Interactions between small heat shock protein subunits and substrate in small heat shock protein-substrate complexes

Expression analysis of nine rice heat shock protein genes under abiotic stresses and ABA treatment

Expression profiles of nine rice heat shock protein genes (OsHSPs) were analyzed by semi-quantitative reverse transcriptase polymerase chain reaction (RT-PCR). The nine genes exhibited distinctive expression in different organs. Expression of nine OsHSP genes was affected differentially by abiotic stresses and abscisic acid (ABA). All nine OsHSP genes were induced strongly by heat shock treatment, whereas none of them were induced by cold. The transcripts of OsHSP80.2, OsHSP71.1 and OsHSP23.7 were increased during salt tress treatment. Expression of OsHSP80.2 and OsHSP24.1 genes were enhanced while treated with 10% PEG. Only OsHSP71.1 was induced by ABA while OsHSP24.1 was suppressed by ABA. These observations imply that the nine OsHSP genes may play different roles in plant development and abiotic stress responses.

Identification and characterization of proteins associated with plant tolerance to heat stress

Heat stress is a major abiotic stress limiting plant growth and productivity in many areas of the world. Understanding mechanisms of plant adaptation to heat stress would facilitate the development of heat-tolerant cultivars for improving productivity in warm climatic regions. Protein metabolism involving protein synthesis and degradation is one of the most sensitive processes to heat stress. Changes in the level and expression pattern of some proteins may play an important role in plant adaptation to heat stress. The identification of stress-responsive proteins and pathways has been facilitated by an increasing number of tools and resources, including two-dimensional electrophoresis and mass spectrometry, and the rapidly expanding nucleotide and amino acid sequence databases. Heat stress may induce or enhance protein expression or cause protein degradation. The induction of heat-responsive proteins, particularly heat shock proteins (HSPs), plays a key role in plant tolerance to heat stress. Protein degradation involving various proteases is also important in regulating plant responses to heat stress. This review provides an overview of recent research on proteomic profiling for the identification of heat-responsive proteins associated with heat tolerance, heat induction and characteristics of HSPs, and protein degradation in relation to plant responses to heat stress.

Insights into small heat shock protein and substrate structure during chaperone action derived from hydrogen/deuterium exchange and mass spectrometry

Small heat shock proteins (sHSPs) and the related alpha-crystallins are ubiquitous chaperones linked to neurodegenerative diseases, myopathies, and cataract. To better define their mechanism of chaperone action, we used hydrogen/deuterium exchange and mass spectrometry (HXMS) to monitor conformational changes during complex formation between the structurally defined sHSPs, pea PsHsp18.1, and wheat TaHsp16.9, and the heat-denatured model substrates malate dehydrogenase (MDH) and firefly luciferase. Remarkably, we found that even when complexed with substrate, the highly dynamic local structure of the sHSPs, especially in the N-terminal arm (>70% exchange in 5 s), remains unchanged. These results, coupled with sHSP-substrate complex stability, indicate that sHSPs do not adopt new secondary structure when binding substrate and suggest sHSPs are tethered to substrate at multiple sites that are locally dynamic, a feature that likely facilitates recognition and refolding of sHSP-bound substrate by the Hsp70/DnaK chaperone system. Both substrates were found to be stabilized in a partially unfolded state that is observed only in the presence of sHSP. Furthermore, peptide-level HXMS showed MDH was substantially protected in two core regions (residues 95-156 and 228-252), which overlap with the MDH structure protected in the GroEL-bound MDH refolding intermediate. Significantly, despite differences in the size and structure of TaHsp16.9-MDH and PsHsp18.1-MDH complexes, peptide-level HXMS patterns for MDH in both complexes are virtually identical, indicating that stabilized MDH thermal unfolding intermediates are not determined by the identity of the sHSP.

The plant sHSP superfamily: five new members in Arabidopsis thaliana with unexpected properties

The small heat shock proteins (sHsps), which are ubiquitous stress proteins proposed to act as chaperones, are encoded by an unusually complex gene family in plants. Plant sHsps are classified into different subfamilies according to amino acid sequence similarity and localization to distinct subcellular compartments. In the whole Arabidopsis thaliana genome, 19 genes were annotated to encode sHsps, of which 14 belong to previously defined plant sHsp families. In this paper, we report studies of the five additional sHsp genes in A. thaliana, which can now be shown to represent evolutionarily distinct sHsp subfamilies also found in other plant species. While two of these five sHsps show expression patterns typical of the other 14 genes, three have unusual tissue specific and developmental profiles and do not respond to heat induction. Analysis of intracellular targeting indicates that one sHsp represents a new class of mitochondrion-targeted sHsps, while the others are cytosolic/nuclear, some of which may cooperate with other sHsps in formation of heat stress granules. Three of the five new proteins were purified and tested for chaperone activity in vitro. Altogether, these studies complete our basic understanding of the sHsp chaperone family in plants.

Activation of the chaperone Hsp26 is controlled by the rearrangement of its thermosensor domain

Cells respond to a sudden increase in temperature with the transcription of a special set of genes, a phenomenon known as the heat shock response. In the yeast S. cerevisiae, the molecular chaperone Hsp26 is one component of the heat shock response. Hsp26 has the remarkable ability to sense increases in temperature directly and can switch from an inactive to a chaperone-active state. The underlying principle of this temperature regulation has remained enigmatic. Hsp26 variants with altered spectroscopic properties allowed us to identify structural elements controlling this activation process. We show that temperature sensing by Hsp26 is a feature of its middle domain that changes its conformation within a narrow temperature range. This structural rearrangement allows Hsp26 to respond autonomously and directly to heat stress by reversibly unleashing its chaperone activity. Thus, the Hsp26 middle domain is a thermosensor and intrinsic regulator of chaperone activity.

Chemical cross-linking of the chloroplast localized small heat-shock protein, Hsp21, and the model substrate citrate synthase

The molecular mechanism whereby the small heat-shock protein (sHsp) chaperones interact with and prevent aggregation of other proteins is not fully understood. We have characterized the sHsp-substrate protein interaction at normal and increased temperatures utilizing a model substrate protein, citrate synthase (CS), widely used in chaperone assays, and a dodecameric plant sHsp, Hsp21, by chemical cross-linking with 3,3'-Dithiobis[sulfosuccinimidylpropionate] (DTSSP) and mass spectrometric peptide mapping. In the absence of CS, the cross-linker captured Hsp21 in dodecameric form, even at increased temperature (47 degrees C). In the presence of equimolar amounts of CS, no Hsp21 dodecamer was captured, indicating a substrate-induced Hsp21 dodecamer dissociation by equimolar amounts of CS. Cross-linked Hsp21-Hsp21 dipeptides indicated an exposure of the Hsp21 C-terminal tails and substrate-binding sites normally covered by the C terminus. Cross-linked Hsp21-CS dipeptides mapped to several sites on the surface of the CS dimer, indicating that there are numerous weak and short-lived interactions between Hsp21 and CS, even at normal temperatures. The N-terminal arms especially interacted with a motif in the CS dimer, which is absent in thermostable forms of CS. The cross-linking data suggest that the presence of substrate rather than temperature influences the conformation of Hsp21.

Complexity of the heat stress response in plants

Plants have evolved a variety of responses to elevated temperatures that minimize damage and ensure protection of cellular homeostasis. New information about the structure and function of heat stress proteins and molecular chaperones has become available. At the same time, transcriptome analysis of Arabidopsis has revealed the involvement of factors other than classical heat stress responsive genes in thermotolerance. Recent reports suggest that both plant hormones and reactive oxygen species also contribute to heat stress signaling. Additionally, an increasing number of mutants that have altered thermotolerance have extended our understanding of the complexity of the heat stress response in plants.

Thermally induced and developmentally regulated expression of a small heat shock protein in Trichinella spiralis

A cDNA encoding a small heat shock protein of Trichinella spiralis, Ts-sHsp, was cloned and expressed and is herein characterized. This cDNA encoded a predicted protein of 165 amino acids, which had a high sequence identity in alpha crystallin domain with various small heat shock proteins of other organisms. A Western blot analysis indicated that anti-Ts-sHsp recombinant antibody recognized the protein of adults and larvae migrating at about 19 kDa. An in situ localization study showed the protein to be abundantly present in the body wall muscle cells, hypodermis, stichocytes, and esophagus of muscle larvae. The Ts-sHsp recombinant protein possessed chaperone activity to suppress the thermally-induced aggregation of citrate synthase. This sHsp was expressed at various developmental stages of T. spiralis, but at different levels. A high level was observed in mature muscle larvae (infective larvae), which was much higher than the levels seen in adults, newborn larvae, or immature muscle larvae. The expression of the sHsp gene was thermal inducible, thus responding to both cold (0 degrees C) and heat shock (43 degrees C) stress; however, at different patterns. The expression of Ts-sHsp increased gradually from 3 to 72 h after cold stress, while the expression was elevated to its highest after 3 h heat stress and then decreased. These results suggest that this small heat shock protein likely plays a role in the tolerance to both chemical and physical stresses, thereby enhancing the survival ability of Trichinella muscle larvae.

The N-terminal arm of small heat shock proteins is important for both chaperone activity and substrate specificity

Small heat shock proteins (sHSPs) are a ubiquitous class of molecular chaperones that interacts with substrates to prevent their irreversible insolubilization during denaturation. How sHSPs interact with substrates remains poorly defined. To investigate the role of the conserved C-terminal alpha-crystallin domain versus the variable N-terminal arm in substrate interactions, we compared two closely related dodecameric plant sHSPs, Hsp18.1 and Hsp16.9, and four chimeras of these two sHSPs, in which all or part of the N-terminal arm was switched. The efficiency of substrate protection and formation of sHSP-substrate complexes by these sHSPs with three different model substrates, firefly luciferase, citrate synthase, and malate dehydrogenase (MDH) provide new insights into sHSP/substrate interactions. Results indicate that different substrates have varying affinities for different domains of the sHSP. For luciferase and citrate synthase, the efficiency of substrate protection was determined by the identity of the N-terminal arm in the chimeric proteins. In contrast, for MDH, efficient protection clearly required interactions with the alpha-crystallin domain in addition to the N-terminal arm. Furthermore, we show that sHSP-substrate complexes with varying stability and composition can protect substrate equally, and substrate protection is not correlated with sHSP oligomeric stability for all substrates. Protection of MDH by the dimeric chimera composed of the Hsp16.9 N-terminal arm and Hsp18.1 alpha-crystallin domain supports the model that a dimeric form of the sHSP can bind and protect substrate. In total, results demonstrate that sHSP-substrate interactions are complex, likely involve multiple sites on the sHSP, and vary depending on substrate.

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.

Subunit Exchange of MjHsp16.5 Studied by Single-Molecule Imaging and Fluorescence Resonance Energy Transfer

MjHsp16.5 was separately labeled by fluorescent dye Cy3 and Cy5.5. The dissociation event of a single 24-mer MjHsp16.5 molecule was captured by single-molecule imaging (SMI). Temperature-regulated subunit exchange was revealed by the real-time fluorescence resonance energy transfer (FRET). The combination of single-molecular statistics and kinetic parameters from FRET experiments leads to the conclusion that below 75 degrees C the rate-determining step of the subunit exchange was the dissociation of the dye-labeled 24-mer in which the dimer was intact, whereas above 75 degrees C, smaller units emerged in the exchange and the rate-determining step had the character of a bimolecular reaction.

The molecular genetics of non-ALS motor neuron diseases

Hereditary disorders of voluntary motor neurons are individually relatively uncommon, but have the potential to provide significant insights into motor neuron function in general and into the mechanisms underlying the more common form of sporadic Amyotrophic Lateral Sclerosis. Recently, mutations in a number of novel genes have been associated with Lower Motor Neuron (HSPB1, HSPB8, GARS, Dynactin), Upper Motor Neuron (Spastin, Atlastin, Paraplegin, HSP60, KIF5A, NIPA1) or mixed ALS-like phenotypes (Alsin, Senataxin, VAPB, BSCL2). In comparison to sporadic ALS these conditions are usually associated with slow progression, but as experience increases, a wide variation in clinical phenotype has become apparent. At the molecular level common themes are emerging that point to areas of specific vulnerability for motor neurons such as axonal transport, endosomal trafficking and RNA processing. We review the clinical and molecular features of this diverse group of genetically determined conditions and consider the implications for the broad group of motor neuron diseases in general.

Fungal heat-shock proteins in human disease

Heat-shock proteins (hsps) have been identified as molecular chaperones conserved between microbes and man and grouped by their molecular mass and high degree of amino acid homology. This article reviews the major hsps of Saccharomyces cerevisiae, their interactions with trehalose, the effect of fermentation and the role of the heat-shock factor. Information derived from this model, as well as from Neurospora crassa and Achlya ambisexualis, helps in understanding the importance of hsps in the pathogenic fungi, Candida albicans, Cryptococcus neoformans, Aspergillus spp., Histoplasma capsulatum, Paracoccidioides brasiliensis, Trichophyton rubrum, Phycomyces blakesleeanus, Fusarium oxysporum, Coccidioides immitis and Pneumocystis jiroveci. This has been matched with proteomic and genomic information examining hsp expression in response to noxious stimuli. Fungal hsp90 has been identified as a target for immunotherapy by a genetically recombinant antibody. The concept of combining this antibody fragment with an antifungal drug for treating life-threatening fungal infection and the potential interactions with human and microbial hsp90 and nitric oxide is discussed.

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

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

Mass spectrometric analysis of intact macromolecular chaperone complexes

Electrospray ionization mass spectrometry enables the efficient and sensitive analysis of small and very large biomolecules. The gentle phase transfer from solution into the gas phase in combination with the seemingly unlimited mass range enables the study of intact homo- and heterogeneous protein complexes, providing an innovative tool in structural biology. Here we highlight recent progress in this field of 'native mass spectrometry' on noncovalent complexes, focusing on several chaperone complexes involved in protein folding.:

Some like it hot: the structure and function of small heat-shock proteins

Small heat-shock proteins (sHsps) are a widespread and diverse class of molecular chaperones. Recent evidence suggests that they maintain protein homeostasis by binding proteins in non-native conformations, thereby preventing substrate aggregation. Some members of the sHsp family are inactive or only partially active under physiological conditions, and transition toward the active state is induced by specific triggers, such as elevated temperature. Release of substrate proteins bound to sHsps requires cooperation with ATP-dependent chaperones, suggesting that sHsps create a reservoir of non-native proteins for subsequent refolding.

Characterization of novel sequence motifs within N- and C-terminal extensions of p26, a small heat shock protein from Artemia franciscana

The small heat shock proteins function as molecular chaperones, an activity often requiring reversible oligomerization and which protects against irreversible protein denaturation. An abundantly produced small heat shock protein termed p26 is thought to contribute to the remarkable stress resistance exhibited by encysted embryos of the crustacean, Artemia franciscana. Three novel sequence motifs termed G, R and TS were individually deleted from p26 by site-directed mutagenesis. G encompasses residues G8-G29, a glycine-enriched region, and R includes residues R36-R45, an arginine-enhanced sequence, both in the amino terminus. TS, composed of residues T169-T186, resides in the carboxy-extension and is augmented in threonine and serine. Deletion of R had more influence than removal of G on p26 oligomerization and chaperoning, the latter determined by thermotolerance induction in Escherichia coli, protection of insulin and citrate synthase from dithiothreitol- and heat-induced aggregation, respectively, and preservation of citrate synthase activity upon heating. Oligomerization of the TS and R variants was similar, but the TS deletion was slightly more effective than R as a chaperone. The extent of p26 structural perturbation introduced by internal deletions, including modification of intrinsic fluorescence, 1-anilino-8-naphthalene-sulphonate binding and secondary structure, paralleled reductions in oligomerization and chaperoning. Three-dimensional modeling of p26 based on wheat Hsp16.9 crystal structure indicated many similarities between the two proteins, including peptide loops associated with secondary structure elements. Loop 1 of p26 was deleted in the G variant with minimal effect on oligomerization and chaperoning, whereas loop 3, containing beta-strand 6 was smaller than the corresponding loop in Hsp16.9, which may influence p26 function.

Chaperone-Like Activity of Mycobacterium tuberculosis Hsp16.3 Does Not Require Its Intact (Native) Structures

Small heat shock proteins (sHsps) were found to exhibit efficient chaperone-like activities under stress conditions although their native structures are severely disturbed. Here, using an alternative approach (site-directed mutagenesis), we obtained two structurally and functionally distinct Mycobacterium tuberculosis Hsp16.3 single-site mutant proteins. The G59W mutant protein (with Gly59 substituted by Trp) is capable of exhibiting efficient chaperone-like activity even under non-stress conditions although its secondary, tertiary, and quaternary structures are very different from that of the wild type protein. By contrast, the G59A mutant protein (with Gly59 substituted by Ala) resembles with the wild type protein in structure and function. These observations suggest that the Gly59 of the Hsp16.3 protein is critical for its folding and assembly. In particular, we propose that the exhibition of chaperone-like activity for Hsp16.3 does not require its intact (native) structures but requires the disturbance of its native structures (i.e., the native structure-disturbed Hsp16.3 retains its chaperone-like activity or even becomes more active). In addition, the behavior of such an active mutant protein (G59W) also strongly supports our previous suggestion that Hsp16.3 exhibits chaperone-like activity via oligomeric dissociation.

Detection of oligomerisation and substrate recognition sites of small heat shock proteins by peptide arrays

Small heat shock proteins (sHsps) form large oligomers that are characterised by their dynamic behaviour, e.g., complex disassembly/reassembly and extensive subunit exchange. These processes are interrelated with sHsp/substrate interaction. sHsps bind a broad spectrum of unrelated substrate proteins under denaturing conditions. Detailed knowledge about the binding process and regions critical for sHsp/substrate interaction is missing. In this study, we screened cellulose-bound peptide spot libraries derived from a bacterial sHsp and the model-substrate citrate synthase to detect oligomerisation and substrate interaction sites, respectively. In line with previous results, it was demonstrated that multiple contacts involving the N- and C-terminal extensions and the central alpha-crystallin domain are required for oligomerisation. Incubation of the citrate synthase membrane with sHsps revealed a putative substrate interaction site. A soluble peptide with the sequence RTKYWELIYEDCMDL (CS(191-205)) corresponding to that site inhibited chaperone activity of sHsps, presumably by blocking their substrate-binding sites.

The small heat shock protein IbpA of Escherichia coli cooperates with IbpB in stabilization of thermally aggregated proteins in a disaggregation competent state

The small heat shock proteins are ubiquitous stress proteins proposed to increase cellular tolerance to heat shock conditions. We isolated IbpA, the Escherichia coli small heat shock protein, and tested its ability to keep thermally inactivated substrate proteins in a disaggregation competent state. We found that the presence of IbpA alone during substrate thermal inactivation only weakly influences the ability of the bi-chaperone Hsp70-Hsp100 system to disaggregate aggregated substrate. Similar minor effects were observed for IbpB alone, the other E. coli small heat shock protein. However, when both IbpA and IbpB are simultaneously present during substrate inactivation they efficiently stabilize thermally aggregated proteins in a disaggregation competent state. The properties of the aggregated protein substrates are changed in the presence of IbpA and IbpB, resulting in lower hydrophobicity and the ability of aggregates to withstand sizing chromatography conditions. IbpA and IbpB form mixed complexes, and IbpA stimulates association of IbpB with substrate.

Comparative analysis of the hspA mutant and wild-type Synechocystis sp. strain PCC 6803 under salt stress: evaluation of the role of hspA in salt-stress management

DNA microarray analysis has previously revealed that hspA, which encodes a small heat-shock protein, is the second most highly expressed gene under salt stress in Synechocystis sp. strain PCC 6803. Consequently, an hspA deletion mutant was studied under various salt stresses in order to identify a potential role of HspA in salt stress management. The mutant had a growth disadvantage under moderate salt stress. It lost the ability to develop tolerance to a lethal salt treatment by a moderate salt pre-treatment when the tolerance was evaluated by cell survival and the level of major soluble proteins, phycocyanins, while the wild-type acquired tolerance. Under various salt stresses, the mutant failed to undergo the ultrastructural changes characteristic of wild-type cells. The mutant, which showed higher survival than the wild-type after a direct shift to lethal salt conditions, accumulated higher levels of groESL1 and groEL2 transcripts and the corresponding proteins, GroES, GroEL1, and GroEL2, suggesting a role for these heat-shock proteins in conferring basal salt tolerance. Under salt stress, heat-shock genes, such as hspA, groEL2, and dnaK2, were transcriptionally induced and greatly stabilized, indicating a transcriptional and post-transcriptional mechanism of acclimation to salt stress involving these heat-shock genes.

Oligomerization, chaperone activity, and nuclear localization of p26, a small heat shock protein from Artemia franciscana

Artemia franciscana embryos undergo encystment, developmental arrest and diapause, the last characterized by profound metabolic dormancy and extreme stress resistance. Encysted embryos contain an abundant small heat shock protein termed p26, a molecular chaperone that undoubtedly has an important role in development. To understand better the role of p26 in Artemia embryos, the structural and functional characteristics of full-length and truncated p26 expressed in Escherichia coli and COS-1 cells were determined. p26 chaperone activity declined with increasing truncation of the protein, and those deletions with the greatest adverse effect on protection of citrate synthase during thermal stress had the most influence on oligomerization. When produced in either prokaryotic or eukaryotic cells the p26 alpha-crystallin domain consisting of amino acid residues 61-152 existed predominantly as monomers, and p26 variants lacking the amino-terminal domain but with intact carboxyl-terminal extensions were mainly monomers and dimers. The amino terminus was, therefore, required for efficient dimer formation. Assembly of higher order oligomers was enhanced by the carboxyl-terminal extension, although removing the 10 carboxyl-terminal residues had relatively little effect on oligomerization and chaperoning. Full-length and carboxyl-terminal truncated p26 resided in the cytoplasm of transfected COS-1 cells; however, variants missing the complete amino-terminal domain and existing predominantly as monomers/dimers entered the nuclei. A mechanism whereby oligomer disassembly assisted entry of p26 into nuclei was suggested, this of importance because p26 translocates into Artemia embryo nuclei during development and stress. However, when examined in Artemia, the p26 oligomer size was unchanged under conditions that allowed movement into nuclei, suggesting a process more complex than just oligomer dissociation.

Phosphorylation of αB-Crystallin Alters Chaperone Function through Loss of Dimeric Substructure

Phosphorylation is the most common posttranslational modification of the alpha-crystallins in the human lens. These phosphorylated forms are not only important because of their abundance in aging lenses and the implications for cataract but also because they have been identified in patients with degenerative brain disease. By using mimics corresponding to the reported in vivo phosphorylation sites in the human lens, we have examined the effects of phosphorylation upon the chaperone-like properties and structure of alphaB-crystallin. Here we show that phosphorylation of alphaB-crystallin at Ser-45 results in uncontrolled aggregation. By using an innovative tandem mass spectrometry approach, we demonstrate how this alteration in behavior stems from disruption of dimeric substructure within the polydisperse alphaB-crystallin assembly. This structural perturbation appears to disturb the housekeeping role of alphaB-crystallin and consequently has important implications for the disease states caused by protein aggregation in the lens and deposition in non-lenticular tissue.

Temperature and concentration-controlled dynamics of rhizobial small heat shock proteins

A hallmark of alpha-crystallin-type small heat shock proteins (sHsps) is their highly dynamic oligomeric structure which promotes intermolecular interactions involved in subunit exchange and substrate binding (chaperone-like activity). We studied the oligomeric features of two classes of bacterial sHsps by size exclusion chromatography and nanoelectrospray mass spectrometry. Proteins of both classes formed large complexes that rapidly dissociated upon dilution and at physiologically relevant heat shock temperatures. As the secondary structure was not perturbed, temperature- and concentration-dependent dissociations were fully reversible. Complexes formed between sHsps and the model substrate citrate synthase were stable and exceeded the size of sHsp oligomers. Small Hsps, mutated in a highly conserved glycine residue at the C-terminal end of the alpha-crystallin domain, formed labile complexes that disassembled more readily than the corresponding wild-type proteins. Reduced complex stability coincided with reduced chaperone activity.

Mutants in a small heat shock protein that affect the oligomeric state. Analysis and allele-specific suppression

Oligomerization is an essential property of small heat shock proteins (sHSPs) that appears to regulate their chaperone activity. We have examined the role of conserved hydrophobic residues that are postulated to stabilize sHSP oligomers. We identified a mutation of Synechocystis Hsp16.6 that impairs function in vivo and in vitro. The V143A mutation is in the C-terminal extension, a region predicted to form an oligomeric interaction with a hydrophobic region that includes the site of a previously characterized mutation, L66A. Both mutants were dimeric, but V143A had a stronger oligomerization defect than L66A. However, V143A protected a model substrate better than L66A. This suggests that although the two regions both play a role in oligomerization, they are not equivalent. Nevertheless, the addition of either dimeric sHSP enhanced the in vitro chaperone activity of wild type Hsp16.6, consistent with models that the sHSP dimers initiate interactions with substrates. Suppressor analysis of V143A identified mutations in the N terminus that restored activity by restabilizing the oligomer. These mutants were allele-specific and unable to suppress L66A, although they suppressed a dimeric C-terminal truncation of Hsp16.6. Conversely, suppressors of L66A were unable to suppress either V143A or the truncation, although they, like suppressors of V143A, stabilize the Hsp16.6 oligomer. We interpret these data as evidence that the mutations V143A and L66A stabilize two different dimeric structures and as further support that sHSP dimers are active species.

Chaperone activity of cytosolic small heat shock proteins from wheat

Small Hsps (sHsps) and the structurally related eye lens alpha-crystallins are ubiquitous stress proteins that exhibit ATP-independent molecular chaperone activity. We studied the chaperone activity of dodecameric wheat TaHsp16.9C-I, a class I cytosolic sHsp from plants and the only eukaryotic sHsp for which a high resolution structure is available, along with the related wheat protein TaHsp17.8C-II, which represents the evolutionarily distinct class II plant cytosolic sHsps. Despite the available structural information on TaHsp16.9C-I, there is minimal data on its chaperone activity, and likewise, data on activity of the class II proteins is very limited. We prepared purified, recombinant TaHsp16.9C-I and TaHsp17.8C-II and find that the class II protein comprises a smaller oligomer than the dodecameric TaHsp16.9C-I, suggesting class II proteins have a distinct mode of oligomer assembly as compared to the class I proteins. Using malate dehydrogenase as a substrate, TaHsp16.9C-I was shown to be a more effective chaperone than TaHsp17.8C-II in preventing heat-induced malate dehydrogenase aggregation. As observed by EM, morphology of sHsp/substrate complexes depended on the sHsp used and on the ratio of sHsp to substrate. Surprisingly, heat-denaturing firefly luciferase did not interact significantly with TaHsp16.9C-I, although it was fully protected by TaHsp17.8C-II. In total the data indicate sHsps show substrate specificity and suggest that N-terminal residues contribute to substrate interactions.

The identity of proteins associated with a small heat shock protein during heat stress in vivo indicates that these chaperones protect a wide range of cellular functions

The small heat shock proteins (sHSPs) are a ubiquitous class of ATP-independent chaperones believed to prevent irreversible protein aggregation and to facilitate subsequent protein renaturation in cooperation with ATP-dependent chaperones. Although sHSP chaperone activity has been studied extensively in vitro, understanding the mechanism of sHSP function requires identification of proteins that are sHSP substrates in vivo. We have used both immunoprecipitation and affinity chromatography to recover 42 proteins that specifically interact with Synechocystis Hsp16.6 in vivo during heat treatment. These proteins can all be released from Hsp16.6 by the ATP-dependent activity of DnaK and co-chaperones and are heat-labile. Thirteen of the putative substrate proteins were identified by mass spectrometry and reveal the potential for sHSPs to protect cellular functions as diverse as transcription, translation, cell signaling, and secondary metabolism. One of the putative substrates, serine esterase, was purified and tested directly for interaction with purified Hsp16.6. Hsp16.6 effectively formed soluble complexes with serine esterase in a heat-dependent fashion, thereby preventing formation of insoluble serine esterase aggregates. These data offer critical insights into the characteristics of native sHSP substrates and extend and provide in vivo support for the chaperone model of sHSP function.