Unique structural features of a novel class of small heat shock proteins

The major inducible small heat shock protein HSP20-3 in the tardigrade Ramazzottius varieornatus forms filament-like structures and is an active chaperone

The tardigrade Ramazzottius varieornatus has remarkable resilience to a range of environmental stresses. In this study, we have characterised two members of the small heat shock protein (sHSP) family in R. varieornatus, HSP20-3 and HSP20-6. These are the most highly upregulated sHSPs in response to a 24 h heat shock at 35 0C of adult tardigrades with HSP20-3 being one of the most highly upregulated gene in the whole transcriptome. Both R. varieornatus sHSPs and the human sHSP, CRYAB (HSPB5), were produced recombinantly for comparative structure-function studies. HSP20-3 exhibited a superior chaperone activity than human CRYAB in a heat-induced protein aggregation assay. Both tardigrade sHSPs also formed larger oligomers than CRYAB as assessed by size exclusion chromatography and transmission electron microscopy of negatively stained samples. Whilst both HSP20-3 and HSP20-6 formed particles that were variable in size and larger than the particles formed by CRYAB, only HSP20-3 formed filament-like structures. The particles and filament-like structures formed by HSP20-3 appear inter-related as the filament-like structures often had particles located at their ends. Sequence analyses identified two unique features; an insertion in the middle region of the N-terminal domain (NTD) and preceding the critical-sequence identified in CRYAB, as well as a repeated QNTN-motif located in the C-terminal domain of HSP20-3. The NTD insertion is expected to affect protein-protein interactions and subunit oligomerisation. Removal of the repeated QNTN-motif abolished HSP20-3 chaperone activity and also affected the assembly of the filament-like structures. We discuss the potential contribution of HSP20-3 to protein condensate formation.

Neuroprotective effects of a medium chain fatty acid, decanoic acid, isolated from H. leucospilota against Parkinsonism in C. elegans PD model

Sea cucumbers are marine organism that have long been used for food and traditional medicine in Asian countries. Recently, we have shown that ethyl acetate fraction (HLEA) of the crude extract of the black sea cucumber, Holothuria leucospilota, could alleviate Parkinsonism in Caenorhabditis elegans PD models. In this study, we found that the effective neuroprotective activity is attributed to HLEA-P1 compound chemically isolated and identified in H. leucospilota ethyl acetate. We reported here that HLEA-P1 could attenuate DAergic neurodegeneration, improve DAergic-dependent behaviors, reduce oxidative stress in 6-OHDA-induced C. elegans. In addition, HLEA-P1 reduced α-synuclein aggregation, improved behavior deficit and recovered lipid deposition in transgenic C. elegans overexpressing α-synuclein. We also found that HLEA-P1 activates nuclear localization of DAF-16 transcription factor of insulin/IGF-1 signaling (IIS) pathway. Treatment with 25 μg/ml of HLEA-P1 upregulated transcriptional activity of DAF-16 target genes including anti-oxidant genes (such as sod-3) and small heat shock proteins (such as hsp16.1, hsp16.2, and hsp12.6) in 6-OHDA-induced worms. In α-synuclein-overexpressed C. elegans strain, treatment with 5 μg/ml of HLEA-P1 significantly activated mRNA expression of sod-3 and hsp16.2. Chemical analysis demonstrated that HLEA-P1 compound is decanoic acid/capric acid. Taken together, our findings revealed that decanoic acid isolated from H. leucospilota exerts anti-Parkinson effect in C. elegans PD models by partly modulating IIS/DAF-16 pathway.

The Caenorhabditis elegans 12-kDa small heat shock proteins with little in vitro chaperone activity play crucial roles for its dauer formation, longevity, and reproduction

Small heat shock proteins (sHSPs) are known to exhibit in vitro chaperone activity by suppressing the aggregation of misfolded proteins. The 12-kDa sHSPs (Hsp12s) subfamily members from Caenorhabditis elegans, including Hsp12.2, Hsp12.3, and Hsp12.6, however, are devoid of such chaperone activity, and their in vivo functions are poorly understood. Here we verified that Hsp12.1, similar to its homologs Hsp12.2, Hsp12.3, and Hsp12.6, hardly exhibited any chaperone activity. Strikingly, we demonstrated that these Hsp12s seem to play crucial physiological roles in C. elegans, for suppressing dauer formation and promoting both longevity and reproduction. A unique sHSP gene from Filarial nematode worm Brugia malayi was identified such that it encodes two products, one as a full-length Hsp12.6 protein and the other one having an N-terminal arm of normal length but lacks the C-terminal extension. This gene may represent an intermediate form in evolution from a common sHSP to a Hsp12. Together, our study offers insights on what biological functions the chaperone-defective sHSPs may exhibit and also implicates an evolutionary scenario for the unique Hsp12s subfamily.

The noncanonical small heat shock protein HSP-17 from Caenorhabditis elegans is a selective protein aggregase

Small heat shock proteins (sHsps) are conserved, ubiquitous members of the proteostasis network. Canonically, they act as "holdases" and buffer unfolded or misfolded proteins against aggregation in an ATP-independent manner. Whereas bacteria and yeast each have only two sHsps in their genomes, this number is higher in metazoan genomes, suggesting a spatiotemporal and functional specialization in higher eukaryotes. Here, using recombinantly expressed and purified proteins, static light-scattering analysis, and disaggregation assays, we report that the noncanonical sHsp HSP-17 of Caenorhabditis elegans facilitates aggregation of model substrates, such as malate dehydrogenase (MDH), and inhibits disaggregation of luciferase in vitro Experiments with fluorescently tagged HSP-17 under the control of its endogenous promoter revealed that HSP-17 is expressed in the digestive and excretory organs, where its overexpression promotes the aggregation of polyQ proteins and of the endogenous kinase KIN-19. Systemic depletion of hsp-17 shortens C. elegans lifespan and severely reduces fecundity and survival upon prolonged heat stress. HSP-17 is an abundant protein exhibiting opposing chaperone activities on different substrates, indicating that it is a selective protein aggregase with physiological roles in development, digestion, and osmoregulation.

Small heat shock proteins: Simplicity meets complexity

Small heat shock proteins (sHsps) are a ubiquitous and ancient family of ATP-independent molecular chaperones. A key characteristic of sHsps is that they exist in ensembles of iso-energetic oligomeric species differing in size. This property arises from a unique mode of assembly involving several parts of the subunits in a flexible manner. Current evidence suggests that smaller oligomers are more active chaperones. Thus, a shift in the equilibrium of the sHsp ensemble allows regulating the chaperone activity. Different mechanisms have been identified that reversibly change the oligomer equilibrium. The promiscuous interaction with non-native proteins generates complexes that can form aggregate-like structures from which native proteins are restored by ATP-dependent chaperones such as Hsp70 family members. In recent years, this basic paradigm has been expanded, and new roles and new cofactors, as well as variations in structure and regulation of sHsps, have emerged.

Proteomic Analysis of the Schistosoma mansoni Miracidium

Despite extensive control efforts, schistosomiasis continues to be a major public health problem in developing nations in the tropics and sub-tropics. The miracidium, along with the cercaria, both of which are water-borne and free-living, are the only two stages in the life-cycle of Schistosoma mansoni which are involved in host invasion. Miracidia penetrate intermediate host snails and develop into sporocysts, which lead to cercariae that can infect humans. Infection of the snail host by the miracidium represents an ideal point at which to interrupt the parasite’s life-cycle. This research focuses on an analysis of the miracidium proteome, including those proteins that are secreted. We have identified a repertoire of proteins in the S. mansoni miracidium at 2 hours post-hatch, including proteases, venom allergen-like proteins, receptors and HSP70, which might play roles in snail-parasite interplay. Proteins involved in energy production and conservation were prevalent, as were proteins predicted to be associated with defence. This study also provides a strong foundation for further understanding the roles that neurohormones play in host-seeking by schistosomes, with the potential for development of novel anthelmintics that interfere with its various life-cycle stages.

Genome-wide survey and characterization of the small heat shock protein gene family in Bursaphelenchus xylophilus

Temperatures directly influence the distribution and intensity of pine wilt disease, which is caused by the pine wood nematode Bursaphelenchus xylophilus. Small heat shock proteins (sHSPs) are molecular chaperones that contribute to nematode survival during the stress response to high temperatures. Seven B. xylophilus sHSPs (Bx-sHSPs) were identified and studied in a whole-genome shotgun project. The replacement of aromatic amino acids with aliphatic amino acids in motifs was the most significant difference between Bx-sHSPs and Caenorhabditis elegans sHSPs (Ce-sHSPs). In Bx-sHSPs, two motifs showed consensus sequences similar to the known palindromic nGAAn sequence or variants of this sequence. A phylogenetic tree of Bx-sHSPs and corresponding Ce-sHSPs suggests the existence of a one-to-one orthologous relationship for all sHSPs. Gene evolution patterns corresponding to both purifying selection and positive selection were found in orthologous pairs of Ce-sHSPs and Bx-sHSPs. The upregulation of Bx-sHSPs in response to heat stress (30°C) suggests that these proteins play a role in thermoregulation.

Chaperone function and mechanism of small heat-shock proteins

Small heat-shock proteins (sHSPs) are ubiquitous ATP-independent molecular chaperones that play crucial roles in protein quality control in cells. They are able to prevent the aggregation and/or inactivation of various non-native substrate proteins and assist the refolding of these substrates independently or under the help of other ATP-dependent chaperones. Substrate recognition and binding by sHSPs are essential for their chaperone functions. This review focuses on what natural substrate proteins an sHSP protects and how it binds the substrates in cells under fluctuating conditions. It appears that sHSPs of prokaryotes, although being able to bind a wide range of cellular proteins, preferentially protect certain classes of functional proteins, such as translation-related proteins and metabolic enzymes, which may well explain why they could increase the resistance of host cells against various stresses. Mechanistically, the sHSPs of prokaryotes appear to possess numerous multi-type substrate-binding residues and are able to hierarchically activate these residues in a temperature-dependent manner, and thus act as temperature-regulated chaperones. The mechanism of hierarchical activation of substrate-binding residues is also discussed regarding its implication for eukaryotic sHSPs.

A tricistronic heat shock operon is important for stress tolerance of Pseudomonas putida and conserved in many environmental bacteria

Small heat shock proteins (sHsps) including the well-studied IbpA protein from Escherichia coli are molecular chaperones that bind to non-native proteins and prevent them from aggregation. We discovered an entirely unexplored tricistronic small heat shock gene cluster in Pseudomonas putida. The genes pp3314, pp3313 and pp3312 (renamed to hspX, hspY and hspZ respectively) are transcribed in a single transcript. In addition to σ(32) -dependent transcriptional control, translation of the first and second gene of the operon is controlled by RNA thermometers with novel architectures. Biochemical analysis of HspY, HspZ and P. putida IbpA demonstrated that they assemble into homo-oligomers of different sizes whose quaternary structures alter in a temperature-dependent manner. IbpA and HspY are able to prevent the model substrate citrate synthase from thermal aggregation in vitro. Increased stress sensitivity of a P. putida strain lacking HspX, HspY and HspZ revealed an important role of these sHsps in stress adaptation. The hspXYZ operon is conserved among metabolically related bacteria that live in hostile environments including polluted soils. This heat shock operon might act as a protective system to promote survival in such ecological niches.

A neuronal GPCR is critical for the induction of the heat shock response in the nematode C. elegans

In the nematode Caenorhabditis elegans, the heat shock response (HSR) is regulated at the organismal level by a network of thermosensory neurons that senses elevated temperatures and activates the HSR in remote tissues. Which neuronal receptors are required for this signaling mechanism and in which neurons they function are largely unanswered questions. Here we used worms that were engineered to exhibit RNA interference hypersensitivity in neurons to screen for neuronal receptors that are required for the activation of the HSR and identified a putative G-protein coupled receptor (GPCR) as a novel key component of this mechanism. This gene, which we termed GPCR thermal receptor 1 (gtr-1), is expressed in chemosensory neurons and has no role in heat sensing but is critically required for the induction of genes that encode heat shock proteins in non-neural tissues upon exposure to heat. Surprisingly, the knock-down of gtr-1 by RNA interference protected worms expressing the Alzheimer's-disease-linked aggregative peptide Aβ3-42 from proteotoxicity but had no effect on lifespan. This study provides several novel insights: (1) it shows that chemosensory neurons play important roles in the nematode's HSR-regulating mechanism, (2) it shows that lifespan and heat stress resistance are separable, and (3) it strengthens the emerging notion that the ability to respond to heat comes at the expense of protein homeostasis (proteostasis).

The specificity of the interaction between αB-crystallin and desmin filaments and its impact on filament aggregation and cell viability

CRYAB (αB-crystallin) is expressed in many tissues and yet the R120G mutation in CRYAB causes tissue-specific pathologies, namely cardiomyopathy and cataract. Here, we present evidence to demonstrate that there is a specific functional interaction of CRYAB with desmin intermediate filaments that predisposes myocytes to disease caused by the R120G mutation. We use a variety of biochemical and biophysical techniques to show that plant, animal and ascidian small heat-shock proteins (sHSPs) can interact with intermediate filaments. Nevertheless, the mutation R120G in CRYAB does specifically change that interaction when compared with equivalent substitutions in HSP27 (R140G) and into the Caenorhabditis elegans HSP16.2 (R95G). By transient transfection, we show that R120G CRYAB specifically promotes intermediate filament aggregation in MCF7 cells. The transient transfection of R120G CRYAB alone has no significant effect upon cell viability, although bundling of the endogenous intermediate filament network occurs and the mitochondria are concentrated into the perinuclear region. The combination of R120G CRYAB co-transfected with wild-type desmin, however, causes a significant reduction in cell viability. Therefore, we suggest that while there is an innate ability of sHSPs to interact with and to bind to intermediate filaments, it is the specific combination of desmin and CRYAB that compromises cell viability and this is potentially the key to the muscle pathology caused by the R120G CRYAB.

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.

Small heat shock protein expression and functions during development

The expression of small heat shock proteins is tightly regulated during development in multiple organisms. As housekeeping proteins, small heat shock proteins help protect cells from apoptosis, stabilize the cytoskeleton and contribute to proteostasis. Consistently, depletion of one small heat shock protein is usually not detrimental due to a certain level of redundancy between the functions of each small heat shock protein. However, while their stress-induced expression is regulated by heat shock factors, their constitutive expression is under the control of other specific transcription factors, suggesting the existence of very specialized functions. This review focuses on the expression patterns and functions of small heat shock proteins in various organisms during development. This article is part of a Directed Issue entitled: Small HSPs in physiology and pathology.

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.

A comparative proteomic study of the undeveloped and developed Schistosoma mansoni egg and its contents: the miracidium, hatch fluid and secretions

The schistosome egg is the key agent responsible both for transmission of the parasite from human to molluscan host, and is the primary cause of pathogenesis in schistosomiasis. Characterisation of its proteome is a crucial step in understanding the egg's interactions with the mammalian host. We devised a scheme to isolate undeveloped eggs from mature schistosome eggs by Percoll gradient and then fractionate the mature egg into miracidial, hatch fluid and secreted protein preparations. The soluble proteins contained within the five preparations were separated by two-dimensional electrophoresis and their spot patterns compared by image analysis. Large numbers of representative spots were then excised and subjected to tandem mass spectrometry to obtain identities. In this way, the principal components of each sub-proteome were established. Chaperones were the most abundant category, with heat shock protein 70 (HSP70) dominant in the undeveloped egg and Schistosoma mansoni protein 40 (Smp-40) in the miracidium. Cytoskeletal proteins were expressed at similar levels in the undeveloped egg and miracidium, with tubulins the most abundant. The proteins of energy metabolism reflected the change from anaerobic to aerobic metabolism as the miracidium developed. None of the above categories was abundant in the hatch fluid but this peri-miracidial compartment was highly enriched for defence proteins such as thioredoxin. Hatch fluid also contained several host proteins and schistosome proteins of unknown function, highlighting its distinct nature and potentially its role. The egg secretions could not be compared with the other preparations due to their unique composition featuring the previously characterised IL-4-inducing principal of S. mansoni eggs (IPSE), Omega-1, egg secreted protein 15 (ESP15), a micro-exon gene 2 (MEG-2) protein and two members of the recently described MEG-3 family. This last preparation contains the subset of egg proteins that probably enables eggs to escape from host tissues and may also initiate granuloma formation, emphasising the need to establish fully the roles of its components in schistosome biology.

Blockade of Hsp20 phosphorylation exacerbates cardiac ischemia/reperfusion injury by suppressed autophagy and increased cell death

RATIONALE

The levels of a small heat shock protein (Hsp)20 and its phosphorylation are increased on ischemic insults, and overexpression of Hsp20 protects the heart against ischemia/reperfusion injury. However, the mechanism underlying cardioprotection of Hsp20 and especially the role of its phosphorylation in regulating ischemia/reperfusion-induced autophagy, apoptosis, and necrosis remain to be clarified.

OBJECTIVE

Herein, we generated a cardiac-specific overexpression model, carrying nonphosphorylatable Hsp20, where serine 16 was substituted with alanine (Hsp20(S16A)). By subjecting this model to ischemia/reperfusion, we addressed whether: (1) the cardioprotective effects of Hsp20 are associated with serine 16 phosphorylation; (2) blockade of Hsp20 phosphorylation influences the balance between autophagy and cell death; and (3) the aggregation pattern of Hsp20 is altered by its phosphorylation.

METHODS AND RESULTS

Our results demonstrated that Hsp20(S16A) hearts were more sensitive to ischemia/reperfusion injury, evidenced by lower recovery of contractile function and increased necrosis and apoptosis, compared with non-TG hearts. Interestingly, autophagy was activated in non-TG hearts but significantly inhibited in Hsp20(S16A) hearts following ischemia/reperfusion. Accordingly, pretreatment of Hsp20(S16A) hearts with rapamycin, an activator of autophagy, resulted in improvement of functional recovery, compared with saline-treated Hsp20(S16A) hearts. Furthermore, on ischemia/reperfusion, the oligomerization pattern of Hsp20 appeared to shift to higher aggregates in Hsp20(S16A) hearts.

CONCLUSIONS

Collectively, these data indicate that blockade of Ser16-Hsp20 phosphorylation attenuates the cardioprotective effects of Hsp20 against ischemia/reperfusion injury, which may be attributable to suppressed autophagy and increased cell death. Therefore, phosphorylation of Hsp20 at serine 16 may represent a potential therapeutic target in ischemic heart disease.

Microbial small heat shock proteins and their use in biotechnology

Small heat shock proteins (sHsps) exist in almost all organisms. Most organisms have more than one sHsp, but their number can be as high as 65, as found in the eukaryote, Vitis vinifera. The function of sHsps is well-known; they confer thermotolerance to cellular cultures and proteins in cellular extracts during prolonged incubations at elevated temperatures. This demonstrates the ability of sHsps to protect cellular proteins, and to maintain cellular viability under conditions of intensive stress, such as heat shock or chemical treatments. sHsps have several properties that distinguish them from heat shock proteins (Hsps): they function as ATP-independent chaperones, require the flexible assembly and reassembly of oligomeric complex structures for their activation, and exhibit a wide range of substrate-binding capacities. Recent studies indicate that sHsps have important biological functions in thermostability, disaggregation, and proteolysis inhibition. These functions can be harnessed for various applications, including nanobiotechnology, proteomics, bioproduction, and bioseparation. In this review, we discuss the properties and diversity of microbial sHsps, as well as their potential uses in the biotechnology industry.

Role of the IXI/V motif in oligomer assembly and function of StHsp14.0, a small heat shock protein from the acidothermophilic archaeon, Sulfolobus tokodaii strain 7

Small heat shock proteins (sHsps) are one of the most ubiquitous molecular chaperones. They are grouped together based on a conserved domain, the alpha-crystallin domain. Generally, sHsps exist as oligomers of 9-40 subunits, and the oligomers undergo reversible temperature-dependent dissociation into smaller species as dimers, which interact with denaturing substrate proteins. Previous studies have shown that the C-terminal region, especially the consensus IXI/V motif, is responsible for oligomer assembly. In this study, we examined deletions or mutations in the C-terminal region on the oligomer assembly and function of StHsp14.0, an sHsp from an acidothermophilic archaeon, Sulfolobus tokodaii strain 7. Mutated StHsp14.0 with C-terminal deletion or replacement of IIe residues in the IXI/V motif to Ala, Ser, or Phe residues could not form large oligomers and lost chaperone activity. StHsp14.0WKW, whose Ile residues in the IXI/V motif are changed to Trp, existed as an oligomer like that of the wild type. However, it dissociates to small oligomers and exhibits chaperone activity at relatively lowered temperature. Replacement of two Ile residues in the motif to relatively small residues, Ala or Ser, also resulted in the change of beta-sheet rich secondary structure and decrease of hydrophobicity. Interestingly, StHsp14.0 mutant with amino acid replacements to Phe kept almost the same secondary structure and relatively high hydrophobicity despite that it could not form an oligomeric structure. The results show that hydrophobicity and size of the amino acids in the IXI/V motif in the C-terminal region are responsible not only for assembly of the oligomer but also for the maintenance of beta-sheet rich secondary structure and hydrophobicity, which are important for the function of sHsp.

A novel small heat shock protein 12.6 (HSP12.6) from Brugia malayi functions as a human IL-10 receptor binding protein

Phage display cDNA expression library of the third stage larvae (L3) of Brugia malayi was screened for identifying target(s) that bound to the human interleukin-10 receptor (huIL10R). This iterative screening identified an insert that showed significant homology to Caenorhabditis elegans HSP12.6. The gene was designated B. malayi HSP12.6 (BmHSP12.6) and has orthologues in several gastrointestinal nematode genome (Ancylostoma caninum, Ascaris lumbricoides and Ascaris suum) but the gene or gene product has not been studied further in these parasites. Structural analyses of BmHSP12.6 showed that it has a highly conserved alpha-crystallin central domain that is characteristic of other small heat shock proteins (HSPs). BmHSP12.6 has a short N-terminal domain and an unusually small C-terminal domain flanking the crystallin domain suggesting that this protein belongs to a novel class of small HSPs. BmHSP12.6 appears to be differentially transcribed with highest expression in the vertebrate stages of the parasite (L4, adult and mf) compared to its mosquito vector stage (L3). More importantly recombinant BmHSP12.6 bound to huIL10R in a dose dependent fashion and inhibited the binding of human IL-10 (huIL10) to huIL10R in vitro. rBmHSP12.6 also enhanced the growth and proliferation of MC/9 mast cells in vitro similar to huIL10. This study thus describes a novel small HSP from B. malayi that has the capacity to bind to huIL10R, block binding of huIL10 to huIL10R and function similar to huIL10.

Confocal spectrofluorimetric evidence for the hetero-aggregation of sequence-scrambled forms of two model all-beta sheet proteins

Can two unrelated proteins with deliberately compromised folding abilities, marked propensities to aggregate upon increase of protein concentration, and proclivities towards beta sheet formation, be caused to hetero-aggregate? We address this question here using the 'designer' backbone-reversed forms of two model all-beta sheet proteins, E. coli CspA and C. elegans HSP12.6, both earlier created and characterized by our group. These were covalently labeled with fluorescent dyes of well-resolved spectral characteristics [retro-CspA with FITC, and retro-HSP12.6 with TRITC] and then allowed to aggregate within the same reaction vessel. The resultant aggregates are shown by spectrofluorimetry-coupled confocal laser scanning microscopy to constitute uniform mixtures of both proteins, existing within every cylindrical volume element of approximately 200nm diameter, and comparable height, in all sections of the co-aggregated material suggesting that the two proteins do not selectively associate with copies of themselves during aggregation. Thus, it would appear that aggregation can occur without reference to protein molecular identity.

A small heat shock protein of Ostertagia ostertagi: stage-specific expression, heat inducibility, and protection trial

In this study, we isolated and analyzed a small heat shock protein (HSP) of Ostertagia ostertagi (Oo-HSP18). Oo-hsp18 is encoded by a single-copy gene and the full-length cDNA represents an 18-kDa protein. The expression of Oo-hsp18 is highly stage specific and restricted to the adult stage. The protein is synthesized in a tissue-specific manner and localized in the body muscle layer. The levels of Oo-hsp18 mRNAs are sharply induced by heat shock but not by other stressors such as levamisole and H2O2. A vaccination trial with recombinant Oo-HSP18 failed to protect calves against a challenge infection.

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.

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.

Alternate metabolism during the dauer stage of the nematode Caenorhabditis elegans

When environmental conditions are unsuitable to support nematode reproduction, Caenorhabditis elegans arrests development before the onset of sexual maturity and specialised 'dauer' larvae, adapted for dispersal, and extended diapause are formed. Dauer larvae do not feed and their metabolism is dependent on internal food reserves. Adult worms which express defects in the insulin/insulin-like growth factor receptor DAF-2 also display enhanced longevity. Whole genome mRNA expression profiling has demonstrated that C. elegans dauer larvae and daf-2 adults have similar transcription profiles for a cohort of longevity genes. Important components of this enhanced longevity system are the alpha-crystallin family of small heat shock proteins, anti-ROS defence systems, increased activity of cellular detoxification processes and possibly also increased chromatin stability and decreased protein turnover. Anaerobic fermentation pathways are upregulated in dauer larvae, while long-lived daf-2 adults appear to have normal oxidative metabolism. Anabolic pathways are down regulated in dauer larvae (and possibly in daf-2 adults as well), and energy consumption appears to be diverted to enhanced cellular maintenance and detoxification processes in both systems.

Interaction of a Small Heat Shock Protein of the Fission Yeast, Schizosaccharomyces pombe, with a Denatured Protein at Elevated Temperature

We have expressed, purified, and characterized one small heat shock protein of the fission yeast Schizosaccharomyces pombe, SpHsp16.0. SpHsp16.0 was able to protect citrate synthase from thermal aggregation at 45 degrees C with high efficiency. It existed as a hexadecameric globular oligomer near the physiological growth temperature. At elevated temperatures, the oligomer dissociated into small species, probably dimers. The dissociation was completely reversible, and the original oligomer reformed immediately after the temperature dropped. Large complexes of SpHsp16.0 and denatured citrate synthase were observed by size exclusion chromatography and electron microscopy following incubation at 45 degrees C and then cooling. However, such large complexes did not elute from the size exclusion column incubated at 45 degrees C. The denatured citrate synthase protected from aggregation was trapped by a GroEL trap mutant at 45 degrees C. These results suggest that the complex of SpHsp16.0 and denatured citrate synthase at elevated temperatures is in the transient state and has a hydrophobic nature. Analyses of the interaction between SpHsp16.0 and denatured citrate synthase by fluorescence cross-correlation spectrometry have also shown that the characteristics of SpHsp16.0-denatured citrate synthase complex at the elevated temperature are different from those of the large complex obtained after the shift to lowered temperatures.

Taenia solium: characterization of a small heat shock protein (Tsol-sHSP35.6) and its possible relevance to the diagnosis and pathogenesis of neurocysticercosis

A cDNA encoding for a predicted small heat shock protein (sHSP), Tsol-sfISP35.6, has been isolated by antibody screening of a Taenia solium c-DNA library. The clone was a full-length sequence (1172 bp) with an open reading frame of 945 bp and encoded for a 314 amino acid protein with deduced molecular mass of 35.6 kDa, isoelectric point of 5.6 arid the characteristic HSP20/alpha-crystallin domain duplicated. It was highly conserved, with a high sequence similarity with other platyhelminth sHSPs. Western blot analysis, using serum from neurocysticercosis patients (NCC), indicated that the purified Tsol-sHSP35.6 expression product was immunogenic, while in indirect ELISA, using the purified Tsol-sHSP35.6 expression product as antigen and serum samples from pigs and humans, 80% of T. solium infected pigs and 84% of patients with active, or 71% of patients with inactive NCC were sero-positive. The possible relevance of Tsol-sHSP35.6 in the diagnosis and pathogenesis of NCC is discussed.

Investigating hookworm genomes by comparative analysis of two Ancylostoma species

Background

Hookworms, infecting over one billion people, are the mostly closely related major human parasites to the model nematode Caenorhabditis elegans. Applying genomics techniques to these species, we analyzed 3,840 and 3,149 genes from Ancylostoma caninum and A. ceylanicum.

Results

Transcripts originated from libraries representing infective L3 larva, stimulated L3, arrested L3, and adults. Most genes are represented in single stages including abundant transcripts like hsp-20 in infective L3 and vit-3 in adults. Over 80% of the genes have homologs in C. elegans, and nearly 30% of these were with observable RNA interference phenotypes. Homologies were identified to nematode-specific and clade V specific gene families. To study the evolution of hookworm genes, 574 A. caninum / A. ceylanicum orthologs were identified, all of which were found to be under purifying selection with distribution ratios of nonsynonymous to synonymous amino acid substitutions similar to that reported for C. elegans / C. briggsae orthologs. The phylogenetic distance between A. caninum and A. ceylanicum is almost identical to that for C. elegans / C. briggsae.

Conclusion

The genes discovered should substantially accelerate research toward better understanding of the parasites' basic biology as well as new therapies including vaccines and novel anthelmintics.

Mammalian Hsp22 is a heat-inducible small heat-shock protein with chaperone-like activity

A newly identified 22 kDa protein that interacts with Hsp27 (heat-shock protein 27) was shown to possess the characteristic alpha-crystallin domain, hence named Hsp22, and categorized as a member of the sHsp (small Hsp) family. Independent studies from different laboratories reported the protein with different names such as Hsp22, H11 kinase, E2IG1 and HspB8. We have identified, on the basis of the nucleotide sequence analysis, putative heat-shock factor 1 binding sites upstream of the Hsp22 translation start site. We demonstrate that indeed Hsp22 is heat-inducible. We show, in vitro, chaperone-like activity of Hsp22 in preventing dithiothreitol-induced aggregation of insulin and thermal aggregation of citrate synthase. We have cloned rat Hsp22, overexpressed and purified the protein to homogeneity and studied its structural and functional aspects. We find that Hsp22 fragments on storage. MS analysis of fragments suggests that the fragmentation might be due to the presence of labile peptide bonds. We have established conditions to improve its stability. Far-UV CD indicates a randomly coiled structure for Hsp22. Quaternary structure analyses by glycerol density-gradient centrifugation and gel filtration chromatography show that Hsp22 exists as a monomer in vitro, unlike other members of the sHsp family. Hsp22 exhibits significantly exposed hydrophobic surfaces as reported by bis-8-anilinonaphthalene-l-sulphonic acid fluorescence. We find that the chaperone-like activity is temperature dependent. Thus Hsp22 appears to be a true member of the sHsp family, which exists as a monomer in vitro and exhibits chaperone-like activity.

Structure function relationship among α-crystallin related small heat shock proteins

A sequence alignment is presented which permits the detection of the sequence and structural homology among alpha-crystallin subunits, alphaA and alphaB, and distantly related sHsps, MjHsp16.5 and wheat Hsp16.9. This alignment shows that homology extends beyond the alpha-crystallin domain. Variations in the polydisperse quaternary structure appear, in part, dependent upon the N-terminal 18 and 19 amino acids that are essential for subunit interactions in polydisperse sHsps. The hydrophobic sequence that follows these N-terminal amino acids shares a number of aromatic amino acids and has significant homology with MjHsp16.5. In the second exon of alpha-crystallin, sequence homology is concentrated in a region with chaperone and ANS binding sites. It is clear that the binding site for ANS and its derivative, bis-ANS, requires both positively charged amino acids and hydrophobic interactions. Therefore, its binding is not a true measure of hydrophobic surface exposure. The limited homology and secondary structure in the following C-terminal sequences is related to the pattern of association of other sHsp subunits and/or functional differences. Our study suggests that alphaA has evolved in the lens to chaperone exposed beta-sheet edges of the betagamma crystallins and their proteolytic fragments. Also, both time and a harsh environment such as that in the lens interior, beta-sheet proteins would naturally generate beta-sheet edges. The interaction between such edges results in insoluble, abnormal protein aggregation and in the lens, light scattering elements that cause cataract.

A dual role for the N-terminal region of Mycobacterium tuberculosis Hsp16.3 in self-oligomerization and binding denaturing substrate proteins

The N-terminal regions, which are highly variable in small heat-shock proteins, were found to be structurally disordered in all the 24 subunits of Methanococcus jannaschii Hsp16.5 oligomer and half of the 12 subunits of wheat Hsp16.9 oligomer. The structural and functional roles of the corresponding region (potentially disordered) in Mycobacterium tuberculosis Hsp16.3, existing as nonamers, were investigated in this work. The data demonstrate that the mutant Hsp16.3 protein with 35 N-terminal residues removed (DeltaN35) existed as trimers/dimers rather than as nonamers, failing to bind the hydrophobic probe (1,1'-bi(4-anilino)naphthalene-5,5'-disulfonic acid) and exhibiting no chaperone-like activity. Nevertheless, another mutant protein with the C-terminal extension (of nine residues) removed, although existing predominantly as dimers, exhibited efficient chaperone-like activity even at room temperatures, indicating that pre-existence as nonamers is not a prerequisite for its chaperone-like activity. Meanwhile, the mutant protein with both the N- and C-terminal ends removed fully exists as a dimer lacking any chaperone-like activity. Furthermore, the N-terminal region alone, either as a synthesized peptide or in fusion protein with glutathione S-transferase, was capable of interacting with denaturing proteins. These observations strongly suggest that the N-terminal region of Hsp16.3 is not only involved in self-oligomerization but also contains the critical site for substrate binding. Such a dual role for the N-terminal region would provide an effective mechanism for the small heat-shock protein to modulate its chaperone-like activity through oligomeric dissociation/reassociation. In addition, this study demonstrated that the wild-type protein was able to form heterononamers with DeltaN35 via subunit exchange at a subunit ratio of 2:1. This implies that the 35 N-terminal residues in three of the nine subunits in the wild-type nonamer are not needed for the assembly of nonamers from trimers and are thus probably structurally disordered.

Expression and purification of a small heat shock protein from the plant pathogen Xylella fastidiosa

The small heat shock proteins (smHSPs) belong to a family of proteins that function as molecular chaperones by preventing protein aggregation and are also known to contain a conserved region termed alpha-crystallin domain. Here, we report the expression, purification, and partial characterization of a novel smHSP (HSP17.9) from the phytopathogen Xylella fastidiosa, causal agent of the citrus variegated chlorosis (CVC). The gene was cloned into a pET32-Xa/LIC vector to over-express the protein coupled with fusion tags in Escherichia coli BL21(DE3). The expressed HSP17.9 was purified by immobilized metal affinity chromatography (IMAC) and had its identity determined by mass spectrometry (MALDI-TOF). The correct folding of the purified recombinant protein was verified by circular dichroism spectroscopy. Finally, the HSP17.9 protein also proved to efficiently prevent induced aggregation of insulin, strongly indicating a chaperone-like activity.

Tsp36, a tapeworm small heat-shock protein with a duplicated α-crystallin domain, forms dimers and tetramers with good chaperone-like activity

Small heat shock proteins (sHSPs), which range in monomer size between 12 and 42 kDa, are characterized by a conserved C-terminal alpha-crystallin domain of 80-100 residues. They generally form large homo- or heteromeric complexes, and typically have in vitro chaperone-like activity, keeping unfolding proteins in solution. A special type of sHSP, with a duplicated alpha-crystallin domain, is present in parasitic flatworms (Platyhelminthes). Considering that an alpha-crystallin domain is essential for the oligomerization and chaperone-like properties of sHSPs, we characterized Tsp36 from the tapeworm Taenia saginata. Both wild-type Tsp36 and a mutant (Tsp36C-->R) in which the single cysteine has been replaced by arginine were expressed and purified. Far-UV CD measurements of Tsp36 were in agreement with secondary structure predictions, which indicated alpha-helical structure in the N-terminal region and the expected beta-sandwich structure for the two alpha-crystallin domains. Gel permeation chromatography and nano-ESI-MS showed that wild type Tsp36 forms dimers in a reducing environment, and tetramers in a non-reducing environment. The tetramers are stabilized by disulfide bridges involving a large proportion of the Tsp36 monomers. Tsp36C-->R exclusively occurs as dimers according to gel permeation chromatography, while the nondisulfide bonded fraction of wild type Tsp36 dissociates from tetramers into dimers under nonreducing conditions at increased temperature (43 degrees C). The tetrameric form of Tsp36 has a greater chaperone-like activity than the dimeric form.

Role of the N-terminal region of the crenarchaeal sHsp, StHsp14.0, in thermal-induced disassembly of the complex and molecular chaperone activity

Small heat shock protein is a ubiquitous molecular chaperone, which consists of a non-conserved N-terminal region followed by a conserved alpha-crystallin domain. To understand the role of the N-terminal region, we constructed N-terminal truncation mutants of StHsp14.0, the sHsp from Sulfolobus tokodaii strain 7. All the mutants formed a stable oligomeric complex similar to that of the wild type. Electron microscopy and size exclusion chromatography-multiangle light scattering showed that the N-terminal region should locate in the center of the oligomeric particle. The mutants exhibited reduced chaperone activity for the protection of 3-isopropylmalate dehydrogenase from thermal aggregation. This reduction correlates with lowered subunit exchange efficiency. The oligomeric structure was retained even after incubation at 90 degrees C. These results suggest that the N-terminal region of StHsp14.0 functions in the thermally induced disassembly of the complex.

Small heat shock proteins from extremophiles: a review

Many microorganisms from extreme environments have been well characterized, and increasing access to genomic sequence data has recently allowed the analysis of the protein families related to stress responses. Heat shock proteins appear to be ubiquitous in extremophiles. In this review, we focus on the family of small heat shock proteins (sHSPs) from extremophiles, which are alpha-crystallin homologues. Like the alpha-crystallin eye lens proteins, sHSPs act as molecular chaperones and prevent aggregation of denatured proteins under heat and desiccation stress. Many putative sHSP homologues have been identified in the genomic sequences of all classes of extremophiles. Current studies of shsp gene expression have revealed mechanisms of regulation and activity distinct from other known hsp gene regulation systems. Biochemical studies on sHSPs are limited to thermophilic and hyperthermophilic organisms, and the only two available crystal structures of sHSPs from Methanocaldococcus jannaschii, a hyperthermophilic archaeon and a mesophilic eukaryote, have contributed significantly to an understanding of the mechanisms of action of sHSPs, although many aspects remain unclear.

Molecular chaperones, stress resistance and development in Artemia franciscana

Embryos of the brine shrimp, Artemia franciscana, either develop directly into swimming larvae or are released from females as encysted gastrulae (cysts) which enter diapause, a reversible state of dormancy. Metabolic activity in diapause cysts is very low and these embryos are remarkably resistant to physiological stresses. Encysting embryos, but not those undergoing uninterrupted development, synthesize large amounts of two proteins, namely p26 and artemin. Cloning and sequencing demonstrated p26 is a small heat shock/alpha-crystallin protein while artemin has structural similarity to ferritin. p26 exhibits molecular chaperone activity in vitro, moves reversibly into nuclei during stress and confers thermotolerance on transformed organisms, suggesting critical roles in cyst development. The function of artemin is unknown. Encysted Artemia also contain an abundance of trehalose, a disaccharide capable of protecting embryos. Artemia represent a novel experimental system where the developmental functions of small heat shock/alpha-crystallin proteins and other stress response elements can be explored.

A backbone-reversed form of an all-beta alpha-crystallin domain from a small heat-shock protein (retro-HSP12.6) folds and assembles into structured multimers

The structural consequences of polypeptide backbone reversal ("retro" modification) remain largely unexplored, in particular, for the retro forms of globular all-beta-sheet proteins. To examine whether the backbone-reversed form of a model all-beta-sheet protein can fold and adopt secondary and tertiary structure, we created and examined the recombinant retro form of a 110-residue-long polypeptide, an alpha-crystallin-like small heat-shock protein, HSP12.6, from C. elegans. Following intracellular overexpression in fusion with a histidine affinity tag in Escherichia coli, purification under denaturing conditions, and removal of denaturant through dialysis, retro-HSP12.6 was found to fold to a soluble state. The folded protein was examined using fluorescence and CD spectroscopy, gel filtration chromatography, non-denaturing electrophoresis, differential scanning calorimetry, and electron microscopy and confirmed to have adopted secondary structure and assembled into a multimer. Interestingly, like its parent polypeptide, retro-HSP12.6 did not aggregate upon heating; rather, heating led to a dramatic increase in structural content and the adoption of what would appear to be a very well folded state at high temperatures. However, this was essentially reversed upon cooling with some hysteresis being observed resulting in greater structural content in the heated-cooled protein than in the unheated protein. The heated-cooled samples displayed CD spectra indicative of structural content comparable to that of any naturally occurring globular protein. Attempts are being made to refine crystallization conditions for the folded protein.

Tissue-specific expression, heat inducibility, and biological roles of two hsp16 genes in Caenorhabditis elegans

In this report we have examined two new heat shock protein (HSP16) proteins in the nematode Caenorhabditis elegans encoded by the open reading frames F08H9.3 and F08H9.4. The F08H9.3 and F08H9.4 genes are oriented in the same direction next to each other on the chromosome, not sharing any promoter region, unlike other hsp16 genes that share common promoters in pairs. The F08H9.3 and F08H9.4 proteins were expressed in a tissue-specific manner, unlike the other four HSP16 proteins. F08H9.3 was expressed in the pharynx, and F08H9.4 in the excretory canal and a few neuronal cells. While F08H9.3 was weakly induced by heat shock only in the same tissue as under the normal condition, F08H9.4 was newly induced in the intestine. RNA interference experiments showed that these two proteins are required for survival under the heat shock condition.

A critical motif for oligomerization and chaperone activity of bacterial alpha-heat shock proteins

Oligomerization into multimeric complexes is a prerequisite for the chaperone function of almost all alpha-crystallin type heat shock proteins (alpha-Hsp), but the molecular details of complex assembly are poorly understood. The alpha-Hsp proteins from Bradyrhizobium japonicum are suitable bacterial models for structure-function studies of these ubiquitous stress proteins. They fall into two distinct classes, A and B, display chaperone activity in vitro and form oligomers of approximately 24 subunits. We constructed 19 derivatives containing truncations or point mutations within the N- and C-terminal regions and analyzed them by gel filtration, citrate synthase assay and coaffinity purification. Truncation of more than the initial few amino acids of the N-terminal region led to the formation of distinct dimeric to octameric structures devoid of chaperone activity. In the C-terminal extension, integrity of an isoleucine-X-isoleucine (I-X-I) motif was imperative for alpha-Hsp functionality. This I-X-I motif is one of the characteristic consensus motifs of the alpha-Hsp family, and here we provide experimental evidence of its structural and functional importance. alpha-Hsp proteins lacking the C-terminal extension were inactive, but still able to form dimers. Here, we demonstrate that the central alpha-crystallin domain alone is not sufficient for dimerization. Additional residues at the end of the N-terminal region were required for the assembly of two subunits.

Alpha-crystallin-type heat shock proteins: socializing minichaperones in the context of a multichaperone network

Alpha-crystallins were originally recognized as proteins contributing to the transparency of the mammalian eye lens. Subsequently, they have been found in many, but not all, members of the Archaea, Bacteria, and Eucarya. Most members of the diverse alpha-crystallin family have four common structural and functional features: (i) a small monomeric molecular mass between 12 and 43 kDa; (ii) the formation of large oligomeric complexes; (iii) the presence of a moderately conserved central region, the so-called alpha-crystallin domain; and (iv) molecular chaperone activity. Since alpha-crystallins are induced by a temperature upshift in many organisms, they are often referred to as small heat shock proteins (sHsps) or, more accurately, alpha-Hsps. Alpha-crystallins are integrated into a highly flexible and synergistic multichaperone network evolved to secure protein quality control in the cell. Their chaperone activity is limited to the binding of unfolding intermediates in order to protect them from irreversible aggregation. Productive release and refolding of captured proteins into the native state requires close cooperation with other cellular chaperones. In addition, alpha-Hsps seem to play an important role in membrane stabilization. The review compiles information on the abundance, sequence conservation, regulation, structure, and function of alpha-Hsps with an emphasis on the microbial members of this chaperone family.