Disassembling protein aggregates in the yeast cytosol. The cooperation of Hsp26 with Ssa1 and Hsp104

Hsp27 overexpression in the R6/2 mouse model of Huntington's disease: chronic neurodegeneration does not induce Hsp27 activation

Huntington's disease (HD) is caused by an expanded polyglutamine tract in the huntingtin protein. Mitochondrial dysfunction and free radical damage occur in both R6/2 mice and HD patient brains and might play a role in disease pathogenesis. In cell culture systems, heat-shock protein 27 (Hsp27), a small molecular chaperone, suppresses mutant huntingtin-induced reactive oxygen species formation and cell death. To investigate this in vivo, we conducted an extensive phenotypic characterization of mice arising from a cross between R6/2 mice and Hsp27 transgenic mice but did not observe an improvement of the R6/2 phenotype. Hsp27 overexpression had no effect in reducing oxidative stress in the R6/2 brain, assessed by measuring striatal aconitase activity and protein carbonylation levels. Native protein gel analysis revealed that transgenic Hsp27 forms active, large oligomeric species in heat-shocked brain lysates, demonstrating that it is efficiently activated upon stress. In contrast, Hsp27 in double transgenic brains exists predominantly as a low molecular weight, inactive species. This suggests that Hsp27, which is otherwise activatable upon heat shock, remains inactive in the R6/2 model of chronic neurodegeneration. Hsp27 transgenics had been previously shown to be protected from acute stresses such as kainate administration, ischemia/reperfusion heart injury and neonatal nerve injury. Our study is the first to suggest a differential modulation of Hsp27 activation in vivo and, importantly, it illustrates the diverse effect of Hsp27 on acute versus chronic models of disease.

ATP-independent thermoprotective activity of Nicotiana tabacum heat shock protein 70 in Escherichia coli

To study the functioning of HSP70 in Escherichia coli, we selected NtHSP70-2 (AY372070) from among three genomic clones isolated in Nicotiana tabacum. Recombinant NtHSP70-2, containing a hexahistidine tag at the amino-terminus, was constructed, expressed in E. coli, and purified by Ni(2+) affinity chromatography and Q Sepharose Fast Flow anion exchange chromatography. The expressed fusion protein, H(6)NtHSP70-2 (hexahistidine-tagged Nicotiana tabacum heat shock protein 70-2), maintained the stability of E. coli proteins up to 90 degrees C. Measuring the light scattering of luciferase (luc) revealed that NtHSP70-2 prevents the aggregation of luc without ATP during high-temperature stress. In a functional bioassay (1 h at 50 degrees C) for recombinant H(6)NtHSP70-2, E. coli cells overexpressing H(6)NtHSP70-2 survived about seven times longer than those lacking H(6)NtHSP70-2. After 2 h at 50 degrees C, only the E. coli overexpressing H(6)NtHSP70-2 survived under such conditions. Our NtHSP70-2 bioassays, as well as in vitro studies, strongly suggest that HSP70 confers thermo-tolerance to E. coli.

Asymmetric deceleration of ClpB or Hsp104 ATPase activity unleashes protein-remodeling activity

Two members of the AAA+ superfamily, ClpB and Hsp104, collaborate with Hsp70 and Hsp40 to rescue aggregated proteins. However, the mechanisms that elicit and underlie their protein-remodeling activities remain unclear. We report that for both Hsp104 and ClpB, mixtures of ATP and ATP-gammaS unexpectedly unleash activation, disaggregation and unfolding activities independent of cochaperones. Mutations reveal how remodeling activities are elicited by impaired hydrolysis at individual nucleotide-binding domains. However, for some substrates, mixtures of ATP and ATP-gammaS abolish remodeling, whereas for others, ATP binding without hydrolysis is sufficient. Remodeling of different substrates necessitates a diverse balance of polypeptide 'holding' (which requires ATP binding but not hydrolysis) and unfolding (which requires ATP hydrolysis). We suggest that this versatility in reaction mechanism enables ClpB and Hsp104 to reactivate the entire aggregated proteome after stress and enables Hsp104 to control prion inheritance.

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.

Modulation of Abeta42 low-n oligomerization using a novel yeast reporter system

Background

While traditional models of Alzheimer's disease focused on large fibrillar deposits of the Aβ₄₂ amyloid peptide in the brain, recent work suggests that the major pathogenic effects may be attributed to SDS-stable oligomers of Aβ₄₂. These Aβ₄₂ oligomers represent a rational target for therapeutic intervention, yet factors governing their assembly are poorly understood.

Results

We describe a new yeast model system focused on the initial stages of Aβ₄₂ oligomerization. We show that the activity of a fusion of Aβ₄₂ to a reporter protein is compromised in yeast by the formation of SDS-stable low-n oligomers. These oligomers are reminiscent of the low-n oligomers formed by the Aβ₄₂ peptide in vitro, in mammalian cell culture, and in the human brain. Point mutations previously shown to inhibit Aβ₄₂ aggregation in vitro, were made in the Aβ₄₂ portion of the fusion protein. These mutations both inhibited oligomerization and restored activity to the fusion protein. Using this model system, we found that oligomerization of the fusion protein is stimulated by millimolar concentrations of the yeast prion curing agent guanidine. Surprisingly, deletion of the chaperone Hsp104 (a known target for guanidine) inhibited oligomerization of the fusion protein. Furthermore, we demonstrate that Hsp104 interacts with the Aβ₄₂-fusion protein and appears to protect it from disaggregation and degradation.

Conclusion

Previous models of Alzheimer's disease focused on unravelling compounds that inhibit fibrillization of Aβ₄₂, i.e. the last step of Aβ₄₂ assembly. However, inhibition of fibrillization may lead to the accumulation of toxic oligomers of Aβ₄₂. The model described here can be used to search for and test proteinacious or chemical compounds for their ability to interfere with the initial steps of Aβ₄₂ oligomerization. Our findings suggest that yeast contain guanidine-sensitive factor(s) that reduce the amount of low-n oligomers of Aβ₄₂. As many yeast proteins have human homologs, identification of these factors may help to uncover homologous proteins that affect Aβ₄₂ oligomerization in mammals.

Diversity, structure, and expression of the gene for p26, a small heat shock protein from Artemia

p26, a small heat shock protein, is thought to protect Artemia embryos from stress during encystment and diapause. Full-length p26 cDNAs were compared and used to determine phylogenetic relationships between several Artemia species. The alpha-crystallin domain of p26 was the most conserved region of the protein and p26 from each Artemia species contained characteristic amino-terminal WD/EPF and carboxy-terminal VPI motifs. Sequence conservation suggested the importance of p26 to oviparously developing Artemia embryos and indicated common functions for the protein during development and stress resistance, although as shown by modeling some species-specific p26 amino acid substitutions may have adaptive significance. The p26 gene obtained from A. franciscana exhibited a unique sHSP intron arrangement with an intron in the 5'-untranslated region. Computer-assisted analysis revealed heat shock elements and other putative cis regulatory sequences but their role in gene regulation is unknown. In contrast to previous results for which Northern blots were analyzed, p26 gene expression was observed in ovoviviparous embryos by use of PCR-based methodology, but the p26 protein was not detected.

Multiple Distinct Assemblies Reveal Conformational Flexibility in the Small Heat Shock Protein Hsp26

Small heat shock proteins are a superfamily of molecular chaperones that suppress protein aggregation and provide protection from cell stress. A key issue for understanding their action is to define the interactions of subunit domains in these oligomeric assemblies. Cryo-electron microscopy of yeast Hsp26 reveals two distinct forms, each comprising 24 subunits arranged in a porous shell with tetrahedral symmetry. The subunits form elongated, asymmetric dimers that assemble via trimeric contacts. Modifications of both termini cause rearrangements that yield a further four assemblies. Each subunit contains an N-terminal region, a globular middle domain, the alpha-crystallin domain, and a C-terminal tail. Twelve of the C termini form 3-fold assembly contacts which are inserted into the interior of the shell, while the other 12 C termini form contacts on the surface. Hinge points between the domains allow a variety of assembly contacts, providing the flexibility required for formation of supercomplexes with non-native proteins.

Distinct yet linked: chaperone networks in the eukaryotic cytosol

The terms chaperone and heat-shock protein are frequently used as synonyms, but this is an oversimplification. Although one subset of chaperones is induced by heat stress, a distinct group fails to respond in the same manner. Recent work reveals that this latter group is linked to the translational apparatus and functions in co-translational processes.

The molecular chaperone Hsp104--a molecular machine for protein disaggregation

At the Cold Spring Harbor Meeting on 'Molecular Chaperones and the Heat Shock Response' in May 1996, Susan Lindquist presented evidence that a chaperone of yeast termed Hsp104, which her group had been investigating for several years, is able to dissolve protein aggregates (Glover, J.R., Lindquist, S., 1998. Hsp104, Hsp70, and Hsp40: a novel chaperone system that rescues previously aggregated proteins. Cell 94, 73-82). Among many of the participants this news stimulated reactions reaching from decided skepticism to utter disbelief because protein aggregation was widely considered to be an irreversible process. Several years and publications later, it is undeniable that Susan had been right. Hsp104 is an ATP dependent molecular machine that-in cooperation with Hsp70 and Hsp40-extracts polypeptide chains from protein aggregates and facilitates their refolding, although the molecular details of this process are still poorly understood. Meanwhile, close homologues of Hsp104 have been identified in bacteria (ClpB), in mitochondria (Hsp78), and in the cytosol of plants (Hsp101), but intriguingly not in the cytosol of animal cells (Mosser, D.D., Ho, S., Glover, J.R., 2004. Saccharomyces cerevisiae Hsp104 enhances the chaperone capacity of human cells and inhibits heat stress-induced proapoptotic signaling. Biochemistry 43, 8107-8115). Observations that Hsp104 plays an essential role in the maintenance of yeast prions (see review by James Shorter in this issue) have attracted even more attention to the molecular mechanism of this ATP dependent chaperone (Chernoff, Y.O., Lindquist, S.L., Ono, B., Inge-Vechtomov, S.G., Liebman, S.W., 1995. Role of the chaperone protein Hsp104 in propagation of the yeast prion-like factor [PSI+]. Science 268, 880-884).

Evidence for an essential function of the N terminus of a small heat shock protein in vivo, independent of in vitro chaperone activity

To investigate the mechanism of small heat shock protein (sHsp) function, unbiased by current models of sHsp chaperone activity, we performed a screen for mutations of Synechocystis Hsp16.6 that reduced the ability of the protein to provide thermotolerance in vivo. Missense mutations at 17 positions throughout the protein and a C-terminal truncation of 5 aa were identified, representing the largest collection of sHsp mutants impaired in function in vivo. Ten mutant proteins were purified and tested for alterations in native oligomeric structure and in vitro chaperone activity. These biochemical assays separated the mutants into two groups. The C-terminal truncation and six mutations in the alpha-crystallin domain destabilized the sHsp oligomer and reduced in vitro chaperone activity. In contrast, the other three mutations had little effect on oligomer stability or chaperone activity in vitro. These mutations were clustered in the N terminus of Hsp16.6, pointing to a previously unrecognized, important function for this evolutionarily variable domain. Furthermore, the fact that the N-terminal mutations were impaired in function in vivo, but active as chaperones in vitro, indicates that current biochemical assays do not adequately measure essential features of the sHsp mechanism of action.

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.

Small heat-shock proteins function in the insoluble protein complex

Small heat-shock proteins (sHSPs) represent an abundant and ubiquitous family of molecular chaperones. The current model proposes that sHSPs function to prevent irreversible aggregation of non-native proteins by forming soluble complex. The chaperone activity of sHSPs is usually determined by the capacity to suppress thermally or chemically induced protein aggregation. However, sHSPs were frequently found in the insoluble complex particularly in vivo. In this report, it is clearly revealed that the insoluble sHSP/substrate complex is formed when sHSP is overloaded with non-native substrates, which is the very case under in vivo conditions. The proposal that sHSPs function to prevent the protein aggregation seems misleading. sHSPs appear to promote the elimination of protein aggregates by incorporating into the insoluble protein complex.

The stress response against denatured proteins in the deletion of cytosolic chaperones SSA1/2 is different from heat-shock response in Saccharomyces cerevisiae

Background

A yeast strain lacking the two genes SSA1 and SSA2, which encode cytosolic molecular chaperones, acquires thermotolerance as well as the mild heat-shocked wild-type yeast strain. We investigated the genomic response at the level of mRNA expression to the deletion of SSA1/2 in comparison with the mild heat-shocked wild-type using cDNA microarray.

Results

Yeast cDNA microarray analysis revealed that genes involved in the stress response, including molecular chaperones, were up-regulated in a similar manner in both the ssa1/2 deletion mutant and the mild heat-shocked wild-type. Genes involved in protein synthesis were up-regulated in the ssa1/2 deletion mutant, but were markedly suppressed in the mild heat-shocked wild-type. The genes involved in ubiquitin-proteasome protein degradation were also up-regulated in the ssa1/2 deletion mutant, whereas the unfolded protein response (UPR) genes were highly expressed in the mild heat-shocked wild-type. RT-PCR confirmed that the genes regulating protein synthesis and cytosolic protein degradation were up-regulated in the ssa1/2 deletion mutant. At the translational level, more ubiquitinated proteins and proteasomes were detected in the ssa1/2 deletion mutant, than in the wild-type, confirming that ubiquitin-proteasome protein degradation was up-regulated by the deletion of SSA1/2.

Conclusion

These results suggest that the mechanism for rescue of denatured proteins in the ssa1/2 deletion mutant is different from that in the mild heat-shocked wild-type: Activated protein synthesis in the ssa1/2 deletion mutant supplies a deficiency of proteins by their degradation, whereas mild heat-shock induces UPR.

Loss of Hsp70 in Drosophila is pleiotropic, with effects on thermotolerance, recovery from heat shock and neurodegeneration

The heat-shock response is a programmed change in gene expression carried out by cells in response to environmental stress, such as heat. This response is universal and is characterized by the synthesis of a small group of conserved protein chaperones. In Drosophila melanogaster the Hsp70 chaperone dominates the profile of protein synthesis during the heat-shock response. We recently generated precise deletion alleles of the Hsp70 genes of D. melanogaster and have used those alleles to characterize the phenotypes of Hsp70-deficient flies. Flies with Hsp70 deletions have reduced thermotolerance. We find that Hsp70 is essential to survive a severe heat shock, but is not required to survive a milder heat shock, indicating that a significant degree of thermotolerance remains in the absence of Hsp70. However, flies without Hsp70 have a lengthened heat-shock response and an extended developmental delay after a non-lethal heat shock, indicating Hsp70 has an important role in recovery from stress, even at lower temperatures. Lack of Hsp70 also confers enhanced sensitivity to a temperature-sensitive lethal mutation and to the neurodegenerative effects produced by expression of a human polyglutamine disease protein.

Wrapping the α-Crystallin Domain Fold in a Chaperone Assembly

Small heat shock proteins (sHsps) are oligomers that perform a protective function by binding denatured proteins. Although ubiquitous, they are of variable sequence except for a C-terminal approximately 90-residue "alpha-crystallin domain". Unlike larger stress response chaperones, sHsps are ATP-independent and generally form polydisperse assemblies. One proposed mechanism of action involves these assemblies breaking into smaller subunits in response to stress, before binding unfolding substrate and reforming into larger complexes. Two previously solved non-metazoan sHsp multimers are built from dimers formed by domain swapping between the alpha-crystallin domains, adding to evidence that the smaller subunits are dimers. Here, the 2.5A resolution structure of an sHsp from the parasitic flatworm Taenia saginata Tsp36, the first metazoan crystal structure, shows a new mode of dimerization involving N-terminal regions, which differs from that seen for non-metazoan sHsps. Sequence differences in the alpha-crystallin domains between metazoans and non-metazoans are critical to the different mechanism of dimerization, suggesting that some structural features seen for Tsp36 may be generalized to other metazoan sHsps. The structure also indicates scope for flexible assembly of subunits, supporting the proposed process of oligomer breakdown, substrate binding and reassembly as the chaperone mechanism. It further shows how sHsps can bind coil and secondary structural elements by wrapping them around the alpha-crystallin domain. The structure also illustrates possible roles for conserved residues associated with disease, and suggests a mechanism for the sHsp-related pathogenicity of some flatworm infections. Tsp36, like other flatworm sHsps, possesses two divergent sHsp repeats per monomer. Together with the two previously solved structures, a total of four alpha-crystallin domain structures are now available, giving a better definition of domain boundaries for sHsps.

A chaperone pathway in protein disaggregation. Hsp26 alters the nature of protein aggregates to facilitate reactivation by Hsp104

Cellular protein folding is challenged by environmental stress and aging, which lead to aberrant protein conformations and aggregation. One way to antagonize the detrimental consequences of protein misfolding is to reactivate vital proteins from aggregates. In the yeast Saccharomyces cerevisiae, Hsp104 facilitates disaggregation and reactivates aggregated proteins with assistance from Hsp70 (Ssa1) and Hsp40 (Ydj1). The small heat shock proteins, Hsp26 and Hsp42, also function in the recovery of misfolded proteins and prevent aggregation in vitro, but their in vivo roles in protein homeostasis remain elusive. We observed that after a sublethal heat shock, a majority of Hsp26 becomes insoluble. Its return to the soluble state during recovery depends on the presence of Hsp104. Further, cells lacking Hsp26 are impaired in the disaggregation of an easily assayed heat-aggregated reporter protein, luciferase. In vitro, Hsp104, Ssa1, and Ydj1 reactivate luciferase:Hsp26 co-aggregates 20-fold more efficiently than luciferase aggregates alone. Small Hsps also facilitate the Hsp104-mediated solubilization of polyglutamine in yeast. Thus, Hsp26 renders aggregates more accessible to Hsp104/Ssa1/Ydj1. Small Hsps partially suppress toxicity, even in the absence of Hsp104, potentially by sequestering polyglutamine from toxic interactions with other proteins. Hence, Hsp26 plays an important role in pathways that defend cells against environmental stress and the types of protein misfolding seen in neurodegenerative disease.