Domain structure and evolution in alpha-crystallins and small heat-shock proteins

The α-crystallin Chaperones Undergo a Quasi-ordered Co-aggregation Process in Response to Saturating Client Interaction

Small heat shock proteins (sHSPs) are ATP-independent chaperones vital to cellular proteostasis, preventing protein aggregation events linked to various human diseases including cataract. The α-crystallins, αA-crystallin (αAc) and αB-crystallin (αBc), represent archetypal sHSPs that exhibit complex polydispersed oligomeric assemblies and rapid subunit exchange dynamics. Yet, our understanding of how this plasticity contributes to chaperone function remains poorly understood. Using biochemical and biophysical analyses combined with single-particle electron microscopy (EM), we examined structural changes in αAc, αBc and native heteromeric lens α-crystallins (αLc) in their apo-states and at varying degree of chaperone saturation leading to co-aggregation, using lysozyme and insulin as model clients. Quantitative single-particle analysis unveiled a continuous spectrum of oligomeric states formed during the co-aggregation process, marked by significant client-triggered expansion and quasi-ordered elongation of the sHSP oligomeric scaffold, whereby the native cage-like sHSP assembly displays a directional growth to accommodate saturating conditions of client sequestration. These structural modifications culminated in an apparent amorphous collapse of chaperone-client complexes, resulting in the creation of co-aggregates capable of scattering visible light. Intriguingly, these co-aggregates maintain internal morphological features of highly elongated sHSP oligomers with striking resemblance to polymeric α-crystallin species isolated from aged lens tissue. This mechanism appears consistent across αAc, αBc and αLc, albeit with varying degrees of susceptibility to client-induced co-aggregation. Importantly, our findings suggest that client-induced co-aggregation follows a distinctive mechanistic and quasi-ordered trajectory, distinct from a purely amorphous process. These insights reshape our understanding of the physiological and pathophysiological co-aggregation processes of α-crystallins, carrying potential implications for a pathway toward cataract formation.

The α-crystallin chaperones undergo a quasi-ordered co-aggregation process in response to saturating client interaction

Small heat shock proteins (sHSPs) are ATP-independent chaperones vital to cellular proteostasis, preventing protein aggregation events linked to various human diseases including cataract. The α-crystallins, αA-crystallin (αAc) and αB-crystallin (αBc), represent archetypal sHSPs that exhibit complex polydispersed oligomeric assemblies and rapid subunit exchange dynamics. Yet, our understanding of how this plasticity contributes to chaperone function remains poorly understood. This study investigates structural changes in αAc and αBc during client sequestration under varying degree of chaperone saturation. Using biochemical and biophysical analyses combined with single-particle electron microscopy (EM), we examined αAc and αBc in their apo-states and at various stages of client-induced co-aggregation, using lysozyme as a model client. Quantitative single-particle analysis unveiled a continuous spectrum of oligomeric states formed during the co-aggregation process, marked by significant client-triggered expansion and quasi-ordered elongation of the sHSP scaffold. These structural modifications culminated in an apparent amorphous collapse of chaperone-client complexes, resulting in the creation of co-aggregates capable of scattering visible light. Intriguingly, these co-aggregates maintain internal morphological features of highly elongated sHSP scaffolding with striking resemblance to polymeric α-crystallin species isolated from aged lens tissue. This mechanism appears consistent across both αAc and αBc, albeit with varying degrees of susceptibility to client-induced co-aggregation. Importantly, our findings suggest that client-induced co-aggregation follows a distinctive mechanistic and quasi-ordered trajectory, distinct from a purely amorphous process. These insights reshape our understanding of the physiological and pathophysiological co-aggregation processes of sHSPs, carrying potential implications for a pathway toward cataract formation.

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.

Investigating the role of double mutations R12C/P20R, and R12C/R69C on structure, chaperone-like activity, and amyloidogenic properties of human αB-crystallin

α-crystallin is a structurally essential small heat shock protein (sHSP) with a chaperone-like activity which maintains transparency of the lenticular tissues during a period of time that is as long as human life. α-crystallin is a multimeric protein consisting of αA and αB subunits, with 57 % homology. The CRYAB gene on chromosome 11 encodes human αB-crystallin (αB-Cry), which contains 175 amino acid residues. In the current study, the cataractogenic mutations R12C, P20R, R69C, and double mutations R12C/P20R and R12C/P20R were embedded into the human CRYAB gene. Following successful expression in the prokaryotic system and purification, a number of spectroscopic techniques, gel electrophoresis, dynamic light scattering (DLS), and transmission electron microscopy (TEM) were applied to assess the role of these mutations on the structure, amyloidogenicity, and biological function of human αB-Cry. The created mutations caused significant changes in the structure, and oligomeric state of human αB-Cry. These mutations, particularly R12C, R12C/P20R, and R12C/R69C, dramatically enhanced the tendency of this protein for the amyloid fibril formation and reduced its chaperone-like activity. Since double mutations R12C/P20R and R12C/P20R were able to intensely change the protein's structure and chaperone function, it can be suggested that they may play a destructive role in a cumulative manner. Our findings indicated that the simultaneous presence of two pathogenic mutations may have a cumulative destructive impacts on the structure and function of human αB-Cry and this observation is likely related to the disease severity of the mutated proteins.

The impact of Hydrogen peroxide on structure, stability and functional properties of Human R12C mutant αA-crystallin: The imperative insights into pathomechanism of the associated congenital cataract incidence

The oxidative stress in eye lens which occurs during inflammation and under chronic hyperglycemia has been already indicated in the pathogenesis of cataract disorders. The aim of this study was to examine structural and functional properties of R12C mutant αA-Crystallin (αA-Cry) in the presence of hydrogen peroxide. The study was done using different spectroscopic techniques and gel mobility shift assay. According to results of our study, H2O2 oxidation strongly compromises the chaperone function of the R12C mutant but not of wild-type αA-Cry. Also, it affects the structural properties of both wild-type and mutant proteins, albeit to different degree. The H2O2 exposure promotes extensive disulfide mediated oligomerization of the R12C mutant but not of the wild-type as revealed by gel mobility shift assay and dynamic light scattering. Moreover, in the presence of hydrogen peroxide, the mutant protein demonstrates severe conformational and protease instability and increased amyloidogenic propensity. The obtained results suggest that incubation of R12C mutant recombinant αA-Cry with hydrogen peroxide accelerates the molecular events which have been already implicated in the pathomechanism of cataract development. Taken together these results suggest that individuals carrying the R12C mutation are at an increased risk to develop early-onset cataract under condition of oxidative stress.

Different transcriptional responses of heat shock protein 20 in the marine diatom Ditylum brightwellii exposed to metals and endocrine-disrupting chemicals

Diatoms are sensitive indicators of water quality, and hence used for environmental hazard assessments; however, their toxicogenomic studies have been insufficiently attempted. In the present study, we determined the cDNA sequence of heat shock protein 20 (Hsp20) gene from the diatom Ditylum brightwellii, and examined the transcriptional responses of the gene after exposing it to environmental stressors such as thermal shock, metals, and endocrine-disrupting chemicals (EDCs). The open reading frame (ORF) of DbHsp20 was 531 bp long, encoding 177 amino acid residues (19.49 kDa) with a conserved C-terminal and α-crystallin domain. The genomic region of DbHsp20 did not contain introns. Phylogeny of eukaryotic Hsp20s showed D. brightwellii was closely related to other diatoms. With regard to the gene expressional profile, real-time PCR results showed that the gene was significantly upregulated (P < 0.001) under thermal stress, with the highest change of 3.2-fold. Metals' (copper and nickel) treatments showed that it was induced after a certain point of treated concentration. On the contrary, EDCs did not display noticeable change on the expression of DbHsp20. These results suggest that the diatom Hsp20 basically responds to thermal stress, but may differentially respond to toxic substances such as metals and organic compounds such as EDCs.

Quaternary structural parameters of the congenital cataract causing mutants of αA-crystallin

Pediatric cataract of the congenital type is the most common form of childhood blindness and it is clinically and genetically heterogeneous. Mutations in 22 different genes have been identified to be associated with congenital cataracts, and among them, eight mutants belong to αA-crystallin. To explain how mutations in αA-crystallin lead to the development of cataract, quaternary structural parameters, and chaperone function have been investigated in αA-wt and in the following mutants: R12C, R21L, R21W, R49C, R54C, R116C, and R116H. Average molar mass, mass at the RI peak, mass across the peak, hydrodynamic radius (R(h)), and polydispersity index (PDI) were determined by dynamic light-scattering measurements. The average molar mass and mass across the peak showed major increase in R116C and R116H, moderate increase in R12C, R21W, and R54C, and no increase in R21L and R49C as compared to αA-wt. PDI and R(h) values were significantly increased only in R116C and R116H. Significant secondary structural changes, as determined by CD measurements, were seen in R21W, R21L, R116C, and R116H, and tertiary structural changes were evident in R21W, R54C, R116C, and R116H. Non-reducing SDS-PAGE has shown the presence of dimers presumably formed by inter-polypeptide disulfide bonds. Chaperone activity, as measured with ADH as the target protein, appeared normal in R49C and R54C, while R12C, R21L, and R21W showed moderate loss and R116C and R116H showed significant loss. Although a specific change in the αA-crystallin behavior that is common to all the mutants was not evident, each mutant showed one or more perturbation as the end effect that leads to cataract.

Heat shock proteins in the human eye

Heat shock proteins (Hsps) are believed to primarily protect and maintain cell viability under stressful conditions such as those occurring during thermal and oxidative challenges chiefly by refolding and stabilizing proteins. Hsps are found throughout the various tissues of the eye where they are thought to confer protection from disease states such as cataract, glaucoma, and cancer. This minireview summarizes the placement, properties, and roles of Hsps in the eye and aims to provide a better comprehension of their function and involvement in ocular disease pathogenesis.

Functional insights by comparison of modeled structures of 18kDa small heat shock protein and its mutant in Mycobacterium leprae

In this work we are proposing Homology modeled structures of Mycobacterium leprae 18kDa heat shock protein and its mutant. The more closely related structure of the small heat shock protein (sHSP) belonging to the eukaryotic species from wheat sHSP16.9 and 16.3kDa ACR1 protein from Mycobacterium tuberculosis were used as template structures. Each model contains an N-terminal domain, alpha-crystalline domain and a C-terminal tail. The models showed that a single point mutation from serine to proline at 52(nd) position causes structural changes. The structural changes are observed in N-terminal region and alpha-crystalline domains. Serine in 52(nd) position is observed in beta4 strand and Proline in 52(nd) position is observed in loop. The number of residues contributing alpha helix at N-terminal region varies in both models. In 18S more number of residues is present in alpha helix when compared to 18P. The loop regions between beta3 and beta4 strands of both models vary in number of residues present in it. Number of residues contributing beta4 strand in both models vary. beta6 strand is absent in both models. Major functional peptide region of alpha crystalline domains of both models varies. These differences observed in secondary structures support their distinct functional roles. It also emphasizes that a point mutation can cause structural variation.

Interaction of C-terminal truncated human alphaA-crystallins with target proteins

BACKGROUND

Significant portion of alphaA-crystallin in human lenses exists as C-terminal residues cleaved at residues 172, 168, and 162. Chaperone activity, determined with alcohol dehydrogenase (ADH) and betaL-crystallin as target proteins, was increased in alphaA(1-172) and decreased in alphaA(1-168) and alphaA(1-162). The purpose of this study was to show whether the absence of the C-terminal residues influences protein-protein interactions with target proteins.

METHODOLOGY/PRINCIPAL FINDINGS

Our hypothesis is that the chaperone-target protein binding kinetics, otherwise termed subunit exchange rates, are expected to reflect the changes in chaperone activity. To study this, we have relied on fluorescence resonance energy transfer (FRET) utilizing amine specific and cysteine specific fluorescent probes. The subunit exchange rate (k) for ADH and alphaA(1-172) was nearly the same as that of ADH and alphaA-wt, alphaA(1-168) had lower and alphaA(1-162) had the lowest k values. When betaL-crystallin was used as the target protein, alphaA(1-172) had slightly higher k value than alphaA-wt and alphaA(1-168) and alphaA(1-162) had lower k values. As expected from earlier studies, the chaperone activity of alphaA(1-172) was slightly better than that of alphaA-wt, the chaperone activity of alphaA(1-168) was similar to that of alphaA-wt and alphaA(1-162) had substantially decreased chaperone activity.

CONCLUSIONS/SIGNIFICANCE

Cleavage of eleven C-terminal residues including Arg-163 and the C-terminal flexible arm significantly affects the interaction with target proteins. The predominantly hydrophilic flexible arm appears to be needed to keep the chaperone-target protein complex soluble.

Analysis of the alphaB-crystallin domain responsible for inhibiting tubulin aggregation

The cytoskeleton has a unique property such that changes of conformation result in polymerization into a filamentous form. alphaB-Crystallin, a small heat shock protein (sHsp), has chaperone activities for various substrates, including proteins constituting the cytoskeleton, such as actin; intermediate filament; and tubulin. However, it is not clear whether the "alpha-crystallin domain" common to sHsps also has chaperone activity for the protein cytoskeleton. To investigate the possibility that the C-terminal alpha-crystallin domain of alpha-crystallin has the aggregation-preventing ability for tubulin, we constructed an N-terminal domain deletion mutant of alphaB-crystallin. We characterized its structural properties and chaperone activities. Far-ultraviolet (UV) circular dichroism measurements showed that secondary structure in the alpha-crystallin domain of the deletion mutant is maintained. Ultracentrifuge analysis of molecular masses indicated that the deletion mutant formed smaller oligomers than did the full-length protein. Chaperone activity assays demonstrated that the N-terminal domain deletion mutant suppressed heat-induced aggregation of tubulin well. Comparison of chaperone activities for 2 other substrates (citrate synthase and alcohol dehydrogenase) showed that it was less effective in the suppression of their aggregation. These results show that alphaB-crystallin recognizes a variety of substrates and especially that alpha-crystallin domain binds free cytoskeletal proteins. We suggest that this feature would be advantageous in its functional role of holding or folding multiple proteins denatured simultaneously under stress conditions.

Cleavage of the C-Terminal Serine of Human αA-Crystallin Produces αA1-172 with Increased Chaperone Activity and Oligomeric Size

This study aimed to study the oligomeric size, structure, hydrodynamic properties, and chaperone function of the C-terminally truncated human alphaA-crystallin mutants with special emphasis on alphaA1-172 which is the cleavage product of the Ser172-Ser173 bond, unique to human lenses and constituting a major part of alphaA-crystallin. Various truncated forms of human alphaA-crystallins were prepared by site-directed mutagenesis. The proteins were expressed in Escherichia coli BL21(DE3) pLysS cells and purified by size exclusion column chromatography. Molecular masses and the other hydrodynamic properties were determined by dynamic light scattering measurements. The secondary and tertiary structural changes were assessed by far- and near-UV CD spectra measurements, respectively. Chaperone activity was determined by using ADH, insulin, and betaL-crystallin as the target proteins. alphaAlpha1-172 exhibited a significant increase in oligomeric size, i.e., 866 kDa by light scattering measurements as compared to 702 kDa in alphaA-wt. alphaAlpha1-172 and alphaA-wt had similar secondary structure, but the former exhibited slightly altered tertiary structure. The most interesting observation was that alphaAlpha1-172 behaved as a 28-46% better chaperone than alphaA-wt. The oligomeric size and structure of alphaAlpha1-168 were similar to those of alphaA-wt, while the chaperone activity was decreased by 12-23%. alphaAlpha1-162, on the other hand, had an oligomeric size of 400 kDa, a decrease in chaperone activity of 80-100%, and significantly altered secondary and tertiary structures. The data show that the overall chaperone function of alphaA-crystallin will be significantly improved by the presence of the major truncated product alphaAlpha1-172. This will be beneficial to the lens undergoing oxidative stress. Since alphaAlpha1-168 and alphaAlpha1-162 are present only in small amounts, their effect would be minimal.

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.

Insights into the domains required for dimerization and assembly of human alphaB crystallin

Protein pin array technology was used to identify subunit-subunit interaction sites in the small heat shock protein (sHSP) alphaB crystallin. Subunit-subunit interaction sites were defined as consensus sequences that interacted with both human alphaA crystallin and alphaB crystallin. The human alphaB crystallin protein pin array consisted of contiguous and overlapping peptides, eight amino acids in length, immobilized on pins that were in a 96-well ELISA plate format. The interaction of alphaB crystallin peptides with physiological partner proteins, alphaA crystallin and alphaB crystallin, was detected using antibodies and recorded using spectrophotometric absorbance. Five peptide sequences including 37LFPTSTSLSPFYLRPPSF54 in the N terminus, 75FSVNLDVK82)(beta3), 131LTITSSLS138 (beta8) and 141GVLTVNGP148 (beta9) that form beta strands in the conserved alpha crystallin core domain, and 155PERTIPITREEK166 in the C-terminal extension were identified as subunit-subunit interaction sites in human alphaB crystallin using the novel protein pin array assay. The subunit-subunit interaction sites were mapped to a three-dimensional (3D) homology model of wild-type human alphaB crystallin that was based on the crystal structure of wheat sHSP16.9 and Methanococcus jannaschi sHSP16.5 (Mj sHSP16.5). The subunit-subunit interaction sites identified and mapped onto the homology model were solvent-exposed and had variable secondary structures ranging from beta strands to random coils and short alpha helices. The subunit-subunit interaction sites formed a pattern of hydrophobic patches on the 3D surface of human alphaB crystallin.

Cloning and characterization of a thermostable catfish alphaB-crystallin with chaperone-like activity at high temperatures

We have cloned, expressed and characterized catfish alphaB-crystallin (FalphaB). Genomic sequence comparison has revealed conservation of intron splicing sites and coding regions, however, the two intron sequences, 5'- and 3'-untranslated regions of FalphaB gene are shorter than those reported for other vertebrates. In contrast to mammalian homologues with a subunit association ratio (alphaA-crystallin/alphaB-crystallin) of 3:1, alpha-crystallin from catfish lens showed a ratio of 19:1. The biophysical properties and chaperone-like activity of recombinant FalphaB and porcine alphaB-crystallin (PalphaB) were studied and compared by heat denaturation, circular dichroism, intrinsic and dye-binding fluorescence, gel-filtration, and analytical ultracentrifugation. FalphaB shows 50% precipitation occurring at 72 degrees C that is higher than PalphaB at 66 degrees C. Even though FalphaB also possesses more surface hydrophilic regions than PalphaB, FalphaB still possesses higher chaperone activity to prevent aggregation of alcohol dehydrogenase at 60 degrees C. The molecular mass of FalphaB showed a smaller size (450 kDa) than PalphaB (550 kDa), which is also confirmed by analytical ultracentrifugation. In addition, FalphaB possesses better refolding potential after preheating treatment than PalphaB. FalphaB also exhibits higher chaperone-like activity than PalphaB to prevent insulin aggregation induced by dithiothreitol. In contrast to the prevalent notion that fish crystallins generally denature easily, FalphaB with chaperone-like activity appears to be more stable than mammalian homologues towards thermal and chemical denaturation.

Essential Role of the NH2-terminal WD/EPF Motif in the Phosphorylation-activated Protective Function of Mammalian Hsp27

Hsp27 is expressed at high levels after mild heat shock and contributes to making cells extremely resistant to subsequent treatments. The activity of the protein is regulated at the transcriptional level, but also by phosphorylation, which occurs rapidly during stress and is responsible for causing the dissociation of large 700-kDa Hsp27 oligomers into dimers. We investigated the mechanism by which phosphorylation and oligomerization modulate the protective activity of Chinese hamster Hsp27. In contrast to oligomer dissociation, which only required Ser90 phosphorylation, activation of Hsp27 thermoprotective activity required the phosphorylation of both Ser90 and Ser15. Replacement of Ser90 by Ala90, which prevented the dissociation of the oligomer upon stress, did cause a severe defect in the protective activity. Dissociation was, however, not a sufficient condition to activate the protein because replacement of Ser15 by Ala15, which caused little effect in the oligomeric organization of the protein, also yielded an inactive protein. Analyzes of mutants with short deletions in the NH2 terminus identified the Hsp27 WD/EPF or PF-rich domain as essential for protection, maintenance of the oligomeric structure, and in vitro chaperone activity of the protein. In light of a three-dimensional model of Hsp27 based on the crystallographic structure of wheat Hsp16.9, we propose that the conserved WD/EPF motif of mammalian Hsp27 mediates important intramolecular interactions with hydrophic surfaces of the alpha-crystallin domain of the protein. These interactions are destabilized by Ser90 phosphorylation, making the motif free to interact with heterologous molecular targets upon the additional phosphorylation of the nearby Ser15.

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.

Cell type-specific activation of intracellular transglutaminase 2 by oxidative stress or ultraviolet irradiation: implications of transglutaminase 2 in age-related cataractogenesis

Transglutaminase (TGase) 2 is a ubiquitously expressed enzyme that modifies proteins by cross-linking or polyamination. An aberrant activity of TGase 2 has implicated its possible roles in a variety of diseases including age-related cataracts. However, the molecular mechanism by which TGase 2 is activated has not been elucidated. In this report, we showed that oxidative stress or UV irradiation elevates in situ TGase 2 activity. Neither the expression level nor the in vitro activity of TGase 2 appeared to correlate with the observed elevation of in situ TGase 2 activity. Screening a number of cell lines revealed that the level of TGase 2 activation depends on the cell type and also the environmental stress, suggesting that unrecognized cellular factor(s) may specifically regulate in situ TGase 2 activity. Concomitantly, we observed that human lens epithelial cells (HLE-B3) exhibited about 3-fold increase in in situ TGase 2 activity in response to the stresses. The activated TGase 2 catalyzed the formation of water-insoluble dimers or polymers of alphaB-crystallin, betaB(2)-crystallin, and vimentin in HLE-B3 cells, providing evidence that TGase 2 may play a role in cataractogenesis. Thus, our findings indicate that in situ TGase 2 activity must be evaluated instead of in vitro activity to study the regulation mechanism and function of TGase 2 in biological and pathological processes.

C-terminal lysine truncation increases thermostability and enhances chaperone-like function of porcine alphaB-crystallin

The carboxyl-terminal segment of alpha-crystallin, a major lens protein of all vertebrates, has a short and flexible peptide extension of about 20 amino acid residues that are very susceptible to proteolytic truncation and modifications under physiological conditions. To investigate its role in crystallin aggregation and chaperone-like activity, we constructed a mutant of porcine alphaB-crystallin with C-terminal lysine truncated end, which unexpectedly showed better chaperone-like function than wild-type alphaB-crystallin. From circular dichroism (CD) spectra, we show that the mutant possesses similar secondary and tertiary structures to those of native purified and recombinant alphaB-crystallins. Analytical ultracentrifugation revealed that the truncated mutant was smaller than wild-type alphaB-crystallin in aggregation size and mass. The observed higher thermostability and anti-thermal aggregation propensity of the truncated alphaB-crystallin mutant than wild-type alphaB-crystallin are in contrast to the prevailing notion that mutations at the C-terminal lysines of alphaB-crystallin result in substantial loss of chaperone-like activity, despite the overall preservation of secondary structure. The detailed characterization of the C-terminal deletion mutants may provide some deeper insight into the chaperoning mechanism of the structurally related small heat-shock protein family.

Subcorneal colocalization of the small heat shock protein, hsp27, with keratins and proteins of the cornified cell envelope

BACKGROUND

hsp27 is a member of the small heat shock protein family. Its expression in epidermal keratinocytes in situ and in tissue culture correlates with differentiation. Experimental evidence points to the fact that hsp27 is a molecular chaperone and is involved in the regulation of cell growth and differentiation.

OBJECTIVES

To investigate whether epidermal hsp27 through its chaperone function plays a role in the assembly of keratin filaments and the cornified cell envelope.

METHODS

We performed double staining immunofluorescence and immunogold microscopy on normal human skin (n = 15). We analysed the colocalization of hsp27 with actin, keratins and proteins of the cornified cell envelope (loricrin, filaggrin, transglutaminase 1).

RESULTS

Actin staining did not reveal detectable colocalization with hsp27. For keratins, transglutaminase, loricrin and filaggrin colocalization was found in more than 60% of the samples. Colocalization was confined to a narrow subcorneal layer with varying patterns of expression. Electron microscopy revealed that loricrin and filaggrin colocalize with hsp27 indirectly through binding to intermediate filaments.

CONCLUSIONS

These results provide morphological evidence that in normal human skin hsp27 might act as a chaperone of cornification. Investigations of the molecular hsp27 interactions with the proteins of the cornified cell envelope are necessary to gain further insight into terminal keratinocyte differentiation and disorders of keratinization.

Functional characterization of Xenopus small heat shock protein, Hsp30C: the carboxyl end is required for stability and chaperone activity

Small heat shock proteins protect cells from stress presumably by acting as molecular chaperones. Here we report on the functional characterization of a developmentally regulated, heat-inducible member of the Xenopus small heat shock protein family, Hsp30C. An expression vector containing the open reading frame of the Hsp30C gene was expressed in Escherichia coli. These bacterial cells displayed greater thermoresistance than wild type or plasmid-containing cells. Purified recombinant protein, 30C, was recovered as multimeric complexes which inhibited heat-induced aggregation of either citrate synthase or luciferase as determined by light scattering assays. Additionally, 30C attenuated but did not reverse heat-induced inactivation of enzyme activity. In contrast to an N-terminal deletion mutant, removal of the last 25 amino acids from the C-terminal end of 30C severely impaired its chaperone activity. Furthermore, heat-treated concentrated solutions of the C-terminal mutant formed nonfunctional complexes and precipitated from solution. Immunoblot and gel filtration analysis indicated that 30C binds with and maintains the solubility of luciferase preventing it from forming heat-induced aggregates. Coimmunoprecipitation experiments suggested that the carboxyl region is necessary for 30C to interact with target proteins. These results clearly indicate a molecular chaperone role for Xenopus Hsp30C and provide evidence that its activity requires the carboxyl terminal region.

Chaperone function of mutant versions of alpha A- and alpha B-crystallin prepared to pinpoint chaperone binding sites

A major stress protein, alpha-crystallin, functions as a chaperone. Site-directed mutagenesis has been used to identify regions of the protein necessary for chaperone function. In this work we have taken some of the previously described mutants produced and assessed their chaperone function by both a traditional heat-induced aggregation method at elevated temperature and using enzyme methods at 37 degrees C. In general the different assays gave parallel results indicating that the same property is being measured. Discrepancies were explicable by the heat lability of some mutants. Most mutants had full chaperone function showing the robust nature of alpha-crystallin. A mutant corresponding to a minor component of rodent alpha A-crystallin, alpha Ains-crystallin, had decreased chaperone function. Decreased chaperone function was also found for human Ser139--> Arg, Thr144-->Arg, Ser59-->Ala mutants of alpha B-crystallin and double mutants Ser45-->Ala/Ser59-->Ala, Lys103--> Leu/His104-->Ile, and Glu110-->His/His111-->Glu. A mutant Phe27-->Arg that was the subject of previous controversy was shown to be fully active at physiological temperatures.

Transcriptional analysis of three Bacillus subtilis genes coding for proteins with the alpha-crystallin domain characteristic of small heat shock proteins

In silico analysis of the complete Bacillus subtilis genome revealed the presence of three genes whose deduced amino acid sequences exhibit an alpha-crystallin domain characteristic for the family of small heat shock proteins: cotM (which has already been identified [Henriques et al. (1997) J. Bacteriol 179, 1887-1897]), yocM, and cotP (formerly ydfT). Analysis of the expression of all three genes by slot-blot experiments and by transcriptional fusions revealed that none of them was heat-inducible. Transcription of cotP was induced late during sporulation by the sporulation-specific sigma factor sigma(K) and negatively controlled by the GerE repressor. No expression of the yocM gene was found under all standard laboratory conditions tested. Both a cotP knockout mutant as well as a cotM cotP double knockout turned out to be viable and form spores and exhibited no germination defect.

Domain swapping in human alpha A and alpha B crystallins affects oligomerization and enhances chaperone-like activity

alphaA and alphaB crystallins, members of the small heat shock protein family, prevent aggregation of proteins by their chaperone-like activity. These two proteins, although very homologous, particularly in the C-terminal region, which contains the highly conserved "alpha-crystallin domain," show differences in their protective ability toward aggregation-prone target proteins. In order to investigate the differences between alphaA and alphaB crystallins, we engineered two chimeric proteins, alphaANBC and alphaBNAC, by swapping the N-terminal domains of alphaA and alphaB crystallins. The chimeras were cloned and expressed in Escherichia coli. The purified recombinant wild-type and chimeric proteins were characterized by fluorescence and circular dichroism spectroscopy and gel permeation chromatography to study the changes in secondary, tertiary, and quaternary structure. Circular dichroism studies show structural changes in the chimeric proteins. alphaBNAC binds more 8-anilinonaphthalene-1-sulfonic acid than the alphaANBC and the wild-type proteins, indicating increased accessible hydrophobic regions. The oligomeric state of alphaANBC is comparable to wild-type alphaB homoaggregate. However, there is a large increase in the oligomer size of the alphaBNAC chimera. Interestingly, swapping domains results in complete loss of chaperone-like activity of alphaANBC, whereas alphaBNAC shows severalfold increase in its protective ability. Our findings show the importance of the N- and C-terminal domains of alphaA and alphaB crystallins in subunit oligomerization and chaperone-like activity. Domain swapping results in an engineered protein with significantly enhanced chaperone-like activity.

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.

Functional characterization of Xenopus small heat shock protein, Hsp30C: the carboxyl end is required for stability and chaperone activity

Small heat shock proteins protect cells from stress presumably by acting as molecular chaperones. Here we report on the functional characterization of a developmentally regulated, heat-inducible member of the Xenopus small heat shock protein family, Hsp30C. An expression vector containing the open reading frame of the Hsp30C gene was expressed in Escherichia coli. These bacterial cells displayed greater thermoresistance than wild type or plasmid-containing cells. Purified recombinant protein, 30C, was recovered as multimeric complexes which inhibited heat-induced aggregation of either citrate synthase or luciferase as determined by light scattering assays. Additionally, 30C attenuated but did not reverse heat-induced inactivation of enzyme activity. In contrast to an N-terminal deletion mutant, removal of the last 25 amino acids from the C-terminal end of 30C severely impaired its chaperone activity. Furthermore, heat-treated concentrated solutions of the C-terminal mutant formed nonfunctional complexes and precipitated from solution. Immunoblot and gel filtration analysis indicated that 30C binds with and maintains the solubility of luciferase preventing it from forming heat-induced aggregates. Coimmunoprecipitation experiments suggested that the carboxyl region is necessary for 30C to interact with target proteins. These results clearly indicate a molecular chaperone role for Xenopus Hsp30C and provide evidence that its activity requires the carboxyl terminal region.

Thermodynamic stability of human lens recombinant alphaA- and alphaB-crystallins

Lens alpha-crystallin is a 600-800-kDa heterogeneous oligomer protein consisting of two subunits, alphaA and alphaB. The homogeneous oligomers (alphaA- and alphaB-crystallins) have been prepared by recombinant DNA technology and shown to differ in the following biophysical/biochemical properties: hydrophobicity, chaperone-like activity, subunit exchange rate, and thermal stability. In this study, we studied their thermodynamic stability by unfolding in guanidine hydrochloride. The unfolding was probed by three spectroscopic parameters: absorbance at 235 nm, Trp fluorescence intensity at 320 nm, and far-UV circular dichroism at 223 nm. Global analysis indicated that a three-state model better describes the unfolding behavior than a two-state model, an indication that there are stable intermediates for both alphaA- and alphaB-crystallins. In terms of standard free energy (DeltaG(NU)(H(2)(O))), alphaA-crystallin is slightly more stable than alphaB-crystallin. The significance of the intermediates may be related to the functioning of alpha-crystallins as chaperone-like molecules.

The cardiomyopathy and lens cataract mutation in alphaB-crystallin alters its protein structure, chaperone activity, and interaction with intermediate filaments in vitro

Desmin-related myopathy and cataract are both caused by the R120G mutation in alphaB-crystallin. Desmin-related myopathy is one of several diseases characterized by the coaggregation of intermediate filaments with alphaB-crystallin, and it identifies intermediate filaments as important physiological substrates for alphaB-crystallin. Using recombinant human alphaB-crystallin, the effects of the disease-causing mutation R120G upon the structure and the chaperone activities of alphaB-crystallin are reported. The secondary, tertiary, and quaternary structural features of alphaB-crystallin are all altered by the mutation as deduced by near- and far-UV circular dichroism spectroscopy, size exclusion chromatography, and chymotryptic digestion assays. The R120G alphaB-crystallin is also less stable than wild type alphaB-crystallin to heat-induced denaturation. These structural changes coincide with a significant reduction in the in vitro chaperone activity of the mutant alphaB-crystallin protein, as assessed by temperature-induced protein aggregation assays. The mutation also significantly altered the interaction of alphaB-crystallin with intermediate filaments. It abolished the ability of alphaB-crystallin to prevent those filament-filament interactions required to induce gel formation while increasing alphaB-crystallin binding to assembled intermediate filaments. These activities are closely correlated to the observed disease pathologies characterized by filament aggregation accompanied by alphaB-crystallin binding. These studies provide important insight into the mechanism of alphaB-crystallin-induced aggregation of intermediate filaments that causes disease.

An atypical form of alphaB-crystallin is present in high concentration in some human cataractous lenses. Identification and characterization of aberrant N- and C-terminal processing

Two unique polypeptides, 22.4 and 16.4 kDa, were prominent in some human cataracts. Both proteins were identified as modified forms of the small heat shock protein, alphaB-crystallin. The concentration of total alphaB-crystallin in most of these cataracts was significantly increased. The 22.4-kDa protein was subsequently designated as alphaB(g). Mass spectrometric analyses of tryptic and Asp-N digests showed alphaB(g) is alphaB-crystallin minus the C-terminal lysine. alphaB(g) constituted 10-90% of the total alphaB-crystallin in these cataracts and was preferentially phosphorylated over the typical form of alphaB-crystallin. Human alphaB(g) and alphaB-crystallin were cloned and expressed in Escherichia coli. The differences in electrophoretic mobility and the large difference in native pI values suggest some structural differences exist. The chaperone-like activity of recombinant human alphaB(g) was comparable to that of recombinant human alphaB-crystallin in preventing the aggregation of lactalbumin induced by dithiothreitol. The mechanism involved in generating alphaB(g) is not known, but a premature termination of the alphaB-crystallin gene was ruled out by sequencing the polymerase chain reaction products of the last exon for the alphaB-crystallin gene from lenses containing alphaB(g). The 16.4-kDa protein was an N-terminally truncated fragment of alphaB(g). The high concentration of alphaB-crystallin in these cataracts is the first observation of this kind in human lenses.

ATP and the core "alpha-Crystallin" domain of the small heat-shock protein alphaB-crystallin

Electrospray ionization mass spectrometry (ESI-LC/MS) of tryptic digests of human alphaB-crystallin in the presence and absence of ATP identified four residues located within the core "alpha-crystallin" domain, Lys(82), Lys(103), Arg(116), and Arg(123), that were shielded from the action of trypsin in the presence of ATP. In control experiments, chymotrypsin was used in place of trypsin. The chymotryptic fragments of human alphaB-crystallin produced in the presence and absence of ATP were analyzed using liquid chromatography-tandem mass spectrometry (LC-MS/MS). Seven chymotryptic cleavage sites, Trp(60), Phe(61), Phe(75), Phe(84), Phe(113), Phe(118), and Tyr(122), located near or within the core alpha-crystallin domain, were shielded from the action of chymotrypsin in the presence of ATP. Chemically similar analogs of ATP were less protective than ATP against proteolysis by trypsin or chymotrypsin. ATP had no effect on the enzymatic activity of trypsin and the K(m) for trypsin was 0.031 mM in the presence of ATP and 0.029 mM in the absence of ATP. The results demonstrated an ATP-dependent structural modification in the core alpha-crystallin domain conserved in nearly all identified small heat-shock proteins that act as molecular chaperones.

Bovine lens crystallins do contain helical structure: a circular dichroism study

In order to settle a recent discussion on the secondary structure of lens crystallins, we have measured the circular dichroism (CD) spectra of alpha-, beta(H)-, and beta(L)-crystallin from 178 to 250 nm and of gamma-crystallin from 168 to 250 nm. The results were analysed by means of a newly developed algorithm that almost doubles the reliability of secondary structure prediction and that allows discrimination between alpha- and 3(10)-helical, and between extended and polyproline beta-type structure. The results indicate that the crystallins studied contain a non-negligible amount of alpha-helical structure, although at least 50% of it is in the form of single and/or distorted loops. In alpha-crystallin, which is related to the chaperones, the helical content is lower than in beta- and gamma-crystallin. In some cases, the helices may play a role in DNA binding by the crystallins.

The unique chaperone operon of Thermotoga maritima: cloning and initial characterization of a functional Hsp70 and small heat shock protein

The hyperthermophilic eubacterium Thermotoga maritima possesses an operon encoding an Hsp70 molecular chaperone protein and a protein with meaningful homology to the small heat shock protein family of chaperones. This represents the first demonstrated co-operon organization for these two important classes of molecular chaperones. We have cloned and initially characterized these proteins as functional chaperones in vitro: the Hsp70 is capable of ATP hydrolysis and substrate binding, and the small heat shock protein can suppress protein aggregation and stably bind a refolding-competent substrate. In addition, the primary sequence of the Hsp70 is used to infer the phylogenetic relationships of T. maritima, one of the deepest-branching eubacteria known.

Small heat shock proteins and protection against injury

The small heat shock proteins alpha B crystallin and HSP27 exert a protective effect in response to simulated ischemia. A model is proposed whereby proteins not in their final folding state bind to the outside of the large oligomeric small heat shock protein complexes thus finding a safe haven during ischemia. After the ischemia is resolved, these proteins may be released and, with the help of HSP70, are shuttled to a productive refolding pathway resulting in proteins in their final folding state, which can assume their normal activity in cells recovered from ischemic injury.

Transcription of the stationary-phase-associated hspX gene of Mycobacterium tuberculosis is inversely related to synthesis of the 16-kilodalton protein

The 16-kDa protein, an alpha-crystallin homologue, is one of the most abundant proteins in stationary-phase Mycobacterium tuberculosis. Here, transcription and translation of the hspX gene, which encodes the 16-kDa protein, have been investigated by Northern blotting analysis, primer extension, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis with a microaerophilic stationary-phase model. Two transcripts of about 2.5 and 1.1 kb were demonstrated by Northern blot analysis and hybridized to the hspX gene probe. Primer extension analysis revealed that the transcription start site is located 33 nucleotides upstream of the hspX gene start codon. The cellular level of the hspX mRNA was maximum in log-phase bacilli and was markedly reduced after 20 days in unagitated culture, when the organisms had entered the stationary phase. A third transcript of 0.5 kb was detected 0.6 kb downstream of the hspX gene; this transcript has a transcriptional pattern completely different from that of the 1.1- and 2.5-kb products, suggesting that there may be another gene in this region. In contrast to the high level of hspX mRNA in log-phase bacilli, 16-kDa protein synthesis was low in log-phase bacteria and rose to its maximum after 20 days. In both log-phase and stationary-phase bacteria the mRNA was unstable, with a half-life of 2 min, which indicated that the transcript stability was growth rate independent and not a general means for controlling the gene expression. However, the cellular content of 16-kDa protein, while low in log-phase bacteria, rose to a maximum at 10 days and remained at this high level for up to 50 days, which indicates that this protein is a stable molecule with a low turnover rate. These data suggest that the regulation of hspX expression during entry into and maintenance of stationary phase involves translation initiation efficiency and protein stability as potential mechanisms.

Identification of a site of Hsp27 binding with Hsp27 and alpha B-crystallin as indicated by the yeast two-hybrid system

The small heat-shock proteins (sHsp), including Hsp27 and alphaB-crystallin, usually form large oligomers in cells. It has been suggested that the sHsp form oligomers by binding either a conserved C-terminal amino acid sequence or the less conserved N-terminal region. However, the site of binding has not been precisely determined. We used the yeast two-hybrid system to investigate binding of full-length rat Hsp27 or fragments of Hsp27 to full-length rat Hsp27 or alphaB-crystallin molecules. A series of cDNAs coding for fragments of Hsp27 were generated and ligated with the coding sequence for the binding domain of the yeast Gal4p transcription factor. These cDNAs were each transfected into yeast that had been transfected to express full-length rat Hsp27 or alphaB-crystallin fused with the DNA binding domain of Gal4p. Yeast cells transfected with both plasmids were assayed, by both a beta-galactosidase (beta-gal) filter assay and a quantitative liquid assay, for activation of Gal4p-driven beta-gal expression. Results indicated that the N-terminal domain of Hsp27 consisting of amino acids 1-124 did not bind to Hsp27 or alphaB-crystallin. The predominant Hsp27-Hsp27/alphaB-crystallin binding domain was the conserved C-terminal region consisting of amino acids 141-206, particularly amino acids 141-176.

Heat shock protein (HSP)27: a further refinement in the diagnosis of suspicious fine needle aspirates of breast

The technique of fine needle aspiration (FNA) is increasingly used in the investigation of breast abnormalities both palpable and impalpable. However, up to 20% of aspirates fall into two categories which require further investigation, because they either show cytological atypia or exhibit features which are suspicious of malignancy (reporting categories C3 and C4). The usefulness of the 27-kD heat shock protein (HSP27) expression in refining these categories of suspicion, and possibly in predicting malignancy, was investigated using an established immunocytochemical staining procedure. Positive HSP27 staining was shown by 73.1% of C4 and 33.3% of C3 FNAs. Statistical analysis showed a significant difference between the results obtained for the C3 and C4 FNAs.

Heat shock proteins and protection against ischemic injury

Heat shock proteins present a complex family of proteins exerting chaperone-like activities that are classified according to their molecular weight. We especially explored protective functions of inducible heat shock protein 70, the mitochondrial heat shock protein 60 and 10, and the small heat shock proteins HSP27 and alphaB-crystallin against ischemic, reoxygenation-mediated injury using transgenic animals and hearts under in vivo conditions and in isolated cardiac myocyte-derived cells using adenoviral vectors. We noted with great interest that differential protective effects are exerted by specific hsps. For example, alpha-B-crystallin and constitutive hsp70 markedly protect microtubular structure in cardiac myocytes from ischemia-induced injury. Inducible hsp70, hsp60 and hsp10 when coexpressed, and hsp27 and alphaB-crystallin have an overall protective effect against ischemic injury as determined by the release of enzymes like creatine kinase and LDH. We did not note inflammatory or immune responses elicited by the expression of hsps in transgenic animals and cardiac myocytes. The specific cell types in which hsps are expressed may contribute to the protective effect of hsps versus their inflammatory and immunogenic effects when expressed in other cell types.

Negative charges in the C-terminal domain stabilize the alphaB-crystallin complex

alphaB-Crystallin is one of the six known mammalian small heat-shock proteins (sHsps). These are characterized by the presence of a conserved sequence of 80-100 residues, which constitutes the putative C-terminal domain. Like other sHsps, alphaB-crystallin forms multimeric globular complexes, often in combination with related sHsps. Here we show that in a yeast two-hybrid system, alphaB-crystallin can specifically interact with itself as well as with alphaA-crystallin and Hsp27. Analyses of the separate domains show that the conserved C-terminal domain (CalphaB) is essential for this interaction between subunits. To try and detect residues that are important in subunit interaction, the CalphaB domain was used in a two-hybrid screen as bait to select randomly mutated CalphaB mutants. In this way we obtained nine mutants that were still able to interact with wild-type CalphaB despite the presence of up to 15 replacements. Similarly, we obtained 16 mutants that were unable to bind, because of the presence of just three to nine replacements. In binding CalphaB mutants, lysine residues were most often replaced by glutamic acid residues, and in non-binding CalphaB mutants, acidic residues were often found to be replaced by non-charged residues. This indicates that negative charges are important for subunit interaction and we propose a model to explain this role of acidic residues. Furthermore, we observed that two homologs of alphaB-crystallin, alphaA-crystallin and Hsp27, generally interact similarly with the binding and non-binding CalphaB mutants as does alphaB-crystallin. This suggests that interactions involved in the complex formation of these three sHsps are largely comparable.

Upregulation of a 23 kDa small heat shock protein transcript during pupal diapause in the flesh fly, Sarcophaga, crassipalpis

A diapause upregulated cDNA clone was isolated from a cDNA library generated from brain mRNA of diapausing Sarcophaga crassipalpis pupae. The clone hybridized to a 1600 bp transcript on a northern blot. The insert is 823 bp in length, has a tentative open reading frame of 615 bp, and codes for a 23 kDa protein. The clone has a high level of identity at the amino acid level with the four small heat shock proteins of Drosophila melanogaster. Northern analysis revealed no detectable expression of the transcript in diapause- or nondiapause-programmed wandering larvae, and only trace expression in nondiapausing pupae. But, the transcript was highly expressed beginning at the onset of diapause and continuing throughout diapause. Expression promptly decreased when diapause was terminated. In nondiapausing individuals the transcript was highly expressed in response to cold shock or heat shock, but temperature stress did not cause greater expression in diapausing pupae. The results imply that expression of this small heat shock protein, a response elicited by temperature stress in nondiapausing individuals, is a normal component of the diapause syndrome. The upregulation of this gene during diapause suggests that it plays an essential role during this overwintering developmental arrest.

Caenorhabditis elegans small heat-shock proteins Hsp12.2 and Hsp12.3 form tetramers and have no chaperone-like activity

Four 12.2-12.6 kDa small heat-shock proteins (sHSPs) of Caenorhabditis elegans are the smallest known members of the sHSP family. They essentially comprise the characteristic C-terminal 'alpha-crystallin domain' of the sHSPs, having a very short N-terminal region, and lacking a C-terminal tail. Recombinant Hsp12.2 and 12.3 are characterized here. Far-UV CD spectra reveal, as for other sHSPs, predominantly a beta-sheet structure. By gel permeation and crosslinking, they are the first sHSPs shown to occur as tetramers, rather than forming the usual large multimeric complexes. Exceptionally, too, both appear devoid of in vitro chaperone-like abilities. This supports the notion that tetramers are the building blocks of sHSP complexes, and that higher multimer formation, mediated through the N-terminal domains, is a prerequisite for chaperone-like activity.

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.

Genealogy of the alpha-crystallin--small heat-shock protein superfamily

Sequences of 40 very diverse representatives of the alpha-crystallin-small heat-shock protein (alpha-Hsp) superfamily are compared. Their characteristic C-terminal 'alpha-crystallin domain' of 80-100 residues contains short consensus sequences that are highly conserved from prokaryotes to eukaryotes. There are, in addition, some positions that clearly distinguish animal from non-animal alpha-Hsps. The alpha-crystallin domain is predicted to consist of two hydrophobic beta-sheet motifs, separated by a hydrophilic region which is variable in length. Combination of a conserved alpha-crystallin domain with a variable N-terminal domain and C-terminal extension probably modulates the properties of the various alpha-Hsps as stress-protective and structural oligomeric proteins. Phylogeny reconstruction indicates that multiple alpha-Hsps were already present in the last common ancestor of pro- and eukaryotes. It is suggested that during eukaryote evolution, animal and non-animal alpha-Hsps originated from different ancestral gene copies. Repeated gene duplications gave rise to the multiple alpha-Hsps present in most organisms.

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.