A chaperone pathway in protein disaggregation: HSP26 alteks the nature of protein aggregates to facilitate reactivation by HSP104

Whitehead Institute for Biomedical Research, Cambridge, Massachusetts 02143, USA.
Journal of Biological Chemistry (Impact Factor: 4.57). 07/2005; 280(25):23869-75. DOI: 10.1074/jbc.M502854200
Source: PubMed

ABSTRACT 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.

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Available from: Martin Duennwald, Aug 11, 2015
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    • "The molecular reasons for the diverse dynamics are unknown. The direct involvement of Hsp42 in CytoQ formation may be responsible for the low dynamics of the trapped molecules since sHsps from different species are known to form highly stable complexes with their substrates [55] [56] [57] [58] "
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    ABSTRACT: An evolutionary conserved response of cells to proteotoxic stress is the organized sequestration of misfolded proteins into subcellular deposition sites. In Saccharomyces cerevisiae three major sequestration sites for misfolded proteins exist, IPOD, INQ (former JUNQ) and CytoQ. IPOD is perivacuolar and predominantly sequesters amyloidogenic proteins. INQ and CytoQs are stress-induced deposits for misfolded proteins residing in the nucleus and the cytosol, respectively, and requiring cell compartment-specific aggregases, nuclear Btn2 and cytosolic Hsp42, for formation. The organized aggregation of misfolded proteins is proposed to serve several purposes collectively increasing cellular fitness and survival under proteotoxic stress. These include (i) shielding of cellular processes from interference by toxic protein conformers; (ii) reducing the substrate burden for protein quality control systems upon immediate stress; (iii) orchestrating chaperone and protease functions for efficient repair or degradation of damaged proteins; this involves initial extraction of aggregated molecules via the Hsp70/Hsp104 bi-chaperone system followed by either refolding or proteasomal degradation or removal of entire aggregates by selective autophagy (aggrephagy) involving the adaptor protein Cue5; (iv) enabling asymmetric retention of protein aggregates during cell division, thereby allowing for damage clearance in daughter cells. Regulated protein aggregation thus serves cytoprotective functions vital for the maintenance of cell integrity and survival even under adverse stress conditions and during aging.
    Journal of Molecular Biology 02/2015; 76(7). DOI:10.1016/j.jmb.2015.02.006 · 4.33 Impact Factor
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    • "In vitro, Hsp26 has its chaperone activity up-regulated at elevated temperatures as a consequence of the temperature-dependent rearrangement of its thermosensor domain (Franzmann et al., 2008). Hsp26 is a promiscuous chaperone able to suppress the aggregation of a broad variety of substrate proteins in vitro, by binding at least 30% of the yeast cytosolic proteins (Haslbeck et al., 2005; Cashikar et al., 2005). Although yeast cells deleted for Hsp26 show no overt heat sensitivity or thermotolerance defects, they do accumulate protein aggregates (Haslbeck et al., 2004). "
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    ABSTRACT: Small heat shock proteins (sHsps) from plants and animals are localized in different intracellular compartments including the nucleus, nucleolus, cytosol, mitochondria, endoplasmic reticulum, chloroplasts, and plant peroxisomes; they can also reversibly translocate from the cytoplasm to the nucleus under heat-stress conditions. The intrinsic fluorescence of the Tg(Hsp26/Gfp) fusion protein was localized in 1-3 cytoplasmic foci when exponential-phase yeast cells were cultured in glucose or in glycerol, or when the cells were heat-shocked. The cytoplasmic localization was confirmed by immunoelectron microscopy, using a specific anti- Hsp26 antibody. During heat shock, Tg(Hsp26/Gfp)p appeared to be initially synthesized free in the cytoplasm, and coalesced into a few cytoplasmic foci over time. Formation of the Tg(Hsp26/Gfp)p-containing foci can be inhibited by guanidinium chloride, a compound that cures all known naturally occurring prions in S. cerevisiae; or by cytochalasin B, a mycotoxin that inhibits the formation of microfilaments, resulting in an even distribution of Tg(Hsp26/Gfp)p in the cytoplasm. An intriguing possibility arose, since Hsp26 could be associated with the prion [PIN+], known to be carried by the S. cerevisiae W303-1AL strain used in this study.
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    • "Overexpression of Hsp104 and Hsp27 resulted in the rescue of striatal dysfunction in E16 rat striatal cultures and HD rat models, with concomitant decrease in the expression of the GABAergic neuronal marker DARPP- 32 [9] [15]. Overexpression of Hsp104 also reduced the formation of huntingtin inclusions in yeast [14], Caenorhabditis elegans [16] and mammalian HD models [17] [18]. Molecules like geldanamycin, which activate the heat shock factor 1, have shown enhanced cell viability and reduced proteotoxicity due to elongated polyQ tract [19]. "
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    ABSTRACT: Inhibition of huntingtin aggregation, either in the nucleus and/or in the cytosol, has been identified as a major strategy to ameliorate the symptoms of Huntington's disease. Chaperones and other protein stabilizers would thus be key players in ensuring the correct folding of the amyloidogenic protein and its expression in the soluble form. By transient activation of the global heat stress response in Saccharomyces cerevisiae BY4742, we show that heterologous expression of mutant huntingtin (103Q-htt) could be modulated so that the protein was partitioned off in the soluble fraction of the cytosol. This led to lower levels of reactive oxygen species and improved cell viability. Previous reports had speculated on the relationship between trehalose and the heat shock response in ensuring enhanced cell survival but no direct evidence of such an interaction was available. Using mutants of an isogenic strain which do not express the major trehalose synthetic or metabolising enzymes or the chaperone, heat shock protein 104 (Hsp104), we were able to identify the functions of Hsp104 and the osmoprotectant trehalose in solubilising mutant huntingtin. We propose that the beneficial effect of the protein refolding machinery in solubilising the aggregation-prone protein is exerted by maintaining a tight balance between the trehalose synthetic enzyme, trehalose-6-phosphate synthase 1 and Hsp104. This ensures that the level of the osmoprotectant, trehalose, does not exceed the limit beyond which it is reported to inhibit protein refolding.
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