Evolution and diversity of prokaryotic small heat shock proteins

Comprehensive genome‑wide analysis of the chicken heat shock protein family: identification, genomic organization, and expression profiles in indigenous chicken with highly pathogenic avian influenza infection

BACKGROUND

Heat shock proteins (HSPs) function as molecular chaperones with critical roles in chicken embryogenesis, immune response to infectious diseases, and response to various environmental stresses. However, little is known on HSP genes in chicken. In this study, to understand the roles of chicken HSPs, we performed genome-wide identification, expression, and functional analyses of the HSP family genes in chicken.

RESULTS

A total of 76 HSP genes were identified in the chicken genome, which were further classified into eight distinct groups (I-VIII) based on phylogenetic tree analysis. The gene-structure analysis revealed that the members of each clade had the same or similar exon-intron structures. Chromosome mapping suggested that HSP genes were widely dispersed across the chicken genome, except in chromosomes 16, 18, 22, 25, 26, and 28-32, which lacked chicken HSP genes. On the other hand, the interactions among chicken HSPs were limited, indicating that the remaining functions of HSPs could be investigated in chicken. Moreover, KEGG pathway analysis showed that the HSP gene family was involved in the regulation of heat stress, apoptotic, intracellular signaling, and immune response pathways. Finally, RNA sequencing data revealed that, of the 76 chicken HSP genes, 46 were differentially expressed at 21 different growth stages in chicken embryos, and 72 were differentially expressed on post-infection day 3 in two indigenous Ri chicken lines infected with highly pathogenic avian influenza.

CONCLUSIONS

This study provides significant insights into the potential functions of HSPs in chicken, including the regulation of apoptosis, heat stress, chaperone activity, intracellular signaling, and immune response to infectious diseases.

Is the lipochaperone activity of sHSP a key to the stress response encoded in its primary sequence?

Several strategies have been put in place by organisms to adapt to their environment. One of these strategies is the production of stress proteins such as sHSPs, which have been widely described over the last 30 years for their role as molecular chaperones. Some sHSPs have, in addition, the particularity to exert a lipochaperone role by interacting with membrane lipids to maintain an optimal membrane fluidity. However, the mechanisms involved in this sHSP-lipid interaction remain poorly understood and described rather sporadically in the literature. This review gathers the information concerning the structure and function of these proteins available in the literature in order to highlight the mechanism involved in this interaction. In addition, analysis of primary sequence data of sHSPs available in database shows that sHSPs can interact with lipids via certain amino acid residues present on some β sheets of these proteins. These residues could have a key role in the structure and/or oligomerization dynamics of sHPSs, which is certainly essential for interaction with membrane lipids and consequently for maintaining optimal cell membrane fluidity.

The permanently chaperone-active small heat shock protein Hsp17 from Caenorhabditis elegans exhibits topological separation of its N-terminal regions

Small Heat shock proteins (sHsps) are a family of molecular chaperones that bind nonnative proteins in an ATP-independent manner. Caenorhabditis elegans encodes 16 different sHsps, among them Hsp17, which is evolutionarily distinct from other sHsps in the nematode. The structure and mechanism of Hsp17 and how these may differ from other sHsps remain unclear. Here, we find that Hsp17 has a distinct expression pattern, structural organization, and chaperone function. Consistent with its presence under nonstress conditions, and in contrast to many other sHsps, we determined that Hsp17 is a mono-disperse, permanently active chaperone in vitro, which interacts with hundreds of different C. elegans proteins under physiological conditions. Additionally, our cryo-EM structure of Hsp17 reveals that in the 24-mer complex, 12 N-terminal regions are involved in its chaperone function. These flexible regions are located on the outside of the spherical oligomer, whereas the other 12 N-terminal regions are engaged in stabilizing interactions in its interior. This allows the same region in Hsp17 to perform different functions depending on the topological context. Taken together, our results reveal structural and functional features that further define the structural basis of permanently active sHsps.

Understanding Diversity, Evolution, and Structure of Small Heat Shock Proteins in Annelida Through in Silico Analyses

Small heat shock proteins (sHsps) are oligomeric stress proteins characterized by an α-crystallin domain (ACD). These proteins are localized in different subcellular compartments and play critical roles in the stress physiology of tissues, organs, and whole multicellular eukaryotes. They are ubiquitous proteins found in all living organisms, from bacteria to mammals, but they have never been studied in annelids. Here, a data set of 23 species spanning the annelid tree of life, including mostly transcriptomes but also two genomes, was interrogated and 228 novel putative sHsps were identified and manually curated. The analysis revealed very high protein diversity and showed that a significant number of sHsps have a particular dimeric architecture consisting of two tandemly repeated ACDs. The phylogenetic analysis distinguished three main clusters, two of them containing both monomeric sHsps, and ACDs located downstream in the dimeric sHsps, and the other one comprising the upstream ACDs from those dimeric forms. Our results support an evolutionary history of these proteins based on duplication events prior to the Spiralia split. Monomeric sHsps 76) were further divided into five subclusters. Physicochemical properties, subcellular location predictions, and sequence conservation analyses provided insights into the differentiating elements of these putative functional groups. Strikingly, three of those subclusters included sHsps with features typical of metazoans, while the other two presented characteristics resembling non-metazoan proteins. This study provides a solid background for further research on the diversity, evolution, and function in the family of the sHsps. The characterized annelid sHsps are disclosed as essential for improving our understanding of this important family of proteins and their pleotropic functions. The features and the great diversity of annelid sHsps position them as potential powerful molecular biomarkers of environmental stress for acting as prognostic tool in a diverse range of environments.

Heat shock proteins and the calcineurin-crz1 signaling regulate stress responses in fungi

The heat shock proteins (Hsps) act as a molecular chaperone to stabilize client proteins involved in various cell functions in fungi. Hsps are classified into different families such as HSP90, HSP70, HSP60, HSP40, and small HSPs (sHsps). Hsp90, a well-studied member of the Hsp family proteins, plays a role in growth, cell survival, and pathogenicity in fungi. Hsp70 and sHsps are involved in the development, tolerance to stress conditions, and drug resistance in fungi. Hsp60 is a mitochondrial chaperone, and Hsp40 regulates fungal ATPase machinery. In this review, we describe the cell functions, regulation, and the molecular link of the Hsps with the calcineurin-crz1 calcium signaling pathway for their role in cell survival, growth, virulence, and drug resistance in fungi and related organisms.

Phosphorylation activates the yeast small heat shock protein Hsp26 by weakening domain contacts in the oligomer ensemble

Hsp26 is a small heat shock protein (sHsp) from S. cerevisiae. Its chaperone activity is activated by oligomer dissociation at heat shock temperatures. Hsp26 contains 9 phosphorylation sites in different structural elements. Our analysis of phospho-mimetic mutations shows that phosphorylation activates Hsp26 at permissive temperatures. The cryo-EM structure of the Hsp26 40mer revealed contacts between the conserved core domain of Hsp26 and the so-called thermosensor domain in the N-terminal part of the protein, which are targeted by phosphorylation. Furthermore, several phosphorylation sites in the C-terminal extension, which link subunits within the oligomer, are sensitive to the introduction of negative charges. In all cases, the intrinsic inhibition of chaperone activity is relieved and the N-terminal domain becomes accessible for substrate protein binding. The weakening of domain interactions within and between subunits by phosphorylation to activate the chaperone activity in response to proteotoxic stresses independent of heat stress could be a general regulation principle of sHsps.

Multiple nanocages of a cyanophage small heat shock protein with icosahedral and octahedral symmetries

The structures of a cyanophage small heat shock protein (sHSP) were determined as octahedrons of 24-mers and 48-mers and as icosahedrons of 60-mers. An N-terminal deletion construct of an 18 kDa sHSP of Synechococcus sp. phage S-ShM2 crystallized as a 24-mer and its structure was determined at a resolution of 7 Å. The negative stain electron microscopy (EM) images showed that the full-length protein is a mixture of a major population of larger and a minor population of smaller cage-like particles. Their structures have been determined by electron cryomicroscopy 3D image reconstruction at a resolution of 8 Å. The larger particles are 60-mers with icosahedral symmetry and the smaller ones are 48-mers with octahedral symmetry. These structures are the first of the viral/phage origin and the 60-mer is the largest and the first icosahedral assembly to be reported for sHSPs.

αB-Crystallin Alleviates Endotoxin-Induced Retinal Inflammation and Inhibits Microglial Activation and Autophagy

αB-Crystallin, a member of the small heat shock protein (sHSP) family, plays an immunomodulatory and neuroprotective role by inhibiting microglial activation in several diseases. However, its effect on endotoxin-induced uveitis (EIU) is unclear. Autophagy may be associated with microglial activation, and αB-crystallin is involved in the regulation of autophagy in some cells. The role of αB-crystallin in microglial autophagy is unknown. This study aimed to explore the role of αB-crystallin on retinal microglial autophagy, microglial activation, and neuroinflammation in both cultured BV2 cells and the EIU mouse model. Our results show that αB-crystallin reduced the release of typical proinflammatory cytokines at both the mRNA and protein level, inhibited microglial activation in morphology, and suppressed the expression of autophagy-related molecules and the number of autophagolysosomes in vitro. In the EIU mouse model, αB-crystallin treatment alleviated the release of ocular inflammatory cytokines and the representative signs of inflammation, reduced the apoptosis of ganglion cells, and rescued retinal inflammatory structural and functional damage, as evaluated by optical coherence tomographic and electroretinography. Taken together, these results indicate that αB-crystallin inhibits the activation of microglia and supresses microglial autophagy, ultimately reducing endotoxin-induced neuroinflammation. In conclusion, αB-crystallin provides a novel and promising option for affecting microglial autophagy and alleviating symptoms of ocular inflammatory diseases.

The Chloroplastic Small Heat Shock Protein Gene KvHSP26 Is Induced by Various Abiotic Stresses in Kosteletzkya virginica

Small heat shock proteins (sHSPs) are a group of chaperone proteins existed in all organisms. The functions of sHSPs in heat and abiotic stress responses in many glycophyte plants have been studied. However, their possible roles in halophyte plants are still largely known. In this work, a putative sHSP gene KvHSP26 was cloned from K. virginica. Bioinformatics analyses revealed that KvHSP26 encoded a chloroplastic protein with the typical features of sHSPs. Amino acid sequence alignment and phylogenetic analysis demonstrated that KvHSP26 shared 30%-77% homology with other sHSPs from Arabidopsis, cotton, durian, salvia, and soybean. Quantitative real-time PCR (qPCR) assays exhibited that KvHSP26 was constitutively expressed in different tissues such as leaves, stems, and roots, with a relatively higher expression in leaves. Furthermore, expression of KvHSP26 was strongly induced by salt, heat, osmotic stress, and ABA in K. virginica. All these results suggest that KvHSP26 encodes a new sHSP, which is involved in multiple abiotic stress responses in K. virginica, and it has a great potential to be used as a candidate gene for the breeding of plants with improved tolerances to various abiotic stresses.

Ethylene and RIPENING INHIBITOR Modulate Expression of SlHSP17.7A, B Class I Small Heat Shock Protein Genes During Tomato Fruit Ripening

Heat shock proteins (HSPs) are ubiquitous and highly conserved in nature. Heat stress upregulates their gene expression and now it is known that they are also developmentally regulated. We have studied regulation of small HSP genes during ripening of tomato fruit. In this study, we identify two small HSP genes, SlHSP17.7A and SlHSP17.7B, localized on tomato Chr.6 and Chr.9, respectively. Each gene encodes proteins constituting 154 amino acids and has characteristic domains as in other sHSP genes. We found that SlHSP17.7A and SlHSP17.7B gene expression is low in the vegetative tissues as compared to that in the fruit. These sHSP genes are characteristically expressed in a fruit-ripening fashion, being upregulated during the ripening transition of mature green to breaker stage. Their expression patterns mirror that of the rate-limiting ethylene biosynthesis gene ACC (1-aminocyclopropane-1-carboxylic acid) synthase, SlACS2, and its regulator SlMADS-RIN. Exogenous application of ethylene to either mature green tomato fruit or tomato leaves suppressed the expression of both the SlHSP17.7A, B genes. Notably and characteristically, a transgenic tomato line silenced for SlACS2 gene and whose fruits produce ~50% less ethylene in vivo, had higher expression of both the sHSP genes at the fruit ripening transition stages [breaker (BR) and BR+3] than the control fruit. Moreover, differential gene expression of SlHSP17.7A versus SlHSP17.7B gene was apparent in the tomato ripening mutants-rin/rin, nor/nor, and Nr/Nr, with the expression of SlHSP17.7A being significantly reduced but that of SlHSP17.7B significantly upregulated as compared to the wild type (WT). These data indicate that ethylene negatively regulates transcriptional abundance of both these sHSPs. Transient overexpression of the ripening regulator SlMADS-RIN in WT and ACS2-AS mature green tomato fruits suppressed the expression of SlHSP17.7A but not that of SlHSP17.7B. Thus, ethylene directly or in tune with SlMADS-RIN regulates the transcript abundance of both these sHSP genes.

Small Heat Shock Proteins, Amyloid Fibrils, and Nicotine Stimulate a Common Immune Suppressive Pathway with Implications for Future Therapies

The α7 nicotinic acetylcholine receptor (α7nAChR) is central to the anti-inflammatory function of the vagus nerve in a physiological mechanism termed the inflammatory reflex. Studies on the inflammatory reflex have been instrumental for the current development of the field of bioelectronic medicine. An independent investigation of the biological role of αB-crystallin (HspB5), the most abundant gene transcript present in active multiple sclerosis lesions in human brains, also led to α7nAChR. Induction of experimental autoimmune encephalomyelitis (EAE) in HspB5^-/- mice results in greater paralytic signs, increased levels of proinflammatory cytokines, and T-lymphocyte activation relative to wild-type animals. Administration of HspB5 was therapeutic in animal models of multiple sclerosis, retinal and cardiac ischemia, and stroke. Structure-activity studies established that residues 73-92 were as potent as the parent protein, but only when it formed amyloid fibrils. Amyloid fibrils and small heat shock proteins (sHsps) selectively bound α7nAChR on peritoneal macrophages (MΦs) and B lymphocytes, converting the MΦs to an immune suppressive phenotype and mobilizing the migration of both cell types from the peritoneum to secondary lymph organs. Here, we review multiple aspects of this work, which may be of interest for developing future therapeutic approaches for multiple sclerosis and other disorders.

Functional principles and regulation of molecular chaperones

To be able to perform their biological function, a protein needs to be correctly folded into its three dimensional structure. The protein folding process is spontaneous and does not require the input of energy. However, in the crowded cellular environment where there is high risk of inter-molecular interactions that may lead to protein molecules sticking to each other, hence forming aggregates, protein folding is assisted. Cells have evolved robust machinery called molecular chaperones to deal with the protein folding problem and to maintain proteins in their functional state. Molecular chaperones promote efficient folding of newly synthesized proteins, prevent their aggregation and ensure protein homeostasis in cells. There are different classes of molecular chaperones functioning in a complex interplay. In this review, we discuss the principal characteristics of different classes of molecular chaperones, their structure-function relationships, their mode of regulation and their involvement in human disorders.

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.

Computational genome-wide identification of heat shock protein genes in the bovine genome

Background: Heat shock proteins (HSPs) are molecular chaperones known to bind and sequester client proteins under stress.

Methods: To identify and better understand some of these proteins, we carried out a computational genome-wide survey of the bovine genome. For this, HSP sequences from each subfamily (sHSP, HSP40, HSP70 and HSP90) were used to search the Pfam (Protein family) database, for identifying exact HSP domain sequences based on the hidden Markov model. ProtParam tool was used to compute potential physico-chemical parameters detectable from a protein sequence. Evolutionary trace (ET) method was used to extract evolutionarily functional residues of a homologous protein family.

Results: We computationally identified 67 genes made up of 10, 43, 10 and 4 genes belonging to small HSP, HSP40, HSP70 and HSP90 families respectively. These genes were widely dispersed across the bovine genome, except in chromosomes 24, 26 and 27, which lack bovine HSP genes. We found an uncharacterized outer dense fiber ( ODF1) gene in cattle with an intact alpha crystallin domain, like other small HSPs. Physico-chemical characteristic of aliphatic index was higher in HSP70 and HSP90 gene families, compared to small HSP and HSP40. Grand average hydropathy showed that small HSP (sHSP), HSP40, HSP70 and HSP90 genes had negative values except for DNAJC22, a member of HSP40 gene family. The uniqueness of DNAJA3 and DNAJB13 among HSP40 members, based on multiple sequence alignment, evolutionary trace analysis and sequence identity dendrograms, suggests evolutionary distinct structural and functional features, with unique roles in substrate recognition and chaperone functions. The monophyletic pattern of the sequence identity dendrograms of cattle, human and mouse HSP sequences suggests functional similarities.

Conclusions: Our computational results demonstrate the first-pass in-silico identification of heat shock proteins and calls for further investigation to better understand their functional roles and mechanisms in Bovidae.

Chaperonomics in leptospirosis

INTRODUCTION

Knowledge of the function of molecular chaperones is required for a better understanding of cellular proteostasis. Nevertheless, such information is currently dispersed as most of previous studies investigated chaperones on a single-angle basis. Recently, a new subdiscipline of chaperonology, namely 'chaperonomics' (defined as 'systematic analysis of chaperone genes, transcripts, proteins, or their interaction networks using omics technologies'), has been emerging to better understand biological, physiological, and pathological roles of chaperones. Areas covered: This review provides broad overviews of bacterial chaperones, heat shock proteins (HSPs), and leptospirosis, and then focuses on recent progress of chaperonomics applied to define roles of HSPs in various pathogenic and saprophytic leptospiral species and serovars. Expert commentary: Comprehensive analysis of leptospiral chaperones/HSPs using a chaperonomics approach holds great promise for better understanding of functional roles of chaperones/HSPs in bacterial survival and disease pathogenesis. Moreover, this new approach may also lead to further development of chaperones/HSPs-based diagnostics and/or vaccine discovery for leptospirosis.

Molecular cloning, bioinformatics analysis, and expression of small heat shock protein beta-1 from Camelus dromedarius, Arabian camel

Small heat shock protein beta-1 (HSPB-1) plays an essential role in the protection of cells against environmental stress.Elucidation of its molecular, structural, and biological characteristics in a naturally wild-type model is essential. Although the sequence information of the HSPB-1 gene is available for many mammalian species, the HSPB-1 gene of Arabian camel (Arabian camel HSPB-1) has not yet been structurally characterized. We cloned and functionally characterized a full-length of Arabian camel HSPB-1 cDNA. It is 791 bp long, with a 5′-untranslated region (UTR) of 34 bp, a 3′-UTR of 151 bp with a poly(A) tail, and an open reading frame (ORF) of 606 bp encoding a protein of 201 amino acids (accession number: MF278354). The tissue-specific expression analysis of Arabian camel HSPB-1 mRNA was examined using quantitative real-time PCR (qRT-PCR); which suggested that Arabian camel HSPB-1 mRNA was constitutionally expressed in all examined tissues of Arabian camel, with the predominately level in the esophagus tissue. Peptide mass fingerprint-mass spectrometry (PMF-MS) analysis of the purified Arabian camel HSPB-1 protein confirmed the identity of this protein. Phylogenetic analysis showed that the HSPB-1 protein of Arabian camel is grouped together with those of Bactrian camel and Alpaca. Comparing the modelled 3D structure of Arabian camel HSPB-1 protein with the available protein 3D structure of HSPB-1 from human confirmed the presence of α-crystallin domain, and high similarities were noted between the two structures by using super secondary structure prediction.

Improvement of outdoor culture efficiency of cyanobacteria by over-expression of stress tolerance genes and its implication as bio-refinery feedstock

This study was undertaken to increase the biomass and carbohydrate productivities of a freshwater cyanobacterium Synechococcus elongatus under hot outdoor conditions through genetic manipulation to facilitate the application of using the cyanobacterial biomass as bio-refinery feedstocks. The stress tolerance genes (hspA, osmotin) were expressed in S. elongatus to improve their growth under various environment stresses of outdoor cultivation. The results revealed that over-expression of hspA and osmotin significantly improved temperature (45°C), high light intensity, and salt tolerances of S. elongatus cells, making it capable of efficiently growing in seawater under outdoor cultivation. The carbohydrate productivity of these stress tolerant strains was also 15-30-fold higher than that of the control strain, although the carbohydrate contents of the recombinant and control strains were similar. Our findings demonstrate that the genetic engineering for improved stresses tolerance in S. elongatus could facilitate the feasibility of using cyanobacteria as feedstock for bio-refinery industry.

Transient regulation of three clustered tomato class-I small heat-shock chaperone genes by ethylene is mediated by SlMADS-RIN transcription factor

Clustered class-I small heat-shock protein (sHSP) chaperone genes, SlHSP17.6, SlHSP20.0 and SlHSP20.1, in tomato are demonstrated to be transcriptionally regulated by ethylene during mature green (MG) fruit transition into ripening. These genes are constitutively expressed at MG fruit stage in two different tomato genotypes as well as in their ripening mutants, including rin, nor and Nr, and an ethylene-deficient transgenic line, ACS2-antisense. Notably, ethylene treatment of the MG fruit led to significant sHSP gene suppression in both wild-types, ACS2-antisense, nor/nor and Nr/Nr, but not the rin/rin mutant. Inability of ethylene to suppress sHSP genes in rin/rin mutant, which harbors MADS-RIN gene mutation, suggests that MADS-RIN transcription factor regulates the expression of these genes. Treatment of the wild type and ACS2-antisense fruit with the ethylene-signaling inhibitor, 1-methylcyclopropane (1-MCP), reversed the sHSP gene suppression. Transcripts of representative ethylene-responsive and ripening-modulated genes confirmed and validated sHSP transcript profile patterns. In silico analysis in conjunction with chromatin immunoprecipitation demonstrated MADS-RIN protein binding to specific CArG motifs present in the promoters of these chaperone genes. The results establish MADS-RIN protein as a transcriptional regulator of these chaperone genes in an ethylene-dependent manner, and that MADS-RIN protein-regulation of sHSPs is integral to tomato fruit ripening.

Crystal structure of a small heat-shock protein from Xylella fastidiosa reveals a distinct high-order structure

Citrus variegated chlorosis is a disease that attacks economically important citrus plantations and is caused by the plant-pathogenic bacterium Xylella fastidiosa. In this work, the structure of a small heat-shock protein from X. fastidiosa (XfsHSP17.9) is reported. The high-order structures of small heat-shock proteins from other organisms are arranged in the forms of double-disc, hollow-sphere or spherical assemblies. Unexpectedly, the structure reported here reveals a high-order architecture forming a nearly square cavity.

The Function of Ile-X-Ile Motif in the Oligomerization and Chaperone-Like Activity of Small Heat Shock Protein AgsA at Room Temperature

Small heat shock proteins assemble as large oligomers in vitro and exhibit ATP-independent chaperone activities. Ile-X-Ile motif is essential in both the function and oligomer formation. AgsA of Salmonella enterica serovar Typhimurium has been demonstrated to adopt large oligomeric structure and possess strong chaperone activity. Size exclusion chromatography, non-denaturing pore gradient PAGE, and negatively stain electron microscopic analysis of the various C-terminal truncated mutants were performed to investigate the role of Ile-X-Ile motif in the oligomer assembly of AgsA. By measuring the ability to prevent insulin from aggregating induced by TCEP, the chaperone-like activity of AgsA and the C-terminal truncated mutants at room temperature were determined. We found that the truncated mutants with Ile-X-Ile motif partially or fully deleted lost the ability to form large oligomers. Contrast to wild type AgsA which displayed weak chaperone-like activity, those mutants shown significantly enhanced activities at room temperature. In summary, biochemical experiment, activity assay and electron microscopic analysis suggested that Ile-X-Ile motif is essential in oligomer assembly of AgsA and might take the role of an inhibitor for its chaperone-like activity at room temperature.

A novel protein quality control mechanism contributes to heat shock resistance of worldwide-distributed Pseudomonas aeruginosa clone C strains

Pseudomonas aeruginosa is a highly successful nosocomial pathogen capable of causing a wide variety of infections with clone C strains most prevalent worldwide. In this study, we initially characterize a molecular mechanism of survival unique to clone C strains. We identified a P. aeruginosa clone C-specific genomic island (PACGI-1) that contains the highly expressed small heat shock protein sHsp20c, the founding member of a novel subclass of class B bacterial small heat shock proteins. sHsp20c and adjacent gene products are involved in resistance against heat shock. Heat stable sHsp20c is unconventionally expressed in stationary phase in a wide temperature range from 20 to 42°C. Purified sHsp20c has characteristic features of small heat shock protein class B as it is monodisperse, forms sphere-like 24-meric oligomers and exhibits significant chaperone activity. As the P. aeruginosa clone C population is significantly more heat shock resistant than genetically unrelated P. aeruginosa strains without sHsp20c, the horizontally acquired shsp20c operon might contribute to the survival of worldwide-distributed clone C strains.

Molecular Cloning and Differential Expression of Cytosolic Class I Small Hsp Gene Family in Pennisetum glaucum (L.)

Small heat shock protein (Hsp) family genes have been reported in several plant species that function as molecular chaperones to protect proteins from being denatured in extreme conditions. As a first step towards the isolation and characterization of genes that contribute to combating abiotic stresses particularly heat stress, construction and screening of the subtracted complementary DNA (cDNA) library is reported here. In this study, a subtractive heat stress cDNA library was constructed that was used to isolate members of small Hsps (sHsps) using PgsHsp17.9A gene as a probe. As a result, a total of 150 cDNA clones were isolated from the subtracted cDNA library screening, leading to 121 high-quality expressed sequence tags (ESTs), with an average size of 450 bp, comprising of 15 contigs, and majority of these isolated sHsp genes belong to cytosolic class I (CI) family. In silico sequence analysis of CI-sHsp family genes revealed that the length of sHsp proteins varied from 151 to 159 amino acids and showed large variation in isoelectric point value (5.03 to 10.05) and a narrow range of molecular weight (16.09 to 17.94 kDa). The real-time PCR results demonstrated that CI-sHsp genes are differentially expressed in Pennisetum leaves under different abiotic stress conditions particularly at high temperature. The results presented in this study provide basic information on PgCI-sHsp family genes and form the foundation for future functional studies of these genes.

Tobacco class I cytosolic small heat shock proteins are under transcriptional and translational regulations in expression and heterocomplex prevails under the high-temperature stress condition in vitro

Seven genomic clones of tobacco (Nicotiana tabacum W38) cytosolic class I small heat shock proteins (sHSPs), probably representing all members in the class, were isolated and found to have 66 to 92% homology between their nucleotide sequences. Even though all seven sHSP genes showed heat shock-responsive accumulation of their transcripts and proteins, each member showed discrepancies in abundance and timing of expression upon high-temperature stress. This was mainly the result of transcriptional regulation during mild stress conditions and transcriptional and translational regulation during strong stress conditions. Open reading frames (ORFs) of these genomic clones were expressed in Escherichia coli and the sHSPs were purified from E. coli. The purified tobacco sHSPs rendered citrate synthase and luciferase soluble under high temperatures. At room temperature, non-denaturing pore exclusion polyacrylamide gel electrophoresis on three sHSPs demonstrated that the sHSPs spontaneously formed homo-oligomeric complexes of 200 ∼ 240 kDa. However, under elevated temperatures, hetero-oligomeric complexes between the sHSPs gradually prevailed. Atomic force microscopy showed that the hetero-oligomer of NtHSP18.2/NtHSP18.3 formed a stable oligomeric particle similar to that of the NtHSP18.2 homo-oligomer. These hetero-oligomers positively influenced the revival of thermally inactivated luciferase. Amino acid residues mainly in the N-terminus are suggested for the exchange of the component sHSPs and the formation of dominant hetero-oligomers under high temperatures.

A first line of stress defense: small heat shock proteins and their function in protein homeostasis

Small heat shock proteins (sHsps) are virtually ubiquitous molecular chaperones that can prevent the irreversible aggregation of denaturing proteins. sHsps complex with a variety of non-native proteins in an ATP-independent manner and, in the context of the stress response, form a first line of defense against protein aggregation in order to maintain protein homeostasis. In vertebrates, they act to maintain the clarity of the eye lens, and in humans, sHsp mutations are linked to myopathies and neuropathies. Although found in all domains of life, sHsps are quite diverse and have evolved independently in metazoans, plants and fungi. sHsp monomers range in size from approximately 12 to 42kDa and are defined by a conserved β-sandwich α-crystallin domain, flanked by variable N- and C-terminal sequences. Most sHsps form large oligomeric ensembles with a broad distribution of different, sphere- or barrel-like oligomers, with the size and structure of the oligomers dictated by features of the N- and C-termini. The activity of sHsps is regulated by mechanisms that change the equilibrium distribution in tertiary features and/or quaternary structure of the sHsp ensembles. Cooperation and/or co-assembly between different sHsps in the same cellular compartment add an underexplored level of complexity to sHsp structure and function.

Cell biological roles of αB-crystallin

αB-crystallin, also called HspB5, is a molecular chaperone able to interact with unfolding proteins. By interacting, it inhibits further unfolding, thereby preventing protein aggregation and allowing ATP-dependent chaperones to refold the proteins. αB-crystallin belongs to the family of small heat-shock proteins (sHsps), which in humans consists of 10 different members. The protein forms large oligomeric complexes, containing up to 40 or more subunits, which in vivo consist of heterooligomeric complexes formed by a mixture of αB-crystallin and other sHsps. αB-crystallin is highly expressed in the lens and to a lesser extent in several other tissues, among which heart, skeletal muscle and brain. αB-crystallin plays a role in several cellular processes, such as signal transduction, protein degradation, stabilization of cytoskeletal structures and apoptosis. Mutations in the αB-crystallin gene can have detrimental effects, leading to pathologies such as cataract and cardiomyopathy. This review describes the biological roles of αB-crystallin, with a special focus on its function in the eye lens, heart muscle and brain. In addition its therapeutic potential is discussed.

Analysis and phylogeny of small heat shock proteins from marine viruses and their cyanobacteria host

Small heat shock proteins (sHSPs) are oligomeric stress proteins characterized by an α-crystallin domain (ACD) surrounded by a N-terminal arm and C-terminal extension. Publications on sHSPs have reported that they exist in prokaryotes and eukaryotes but, to our knowledge, not in viruses. Here we show that sHSPs are present in some cyanophages that infect the marine unicellular cyanobacteria, Synechococcus and Prochlorococcus. These phage sHSPs contain a conserved ACD flanked by a relatively conserved N-terminal arm and a short C-terminal extension with or without the conserved C-terminal anchoring module (CAM) L-X-I/V, suggested to be implicated in the oligomerization. In addition, cyanophage sHSPs have the signature pattern, P-P-[YF]-N-[ILV]-[IV]-x(9)-[EQ], in the predicted β2 and β3 strands of the ACD. Phylogenetically, cyanophage sHSPs form a monophyletic clade closer to bacterial class A sHSPs than to cyanobacterial sHSPs. Furthermore, three sHSPs from their cellular host, Synechococcus, are phylogenetically close to plants sHSPs. Implications of evolutionary relationships between the sHSPs of cyanophages, bacterial class A, cyanobacteria, and plants are discussed.

The evolution, function, structure, and expression of the plant sHSPs

Small heat shock proteins are a diverse, ancient, and important family of proteins. All organisms possess small heat shock proteins (sHSPs), indicating that these proteins evolved very early in the history of life prior to the divergence of the three domains of life (Archaea, Bacteria, and Eukarya). Comparing the structures of sHSPs from diverse organisms across these three domains reveals that despite considerable amino acid divergence, many structural features are conserved. Comparisons of the sHSPs from diverse organisms reveal conserved structural features including an oligomeric form with a β-sandwich that forms a hollow ball. This conservation occurs despite significant divergence in primary sequences. It is well established that sHSPs are molecular chaperones that prevent misfolding and irreversible aggregation of their client proteins. Most notably, the sHSPs are extremely diverse and variable in plants. Some plants have >30 individual sHSPs. Land plants, unlike other groups, possess distinct sHSP subfamilies. Most are highly up-regulated in response to heat and other stressors. Others are selectively expressed in seeds and pollen, and a few are constitutively expressed. As a family, sHSPs have a clear role in thermotolerance, but attributing specific effects to individual proteins has proved challenging. Considerable progress has been made during the last 15 years in understanding the sHSPs. However, answers to many important questions remain elusive, suggesting that the next 15 years will be at least equally rewarding.

Alternative bacterial two-component small heat shock protein systems

Small heat shock proteins (sHsps) are molecular chaperones that prevent the aggregation of nonnative proteins. The sHsps investigated to date mostly form large, oligomeric complexes. The typical bacterial scenario seemed to be a two-component sHsps system of two homologous sHsps, such as the Escherichia coli sHsps IbpA and IbpB. With a view to expand our knowledge on bacterial sHsps, we analyzed the sHsp system of the bacterium Deinococcus radiodurans, which is resistant against various stress conditions. D. radiodurans encodes two sHsps, termed Hsp17.7 and Hsp20.2. Surprisingly, Hsp17.7 forms only chaperone active dimers, although its crystal structure reveals the typical α-crystallin fold. In contrast, Hsp20.2 is predominantly a 36mer that dissociates into smaller oligomeric assemblies that bind substrate proteins stably. Whereas Hsp20.2 cooperates with the ATP-dependent bacterial chaperones in their refolding, Hsp17.7 keeps substrates in a refolding-competent state by transient interactions. In summary, we show that these two sHsps are strikingly different in their quaternary structures and chaperone properties, defining a second type of bacterial two-component sHsp system.

Chaperone activity of small heat shock proteins underlies therapeutic efficacy in experimental autoimmune encephalomyelitis

To determine whether the therapeutic activity of αB crystallin, small heat shock protein B5 (HspB5), was shared with other human sHsps, a set of seven human family members, a mutant of HspB5 G120 known to exhibit reduced chaperone activity, and a mycobacterial sHsp were expressed and purified from bacteria. Each of the recombinant proteins was shown to be a functional chaperone, capable of inhibiting aggregation of denatured insulin with varying efficiency. When injected into mice at the peak of disease, they were all effective in reducing the paralysis in experimental autoimmune encephalomyelitis. Additional structure activity correlations between chaperone activity and therapeutic function were established when linear regions within HspB5 were examined. A single region, corresponding to residues 73-92 of HspB5, forms amyloid fibrils, exhibited chaperone activity, and was an effective therapeutic for encephalomyelitis. The linkage of the three activities was further established by demonstrating individual substitutions of critical hydrophobic amino acids in the peptide resulted in the loss of all of the functions.

Novel roles for α-crystallins in retinal function and disease

α-Crystallins are key members of the superfamily of small heat shock proteins that have been studied in detail in the ocular lens. Recently, novel functions for α-crystallins have been identified in the retina and in the retinal pigmented epithelium (RPE). αB-Crystallin has been localized to multiple compartments and organelles including mitochondria, golgi apparatus, endoplasmic reticulum and nucleus. α-Crystallins are regulated by oxidative and endoplasmic reticulum stress, and inhibit apoptosis-induced cell death. α-Crystallins interact with a large number of proteins that include other crystallins, and apoptotic, cytoskeletal, inflammatory, signaling, angiogenic, and growth factor molecules. Studies with RPE from αB-crystallin deficient mice have shown that αB-crystallin supports retinal and choroidal angiogenesis through its interaction with vascular endothelial growth factor. αB-Crystallin has also been shown to have novel functions in the extracellular space. In RPE, αB-crystallin is released from the apical surface in exosomes where it accumulates in the interphotoreceptor matrix and may function to protect neighboring cells. In other systems administration of exogenous recombinant αB-crystallin has been shown to be anti-inflammatory. Another newly described function of αB-crystallin is its ability to inhibit β-amyloid fibril formation. α-Crystallin minichaperone peptides have been identified that elicit anti-apoptotic function in addition to being efficient chaperones. Generation of liposomal particles and other modes of nanoencapsulation of these minipeptides could offer great therapeutic advantage in ocular delivery for a wide variety of retinal degenerative, inflammatory and vascular diseases including age-related macular degeneration and diabetic retinopathy.

Dissecting heterogeneous molecular chaperone complexes using a mass spectrum deconvolution approach

Small heat-shock proteins (sHSPs) are molecular chaperones that prevent irreversible aggregation through binding nonnative target proteins. Due to their heterogeneity, these sHSP:target complexes remain poorly understood. We present a nanoelectrospray mass spectrometry analysis algorithm for estimating the distribution of stoichiometries comprising a polydisperse ensemble of oligomers. We thus elucidate the organization of complexes formed between sHSPs and different target proteins. We find that binding is mass dependent, with the resultant complexes reflecting the native quaternary architecture of the target, indicating that protection happens early in the denaturation. Our data therefore explain the apparent paradox of how variable complex morphologies result from the generic mechanism of protection afforded by sHSPs. Our approach is applicable to a range of polydisperse proteins and provides a means for the automated and accurate interpretation of mass spectra derived from heterogeneous protein assemblies.

Features of a unique intronless cluster of class I small heat shock protein genes in tandem with box C/D snoRNA genes on chromosome 6 in tomato (Solanum lycopersicum)

Physical clustering of genes has been shown in plants; however, little is known about gene clusters that have different functions, particularly those expressed in the tomato fruit. A class I 17.6 small heat shock protein (Sl17.6 shsp) gene was cloned and used as a probe to screen a tomato (Solanum lycopersicum) genomic library. An 8.3-kb genomic fragment was isolated and its DNA sequence determined. Analysis of the genomic fragment identified intronless open reading frames of three class I shsp genes (Sl17.6, Sl20.0, and Sl20.1), the Sl17.6 gene flanked by Sl20.1 and Sl20.0, with complete 5' and 3' UTRs. Upstream of the Sl20.0 shsp, and within the shsp gene cluster, resides a box C/D snoRNA cluster made of SlsnoR12.1 and SlU24a. Characteristic C and D, and C' and D', boxes are conserved in SlsnoR12.1 and SlU24a while the upstream flanking region of SlsnoR12.1 carries TATA box 1, homol-E and homol-D box-like cis sequences, TM6 promoter, and an uncharacterized tomato EST. Molecular phylogenetic analysis revealed that this particular arrangement of shsps is conserved in tomato genome but is distinct from other species. The intronless genomic sequence is decorated with cis elements previously shown to be responsive to cues from plant hormones, dehydration, cold, heat, and MYC/MYB and WRKY71 transcription factors. Chromosomal mapping localized the tomato genomic sequence on the short arm of chromosome 6 in the introgression line (IL) 6-3. Quantitative polymerase chain reaction analysis of gene cluster members revealed differential expression during ripening of tomato fruit, and relatively different abundances in other plant parts.

Regulation of the 18 kDa heat shock protein in Mycobacterium ulcerans: an alpha-crystallin orthologue that promotes biofilm formation

Mycobacterium ulcerans is the causative agent of the debilitating skin disease Buruli ulcer, which is most prevalent in Western and Central Africa. M. ulcerans shares >98% DNA sequence identity with Mycobacterium marinum, however, M. marinum produces granulomatous, but not ulcerative, lesions in humans and animals. Here we report the differential expression of a small heat shock protein (Hsp18) between strains of M. ulcerans (Hsp18(+) ) and M. marinum (Hsp18(-) ) and describe the molecular basis for this difference. We show by gene deletion and GFP reporter assays in M. marinum that a divergently transcribed gene called hspR_2, immediately upstream of hsp18, encodes a MerR-like regulatory protein that represses hsp18 transcription while promoting its own expression. Naturally occurring mutations within a 70 bp segment of the 144 bp hspR_2-hsp18 intergenic region among M. ulcerans strains inhibit hspR_2 transcription and explain the Hsp18(+) phenotype. We also propose a biological role for Hsp18, as we show that this protein significantly enhances bacterial attachment or aggregation during biofilm formation. This study has uncovered a new member of the MerR family of transcriptional regulators and suggests that upregulation of hsp18 expression was an important pathoadaptive response in the evolution of M. ulcerans from a M. marinum-like ancestor.

Characterization of Xanthomonas campestris pv. campestris heat shock protein A (HspA), which possesses an intrinsic ability to reactivate inactivated proteins

hspA encodes a small heat shock protein (sHSP) in Xanthomonas campestris pv. campestris, the causative agent of black rot in cruciferous plants. In this study, two-dimensional gel electrophoresis, promoter activity assays, and Northern hybridization results revealed that HspA expression was induced by heat shock but not by other stresses, although low-level expression was detectable by reverse transcription-polymerase chain reaction (RT-PCR) under normal culture conditions. An hspA mutant exhibited reduced tolerance to heat, especially in the presence of MgSO4, but no change in pathogenicity. Results of size-exclusion chromatography indicated that purified HspA(his), containing six C-terminal histidine residues, formed two different size classes of oligomeric complexes--410 and 820 kDa. In contrast, HspA(ter), the unmodified protein translated from the original hspA gene, formed only the 820-kDa complex. These results suggest that the C-terminus of HspA is important for oligomerization. Both HspA820(his) and HspA410(his) were able to partially protect luciferase against heat-induced aggregation. Unlike other reported sHSPs that commonly capture denaturing proteins in refoldable states until refolded by adenosine triphosphate-dependent chaperone systems, HspA(his) alone was capable of reactivating heat-inactivated EcoRI. Thus, Xanthomonas campestris pv. campestris HspA has potential application as a protective agent during the storage of proteins.

Distinct amino acids of the Oenococcus oeni small heat shock protein Lo18 are essential for damaged protein protection and membrane stabilization

The small heat shock protein (smHsp) Lo18 from lactic acid bacteria Oenococcus oeni reduces in vitro thermal aggregation of proteins and modulates the membrane fluidity of native liposomes. An absence of information relating to the way in which the smHsp demonstrates a stabilizing effect for both proteins and membranes prompted this study. We expressed three Lo18 proteins with amino acid substitutions in Escherichia coli to investigate their ability to prevent E. coli protein aggregation and their capacity to stabilize E. coli whole-cell membranes. Our results showed that the alanine 123 to serine substitution induces a decrease in chaperone activity in denaturated proteins, and that the tyrosine 107 is required for membrane stabilization. Moreover, this study revealed that the oligomeric structures of proteins with amino acid substitutions do not appear to be modified. Our data strongly suggest that different amino acids are involved in the thermostabilization of proteins and in membrane fluidity regulation and are localized in the alpha-crystallin domain.

Quaternary dynamics and plasticity underlie small heat shock protein chaperone function

Small Heat Shock Proteins (sHSPs) are a diverse family of molecular chaperones that prevent protein aggregation by binding clients destabilized during cellular stress. Here we probe the architecture and dynamics of complexes formed between an oligomeric sHSP and client by employing unique mass spectrometry strategies. We observe over 300 different stoichiometries of interaction, demonstrating that an ensemble of structures underlies the protection these chaperones confer to unfolding clients. This astonishing heterogeneity not only makes the system quite distinct in behavior to ATP-dependent chaperones, but also renders it intractable by conventional structural biology approaches. We find that thermally regulated quaternary dynamics of the sHSP establish and maintain the plasticity of the system. This extends the paradigm that intrinsic dynamics are crucial to protein function to include equilibrium fluctuations in quaternary structure, and suggests they are integral to the sHSPs' role in the cellular protein homeostasis network.

Identification of the key structural motifs involved in HspB8/HspB6-Bag3 interaction

The molecular chaperone HspB8 [Hsp (heat-shock protein) B8] is member of the B-group of Hsps. These proteins bind to unfolded or misfolded proteins and protect them from aggregation. HspB8 has been reported to form a stable molecular complex with the chaperone cohort protein Bag3 (Bcl-2-associated athanogene 3). In the present study we identify the binding regions in HspB8 and Bag3 crucial for their interaction. We present evidence that HspB8 binds to Bag3 through the hydrophobic groove formed by its strands beta4 and beta8, a region previously known to be responsible for the formation and stability of higher-order oligomers of many sHsps (small Hsps). Moreover, we demonstrate that two conserved IPV (Ile-Pro-Val) motifs in Bag3 mediate its binding to HspB8 and that deletion of these motifs suppresses HspB8 chaperone activity towards mutant Htt43Q (huntingtin exon 1 fragment with 43 CAG repeats). In addition, we show that Bag3 can bind to the molecular chaperone HspB6. The interaction between HspB6 and Bag3 requires the same regions that are involved in the HspB8-Bag3 association and HspB6-Bag3 promotes clearance of aggregated Htt43Q. Our findings suggest that the co-chaperone Bag3 might prevent the accumulation of denatured proteins by regulating sHsp activity and by targeting their substrate proteins for degradation. Interestingly, a mutation in one of Bag3 IPV motifs has recently been associated with the development of severe dominant childhood muscular dystrophy, suggesting a possible important physiological role for HspB-Bag3 complexes in this disease.

A molecular trio in relapse and remission in multiple sclerosis

Two thirds of patients with multiple sclerosis have the relapsing-remitting form, which often progresses to more debilitating disease. Striking clinical recovery, termed remission, often follows these periodic neurological defects, termed relapses. Recent work has revealed the role of three key molecules in relapse and remission: alpha4beta1 integrin (also known as VLA4) is an adhesion molecule that mediates T cell migration from the blood into the brain; osteopontin binds to alpha4beta1 integrin, stimulating the production of pro-inflammatory cytokines and inhibiting apoptosis; and alphaB crystallin inhibits inflammation in the brain. This Review discusses how this molecular trio interacts to initiate relapses (in the case of osteopontin and alpha4beta1 integrin) and then to terminate them as remissions in multiple sclerosis (in the case of alphaB crystallin).

Structural and functional diversity in the family of small heat shock proteins from the parasite Toxoplasma gondii

Small heat shock proteins (sHsps) are ubiquitous molecular chaperones which prevent the nonspecific aggregation of non-native proteins. Five potential sHsps exist in the parasite Toxoplasma gondii. They are located in different intracellular compartments including mitochondria and are differentially expressed during the parasite's life cycle. Here, we analyzed the structural and functional properties of all five proteins. Interestingly, this first in vitro characterization of sHsps from protists showed that all T. gondii sHsps exhibit the characteristic properties of sHsps such as oligomeric structure and chaperone activity. However, differences in their quaternary structure and in their specific chaperone properties exist. On the structural level, the T. gondii sHsps can be divided in small (12-18 subunits) and large (24-32 subunits) oligomers. Furthermore, they differ in their interaction with non-native proteins. While some bind substrates tightly, others interact more transiently. The chaperone activity of the three more mono-disperse T. gondii sHsps is regulated by temperature with a decrease in temperature leading to the activation of chaperone activity, suggesting an adaption to specific steps of the parasite's life cycle.

A small heat-shock protein confers stress tolerance and stabilizes thylakoid membrane proteins in cyanobacteria under oxidative stress

Small heat-shock proteins are molecular chaperones that bind and prevent aggregation of nonnative proteins. They also associate with membranes. In this study, we show that the small heat-shock protein HspA plays a protective role under oxidative stress in the cyanobacterium Synechococcus elongatus strain ECT16-1, which constitutively expresses HspA. Compared with the reference strain ECT, ECT16-1 showed much better growth and viability in the presence of hydrogen peroxide. Under the peroxide stress, pigments in thylakoid membrane, chlorophyll, carotenoids, and phycocyanins, were continuously reduced in ECT, but in ECT16-1 they decreased only during the first 24 h of stress; thereafter no further reduction was observed. For comparison, we analyzed a wild type and an hspA deletion strain from Synechocystis sp. PCC 6803 and found that lack of hspA significantly affected the viability of the cell and the pigment content in the presence of methyl viologen, suggesting that HspA stabilizes membrane proteins such as the photosystems and phycobilisomes from oxidative damage. In vitro pull down assays showed a direct interaction of HspA with components of phycobilisomes. These results show that HspA and small heat-shock proteins in general play an important role in the acclimation to oxidative stress in cyanobacteria.

Functional characterization of the DnaK chaperone system from the archaeon Methanothermobacter thermautotrophicus DeltaH

We characterized the biochemical and functional properties of the DnaK system from the archaeon Methanothermobacter thermautotrophicus DeltaH. In contrast to the eubacterial chaperone components the archaeal Hsp70 system shows thermal transitions only slightly above the optimal environmental temperature (65 degrees C). Nevertheless, it prevents aggregation of luciferase in the physiological temperature range of the organism, but is also fully functional at 30 degrees C in luciferase refolding. Additionally, GrpE(M.th.) and DnaJ(M.th.) substitute their eubacterial counterparts whereas DnaK(M.th.) is only functional with its native cochaperones which could be attributed to a functional specialization of the eubacterial chaperones during evolution.

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.

Structural dynamics of archaeal small heat shock proteins

Small heat shock proteins (sHsps) are a widespread and diverse class of molecular chaperones. In vivo, sHsps contribute to thermotolerance. Recent evidence suggests that their function in the cellular chaperone network is to maintain protein homeostasis by complexing a variety of non-native proteins. One of the most characteristic features of sHsps is their organization into large, sphere-like structures commonly consisting of 12 or 24 subunits. Here, we investigated the functional and structural properties of Hsp20.2, an sHsp from Archaeoglobus fulgidus, in comparison to its relative, Hsp16.5 from Methanocaldococcus jannaschii. Hsp20.2 is active in suppressing the aggregation of different model substrates at physiological and heat-stress temperatures. Electron microscopy showed that Hsp20.2 forms two distinct types of octahedral oligomers of slightly different sizes, indicating certain structural flexibility of the oligomeric assembly. By three-dimensional analysis of electron microscopic images of negatively stained specimens, we were able to reconstitute 3D models of the assemblies at a resolution of 19 A. Under conditions of heat stress, the distribution of the structurally different Hsp20.2 assemblies changed, and this change was correlated with an increased chaperone activity. In analogy to Hsp20.2, Hsp16.5 oligomers displayed structural dynamics and exhibited increased chaperone activity under conditions of heat stress. Thus, temperature-induced conformational regulation of the activity of sHsps may be a general phenomenon in thermophilic archaea.

Genome-wide analysis and expression profiling of the small heat shock proteins in zebrafish

Small Heat Shock Proteins (sHSPs) have important roles in preventing disease and promoting resistance to environmental stressors. Mutations in any one of a number of sHSPs, including HSP27 (HSPB1), HSP22 (HSPB8), alphaA-crystallin (HSPB4), or alphaB-crystallin (HSPB5) can result in neuronal degeneration, myopathy, and/or cataract in humans. Ten sHSPs are known in humans, and thirteen have been identified in teleost fish. Here we report the identification of thirteen zebrafish sHSPs. Using a combination of phylogenetic analysis and analysis of synteny, we have determined that ten are likely orthologs of human sHSPs. We have used quantitative RT-PCR to determine the relative expression levels of all thirteen sHSPs during development and in response to heat shock. Our findings indicate that most of the zebrafish sHSPs are expressed during development, and five of these genes are transcriptionally upregulated by heat shock at one or more stages of development.

The cellular "networking" of mammalian Hsp27 and its functions in the control of protein folding, redox state and apoptosis

Cells possess effective mechanisms to cope with chronic or acute disturbance of homeostasis. Key roles in maintaining or restoring homeostasis are played by the various heat shock or stress proteins (Hsps). Among the Hsps, the group of proteins characterized by low molecular masses (between 20 to 30 kDa) and homology to alpha-crystallin are called small stress proteins (denoted sHsps). The present chapter summarizes the actual knowledge of the protective mechanisms generated by the expression of mammalian Hsp27 (also denoted HspB1 in human) against the cytotoxicity induced by heat shock and oxidative stress. It also describes the anti-apoptotic properties of Hsp27 and their putative consequences in different pathological conditions.

Identification of bis-ANS binding sites in Mycobacterium tuberculosis small heat shock protein Hsp16.3: Evidences for a two-step substrate-binding mechanism

Small heat shock proteins (sHSPs), as one important subclass of molecular chaperones, are able to specifically bind to denatured substrate proteins rather than to native proteins, of which their substrate-binding sites are far from clear. Our previous study showed an overlapping nature of the sites for both hydrophobic probe 1,1'-Bi(4-anilino)naphthalene-5,5'-disulfonic acid (bis-ANS) binding and substrate binding in Mycobacterium tuberculosis Hsp16.3 [X. Fu, H. Zhang, X. Zhang, Y. Cao, W. Jiao, C. Liu, Y. Song, A. Abulimiti, Z. Chang, A dual role for the N-terminal region of M. tuberculosis Hsp16.3 in self-oligomerization and binding denaturing substrate proteins, J. Biol. Chem. 280 (2005) 6337-6348]. In this work, two bis-ANS binding sites in Hsp16.3 were identified by a combined use of reverse phase HPLC, mass spectroscopy and N-terminal protein sequencing. One site is in the N-terminal region and the other one in the N-terminus of alpha-crystallin domain, both of which are similar to those identified so far in sHSPs. However, accumulating data suggest that these two sites differentially function in binding substrate proteins. With regard to this difference, we proposed a two-step mechanism by which Hsp16.3 binds substrate proteins, i.e., substrate proteins are recognized and initially captured by the N-terminal region that is exposed in the dissociated Hsp16.3 oligomers, and then the captured substrate proteins are further stabilized in the complex by the subsequent binding of the N-terminus of alpha-crystallin domain.