Crystallization and preliminary X-ray diffraction analysis of XAC1151, a small heat-shock protein from Xanthomonas axonopodis pv. citri belonging to the alpha-crystallin family

Two Bacterial Small Heat Shock Proteins, IbpA and IbpB, Form a Functional Heterodimer

Small heat shock proteins (sHsps) are a conserved class of ATP-independent chaperones which in stress conditions bind to unfolded protein substrates and prevent their irreversible aggregation. Substrates trapped in sHsps-containing aggregates are efficiently refolded into native structures by ATP-dependent Hsp70 and Hsp100 chaperones. Most γ-proteobacteria possess a single sHsp (IbpA), while in a subset of Enterobacterales, as a consequence of ibpA gene duplication event, a two-protein sHsp (IbpA and IbpB) system has evolved. IbpA and IbpB are functionally divergent. Purified IbpA, but not IbpB, stably interacts with aggregated substrates, yet both sHsps are required to be present at the substrate denaturation step for subsequent efficient Hsp70-Hsp100-dependent substrate refolding. IbpA and IbpB interact with each other, influence each other's expression levels and degradation rates. However, the crucial information on how these two sHsps interact and what is the basic building block required for proper sHsps functioning was missing. Here, based on NMR, mass spectrometry and crosslinking studies, we show that IbpA-IbpB heterodimer is a dominating functional unit of the two sHsp system in Enterobacterales. The principle of heterodimer formation is similar to one described for homodimers of single bacterial sHsps. β-hairpins formed by strands β5 and β7 of IbpA or IbpB crystallin domains associate with the other one's β-sandwich in the heterodimer structure. Relying on crosslinking and molecular dynamics studies, we also propose the orientation of two IbpA-IbpB heterodimers in a higher order tetrameric structure.

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.

Identification of HSP20 gene family in wheat and barley and their differential expression profiling under heat stress

Small heat shock proteins (sHSPs) are chaperones that play an important role in various developmental, biotic and abiotic stresses. The sHSP family possess a conserved domain of approximately 80 to 100 amino acids called alpha-crystalline domain (ACD), flanked by N- and C-terminal regions. Search for complete proteomes and expressed sequenced tag (EST) database of wheat and barley using Hidden Markov Model and BLAST algorithm was conducted. Here, we report genome-wide identification and characterization of 27 newly TaHSP20 candidate genes in wheat and 13 HvHSP20 in barley, describing structures, phylogenetic relationships, conserved protein motifs, and expression patterns. The structural analysis highlights that this gene family possesses a conserved ACD region at the C-terminal. Detailed pattern analysis of HSP20 revealed presence of P-G doublet and I/V/L-X-I/V/L motif that helps in oligomerization. Identification of conserved motif sequences of wheat and barley HSP20 strongly supported their identity as sHSP families. This study illustrates for the first time 3D model prediction of full-length wheat HSP20 (TaHSP20) protein and ACD region. Digital expression analysis was also carried out in order to reveal a widespread distribution of the sHSP family genes at various developmental stages of wheat and barley. In addition, five selected transcripts of both wheat and barley were validated for their expression profile under 35 °C and 42 °C heat stress conditions. Results indicate up-regulation of all the transcripts under heat stress condition except TaCBM38894 candidate, which showed down-regulation in wheat.

Evolution and functional diversification of the small heat shock protein/α-crystallin family in higher plants

Small heat shock proteins (sHSPs) are chaperones that play an important role in stress tolerance. They consist of an alpha-crystallin domain (ACD) flanked by N- and C-terminal regions. However, not all proteins that contain an ACD, hereafter referred to as ACD proteins, are sHSPs because certain ACD proteins are known to have different functions. Furthermore, since not all ACD proteins have been identified yet, current classifications are incomplete. A total of 17 complete plant proteomes were screened for the presence of ACD proteins by HMMER profiling and the identified ACD protein sequences were classified by maximum likelihood phylogeny. Differences among and within groups were analysed, and levels of functional constraint were determined. There are 29 different classes of ACD proteins, eight of which contain classical sHSPs and five likely chaperones. The other classes contain proteins with uncharacterised or poorly characterised functions. N- and C-terminal sequences are conserved within the phylogenetic classes. Phylogenetics suggests a single duplication of the CI sHSP ancestor that occurred prior to the speciation of mono- and dicotyledons. This was followed by a number of more recent duplications that resulted in the presence of many paralogues. The results suggest that N- and C-terminal sequences of sHSPs play a role in class-specific functionality and that non-sHSP ACD proteins have conserved but unexplored functions, which are mainly determined by subsequences other than that of the ACD.

Evolution and functional diversification of the small heat shock protein/α-crystallin family in higher plants

Small heat shock proteins (sHSPs) are chaperones that play an important role in stress tolerance. They consist of an alpha-crystallin domain (ACD) flanked by N- and C-terminal regions. However, not all proteins that contain an ACD, hereafter referred to as ACD proteins, are sHSPs because certain ACD proteins are known to have different functions. Furthermore, since not all ACD proteins have been identified yet, current classifications are incomplete. A total of 17 complete plant proteomes were screened for the presence of ACD proteins by HMMER profiling and the identified ACD protein sequences were classified by maximum likelihood phylogeny. Differences among and within groups were analysed, and levels of functional constraint were determined. There are 29 different classes of ACD proteins, eight of which contain classical sHSPs and five likely chaperones. The other classes contain proteins with uncharacterised or poorly characterised functions. N- and C-terminal sequences are conserved within the phylogenetic classes. Phylogenetics suggests a single duplication of the CI sHSP ancestor that occurred prior to the speciation of mono- and dicotyledons. This was followed by a number of more recent duplications that resulted in the presence of many paralogues. The results suggest that N- and C-terminal sequences of sHSPs play a role in class-specific functionality and that non-sHSP ACD proteins have conserved but unexplored functions, which are mainly determined by subsequences other than that of the ACD.

Initial crystallographic studies of a small heat-shock protein from Xylella fastidiosa

The ORF XF2234 in the Xylella fastidiosa genome was identified as encoding a small heat-shock protein of 17.9 kDa (HSP17.9). HSP17.9 was found as one of the proteins that are induced during X. fastidiosa proliferation and infection in citrus culture. Recombinant HSP17.9 was crystallized and surface atomic force microscopy experiments were conducted with the aim of better characterizing the HSP17.9 crystals. X-ray diffraction data were collected at 2.7 Å resolution. The crystal belonged to space group P4(3)22, with unit-cell parameters a = 68.90, b = 68.90, c = 72.51 Å, and is the first small heat-shock protein to crystallize in this space group.

Small heat-shock proteins: paramedics of the cell

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

Small heat shock proteins and α-crystallins: dynamic proteins with flexible functions

The small heat shock proteins (sHSPs) and the related α-crystallins (αCs) are virtually ubiquitous proteins that are strongly induced by a variety of stresses, but that also function constitutively in multiple cell types in many organisms. Extensive research has demonstrated that a majority of sHSPs and αCs can act as ATP-independent molecular chaperones by binding denaturing proteins and thereby protecting cells from damage due to irreversible protein aggregation. As a result of their diverse evolutionary history, their connection to inherited human diseases, and their novel protein dynamics, sHSPs and αCs are of significant interest to many areas of biology and biochemistry. However, it is increasingly clear that no single model is sufficient to describe the structure, function or mechanism of action of sHSPs and αCs. In this review, we discuss recent data that provide insight into the variety of structures of these proteins, their dynamic behavior, how they recognize substrates, and their many possible cellular roles.

Crystal structures of Xanthomonas small heat shock protein provide a structural basis for an active molecular chaperone oligomer

Small heat shock proteins (sHsps) are ubiquitous low-molecular-weight chaperones that prevent protein aggregation under cellular stresses. sHsps contain a structurally conserved α-crystallin domain (ACD) of about 100 amino acid residues flanked by varied N- and C-terminal extensions and usually exist as oligomers. Oligomerization is important for the biological functions of most sHsps. However, the active oligomeric states of sHsps are not defined yet. We present here crystal structures (up to 1.65 Å resolution) of the sHspA from the plant pathogen Xanthomonas (XaHspA). XaHspA forms closed or open trimers of dimers (hexamers) in crystals but exists predominantly as 36mers in solution as estimated by size-exclusion chromatography. The XaHspA monomer structures mainly consist of α-crystallin domain with disordered N- and C-terminal extensions, indicating that the extensions are flexible and not essential for the formation of dimers and 36mers. Under reducing conditions where α-lactalbumin (LA) unfolds and aggregates, XaHspA 36mers formed complexes with one LA per XaHspA dimer. Based on XaHspA dimer-dimer interactions observed in crystals, we propose that XaHspA 36mers have four possible conformations, but only XaHspA 36merB, which is formed by open hexamers in 12mer-6mer-6mer-12mer with protruding dimers accessible for substrate (unfolding protein) binding, can bind to 18 reduced LA molecules. Together, our results unravel the structural basis of an active sHsp oligomer.

Crystal structures of truncated alphaA and alphaB crystallins reveal structural mechanisms of polydispersity important for eye lens function

Small heat shock proteins alphaA and alphaB crystallin form highly polydisperse oligomers that frustrate protein aggregation, crystallization, and amyloid formation. Here, we present the crystal structures of truncated forms of bovine alphaA crystallin (AAC(59-163)) and human alphaB crystallin (ABC(68-162)), both containing the C-terminal extension that functions in chaperone action and oligomeric assembly. In both structures, the C-terminal extensions swap into neighboring molecules, creating runaway domain swaps. This interface, termed DS, enables crystallin polydispersity because the C-terminal extension is palindromic and thereby allows the formation of equivalent residue interactions in both directions. That is, we observe that the extension binds in opposite directions at the DS interfaces of AAC(59-163) and ABC(68-162). A second dimeric interface, termed AP, also enables polydispersity by forming an antiparallel beta sheet with three distinct registration shifts. These two polymorphic interfaces enforce polydispersity of alpha crystallin. This evolved polydispersity suggests molecular mechanisms for chaperone action and for prevention of crystallization, both necessary for transparency of eye lenses.

Detection and architecture of small heat shock protein monomers

Background

Small Heat Shock Proteins (sHSPs) are chaperone-like proteins involved in the prevention of the irreversible aggregation of misfolded proteins. Although many studies have already been conducted on sHSPs, the molecular mechanisms and structural properties of these proteins remain unclear. Here, we propose a better understanding of the architecture, organization and properties of the sHSP family through structural and functional annotations. We focused on the Alpha Crystallin Domain (ACD), a sandwich fold that is the hallmark of the sHSP family.

Methodology/Principal Findings

We developed a new approach for detecting sHSPs and delineating ACDs based on an iterative Hidden Markov Model algorithm using a multiple alignment profile generated from structural data on ACD. Using this procedure on the UniProt databank, we found 4478 sequences identified as sHSPs, showing a very good coverage with the corresponding PROSITE and Pfam profiles. ACD was then delimited and structurally annotated. We showed that taxonomic-based groups of sHSPs (animals, plants, bacteria) have unique features regarding the length of their ACD and, more specifically, the length of a large loop within ACD. We detailed highly conserved residues and patterns specific to the whole family or to some groups of sHSPs. For 96% of studied sHSPs, we identified in the C-terminal region a conserved I/V/L-X-I/V/L motif that acts as an anchor in the oligomerization process. The fragment defined from the end of ACD to the end of this motif has a mean length of 14 residues and was named the C-terminal Anchoring Module (CAM).

Conclusions/Significance

This work annotates structural components of ACD and quantifies properties of several thousand sHSPs. It gives a more accurate overview of the architecture of sHSP monomers.

Increased monomerization of mutant HSPB1 leads to protein hyperactivity in Charcot-Marie-Tooth neuropathy

Small heat shock proteins are molecular chaperones capable of maintaining denatured proteins in a folding-competent state. We have previously shown that missense mutations in the small heat shock protein HSPB1 (HSP27) cause distal hereditary motor neuropathy and axonal Charcot-Marie-Tooth disease. Here we investigated the biochemical consequences of HSPB1 mutations that are known to cause peripheral neuropathy. In contrast to other chaperonopathies, our results revealed that particular HSPB1 mutations presented higher chaperone activity compared with wild type. Hyperactivation of HSPB1 was accompanied by a change from its wild-type dimeric state to a monomer without dissociation of the 24-meric state. Purification of protein complexes from wild-type and HSPB1 mutants showed that the hyperactive isoforms also presented enhanced binding to client proteins. Furthermore, we show that the wild-type HSPB1 protein undergoes monomerization during heat-shock activation, strongly suggesting that the monomer is the active form of the HSPB1 protein.