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

Genome-Wide Identification of the DnaJ Gene Family in Citrus and Functional Characterization of ClDJC24 in Response to Citrus Huanglongbing

Citrus Huanglongbing (HLB) is the most destructive citrus disease worldwide. The etiological agent responsible for this disease is "Candidatus Liberibacter asiaticus" (CLas), a phloem-restricted bacterium transmitted by psyllid vectors. To date, effective practical strategies for curing Citrus HLB remain elusive. Additionally, no susceptibility genes associated with HLB have been identified in Citrus species, thereby complicating the application of gene-editing techniques such as CRISPR-Cas9 to enhance resistance to HLB. The co-chaperone DnaJ plays a crucial role in protein folding and the regulation of various physiological activities, and it is also associated with multiple pathological processes. DnaJ has been extensively studied in many species, including Arabidopsis, rice, and wheat. However, there is limited information available regarding the DnaJ gene family in citrus. In this study, we conducted a comprehensive genome-wide analysis of the DnaJ family genes in various Citrus species. The Citrus genome was identified to contain 86 DnaJ genes, which were unevenly distributed across nine chromosomes. Phylogenetic analysis indicated that these genes could be classified into six distinct groups. Furthermore, transcriptomic analysis revealed that nine DnaJ genes exhibited significantly higher induction in HLB-infected samples relative to non-HLB-infected Citrus. Cis-acting elements within the promoters of DnaJ genes were also examined, revealing the presence of hormone and defense/stress responsiveness elements (TC-rich) distributed on the ClDJC24 gene. The results were validated using quantitative real-time PCR (qRT-PCR). Additionally, the silencing of ClDJC24 suggested that this gene negatively regulates disease resistance in Citrus. Our study provided useful clues for further functional characterization and constructed a theoretical foundation for disease-resistant breeding in Citrus.

Unique characteristics of the J-domain proximal regions of Hsp70 cochaperone Apj1 in prion propagation/elimination and its overlap with Sis1 function

J-domain proteins (JDPs) are obligate cochaperones of Hsp70s. The Class A JDP Apj1 of the yeast cytosol has an unusually complex region between the N-terminal J-domain and the substrate binding region-often called the Grich or GF region in Class A and B JDPs because of its typical abundance of glycine. The N-terminal 161-residue Apj1 fragment is known to be sufficient for Apj1 function in prion curing, driven by the overexpression of Hsp104. Further analyzing the N-terminal segment of Apj1, we found that a 90-residue fragment that includes the 70-residue J-domain and the adjacent 12-residue glutamine/alanine (Q/A) segment is sufficient for curing. Furthermore, the 121-residue fragment that includes the Grich region was sufficient to not only sustain the growth of cells lacking the essential Class B JDP Sis1 but also enabled the maintenance of several prions normally dependent on Sis1 for propagation. A J-domain from another cytosolic JDP could substitute for the Sis1-related functions but not for Apj1 in prion curing. Together, these results separate the functions of JDPs in prion biology and underscore the diverse functionality of multi-domain cytosolic JDPs in yeast.

Human proteins curing yeast prions

Recognition that common human amyloidoses are prion diseases makes the use of the Saccharomyces cerevisiae prion model systems to screen for possible anti-prion components of increasing importance. [PSI+] and [URE3] are amyloid-based prions of Sup35p and Ure2p, respectively. Yeast has at least six anti-prion systems that together cure nearly all [PSI+] and [URE3] prions arising in their absence. We made a GAL-promoted bank of 14,913 human open reading frames in a yeast shuttle plasmid and isolated 20 genes whose expression cures [PSI+] or [URE3]. PRPF19 is an E3 ubiquitin ligase that cures [URE3] if its U-box is intact. DNAJA1 is a J protein that cures [PSI+] unless its interaction with Hsp70s is defective. Human Bag5 efficiently cures [URE3] and [PSI+]. Bag family proteins share a 110 to 130 residue "BAG domain"; Bag 1, 2, 3, 4, and 6 each have one BAG domain while Bag5 has five BAG domains. Two BAG domains are necessary for curing [PSI+], but one can suffice to cure [URE3]. Although most Bag proteins affect autophagy in mammalian cells, mutations blocking autophagy in yeast do not affect Bag5 curing of [PSI+] or [URE3]. Curing by Bag proteins depends on their interaction with Hsp70s, impairing their role, with Hsp104 and Sis1, in the amyloid filament cleavage necessary for prion propagation. Since Bag5 curing is reduced by overproduction of Sis1, we propose that Bag5 cures prions by blocking Sis1 access to Hsp70s in its role with Hsp104 in filament cleavage.

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.

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.

Client processing is altered by novel myopathy-causing mutations in the HSP40 J domain

The misfolding and aggregation of proteins is often implicated in the development and progression of degenerative diseases. Heat shock proteins (HSPs), such as the ubiquitously expressed Type II Hsp40 molecular chaperone, DNAJB6, assist in protein folding and disaggregation. Historically, mutations within the DNAJB6 G/F domain have been associated with Limb-Girdle Muscular Dystrophy type 1D, now referred to as LGMDD1, a dominantly inherited degenerative disease. Recently, novel mutations within the J domain of DNAJB6 have been reported in patients with LGMDD1. Since novel myopathy-causing mutations in the Hsp40 J domain have yet to be characterized and both the function of DNAJB6 in skeletal muscle and the clients of this chaperone are unknown, we set out to assess the effect of these mutations on chaperone function using the genetically tractable yeast system. The essential yeast Type II Hsp40, Sis1, is homologous to DNAJB6 and is involved in the propagation of yeast prions. Using phenotypic, biochemical, and functional assays we found that homologous mutations in the Sis1 J domain differentially alter the processing of specific yeast prion strains, as well as a non-prion substrate. These data suggest that the newly-identified mutations in the J domain of DNAJB6 cause aberrant chaperone function that leads to the pathogenesis in LGMDD1.

Yeast Models for Amyloids and Prions: Environmental Modulation and Drug Discovery

Amyloids are self-perpetuating protein aggregates causing neurodegenerative diseases in mammals. Prions are transmissible protein isoforms (usually of amyloid nature). Prion features were recently reported for various proteins involved in amyloid and neural inclusion disorders. Heritable yeast prions share molecular properties (and in the case of polyglutamines, amino acid composition) with human disease-related amyloids. Fundamental protein quality control pathways, including chaperones, the ubiquitin proteasome system and autophagy are highly conserved between yeast and human cells. Crucial cellular proteins and conditions influencing amyloids and prions were uncovered in the yeast model. The treatments available for neurodegenerative amyloid-associated diseases are few and their efficiency is limited. Yeast models of amyloid-related neurodegenerative diseases have become powerful tools for high-throughput screening for chemical compounds and FDA-approved drugs that reduce aggregation and toxicity of amyloids. Although some environmental agents have been linked to certain amyloid diseases, the molecular basis of their action remains unclear. Environmental stresses trigger amyloid formation and loss, acting either via influencing intracellular concentrations of the amyloidogenic proteins or via heterologous inducers of prions. Studies of environmental and physiological regulation of yeast prions open new possibilities for pharmacological intervention and/or prophylactic procedures aiming on common cellular systems rather than the properties of specific amyloids.

Three J-proteins impact Hsp104-mediated variant-specific prion elimination: a new critical role for a low-complexity domain

Prions are self-propagating protein isoforms that are typically amyloid. In Saccharomyces cerevisiae, amyloid prion aggregates are fragmented by a trio involving three classes of chaperone proteins: Hsp40s, also known as J-proteins, Hsp70s, and Hsp104. Hsp104, the sole Hsp100-class disaggregase in yeast, along with the Hsp70 Ssa and the J-protein Sis1, is required for the propagation of all known amyloid yeast prions. However, when Hsp104 is ectopically overexpressed, only the prion [PSI⁺] is efficiently eliminated from cell populations via a highly debated mechanism that also requires Sis1. Recently, we reported roles for two additional J-proteins, Apj1 and Ydj1, in this process. Deletion of Apj1, a J-protein involved in the degradation of sumoylated proteins, partially blocks Hsp104-mediated [PSI⁺] elimination. Apj1 and Sis1 were found to have overlapping functions, as overexpression of one compensates for loss of function of the other. In addition, overexpression of Ydj1, the most abundant J-protein in the yeast cytosol, completely blocks Hsp104-mediated curing. Yeast prions exhibit structural polymorphisms known as “variants”; most intriguingly, these J-protein effects were only observed for strong variants, suggesting variant-specific mechanisms. Here, we review these results and present new data resolving the domains of Apj1 responsible, specifically implicating the involvement of Apj1’s Q/S-rich low-complexity domain.

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.

Biomolecular Assemblies: Moving from Observation to Predictive Design

Biomolecular assembly is a key driving force in nearly all life processes, providing structure, information storage, and communication within cells and at the whole organism level. These assembly processes rely on precise interactions between functional groups on nucleic acids, proteins, carbohydrates, and small molecules, and can be fine-tuned to span a range of time, length, and complexity scales. Recognizing the power of these motifs, researchers have sought to emulate and engineer biomolecular assemblies in the laboratory, with goals ranging from modulating cellular function to the creation of new polymeric materials. In most cases, engineering efforts are inspired or informed by understanding the structure and properties of naturally occurring assemblies, which has in turn fueled the development of predictive models that enable computational design of novel assemblies. This Review will focus on selected examples of protein assemblies, highlighting the story arc from initial discovery of an assembly, through initial engineering attempts, toward the ultimate goal of predictive design. The aim of this Review is to highlight areas where significant progress has been made, as well as to outline remaining challenges, as solving these challenges will be the key that unlocks the full power of biomolecules for advances in technology and medicine.

Cell surface nucleolin interacts with and internalizes Bothrops asper Lys49 phospholipase A2 and mediates its toxic activity

Phospholipases A2 are a major component of snake venoms. Some of them cause severe muscle necrosis through an unknown mechanism. Phospholipid hydrolysis is a possible explanation of their toxic action, but catalytic and toxic properties of PLA2s are not directly connected. In addition, viperid venoms contain PLA2-like proteins, which are very toxic even if they lack catalytic activity due to a critical mutation in position 49. In this work, the PLA2-like Bothrops asper myotoxin-II, conjugated with the fluorophore TAMRA, was found to be internalized in mouse myotubes, and in RAW264.7 cells. Through experiments of protein fishing and mass spectrometry analysis, using biotinylated Mt-II as bait, we found fifteen proteins interacting with the toxin and among them nucleolin, a nucleolar protein present also on cell surface. By means of confocal microscopy, Mt-II and nucleolin were shown to colocalise, at 4 °C, on cell membrane where they form Congo-red sensitive assemblies, while at 37 °C, 20 minutes after the intoxication, they colocalise in intracellular spots going from plasmatic membrane to paranuclear and nuclear area. Finally, nucleolin antagonists were found to inhibit the Mt-II internalization and toxic activity and were used to identify the nucleolin regions involved in the interaction with the toxin.

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.

Yeast Short-Lived Actin-Associated Protein Forms a Metastable Prion in Response to Thermal Stress

Self-perpetuating ordered protein aggregates (amyloids and prions) are associated with a variety of neurodegenerative disorders. Although environmental agents have been linked to certain amyloid diseases, the molecular basis of their action remains unclear. We have employed endogenous yeast prions as a model system to study environmental control of amyloid formation. A short-lived actin-associated yeast protein Lsb2 can trigger prion formation by other proteins in a mode regulated by the cytoskeleton and ubiquitin-dependent processes. Here, we show that such a heterologous prion induction is due to the ability of Lsb2 to form a transient prion state, generated in response to thermal stress. Evolutionary acquisition of prion-inducing activity by Lsb2 is traced to a single amino acid change, coinciding with the acquisition of thermotolerance in the Saccharomyces yeast lineage. This raises the intriguing possibility that the transient prion formation could aid in functioning of Lsb2 at higher temperatures.

The small heat shock protein Hsp31 cooperates with Hsp104 to modulate Sup35 prion aggregation

The yeast homolog of DJ-1, Hsp31, is a multifunctional protein that is involved in several cellular pathways including detoxification of the toxic metabolite methylglyoxal and as a protein deglycase. Prior studies ascribed Hsp31 as a molecular chaperone that can inhibit α-Syn aggregation in vitro and alleviate its toxicity in vivo. It was also shown that Hsp31 inhibits Sup35 aggregate formation in yeast, however, it is unknown if Hsp31 can modulate [PSI⁺] phenotype and Sup35 prionogenesis. Other small heat shock proteins, Hsp26 and Hsp42 are known to be a part of a synergistic proteostasis network that inhibits Sup35 prion formation and promotes its disaggregation. Here, we establish that Hsp31 inhibits Sup35 [PSI⁺] prion formation in collaboration with a well-known disaggregase, Hsp104. Hsp31 transiently prevents prion induction but does not suppress induction upon prolonged expression of Sup35 indicating that Hsp31 can be overcome by larger aggregates. In addition, elevated levels of Hsp31 do not cure [PSI⁺] strains indicating that Hsp31 cannot intervene in a pre-existing prion oligomerization cycle. However, Hsp31 can modulate prion status in cooperation with Hsp104 because it inhibits Sup35 aggregate formation and potentiates [PSI⁺] prion curing upon overexpression of Hsp104. The absence of Hsp31 reduces [PSI⁺] prion curing by Hsp104 without influencing its ability to rescue cellular thermotolerance. Hsp31 did not synergize with Hsp42 to modulate the [PSI⁺] phenotype suggesting that both proteins act on similar stages of the prion cycle. We also showed that Hsp31 physically interacts with Hsp104 and together they prevent Sup35 prion toxicity to greater extent than if they were expressed individually. These results elucidate a mechanism for Hsp31 on prion modulation that suggest it acts at a distinct step early in the Sup35 aggregation process that is different from Hsp104. This is the first demonstration of the modulation of [PSI⁺] status by the chaperone action of Hsp31. The delineation of Hsp31's role in the chaperone cycle has implications for understanding the role of the DJ-1 superfamily in controlling misfolded proteins in neurodegenerative disease and cancer.

Evolutionary Conservation and Emerging Functional Diversity of the Cytosolic Hsp70:J Protein Chaperone Network of Arabidopsis thaliana

Heat shock proteins of 70 kDa (Hsp70s) partner with structurally diverse Hsp40s (J proteins), generating distinct chaperone networks in various cellular compartments that perform myriad housekeeping and stress-associated functions in all organisms. Plants, being sessile, need to constantly maintain their cellular proteostasis in response to external environmental cues. In these situations, the Hsp70:J protein machines may play an important role in fine-tuning cellular protein quality control. Although ubiquitous, the functional specificity and complexity of the plant Hsp70:J protein network has not been studied. Here, we analyzed the J protein network in the cytosol of Arabidopsis thaliana and, using yeast genetics, show that the functional specificities of most plant J proteins in fundamental chaperone functions are conserved across long evolutionary timescales. Detailed phylogenetic and functional analysis revealed that increased number, regulatory differences, and neofunctionalization in J proteins together contribute to the emerging functional diversity and complexity in the Hsp70:J protein network in higher plants. Based on the data presented, we propose that higher plants have orchestrated their "chaperome," especially their J protein complement, according to their specialized cellular and physiological stipulations.

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.

Analysis of [SWI+ ] formation and propagation events

The budding yeast, Saccharomyces cerevisiae, harbors several prions that are transmitted as altered, heritable protein conformations. [SWI⁺ ] is one such prion whose determinant is Swi1, a subunit of the evolutionarily conserved chromatin-remodeling complex SWI/SNF. Despite the importance of Swi1, the molecular events that lead to [SWI⁺ ] prionogenesis remain poorly understood. In this study, we have constructed floccullin-promoter-based URA3 reporters for [SWI⁺ ] identification. Using these reporters, we show that the spontaneous formation frequency of [SWI⁺ ] is significantly higher than that of [PSI⁺ ] (prion form of Sup35). We also show that preexisting [PSI⁺ ] or [PIN⁺ ] (prion form of Rnq1), or overproduction of Swi1 prion-domain (PrD) can considerably promote Swi1 prionogenesis. Moreover, our data suggest a strain-specific effect of overproduction of Sse1 - a nucleotide exchange factor of the molecular chaperone Hsp70, and its interaction with another molecular chaperone Hsp104 on [SWI⁺ ] maintenance. Additionally, we show that Swi1 aggregates are initially ring/ribbon-like then become dot-like in mature [SWI⁺ ] cells. In the presence of [PSI⁺ ] or [PIN⁺ ], Swi1 ring/ribbon-like aggregates predominantly colocalize with the Sup35 or Rnq1 aggregates; without a preexisting prion, however, such colocalizations are rarely seen during Swi1-PrD overproduction-promoted Swi1 prionogenesis. We have thus demonstrated a complex interacting mechanism of yeast prionogenesis.

Potential mechanisms and implications for the formation of tau oligomeric strains

The culmination of many years of increasing research into the toxicity of tau aggregation in neurodegenerative disease has led to the consensus that soluble, oligomeric forms of tau are likely the most toxic entities in disease. While tauopathies overlap in the presence of tau pathology, each disease has a unique combination of symptoms and pathological features; however, most study into tau has grouped tau oligomers and studied them as a homogenous population. Established evidence from the prion field combined with the most recent tau and amyloidogenic protein research suggests that tau is a prion-like protein, capable of seeding the spread of pathology throughout the brain. Thus, it is likely that tau may also form prion-like strains or diverse conformational structures that may differ by disease and underlie some of the differences in symptoms and pathology in neurodegenerative tauopathies. The development of techniques and new technology for the detection of tau oligomeric strains may, therefore, lead to more efficacious diagnostic and treatment strategies for neurodegenerative disease. [Formula: see text].

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.

Determination of Growth-Phase Dependent Influences Exerted by Prions on Yeast Lipid Content Using HPTLC-Densitometry

Prions of the baker's yeast Saccharomyces cerevisiae allow for the inheritance of complex traits based solely on the acquisition of cytoplasmic protein aggregates and confer distinctive phenotypes to the cells which harbor them, creating heterogeneity within an otherwise clonal cell population. These phenotypes typically arise from a loss-of-function of the prion-forming protein that is unable to perform its normal cellular function(s) while sequestered in prion amyloid aggregates, but the specific biochemical consequences of prion infection are poorly understood. To begin to address this issue, we initiated a direct investigation into the potential control that yeast prions exert over fungal lipid content by utilizing the prions [URE3] and [PSI+], the first two prions discovered in yeast. We utilized silica gel high-performance thin-layer chromatography (HPTLC)-densitometry to conduct pair-wise quantifications of the relative levels of free sterols, free fatty acids, and triacylglycerols [petroleum ether-diethyl ether-acetic acid (80:20:1) mobile phase, phosphomolybdic acid (PMA) detection reagent]; steryl esters and squalene (hexane-petroleum ether-diethyl ether-acetic acid (50:20;5:1), PMA]; and phosphatidylethanolamine, phosphatidylcholine, and phosphatidylinositol (chloroform-diethyl ether-acetic acid (65:25:4.5), cupric sulfate-phosphoric acid) in otherwise clonal prion-infected ([PSI+] or [URE3]) and prion-free ([psi-] or [ure-o]) cells in two growth phases: log-phase and stationary phase. Our analysis revealed multiple statistically significant differences (p < 0.00625) between prion-infected and prion-free cells. Interestingly, prion-induced changes varied dramatically by growth phase, indicating that prions exert differential influences on cell physiology between log and stationary growth. Further experimental replication and extension of the analysis to other prions is expected to resolve additional physiological effects of prion infection. This investigation demonstrates that HPTLC-densitometry is an effective method for studying prion-induced alterations in lipid content in yeast.

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.