Is the prion domain of soluble Ure2p unstructured?

The [URE3] prion is a self-propagating amyloid form of the Ure2 protein of Saccharomyces cerevisiae. Deletions in the C-terminal nitrogen regulation domain of Ure2p increase the frequency with which the N-terminal prion domain polymerizes into the prion form, suggesting that the C-terminus stabilizes the prion domain or that the structured C-terminal region sterically impairs amyloid formation. We find by in vivo two-hybrid analysis no evidence of interaction of prion domain and C-terminal domain. Furthermore, surface plasmon resonance spectrometry shows no evidence of interaction of prion domain and C-terminal domain, and cleavage at a specific site between the domains frees the two fragments. Our NMR analysis indicates that most residues of the prion domain are in fact disordered in the soluble form of Ure2p. Deleting the tether holding the C-terminal structured region to the amyloid core does not impair prion formation, arguing against steric impairment of amyloid formation. These results suggest that the N-terminal prion domain is unstructured in the soluble protein and does not have a specific interaction with the C-terminus.     

Hierarchical organization in the amyloid core of yeast prion protein Ure2

Formation of amyloid fibrils is involved in a range of fatal human disorders including Alzheimer, Parkinson, and prion diseases. Yeast prions, despite differences in sequence from their mammalian counterparts, share similar features with mammalian prions including infectivity, prion strain phenomenon, and species barrier and thus are good model systems for human prion diseases. Yeast prions normally have long prion domains that presumably form multiple β strands in the fibril, and structural knowledge about the yeast prion fibrils has been limited. Here we use site-directed spin labeling and electron paramagnetic resonance (EPR) spectroscopy to investigate the structures of amyloid fibrils of Ure2 prion domain. We show that 15 spin-labeled Ure2 mutants, with spin labels at every 5th residue from position 5 to position 75, show a single-line or nearly single-line feature in their EPR spectra as a result of strong spin exchange interactions. These results suggest that a parallel in-register β structure exists at these spin-labeled positions. More interestingly, we also show that residues in the segment 30-65 have stronger spin exchange interactions, higher local stability, and lower solvent accessibility than segments 5-25 and 70-75, suggesting different local environment at these segments. We propose a hierarchical organization in the amyloid core of Ure2, with the segment 30-65 forming an inner core and the segments 5-25 and 70-75 forming an outer core. The hierarchical organization in the amyloid core may be a structural origin for polymorphism in fibrils and prion strains.     

Prions in Saccharomyces and Podospora spp.: Protein-Based Inheritance

Genetic evidence showed two non-Mendelian genetic elements of Saccharomyces cerevisiae, called [URE3] and [PSI], to be prions of Ure2p and Sup35p, respectively. [URE3] makes cells derepressed for nitrogen catabolism, while [PSI] elevates the efficiency of weak suppressor tRNAs. The same approach led to identification of the non-Mendelian element [Het-s] of the filamentous fungus Podospora anserina, as a prion of the het-s protein. The prion form of the het-s protein is required for heterokaryon incompatibility, a normal fungal function, suggesting that other normal cellular functions may be controlled by prions. [URE3] and [PSI] involve a self-propagating aggregation of Ure2p and Sup35p, respectively. In vitro, Ure2p and Sup35p form amyloid, a filamentous protein structure, high in β-sheet with a characteristic green birefringent staining by the dye Congo Red. Amyloid deposits are a cardinal feature of Alzheimer’s disease, non-insulin-dependent diabetes mellitus, the transmissible spongiform encephalopathies, and many other diseases. The prion domain of Ure2p consists of Asn-rich residues 1 to 80, but two nonoverlapping fragments of the molecule can, when overproduced, induce the de nova appearance of [URE3]. The prion domain of Sup35 consists of residues 1 to 114, also rich in Asn and Gln residues. While runs of Asn and Gln are important for [URE3] and [PSI], no such structures are found in PrP or the Het-s protein. Either elevated or depressed levels of the chaperone Hsp104 interfere with propagation of [PSI]. Both [URE3] and [PSI] are cured by growth of cells in millimolar guanidine HCl. [URE3] is also cured by overexpression of fragments of Ure2p or fusion proteins including parts of Ure2p.     

Scrambled prion domains form prions and amyloid

The [URE3] prion of Saccharomyces cerevisiae is a self-propagating amyloid form of Ure2p. The amino-terminal prion domain of Ure2p is necessary and sufficient for prion formation and has a high glutamine (Q) and asparagine (N) content. Such Q/N-rich domains are found in two other yeast prion proteins, Sup35p and Rnq1p, although none of the many other yeast Q/N-rich domain proteins have yet been found to be prions. To examine the role of amino acid sequence composition in prion formation, we used Ure2p as a model system and generated five Ure2p variants in which the order of the amino acids in the prion domain was randomly shuffled while keeping the amino acid composition and C-terminal domain unchanged. Surprisingly, all five formed prions in vivo, with a range of frequencies and stabilities, and the prion domains of all five readily formed amyloid fibers in vitro. Although it is unclear whether other amyloid-forming proteins would be equally resistant to scrambling, this result demonstrates that [URE3] formation is driven primarily by amino acid composition, largely independent of primary sequence.     

Architecture of Ure2p prion filaments: the N-terminal domains form a central core fiber

The [URE3] prion is an inactive, self-propagating, filamentous form of the Ure2 protein, a regulator of nitrogen catabolism in yeast. The N-terminal "prion" domain of Ure2p determines its in vivo prion properties and in vitro amyloid-forming ability. Here we determined the overall structures of Ure2p filaments and related polymers of the prion domain fused to other globular proteins. Protease digestion of 25-nm diameter Ure2p filaments trimmed them to 4-nm filaments, which mass spectrometry showed to be composed of prion domain fragments, primarily residues approximately 1-70. Fusion protein filaments with diameters of 14-25 nm were also reduced to 4-nm filaments by proteolysis. The prion domain transforms from the most to the least protease-sensitive part upon filament formation in each case, implying that it undergoes a conformational change. Intact filaments imaged by cryo-electron microscopy or after vanadate staining by scanning transmission electron microscopy (STEM) revealed a central 4-nm core with attached globular appendages. STEM mass per unit length measurements of unstained filaments yielded 1 monomer per 0.45 nm in each case. These observations strongly support a unifying model whereby subunits in Ure2p filaments, as well as in fusion protein filaments, are connected by interactions between their prion domains, which form a 4-nm amyloid filament backbone, surrounded by the corresponding C-terminal moieties.     

Structure of the globular region of the prion protein Ure2 from the yeast Saccharomyces cerevisiae

BACKGROUND: The [URE3] non-Mendelian element of the yeast S. cerevisiae is due to the propagation of a transmissible form of the protein Ure2. The infectivity of Ure2p is thought to originate from a conformational change of the normal form of the prion protein. This conformational change generates a form of Ure2p that assembles into amyloid fibrils. Hence, knowledge of the three-dimensional structure of prion proteins such as Ure2p should help in understanding the mechanism of amyloid formation associated with a number of neurodegenerative diseases. RESULTS: Here we report the three-dimensional crystal structure of the globular region of Ure2p (residues 95--354), also called the functional region, solved at 2.5 A resolution by the MAD method. The structure of Ure2p 95--354 shows a two-domain protein forming a globular dimer. The N-terminal domain is composed of a central 4 strand beta sheet flanked by four alpha helices, two on each side. In contrast, the C-terminal domain is entirely alpha-helical. The fold of Ure2p 95--354 resembles that of the beta class glutathione S-transferases (GST), in line with a weak similarity in the amino acid sequence that exists between these proteins. Ure2p dimerizes as GST does and possesses a potential ligand binding site, although it lacks GST activity. CONCLUSIONS: The structure of the functional region of Ure2p is the first crystal structure of a prion protein. Structure comparisons between Ure2p 95--354 and GST identified a 32 amino acid residues cap region in Ure2p exposed to the solvent. The cap region is highly flexible and may interact with the N-terminal region of the partner subunit in the dimer. The implication of this interaction in the assembly of Ure2p into amyloid fibrils is discussed.     

Molecular chaperone Hsp104 can promote yeast prion generation

[URE3] is an amyloid-based prion of Ure2p, a regulator of nitrogen catabolism in Saccharomyces cerevisiae. The Ure2p of the human pathogen Candida albicans can also be a prion in S. cerevisiae. We find that overproduction of the disaggregating chaperone, Hsp104, increases the frequency of de novo [URE3] prion formation by the Ure2p of S. cerevisiae and that of C. albicans. This stimulation is strongly dependent on the presence of the [PIN(+)] prion, known from previous work to enhance [URE3] prion generation. Our data suggest that transient Hsp104 overproduction enhances prion generation through persistent effects on Rnq1 amyloid, as well as during overproduction by disassembly of amorphous Ure2 aggregates (generated during Ure2p overproduction), driving the aggregation toward the amyloid pathway. Overproduction of other major cytosolic chaperones of the Hsp70 and Hsp40 families (Ssa1p, Sse1p, and Ydj1p) inhibit prion formation, whereas another yeast Hsp40, Sis1p, modulates the effects of Hsp104p on both prion induction and prion curing in a prion-specific manner. The same factor may both enhance de novo prion generation and destabilize existing prion variants, suggesting that prion variants may be selected by changes in the chaperone network.     

In vitro analysis of SpUre2p, a prion-related protein, exemplifies the relationship between amyloid and prion

The yeast Saccharomyces cerevisiae contains in its proteome at least three prion proteins. These proteins (Ure2p, Sup35p, and Rnq1p) share a set of remarkable properties. In vivo, they form aggregates that self-perpetuate their aggregation. This aggregation is controlled by Hsp104, which plays a major role in the growth and severing of these prions. In vitro, these prion proteins form amyloid fibrils spontaneously. The introduction of such fibrils made from Ure2p or Sup35p into yeast cells leads to the prion phenotypes [URE3] and [PSI], respectively. Previous studies on evolutionary biology of yeast prions have clearly established that [URE3] is not well conserved in the hemiascomycetous yeasts and particularly in S. paradoxus. Here we demonstrated that the S. paradoxus Ure2p is able to form infectious amyloid. These fibrils are more resistant than S. cerevisiae Ure2p fibrils to shear force. The observation, in vivo, of a distinct aggregation pattern for GFP fusions confirms the higher propensity of SpUre2p to form fibrillar structures. Our in vitro and in vivo analysis of aggregation propensity of the S. paradoxus Ure2p provides an explanation for its loss of infective properties and suggests that this protein belongs to the non-prion amyloid world.     

Structure and Assembly Properties of the N-Terminal Domain of the Prion Ure2p in Isolation and in Its Natural Context

Background

The aggregation of the baker''s yeast prion Ure2p is at the origin of the [URE3] trait. The Q- and N-rich N-terminal part of the protein is believed to drive Ure2p assembly into fibrils of amyloid nature and the fibrillar forms of full-length Ure2p and its N-terminal part generated in vitro have been shown to induce [URE3] occurrence when introduced into yeast cells. This has led to the view that the fibrillar form of the N-terminal part of the protein is sufficient for the recruitment of constitutive Ure2p and that it imprints its amyloid structure to full-length Ure2p.

Results

Here we generate a set of Ure2p N-terminal fragments, document their assembly and structural properties and compare them to that of full-length Ure2p. We identify the minimal region critical for the assembly of Ure2p N-terminal part into amyloids and show that such fibrils are unable to seed the assembly of full length Ure2p unlike fibrils made of intact Ure2p.

Conclusion

Our results clearly indicate that fibrillar Ure2p shares no structural similarities with the amyloid fibrils made of Ure2p N-terminal part. Our results further suggest that the induction of [URE3] by fibrils made of full-length Ure2p is likely the consequence of fibrils growth by depletion of cytosolic Ure2p while it is the consequence of de novo formation of prion particles following, for example, titration within the cells of a specific set of molecular chaperones when fibrils made of Ure2p N-terminal domain are introduced within the cytoplasm.     

Structure of the prion Ure2p in protein fibrils assembled in vitro

The Ure2 protein from the yeast Saccharomyces cerevisiae has prion properties. In vitro and at neutral pH, soluble Ure2p spontaneously forms long, straight, insoluble protein fibrils. Two models have been proposed to account for the assembly of Ure2p into protein fibrils. The "amyloid backbone" model postulates that a segment ranging from 40 to 70 amino acids in the flexible N-terminal domain from different Ure2p molecules forms a parallel superpleated beta-structure running along the fibrils. The second model hypothesizes that assembly of full-length Ure2p is driven by limited conformational rearrangements and non-native inter- and/or intramolecular interactions between Ure2p monomers. Here, we performed a cysteine scan on residues located in the N- and C-terminal parts of Ure2p to determine whether these domains interact. Amino acid sequences centered around residue 6 in the N-terminal domain of Ure2p and residue 137 in the C-terminal moiety interacted at least transiently via intramolecular interactions. We documented the assembly properties of a Ure2p variant in which a disulfide bond was established between the N- and C-terminal domains and showed that it possesses assembly properties indistinguishable from those of wild-type Ure2p. We probed the structure of Ure2pC6C137 within the fibrils and demonstrate that the polypeptide is in a conformation similar to that of its soluble assembly-competent state. Our results constitute the first structural characterization of the N-terminal domain of Ure2p in both its soluble assembly-competent and fibrillar forms. Our data indicate that the flexibility of the N-terminal domain and conformational changes within this domain are essential for fibril formation and provide new insight into the conformational rearrangements that lead to the assembly of Ure2p into fibrils and the propagation of the [URE3] phenotype in yeast.     

Prion Variants and Species Barriers Among Saccharomyces Ure2 Proteins

As hamster scrapie cannot infect mice, due to sequence differences in their PrP proteins, we find “species barriers” to transmission of the [URE3] prion in Saccharomyces cerevisiae among Ure2 proteins of S. cerevisiae, paradoxus, bayanus, cariocanus, and mikatae on the basis of differences among their Ure2p prion domain sequences. The rapid variation of the N-terminal Ure2p prion domains results in protection against the detrimental effects of infection by a prion, just as the PrP residue 129 Met/Val polymorphism may have arisen to protect humans from the effects of cannibalism. Just as spread of bovine spongiform encephalopathy prion variant is less impaired by species barriers than is sheep scrapie, we find that some [URE3] prion variants are infectious to another yeast species while other variants (with the identical amino acid sequence) are not. The species barrier is thus prion variant dependent as in mammals. [URE3] prion variant characteristics are maintained even on passage through the Ure2p of another species. Ure2p of Saccharomyces castelli has an N-terminal Q/N-rich “prion domain” but does not form prions (in S. cerevisiae) and is not infected with [URE3] from Ure2p of other Saccharomyces. This implies that conservation of its prion domain is not for the purpose of forming prions. Indeed the Ure2p prion domain has been shown to be important, though not essential, for the nitrogen catabolism regulatory role of the protein.     

Ure2p function is enhanced by its prion domain in Saccharomyces cerevisiae

The Ure2 protein of Saccharomyces cerevisiae can become a prion (infectious protein). At very low frequencies Ure2p forms an insoluble, infectious amyloid known as [URE3], which is efficiently transmitted to progeny cells or mating partners that consequently lose the normal Ure2p nitrogen regulatory function. The [URE3] prion causes yeast cells to grow slowly, has never been identified in the wild, and confers no obvious phenotypic advantage. An N-terminal asparagine-rich domain determines Ure2p prion-forming ability. Since ure2Delta strains are complemented by plasmids that overexpress truncated forms of Ure2p lacking the prion domain, the existence of the [URE3] prion and the evolutionary conservation of an N-terminal extension have remained mysteries. We find that Ure2p function is actually compromised in vivo by truncation of the prion domain. Moreover, Ure2p stability is diminished without the full-length prion domain. Mca1p, like Ure2p, has an N-terminal Q/N-rich domain whose deletion reduces its steady-state levels. Finally, we demonstrate that the prion domain may affect the interaction of Ure2p with other components of the nitrogen regulation system, specifically the negative regulator of nitrogen catabolic genes, Gzf3p.     

The [URE3] yeast prion results from protein aggregates that differ from amyloid filaments formed in vitro

The [URE3] yeast prion is a self-propagating inactive form of the Ure2 protein. Ure2p is composed of two domains, residues 1-93, the prion-forming domain, and the remaining C-terminal part of the protein, which forms the functional domain involved in nitrogen catabolite repression. In vitro, Ure2p forms amyloid filaments that have been proposed to be the aggregated prion form found in vivo. Here we showed that the biochemical characteristics of these two species differ. Protease digestions of Ure2p filaments and soluble Ure2p are comparable when analyzed by Coomassie staining as by Western blot. However, this finding does not explain the pattern specifically observed in [URE3] strains. Antibodies raised against the C-terminal part of Ure2p revealed the existence of proteolysis sites efficiently cleaved when [URE3], but not wild-type crude extracts, were submitted to limited proteolysis. The same antibodies lead to an equivalent digestion pattern when recombinant Ure2p (either soluble or amyloid) was analyzed in the same way. These results strongly suggest that aggregated Ure2p in [URE3] yeast cells is different from the amyloid filaments generated in vitro.     

Amyloid of the Candida albicans Ure2p prion domain is infectious and has an in-register parallel β-sheet structure

Ure2p of Candida albicans (Ure2(albicans) or CaUre2p) can be a prion in Saccharomyces cerevisiae, but Ure2p of Candida glabrata (Ure2(glabrata)) cannot, even though the Ure2(glabrata) N-terminal domain is more similar to that of the S. cerevisiae Ure2p (Ure2(cerevisiae)) than Ure2(albicans) is. We show that the N-terminal N/Q-rich prion domain of Ure2(albicans) forms amyloid that is infectious, transmitting [URE3alb] to S. cerevisiae cells expressing only C. albicans Ure2p. Using solid-state nuclear magnetic resonance of selectively labeled C. albicans Ure2p(1-90), we show that this infectious amyloid has an in-register parallel β-sheet structure, like that of the S. cerevisiae Ure2p prion domain and other S. cerevisiae prion amyloids. In contrast, the N/Q-rich N-terminal domain of Ure2(glabrata) does not readily form amyloid, and that formed upon prolonged incubation is not infectious.     

The yeast prion Ure2p native-like assemblies are toxic to mammalian cells regardless of their aggregation state

The yeast prion Ure2p assembles in vitro into oligomers and fibrils retaining the alpha-helix content and binding properties of the soluble protein. Here we show that the different forms of Ure2p native-like assemblies (dimers, oligomers, and fibrils) are similarly toxic to murine H-END cells when added to the culture medium. Interestingly, the amyloid fibrils obtained by heat treatment of the toxic native-like fibrils appear harmless. Moreover, the Ure2p C-terminal domain, lacking the N-terminal segment necessary for aggregation but containing the glutathione binding site, is not cytotoxic. This finding strongly supports the idea that Ure2p toxicity depends on the structural properties of the flexible N-terminal prion domain and can therefore be considered as an inherent feature of the protein, unrelated to its aggregation state but rather associated with a basic toxic fold shared by all of the Ure2p native-like assemblies. Indeed, the latter are able to interact with the cell surface, leading to alteration of calcium homeostasis, membrane permeabilization, and oxidative stress, whereas the heat-treated amyloid fibrils do not. Our results support the idea of a general mechanism of toxicity of any protein/peptide aggregate endowed with structural features, making it able to interact with cell membranes and to destabilize them. This evidence extends the widely accepted view that the toxicity by protein aggregates is restricted to amyloid prefibrillar aggregates and provides new insights into the mechanism by which native-like oligomers compromise cell viability.     

Amyloid nucleation and hierarchical assembly of Ure2p fibrils. Role of asparagine/glutamine repeat and nonrepeat regions of the prion domains

The yeast prion protein Ure2 forms amyloid-like filaments in vivo and in vitro. This ability depends on the N-terminal prion domain, which contains Asn/Gln repeats, a motif thought to cause human disease by forming stable protein aggregates. The Asn/Gln region of the Ure2p prion domain extends to residue 89, but residues 15-42 represent an island of "normal" random sequence, which is highly conserved in related species and is relatively hydrophobic. We compare the time course of structural changes monitored by thioflavin T (ThT) binding fluorescence and atomic force microscopy for Ure2 and a series of prion domain mutants under a range of conditions. Atomic force microscopy height images at successive time points during a single growth experiment showed the sequential appearance of at least four fibril types that could be readily differentiated by height (5, 8, 12, or 9 nm), morphology (twisted or smooth), and/or time of appearance (early or late in the plateau phase of ThT binding). The Ure2 dimer (h = 2.6 +/- 0.5 nm) and granular particles corresponding to higher order oligomers (h = 4-12 nm) could also be detected. The mutants 15Ure2 and Delta 15-42Ure2 showed the same time-dependent variation in fibril types but with an increased lag time detected by ThT binding compared with wild-type Ure2. In addition, Delta 15-42Ure2 showed reduced binding to ThT. The results imply a role of the conserved region in both amyloid nucleation and formation of the binding surface recognized by ThT. Further, Ure2 amyloid formation is a multistep process via a series of fibrillar intermediates.     

The mechanisms of [URE3] prion elimination demonstrate that large aggregates of Ure2p are dead-end products

The yeast prion [URE3] is a self-propagating inactive form (the propagon) of the Ure2 protein. Ure2p is composed of two domains: residues 1-93--the prion-forming domain (PFD)--and the remaining C-terminal part of the protein, which forms the functional domain involved in nitrogen catabolite repression. Guanidine hydrochloride, and the overproduction of Ure2p 1-65 or Ure2-GFP have been shown to induce the elimination of [URE3]. We demonstrate here, two different curing mechanisms: the inhibition of [URE3] replication by guanidine hydrochloride and its destruction by Ure2p aggregation. Such aggregation is observed if PFD or Ure2-GFP are overproduced and in heterozygous URE2/URE2-GFP, [URE3] diploids. We found that the GFP foci associated with the presence of the prion were dead-end products, the propagons remaining soluble. Surprisingly, [URE3] propagated via the Ure2-GFP fusion protein alone is resistant to these two curing mechanisms and cannot promote the formation of foci. The relationship between aggregation, prion and Hsp104 gives rise to a model in which the propagon is in equilibrium with larger aggregates and functional protein.     

Flexibility of the Ure2 prion domain is important for amyloid fibril formation

Ure2, the protein determinant of the Saccharomyces cerevisiae prion [URE3], has a natively disordered N-terminal domain that is important for prion formation in vivo and amyloid formation in vitro; the globular C-domain has a glutathione transferase-like fold. In the present study, we swapped the position of the N- and C-terminal regions, with or without an intervening peptide linker, to create the Ure2 variants CLN-Ure2 and CN-Ure2 respectively. The native structural content and stability of the variants were the same as wild-type Ure2, as indicated by enzymatic activity, far-UV CD analysis and equilibrium denaturation. CLN-Ure2 was able to form amyloid-like fibrils, but with a significantly longer lag time than wild-type Ure2; and the two proteins were unable to cross-seed. Under the same conditions, CN-Ure2 showed limited ability to form fibrils, but this was improved after addition of 0.03?M guanidinium chloride. As for wild-type Ure2, allosteric enzyme activity was observed in fibrils of CLN-Ure2 and CN-Ure2, consistent with retention of the native-like dimeric structure of the C-domains within the fibrils. Proteolytically digested fibrils of CLN-Ure2 and CN-Ure2 showed the same residual fibril core morphology as wild-type Ure2. The results suggest that the position of the prion domain affects the ability of Ure2 to form fibrils primarily due to effects on its flexibility.     

Solid-support electron paramagnetic resonance (EPR) studies of Aβ40 monomers reveal a structured state with three ordered segments

Alzheimer disease is associated with the pathological accumulation of amyloid-β peptide (Aβ) in the brain. Soluble Aβ oligomers formed during early aggregation process are believed to be neurotoxins and causative agents in Alzheimer disease. Aβ monomer is the building block for amyloid assemblies. A comprehensive understanding of the structural features of Aβ monomer is crucial for delineating the mechanism of Aβ oligomerization. Here we investigated the structures of Aβ40 monomer using a solid-support approach, in which Aβ40 monomers are tethered on the solid support via an N-terminal His tag to prevent further aggregation. EPR spectra of tethered Aβ40 with spin labels at 18 different positions show that Aβ40 monomers adopt a completely disordered structure under denaturing conditions. Under native conditions, however, EPR spectra suggest that Aβ40 monomers adopt both a disordered state and a structured state. The structured state of Aβ40 monomer has three more ordered segments at 14-18, 29-30, and 38-40. Interactions between these segments may stabilize the structured state, which likely plays an important role in Aβ aggregation.     

The [URE3] phenotype: evidence for a soluble prion in yeast

The aggregation of the two yeast proteins Sup35p and Ure2p is widely accepted as a model for explaining the prion propagation of the phenotypes [PSI⁺] and [URE3], respectively. Here, we demonstrate that the propagation of [URE3] cannot simply be the consequence of generating large aggregates of Ure2p, because such aggregation can be found in some conditions that are not related to the prion state of Ure2p. A comparison of [PSI⁺] and [URE3] aggregation demonstrates differences between these two prion mechanisms. Our findings lead us to propose a new unifying model for yeast prion propagation.