Terminal Regions Confer Plasticity to the Tetrameric Assembly of Human HspB2 and HspB3

Mechanism of small heat shock protein client sequestration and induced polydispersity

Small heat shock proteins (sHSPs) act as first responders during cellular stress by recognizing and sequestering destabilized proteins (clients), preventing their aggregation and facilitating downstream refolding or degradation1-3. This chaperone function is critically important to proteostasis, conserved across all kingdoms of life, and associated with various protein misfolding diseases in humans4,5. Mechanistic insights into how sHSPs sequester destabilized clients have been limited due to the extreme molecular plasticity and client-induced polydispersity of sHSP/client complexes6-8. Here, we present high-resolution cryo-EM structures of the sHSP from Methanocaldococcus jannaschii (mjHSP16.5) in both the apo-state and in an ensemble of client-bound states. The ensemble not only reveals key molecular mechanisms by which sHSPs respond to and sequester client proteins, but also provides insights into the cooperative nature of chaperone-client interactions. Engagement with destabilized client induces a polarization of stability across the mjHSP16.5 scaffold, proposed to facilitate higher-order assembly and enhance client sequestration capacity. Some higher-order sHSP oligomers appear to form through simple insertion of dimeric subunits into new geometrical features, while other higher-order states suggest multiple sHSP/client assembly pathways. Together, these results provide long-sought insights into the chaperone function of sHSPs and highlight the relationship between polydispersity and client sequestration under stress conditions.

Catchers of folding gone awry: a tale of small heat shock proteins

Small heat shock proteins (sHsps) are an important part of the cellular system maintaining protein homeostasis under physiological and stress conditions. As molecular chaperones, they form complexes with different non-native proteins in an ATP-independent manner. Many sHsps populate ensembles of energetically similar but different-sized oligomers. Regulation of chaperone activity occurs by changing the equilibrium of these ensembles. This makes sHsps a versatile and adaptive system for trapping non-native proteins in complexes, allowing recycling with the help of ATP-dependent chaperones. In this review, we discuss progress in our understanding of the structural principles of sHsp oligomers and their functional principles, as well as their roles in aging and eye lens transparency.

Dynamic fibrillar assembly of αB-crystallin induced by perturbation of the conserved NT-IXI motif resolved by cryo-EM

αB-crystallin is an archetypical member of the small heat shock proteins (sHSPs) vital for cellular proteostasis and mitigating protein misfolding diseases. Gaining insights into the principles defining their molecular organization and chaperone function have been hindered by intrinsic dynamic properties and limited high-resolution structural analysis. To disentangle the mechanistic underpinnings of these dynamical properties, we ablate a conserved IXI-motif located within the N-terminal (NT) domain of human αB-crystallin implicated in subunit exchange dynamics and client sequestration. This results in a profound structural transformation, from highly polydispersed caged-like native assemblies into an elongated fibril state amenable to high-resolution cryo-EM analysis. The reversible nature of this variant facilitates interrogation of functional effects due to perturbation of the NT-IXI motif in both the native-like oligomer and fibril states. Together, our investigations unveil several features thought to be key mechanistic attributes to sHSPs and point to a critical significance of the NT-IXI motif in αB-crystallin assembly, polydispersity, and chaperone activity.

Interaction of the C-terminal immunoglobulin-like domains (Ig 22-24) of filamin C with human small heat shock proteins

Small heat shock proteins are the well-known regulators of the cytoskeleton integrity, yet their complexes with actin-binding proteins are underexplored. Filamin C, a dimeric 560 kDa protein, abundant in cardiac and skeletal muscles, crosslinks actin filaments and contributes to Z-disc formation and membrane-cytoskeleton attachment. Here, we analyzed the interaction of a human filamin C fragment containing immunoglobulin-like domains 22-24 (FLNC22-24) with five small heat shock proteins (HspB1, HspB5, HspB6, HspB7, HspB8) and their α-crystallin domains. On size-exclusion chromatography, only HspB7 or its α-crystallin domain formed complexes with FLNC22-24. Despite similar isoelectric points of the small heat shock proteins analyzed, only HspB7 and its α-crystallin domain interacted with FLNC22-24 on native gel electrophoresis. Crosslinking with glutaraldehyde confirmed the formation of complexes between HspB7 (or its α-crystallin domain) and the filamin С fragment, inhibiting intersubunit FLNC crosslinking. These data are consistent with the structure modeling using Alphafold. Thus, the C-terminal fragment (immunoglobulin-like domains 22-24) of filamin C contains the site for HspB7 (or its α-crystallin domain) interaction, which competes with FLNC22-24 dimerization and its probable interaction with different target proteins.

Dynamic fibrillar assembly of αB-crystallin induced by perturbation of the conserved NT-IXI motif resolved by cryo-EM

αB-crystallin is an archetypical member of the small heat-shock proteins (sHSPs) vital for cellular proteostasis and mitigating protein misfolding diseases. Gaining insights into the principles defining their molecular organization and chaperone function have been hindered by intrinsic dynamic properties and limited high-resolution structural analysis. To disentangle the mechanistic underpinnings of these dynamical properties, we mutated a conserved IXI-motif located within the N-terminal (NT) domain of human αB-crystallin. This resulted in a profound structural transformation, from highly polydispersed caged-like native assemblies into a comparatively well-ordered helical fibril state amenable to high-resolution cryo-EM analysis. The reversible nature of the induced fibrils facilitated interrogation of functional effects due to perturbation of the NT-IXI motif in both the native-like oligomer and fibril states. Together, our investigations unveiled several features thought to be key mechanistic attributes to sHSPs and point to a critical significance of the NT-IXI motif in αB-crystallin assembly, dynamics and chaperone activity.

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

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

PINK1 and Parkin rescue motor defects and mitochondria dysfunction induced by a patient-derived HSPB3 mutant in Drosophila models

Small heat shock proteins (sHSPs) are ATP-independent molecular chaperones with the α-crystalline domain that is critical to their chaperone activity. Within the sHSP family, three (HSPB1, HSPB3, and HSPB8) proteins are linked with inherited peripheral neuropathies, including distal hereditary motor neuropathy (dHMN) and Charco-Marie-Tooth disease (CMT). In this study, we introduced the HSPB3 Y118H (HSPB3Y118H) mutant gene identified from the CMT2 family in Drosophila. With a missense mutation on its α-crystalline domain, this human HSPB3 mutant gene induced a loss of motor activity accompanied by reduced mitochondrial membrane potential in fly neuronal tissues. Moreover, mitophagy, a critical mechanism of mitochondrial quality control, is downregulated in fly motor neurons expressing HSPB3Y118H. Surprisingly, PINK1 and Parkin, the core regulators of mitophagy, successfully rescued these motor and mitochondrial abnormalities in HSPB3 mutant flies. Results from the first animal model of HSPB3 mutations suggest that mitochondrial dysfunction plays a critical role in HSPB3-associated human pathology.

Dynamics and composition of small heat shock protein condensates and aggregates

Small heat shock proteins (sHSPs) are essential ATP-independent chaperones that protect the cellular proteome. These proteins assemble into polydisperse oligomeric structures, the composition of which dramatically affects their chaperone activity. The biomolecular consequences of variations in sHSP ratios, especially inside living cells, remain elusive. Here, we study the consequences of altering the relative expression levels of HspB2 and HspB3 in HEK293T cells. These chaperones are partners in a hetero-oligomeric complex, and genetic mutations that abolish their mutual interaction are associated with myopathic disorders. HspB2 displays three distinct phenotypes when co-expressed with HspB3 at varying ratios. Expression of HspB2 alone leads to formation of liquid nuclear condensates, while shifting the stoichiometry towards HspB3 resulted in the formation of large solid-like aggregates. Only cells co-expressing HspB2 with a limited amount of HspB3 formed fully soluble complexes that were distributed homogeneously throughout the nucleus. Strikingly, both condensates and aggregates were reversible, as shifting the HspB2:HspB3 balance in situ resulted in dissolution of these structures. To uncover the molecular composition of HspB2 condensates and aggregates, we used APEX-mediated proximity labelling. Most proteins interact transiently with the condensates and were neither enriched nor depleted in these cells. In contrast, we found that HspB2:HspB3 aggregates sequestered several disordered proteins and autophagy factors, suggesting that the cell is actively attempting to clear these aggregates. This study presents a striking example of how changes in the relative expression levels of interacting proteins affects their phase behavior. Our approach could be applied to study the role of protein stoichiometry and the influence of client binding on phase behavior in other biomolecular condensates and aggregates.

Small heat shock proteins operate as molecular chaperones in the mitochondrial intermembrane space

Mitochondria are complex organelles with different compartments, each harbouring their own protein quality control factors. While chaperones of the mitochondrial matrix are well characterized, it is poorly understood which chaperones protect the mitochondrial intermembrane space. Here we show that cytosolic small heat shock proteins are imported under basal conditions into the mitochondrial intermembrane space, where they operate as molecular chaperones. Protein misfolding in the mitochondrial intermembrane space leads to increased recruitment of small heat shock proteins. Depletion of small heat shock proteins leads to mitochondrial swelling and reduced respiration, while aggregation of aggregation-prone substrates is countered in their presence. Charcot-Marie-Tooth disease-causing mutations disturb the mitochondrial function of HSPB1, potentially linking previously observed mitochondrial dysfunction in Charcot-Marie-Tooth type 2F to its role in the mitochondrial intermembrane space. Our results reveal that small heat shock proteins form a chaperone system that operates in the mitochondrial intermembrane space.

Disordered region encodes α-crystallin chaperone activity toward lens client γD-crystallin

Small heat-shock proteins (sHSPs) are a widely expressed family of ATP-independent molecular chaperones that are among the first responders to cellular stress. Mechanisms by which sHSPs delay aggregation of client proteins remain undefined. sHSPs have high intrinsic disorder content of up to ~60% and assemble into large, polydisperse homo- and hetero-oligomers, making them challenging structural and biochemical targets. Two sHSPs, HSPB4 and HSPB5, are present at millimolar concentrations in eye lens, where they are responsible for maintaining lens transparency over the lifetime of an organism. Together, HSPB4 and HSPB5 compose the hetero-oligomeric chaperone known as α-crystallin. To identify the determinants of sHSP function, we compared the effectiveness of HSPB4 and HSPB5 homo-oligomers and HSPB4/HSPB5 hetero-oligomers in delaying the aggregation of the lens protein γD-crystallin. In chimeric versions of HSPB4 and HSPB5, chaperone activity tracked with the identity of the 60-residue disordered N-terminal regions (NTR). A short 10-residue stretch in the middle of the NTR ("Critical sequence") contains three residues that are responsible for high HSPB5 chaperone activity toward γD-crystallin. These residues affect structure and dynamics throughout the NTR. Abundant interactions involving the NTR Critical sequence reveal it to be a hub for a network of interactions within oligomers. We propose a model whereby the NTR critical sequence influences local structure and NTR dynamics that modulate accessibility of the NTR, which in turn modulates chaperone activity.

Heat shock proteins: Biological functions, pathological roles, and therapeutic opportunities

The heat shock proteins (HSPs) are ubiquitous and conserved protein families in both prokaryotic and eukaryotic organisms, and they maintain cellular proteostasis and protect cells from stresses. HSP protein families are classified based on their molecular weights, mainly including large HSPs, HSP90, HSP70, HSP60, HSP40, and small HSPs. They function as molecular chaperons in cells and work as an integrated network, participating in the folding of newly synthesized polypeptides, refolding metastable proteins, protein complex assembly, dissociating protein aggregate dissociation, and the degradation of misfolded proteins. In addition to their chaperone functions, they also play important roles in cell signaling transduction, cell cycle, and apoptosis regulation. Therefore, malfunction of HSPs is related with many diseases, including cancers, neurodegeneration, and other diseases. In this review, we describe the current understandings about the molecular mechanisms of the major HSP families including HSP90/HSP70/HSP60/HSP110 and small HSPs, how the HSPs keep the protein proteostasis and response to stresses, and we also discuss their roles in diseases and the recent exploration of HSP related therapy and diagnosis to modulate diseases. These research advances offer new prospects of HSPs as potential targets for therapeutic intervention.

Role of Small Heat Shock Proteins in the Remodeling of Actin Microfilaments

Small heat shock proteins (sHsps) play an important role in the maintenance of proteome stability and, particularly, in stabilization of the cytoskeleton and cell contractile apparatus. Cell exposure to different types of stress is accompanied by the translocation of sHsps onto actin filaments; therefore, it is commonly believed that the sHsps are true actin-binding proteins. Investigations of last years have shown that this assumption is incorrect. Stress-induced translocation of sHsp to actin filaments is not the result of direct interaction of these proteins with intact actin, but results from the chaperone-like activity of sHsps and their interaction with various actin-binding proteins. HspB1 and HspB5 interact with giant elastic proteins titin and filamin thus providing an integrity of the contractile apparatus and its proper localization in the cell. HspB6 binds to the universal adapter protein 14-3-3 and only indirectly affects the structure of actin filament. HspB7 interacts with filamin C and controls actin filament assembly. HspB8 forms tight complex with the universal regulatory and adapter protein Bag3 and participates in the chaperone-assisted selective autophagy (CASA) of actin-binding proteins (e.g., filamin), as well as in the actin-depending processes taking place in mitoses. Hence, the mechanisms of sHsp participation in the maintenance of the contractile apparatus and cytoskeleton are much more complicated and diverse than it has been postulated earlier and are not limited to direct interactions of sHsps with actin. The old hypothesis on the direct binding of sHsps to intact actin should be revised and further detailed investigation on the sHsp interaction with minor proteins participating in the formation and remodeling of actin filaments is required.

Are casein micelles extracellular condensates formed by liquid-liquid phase separation?

Casein micelles are extracellular polydisperse assemblies of unstructured casein proteins. Caseins are the major component of milk. Within casein micelles, casein molecules are stabilised by binding to calcium phosphate nanoclusters and, by acting as molecular chaperones, through multivalent interactions. In light of such interactions, we discuss whether casein micelles can be considered as extracellular condensates formed by liquid-liquid phase separation. We analyse the sequence, structure and interactions of caseins in comparison to proteins forming intracellular condensates. Furthermore, we review the similarities between caseins and small heat-shock proteins whose chaperone activity is linked to phase separation of proteins. By bringing these observations together, we describe a regulatory mechanism for protein condensates, as exemplified by casein micelles.

Insights on Human Small Heat Shock Proteins and Their Alterations in Diseases

The family of the human small Heat Shock Proteins (HSPBs) consists of ten members of chaperones (HSPB1-HSPB10), characterized by a low molecular weight and capable of dimerization and oligomerization forming large homo- or hetero-complexes. All HSPBs possess a highly conserved centrally located α-crystallin domain and poorly conserved N- and C-terminal domains. The main feature of HSPBs is to exert cytoprotective functions by preserving proteostasis, assuring the structural maintenance of the cytoskeleton and acting in response to cellular stresses and apoptosis. HSPBs take part in cell homeostasis by acting as holdases, which is the ability to interact with a substrate preventing its aggregation. In addition, HSPBs cooperate in substrates refolding driven by other chaperones or, alternatively, promote substrate routing to degradation. Notably, while some HSPBs are ubiquitously expressed, others show peculiar tissue-specific expression. Cardiac muscle, skeletal muscle and neurons show high expression levels for a wide variety of HSPBs. Indeed, most of the mutations identified in HSPBs are associated to cardiomyopathies, myopathies, and motor neuropathies. Instead, mutations in HSPB4 and HSPB5, which are also expressed in lens, have been associated with cataract. Mutations of HSPBs family members encompass base substitutions, insertions, and deletions, resulting in single amino acid substitutions or in the generation of truncated or elongated proteins. This review will provide an updated overview of disease-related mutations in HSPBs focusing on the structural and biochemical effects of mutations and their functional consequences.

The Diverse Functions of Small Heat Shock Proteins in the Proteostasis Network

The protein quality control (PQC) system maintains protein homeostasis by counteracting the accumulation of misfolded protein conformers. Substrate degradation and refolding activities executed by ATP-dependent proteases and chaperones constitute major strategies of the proteostasis network. Small heat shock proteins represent ATP-independent chaperones that bind to misfolded proteins, preventing their uncontrolled aggregation. sHsps share the conserved α-crystallin domain (ACD) and gain functional specificity through variable and largely disordered N- and C-terminal extensions (NTE, CTE). They form large, polydisperse oligomers through multiple, weak interactions between NTE/CTEs and ACD dimers. Sequence variations of sHsps and the large variability of sHsp oligomers enable sHsps to fulfill diverse tasks in the PQC network. sHsp oligomers represent inactive yet dynamic resting states that are rapidly deoligomerized and activated upon stress conditions, releasing substrate binding sites in NTEs and ACDs Bound substrates are usually isolated in large sHsp/substrate complexes. This sequestration activity of sHsps represents a third strategy of the proteostasis network. Substrate sequestration reduces the burden for other PQC components during immediate and persistent stress conditions. Sequestered substrates can be released and directed towards refolding pathways by ATP-dependent Hsp70/Hsp100 chaperones or sorted for degradation by autophagic pathways. sHsps can also maintain the dynamic state of phase-separated stress granules (SGs), which store mRNA and translation factors, by reducing the accumulation of misfolded proteins inside SGs and preventing unfolding of SG components. This ensures SG disassembly and regain of translational capacity during recovery periods.

Replacement of Arg in the conserved N-terminal RLFDQxFG motif affects physico-chemical properties and chaperone-like activity of human small heat shock protein HspB8 (Hsp22)

The small heat shock protein (sHsp) called HspB8 (formerly, Hsp22) is one of the least typical sHsp members, whose oligomerization status remains debatable. Here we analyze the effect of mutations in a highly conservative sequence located in the N-terminal domain of human HspB8 on its physico-chemical properties and chaperone-like activity. According to size-exclusion chromatography coupled to multi-angle light scattering, the wild type (WT) HspB8 is present as dominating monomeric species (~24 kDa) and a small fraction of oligomers (~60 kDa). The R29A amino acid substitution leads to the predominant formation of 60-kDa oligomers, leaving only a small fraction of monomers. Deletion of the 28–32 pentapeptide (Δ mutant) results in the formation of minor quantities of dimers (~49 kDa) and large quantities of the 24-kDa monomers. Both the WT protein and its Δ mutant efficiently bind a hydrophobic probe bis-ANS and are relatively rapidly hydrolyzed by chymotrypsin, whereas the R29A mutant weakly binds bis-ANS and resists chymotrypsinolysis. In contrast to HspB8 WT and its Δ mutant, which are well phosphorylated by cAMP-dependent and ERK1 protein kinases, the R29A mutant is poorly phosphorylated. R29A mutation affects the chaperone-like activity of HspB8 measured in vitro. It is concluded that the irreplaceable Arg residue located in the only highly conservative motif in the N-terminal domain of all sHsp proteins affects the oligomeric structure and key properties of HspB8.

A novel deletion in the C-terminal region of HSPB8 in a family with rimmed vacuolar myopathy

Heat shock protein family B member 8, encoded by HSPB8, is an essential component of the chaperone-assisted selective autophagy complex, which maintains muscle function by degrading damaged proteins in the cells. Mutations in HSPB8 have been reported to cause Charcot-Marie-Tooth type 2L, distal hereditary motor neuropathy IIa, and rimmed vacuolar myopathies (RVM). In this study, we identified a novel heterozygous frameshift variant c.525_529del in HSPB8 in a large Japanese family with RVM, using whole exome sequencing. Three affected individuals had severe respiratory failure, which has not been addressed by previous studies. Muscle atrophy in the paraspinal muscles was also a clinical feature of the individuals affected with RVM in this study. The frameshift mutation was located in the last coding exon, and the mutated protein was predicted to harbor an isoleucine-leucine-valine (ILV) sequence, which corresponds to the IXI/V (isoleucine, X amino acids, and isoleucine or valine) motif. The IXI/V motif is essential for assembly into larger oligomers in other small heat shock proteins and all frameshift mutants of HSPB8 were predicted to share the ILV sequence in the C-terminal extension. The in silico prediction tools showed low protein solubility and increased aggregation propensity for the region around the ILV sequence. The IXI/V motif might be associated with the pathogenesis of HSPB8-related RVM.

O-GlcNAc modification of small heat shock proteins enhances their anti-amyloid chaperone activity

A major role for the intracellular post-translational modification O-GlcNAc appears to be the inhibition of protein aggregation. Most of the previous studies in this area focused on O-GlcNAc modification of the amyloid-forming proteins themselves. Here we used synthetic protein chemistry to discover that O-GlcNAc also activates the anti-amyloid activity of certain small heat shock proteins (sHSPs), a potentially more important modification event that can act broadly and substoichiometrically. More specifically, we found that O-GlcNAc increases the ability of sHSPs to block the amyloid formation of both α-synuclein and Aβ(1-42). Mechanistically, we show that O-GlcNAc near the sHSP IXI-domain prevents its ability to intramolecularly compete with substrate binding. Finally, we found that, although O-GlcNAc levels are globally reduced in Alzheimer's disease brains, the modification of relevant sHSPs is either maintained or increased, which suggests a mechanism to maintain these potentially protective O-GlcNAc modifications. Our results have important implications for neurodegenerative diseases associated with amyloid formation and potentially other areas of sHSP biology.

Analysis of insect nuclear small heat shock proteins and interacting proteins

The small heat shock proteins (sHsps) are a ubiquitous family of ATP-independent stress proteins found in all domains of life. Drosophila melanogaster Hsp27 (DmHsp27) is the only known nuclear sHsp in insect. Here analyzing sequences from HMMER, we identified 56 additional insect sHsps with conserved arginine-rich nuclear localization signal (NLS) in the N-terminal region. At this time, the exact role of nuclear sHsps remains unknown. DmHsp27 protein-protein interaction analysis from iRefIndex database suggests that this protein, in addition to a putative role of molecular chaperone, is likely involved in other nuclear processes (i.e., chromatin remodeling and transcription). Identification of DmHsp27 interactors should provide key insights on the cellular and molecular functions of this nuclear chaperone.

Proteinaceous Transformers: Structural and Functional Variability of Human sHsps

The proteostasis network allows organisms to support and regulate the life cycle of proteins. Especially regarding stress, molecular chaperones represent the main players within this network. Small heat shock proteins (sHsps) are a diverse family of ATP-independent molecular chaperones acting as the first line of defense in many stress situations. Thereby, the promiscuous interaction of sHsps with substrate proteins results in complexes from which the substrates can be refolded by ATP-dependent chaperones. Particularly in vertebrates, sHsps are linked to a broad variety of diseases and are needed to maintain the refractive index of the eye lens. A striking key characteristic of sHsps is their existence in ensembles of oligomers with varying numbers of subunits. The respective dynamics of these molecules allow the exchange of subunits and the formation of hetero-oligomers. Additionally, these dynamics are closely linked to the chaperone activity of sHsps. In current models a shift in the equilibrium of the sHsp ensemble allows regulation of the chaperone activity, whereby smaller oligomers are commonly the more active species. Different triggers reversibly change the oligomer equilibrium and regulate the activity of sHsps. However, a finite availability of high-resolution structures of sHsps still limits a detailed mechanistic understanding of their dynamics and the correlating recognition of substrate proteins. Here we summarize recent advances in understanding the structural and functional relationships of human sHsps with a focus on the eye-lens αA- and αB-crystallins.

Structural aspects of the human small heat shock proteins related to their functional activities

Small heat shock proteins function as chaperones by binding unfolding substrate proteins in an ATP-independent manner to keep them in a folding-competent state and to prevent irreversible aggregation. They play crucial roles in diseases that are characterized by protein aggregation, such as neurodegenerative and neuromuscular diseases, but are also involved in cataract, cancer, and congenital disorders. For this reason, these proteins are interesting therapeutic targets for finding molecules that could affect the chaperone activity or compensate specific mutations. This review will give an overview of the available knowledge on the structural complexity of human small heat shock proteins, which may aid in the search for such therapeutic molecules.

Small heat shock proteins in neurodegenerative diseases

Small heat shock proteins are ubiquitously expressed chaperones, yet mutations in some of them cause tissue-specific diseases. Here, we will discuss how small heat shock proteins give rise to neurodegenerative disorders themselves while we will also highlight how these proteins can fulfil protective functions in neurodegenerative disorders caused by protein aggregation. The first half of this paper will be focused on how mutations in HSPB1, HSPB3, and HSPB8 are linked to inherited peripheral neuropathies like Charcot-Marie-Tooth (CMT) disease and distal hereditary motor neuropathy (dHMN). The second part of the paper will discuss how small heat shock proteins are linked to neurodegenerative disorders like Alzheimer's, Parkinson's, and Huntington's disease.

The Heterooligomerization of Human Small Heat Shock Proteins Is Controlled by Conserved Motif Located in the N-Terminal Domain

Ubiquitously expressed human small heat shock proteins (sHsps) HspB1, HspB5, HspB6 and HspB8 contain a conserved motif (S/G)RLFD in their N-terminal domain. For each of them, we prepared mutants with a replacement of the conserved R by A (R/A mutants) and a complete deletion of the pentapeptide (Δ mutants) and analyzed their heterooligomerization with other wild-type (WT) human sHsps. We found that WT HspB1 and HspB5 formed heterooligomers with HspB6 only upon heating. In contrast, both HspB1 mutants interacted with WT HspB6 even at low temperature. HspB1/HspB6 heterooligomers revealed a broad size distribution with equimolar ratio suggestive of heterodimers as building blocks, while HspB5/HspB6 heterooligomers had an approximate 2:1 ratio. In contrast, R/A or Δ mutants of HspB6, when mixed with either HspB1 or HspB5, resulted in heterooligomers with a highly variable molar ratio and a decreased HspB6 incorporation. No heterooligomerization of HspB8 or its mutants with either HspB1 or HspB5 could be detected. Finally, R/A or Δ mutations had no effect on heterooligomerization of HspB1 and HspB5 as analyzed by ion exchange chromatography. We conclude that the conserved N-terminal motif plays an important role in heterooligomer formation, as especially pronounced in HspB6 lacking the C-terminal IXI motif.

Effect of cataract-associated mutations in the N-terminal domain of αB-crystallin (HspB5)

Physico-chemical properties of three cataract-associated missense mutants of αB-crystallin (HspB5) (R11H, P20S, R56W) were analyzed. The oligomers formed by the R11H mutant were smaller, whereas the oligomers of the P20S and R56W mutants were larger than those of the wild-type protein. The P20S mutant possessed lower thermal stability than the wild-type HspB5 or two other HspB5 mutants. All HspB5 mutants were able to form heterooligomeric complexes with αA-crystallin (HspB4), a genuine component of eye lens. However, the P20S and R56W mutants were less effective in the formation of these complexes and properties of heterooligomeric complexes formed by these mutants and HspB4 and analyzed by ion-exchange chromatography were different from those formed by the wild-type HspB5 and HspB4. All HspB5 variants also heterooligomerized with another partner protein, HspB6. Specifically for the P20S mutant forming two distinct sizes of homooligomers, only the smaller homooligomer population was able to interact with HspB6. P20S and R56W mutants possessed lower chaperone-like activity than the wild-type HspB5 when UV-irradiated β_L-crystallin was used as a model substrate. Importantly, all three mutations are localized in three earlier postulated short α-helical regions present in the N-terminal domain of αB-crystallin. These observations suggest an important structural and functional role of these regions. Correspondingly, therein localized mutations ultimately result in clinically relevant cataracts.

Conditional Disorder in Small Heat-shock Proteins

Small heat-shock proteins (sHSPs) are molecular chaperones that respond to cellular stresses to combat protein aggregation. HSP27 is a critical human sHSP that forms large, dynamic oligomers whose quaternary structures and chaperone activities depend on environmental factors. Upon exposure to cellular stresses, such as heat shock or acidosis, HSP27 oligomers can dissociate into dimers and monomers, which leads to significantly enhanced chaperone activity. The structured core of the protein, the α-crystallin domain (ACD), forms dimers and can prevent the aggregation of substrate proteins to a similar degree as the full-length protein. When the ACD dimer dissociates into monomers, it partially unfolds and exhibits enhanced activity. Here, we used solution-state NMR spectroscopy to characterize the structure and dynamics of the HSP27 ACD monomer. Web show that the monomer is stabilized at low pH and that its backbone chemical shifts, 15N relaxation rates, and 1H-15N residual dipolar couplings suggest structural changes and rapid motions in the region responsible for dimerization. By analyzing the solvent accessible and buried surface areas of sHSP structures in the context of a database of dimers that are known to dissociate into disordered monomers, we predict that ACD dimers from sHSPs across all kingdoms of life may partially unfold upon dissociation. We propose a general model in which conditional disorder-the partial unfolding of ACDs upon monomerization-is a common mechanism for sHSP activity.

Neuromuscular Diseases Due to Chaperone Mutations: A Review and Some New Results

Skeletal muscle and the nervous system depend on efficient protein quality control, and they express chaperones and cochaperones at high levels to maintain protein homeostasis. Mutations in many of these proteins cause neuromuscular diseases, myopathies, and hereditary motor and sensorimotor neuropathies. In this review, we cover mutations in DNAJB6, DNAJB2, αB-crystallin (CRYAB, HSPB5), HSPB1, HSPB3, HSPB8, and BAG3, and discuss the molecular mechanisms by which they cause neuromuscular disease. In addition, previously unpublished results are presented, showing downstream effects of BAG3 p.P209L on DNAJB6 turnover and localization.

Release of a disordered domain enhances HspB1 chaperone activity toward tau

Small heat shock proteins (sHSPs) are a class of ATP-independent molecular chaperones that play vital roles in maintaining protein solubility and preventing aberrant protein aggregation. They form highly dynamic, polydisperse oligomeric ensembles and contain long intrinsically disordered regions. Experimental challenges posed by these properties have greatly impeded our understanding of sHSP structure and mechanism of action. Here we characterize interactions between the human sHSP HspB1 (Hsp27) and microtubule-associated protein tau, which is implicated in multiple dementias, including Alzheimer's disease. We show that tau binds both to a well-known binding groove within the structured alpha-crystallin domain (ACD) and to sites within the enigmatic, disordered N-terminal region (NTR) of HspB1. However, only interactions involving the NTR lead to productive chaperone activity, whereas ACD binding is uncorrelated with chaperone function. The tau-binding groove in the ACD also binds short hydrophobic regions within HspB1 itself, and HspB1 mutations that disrupt these intrinsic ACD-NTR interactions greatly enhance chaperone activity toward tau. This leads to a mechanism in which the release of the disordered NTR from a binding groove on the ACD enhances chaperone activity toward tau. The study advances understanding of the mechanisms by which sHSPs achieve their chaperone activity against amyloid-forming clients and how cells defend against pathological tau aggregation. Furthermore, the resulting mechanistic model points to ways in which sHSP chaperone activity may be increased, either by native factors within the cell or by therapeutic intervention.

Interplay of disordered and ordered regions of a human small heat shock protein yields an ensemble of 'quasi-ordered' states

Small heat shock proteins (sHSPs) are nature's 'first responders' to cellular stress, interacting with affected proteins to prevent their aggregation. Little is known about sHSP structure beyond its structured α-crystallin domain (ACD), which is flanked by disordered regions. In the human sHSP HSPB1, the disordered N-terminal region (NTR) represents nearly 50% of the sequence. Here, we present a hybrid approach involving NMR, hydrogen-deuterium exchange mass spectrometry, and modeling to provide the first residue-level characterization of the NTR. The results support a model in which multiple grooves on the ACD interact with specific NTR regions, creating an ensemble of 'quasi-ordered' NTR states that can give rise to the known heterogeneity and plasticity of HSPB1. Phosphorylation-dependent interactions inform a mechanism by which HSPB1 is activated under stress conditions. Additionally, we examine the effects of disease-associated NTR mutations on HSPB1 structure and dynamics, leveraging our emerging structural insights.

Mechanisms of Small Heat Shock Proteins

Small heat shock proteins (sHSPs) are ATP-independent chaperones that delay formation of harmful protein aggregates. sHSPs' role in protein homeostasis has been appreciated for decades, but their mechanisms of action remain poorly understood. This gap in understanding is largely a consequence of sHSP properties that make them recalcitrant to detailed study. Multiple stress-associated conditions including pH acidosis, oxidation, and unusual availability of metal ions, as well as reversible stress-induced phosphorylation can modulate sHSP chaperone activity. Investigations of sHSPs reveal that sHSPs can engage in transient or long-lived interactions with client proteins depending on solution conditions and sHSP or client identity. Recent advances in the field highlight both the diversity of function within the sHSP family and the exquisite sensitivity of individual sHSPs to cellular and experimental conditions. Here, we will present and highlight current understanding, recent progress, and future challenges.

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

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

Cellular Functions and Mechanisms of Action of Small Heat Shock Proteins

Small heat shock proteins (sHsps) constitute a diverse chaperone family that shares the α-crystallin domain, which is flanked by variable, disordered N- and C-terminal extensions. sHsps act as the first line of cellular defense against protein unfolding stress. They form dynamic, large oligomers that represent inactive storage forms. Stress conditions cause a rapid increase in cellular sHsp levels and trigger conformational rearrangements, resulting in exposure of substrate-binding sites and sHsp activation. sHsps bind to early-unfolding intermediates of misfolding proteins in an ATP-independent manner and sequester them in sHsp/substrate complexes. Sequestration protects substrates from further uncontrolled aggregation and facilitates their refolding by ATP-dependent Hsp70-Hsp100 disaggregases. Some sHsps with particularly strong sequestrase activity, such as yeast Hsp42, are critical factors for forming large, microscopically visible deposition sites of misfolded proteins in vivo. These sites are organizing centers for triaging substrates to distinct quality control pathways, preferentially Hsp70-dependent refolding and selective autophagy.

Small heat shock proteins: multifaceted proteins with important implications for life

Small Heat Shock Proteins (sHSPs) evolved early in the history of life; they are present in archaea, bacteria, and eukaryota. sHSPs belong to the superfamily of molecular chaperones: they are components of the cellular protein quality control machinery and are thought to act as the first line of defense against conditions that endanger the cellular proteome. In plants, sHSPs protect cells against abiotic stresses, providing innovative targets for sustainable agricultural production. In humans, sHSPs (also known as HSPBs) are associated with the development of several neurological diseases. Thus, manipulation of sHSP expression may represent an attractive therapeutic strategy for disease treatment. Experimental evidence demonstrates that enhancing the chaperone function of sHSPs protects against age-related protein conformation diseases, which are characterized by protein aggregation. Moreover, sHSPs can promote longevity and healthy aging in vivo. In addition, sHSPs have been implicated in the prognosis of several types of cancer. Here, sHSP upregulation, by enhancing cellular health, could promote cancer development; on the other hand, their downregulation, by sensitizing cells to external stressors and chemotherapeutics, may have beneficial outcomes. The complexity and diversity of sHSP function and properties and the need to identify their specific clients, as well as their implication in human disease, have been discussed by many of the world's experts in the sHSP field during a dedicated workshop in Québec City, Canada, on 26-29 August 2018.

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.