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


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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    • "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.
    Full-text · Article · Feb 2015 · Journal of Molecular Biology
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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.
    Full-text · Article · Oct 2014
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    • "Small heat shock proteins (sHSPs), as a conserved family of molecular chaperone, are characterized by a low molecular mass of 12–43 kDa, formation of large oligomers and exhibition of ATPindependent chaperone activity [3] [4] [5] [6] [7] [8] [9] [10] [11]. They are able to bind non-native substrate proteins and facilitate the substrate-refolding under the help of other ATP-dependent molecular chaperones (e.g., Hsp70/DnaK and Hsp100/ClpB) under both in vitro and in vivo conditions [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]. "
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    ABSTRACT: Small heat shock proteins (sHSPs), as a conserved family of ATP-independent molecular chaperones, are known to bind non-native substrate proteins and facilitate the substrate refolding in cooperation with ATP-dependent chaperones (e.g., DnaK and ClpB). However, how different sHSPs function in coordination is poorly understood. Here we report that IbpA and IbpB, the two sHSPs of Escherichia coli, are coordinated by synchronizing their differential in vivo degradation. Whereas the individually expressed IbpA and IbpB are respectively degraded slowly and rapidly in cells cultured under both heat shock and normal conditions, their simultaneous expression leads to a synchronized degradation at a moderate rate. Apparently, such synchronization is linked to their hetero-oligomerization and cooperation in binding substrate proteins. In addition, truncation of the flexible N- and C-terminal tails dramatically suppresses the IbpB degradation, and somehow accelerates the IbpA degradation. In view of these in vivo data, we propose that the synchronized degradation for IbpA and IbpB are crucial for their synergistic promoting effect on DnaK/ClpB-mediated substrate refolding, conceivably via the formation of IbpA-IbpB-substrate complexes. This scenario may be common for different sHSPs that interact with each other in cells.
    Preview · Article · Aug 2014 · Biochemical and Biophysical Research Communications
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