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

Study of The Molecular Nature of Congenital Cataracts in Patients from The Volga-Ural Region

Hereditary cataracts are characterized by significant clinical and genetic heterogeneity, which can pose challenges for early DNA diagnosis. To comprehensively address this problem, it is essential to investigate the epidemiology of the disease, perform population studies to determine the spectrum and frequencies of mutations in the responsible genes, and examine clinical and genetic correlations. Based on modern concepts, non-syndromic hereditary cataracts are predominantly caused by genetic disease forms associated with mutations in crystallin and connexin genes. Therefore, a comprehensive approach to studying hereditary cataracts is necessary for early diagnosis and improved treatment outcomes. The crystallin (CRYAA, CRYAB, CRYGC, CRYGD, and CRYBA1) and connexin (GJA8, GJA3) genes were analyzed in 45 unrelated families from the Volga-Ural Region (VUR) with hereditary congenital cataracts. Pathogenic and probably pathogenic nucleotide variants were identified in ten unrelated families, nine of which had cataracts in an autosomal dominant pattern of inheritance. Two previously undescribed likely pathogenic missense variants were identified in the CRYAA gene: c.253C > T (p.L85F) in one family and c.291C > G (p.H97Q) in two families. The known mutation c.272_274delGAG (p.G91del) was found in the CRYBA1 gene in one family, while no pathogenic variants were found in the CRYAB, CRYGC, or CRYGD genes in the examined patients. In the GJA8 gene, the known mutation c.68G > C (p.R23T) was found in two families, and previously undescribed variants were identified in two other families: a c.133_142del deletion (p.W45Sfs*72) and a missense variant, c.179G > A (p.G60D). In one patient with a recessive form of cataract, two compound-heterozygous variants were identified-a previously undescribed likely pathogenic missense variant, c.143A > G (p.E48G), and a known variant with uncertain pathogenetic significance, c.741T > G (p.I24M). Additionally, a previously undescribed deletion, c.del1126_1139 (p.D376Qfs*69), was identified in the GJA3 gene in one family. In all families where mutations were identified, cataracts were diagnosed either immediately after birth or during the first year of life. The clinical presentation of the cataracts varied depending on the type of lens opacity, resulting in various clinical forms. This information emphasizes the importance of early diagnosis and genetic testing for hereditary congenital cataracts to guide appropriate management and improve outcomes.

α‐crystallin: a review of its structure and function

alpha-crystallin, the major protein of the mammalian lens in most species, is an aggregate assembled from two polypeptides, each with a molecular weight around 20,000 Da. It is polydisperse and can be isolated in a variety of forms, including spherical particles with molecular weights ranging upwards from about 200 kDa. Sequence comparisons reveal that it is a member of the small heat shock protein (shsp) family. These proteins are aggregates assembled from polypeptides of 10 to 25 kDa that share a common central domain of about 90 residues (the 'alpha-crystallin domain') with variable N- and C-terminal extensions. alpha-crystallin has been intensively studied for more than 50 years but its three-dimensional structure remains unknown because it has not been possible to obtain crystals for X-ray studies and it is too large for NMR measurements. Structural information has been derived from a variety of solution studies. Because of the protein's polydispersity, interpretation of data has been difficult. This led to different viewpoints and vigorous debate on its structure and properties. Recently, the crystal structures of two closely-related small heat shock proteins have been determined. These have provided some insight into the structure of a-crystallin and explanations of previous observations. Like many other heat shock proteins, alpha-crystallin exhibits chaperone-like properties, including the ability to prevent the precipitation of denatured proteins and to increase cellular tolerance to stress. It has been suggested that these functions are important for the maintenance of lens transparency and the prevention of cataract.

Clinical characteristics of congenital lamellar cataract and myopia in a Chinese family

To investigate the clinical characteristics and the genetic defect in a Chinese family with congenital lamellar cataract with myopia. Three generations of a single family were recruited in the present study. A detailed family history and clinical data were recorded. A total of 100 unrelated ethnically matched controls without family history of congenital cataracts and myopia were also recruited. Genomic DNA was extracted from peripheral blood leukocytes. The sequencing of candidate genes was performed to screen out the disease-causing mutation. The effects of amino acid changes on the structure of proteins were predicted by bioinformatics analysis. Affected individuals presented lamellar lens opacities and myopia. Direct sequencing revealed a heterozygous c. 34 C>T variation in the αA-crystallin protein (CRYAA) gene, which resulted in the replacement of a highly conserved arginine by cystine at codon 12 (p.R12C). This mutation co-segregated with all affected individuals and was not observed in unaffected members or the 100 normal controls. Bioinformatic analysis showed that a highly conserved region was located around Arg12, an increase in local hydrophobicity was shown around the substitution site and the secondary structure of the mutant CRYAA protein has been changed. This is the case of a congenital lamellar cataract phenotype with myopia associated with the mutation of Arg12Cys (p.R12C) in CRYAA. Our finding confirms the high rate of mutations at this dinucleotide. In addition, these results demonstrate a myopia susceptibility locus in this region, which might also be associated with the mutation in CRYAA.

The impact of different mutations at arginine141 on the structure, subunit exchange dynamics and chaperone activity of Hsp16.3

Hsp16.3, a molecular chaperone, plays a vital role in the growth and survival of Mycobacterium tuberculosis inside the host. We previously reported that deletion of three amino acid residues (¹⁴² STN¹⁴⁴ ) from C-terminal extension (CTE) of Hsp16.3 triggers its structural perturbation and increases its chaperone activity, which reaches its apex upon the deletion of its entire CTE (¹⁴¹ RSTN¹⁴⁴ ). Thus, we hypothesized that Arg141 (R141) and Ser142 (S142) in the CTE of Hsp16.3 possibly hold the key in maintaining its native-like structure and chaperone activity. To test this hypothesis, we generated two deletion mutants in which R141 and S142 were deleted individually (Hsp16.3ΔR141 and Hsp16.3ΔS142) and three substitution mutants in which R141 was replaced by lysine (Hsp16.3R141K), alanine (Hsp16.3R141A), and glutamic acid (Hsp16.3R141E), respectively. Hsp16.3ΔS142 or Hsp16.3R141K mutant has native-like structure and chaperone activity. Deletion of R141 from the CTE (Hsp16.3ΔR141) perturbs the secondary and tertiary structure, lowers the subunit exchange dynamics and decreases the chaperone activity of Hsp16.3. But, the substitution of R141 with alanine (Hsp16.3R141A) or glutamic acid (Hsp16.3R141E) perturbs its secondary and tertiary structure. Surprisingly, such charge tampering of R141 enhances the subunit exchange dynamics and chaperone activity of Hsp16.3. Interestingly, neither the deletion of R141/S142 nor the substitution of R141 with lysine, alanine and glutamic acid affects the oligomeric mass/size of Hsp16.3. Overall, our study suggests that R141 (especially the positive charge on R141) plays a crucial role in maintaining the native-like structure as well as in regulating subunit exchange dynamics and chaperone activity of Hsp16.3.

Transgenic zebrafish models reveal distinct molecular mechanisms for cataract-linked αA-crystallin mutants

Mutations in the small heat shock proteins α-crystallins have been linked to autosomal dominant cataracts in humans. Extensive studies in vitro have revealed a spectrum of alterations to the structure and function of these proteins including shifts in the size of the oligomer, modulation of subunit exchange and modification of their affinity to client proteins. Although mouse models of these mutants were instrumental in identifying changes in cellular proliferation and lens development, a direct comparative analysis of their effects on lens proteostasis has not been performed. Here, we have transgenically expressed cataract-linked mutants of αA- and αB-crystallin in the zebrafish lens to dissect the underlying molecular changes that contribute to the loss of lens optical properties. Zebrafish lines expressing these mutants displayed a range of morphological lens defects. Phenotype penetrance and severity were dependent on the mutation even in fish lines lacking endogenous α-crystallin. The mechanistic origins of these differences were investigated by the transgenic co-expression of a destabilized human γD-crystallin mutant. We found that the R49C but not the R116C mutant of αA-crystallin drove aggregation of γD-crystallin, although both mutants have similar affinity to client proteins in vitro. Our working model attributes these differences to the propensity of R49C, located in the buried N-terminal domain of αA-crystallin, to disulfide crosslinking as previously demonstrated in vitro. Our findings complement and extend previous work in mouse models and emphasize the need of investigating chaperone/client protein interactions in appropriate cellular context.

Initial characterization of newly identified mitochondrial and chloroplast small HSPs from sugarcane shows that these chaperones have different oligomerization states and substrate specificities

Chaperones belonging to the small heat shock protein (sHSP) family are ubiquitous and exhibit elevated expression under stresses conditions to protect proteins against aggregation, thereby contributing to the stress tolerance of the organism. Tropical plants are constantly exposed to high temperatures, and the mechanisms by which these plants tolerate heat stress are of foremost importance to basic science as well as applied agrobiotechnology. Therefore, this study aims to characterize sHSPs from different organelles from sugarcane, an important crop that is associated with sugar and bioenergy production. An expression sequence tag database of sugarcane was searched, and sHsp genes of mitochondrial and chloroplast organelles were selected and cloned. The proteins were expressed in Escherichia coli and isolated and purified by two chromatographic steps with high purity as single species. Circular dichroism and fluorescence spectroscopy showed that both proteins were purified in their folded states with a predominant β-sheet secondary structure. Determination of the molecular weight, diffusion coefficient and Stokes radius parameters showed that both chaperones form large spherical-like oligomers in solution. The two sHSPs had different oligomeric states and substrate specificities. The mitochondrial sHSP was a 20-mer with ability to protect model substrates that differ from that of the 16-meric sHSP from chloroplasts. These results indicate that both sHSPs are key agents to protect against stress confirming the importance of the great diversity of sHSP chaperones in plants for homeostasis maintenance. Moreover, to our knowledge, this is the first report about small HSPs from sugarcane organelles.

Engineering of a Polydisperse Small Heat-Shock Protein Reveals Conserved Motifs of Oligomer Plasticity

Small heat-shock proteins (sHSPs) are molecular chaperones that bind partially and globally unfolded states of their client proteins. Previously, we discovered that the archaeal Hsp16.5, which forms ordered and symmetric 24-subunit oligomers, can be engineered to transition to an ordered and symmetric 48-subunit oligomer by insertion of a peptide from human HspB1 (Hsp27). Here, we uncovered the existence of an array of oligomeric states (30-38 subunits) that can be populated as a consequence of altering the sequence and length of the inserted peptide. Polydisperse Hsp16.5 oligomers displayed higher affinity to a model client protein consistent with a general mechanism for recognition and binding that involves increased access of the hydrophobic N-terminal region. Our findings, which integrate structural and functional analyses from evolutionarily distant sHSPs, support a model wherein the modular architecture of these proteins encodes motifs of oligomer polydispersity, dissociation, and expansion to achieve functional diversity and regulation.

Immunodominant protein MIP_05962 from Mycobacterium indicus pranii displays chaperone activity

Tuberculosis, a contagious disease of infectious origin is currently a major cause of deaths worldwide. Mycobacterium indicus pranii (MIP), a saprophytic nonpathogen and a potent immunomodulator is currently being investigated as an intervention against tuberculosis along with many other diseases with positive outcome. The apparent paradox of multiple chaperones in mycobacterial species and enigma about the cellular functions of the client proteins of these chaperones need to be explored. Chaperones are the known immunomodulators; thus, there is need to exploit the proteome of MIP for identification and characterization of putative chaperones. One of the immunogenic proteins, MIP_05962 is a member of heat shock protein (HSP) 20 family due to the presence of α-crystallin domain, and has amino acid similarity with Mycobacterium lepraeHSP18 protein. The diverse functions of M. lepraeHSP18 in stress conditions implicate MIP_05962 as an important protein that needs to be explored. Biophysical and biochemical characterization of the said protein proved it to be a chaperone. The observations of aggregation prevention and refolding of substrate proteins in the presence of MIP_05962 along with interaction with non-native proteins, surface hydrophobicity, formation of large oligomers, in-vivo thermal rescue of Escherichia coli expressing MIP_05962, enhancing solubility of insoluble protein maltodextrin glucosidase (MalZ) under in-vivo conditions, and thermal stability and reversibility confirmed MIP_05962 as a molecular chaperone.

Structure, Chaperone Activity, and Aggregation of Wild-Type and R12C Mutant αB-Crystallins in the Presence of Thermal Stress and Calcium Ion - Implications for Role of Calcium in Cataract Pathogenesis

The current study was performed with the aim to evaluate the chaperoning ability, structural features, and aggregation propensity of wild-type and R12C mutant αB-crystallins (αB-Cry) under thermal stress and in the presence of calcium ion. The results of different spectroscopic analyses suggest that wild-type and mutant αB-Cry have dissimilar secondary and tertiary structures. Moreover, αB-Cry indicates slightly improved chaperone activity upon the R12C mutation. Thermal stress and calcium, respectively, enhance and reduce the extent of solvent-exposed hydrophobic surfaces accompanying formation of ordered and non-ordered aggregate entities in both proteins. Compared to the wild-type protein, the R12C mutant counterpart shows significant resistance against thermal and calcium-induced aggregation. In addition, in the presence of calcium, significant structural variation was accompanied by reduction in the solvent-exposed hydrophobic patches and attenuation of chaperone activity in both proteins. Additionally, gel mobility shift assay indicates the intrinsic propensity of R12C mutant αB-Cry for disulfide bridge-mediated protein dimerization. Overall, the results of this study are of high significance for understanding the molecular details of different factors that are involved in the pathomechanism of cataract disorders.

Species-Specific Structural and Functional Divergence of α-Crystallins: Zebrafish αBa- and Rodent αA(ins)-Crystallin Encode Activated Chaperones

In addition to contributing to lens optical properties, the α-crystallins are small heat shock proteins that possess chaperone activity and are predicted to bind and sequester destabilized proteins to delay cataract formation. The current model of α-crystallin chaperone mechanism envisions a transition from the native oligomer to an activated form that has higher affinity to non-native states of the substrate. Previous studies have suggested that this oligomeric plasticity is encoded in the primary sequence and controls access to high affinity binding sites within the N-terminal domain. Here, we further examined the role of sequence variation in the context of species-specific α-crystallins from rat and zebrafish. Alternative splicing of the αA gene in rodents produces αA(ins), which is distinguished by a longer N-terminal domain. The zebrafish genome includes duplicate αB-crystallin genes, αBa and αBb, which display divergent primary sequence and tissue expression patterns. Equilibrium binding experiments were employed to quantitatively define chaperone interactions with a destabilized model substrate, T4 lysozyme. In combination with multiangle light scattering, we show that rat αA(ins) and zebrafish α-crystallins display distinct global structural properties and chaperone activities. Notably, we find that αA(ins) and αBa demonstrate substantially enhanced chaperone function relative to other α-crystallins, binding the same substrate more than 2 orders of magnitude higher affinity and mimicking the activity of fully activated mammalian small heat shock proteins. These results emphasize the role of sequence divergence as an evolutionary strategy to tune chaperone function to the requirements of the tissues and organisms in which they are expressed.

Role of Subunit Exchange and Electrostatic Interactions on the Chaperone Activity of Mycobacterium leprae HSP18

Mycobacterium leprae HSP18, a major immunodominant antigen of M. leprae pathogen, is a small heat shock protein. Previously, we reported that HSP18 is a molecular chaperone that prevents aggregation of different chemically and thermally stressed client proteins and assists refolding of denatured enzyme at normal temperature. We also demonstrated that it can efficiently prevent the thermal killing of E. coli at higher temperature. However, molecular mechanism behind the chaperone function of HSP18 is still unclear. Therefore, we studied the structure and chaperone function of HSP18 at normal temperature (25°C) as well as at higher temperatures (31–43°C). Our study revealed that the chaperone function of HSP18 is enhanced significantly with increasing temperature. Far- and near-UV CD experiments suggested that its secondary and tertiary structure remain intact in this temperature range (25–43°C). Besides, temperature has no effect on the static oligomeric size of this protein. Subunit exchange study demonstrated that subunits of HSP18 exchange at 25°C with a rate constant of 0.018 min^-1. Both rate of subunit exchange and chaperone activity of HSP18 is found to increase with rise in temperature. However, the surface hydrophobicity of HSP18 decreases markedly upon heating and has no correlation with its chaperone function in this temperature range. Furthermore, we observed that HSP18 exhibits diminished chaperone function in the presence of NaCl at 25°C. At elevated temperatures, weakening of interactions between HSP18 and stressed client proteins in the presence of NaCl results in greater reduction of its chaperone function. The oligomeric size, rate of subunit exchange and structural stability of HSP18 were also found to decrease when electrostatic interactions were weakened. These results clearly indicated that subunit exchange and electrostatic interactions play a major role in the chaperone function of HSP18.

Exploring protein solution structure: Second moments of fluorescent spectra report heterogeneity of tryptophan rotamers

Trp fluorescent spectra appear as a log-normal function but are usually analyzed with λmax, full width at half maximum, and the first moment of incomplete spectra. Log-normal analyses have successfully separated fluorescence contributions from some multi-Trp proteins but deviations were observed in single Trp proteins. The possibility that disparate rotamer environments might account for these deviations was explored by moment spectral analysis of single Trp mutants spanning the sequence of tear lipocalin as a model. The analysis required full width Trp spectra. Composite spectra were constructed using log-normal analysis to derive the inaccessible blue edge, and the experimentally obtained spectra for the remainder. First moments of the composite spectra reflected the site-resolved secondary structure. Second moments were most sensitive for spectral deviations. A novel parameter, derived from the difference of the second moments of composite and simulated log-normal spectra correlated with known multiple heterogeneous rotamer conformations. Buried and restricted side chains showed the most heterogeneity. Analyses applied to other proteins further validated the method. The rotamer heterogeneity values could be rationalized by known conformational properties of Trp residues and the distribution of nearby charged groups according to the internal Stark effect. Spectral heterogeneity fits the rotamer model but does not preclude other contributing factors. Spectral moment analysis of full width Trp emission spectra is accessible to most laboratories. The calculations are informative of protein structure and can be adapted to study dynamic processes.

Differential role of arginine mutations on the structure and functions of α-crystallin

BACKGROUND

α-Crystallin is a major protein of the eye lens in vertebrates. It is composed of two subunits, αA- and αB-crystallin. α-Crystallin is an oligomeric protein having these two subunits in 3:1 ratio. It belongs to small heat shock protein family and exhibits molecular chaperone function, which plays an important role in maintaining the lens transparency. Apart from chaperone function, both subunits also exhibit anti-apoptotic property. Comparison of their primary sequences reveals that αA- and αB-crystallin posses 13 and 14 arginine residues, respectively. Several of them undergo mutations which eventually lead to various eye diseases such as congenital cataract, juvenile cataract, and retinal degeneration. Interestingly, many arginine residues of these subunits are modified during glycation and even some are truncated during aging. All these facts indicate the importance of arginine residues in α-crystallin.

SCOPE OF REVIEW

In this review, we will emphasize the recent in vitro and in vivo findings related to congenital cataract causing arginine mutations in α-crystallin.

MAJOR CONCLUSIONS

Congenital cataract causing arginine mutations alters the structure and decreases the chaperone function of α-crystallin. These mutations also affect the lens morphology and phenotypes. Interestingly, non-natural arginine mutations (generated for mimicking the glycation and truncation environment) improve the chaperone function of α-crystallin which may play an important role in maintaining the eye lens transparency during aging.

GENERAL SIGNIFICANCE

The neutralization of positive charge on the guanidino group of arginine residues is not always detrimental to the functionality of α-crystallin. This article is part of a Special Issue entitled Crystallin Biochemistry in Health and Disease.

Small heat-shock proteins: important players in regulating cellular proteostasis

Small heat-shock proteins (sHsps) are a diverse family of intra-cellular molecular chaperone proteins that play a critical role in mitigating and preventing protein aggregation under stress conditions such as elevated temperature, oxidation and infection. In doing so, they assist in the maintenance of protein homeostasis (proteostasis) thereby avoiding the deleterious effects that result from loss of protein function and/or protein aggregation. The chaperone properties of sHsps are therefore employed extensively in many tissues to prevent the development of diseases associated with protein aggregation. Significant progress has been made of late in understanding the structure and chaperone mechanism of sHsps. In this review, we discuss some of these advances, with a focus on mammalian sHsp hetero-oligomerisation, the mechanism by which sHsps act as molecular chaperones to prevent both amorphous and fibrillar protein aggregation, and the role of post-translational modifications in sHsp chaperone function, particularly in the context of disease.

Cryoelectron microscopy analysis of small heat shock protein 16.5 (Hsp16.5) complexes with T4 lysozyme reveals the structural basis of multimode binding

Small heat shock proteins (sHSPs) are ubiquitous chaperones that bind and sequester non-native proteins preventing their aggregation. Despite extensive studies of sHSPs chaperone activity, the location of the bound substrate within the sHSP oligomer has not been determined. In this paper, we used cryoelectron microscopy (cryoEM) to visualize destabilized mutants of T4 lysozyme (T4L) bound to engineered variants of the small heat shock protein Hsp16.5. In contrast to wild type Hsp16.5, binding of T4L to these variants does not induce oligomer heterogeneity enabling cryoEM analysis of the complexes. CryoEM image reconstruction reveals the sequestration of T4L in the interior of the Hsp16.5 oligomer primarily interacting with the buried N-terminal domain but also tethered by contacts with the α-crystallin domain shell. Analysis of Hsp16.5-WT/T4L complexes uncovers oligomer expansion as a requirement for high affinity binding. In contrast, a low affinity mode of binding is found to involve T4L binding on the outer surface of the oligomer bridging the formation of large complexes of Hsp16.5. These mechanistic principles were validated by cryoEM analysis of an expanded variant of Hsp16.5 in complex with T4L and Hsp16.5-R107G, which is equivalent to a mutant of human αB-crystallin linked to cardiomyopathy. In both cases, high affinity binding is found to involve conformational changes in the N-terminal region consistent with a central role of this region in substrate recognition.

Crystal structure of an activated variant of small heat shock protein Hsp16.5

How does the sequence of a single small heat shock protein (sHSP) assemble into oligomers of different sizes? To gain insight into the underlying structural mechanism, we determined the crystal structure of an engineered variant of Methanocaldococcus jannaschii Hsp16.5 wherein a 14 amino acid peptide from human heat shock protein 27 (Hsp27) was inserted at the junction of the N-terminal region and the α-crystallin domain. In response to this insertion, the oligomer shell expands from 24 to 48 subunits while maintaining octahedral symmetry. Oligomer rearrangement does not alter the fold of the conserved α-crystallin domain nor does it disturb the interface holding the dimeric building block together. Rather, the flexible C-terminal tail of Hsp16.5 changes its orientation relative to the α-crystallin domain which enables alternative packing of dimers. This change in orientation preserves a peptide-in-groove interaction of the C-terminal tail with an adjacent β-sandwich, thereby holding the assembly together. The interior of the expanded oligomer, where substrates presumably bind, retains its predominantly nonpolar character relative to the outside surface. New large windows in the outer shell provide increased access to these substrate-binding regions, thus accounting for the higher affinity of this variant to substrates. Oligomer polydispersity regulates sHSPs chaperone activity in vitro and has been implicated in their physiological roles. The structural mechanism of Hsp16.5 oligomer flexibility revealed here, which is likely to be highly conserved across the sHSP superfamily, explains the relationship between oligomer expansion observed in disease-linked mutants and changes in chaperone activity.

The family of mammalian small heat shock proteins (HSPBs): implications in protein deposit diseases and motor neuropathies

A number of neurological and muscular disorders are characterized by the accumulation of aggregate-prone proteins and are referred to as protein deposit or protein conformation diseases. Besides some sporadic forms, most of them are genetically inherited in an autosomal dominant manner, although recessive forms also exist. Although genetically very heterogeneous, some of these diseases are the result of mutations in some members of the mammalian small heat shock protein family (sHSP/HSPB), which are key players of the protein quality control system and participate, together with other molecular chaperones and co-chaperones, in the maintenance of protein homeostasis. Thus, on one hand upregulation of specific members of the HSPB family can exert protective effects in protein deposit diseases, such as the polyglutamine diseases. On the other hand, mutations in the HSPBs lead to neurological and muscular disorders, which may be due to a loss-of-function in protein quality control and/or to a gain-of-toxic function, resulting from the aggregation-proneness of the mutants. In this review we summarize the current knowledge about some of the best characterized functions of the HSPBs (e.g. role in cytoskeleton stabilization, chaperone function, anti-aggregation and anti-apoptotic activities), also highlighting differences in the properties of the various HSPBs and how these may counteract protein aggregation diseases. We also describe the mutations in the various HSPBs associated with neurological and muscular disorders and we discuss how gain-of-toxic function mechanisms (e.g. due to the mutated HSPB protein instability and aggregation) and/or loss-of-function mechanisms can contribute to HSPB-associated pathologies. This article is part of a Directed Issue entitled: Small HSPs in physiology and pathology.

Sequence, structure, and dynamic determinants of Hsp27 (HspB1) equilibrium dissociation are encoded by the N-terminal domain

Human small heat shock protein 27 (Hsp27) undergoes concentration-dependent equilibrium dissociation from an ensemble of large oligomers to a dimer. This phenomenon plays a critical role in Hsp27 chaperone activity in vitro enabling high affinity binding to destabilized proteins. In vivo dissociation, which is regulated by phosphorylation, controls Hsp27 role in signaling pathways. In this study, we explore the sequence determinants of Hsp27 dissociation and define the structural basis underlying the increased affinity of Hsp27 dimers to client proteins. A systematic cysteine mutagenesis is carried out to identify residues in the N-terminal domain important for the equilibrium between Hsp27 oligomers and dimers. In addition, spin-labels were attached to the cysteine mutants to enable electron paramagnetic resonance (EPR) analysis of residue environment and solvent accessibility in the context of the large oligomers, upon dissociation to the dimer, and following complex formation with the model substrate T4 Lysozyme (T4L). The mutagenic analysis identifies residues that modulate the equilibrium dissociation in favor of the dimer. EPR analysis reveals that oligomer dissociation disrupts subunit contacts leading to the exposure of Hsp27 N-terminal domain to the aqueous solvent. Moreover, regions of this domain are highly dynamic with no evidence of a packed core. Interaction between T4L and sequences in this domain is inferred from transition of spin-labels to a buried environment in the substrate/Hsp27 complex. Together, the data provide the first structural analysis of sHSP dissociation and support a model of chaperone activity wherein unstructured and highly flexible regions in the N-terminal domain are critical for substrate binding.

Cataract-linked γD-crystallin mutants have weak affinity to lens chaperones α-crystallins

To test the hypothesis that α-crystallin chaperone activity plays a central role in maintenance of lens transparency, we investigated its interactions with γ-crystallin mutants that cause congenital cataract in mouse models. Although the two substitutions, I4F and V76D, stabilize a partially unfolded γD-crystallin intermediate, their affinities to α-crystallin are marginal even at relatively high concentrations. Detectable binding required further reduction of γD-crystallin stability which was achieved by combining the two mutations. Our results demonstrate that mutants and possibly age-damaged γ-crystallin can escape quality control by lens chaperones rationalizing the observation that they nucleate protein aggregation and lead to cataract.

Identification of the p. R116H mutation in a Chinese family with novel variable cataract phenotype: evidence for a mutational hot spot in αA-crystallin gene

PURPOSE

To report the recurrent p.R116H mutation in the αA-crystallin gene (CRYAA) which causes a novel variable cataract phenotype, and to determine whether this mutation represents a mutational hot spot.

METHODS

Family history and clinical data were recorded. The genomic DNA was extracted from peripheral blood leukocytes. Microsatellite markers at loci considered to be associated with autosomal dominant cataracts were selected and genotyped for two-point linkage analysis. Direct sequencing was performed to identify the disease-causing mutation. Haplotype analysis was constructed to compare the affected haplotype in this family and in another Chinese family previously reported by us.

RESULTS

Clinical features of cataract in this family were asymmetric in two eyes of some affected subjects. Evidence of linkage was obtained with marker D21S1411 (logarithm of odds [LOD] score [Z] = 2.42, recombination fraction [θ] = 0.0). Sequencing of the candidate CRYAA gene revealed a single base alteration c.347 G > A in exon 3, which resulted in the substitution of highly conserved arginine by histidine at codon 116 (p.R116H). This mutation co-segregated with all affected individuals and was not observed in unaffected family members or 100 normal unrelated individuals. The comparative haplotype analysis showed that the affected haplotypes in the two families were different.

CONCLUSIONS

This study identified a novel cataract-microcornea phenotype caused by the recurrent mutation p.R116H in CRYAA, and suggested that this mutation site is not likely the consequence of a founder effect, but probably a result of a mutational hot spot.

Non-3D domain swapped crystal structure of truncated zebrafish alphaA crystallin

In previous work on truncated alpha crystallins (Laganowsky et al., Protein Sci 2010; 19:1031-1043), we determined crystal structures of the alpha crystallin core, a seven beta-stranded immunoglobulin-like domain, with its conserved C-terminal extension. These extensions swap into neighboring cores forming oligomeric assemblies. The extension is palindromic in sequence, binding in either of two directions. Here, we report the crystal structure of a truncated alphaA crystallin (AAC) from zebrafish (Danio rerio) revealing C-terminal extensions in a non three-dimensional (3D) domain swapped, "closed" state. The extension is quasi-palindromic, bound within its own zebrafish core domain, lying in the opposite direction to that of bovine AAC, which is bound within an adjacent core domain (Laganowsky et al., Protein Sci 2010; 19:1031-1043). Our findings establish that the C-terminal extension of alpha crystallin proteins can be either 3D domain swapped or non-3D domain swapped. This duality provides another molecular mechanism for alpha crystallin proteins to maintain the polydispersity that is crucial for eye lens transparency.

Characterization of a mutant R11H αB-crystallin associated with human inherited cataract

αB-Crystallin plays an important part in cataract development. A novel mutation (R11H) was previously detected by our group. In the present study, we set out to investigate the possible molecular mechanism by which the R11H mutation causes cataract. We found that the mutant αB-crystallin exhibits folding defects, decreased surface hydrophobicity and enhanced chaperone-like activity compared with the wild-type αB-crystallin. The mutant protein shows nearly the same molecular mass and thermal stability as the wild-type form. Transfection studies revealed that the R11H mutant was remarkably similar to the wild-type protein in its subcellular distribution, but has an abnormal ability to induce cell apoptosis. These results suggest that the changes in hydrophobic exposure and the abnormal ability to induce programmed cell death of the mutant protein are likely to be responsible for the onset of cataract.

Algorithm for selection of optimized EPR distance restraints for de novo protein structure determination

A hybrid protein structure determination approach combining sparse Electron Paramagnetic Resonance (EPR) distance restraints and Rosetta de novo protein folding has been previously demonstrated to yield high quality models (Alexander et al. (2008)). However, widespread application of this methodology to proteins of unknown structures is hindered by the lack of a general strategy to place spin label pairs in the primary sequence. In this work, we report the development of an algorithm that optimally selects spin labeling positions for the purpose of distance measurements by EPR. For the α-helical subdomain of T4 lysozyme (T4L), simulated restraints that maximize sequence separation between the two spin labels while simultaneously ensuring pairwise connectivity of secondary structure elements yielded vastly improved models by Rosetta folding. 54% of all these models have the correct fold compared to only 21% and 8% correctly folded models when randomly placed restraints or no restraints are used, respectively. Moreover, the improvements in model quality require a limited number of optimized restraints, which is determined by the pairwise connectivities of T4L α-helices. The predicted improvement in Rosetta model quality was verified by experimental determination of distances between spin labels pairs selected by the algorithm. Overall, our results reinforce the rationale for the combined use of sparse EPR distance restraints and de novo folding. By alleviating the experimental bottleneck associated with restraint selection, this algorithm sets the stage for extending computational structure determination to larger, traditionally elusive protein topologies of critical structural and biochemical importance.

Crystal structures of truncated alphaA and alphaB crystallins reveal structural mechanisms of polydispersity important for eye lens function

Small heat shock proteins alphaA and alphaB crystallin form highly polydisperse oligomers that frustrate protein aggregation, crystallization, and amyloid formation. Here, we present the crystal structures of truncated forms of bovine alphaA crystallin (AAC(59-163)) and human alphaB crystallin (ABC(68-162)), both containing the C-terminal extension that functions in chaperone action and oligomeric assembly. In both structures, the C-terminal extensions swap into neighboring molecules, creating runaway domain swaps. This interface, termed DS, enables crystallin polydispersity because the C-terminal extension is palindromic and thereby allows the formation of equivalent residue interactions in both directions. That is, we observe that the extension binds in opposite directions at the DS interfaces of AAC(59-163) and ABC(68-162). A second dimeric interface, termed AP, also enables polydispersity by forming an antiparallel beta sheet with three distinct registration shifts. These two polymorphic interfaces enforce polydispersity of alpha crystallin. This evolved polydispersity suggests molecular mechanisms for chaperone action and for prevention of crystallization, both necessary for transparency of eye lenses.

The pivotal role of the beta 7 strand in the intersubunit contacts of different human small heat shock proteins

Human alpha B-crystallin and small heat shock proteins HspB6 and HspB8 were mutated so that all endogenous Cys residues were replaced by Ser and the single Cys residue was inserted in a position homologous to that of Cys137 of human HspB1, i.e. in a position presumably located in the central part of beta 7 strand of the alpha-crystallin domain. The secondary, tertiary, and quaternary structures of thus obtained Cys-mutants as well as their chaperone-like activity were similar to those of their wild-type counterparts. Mild oxidation of Cys-mutants leads to formation of disulfide bond crosslinking neighboring monomers thus indicating participation of the beta 7 strand in intersubunit interaction. Oxidation weakly affects the secondary and tertiary structure, does not affect the quaternary structure of alpha B-crystallin and HspB6, and shifts equilibrium between monomer and dimer of HspB8 towards dimer formation. It is concluded that the beta 7 strand participates in the intersubunit interaction of four human small heat shock proteins (alpha B-crystallin, HspB1, HspB6, HspB8) having different structure of beta2 strand of alpha-crystallin domain and different length and composition of variable N- and C-terminal tails.

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

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

Glutamic acid residues in the C-terminal extension of small heat shock protein 25 are critical for structural and functional integrity

Small heat shock proteins (sHsps) are intracellular molecular chaperones that prevent the aggregation and precipitation of partially folded and destabilized proteins. sHsps comprise an evolutionarily conserved region of 80-100 amino acids, denoted the alpha-crystallin domain, which is flanked by regions of variable sequence and length: the N-terminal domain and the C-terminal extension. Although the two domains are known to be involved in the organization of the quaternary structure of sHsps and interaction with their target proteins, the role of the C-terminal extension is enigmatic. Despite the lack of sequence similarity, the C-terminal extension of mammalian sHsps is typically a short, polar segment which is unstructured and highly flexible and protrudes from the oligomeric structure. Both the polarity and flexibility of the C-terminal extension are important for the maintenance of sHsp solubility and for complexation with its target protein. In this study, mutants of murine Hsp25 were prepared in which the glutamic acid residues in the C-terminal extension at positions 190, 199 and 204 were each replaced with alanine. The mutants were found to be structurally altered and functionally impaired. Although there were no significant differences in the environment of tryptophan residues in the N-terminal domain or in the overall secondary structure, an increase in exposed hydrophobicity was observed for the mutants compared with wild-type Hsp25. The average molecular masses of the E199A and E204A mutants were comparable with that of the wild-type protein, whereas the E190A mutant was marginally smaller. All mutants displayed markedly reduced thermostability and chaperone activity compared with the wild-type. It is concluded that each of the glutamic acid residues in the C-terminal extension is important for Hsp25 to act as an effective molecular chaperone.

Mechanism of cataract formation in alphaA-crystallin Y118D mutation

PURPOSE

The aim of this study was to elucidate the molecular mechanisms that lead to a dominant nuclear cataract in a mouse harboring the Y118D mutation in the alphaA-crystallin gene.

METHODS

The physicochemical properties of alpha-crystallin obtained from mouse lenses with the Y118D mutation as well as a recombinant Y118D alphaA-crystallin were studied using gel filtration, two-dimensional (2D) gel electrophoresis, multi-angle light scattering, circular dichroism, fluorescence, and chaperone activities.

RESULTS

Both native alpha-crystallin from mutant lens and recombinant alphaA-Y118D displayed higher molecular mass distribution than the wild-type. Circular dichroism spectra indicated changes in the secondary structures of alphaA-Y118D. The alphaA-Y118D protein prevented nonspecific protein aggregation more effectively than wild-type alphaA-crystallin. The gel filtration and 2D gel electrophoresis analysis showed a significant reduction of Y118D mutant protein in comparison with wild-type alphaA protein of heterozygous mutant lenses. Quantitative RT-PCR results confirmed a decrease in alphaA and alphaB transcripts in the homozygous mutant alpha A(Y118D/Y118D) lenses.

CONCLUSIONS

The alphaA-Y118D mutant protein itself displays an increased chaperone-like activity. However, the dominant nuclear cataract is associated with a significant decrease in the amount of alphaA-crystallin, leading to a reduction in total chaperone capacity needed for maintaining lens transparency.

Disulfide cross-links in the interaction of a cataract-linked alphaA-crystallin mutant with betaB1-crystallin

A number of alphaA-crystallin mutants are associated with hereditary cataract including cysteine substitution at arginine 49. We report the formation of affinity-driven disulfide bonds in the interaction of alphaA-R49C with betaB1-crystallin. To mimic cysteine thiolation in the lens, betaB1-crystallin was modified by a bimane probe through a disulfide linkage. Our data suggest a mechanism whereby a transient disulfide bond occurs between alphaA- and betaB1-crystallin followed by a disulfide exchange with cysteine 49 of a neighboring alphaA-crystallin subunit. This is the first investigation of disulfide bonds in the confine of the chaperone/substrate complex where reaction rates are favored by orders of magnitude. Covalent protein cross-links are a hallmark of age-related cataract and may be a factor in its inherited form.

alphaB-crystallin: a hybrid solid-state/solution-state NMR investigation reveals structural aspects of the heterogeneous oligomer

Atomic-level structural information on alphaB-Crystallin (alphaB), a prominent member of the small heat-shock protein family, has been a challenge to obtain due its polydisperse oligomeric nature. We show that magic-angle spinning solid-state NMR can be used to obtain high-resolution information on an approximately 580-kDa human alphaB assembled from 175-residue 20-kDa subunits. An approximately 100-residue alpha-crystallin domain is common to all small heat-shock proteins, and solution-state NMR was performed on two different alpha-crystallin domain constructs isolated from alphaB. In vitro, the chaperone-like activities of full-length alphaB and the isolated alpha-crystallin domain are identical. Chemical shifts of the backbone and C(beta) resonances have been obtained for residues 64-162 (alpha-crystallin domain plus part of the C-terminus) in alphaB and the isolated alpha-crystallin domain by solid-state and solution-state NMR, respectively. Both sets of data strongly predict six beta-strands in the alpha-crystallin domain. A majority of residues in the alpha-crystallin domain have similar chemical shifts in both solid-state and solution-state, indicating similar structures for the domain in its isolated and oligomeric forms. Sites of intersubunit interaction are identified from chemical shift differences that cluster to specific regions of the alpha-crystallin domain. Multiple signals are observed for the resonances of M68 in the oligomer, identifying the region containing this residue as existing in heterogeneous environments within alphaB. Evidence for a novel dimerization motif in the human alpha-crystallin domain is obtained by a comparison of (i) solid-state and solution-state chemical shift data and (ii) (1)H-(15)N heteronuclear single quantum coherence spectra as a function of pH. The isolated alpha-crystallin domain undergoes a dimer-monomer transition over the pH range 7.5-6.8. This steep pH-dependent switch may be important for alphaB to function optimally (e.g., to preserve the filament integrity of cardiac muscle proteins such as actin and desmin during cardiac ischemia, which is accompanied by acidosis).

Alpha crystallin: the quest for a homogeneous quaternary structure

Alpha A and alpha B crystallins are key members of the small heat-shock protein family. In addition to being a major structural protein of the lens, they are constitutively found in many other cells, where their function is not completely understood. Alpha B crystallin is also known to be over-expressed in many neurological diseases. To date, all efforts to crystallize alpha A or alpha B have failed. Thus, high-resolution data on the tertiary and quaternary structures of alpha crystallin is not available. The main reason for this failure seems to be the polydisperse nature of alpha crystallin. This review deals mainly with the polydisperse properties of alpha crystallin and the influence of post-translational modification, chemical modifications, truncations and mutation on its quaternary structure.

Ligand binding site of tear lipocalin: contribution of a trigonal cluster of charged residues probed by 8-anilino-1-naphthalenesulfonic acid

Human tear lipocalin (TL) exhibits diverse functions, most of which are linked to ligand binding. To map the binding site of TL for some amphiphilic ligands, we capitalized on the hydrophobic and hydrophilic properties of 8-anilino-1-naphthalenesulfonic acid (ANS). In single Trp mutants, resonance energy transfer from Trp to ANS indicates that the naphthalene group of ANS is proximate to Leu105 in the cavity. Binding energies of TL to ANS and its analogues reveal contributions from electrostatic interactions. The sulfonate group of ANS interacts strongly with the nonconserved intracavitary residue Lys114 and less with neighboring residues His84 and Glu34. This trigonal cluster of residues may play a role in the ligand recognition site for some negatively charged ligands. Because many drugs possess sulfonate groups, the trigonal cluster-sulfonate interaction can also be exploited as a lipocalin-based drug delivery mechanism. The binding of lauric acid and its analogues shows that fatty acids assume heterogeneous orientations in the cavity of TL. Predominantly, the hydrocarbon tail is buried in the cavity of TL and the carboxyl group is oriented toward the mouth. However, TL can also interact, albeit relatively weakly, with fatty acids oriented in the opposite direction. As the major lipid binding protein of tears, the ability to accommodate fatty acids in two opposing orientations may have functional implications for TL. At the aqueous-lipid interface, fatty acids whose carboxyl groups are positioned toward the aqueous phase are available for interaction with TL that could augment stability of the tear film.

The necessity of functional proteomics: protein species and molecular function elucidation exemplified by in vivo alpha A crystallin N-terminal truncation

Ten years after the establishment of the term proteome, the science surrounding it has yet to fulfill its potential. While a host of technologies have generated lists of protein names, there are only a few reported studies that have examined the individual proteins at the covalent chemical level defined as protein species in 1997 and their function. In the current study, we demonstrate that this is possible with two-dimensional gel electrophoresis (2-DE) and mass spectrometry by presenting clear evidence of in vivo N-terminal alpha A crystallin truncation and relating this newly detected protein species to alpha crystallin activity regulation by protease cleavage in the healthy young murine lens. We assess the present state of technology and suggest a shift in resources and paradigm for the routine attainment of the protein species level in proteomics.

Mechanism of a Hereditary Cataract Phenotype

We present a novel hypothesis for the molecular mechanism of autosomal dominant cataract linked to two mutations in the alphaA-crystallin gene of the ocular lens. AlphaA-crystallin is a molecular chaperone that plays a critical role in the suppression of protein aggregation and hence in the long term maintenance of lens optical properties. Using a steady state binding assay in which the chaperone-substrate complex is directly detected, we demonstrate that the mutations result in a substantial increase in the level of binding to non-native states of the model substrate T4 lysozyme. The structural basis of the enhanced binding is investigated through equivalent substitutions in the homologous heat shock protein 27. The mutations shift the oligomeric equilibrium toward a dissociated multimeric form previously shown to be the binding-competent state. In the context of a recent thermodynamic model of chaperone function that proposes the coupling of small heat shock protein activation to the substrate folding equilibrium (Shashidharamurthy, R., Koteiche, H. A., Dong, J., and McHaourab, H. S. (2005) J. Biol. Chem. 280, 5281-5289), the enhanced binding by the alphaA-crystallin mutants is predicted to shift the substrate folding equilibrium toward non-native intermediates, i.e. the mutants promote substrate unfolding. Given the high concentration of alphaA-crystallin in the lens, the molecular basis of pathogenesis implied by our results is a gain of function that leads to the binding of undamaged proteins and subsequent precipitation of the saturated alpha-crystallin complexes in the developing lens of affected individuals.

Structural and functional roles for beta-strand 7 in the alpha-crystallin domain of p26, a polydisperse small heat shock protein from Artemia franciscana

Oviparous development in the extremophile crustacean, Artemia franciscana, generates encysted embryos which enter a profound state of dormancy, termed diapause. Encystment is marked by the synthesis of p26, a polydisperse small heat shock protein thought to protect embryos from stress. In order to elucidate structural/functional relationships within p26 and other polydisperse small heat shock proteins, and to better define the protein's role during diapause, amino acid substitutions R110G, F112R, R114A and Y116D were generated within the p26 alpha-crystallin domain by site-directed mutagenesis. These residues were chosen because they are highly conserved across species boundaries, and molecular modelling indicates that they are part of a key structural interface between dimers. The F112R mutation, which had the greatest impact on oligomerization, placed two charged residues at the p26 dimer-dimer interface, demonstrating the importance of beta-strand 7 in tetramer formation. All mutated versions of p26 were less able than wild-type p26 to confer thermotolerance on transformed bacteria and they exhibited diminished chaperone action in three in vitro assays; however, all variants retained protective activity. This apparent stability of p26 may, by prolonging effective chaperone life in vivo, enhance embryo stress resistance. All substitutions modified p26 intrinsic fluorescence, surface hydrophobicity and secondary structure, and the pronounced changes in variant R114A, as indicated by these physical measurements, correlated with the greatest loss of function. Although mutation R114A had the greatest effect on p26 chaperoning, it had the least on oligomerization. These results demonstrate that in contrast to many other small heat shock proteins, p26 effectiveness as a chaperone is independent of oligomerization. The results also reinforce the idea, occasioned by modelling, that R114 is removed slightly from dimer-dimer interfaces. Moreover, beta-strand 7 is shown to have an important role in oligomerization of p26, a function first proposed for this structural element upon crystallization of wheat Hsp16.9, a small heat shock protein with different quaternary structure.

Influence of the disulfide bond configuration on the dynamics of the spin label attached to cytochrome c

A series of multi-nanosecond molecular dynamics (MD) simulations of wild-type cytochrome c and its spin-labeled variants with the methanethiosulfonate moiety attached at position C102 were performed (1) to elucidate the effect of the spin probe presence on the protein structure and (2) to describe the structure and dynamics of the spin-label moiety. Comparisons with the reference crystal structure of cytochrome c (PDB entry: 1YCC) indicate that the protein secondary structure is well preserved during simulations of the wild-type cytochrome c but slightly changed in simulations of the cytochrome c labeled at position C102. At the time scale covered in our simulations, the spin label exhibits highly dynamical behavior. The number of observed distinct conformations of the spin label moiety is between 3 and 13. The spin probe was found to form short-lived hydrogen bonds with the protein. Temporary hydrophobic interactions between the probe and the protein were also detected. The MD simulations directly show that the disulfide bond in the tether linking a spin probe with a protein strongly influence the behavior of the nitroxide group. The conformational flexibility and interaction with the protein are different for each of the two low energy conformations of the disulfide bond.

N-terminal control of small heat shock protein oligomerization: changes in aggregate size and chaperone-like function

The small heat shock protein superfamily is composed of proteins from throughout the phylogenetic spectrum that are induced upon environmental stress. Their structural stability under stress derives in large part from the central region of the proteins, which forms two beta sheets held together by hydrophobic interactions and appears to be present in all superfamily members. The length, sequence, and amino acid composition of the N- and C-terminals, in contrast, are quite variable. The role of the N-terminal has been hypothesized to control species-specific assembly of subunits into higher level structures. To test this, a set of constructs was designed and expressed: the N-terminal sequences preceding the start of the core regions of alphaA-crystallin and HSP 16.5 from Methanococcus jannaschii were swapped; the N-terminal of each protein was removed, and replaced with a brief N-terminal extension sequence; and two nonsense N-terminal sequences of approximately the same length and hydropathicity as the original replaced the alphaA-crystallin N-terminal. All constructs, plus the original recombinant sequences, could be overexpressed except for the 16.5 N-terminal extension, and all showed chaperone-like activity except for the hybrid with the 16.5 C-terminal. Size and properties of the replacement N-terminal place limits on aggregate size. Additional restrictions are imposed by the structure of the dimer.

R120G alphaB-crystallin promotes the unfolding of reduced alpha-lactalbumin and is inherently unstable

alpha-Crystallin is the principal lens protein which, in addition to its structural role, also acts as a molecular chaperone, to prevent aggregation and precipitation of other lens proteins. One of its two subunits, alphaB-crystallin, is also expressed in many nonlenticular tissues, and a natural missense mutation, R120G, has been associated with cataract and desmin-related myopathy, a disorder of skeletal muscles [Vicart P, Caron A, Guicheney P, Li Z, Prevost MC, Faure A, Chateau D, Chapon F, Tome F, Dupret JM, Paulin D & Fardeau M (1998) Nat Genet20, 92-95]. In the present study, real-time 1H-NMR spectroscopy showed that the ability of R120G alphaB-crystallin to stabilize the partially folded, molten globule state of alpha-lactalbumin was significantly reduced in comparison with wild-type alphaB-crystallin. The mutant showed enhanced interaction with, and promoted unfolding of, reduced alpha-lactalbumin, but showed limited chaperone activity for other target proteins. Using NMR spectroscopy, gel electrophoresis, and MS, we observed that, unlike the wild-type protein, R120G alphaB-crystallin is intrinsically unstable in solution, with unfolding of the protein over time leading to aggregation and progressive truncation from the C-terminus. Light scattering, MS, and size-exclusion chromatography data indicated that R120G alphaB-crystallin exists as a larger oligomer than wild-type alphaB-crystallin, and its size increases with time. It is likely that removal of the positive charge from R120 of alphaB-crystallin causes partial unfolding, increased exposure of hydrophobic regions, and enhances its susceptibility to proteolysis, thus reducing its solubility and promoting its aggregation and complexation with other proteins. These characteristics may explain the involvement of R120G alphaB-crystallin with human disease states.

Deamidation affects structural and functional properties of human alphaA-crystallin and its oligomerization with alphaB-crystallin

To determine the effects of deamidation on structural and functional properties of alphaA-crystallin, three mutants (N101D, N123D, and N101D/N123D) were generated. Deamidated alphaB-crystallin mutants (N78D, N146D, and N78D/N146D), characterized in a previous study (Gupta, R., and Srivastava, O. P. (2004) Invest. Ophthalmol. Vis. Sci. 45, 206-214) were also used. The biophysical and chaperone properties were determined in (a) homoaggregates of alphaA mutants (N101D, N123D, and N101D/N123D) and (b) reconstituted heteroaggregates of alpha-crystallin containing (i) wild type alphaA (WT-alphaA): WT-alphaB crystallins, (ii) individual alphaA-deamidated mutants:WT-alphaB crystallins, and (iii) WT-alphaA:individual alphaB-deamidated mutant crystallins. Compared with the WT-alphaA, the three alphaA-deamidated mutants showed reduced levels of chaperone activity, alterations in secondary and tertiary structures, and larger aggregates. These altered properties were relatively more pronounced in the mutant N101D compared with the mutant N123D. Further, compared with heteroaggregates of WT-alphaA and WT-alphaB, the heteroaggregates containing deamidated subunits of either alphaA- or alphaB-crystallins and their counterpart WT proteins showed higher molecular mass, altered tertiary structures, lower exposed hydrophobic surfaces, and reduced chaperone activity. However, the heteroaggregate containing WT-alphaA and deamidated alphaB subunit showed lower chaperone activity, smaller oligomers, and 3-fold lower subunit exchange rate than heteroaggregate containing deamidated alphaA- and WT-alphaB subunits. Together, the results suggested that (a) both Asn residues (Asn-101 and Asn-123) are required for the structural integrity and chaperone function of alphaA-crystallin and (b) the presence of WT-alphaB in the alpha-crystallin heteroaggregate leads to packing-induced structural changes which influences the oligomerization and modulate chaperone activity.

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

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

Role of the Conserved SRLFDQFFG Region of α-Crystallin, a Small Heat Shock Protein

Small heat shock proteins (sHsps) are necessary for several cellular functions and in stress tolerance. Most sHsps are oligomers; intersubunit interactions leading to changes in oligomeric structure and exposure of specific regions may modulate their functioning. Many sHsps, including alpha A- and alpha B-crystallin, contain a well conserved SRLFDQFFG sequence motif in the N-terminal region. Sequence-based prediction shows that it exhibits helical propensity with amphipathic character, suggesting that it plays a critical role in the structure and function of alpha-crystallins. In order to investigate the role of this motif in the structure and function of sHsps, we have made constructs deleting this sequence from alpha A- and alpha B-crystallin, overexpressed, purified, and studied these engineered proteins. Circular dichroism spectroscopic studies show changes in tertiary and secondary structure on deletion of the sequence. Glycerol density gradient centrifugation and dynamic light scattering studies show that the multimeric size of the mutant proteins is significantly reduced, indicating a role for this motif in higher order organization of the subunits. Both deletion mutants exhibit similar oligomeric size and increased chaperone-like activity. Urea-induced denaturation study shows that the SRLFDQFFG sequence contributes significantly to the structural stability. Fluorescence resonance energy transfer studies show that the rate of exchange of the subunits in the alpha Adel-crystallin oligomer is higher compared with that in the alpha A-crystallin oligomer, suggesting that this region contributes to the oligomer dynamics in addition to the higher order assembly and structural stability. Thus, our study shows that the SRLFDQFFG sequence is one of the critical motifs in structure-function regulation of alpha A- and alpha B-crystallin.

Structural and functional defects caused by point mutations in the alpha-crystallin domain of a bacterial alpha-heat shock protein

The diverse family of alpha-crystallin-type small heat shock proteins (alpha-Hsps or sHsps) is characterised by a central, moderately conserved alpha-crystallin domain. Oligomerisation followed by dissociation of subparticles is thought to be a prerequisite for chaperone function. We demonstrate that HspH, a bacterial alpha-Hsp from the soybean-symbiont Bradyrhizobium japonicum, assembles into dynamic complexes freely exchanging subunits with homologous and heterologous complexes. The importance of the alpha-crystallin domain for oligomerisation and chaperone activity was tested by site-directed mutagenesis of 12 different residues. In contrast to mammalian alpha-Hsps, the majority of these mutations elicited severe structural and functional defects in HspH. The individual exchange of five amino acid residues throughout the alpha-crystallin domain was found to compromise oligomerisation to various degrees. Assembly defects resulting in complexes of reduced size correlated with greatly decreased or abolished chaperone activity, reinforcing that complete oligomerisation is required for functionality. Mutation of a highly conserved glycine (G114) at the C-terminal end of the alpha-crystallin domain specifically impaired chaperone activity without interfering with oligomerisation properties, indicating that this residue is critical for substrate interaction. The structural and functional importance of this and other residues is discussed in the context of a modeled three-dimensional structure of HspH.

Role of the C-terminal extensions of alpha-crystallins. Swapping the C-terminal extension of alpha-crystallin to alphaB-crystallin results in enhanced chaperone activity

Several small heat shock proteins contain a well conserved alpha-crystallin domain, flanked by an N-terminal domain and a C-terminal extension, both of which vary in length and sequence. The structural and functional role of the C-terminal extension of small heat shock proteins, particularly of alphaA- and alphaB-crystallins, is not well understood. We have swapped the C-terminal extensions between alphaA- and alphaB-crystallins and generated two novel chimeric proteins, alphaABc and alphaBAc. We have investigated the domain-swapped chimeras for structural and functional alterations. We have used thermal and non-thermal models of protein aggregation and found that the chimeric alphaB with the C-terminal extension of alphaA-crystallin, alphaBAc, exhibits dramatically enhanced chaperone-like activity. Interestingly, however, the chimeric alphaA with the C-terminal extension of alphaB-crystallin, alphaABc, has almost lost its activity. Pyrene solubilization and bis-1-anilino-8-naphthalenesulfonate binding studies show that alphaBAc exhibits more solvent-exposed hydrophobic pockets than alphaA, alphaB, or alphaABc. Significant tertiary structural changes are revealed by tryptophan fluorescence and near-UV CD studies upon swapping the C-terminal extensions. The far-UV CD spectrum of alphaBAc differs from that of alphaB-crystallin whereas that of alphaABc overlaps with that of alphaA-crystallin. Gel filtration chromatography shows alteration in the size of the proteins upon swapping the C-terminal extensions. Our study demonstrates that the unstructured C-terminal extensions play a crucial role in the structure and chaperone activity, in addition to generally believed electrostatic "solubilizer" function.

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

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

Subunit exchange, conformational stability, and chaperone-like function of the small heat shock protein 16.5 from Methanococcus jannaschii

Hsp16.5, isolated from the hyperthermophilic Archaea Methanococcus jannaschii, is a member of the small heat-shock protein family. Small Hsps have 12- to 42-kDa subunit sizes and have sequences that are conserved among all organisms. The recently determined crystal structure of Hsp16.5 indicates that it consists discretely of 24 identical subunits. Using fluorescence resonance energy transfer, we show that at temperatures above 60 degrees C, the subunits of MjHsp16.5 freely and reversibly exchange with a rate constant of exchange at 68 degrees C of 0.067 min(-1). The subunit exchange reactions were strongly temperature-dependent, similar to the exchange reactions of the alpha-crystallins. The exchange reaction was specific to MjHsp16.5 subunits, as other sHsps such as alpha-crystallin were not structurally compatible and could not integrate into the MjHsp16.5 oligomer. In addition, we demonstrate that at temperatures as high as 70 degrees C, MjHsp16.5 retains its multimeric structure and subunit organization. Using insulin and alpha-lactalbumin as model target proteins, we also show that MjHsp16.5 at 37 degrees C is a markedly inefficient chaperone compared with other sHsps with these substrates. The results of this study support the hypothesis that MjHsp16.5 has a dynamic quaternary structure at temperatures that are physiologically relevant to M. jannaschii.

Effects of modifications of alpha-crystallin on its chaperone and other properties

The role of alpha-crystallin, a small heat-shock protein and chaperone, may explain how the lens stays transparent for so long. alpha-Crystallin prevents the aggregation of other lens crystallins and proteins that have become unfolded by 'trapping' the protein in a high-molecular-mass complex. However, during aging, the chaperone function of alpha-crystallin becomes compromised, allowing the formation of light-scattering aggregates that can proceed to form cataracts. Within the central part of the lens there is no turnover of damaged protein, and therefore post-translational modifications of alpha-crystallin accumulate that can reduce chaperone function; this is compounded in cataract lenses. Extensive in vitro glycation, carbamylation and oxidation all decrease chaperone ability. In the present study, we report the effect of the modifiers malondialdehyde, acetaldehyde and methylglyoxal, all of which are pertinent to cataract. Also modification by aspirin, which is known to delay cataract and other diseases, has been investigated. Recently, two point mutations of arginine residues were shown to cause congenital cataract. 1,2-Cyclohexanedione modifies arginine residues, and the extent of modification needed for a change in chaperone function was investigated. Only methylglyoxal and extensive modification by 1,2-cyclohexanedione caused a decrease in chaperone function. This highlights the robust nature of alpha-crystallin.

The determinants of the oligomeric structure in Hsp16.5 are encoded in the alpha-crystallin domain

The determinants of the oligomeric assembly of Hsp16.5, a small heat-shock protein (sHSP) from Methanococcus jannaschii, were explored via site-directed truncation and site-directed spin labeling. For this purpose, subunit contacts around the two-, three- and four-fold symmetry axes were fingerprinted using patterns of proximities between nitroxide spin labels introduced at selected sites. The lack of change in this fingerprint in an N-terminal truncation of the protein demonstrates that the interactions are encoded in the alpha-crystallin domain. In contrast, the truncation of the N-terminal domain of Mycobacterium tuberculosis Hsp16.3, a bacterial sHSP with an equally short N-terminal region, results in the dissociation of the oligomer to a trimer. These results, in conjunction with those from previous truncation studies in mammalian sHSP, suggest that as the alpha-crystallin domain evolved to encode a smaller basic unit than the overall oligomer, the control of the assembly and dynamics of the oligomeric structure became encoded in the N-terminal domain.