Structural variants of yeast prions show conformer-specific requirements for chaperone activity

Human J-Domain Protein DnaJB6 Protects Yeast from [PSI+] Prion Toxicity

Human J-domain protein (JDP) DnaJB6 has a broad and potent activity that prevents formation of amyloid by polypeptides such as polyglutamine, A-beta, and alpha-synuclein, related to Huntington's, Alzheimer's, and Parkinson's diseases, respectively. In yeast, amyloid-based [PSI⁺] prions, which rely on the related JDP Sis1 for replication, have a latent toxicity that is exposed by reducing Sis1 function. Anti-amyloid activity of DnaJB6 is very effective against weak [PSI⁺] prions and the Sup35 amyloid that composes them, but ineffective against strong [PSI⁺] prions composed of structurally different amyloid of the same Sup35. This difference reveals limitations of DnaJB6 that have implications regarding its therapeutic use for amyloid disease. Here, we find that when Sis1 function is reduced, DnaJB6 represses toxicity of strong [PSI⁺] prions and inhibits their propagation. Both Sis1 and DnaJB6, which are regulators of protein chaperone Hsp70, counteract the toxicity by reducing excessive incorporation of the essential Sup35 into prion aggregates. However, while Sis1 apparently requires interaction with Hsp70 to detoxify [PSI⁺], DnaJB6 counteracts prion toxicity by a different, Hsp70-independent mechanism.

Hsp40/JDP Requirements for the Propagation of Synthetic Yeast Prions

Yeast prions are protein-based transmissible elements, most of which are amyloids. The chaperone protein network in yeast is inexorably linked to the spreading of prions during cell division by fragmentation of amyloid prion aggregates. Specifically, the core “prion fragmentation machinery” includes the proteins Hsp104, Hsp70 and the Hsp40/J-domain protein (JDP) Sis1. Numerous novel amyloid-forming proteins have been created and examined in the yeast system and occasionally these amyloids are also capable of continuous Hsp104-dependent propagation in cell populations, forming synthetic prions. However, additional chaperone requirements, if any, have not been determined. Here, we report the first instances of a JDP-Hsp70 system requirement for the propagation of synthetic prions. We utilized constructs from a system of engineered prions with prion-forming domains (PrDs) consisting of a polyQ stretch interrupted by a single heterologous amino acid interspersed every fifth residue. These “polyQX” PrDs are fused to the MC domains of Sup35, creating chimeric proteins of which a subset forms synthetic prions in yeast. For four of these prions, we show that SIS1 repression causes prion loss in a manner consistent with Sis1′s known role in prion fragmentation. PolyQX prions were sensitive to Sis1 expression levels to differing degrees, congruent with the variability observed among native prions. Our results expand the scope known Sis1 functionality, demonstrating that Sis1 acts on amyloids broadly, rather than through specific protein–protein interactions with individual yeast prion-forming proteins.

J Proteins Counteract Amyloid Propagation and Toxicity in Yeast

Simple Summary

Dozens of diseases are associated with misfolded proteins that accumulate in highly ordered fibrous aggregates called amyloids. Protein quality control (PQC) factors keep cells healthy by helping maintain the integrity of the cell’s proteins and physiological processes. Yeast has been used widely for years to study how amyloids cause toxicity to cells and how PQC factors help protect cells from amyloid toxicity. The so-called J-domain proteins (JDPs) are PQC factors that are particularly effective at providing such protection. We discuss how PQC factors protect animals, human cells, and yeast from amyloid toxicity, focusing on yeast and human JDPs.

Abstract

The accumulation of misfolded proteins as amyloids is associated with pathology in dozens of debilitating human disorders, including diabetes, Alzheimer’s, Parkinson’s, and Huntington’s diseases. Expressing human amyloid-forming proteins in yeast is toxic, and yeast prions that propagate as infectious amyloid forms of cellular proteins are also harmful. The yeast system, which has been useful for studying amyloids and their toxic effects, has provided much insight into how amyloids affect cells and how cells respond to them. Given that an amyloid is a protein folding problem, it is unsurprising that the factors found to counteract the propagation or toxicity of amyloids in yeast involve protein quality control. Here, we discuss such factors with an emphasis on J-domain proteins (JDPs), which are the most highly abundant and diverse regulators of Hsp70 chaperones. The anti-amyloid effects of JDPs can be direct or require interaction with Hsp70.

Differential Interactions of Molecular Chaperones and Yeast Prions

Baker’s yeast Saccharomyces cerevisiae is an important model organism that is applied to study various aspects of eukaryotic cell biology. Prions in yeast are self-perpetuating heritable protein aggregates that can be leveraged to study the interaction between the protein quality control (PQC) machinery and misfolded proteins. More than ten prions have been identified in yeast, of which the most studied ones include [PSI+], [URE3], and [PIN+]. While all of the major molecular chaperones have been implicated in propagation of yeast prions, many of these chaperones differentially impact propagation of different prions and/or prion variants. In this review, we summarize the current understanding of the life cycle of yeast prions and systematically review the effects of different chaperone proteins on their propagation. Our analysis clearly shows that Hsp40 proteins play a central role in prion propagation by determining the fate of prion seeds and other amyloids. Moreover, direct prion-chaperone interaction seems to be critically important for proper recruitment of all PQC components to the aggregate. Recent results also suggest that the cell asymmetry apparatus, cytoskeleton, and cell signaling all contribute to the complex network of prion interaction with the yeast cell.

Yeast J-protein Sis1 prevents prion toxicity by moderating depletion of prion protein

[PSI+] is a prion of Saccharomyces cerevisiae Sup35, an essential ribosome release factor. In [PSI+] cells, most Sup35 is sequestered into insoluble amyloid aggregates. Despite this depletion, [PSI+] prions typically affect viability only modestly, so [PSI+] must balance sequestering Sup35 into prions with keeping enough Sup35 functional for normal growth. Sis1 is an essential J-protein regulator of Hsp70 required for the propagation of amyloid-based yeast prions. C-terminally truncated Sis1 (Sis1JGF) supports cell growth in place of wild-type Sis1. Sis1JGF also supports [PSI+] propagation, yet [PSI+] is highly toxic to cells expressing only Sis1JGF. We searched extensively for factors that mitigate the toxicity and identified only Sis1, suggesting Sis1 is uniquely needed to protect from [PSI+] toxicity. We find the C-terminal substrate-binding domain of Sis1 has a critical and transferable activity needed for the protection. In [PSI+] cells that express Sis1JGF in place of Sis1, Sup35 was less soluble and formed visibly larger prion aggregates. Exogenous expression of a truncated Sup35 that cannot incorporate into prions relieved [PSI+] toxicity. Together our data suggest that Sis1 has separable roles in propagating Sup35 prions and in moderating Sup35 aggregation that are crucial to the balance needed for the propagation of what otherwise would be lethal [PSI+] prions.

Functional genomics screen identifies proteostasis targets that modulate prion protein (PrP) stability

Prion protein (PrP) adopts either a helical conformation (PrPC) or an alternative, beta sheet-rich, misfolded conformation (PrPSc). The PrPSc form has the ability to "infect" PrPC and force it into the misfolded state. Accumulation of PrPSc is associated with a number of lethal neurodegenerative disorders, including Creutzfeldt-Jacob disease (CJD). Knockout of PrPC protects cells and animals from PrPSc infection; thus, there is interest in identifying factors that regulate PrPC stability, with the therapeutic goal of reducing PrPC levels and limiting infection by PrPSc. Here, we assembled a short-hairpin RNA (shRNA) library composed of 25+ shRNA sequences for each of 133 protein homeostasis (aka proteostasis) factors, such as molecular chaperones and co-chaperones. This Proteostasis shRNA Library was used to identify regulators of PrPC stability in HEK293 Hu129M cells. Strikingly, the screen identified a number of Hsp70 family members and their co-chaperones as putative targets. Indeed, a chemical pan-inhibitor of Hsp70s reduced PrPC levels and limited conversion to PrPSc in N2a cells. These results implicate specific proteostasis sub-networks, especially the Hsp70 system, as potential new targets for the treatment of CJD. More broadly, the Proteostasis shRNA Library might be a useful tool for asking which proteostasis factors are important for a given protein.

Yeast Sup35 Prion Structure: Two Types, Four Parts, Many Variants

The yeast [PSI⁺] prion, formed by the Sup35 (eRF3) protein, has multiple structural variants differing in the strength of nonsense suppressor phenotype. Structure of [PSI⁺] and its variation are characterized poorly. Here, we mapped Sup35 amyloid cores of 26 [PSI⁺] ex vivo prions of different origin using proteinase K digestion and mass spectrometric identification of resistant peptides. In all [PSI⁺] variants the Sup35 amino acid residues 2-32 were fully resistant and the region up to residue 72 was partially resistant. Proteinase K-resistant structures were also found within regions 73-124, 125-153, and 154-221, but their presence differed between [PSI⁺] isolates. Two distinct digestion patterns were observed for region 2-72, which always correlated with the "strong" and "weak" [PSI⁺] nonsense suppressor phenotypes. Also, all [PSI⁺] with a weak pattern were eliminated by multicopy HSP104 gene and were not toxic when combined with multicopy SUP35. [PSI⁺] with a strong pattern showed opposite properties, being resistant to multicopy HSP104 and lethal with multicopy SUP35. Thus, Sup35 prion cores can be composed of up to four elements. [PSI⁺] variants can be divided into two classes reliably distinguishable basing on structure of the first element and the described assays.

Impact of Amyloid Polymorphism on Prion-Chaperone Interactions in Yeast

Yeast prions are protein-based genetic elements found in the baker's yeast Saccharomyces cerevisiae, most of which are amyloid aggregates that propagate by fragmentation and spreading of small, self-templating pieces called propagons. Fragmentation is carried out by molecular chaperones, specifically Hsp104, Hsp70, and Hsp40. Like other amyloid-forming proteins, amyloid-based yeast prions exhibit structural polymorphisms, termed "strains" in mammalian systems and "variants" in yeast, which demonstrate diverse phenotypes and chaperone requirements for propagation. Here, the known differential interactions between chaperone proteins and yeast prion variants are reviewed, specifically those of the yeast prions [PSI⁺], [RNQ⁺]/[PIN⁺], and [URE3]. For these prions, differences in variant-chaperone interactions (where known) with Hsp104, Hsp70s, Hsp40s, Sse1, and Hsp90 are summarized, as well as some interactions with chaperones of other species expressed in yeast. As amyloid structural differences greatly impact chaperone interactions, understanding and accounting for these variations may be crucial to the study of chaperones and both prion and non-prion amyloids.

Prion Variants of Yeast are Numerous, Mutable, and Segregate on Growth, Affecting Prion Pathogenesis, Transmission Barriers, and Sensitivity to Anti-Prion Systems

The known amyloid-based prions of Saccharomyces cerevisiae each have multiple heritable forms, called "prion variants" or "prion strains". These variants, all based on the same prion protein sequence, differ in their biological properties and their detailed amyloid structures, although each of the few examined to date have an in-register parallel folded β sheet architecture. Here, we review the range of biological properties of yeast prion variants, factors affecting their generation and propagation, the interaction of prion variants with each other, the mutability of prions, and their segregation during mitotic growth. After early differentiation between strong and weak stable and unstable variants, the parameters distinguishing the variants has dramatically increased, only occasionally correlating with the strong/weak paradigm. A sensitivity to inter- and intraspecies barriers, anti-prion systems, and chaperone deficiencies or excesses and other factors all have dramatic selective effects on prion variants. Recent studies of anti-prion systems, which cure prions in wild strains, have revealed an enormous array of new variants, normally eliminated as they arise and so not previously studied. This work suggests that defects in the anti-prion systems, analogous to immune deficiencies, may be at the root of some human amyloidoses.

Variant-specific and reciprocal Hsp40 functions in Hsp104-mediated prion elimination

The amyloid-based prions of Saccharomyces cerevisiae are heritable aggregates of misfolded proteins, passed to daughter cells following fragmentation by molecular chaperones including the J-protein Sis1, Hsp70 and Hsp104. Overexpression of Hsp104 efficiently cures cell populations of the prion [PSI+ ] by an alternative Sis1-dependent mechanism that is currently the subject of significant debate. Here, we broadly investigate the role of J-proteins in this process by determining the impact of amyloid polymorphisms (prion variants) on the ability of well-studied Sis1 constructs to compensate for Sis1 and ask whether any other S. cerevisiae cytosolic J-proteins are also required for this process. Our comprehensive screen, examining all 13 members of the yeast cytosolic/nuclear J-protein complement, uncovered significant variant-dependent genetic evidence for a role of Apj1 (antiprion DnaJ) in this process. For strong, but not weak [PSI+ ] variants, depletion of Apj1 inhibits Hsp104-mediated curing. Overexpression of either Apj1 or Sis1 enhances curing, while overexpression of Ydj1 completely blocks it. We also demonstrated that Sis1 was the only J-protein necessary for the propagation of at least two weak [PSI+ ] variants and no J-protein alteration, or even combination of alterations, affected the curing of weak [PSI+ ] variants, suggesting the possibility of biochemically distinct, variant-specific Hsp104-mediated curing mechanisms.

Heterologous prion-forming proteins interact to cross-seed aggregation in Saccharomyces cerevisiae

The early stages of protein misfolding remain incompletely understood, as most mammalian proteinopathies are only detected after irreversible protein aggregates have formed. Cross-seeding, where one aggregated protein templates the misfolding of a heterologous protein, is one mechanism proposed to stimulate protein aggregation and facilitate disease pathogenesis. Here, we demonstrate the existence of cross-seeding as a crucial step in the formation of the yeast prion [PSI ⁺], formed by the translation termination factor Sup35. We provide evidence for the genetic and physical interaction of the prion protein Rnq1 with Sup35 as a predominant mechanism leading to self-propagating Sup35 aggregation. We identify interacting sites within Rnq1 and Sup35 and determine the effects of breaking and restoring a crucial interaction. Altogether, our results demonstrate that single-residue disruption can drastically reduce the effects of cross-seeding, a finding that has important implications for human protein misfolding disorders.

To CURe or not to CURe? Differential effects of the chaperone sorting factor Cur1 on yeast prions are mediated by the chaperone Sis1

Yeast self-perpetuating protein aggregates (prions) provide a convenient model for studying various components of the cellular protein quality control system. Molecular chaperones and chaperone-sorting factors, such as yeast Cur1 protein, play key role in proteostasis via tight control of partitioning and recycling of misfolded proteins. In this study, we show that, despite the previously described ability of Cur1 to antagonize the yeast prion [URE3], it enhances propagation and phenotypic manifestation of another prion, [PSI⁺ ]. We demonstrate that both curing of [URE3] and enhancement of [PSI⁺ ] in the presence of excess Cur1 are counteracted by the cochaperone Hsp40-Sis1 in a dosage-dependent manner, and show that the effect of Cur1 on prions parallels effects of the attachment of nuclear localization signal to Sis1, indicating that Cur1 acts on prions via its previously reported ability to relocalize Sis1 from the cytoplasm to nucleus. This shows that the direction in which Cur1 influences a prion depends on how this specific prion responds to relocalization of Sis1.

Prion-specific Hsp40 function: The role of the auxilin homolog Swa2

Yeast prions are protein-based genetic elements that propagate through cell populations via cytosolic transfer from mother to daughter cell. Molecular chaperone proteins including Hsp70, the Hsp40/J-protein Sis1, and Hsp104 are required for continued prion propagation, however the specific requirements of chaperone proteins differ for various prions. We recently reported that Swa2, the yeast homolog of the mammalian protein auxilin, is specifically required for the propagation of the prion [URE3]. 1 [URE3] propagation requires both a functional J-domain and the tetratricopeptide repeat (TPR) domain of Swa2, but does not require Swa2 clathrin binding. We concluded that the TPR domain determines the specificity of the genetic interaction between Swa2 and [URE3], and that this domain likely interacts with one or more proteins with a C-terminal EEVD motif. Here we extend that analysis to incorporate additional data that supports this hypothesis. We also present new data eliminating Hsp104 as the relevant Swa2 binding partner and discuss our findings in the context of other recent work involving Hsp90. Based on these findings, we propose a new model for Swa2's involvement in [URE3] propagation in which Swa2 and Hsp90 mediate the formation of a multi-protein complex that increases the number of sites available for Hsp104 disaggregation.

Prions and Protein Assemblies that Convey Biological Information in Health and Disease

Prions derived from the prion protein (PrP) were first characterized as infectious agents that transmit pathology between individuals. However, the majority of cases of neurodegeneration caused by PrP prions occur sporadically. Proteins that self-assemble as cross-beta sheet amyloids are a defining pathological feature of infectious prion disorders and all major age-associated neurodegenerative diseases. In fact, multiple non-infectious proteins exhibit properties of template-driven self-assembly that are strikingly similar to PrP. Evidence suggests that like PrP, many proteins form aggregates that propagate between cells and convert cognate monomer into ordered assemblies. We now recognize that numerous proteins assemble into macromolecular complexes as part of normal physiology, some of which are self-amplifying. This review highlights similarities among infectious and non-infectious neurodegenerative diseases associated with prions, emphasizing the normal and pathogenic roles of higher-order protein assemblies. We propose that studies of the structural and cellular biology of pathological versus physiological aggregates will be mutually informative.

Hsp40 function in yeast prion propagation: Amyloid diversity necessitates chaperone functional complexity

Yeast prions are heritable protein-based elements, most of which are formed of amyloid aggregates that rely on the action of molecular chaperones for transmission to progeny. Prions can form distinct amyloid structures, known as 'strains' in mammalian systems, that dictate both pathological progression and cross-species infection barriers. In yeast these same amyloid structural polymorphisms, called 'variants', dictate the intensity of prion-associated phenotypes and stability in mitosis. We recently reported that [PSI(+)] prion variants differ in the fundamental domain requirements for one chaperone, the Hsp40/J-protein Sis1, which are mutually exclusive between 2 different yeast prions, demonstrating a functional plurality for Sis1. Here we extend that analysis to incorporate additional data that collectively support the hypothesis that Sis1 has multiple functional roles that can be accomplished by distinct sets of domains. These functions are differentially required by distinct prions and prion variants. We also present new data regarding Hsp104-mediated prion elimination and show that some Sis1 functions, but not all, are conserved in the human homolog Hdj1/DNAJB1. Importantly, of the 10 amyloid-based prions indentified to date in Saccharomyces cerevisiae, the chaperone requirements of only 4 are known, leaving a great diversity of amyloid structures, and likely modes of amyloid-chaperone interaction, largely unexplored.

Human J-protein DnaJB6b Cures a Subset of Saccharomyces cerevisiae Prions and Selectively Blocks Assembly of Structurally Related Amyloids

Human chaperone DnaJB6, an Hsp70 co-chaperone whose defects cause myopathies, protects cells from polyglutamine toxicity and prevents purified polyglutamine and Aβ peptides from forming amyloid. Yeast prions [URE3] and [PSI(+)] propagate as amyloid forms of Ure2 and Sup35 proteins, respectively. Here we find DnaJB6-protected yeast cells from polyglutamine toxicity and cured yeast of both [URE3] prions and weak variants of [PSI(+)] prions but not strong [PSI(+)] prions. Weak and strong variants of [PSI(+)] differ only in the structural conformation of their amyloid cores. In line with its anti-prion effects, DnaJB6 prevented purified Sup35NM from forming amyloids at 37 °C, which produce predominantly weak [PSI(+)] variants when used to infect yeast, but not at 4 °C, which produces mostly strong [PSI(+)] variants. Thus, structurally distinct amyloids composed of the same protein were differentially sensitive to the anti-amyloid activity of DnaJB6 both in vitro and in vivo. These findings have important implications for strategies using DnaJB6 as a target for therapy in amyloid disorders.

Swa2, the yeast homolog of mammalian auxilin, is specifically required for the propagation of the prion variant [URE3-1]

Yeast prions require a core set of chaperone proteins including Sis1, Hsp70 and Hsp104 to generate new amyloid templates for stable propagation, yet emerging studies indicate that propagation of some prions requires additional chaperone activities, demonstrating chaperone specificity beyond the common amyloid requirements. To comprehensively assess such prion-specific requirements for the propagation of the [URE3] prion variant [URE3-1], we screened 12 yeast cytosolic J-proteins, and here we report a novel role for the J-protein Swa2/Aux1. Swa2 is the sole yeast homolog of the mammalian protein auxilin, which, like Swa2, functions in vesicle-mediated endocytosis by disassembling the structural lattice formed by the protein clathrin. We found that, in addition to Sis1, [URE3-1] is specifically dependent upon Swa2, but not on any of the 11 other J-proteins. Further, we show that [URE3-1] propagation requires both a functional J-domain and the tetratricopeptide repeat (TPR) domain, but surprisingly does not require Swa2-clathrin binding. Because the J-domain of Swa2 can be replaced with the J-domains of other proteins, our data strongly suggest that prion-chaperone specificity arises from the Swa2 TPR domain and supports a model where Swa2 acts through Hsp70, most likely to provide additional access points for Hsp104 to promote prion template generation.

Hsp40s specify functions of Hsp104 and Hsp90 protein chaperone machines

Hsp100 family chaperones of microorganisms and plants cooperate with the Hsp70/Hsp40/NEF system to resolubilize and reactivate stress-denatured proteins. In yeast this machinery also promotes propagation of prions by fragmenting prion polymers. We previously showed the bacterial Hsp100 machinery cooperates with the yeast Hsp40 Ydj1 to support yeast thermotolerance and with the yeast Hsp40 Sis1 to propagate [PSI⁺] prions. Here we find these Hsp40s similarly directed specific activities of the yeast Hsp104-based machinery. By assessing the ability of Ydj1-Sis1 hybrid proteins to complement Ydj1 and Sis1 functions we show their C-terminal substrate-binding domains determined distinctions in these and other cellular functions of Ydj1 and Sis1. We find propagation of [URE3] prions was acutely sensitive to alterations in Sis1 activity, while that of [PIN⁺] prions was less sensitive than [URE3], but more sensitive than [PSI⁺]. These findings support the ideas that overexpressing Ydj1 cures [URE3] by competing with Sis1 for interaction with the Hsp104-based disaggregation machine, and that different prions rely differently on activity of this machinery, which can explain the various ways they respond to alterations in chaperone function.

The cellular chaperone machinery helps proteins adopt and maintain native conformations and protects cells from stress. The yeast Hsp40s Ydj1 and Sis1 are co-chaperones that regulate Hsp70s, which are key components of many chaperone complexes. Both of these Hsp40s are crucial for growth and Ydj1 directs disaggregation activity of the Hsp100-based machinery to provide stress protection while Sis1 directs this activity to promote prion replication. Ydj1 also cures yeast of certain prions when overexpressed. We show that C-terminal domains that possess substrate-binding function of Ydj1 and Sis1 can mediate these and other functional distinctions and that the degree that prions depend on Sis1 activities could underlie differences in how they respond to alterations of chaperones. These findings support a view that Hsp40s regulate and specify functions of the chaperone machinery through substrate discrimination and cooperation with Hsp70. The disproportionate evolutionary expansion of Hsp40s (J-proteins) relative to their Hsp70 partners led to a proposal that this amplification allows increased regulation and fine-tuning of chaperone machines for increasingly complex processes. Our findings support this idea and provide insight into fundamental aspects of this cooperation.