crystallin and myoglobin, have been prepared. Within the lens, gamma-crystallin naturally interacts with alpha-crystallin and myoglobin and alpha-crystallin are present together in muscle tissue. The molten globule states of gamma-crystallin and myoglobin were prepared by reacting gamma-crystallin with glucose 6-phosphate and by removing the haem group of myoglobin. Following spectroscopic characterisation of these modified proteins, their interaction with alpha-crystallin was examined by a variety of spectroscopic and protein chemical techniques. In both cases, there was no interaction with alpha-crystallin that led to complexation. It is concluded that alpha-crystallin does not recognise stable molten globule states of cytosolic 'substrate' proteins and only interacts with molten globule states of proteins that are on the irreversible pathway towards an aggregated and precipitated form.

3-Hydroxykynurenine and 3-hydroxyanthranilic acid generate hydrogen peroxide and promote alpha-crystallin cross-linking by metal ion reduction

The kynurenine pathway catabolite 3-hydroxykynurenine (3HK) and redox-active metals such as copper and iron are implicated in cataractogenesis. Here we investigate the reaction of kynurenine pathway catabolites with copper and iron, as well as interactions with the major lenticular structural proteins, the alpha-crystallins. The o-aminophenol kynurenine catabolites 3HK and 3-hydroxyanthranilic acid (3HAA) reduced Cu(II)>Fe(III) to Cu(I) and Fe(II), respectively, whereas quinolinic acid and the nonphenolic kynurenine catabolites kynurenine and anthranilic acid did not reduce either metal. Both 3HK and 3HAA generated superoxide and hydrogen peroxide in a copper-dependent manner. In addition, 3HK and 3HAA fostered copper-dependent alpha-crystallin cross-linking. 3HK- or 3HAA-modifed alpha-crystallin showed enhanced redox activity in comparison to unmodified alpha-crystallin or ascorbate-modified alpha-crystallin. These data support the possibility that 3HK and 3HAA may be cofactors in the oxidative damage of proteins, such as alpha-crystallin, through interactions with redox-active metals and especially copper. These findings may have relevance for understanding cataractogenesis and other degenerative conditions in which the kynurenine pathway is activated.

Mouse Hsp25, a small shock protein. The role of its C-terminal extension in oligomerization and chaperone action

Under conditions of cellular stress, small heat shock proteins (sHsps), e.g. Hsp25, stabilize unfolding proteins and prevent their precipitation from solution. 1H NMR spectroscopy has shown that mammalian sHsps possess short, polar and highly flexible C-terminal extensions. A mutant of mouse Hsp25 without this extension has been constructed. CD spectroscopy reveals some differences in secondary and tertiary structure between this mutant and the wild-type protein but analytical ultracentrifugation and electron microscopy show that the proteins have very similar oligomeric masses and quaternary structures. The mutant shows chaperone ability comparable to that of wild-type Hsp25 in a thermal aggregation assay using citrate synthase, but does not stabilize alpha-lactalbumin against precipitation following reduction with dithiothreitol. The accessible hydrophobic surface of the mutant protein is less than that of the wild-type protein and the mutant is also less stable at elevated temperature. 1H NMR spectroscopy reveals that deletion of the C-terminal extension of Hsp25 leads to induction of extra C-terminal flexibility in the molecule. Monitoring complex formation between Hsp25 and dithiothreitol-reduced alpha-lactalbumin by 1H NMR spectroscopy indicates that the C-terminal extension of Hsp25 retains its flexibility during this interaction. Overall, these data suggest that a highly flexible C-terminal extension in mammalian sHsps is required for full chaperone activity.

Characterization of alpha-crystallin-plasma membrane binding

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

Clusterin has chaperone-like activity similar to that of small heat shock proteins

Clusterin is a highly conserved protein which is expressed at increased levels by many cell types in response to a broad variety of stress conditions. A genuine physiological function for clusterin has not yet been established. The results presented here demonstrate for the first time that clusterin has chaperone-like activity. At physiological concentrations, clusterin potently protected glutathione S-transferase and catalase from heat-induced precipitation and alpha-lactalbumin and bovine serum albumin from precipitation induced by reduction with dithiothreitol. Enzyme-linked immunosorbent assay data showed that clusterin bound preferentially to heat-stressed glutathione S-transferase and to dithiothreitol-treated bovine serum albumin and alpha-lactalbumin. Size exclusion chromatography and SDS-polyacrylamide gel electrophoresis analyses showed that clusterin formed high molecular weight complexes (HMW) with all four proteins tested. Small heat shock proteins (sHSP) also act in this way to prevent protein precipitation and protect cells from heat and other stresses. The stoichiometric subunit molar ratios of clusterin:stressed protein during formation of HMW complexes (which for the four proteins tested ranged from 1.0:1.3 to 1.0:11) is less than the reported ratios for sHSP-mediated formation of HMW complexes (1.0:1.0 or greater), indicating that clusterin is a very efficient chaperone. Our results suggest that clusterin may play a sHSP-like role in cytoprotection.

Intermolecular interaction of lens crystallins: from rotationally mobile to immobile states at high protein concentrations

The conformation of lens crystallins in vivo or in a highly concentrated solution is not well established. Most studies were carried out in dilute solutions in which protein-protein interaction is minimal. In order to see whether there is conformational change (tertiary and secondary structures) when crystallin solutions are brought to high concentrations, we have performed the following molecular spectroscopic measurements: circular dichroism (CD) and Fourier transform infrared (FTIR). Near-UV CD measurements showed a more than two-fold increase in CD intensity (molar ellipticity) for the total water-soluble (WS) protein from young calf lens nucleus in a highly concentrated solution (> 300 mg/ml in a 0.01-mm cell), when compared with a dilute solution (1000-fold dilution in a 10-mm cell). The individual crystallins in concentrated solutions also showed an increase in CD intensity, but of different magnitude: alpha-crystallin > beta-crystallin > gamma-crystallin. The increased CD indicates that lens crystallins are in a more compact structure in highly concentrated solutions; they likely undergo a transition from a mobile to an immobile state. Change in near-UV CD usually is caused by restricted mobility of aromatic side groups, particularly Trp. The transition involves not only a change in protein tertiary and/or quaternary structure, but also in protein backbone structure. The change of protein backbone structure was drawn from FTIR measurements. FTIR spectra, sensitive to the secondary structure in the amide I region, could be measured for a highly concentrated solution for which far-UV CD measurement is not feasible. The secondary structure that showed prominent change for alpha-crystallin in a highly concentrated solution was beta-conformation: increase in beta-turn with a concomitant decrease of alpha-helix structure.

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

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

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

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

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

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

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

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

A spectroscopic study of glycated bovine alpha-crystallin: investigation of flexibility of the C-terminal extension, chaperone activity and evidence for diglycation

The effect of glycating the C-terminal extensions of alpha-crystallin on their flexibility was investigated. In the course of the study the reaction sites were identified and double glycation of single lysine residues was found. Alpha-crystallin was incubated until approximately one mole of the sugar had reacted per subunit of the crystallin. The reaction sites were investigated by mass spectrometry and H NMR spectroscopy, and were found to be principally in the short and flexible C-terminal extensions. The chaperone ability of alpha-crystallin was unaffected by this limited glycation. There was little effect on the flexibility of the C-terminal extensions. This result supports the view that the flexibility of the C-terminal extensions of alpha-crystallin is important for chaperone activity. As alpha-crystallin consists of a mixture of unmodified and phosphorylated subunits, a detailed investigation was undertaken of the reaction of galactose with peptides comprising the C-terminal extensions of alphaA- and alphaB-crystallin. The alphaA peptide was incubated with galactose until 0.79 mole of sugar was bound per mole of peptide and the alphaB peptide reacted until 2.2 moles of galactose had been incorporated. The purified glycated peptides were examined by NMR and mass spectrometry to identify glycation site(s), and the effect of glycation on the conformation of the peptides. For both peptides, it was found that extensive glycation of the constituent lysine residues occurred. The addition of two galactose molecules to some lysine residues of the peptides was also noted. This diglycation was confirmed in control experiments with N-acetyl-lysine.

Molecular evidence for the involvement of alpha crystallin in the colouration/crosslinking of crystallins in age-related nuclear cataract

The proteins of the lens which become insoluble, crosslinked and coloured as a result of the onset of human nuclear cataract have been studied using a combination of enzymatic digestion and HPLC/mass spectrometry (MS). The objective was to determine if such an approach could provide information on the identities of the polypeptide components involved in the colouration and crosslinking and to discover whether any crystallins predominate in this characteristic post-translational modification process. Initially, coloured high molecular weight peptides were isolated from a tryptic/chymotryptic digest of the 6 M guanidine hydrochloride-insoluble lens protein fraction. These tryptic/chymotryptic peptides were then incubated with pronase and the small peptides released, purified by gel filtration. All but one of the peptides analysed by HPLC/MS/MS were found to contain proline. Peptides derived from alpha-crystallin were found to comprise the great majority of the peptides characterised. No gamma-crystallin peptides were observed. Both alpha A-crystallin and alpha B-crystallin were represented. Further, all but one of these peptides were derived from the N-terminal region of the alpha-crystallin subunits: a region recently implicated in the chaperone activity of alpha-crystallin. This finding suggests that the putative N-terminal domain of alpha-crystallin may be involved at the molecular level in the process of crosslinking and colouration which is known to be characteristic of age-related nuclear cataract. It is, therefore, conceivable that an early stage of these cataractous modifications may involve alpha-crystallin acting as a molecular chaperone.