Antagonistic Interactions between Yeast Chaperones Hsp104 and Hsp70 in Prion Curing

The maintenance of [PSI], a prion-like form of the yeast release factor Sup35, requires a specific concentration of the chaperone protein Hsp104: either deletion or overexpression of Hsp104 will cure cells of [PSI]. A major puzzle of these studies was that overexpression of Hsp104 alone, from a heterologous promoter, cures cells of [PSI] very efficiently, yet the natural induction of Hsp104 with heat shock, stationary-phase growth, or sporulation does not. These observations pointed to a mechanism for protecting the genetic information carried by the [PSI] element from vicissitudes of the environment. Here, we show that simultaneous overexpression of Ssa1, a protein of the Hsp70 family, protects [PSI] from curing by overexpression of Hsp104. Ssa1 protein belongs to the Ssa subfamily, members of which are normally induced with Hsp104 during heat shock, stationary-phase growth, and sporulation. At the molecular level, excess Ssa1 prevents a shift of Sup35 protein from the insoluble (prion) to the soluble (cellular) state in the presence of excess Hsp104. Overexpression of Ssa1 also increases nonsense suppression by [PSI] when Hsp104 is expressed at its normal level. In contrast, hsp104 deletion strains lose [PSI] even in the presence of overproduced Ssa1. Overproduction of the unrelated chaperone protein Hsp82 (Hsp90) neither cured [PSI] nor antagonized the [PSI]-curing effect of overproduced Hsp104. Our results suggest it is the interplay between Hsp104 and Hsp70 that allows the maintenance of [PSI] under natural growth conditions.     

Intermolecular Interactions between Hsp90 and Hsp70

Members of the Hsp90 and Hsp70 families of molecular chaperones are imp\ortant for the maintenance of protein homeostasis and cellular recovery following environmental stresses, such as heat and oxidative stress. Moreover, the two chaperones can collaborate in protein remodeling and activation. In higher eukaryotes, Hsp90 and Hsp70 form a functionally active complex with Hop (Hsp90–Hsp70 organizing protein) acting as a bridge between the two chaperones. In bacteria, which do not contain a Hop homolog, Hsp90 and Hsp70, DnaK, directly interact during protein remodeling. Although yeast possesses a Hop-like protein, Sti1, Hsp90, and Hsp70 can directly interact in yeast in the absence of Sti1. Previous studies showed that residues in the middle domain of Escherichia coli Hsp90 are important for interaction with the J-protein binding region of DnaK. The results did not distinguish between the possibility that (i) these sites were involved in direct interaction and (ii) the residues in these sites participate in conformational changes which are transduced to other sites on Hsp90 and DnaK that are involved in the direct interaction. Here we show by crosslinking experiments that the direct interaction is between a site in the middle domain of Hsp90 and the J-protein binding site of Hsp70 in both E. coli and yeast. Moreover, J-protein promotes the Hsp70–Hsp90 interaction in the presence of ATP, likely by converting Hsp70 into the ADP-bound conformation. The identification of the protein–protein interaction site is anticipated to lead to a better understanding of the collaboration between the two chaperones in protein remodeling.     

The M-Domain Controls Hsp104 Protein Remodeling Activity in an Hsp70/Hsp40-Dependent Manner

Yeast Hsp104 is a ring-forming ATP-dependent protein disaggregase that, together with the cognate Hsp70 chaperone system, has the remarkable ability to rescue stress-damaged proteins from a previously aggregated state. Both upstream and downstream functions for the Hsp70 system have been reported, but it remains unclear how Hsp70/Hsp40 is coupled to Hsp104 protein remodeling activity.Hsp104 is a multidomain protein that possesses an N-terminal domain, an M-domain, and two tandem AAA⁺ domains. The M-domain forms an 85-Å long coiled coil and is a hallmark of the Hsp104 chaperone family. While the three-dimensional structure of Hsp104 has been determined, the function of the M-domain is unclear. Here, we demonstrate that the M-domain is essential for protein disaggregation, but dispensable for Hsp104 ATPase- and substrate-translocating activities. Remarkably, replacing the Hsp104 M-domain with that of bacterial ClpB, and vice versa, switches species specificity so that our chimeras now cooperate with the noncognate Hsp70/DnaK chaperone system. Our results demonstrate that the M-domain controls Hsp104 protein remodeling activities in an Hsp70/Hsp40-dependent manner, which is required to unleash Hsp104 protein disaggregating activity.     

Hsp104, Hsp70 and Hsp40 interplay regulates formation, growth and elimination of Sup35 prions

Self-templating amyloid forms of Sup35 constitute the yeast prion [PSI(+)]. How the protein-remodelling factor, Hsp104, collaborates with other chaperones to regulate [PSI(+)] inheritance remains poorly delineated. Here, we report how the Ssa and Ssb components of the Hsp70 chaperone system directly affect Sup35 prionogenesis and cooperate with Hsp104. We identify the ribosome-associated Ssb1:Zuo1:Ssz1 complex as a potent antagonist of Sup35 prionogenesis. The Hsp40 chaperones, Sis1 and Ydj1, preferentially interact with Sup35 oligomers and fibres compared with monomers, and facilitate Ssa1 and Ssb1 binding. Various Hsp70:Hsp40 pairs block prion nucleation by disassembling molten oligomers and binding mature oligomers. By binding fibres, Hsp70:Hsp40 pairs occlude prion recognition elements and inhibit seeded assembly. These inhibitory activities are partially relieved by the nucleotide exchange factor, Fes1. Low levels of Hsp104 stimulate prionogenesis and alleviate inhibition by some Hsp70:Hsp40 pairs. At high concentrations, Hsp104 eliminates Sup35 prions. This activity is reduced when Ssa1, or enhanced when Ssb1, is incorporated into nascent prions. These findings illuminate several facets of the chaperone interplay that underpins [PSI(+)] inheritance.     

Synergy between Trehalose and Hsp104 for Thermotolerance in Saccharomyces Cerevisiae

We isolated a mutant strain unable to acquire heat shock resistance in stationary phase. Two mutations contributed to this phenotype. One mutation was at the TPS2locus, which encodes trehalose-6-phosphate phosphatase. The mutant fails to make trehalose and accumulates trehalose-6-phosphate. The other mutation was at the HSP104 locus. Gene disruptions showed that tps2 and hsp104 null mutants each produced moderate heat shock sensitivity in stationary phase cells. The two mutations were synergistic and the double mutant had little or no stationary phase-induced heat shock resistance. The same effect was seen in the tps1 (trehalose-6-phosphate synthase) hsp104 double mutant, suggesting that the extreme heat shock sensitivity was due mainly to a lack of trehalose rather than to the presence of trehalose-6-phosphate. However, accumulation of trehalose-6-phosphate did cause some phenotypes in the tps2 mutant, such as temperature sensitivity for growth. Finally, we isolated a high copy number suppressor of the temperature sensitivity of tps2, which we call PMU1, which reduced the levels of trehalose-6-phosphate in tps2 mutants. The encoded protein has a region homologous to the active site of phosphomutases.     

Sti1 Regulation of Hsp70 and Hsp90 Is Critical for Curing of Saccharomyces cerevisiae [PSI+] Prions by Hsp104

Although propagation of Saccharomyces cerevisiae prions requires Hsp104 protein disaggregating activity, overproducing Hsp104 “cures” cells of [PSI⁺] prions. Earlier evidence suggests that the Hsp70 mutant Ssa1-21 impairs [PSI⁺] by a related mechanism. Here, we confirm this link by finding that deletion of STI1 both suppresses Ssa1-21 impairment of [PSI⁺] and blocks Hsp104 curing of [PSI⁺]. Hsp104''s tetratricopeptide repeat (TPR) interaction motif was dispensable for curing; however, cells expressing Sti1 defective in Hsp70 or Hsp90 interaction cured less efficiently, and the Hsp90 inhibitor radicicol abolished curing, implying that Sti1 acts in curing through Hsp70 and Hsp90 interactions. Accordingly, strains lacking constitutive or inducible Hsp90 isoforms cured at reduced rates. We confirm an earlier finding that elevating free ubiquitin levels enhances curing, but it did not overcome inhibition of curing caused by Hsp90 defects, suggesting that Hsp90 machinery is important for the contribution of ubiquitin to curing. We also find curing associated with cell division. Our findings point to crucial roles of Hsp70, Sti1, and Hsp90 for efficient curing by overexpressed Hsp104 and provide evidence supporting the earlier suggestion that destruction of prions by protein disaggregation does not adequately explain the curing.Saccharomyces cerevisiae prions are self-replicating misfolded forms of normal cellular proteins. They are believed to propagate as amyloid, which is a highly ordered fibrous aggregate. What triggers prion formation is uncertain, but in order to be maintained in an expanding yeast population, prions must grow, replicate, and be transmitted to daughter cells during cell division. Growth occurs when soluble protein joins the fiber ends and is converted into the prion form (30, 52, 58). Replication is associated with fragmentation of prion polymers, which generates new prions from preexisting material (37, 50). Transmission is believed to occur by passive diffusion of prions with cytoplasm (57).Although it is uncertain to what extent cellular factors influence growth or transmission of prions, it is clear that the Hsp104 disaggregation machinery is necessary for prion replication (10, 17, 55, 70). Hsp104 is a hexameric AAA+ chaperone that protects cells from a variety of stresses by resolubilizing proteins from aggregates (24, 25, 53). With help from Hsp70 and Hsp40, it extracts monomers from aggregates and extrudes them through its central pore (24, 41, 68). This machinery could act in prion replication by extracting monomers from amyloid fibers (29, 68), which would destabilize the fibers, causing them to break into more numerous pieces that each can continue to propagate the prion.Paradoxically, overexpressing Hsp104 very efficiently “cures” cells of the [PSI⁺] prion, which is composed of the translation termination factor Sup35 (10). A widely held view of this curing is that elevating the cellular protein disaggregation activity causes complete destruction of prions. However, elevating Hsp104 has little or no effect on most other amyloidogenic prions (15, 16, 38, 47, 54, 66), although it can be inferred to cure [MCA] prions in cells also propagating a prion of an Mca1-Sup35 fusion (49). Together, these results suggest that prions of Sup35, and perhaps those of Mca1, are particularly sensitive to Hsp104 disaggregation activity. Alternatively, something in addition to or other than a simple increase in protein disaggregation is involved in the curing.Although protein disaggregation activity of Hsp104 is required for both thermotolerance and prion propagation, we and others have identified mutations in Hsp104 that affect these processes separately (27, 32, 39, 60). The ability of Hsp104 to thread proteins through its central pore, however, is required for both processes (29, 41, 68), so this distinction in Hsp104 function could be due to differences in how Hsp104 interacts with amorphous aggregates of thermally denatured proteins and highly ordered prion aggregates or with cofactors that interact with the different prions as substrates. In any scenario, efficiency and specificity of Hsp104 function are affected by interactions with other components of the disaggregation machinery, in particular the Hsp70s and Hsp40s, which are believed to interact first with substrates to facilitate action of Hsp100 family disaggregases (2, 71, 72).Increasing expression of either ubiquitin (Ub) or Ssb, an Hsp70 that has roles in protein translation and proteasome degradation, enhances Hsp104 curing of [PSI⁺] (3, 11, 12). Predictably, reducing expression of either of them reduces curing efficiency. The mechanisms underlying these effects are unknown, but the combined effects of Ssb and Ub are additive, suggesting that they act in different pathways. The role of Ub is indirect, as Sup35 is neither ubiquitylated nor degraded during curing. Whether other chaperones are involved in the effects of Ub on curing has not been investigated.Earlier we isolated a mutant of the Hsp70 Ssa1, designated Ssa1-21, that weakens and destabilizes [PSI⁺] propagation (33). We later isolated several Hsp104 mutants that suppress this antiprion effect (29). The Hsp104 mutants retain normal functions in thermotolerance, protein disaggregation, and prion propagation, but when overexpressed, they are unable to cure [PSI⁺], even in wild-type cells. These findings argue against a specific hypersensitivity of [PSI⁺] to disaggregation and support the notion that something distinct from or in addition to complete destruction of prions is involved in the curing. They also imply that Ssa1-21 and elevated Hsp104 inhibit [PSI⁺] prions by similar mechanisms. A prediction from this conclusion is that other suppressors of Ssa1-21 will also inhibit curing of [PSI⁺] by overexpressed Hsp104. Indeed, we find here that alterations that suppress Ssa1-21 inhibition of [PSI⁺] do interfere with curing of [PSI⁺] by overexpressed Hsp104. We also provide evidence that Hsp90 has a critical role in this curing and that the ability of Ub to enhance curing depends on proper function of Hsp90 machinery.     

Structural and functional analysis of the Hsp70/Hsp40 chaperone system

As one of the most abundant and highly conserved molecular chaperones, the 70‐kDa heat shock proteins (Hsp70s) play a key role in maintaining cellular protein homeostasis (proteostasis), one of the most fundamental tasks for every living organism. In this role, Hsp70s are inextricably linked to many human diseases, most notably cancers and neurodegenerative diseases, and are increasingly recognized as important drug targets for developing novel therapeutics for these diseases. Hsp40s are a class of essential and universal partners for Hsp70s in almost all aspects of proteostasis. Thus, Hsp70s and Hsp40s together constitute one of the most important chaperone systems across all kingdoms of life. In recent years, we have witnessed significant progress in understanding the molecular mechanism of this chaperone system through structural and functional analysis. This review will focus on this recent progress, mainly from a structural perspective.     

Genetic evidence for the phylogenetic relationship between Na-Dene and Yeniseian speakers

Ruhlen's hypothesis, based on linguistic evidence, for a common phylogenetic origin of Na-Dene and Yeniseian speakers is tested using genetic data. Gene frequency data for the Kets, the only surviving Yeniseian speakers, were collected during a field study in 1993. Data for several Na-Dene groups, as well as other New World and Siberian populations, were compiled from the literature. These data were analyzed using R-matrix, principal components analysis, and Mantel tests. In a comparison of 10 New World and Siberian populations using eight alleles, 55.8% of the variation was accounted for by the first principal component, and 22.1% of the variation was subsumed by the second principal component. Contrary to Ruhlen's interpretation of the linguistic data, analysis of the genetic data shows that the Na-Dene cluster with other Native American populations, while the Kets genetically resemble the surrounding Siberian groups. This conclusion is further supported by correlations that are higher when the Kets are considered unrelated to Na-Dene speakers, and an insignificant partial correlation between genes and language when geography is held constant, indicating that spatial patterning accounts for most of the variation present in these populations.     

Hsp110 is a nucleotide-activated exchange factor for Hsp70

Hsp110 proteins constitute a subfamily of the Hsp70 chaperones and are potent nucleotide exchange factors (NEFs) for canonical Hsp70s of the eukaryotic cytosol. Here, we show that the NEF activity of the yeast Hsp110 homologue Sse1 itself is controlled by nucleotide. Nucleotide binding results in formation of a stabilized conformation of Sse1 that is required for association with the yeast Hsp70 Ssa1. The interaction triggers release of bound ADP from Ssa1, but nucleotide persists bound to Sse1 in the complex. Surprisingly, removal of this nucleotide does not affect the integrity of the complex. Instead, rebinding of ATP to the Hsp70 prompts the dissociation of the complex. Our data demonstrate that in contrast to previously characterized NEFs for Hsp70 chaperones, the NEF activity of Sse1 requires nucleotide binding and let us propose a new model for Hsp110 function.     

The relationship between adrenocortical function and Hsp70 expression in socially isolated Japanese quail

Physiological responses to social isolation stress were compared in 56-day-old male Japanese quail. Birds were fed pretreated diets for 3 days as follows: (i) Basal diet (control); (ii) Basal diet + 1500 mg/kg metyrapone (BM); (iii) Basal diet + 30 mg/kg corticosterone (BCO); (iv) Basal diet + 250 mg/kg ascorbic acid (BC); (v) Basal diet + 250 mg/kg α-tocopherol (BE); (vi) Basal diet + 250 mg/kg ascorbic acid and 250 mg/kg α-tocopherol (BCE). The birds were subsequently socially isolated in individual opaque brown paper box for 2 hours. Plasma corticosterone (CORT) concentration and heart and brain heat shock protein 70 (Hsp 70) expressions were determined before stress and immediately after stress. Two hours of isolation stress elevated CORT concentration significantly in the control and BE birds but not in the BC, BCE and BM birds. There was a significant reduction in CORT concentration after isolation stress in the BCO group. Isolation stress increased Hsp 70 expression in the brain and heart of control and BM birds. However, brain and heart Hsp 70 expressions were not significantly altered in the isolated BC, BCE and BE birds. Although, the CORT concentration of BM birds was not affected by isolation stress, Hsp70 expression in both brain and heart were significantly increased. Moreover, exogenous corticosterone supplementation did not result in elevation of Hsp 70 expression. It can be concluded that, although Hsp 70 induction had not been directly affected by CORT concentration, it may be modulated by the HPA axis function via activation of ACTH.     

Evidence for the Interplay between Trehalose Metabolism and Hsp104 in Yeast

Disruption of the HSP104 gene in a mutant which cannot accumulate trehalose during heat shock treatment caused trehalose accumulation (H. Iwahashi, K. Obuchi, S. Fujii, and Y. Komatsu, Lett. Appl. Microbiol 25:43–47, 1997). This implies that Hsp104 affects trehalose metabolism. Thus, we measured the activities of enzymes involved in trehalose metabolism. The activities of trehalose-synthesizing and -hydrolyzing enzymes are low in the HSP104 disruption mutant during heat shock. This data is correlated with intracellular trehalose and glucose levels observed in the HSP104 disruption mutant. These results suggest that during heat shock, Hsp104 contributes to the simultaneous increase in both accumulation and degradation of trehalose.     

Interplay between recombinant Hsp70 and proteasomes: proteasome activity modulation and ubiquitin-independent cleavage of Hsp70

The heat shock protein 70 (Hsp70, human HSPA1A) plays indispensable roles in cellular stress responses and protein quality control (PQC). In the framework of PQC, it cooperates with the ubiquitin-proteasome system (UPS) to clear damaged and dysfunctional proteins in the cell. Moreover, Hsp70 itself is rapidly degraded following the recovery from stress. It was demonstrated that its fast turnover is mediated via ubiquitination and subsequent degradation by the 26S proteasome. At the same time, the effect of Hsp70 on the functional state of proteasomes has been insufficiently investigated. Here, we characterized the direct effect of recombinant Hsp70 on the activity of 20S and 26S proteasomes and studied Hsp70 degradation by the 20S proteasome in vitro. We have shown that the activity of purified 20S proteasomes is decreased following incubation with recombinant human Hsp70. On the other hand, high concentrations of Hsp70 activated 26S proteasomes. Finally, we obtained evidence that in addition to previously reported ubiquitin-dependent degradation, Hsp70 could be cleaved independent of ubiquitination by the 20S proteasome. The results obtained reveal novel aspects of the interplay between Hsp70 and proteasomes.     

Direct evidence for the intracellular localization of Hsp104 in Saccharomyces cerevisiae by immunoelectron microscopy

To reveal the intracellular localization of Hsp104 in the yeast Saccharomyces cerevisiae before and after heat-shock, we performed immunoelectron microscopy after immunogold labeling with anti-Hsp104 antibody. At normal temperature (25 degrees C), a small amount of Hsp104 was located in the cytoplasm and nucleus. On exposure to mild heat-shock at 40 degrees C, protein aggregates appeared in the cytoplasm and nucleus, and Hsp104 increased around the aggregates with increasing time of the mild heat-shock treatment. Moreover, at lethal heat-shock temperature (51 degrees C) for 20 min after mild heat treatment at 40 degrees C, the intracellular localization of Hsp104 and intracellular structures were similar to those of the mild heat-shocked cells. However, in the lethally heat-shocked cells, certain intracellular structures were destroyed, and Hsp104 was not expressed. In the hsp104 null mutant strain Deltahsp104 which was treated at 40 degrees C, Hsp104 was not localized around the aggregates. Additionally, in the Deltahsp104 strain, even mild heat-shocked cells at 37 degrees C or 40 degrees C, showed destruction of intracellular structure compared to the wild-type strain. Our data suggest the following: (1) Hsp104 is associated closely with protein aggregates during heat-shock treatment, (2) Hsp104 is important for maintenance of the intracellular structure under lethal heat-shock conditions, (3) acquisition of thermotolerance depends on the amount of Hsp104 produced during mild heat-shock treatment.     

Ordered assembly of heat shock proteins, Hsp26, Hsp70, Hsp90, and Hsp104, on expanded polyglutamine fragments revealed by chemical probes

In Saccharomyces cerevisae, expanded polyglutamine (polyQ) fragments are assembled into discrete cytosolic aggregates in a process regulated by the molecular chaperones Hsp26, Hsp70, Hsp90, and Hsp104. To better understand how the different chaperones might cooperate during polyQ aggregation, we used sequential immunoprecipitations and mass spectrometry to identify proteins associated with either soluble (Q25) or aggregation-prone (Q103) fragments at both early and later times after induction of their expression. We found that Hsp26, Hsp70, Hsp90, and other chaperones interact with Q103, but not Q25, within the first 2 h. Further, Hsp70 and Hsp90 appear to be partially released from Q103 prior to the maturation of the aggregates and before the recruitment of Hsp104. To test the importance of this seemingly ordered process, we used a chemical probe to artificially enhance Hsp70 binding to Q103. This treatment retained both Hsp70 and Hsp90 on the polyQ fragment and, interestingly, limited subsequent exchange for Hsp26 and Hsp104, resulting in incomplete aggregation. Together, these results suggest that partial release of Hsp70 may be an essential step in the continued processing of expanded polyQ fragments in yeast.     

Purification of Hsp104, a Protein Disaggregase

Hsp104 is a hexameric AAA+ protein¹ from yeast, which couples ATP hydrolysis to protein disaggregation^2-10 (Fig. 1). This activity imparts two key selective advantages. First, renaturation of disordered aggregates by Hsp104 empowers yeast survival after various protein-misfolding stresses, including heat shock^3,5,11,12. Second, remodeling of cross-beta amyloid fibrils by Hsp104 enables yeast to exploit myriad prions (infectious amyloids) as a reservoir of beneficial and heritable phenotypic variation^13-22. Remarkably, Hsp104 directly remodels preamyloid oligomers and amyloid fibrils, including those comprised of the yeast prion proteins Sup35 and Ure2^23-30. This amyloid-remodeling functionality is a specialized facet of yeast Hsp104. The E. coli orthologue, ClpB, fails to remodel preamyloid oligomers or amyloid fibrils^26,31,32.Hsp104 orthologues are found in all kingdoms of life except, perplexingly, animals. Indeed, whether animal cells possess any enzymatic system that couples protein disaggregation to renaturation (rather than degradation) remains unknown^33-35. Thus, we and others have proposed that Hsp104 might be developed as a therapeutic agent for various neurodegenerative diseases connected with the misfolding of specific proteins into toxic preamyloid oligomers and amyloid fibrils^4,7,23,36-38. There are no treatments that directly target the aggregated species associated with these diseases. Yet, Hsp104 dissolves toxic oligomers and amyloid fibrils composed of alpha-synuclein, which are connected with Parkinson''s Disease²³ as well as amyloid forms of PrP³⁹. Importantly, Hsp104 reduces protein aggregation and ameliorates neurodegeneration in rodent models of Parkinson''s Disease²³ and Huntington''s disease³⁸. Ideally, to optimize therapy and minimize side effects, Hsp104 would be engineered and potentiated to selectively remodel specific aggregates central to the disease in question^4,7. However, the limited structural and mechanistic understanding of how Hsp104 disaggregates such a diverse repertoire of aggregated structures and unrelated proteins frustrates these endeavors^30,40-42.To understand the structure and mechanism of Hsp104, it is essential to study the pure protein and reconstitute its disaggregase activity with minimal components. Hsp104 is a 102kDa protein with a pI of ~5.3, which hexamerizes in the presence of ADP or ATP, or at high protein concentrations in the absence of nucleotide^43-46. Here, we describe an optimized protocol for the purification of highly active, stable Hsp104 from E. coli. The use of E. coli allows simplified large-scale production and our method can be performed quickly and reliably for numerous Hsp104 variants. Our protocol increases Hsp104 purity and simplifies His₆-tag removal compared to a previous purification method from E. coli⁴⁷. Moreover, our protocol is more facile and convenient than two more recent protocols^26,48.Download video file.^{(43M, mov)}     

Selective Hsp70-Dependent Docking of Hsp104 to Protein Aggregates Protects the Cell from the Toxicity of the Disaggregase

Hsp104 is a yeast chaperone that rescues misfolded proteins from aggregates associated with proteotoxic stress and aging. Hsp104 consists of N-terminal domain, regulatory M-domain and two ATPase domains, assembled into a spiral-shaped hexamer. Protein disaggregation involves polypeptide extraction from an aggregate and its translocation through the central channel. This process relies on Hsp104 cooperation with the Hsp70 chaperone, which also plays important role in regulation of the disaggregase. Although Hsp104 protein-unfolding activity enables cells to survive stress, when uncontrolled, it becomes toxic to the cell.In this work, we investigated the significance of the interaction between Hsp70 and the M-domain of Hsp104 for functioning of the disaggregation system. We identified phenylalanine at position 508 in Hsp104 to be the key site of interaction with Hsp70. Disruption of this site makes Hsp104 unable to bind protein aggregates and to confer tolerance in yeast cells. The use of this Hsp104 variant demonstrates that Hsp70 allows successful initiation of disaggregation only as long as it is able to interact with the disaggregase. As reported previously, this interaction causes release of the M-domain-driven repression of Hsp104. Now we reveal that, apart from this allosteric effect, the interaction between the chaperone partners itself contributes to effective initiation of disaggregation and plays important role in cell protection against Hsp104-induced toxicity. Interaction with Hsp70 shifts Hsp104 substrate specificity from non-aggregated, disordered substrates toward protein aggregates. Accordingly, Hsp70-mediated sequestering of the Hsp104 unfoldase in aggregates makes it less toxic and more productive.     

Analysis of monomeric mutants of HSC70: a possible relationship between oligomerization and functional properties

Data of this study showed that alphaD-alphaE helices and the conserved interdomain linker are two interfaces essential not only for the self-association but also for the functional properties of rat HSC70. Self-association which is a conserved property of HSP70 seems to be important for the activity of these proteins.