A novel quaternary structure of the dimeric alpha-crystallin domain with chaperone-like activity

Heat Shock Response and Heat Shock Proteins: Current Understanding and Future Opportunities in Human Diseases

The heat shock response is an evolutionarily conserved mechanism that protects cells or organisms from the harmful effects of various stressors such as heat, chemicals toxins, UV radiation, and oxidizing agents. The heat shock response triggers the expression of a specific set of genes and proteins known as heat shock genes/proteins or molecular chaperones, including HSP100, HSP90, HSP70, HSP60, and small HSPs. Heat shock proteins (HSPs) play a crucial role in thermotolerance and aiding in protecting cells from harmful insults of stressors. HSPs are involved in essential cellular functions such as protein folding, eliminating misfolded proteins, apoptosis, and modulating cell signaling. The stress response to various environmental insults has been extensively studied in organisms from prokaryotes to higher organisms. The responses of organisms to various environmental stressors rely on the intensity and threshold of the stress stimuli, which vary among organisms and cellular contexts. Studies on heat shock proteins have primarily focused on HSP70, HSP90, HSP60, small HSPs, and ubiquitin, along with their applications in human biology. The current review highlighted a comprehensive mechanism of heat shock response and explores the function of heat shock proteins in stress management, as well as their potential as therapeutic agents and diagnostic markers for various diseases.

Desmin and its molecular chaperone, the αB-crystallin: How post-translational modifications modulate their functions in heart and skeletal muscles?

Maintenance of the highly organized striated muscle tissue requires a cell-wide dynamic network through protein-protein interactions providing an effective mechanochemical integrator of morphology and function. Through a continuous and complex trans-cytoplasmic network, desmin intermediate filaments ensure this essential role in heart and in skeletal muscle. Besides their role in the maintenance of cell shape and architecture (permitting contractile activity efficiency and conferring resistance towards mechanical stress), desmin intermediate filaments are also key actors of cell and tissue homeostasis. Desmin participates to several cellular processes such as differentiation, apoptosis, intracellular signalisation, mechanotransduction, vesicle trafficking, organelle biogenesis and/or positioning, calcium homeostasis, protein homeostasis, cell adhesion, metabolism and gene expression. Desmin intermediate filaments assembly requires αB-crystallin, a small heat shock protein. Over its chaperone activity, αB-crystallin is involved in several cellular functions such as cell integrity, cytoskeleton stabilization, apoptosis, autophagy, differentiation, mitochondria function or aggresome formation. Importantly, both proteins are known to be strongly associated to the aetiology of several cardiac and skeletal muscles pathologies related to desmin filaments disorganization and a strong disturbance of desmin interactome. Note that these key proteins of cytoskeleton architecture are extensively modified by post-translational modifications that could affect their functional properties. Therefore, we reviewed in the herein paper the impact of post-translational modifications on the modulation of cellular functions of desmin and its molecular chaperone, the αB-crystallin.

Deletion of Specific Conserved Motifs from the N-Terminal Domain of αB-Crystallin Results in the Activation of Chaperone Functions

Smaller oligomeric chaperones of α-crystallins (αA- and αB-) have received increasing attention due to their improved therapeutic potential in preventing protein aggregating diseases. Our previous study suggested that deleting 54-61 residues from the N-terminal domain (NTD) of αB-crystallin (αBΔ54-61) decreases the oligomer size and increases the chaperone function. Several studies have also suggested that NTD plays a significant role in protein oligomerization and chaperone function. The current study was undertaken to assess the effect of deleting conserved 21-28 residues from the activated αBΔ54-61 (to get αBΔ21-28, Δ54-61) on the structure-function of recombinant αBΔ21-28, Δ54-61. The αBΔ21-28, Δ54-61 mutant shows an 80% reduction in oligomer size and 3- to 25-fold increases in chaperone activity against model substrates when compared to αB-WT. Additionally, the αB∆21-28, ∆54-61 was found to prevent β-amyloid (Aβ1-42) fibril formation in vitro and suppressed Aβ1-42-induced cytotoxicity in ARPE-19 cells in a more effective manner than seen with αB-WT or αB∆54-61. Cytotoxicity and reactive oxygen species (ROS) detection studies with sodium iodate (SI) showed that the double mutant protein has higher anti-apoptotic and anti-oxidative activities than the wild-type or αB∆54-61 in oxidatively stressed cells. Our study shows that the residues 21-28 and 54-61 in αB-crystallin contribute to the oligomerization and modulate chaperone function. The deletion of conserved 21-28 residues further potentiates the activated αBΔ54-61. We propose that increased substrate affinity, altered subunit structure, and assembly leading to smaller oligomers could be the causative factors for the increased chaperone activity of αBΔ21-28, Δ54-61.

Epinephelus coioides Hsp27 negatively regulates innate immune response and apoptosis induced by Singapore grouper iridovirus (SGIV) infection

Heat shock proteins (Hsps) are important for maintaining protein homeostasis and cell survival. In this study, Hsp27 of Epinephelus coioides, an economically important marine fish in China and Southeast Asian countries, was characterized. E. coioides Hsp27 contains the consered ACD_HspB1_like domain and three p38 MAPK phosphorylation sites, located at Thr-13, Thr-60 and Ser-167. E. coioides Hsp27 was distributed in both the cytoplasm and nucleus, its mRNA was detected in all 14 tissues examined, and its expression was up-regulated after challenge with Singapore grouper iridovirus (SGIV), an important E. coioides pathogen. Over-expression of E. coioides Hsp27 significantly upregulated the expressions of the key SGIV genes (VP19, LITAF, MCP, and ICP18), downgraded the expressions of the E. coioides immune factors (IRF3, IRF7, ISG15, and TRAF6) and proinflammatory factors (TNF-α, IL-8), downgraded the activation of nuclear factor kappa-B (NF-κB) and activator protein-1 (AP-1), and substantially inhibited the cell apoptosis induced by SGIV infection. These data illustrated that E. coioides Hsp27 might be involved in SGIV infection by negatively regulating the innate immune response.

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.

The Effect of Oxidized Dopamine on the Structure and Molecular Chaperone Function of the Small Heat-Shock Proteins, αB-Crystallin and Hsp27

Oxidation of the neurotransmitter, dopamine (DA), is a pathological hallmark of Parkinson’s disease (PD). Oxidized DA forms adducts with proteins which can alter their functionality. αB-crystallin and Hsp27 are intracellular, small heat-shock molecular chaperone proteins (sHsps) which form the first line of defense to prevent protein aggregation under conditions of cellular stress. In vitro, the effects of oxidized DA on the structure and function of αB-crystallin and Hsp27 were investigated. Oxidized DA promoted the cross-linking of αB-crystallin and Hsp27 to form well-defined dimer, trimer, tetramer, etc., species, as monitored by SDS-PAGE. Lysine residues were involved in the cross-links. The secondary structure of the sHsps was not altered significantly upon cross-linking with oxidized DA but their oligomeric size was increased. When modified with a molar equivalent of DA, sHsp chaperone functionality was largely retained in preventing both amorphous and amyloid fibrillar aggregation, including fibril formation of mutant (A53T) α-synuclein, a protein whose aggregation is associated with autosomal PD. In the main, higher levels of sHsp modification with DA led to a reduction in chaperone effectiveness. In vivo, DA is sequestered into acidic vesicles to prevent its oxidation and, intracellularly, oxidation is minimized by mM levels of the antioxidant, glutathione. In vitro, acidic pH and glutathione prevented the formation of oxidized DA-induced cross-linking of the sHsps. Oxidized DA-modified αB-crystallin and Hsp27 were not cytotoxic. In a cellular context, retention of significant chaperone functionality by mildly oxidized DA-modified sHsps would contribute to proteostasis by preventing protein aggregation (particularly of α-synuclein) that is associated with PD.

Come Together: Protein Assemblies, Aggregates and the Sarcostat at the Heart of Cardiac Myocyte Homeostasis

Homeostasis in vertebrate systems is contingent on normal cardiac function. This, in turn, depends on intricate protein-based cellular machinery, both for contractile function, as well as, durability of cardiac myocytes. The cardiac small heat shock protein (csHsp) chaperone system, highlighted by αB-crystallin (CRYAB), a small heat shock protein (sHsp) that forms ∼3–5% of total cardiac mass, plays critical roles in maintaining proteostatic function via formation of self-assembled multimeric chaperones. In this work, we review these ancient proteins, from the evolutionarily preserved role of homologs in protists, fungi and invertebrate systems, as well as, the role of sHsps and chaperones in maintaining cardiac myocyte structure and function. We propose the concept of the “sarcostat” as a protein quality control mechanism in the sarcomere. The roles of the proteasomal and lysosomal proteostatic network, as well as, the roles of the aggresome, self-assembling protein complexes and protein aggregation are discussed in the context of cardiac myocyte homeostasis. Finally, we will review the potential for targeting the csHsp system as a novel therapeutic approach to prevent and treat cardiomyopathy and heart failure.

Structural and functional characterization of D109H and R69C mutant versions of human αB-crystallin: The biochemical pathomechanism underlying cataract and myopathy development

In human αB-crystallin (αB-Cry), the highly conserved residues arginine 69 (R69) and aspartate 109 (D109) are located within a critical motif of α-crystallin domain (ACD), contributing to the subunit interactions and oligomeric assembly. Recently, two missense mutations (R69C and D109H) in human αB-Cry have been reported to cause congenital cataract and myopathy disorders. We used various spectroscopic techniques, dynamic light scattering (DLS), small-angle X-ray scattering (SAXS), gel electrophoresis and transmission electron microscopy (TEM) to show how these mutations cause significant changes in structure, amyloidogenic feature and biological function of human αB-Cry. These pathogenic mutations resulted in the important alterations of the secondary, tertiary and oligomeric (quaternary) structures of human αB-Cry. The missense mutations were also capable to significantly increase the amyloidogenic propensity of human αB-Cry and to diminish the chaperone-like activity of this protein. The above mentioned changes were observed more noticeably after D109H mutation. The detrimental effects of D109H mutation may be due to the loss of salt bridge with R120 in the dimeric interface, flagging the anti-aggregation ability of αB-Cry chaperone. In conclusion, the R69C and D109H mutations displayed a significant damaging effect on the structure and chaperone function of human αB-Cry which could be considered as their biochemical pathomechanisms in development of congenital cataract and myopathy disorders.

Small Heat Shock Proteins, Big Impact on Protein Aggregation in Neurodegenerative Disease

Misfolding, aggregation, and aberrant accumulation of proteins are central components in the progression of neurodegenerative disease. Cellular molecular chaperone systems modulate proteostasis, and, therefore, are primed to influence aberrant protein-induced neurotoxicity and disease progression. Molecular chaperones have a wide range of functions from facilitating proper nascent folding and refolding to degradation or sequestration of misfolded substrates. In disease states, molecular chaperones can display protective or aberrant effects, including the promotion and stabilization of toxic protein aggregates. This seems to be dependent on the aggregating protein and discrete chaperone interaction. Small heat shock proteins (sHsps) are a class of molecular chaperones that typically associate early with misfolded proteins. These interactions hold proteins in a reversible state that helps facilitate refolding or degradation by other chaperones and co-factors. These sHsp interactions require dynamic oligomerization state changes in response to diverse cellular triggers and, unlike later steps in the chaperone cascade of events, are ATP-independent. Here, we review evidence for modulation of neurodegenerative disease-relevant protein aggregation by sHsps. This includes data supporting direct physical interactions and potential roles of sHsps in the stewardship of pathological protein aggregates in brain. A greater understanding of the mechanisms of sHsp chaperone activity may help in the development of novel therapeutic strategies to modulate the aggregation of pathological, amyloidogenic proteins. sHsps-targeting strategies including modulators of expression or post-translational modification of endogenous sHsps, small molecules targeted to sHsp domains, and delivery of engineered molecular chaperones, are also discussed.

Targeting Heat Shock Protein 27 in Cancer: A Druggable Target for Cancer Treatment?

Heat shock protein 27 (HSP27), induced by heat shock, environmental, and pathophysiological stressors, is a multi-functional protein that acts as a protein chaperone and an antioxidant. HSP27 plays a significant role in the inhibition of apoptosis and actin cytoskeletal remodeling. HSP27 is upregulated in many cancers and is associated with a poor prognosis, as well as treatment resistance, whereby cells are protected from therapeutic agents that normally induce apoptosis. This review highlights the most recent findings and role of HSP27 in cancer, as well as the strategies for using HSP27 inhibitors for therapeutic purposes.

Cooperative Assembly of Hsp70 Subdomain Clusters

Many molecular chaperones exist as oligomeric complexes in their functional states, yet the physical determinants underlying such self-assembly behavior, as well as the role of oligomerization in the activity of molecular chaperones in inhibiting protein aggregation, have proven to be difficult to define. Here, we demonstrate direct measurements under native conditions of the changes in the average oligomer populations of a chaperone system as a function of concentration and time and thus determine the thermodynamic and kinetic parameters governing the self-assembly process. We access this self-assembly behavior in real time under native-like conditions by monitoring the changes in the micrometer-scale diffusion of the different complexes in time and space using a microfluidic platform. Using this approach, we find that the oligomerization mechanism of the Hsp70 subdomain occurs in a cooperative manner and involves structural constraints that limit the size of the species formed beyond the limits imposed by mass balance. These results illustrate the ability of microfluidic methods to probe polydisperse protein self-assembly in real time in solution and to shed light on the nature and dynamics of oligomerization processes.

Effect of N-terminal region of nuclear Drosophila melanogaster small heat shock protein DmHsp27 on function and quaternary structure

The importance of the N-terminal region (NTR) in the oligomerization and chaperone-like activity of the Drosophila melanogaster small nuclear heat shock protein DmHsp27 was investigated by mutagenesis using size exclusion chromatography and native gel electrophoresis. Mutation of two sites of phosphorylation in the N-terminal region, S58 and S75, did not affect the oligomerization equilibrium or the intracellular localization of DmHsp27 when transfected into mammalian cells. Deletion or mutation of specific residues within the NTR region delineated a motif (FGFG) important for the oligomeric structure and chaperone-like activity of this sHsp. While deletion of the full N-terminal region, resulted in total loss of chaperone-like activity, removal of the (FGFG) at position 29 to 32 or single mutation of F29A/Y, G30R and G32R enhanced oligomerization and chaperoning capacity under non-heat shock conditions in the insulin assay suggesting the importance of this site for chaperone activity. Unlike mammalian sHsps DmHsp27 heat activation leads to enhanced association of oligomers to form large structures of approximately 1100 kDa. A new mechanism of thermal activation for DmHsp27 is presented.

sHSPdb: a database for the analysis of small Heat Shock Proteins

BACKGROUND

small Heat Shock Proteins (sHSP) is a wide proteins family. SHSP are found in all kingdoms and they play critical roles in plant stress tolerance mechanisms (as well as in pathogenic microorganisms and are implicated in human diseases).

RESULTS

sHSPdb (small Heat Shock Proteins database) is an integrated resource containing non-redundant, full-length and curated sequences of sHSP, classified on the basis of amino acids motifs and physico-chemical properties. sHSPdb gathers data about sHSP defined by various databases (Uniprot, PFAM, CDD, InterPro). It provides a browser interface for retrieving information from the whole database and a search interface using various criteria for retrieving a refined subset of entries. Physicochemical properties, amino acid composition and combinations are calculated for each entry. sHSPdb provides automatic statistical analysis of all sHSP properties. Among various possibilities, sHSPdb allows BLAST searches, alignment of selected sequences and submission of sequences.

CONCLUSIONS

sHSPdb is a new database containing information about sHSP from all kingdoms. sHSPdb provides a classification of sHSP, as well as tools and data for the analysis of the structure - function relationships of sHSP. Data are mainly related to various physico-chemical properties of the amino acids sequences of sHSP. sHSPdb is accessible at http://forge.info.univ-angers.fr/~gh/Shspdb/index.php .

Oligomers of Heat-Shock Proteins: Structures That Don't Imply Function

Most proteins must remain soluble in the cytosol in order to perform their biological functions. To protect against undesired protein aggregation, living cells maintain a population of molecular chaperones that ensure the solubility of the proteome. Here we report simulations of a lattice model of interacting proteins to understand how low concentrations of passive molecular chaperones, such as small heat-shock proteins, suppress thermodynamic instabilities in protein solutions. Given fixed concentrations of chaperones and client proteins, the solubility of the proteome can be increased by tuning the chaperone-client binding strength. Surprisingly, we find that the binding strength that optimizes solubility while preventing irreversible chaperone binding also promotes the formation of weakly bound chaperone oligomers, although the presence of these oligomers does not significantly affect the thermodynamic stability of the solution. Such oligomers are commonly observed in experiments on small heat-shock proteins, but their connection to the biological function of these chaperones has remained unclear. Our simulations suggest that this clustering may not have any essential biological function, but rather emerges as a natural side-effect of optimizing the thermodynamic stability of the proteome.

Interaction of βA3-Crystallin with Deamidated Mutants of αA- and αB-Crystallins

Interaction among crystallins is required for the maintenance of lens transparency. Deamidation is one of the most common post-translational modifications in crystallins, which results in incorrect interaction and leads to aggregate formation. Various studies have established interaction among the α- and β-crystallins. Here, we investigated the effects of the deamidation of αA- and αB-crystallins on their interaction with βA3-crystallin using surface plasmon resonance (SPR) and fluorescence lifetime imaging microscopy-fluorescence resonance energy transfer (FLIM-FRET) methods. SPR analysis confirmed adherence of WT αA- and WT αB-crystallins and their deamidated mutants with βA3-crystallin. The deamidated mutants of αA–crystallin (αA N101D and αA N123D) displayed lower adherence propensity for βA3-crystallin relative to the binding affinity shown by WT αA-crystallin. Among αB-crystallin mutants, αB N78D displayed higher adherence propensity whereas αB N146D mutant showed slightly lower binding affinity for βA3-crystallin relative to that shown by WT αB-crystallin. Under the in vivo condition (FLIM-FRET), both αA-deamidated mutants (αA N101D and αA N123D) exhibited strong interaction with βA3-crystallin (32±4% and 36±4% FRET efficiencies, respectively) compared to WT αA-crystallin (18±4%). Similarly, the αB N78D and αB N146D mutants showed strong interaction (36±4% and 22±4% FRET efficiencies, respectively) with βA3-crystallin compared to 18±4% FRET efficiency of WT αB-crystallin. Further, FLIM-FRET analysis of the C-terminal domain (CTE), N-terminal domain (NTD), and core domain (CD) of αA- and αB-crystallins with βA3-crystallin suggested that interaction sites most likely reside in the αA CTE and αB NTD regions, respectively, as these domains showed the highest FRET efficiencies. Overall, results suggest that similar to WT αA- and WTαB-crystallins, the deamidated mutants showed strong interactionfor βA3-crystallin. Variable in vitro and in vivo interactions are most likely due to the mutant’s large size oligomers, reduced hydrophobicity, and altered structures. Together, the results suggest that deamidation of α-crystallin may facilitate greater interaction and the formation of large oligomers with other crystallins, and this may contribute to the cataractogenic mechanism.

Biophysical approaches for the study of interactions between molecular chaperones and protein aggregates

Molecular chaperones are key components of the arsenal of cellular defence mechanisms active against protein aggregation. In addition to their established role in assisting protein folding, increasing evidence indicates that molecular chaperones are able to protect against a range of potentially damaging aspects of protein behaviour, including misfolding and aggregation events that can result in the generation of aberrant protein assemblies whose formation is implicated in the onset and progression of neurodegenerative disorders such as Alzheimer's and Parkinson's diseases. The interactions between molecular chaperones and different amyloidogenic protein species are difficult to study owing to the inherent heterogeneity of the aggregation process as well as the dynamic nature of molecular chaperones under physiological conditions. As a consequence, understanding the detailed microscopic mechanisms underlying the nature and means of inhibition of aggregate formation remains challenging yet is a key objective for protein biophysics. In this review, we discuss recent results from biophysical studies on the interactions between molecular chaperones and protein aggregates. In particular, we focus on the insights gained from current experimental techniques into the dynamics of the oligomerisation process of molecular chaperones, and highlight the opportunities that future biophysical approaches have in advancing our understanding of the great variety of biological functions of this important class of proteins.

Structure and function of α-crystallins: Traversing from in vitro to in vivo

BACKGROUND

The two α-crystallins (αA- and αB-crystallin) are major components of our eye lenses. Their key function there is to preserve lens transparency which is a challenging task as the protein turnover in the lens is low necessitating the stability and longevity of the constituent proteins. α-Crystallins are members of the small heat shock protein family. αB-crystallin is also expressed in other cell types.

SCOPE OF THE REVIEW

The review summarizes the current concepts on the polydisperse structure of the α-crystallin oligomer and its chaperone function with a focus on the inherent complexity and highlighting gaps between in vitro and in vivo studies.

MAJOR CONCLUSIONS

Both α-crystallins protect proteins from irreversible aggregation in a promiscuous manner. In maintaining eye lens transparency, they reduce the formation of light scattering particles and balance the interactions between lens crystallins. Important for these functions is their structural dynamics and heterogeneity as well as the regulation of these processes which we are beginning to understand. However, currently, it still remains elusive to which extent the in vitro observed properties of α-crystallins reflect the highly crowded situation in the lens.

GENERAL SIGNIFICANCE

Since α-crystallins play an important role in preventing cataract in the eye lens and in the development of diverse diseases, understanding their mechanism and substrate spectra is of importance. To bridge the gap between the concepts established in vitro and the in vivo function of α-crystallins, the joining of forces between different scientific disciplines and the combination of diverse techniques in hybrid approaches are necessary. This article is part of a Special Issue entitled Crystallin Biochemistry in Health and Disease.

Lens Biology and Biochemistry

The primary function of the lens resides in its transparency and ability to focus light on the retina. These require both that the lens cells contain high concentrations of densely packed lens crystallins to maintain a refractive index constant over distances approximating the wavelength of the light to be transmitted, and a specific arrangement of anterior epithelial cells and arcuate fiber cells lacking organelles in the nucleus to avoid blocking transmission of light. Because cells in the lens nucleus have shed their organelles, lens crystallins have to last for the lifetime of the organism, and are specifically adapted to this function. The lens crystallins comprise two major families: the βγ-crystallins are among the most stable proteins known and the α-crystallins, which have a chaperone-like function. Other proteins and metabolic activities of the lens are primarily organized to protect the crystallins from damage over time and to maintain homeostasis of the lens cells. Membrane protein channels maintain osmotic and ionic balance across the lens, while the lens cytoskeleton provides for the specific shape of the lens cells, especially the fiber cells of the nucleus. Perhaps most importantly, a large part of the metabolic activity in the lens is directed toward maintaining a reduced state, which shelters the lens crystallins and other cellular components from damage from UV light and oxidative stress. Finally, the energy requirements of the lens are met largely by glycolysis and the pentose phosphate pathway, perhaps in response to the avascular nature of the lens. Together, all these systems cooperate to maintain lens transparency over time.

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.

Preventing α-synuclein aggregation: the role of the small heat-shock molecular chaperone proteins

Protein homeostasis, or proteostasis, is the process of maintaining the conformational and functional integrity of the proteome. The failure of proteostasis can result in the accumulation of non-native proteins leading to their aggregation and deposition in cells and in tissues. The amyloid fibrillar aggregation of the protein α-synuclein into Lewy bodies and Lewy neuritis is associated with neurodegenerative diseases classified as α-synucleinopathies, which include Parkinson's disease and dementia with Lewy bodies. The small heat-shock proteins (sHsps) are molecular chaperones that are one of the cell's first lines of defence against protein aggregation. They act to stabilise partially folded protein intermediates, in an ATP-independent manner, to maintain cellular proteostasis under stress conditions. Thus, the sHsps appear ideally suited to protect against α-synuclein aggregation, yet these fail to do so in the context of the α-synucleinopathies. This review discusses how sHsps interact with α-synuclein to prevent its aggregation and, in doing so, highlights the multi-faceted nature of the mechanisms used by sHsps to prevent the fibrillar aggregation of proteins. It also examines what factors may contribute to α-synuclein escaping the sHsp chaperones in the context of the α-synucleinopathies.

The structured core domain of αB-crystallin can prevent amyloid fibrillation and associated toxicity

Mammalian small heat-shock proteins (sHSPs) are molecular chaperones that form polydisperse and dynamic complexes with target proteins, serving as a first line of defense in preventing their aggregation into either amorphous deposits or amyloid fibrils. Their apparently broad target specificity makes sHSPs attractive for investigating ways to tackle disorders of protein aggregation. The two most abundant sHSPs in human tissue are αB-crystallin (ABC) and HSP27; here we present high-resolution structures of their core domains (cABC, cHSP27), each in complex with a segment of their respective C-terminal regions. We find that both truncated proteins dimerize, and although this interface is labile in the case of cABC, in cHSP27 the dimer can be cross-linked by an intermonomer disulfide linkage. Using cHSP27 as a template, we have designed an equivalently locked cABC to enable us to investigate the functional role played by oligomerization, disordered N and C termini, subunit exchange, and variable dimer interfaces in ABC. We have assayed the ability of the different forms of ABC to prevent protein aggregation in vitro. Remarkably, we find that cABC has chaperone activity comparable to that of the full-length protein, even when monomer dissociation is restricted through disulfide linkage. Furthermore, cABC is a potent inhibitor of amyloid fibril formation and, by slowing the rate of its aggregation, effectively reduces the toxicity of amyloid-β peptide to cells. Overall we present a small chaperone unit together with its atomic coordinates that potentially enables the rational design of more effective chaperones and amyloid inhibitors.

An unusual dimeric small heat shock protein provides insight into the mechanism of this class of chaperones

Small heat shock proteins (sHSPs) are virtually ubiquitous stress proteins that are also found in many normal tissues and accumulate in diseases of protein folding. They generally act as ATP-independent chaperones to bind and stabilize denaturing proteins that can be later reactivated by ATP-dependent Hsp70/DnaK, but the mechanism of substrate capture by sHSPs remains poorly understood. A majority of sHSPs form large oligomers, a property that has been linked to their effective chaperone action. We describe AtHsp18.5 from Arabidopsis thaliana, demonstrating that it is dimeric and exhibits robust chaperone activity, which adds support to the model that suboligomeric sHSP forms are a substrate binding species. Notably, like oligomeric sHSPs, when bound to substrate, AtHsp18.5 assembles into large complexes, indicating that reformation of sHSP oligomeric contacts is not required for assembly of sHSP-substrate complexes. Monomers of AtHsp18.5 freely exchange between dimers but fail to coassemble in vitro with dodecameric plant cytosolic sHSPs, suggesting that AtHsp18.5 does not interact by coassembly with these other sHSPs in vivo. Data from controlled proteolysis and hydrogen-deuterium exchange coupled with mass spectrometry show that the N- and C-termini of AtHsp18.5 are highly accessible and lack stable secondary structure, most likely a requirement for substrate interaction. Chaperone activity of a series of AtHsp18.5 truncation mutants confirms that the N-terminal arm is required for substrate protection and that different substrates interact differently with the N-terminal arm. In total, these data imply that the core α-crystallin domain of the sHSPs is a platform for flexible arms that capture substrates to maintain their solubility.

The oligomer plasticity of the small heat-shock protein Lo18 from Oenococcus oeni influences its role in both membrane stabilization and protein protection

The ability of the small Hsp (heat-shock protein) Lo18 from Oenococcus oeni to modulate the membrane fluidity of liposomes or to reduce the thermal aggregation of proteins was studied as a function of the pH in the range 5-9. We have determined by size-exclusion chromatography and analytical ultracentrifugation that Lo18 assembles essentially as a 16-mer at acidic pH. Its quaternary structure evolves to a mixture of lower molecular mass oligomers probably in dynamic equilibrium when the pH increases. The best Lo18 activities are observed at pH 7 when the particle distribution contains a major proportion of dodecamers. At basic pH, particles corresponding to a dimer prevail and are thought to be the building blocks leading to oligomerization of Lo18. At acidic pH, the dimers are organized in a double-ring of stacked octamers to form the 16-mer as shown by the low-resolution structure determined by electron microscopy. Experiments performed with a modified protein (A123S) shown to preferentially form dimers confirm these results. The α-crystallin domain of Methanococcus jannaschii Hsp16.5, taken as a model of the Lo18 counterpart, fits with the electron microscopy envelope of Lo18.

Small heat-shock proteins: paramedics of the cell

The small heat-shock proteins (sHSPs) comprise a family of molecular chaperones which are widespread but poorly understood. Despite considerable effort, comparatively few high-resolution structures have been determined for the sHSPs, a likely consequence of their tendency to populate ensembles of inter-converting conformational and oligomeric states at equilibrium. This dynamic structure appears to underpin the sHSPs' ability to bind and sequester target proteins rapidly, and renders them the first line of defence against protein aggregation during disease and cellular stress. Here we describe recent studies on the sHSPs, with a particular focus on those which have provided insight into the structure and dynamics of these proteins. The combined literature reveals a picture of a remarkable family of molecular chaperones whose thermodynamic and kinetic properties are exquisitely balanced to allow functional regulation by subtle changes in cellular conditions.

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.

Identification of interaction sites between human betaA3- and alphaA/alphaB-crystallins by mammalian two-hybrid and fluorescence resonance energy transfer acceptor photobleaching methods

Our recent study has shown that betaA3-crystallin along with betaB1- and betaB2-crystallins were part of high molecular weight complex obtained from young, old, and cataractous lenses suggesting potential interactions between alpha- and beta-crystallins (Srivastava, O. P., Srivastava, K., and Chaves, J. M. (2008) Mol. Vis. 14, 1872-1885). To investigate this further, this study was carried out to determine the interaction sites of betaA3-crystallin with alphaA- and alphaB-crystallins. The study employed a mammalian two-hybrid method, an in vivo assay to determine the regions of betaA3-crystallin that interact with alphaA- and alphaB-crystallins. Five regional truncated mutants of betaA3-crystallin were generated using specific primers with deletions of N-terminal extension (NT) (named betaA3-NT), N-terminal extension plus motif I (named betaA3-NT + I), N-terminal extension plus motifs I and II (named betaA3-NT + I + II), motif III plus IV (named betaA3-III + IV), and motif IV (named betaA3-IV). The mammalian two-hybrid studies were complemented with fluorescence resonance energy transfer acceptor photobleaching studies using the above described mutant proteins, fused with DsRed (Red) and AcGFP fluorescent proteins. The results showed that the motifs III and IV of betaA3-crystallin were interactive with alphaA-crystallin, and motifs II and III of betaA3-crystallin primarily interacted with alphaB-crystallin.

ORF-C4 from the early branching eukaryote Giardia lamblia displays characteristics of alpha-crystallin small heat-shock proteins

Giardia lamblia is a medically important protozoan parasite with a basal position in the eukaryotic lineage and is an interesting model to explain the evolution of biochemical events in eukaryotic cells. G. lamblia trophozoites undergo significant changes in order to survive outside the intestine of their host by differentiating into infective cysts. In the present study, we characterize the previously identified Orf-C4 (G. lamblia open reading frame C4) gene, which is considered to be specific to G. lamblia. It encodes a 22 kDa protein that assembles into high-molecular-mass complexes during the entire life cycle of the parasite. ORF-C4 localizes to the cytoplasm of trophozoites and cysts, and forms large spherical aggregates when overexpressed. ORF-C4 overexpression and down-regulation do not affect trophozoite viability; however, differentiation into cysts is slightly delayed when the expression of ORF-C4 is down-regulated. In addition, ORF-C4 protein expression is modified under specific stress-inducing conditions. Neither orthologous proteins nor conserved domains are found in databases by conventional sequence analysis of the predicted protein. However, ORF-C4 contains a region which is similar structurally to the alpha-crystallin domain of sHsps (small heat-shock proteins). In the present study, we show the potential role of ORF-C4 as a small chaperone which is involved in the response to stress (including encystation) in G. lamblia.

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).

Structural and functional diversities between members of the human HSPB, HSPH, HSPA, and DNAJ chaperone families

Heat shock proteins (HSPs) were originally identified as stress-responsive proteins required to deal with proteotoxic stresses. Besides being stress-protective and possible targets for delaying progression of protein folding diseases, mutations in chaperones also have been shown to cause disease (chaperonopathies). The mechanism of action of the "classical", stress-inducible HSPs in serving as molecular chaperones preventing the irreversible aggregation of stress-unfolded or disease-related misfolded proteins is beginning to emerge. However, the human genome encodes several members for each of the various HSP families that are not stress-related but contain conserved domains. Here, we have reviewed the existing literature on the various members of the human HSPB (HSP27), HSPH (HSP110), HSPA (HSP70), and DNAJ (HSP40) families. Apart from structural and functional homologies, several diversities between members and families can be found that not only point to differences in client specificity but also seem to serve differential client handling and processing. How substrate specificity and client processing is determined is far from being understood.

Small heat shock protein Hsp27 is required for proper heart tube formation

The small heat shock protein Hsp27 has been shown to be involved in a diverse array of cellular processes, including cellular stress response, protein chaperone activity, regulation of cellular glutathione levels, apoptotic signaling, and regulation of actin polymerization and stability. Furthermore, mutation within Hsp27 has been associated with the human congenital neuropathy Charcot-Marie Tooth (CMT) disease. Hsp27 is known to be expressed in developing embryonic tissues; however, little has been done to determine the endogenous requirement for Hsp27 in developing embryos. In this study, we show that depletion of XHSP27 protein results in a failure of cardiac progenitor fusion resulting in cardia bifida. Furthermore, we demonstrate a concomitant disorganization of actin filament organization and defects in myofibril assembly. Moreover, these defects are not associated with alterations in specification or differentiation. We have thus demonstrated a critical requirement for XHSP27 in developing cardiac and skeletal muscle tissues.

Truncation of alphaB-crystallin by the myopathy-causing Q151X mutation significantly destabilizes the protein leading to aggregate formation in transfected cells

Here we investigate the effects of a myopathy-causing mutation in alphaB-crystallin, Q151X, upon its structure and function. This mutation removes the C-terminal domain of alphaB-crystallin, which is expected to compromise both its oligomerization and chaperone activity. We compared this to two other alphaB-crystallin mutants (450delA, 464delCT) and also to a series of C-terminal truncations (E164X, E165X, K174X, and A171X). We find that the effects of the Q151X mutation were not always as predicted. Specifically, we have found that although the Q151X mutation decreased oligomerization of alphaB-crystallin and even increased some chaperone activities, it also significantly destabilized alphaB-crystallin causing it to self-aggregate. This conclusion was supported by our analyses of both the other disease-causing mutants and the series of C-terminal truncation constructs of alphaB-crystallin. The 450delA and 464delCT mutants could only be refolded and assayed as a complex with wild type alphaB-crystallin, which was not the case for Q151X alphaB-crystallin. From these studies, we conclude that all three disease-causing mutations (450delA, 464delCT, and Q151X) in the C-terminal extension destabilize alphaB-crystallin and increase its tendency to self-aggregate. We propose that it is this, rather than a catastrophic loss of chaperone activity, which is a major factor in the development of the reported diseases for the three disease-causing mutations studied here. In support of this hypothesis, we show that Q151X alphaB-crystallin is found mainly in the insoluble fraction of cell extracts from transient transfected cells, due to the formation of cytoplasmic aggregates.

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.

Evolutionary analysis of the small heat shock proteins in five complete algal genomes

Small heat shock proteins (sHSPs) are chaperones that are crucial in the heat shock response but also have important nonstress roles within the cell. sHSPs are found in all three domains of life (Bacteria, Archaea, and Eukarya). These proteins are particularly diverse within land plants and the evolutionary origin of the land plant sHSP families is still an open question. Here we describe the identification of 17 small sHSPs from the complete genome sequences of five diverse algae: Chlamydomonas reinhardtii, Cyanidioschyzon merolae, Ostreococcus lucimarinus, Ostreococcus tauri, and Thalassiosira pseudonana. Our analysis indicates that the number and diversity of algal sHSPs are not correlated with adaptation to extreme conditions. While all of the algal sHSPs identified are members of this large and important superfamily, none of these sHSPs are members of the diverse land plant sHSP families. The evolutionary relationships among the algal sHSPs and homologues from bacteria and other eukaryotes are consistent with the hypothesis that the land plant chloroplast and mitochondrion sHSPs did not originate from the endosymbionts of the chloroplast and mitochondria. In addition the evolutionary history of the sHSPs is very different from that of the HSP70s. Finally, our analysis of the algal sHSPs sequences in light of the known sHSP crystal structures and functional data suggests that the sHSPs possess considerable structural and functional diversity.

Conserved F84 and P86 residues in alphaB-crystallin are essential to effectively prevent the aggregation of substrate proteins

Previously, we have shown that residues 73-92 (sequence DRFSVNLDVKHFSPEELKVK) in alphaB-crystallin are involved in preventing the formation of light scattering aggregates by substrate proteins. In this study, we made single substitutions of three conserved amino acid residues (H83 --> A, F84 --> G, and P86 --> A) and a nonconserved amino acid residue (K90 --> C) in the functional region of alphaB-crystallin and evaluated their role in anti-aggregation activity. Mutation of conserved residues led to changes in intrinsic tryptophan intensity, bis-ANS binding, and in the secondary and tertiary structures. The H83A mutation led to a twofold increase in molar mass, while the other mutants did not produce significant changes in the molar mass when compared to that of wild-type protein. The chaperone-like activity of the H83A mutant was enhanced by 15%-20%, and the chaperone-like activity of F84G and P86A mutants was reduced by 50%-65% when compared to the chaperone-like activity of wild-type alphaB-crystallin. The substitution of the nonconserved residue (K90 --> C) did not induce an appreciable change in the structure and function of the mutant protein. Fluorescence resonance energy transfer (FRET) assay demonstrated that destabilized ADH interacted near the K90 region in alphaB-crystallin. The data show that F84 and P86 residues are essential for alphaB-crystallin to effectively prevent the aggregation of substrate proteins. This study further supports the involvement of the residues in the 73-92 region of alphaB-crystallin in substrate protein binding and chaperone-like action.

Structure, stability, and chaperone function of αA-crystallin: Role of N-terminal region

Small heat shock protein alphaA-crystallin, the major protein of the eye lens, is a molecular chaperone. It consists of a highly conserved central domain flanked by the N-terminal and C-terminal regions. In this article we studied the role of the N-terminal domain in the structure and chaperone function of alphaA-crystallin. Using site directed truncation we raised several deletion mutants of alphaA-crystallin and their protein products were expressed in Escherichia coli. Size exclusion chromatography of these purified proteins showed that deletion from the N-terminal beyond the first 20 residues drastically reduced the oligomeric association of alphaA-crystallin and its complete removal resulted in a tetramer. Chaperone activity of alphaA-crystallin, determined by thermal and nonthermal aggregation and refolding assay, decreased with increasing length of deletion and little activity was observed for the tetramer. However it was revealed that N-terminal regions were not responsible for specific recognition of natural substrates and that low affinity substrate binding sites existed in other part of the molecule. The number of exposed hydrophobic sites and the affinity of binding hydrophobic probe bis-ANS as well as protein substrates decreased with N-terminal deletion. The stability of the mutant proteins decreased with increase in the length of deletion. The role of thermodynamic stability, oligomeric size, and surface hydrophobicity in chaperone function is discussed. Detailed analysis showed that the most important role of N-terminal region is to control the oligomerization, which is crucial for the stability and in vivo survival of this protein molecule.

FALS mutations in Cu, Zn superoxide dismutase destabilize the dimer and increase dimer dissociation propensity: a large-scale thermodynamic analysis

Mutations in the dimeric enzyme Cu, Zn superoxide dismutase (SOD1) leading to its aggregation are implicated in the toxicity in familial amyotrophic lateral sclerosis (FALS). We and others have previously shown that aggregation occurs by a pathway involving dimer dissociation, metal-loss from monomers and multimeric assembly of apo-SOD1 monomers. We postulate that FALS mutations cause enhanced aggregation by affecting one or more steps in the pathway, and computationally test this postulate for 75 known mis-sense FALS mutants of SOD1. Based on an extensive thermodynamic analysis of the stability of apo-dimer and apo-monomer forms of these mutants, we classify the mutations into the following groups: 70 out of 75 mutations in SOD1 lead to (i) decreased dimer stability, and/or (ii) increased dimer dissociation, compared to wild type, and four mutations lead to (iii) decreased monomer stability compared to wild type. Our results suggest that enhanced aggregation of SOD1 in FALS occurs due to an increased population of mutant SOD1 apo-monomers compared to wild type. The dissociation of multimeric proteins induced by diverse mutations may be a common theme in several human diseases.

Temperature dependence of chaperone-like activity and oligomeric state of αB-crystallin

The chaperone-like activity and the oligomeric state of alphaB-crystallin were studied at different temperatures and in the presence of urea and thiocyanate. The activity, assessed measuring the ability of alphaB-crystallin to prevent the aggregation of denatured insulin, strongly depends on temperature. While a significant activity increase was detected at 42 degrees C, the presence of urea and thiocyanate does not affect the protein activity in an irreversible way. In-solution SAXS measurements performed in the same experimental conditions showed that alphaB-crystallin forms near-spherical, hollowed, polydisperse oligomers, whose dimensions change above 42 degrees C. Moreover, in the presence of urea and thiocyanate, a global fit analysis confirms the high stability of alphaB-crystallin assemblies in relationship with their variable quaternary structure. In particular, the changes in the inner radius as well as the thickness and dispersion of the protein shell, account for the preservation of the chaperone-like activity.

Studies of alphaB crystallin subunit dynamics by surface plasmon resonance

The molecular chaperone activity of alphaB crystallin, an important stress protein in humans, is regulated by physiological factors, including temperature, pH, Ca2+, and ATP. In this study, the role of these factors in regulating the subunit dynamics of human alphaB crystallin was investigated using surface plasmon resonance (SPR). SPR experiments indicate that at temperatures above 37 degrees C, where alphaB crystallin has been reported to have higher chaperone activity, the subunit dynamics of alphaB crystallin were increased with faster association and dissociation rates. SPR experiments also indicate that interactions between alphaB crystallin subunits were enhanced with much faster association and slower dissociation rates at pH values below 7.0, where alphaB crystallin has been reported to have lower chaperone activity. The results suggest that the dynamic and rapid subunit exchange rate may regulate the chaperone activity of alphaB crystallin. The effect of Ca2+ and ATP on the subunit dynamics of alphaB crystallin was minimal, suggesting that Ca2+ and ATP modulate the chaperone activity of alphaB crystallin without altering the subunit dynamics. Based on the SPR results and previously reported biochemical data for the chaperone activity of alphaB crystallin under different conditions of temperature and pH, a model for the relationship between the subunit dynamics and chaperone activity of alphaB crystallin is established. The model is consistent with previous biochemical data for the chaperone activity and subunit dynamics of small heat shock proteins (sHSPs) and establishes a working hypothesis for the relationship between complex assembly and chaperone activity for sHSPs.

Temperature effects on alpha-crystallin structure probed by 6-bromomethyl-2-(2-furanyl)-3-hydroxychromone, an environmentally sensitive two-wavelength fluorescent dye covalently attached to the single Cys residue

The single Cys residue in the C-terminal domain of bovine eye lens alpha-crystallin was covalently labelled with 6-bromomethyl-2-(2-furanyl)-3-hydroxychromone. This novel SH-reactive two-band ratiometric fluorescent dye is characterized by excited state intramolecular proton transfer reaction yielding two highly emissive N* and T* bands separated by more than 100 nm. Their relative intensities are known to be highly sensitive to the H-bonding ability of the environment. Properties of the environment of the dye attached to the protein were studied under native-like conditions and at a range of elevated temperatures that are known to facilitate alpha-crystallin chaperone-like activity. We observe that on heating, the environment of the dye becomes more flexible and the H-bonding of the dye with the protein vicinity decreases. The spectroscopic properties observed on heating were partially restored after cooling, but the initial values were not reached on the time scale of our experiments (up to 3 h). This suggests that the changes of the dye microenvironment are connected with the rearrangements of alpha-crystallin quaternary structure. Since there is only one Cys residue in alphaA subunit of alpha-crystallin (whereas alphaB subunit contains no Cys), we attributed the observed temperature-induced changes of the dye's microenvironment to the particular site within alpha-crystallin molecule.