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

Sequestrase chaperones protect against oxidative stress-induced protein aggregation and [PSI+] prion formation

Misfolded proteins are usually refolded to their functional conformations or degraded by quality control mechanisms. When misfolded proteins evade quality control, they can be sequestered to specific sites within cells to prevent the potential dysfunction and toxicity that arises from protein aggregation. Btn2 and Hsp42 are compartment-specific sequestrases that play key roles in the assembly of these deposition sites. Their exact intracellular functions and substrates are not well defined, particularly since heat stress sensitivity is not observed in deletion mutants. We show here that Btn2 and Hsp42 are required for tolerance to oxidative stress conditions induced by exposure to hydrogen peroxide. Btn2 and Hsp42 act to sequester oxidized proteins into defined PQC sites following ROS exposure and their absence leads to an accumulation of protein aggregates. The toxicity of protein aggregate accumulation causes oxidant sensitivity in btn2 hsp42 sequestrase mutants since overexpression of the Hsp104 disaggregase rescues oxidant tolerance. We have identified the Sup35 translation termination factor as an in vivo sequestrase substrate and show that Btn2 and Hsp42 act to suppress oxidant-induced formation of the yeast [PSI+] prion, which is the amyloid form of Sup35. [PSI+] prion formation in sequestrase mutants does not require IPOD (insoluble protein deposit) localization which is the site where amyloids are thought to undergo fragmentation and seeding to propagate their heritable prion form. Instead, both amorphous and amyloid Sup35 aggregates are increased in btn2 hsp42 mutants consistent with the idea that prion formation occurs at multiple intracellular sites during oxidative stress conditions in the absence of sequestrase activity. Taken together, our data identify protein sequestration as a key antioxidant defence mechanism that functions to mitigate the damaging consequences of protein oxidation-induced aggregation.

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.

Protein aggregation: A detrimental symptom or an adaptation mechanism?

Protein quality control mechanisms oversee numerous aspects of protein lifetime. From the point of protein synthesis, protein homeostasis machineries take part in folding, solubilization, and/or degradation of impaired proteins. Some proteins follow an alternative path upon loss of their solubility, thus are secluded from the cytosol and form protein aggregates. Protein aggregates differ in their function and composition, rendering protein aggregation a complex phenomenon that continues to receive plenty of attention in the scientific and medical communities. Traditionally, protein aggregates have been associated with aging and a large spectrum of protein folding diseases, such as neurodegenerative diseases, type 2 diabetes, or cataract. However, a body of evidence suggests that they may act as an adaptive mechanism to overcome transient stressful conditions, serving as a sink for the removal of misfolded proteins from the cytosol or storage compartments for machineries required upon stress release. In this review, we present examples and evidence elaborating different possible roles of protein aggregation and discuss their potential roles in stress survival, aging, and disease, as well as possible anti-aggregation interventions.

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

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

The cytoprotective sequestration activity of small heat shock proteins is evolutionarily conserved

The chaperone-mediated sequestration of misfolded proteins into inclusions is a pivotal cellular strategy to maintain proteostasis in Saccharomyces cerevisiae, executed by small heat shock proteins (sHsps) Hsp42 and Btn2. Direct homologs of Hsp42 and Btn2 are absent in other organisms, questioning whether sequestration represents a conserved proteostasis strategy and, if so, which factors are involved. We examined sHsps from Escherchia coli, Caenorhabditis elegans, and humans for their ability to complement the defects of yeast sequestrase mutants. We show that sequestration of misfolded proteins is an original and widespread activity among sHsps executed by specific family members. Sequestrase positive C. elegans' sHsps harbor specific sequence features, including a high content of aromatic and methionine residues in disordered N-terminal extensions. Those sHsps buffer limitations in Hsp70 capacity in C. elegans WT animals and are upregulated in long-lived daf-2 mutants, contributing to lifespan extension. Cellular protection by sequestration of misfolded proteins is, therefore, an evolutionarily conserved activity of the sHsp family.

Identification of a HTT-specific binding motif in DNAJB1 essential for suppression and disaggregation of HTT

Huntington’s disease is a neurodegenerative disease caused by an expanded polyQ stretch within Huntingtin (HTT) that renders the protein aggregation-prone, ultimately resulting in the formation of amyloid fibrils. A trimeric chaperone complex composed of Hsc70, DNAJB1 and Apg2 can suppress and reverse the aggregation of HTTExon1Q₄₈. DNAJB1 is the rate-limiting chaperone and we have here identified and characterized the binding interface between DNAJB1 and HTTExon1Q₄₈. DNAJB1 exhibits a HTT binding motif (HBM) in the hinge region between C-terminal domains (CTD) I and II and binds to the polyQ-adjacent proline rich domain (PRD) of soluble as well as aggregated HTT. The PRD of HTT represents an additional binding site for chaperones. Mutation of the highly conserved H244 of the HBM of DNAJB1 completely abrogates the suppression and disaggregation of HTT fibrils by the trimeric chaperone complex. Notably, this mutation does not affect the binding and remodeling of any other protein substrate, suggesting that the HBM of DNAJB1 is a specific interaction site for HTT. Overexpression of wt DNAJB1, but not of DNAJB1^H244A can prevent the accumulation of HTTExon1Q₉₇ aggregates in HEK293 cells, thus validating the biological significance of the HBM within DNAJB1.

Ayala Mariscal et al have identified and characterized the interface of pathogenic Huntingtin and the molecular chaperone DNAJB1. Histidine-244 of the C-terminal domain of DNAJB1 is a key residues for binding to the poly-proline region of HTT. This binding site is specific for the interaction with Huntingtin.

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.

Come Together: Protein Assemblies, Aggregates and the Sarcostat at the Heart of Cardiac Myocyte Homeostasis

Homeostasis in vertebrate systems is contingent on normal cardiac function. This, in turn, depends on intricate protein-based cellular machinery, both for contractile function, as well as, durability of cardiac myocytes. The cardiac small heat shock protein (csHsp) chaperone system, highlighted by αB-crystallin (CRYAB), a small heat shock protein (sHsp) that forms ∼3–5% of total cardiac mass, plays critical roles in maintaining proteostatic function via formation of self-assembled multimeric chaperones. In this work, we review these ancient proteins, from the evolutionarily preserved role of homologs in protists, fungi and invertebrate systems, as well as, the role of sHsps and chaperones in maintaining cardiac myocyte structure and function. We propose the concept of the “sarcostat” as a protein quality control mechanism in the sarcomere. The roles of the proteasomal and lysosomal proteostatic network, as well as, the roles of the aggresome, self-assembling protein complexes and protein aggregation are discussed in the context of cardiac myocyte homeostasis. Finally, we will review the potential for targeting the csHsp system as a novel therapeutic approach to prevent and treat cardiomyopathy and heart failure.