Amyloidogenesis of bacterial prionoid RepA-WH1 recapitulates dimer to monomer transitions of RepA in DNA replication initiation

Human RAD51 Protein Forms Amyloid-like Aggregates In Vitro

RAD51 is a central protein of homologous recombination and DNA repair processes that maintains genome stability and ensures the accurate repair of double-stranded breaks (DSBs). In this work, we assessed amyloid properties of RAD51 in vitro and in the bacterial curli-dependent amyloid generator (C-DAG) system. Resistance to ionic detergents, staining with amyloid-specific dyes, polarized microscopy, transmission electron microscopy (TEM), X-ray diffraction and other methods were used to evaluate the properties and structure of RAD51 aggregates. The purified human RAD51 protein formed detergent-resistant aggregates in vitro that had an unbranched cross-β fibrillar structure, which is typical for amyloids, and were stained with amyloid-specific dyes. Congo-red-stained RAD51 aggregates demonstrated birefringence under polarized light. RAD51 fibrils produced sharp circular X-ray reflections at 4.7 Å and 10 Å, demonstrating that they had a cross-β structure. Cytoplasmic aggregates of RAD51 were observed in cell cultures overexpressing RAD51. We demonstrated that a key protein that maintains genome stability, RAD51, has amyloid properties in vitro and in the C-DAG system and discussed the possible biological relevance of this observation.

Evaluation of the Role of Bacterial Amyloid on Nucleoid Structure Using Cryo-Soft X-Ray Tomography

Bacterial chromosomal DNA is packed within a non-membranous structure, the nucleoid, thanks to nucleoid associated proteins (NAPs). The role of bacterial amyloid has recently emerged among these NAPs, particularly with the nucleoid-associated protein Hfq that plays a direct role in DNA compaction. In this chapter, we present a 3D imaging technique, cryo-soft X-ray tomography (cryo-SXT) to obtain a detailed 3D visualization of subcellular bacterial structures, especially the nucleoid. Cryo-SXT imaging of native unlabeled cells enables observation of the nucleoid in 3D with a high resolution, allowing to evidence in vivo the role of amyloids on DNA compaction. The precise experimental methods to obtain 3D tomograms will be presented.

Cryo-electron Microscopy to Analyze the Structure of Bacterial Amyloids In Vitro

Amyloid fibrils are aggregates of proteins or peptides. In humans, they are associated with various pathologies ranging from neurodegenerative diseases such as Alzheimer's and Parkinson's to systemic diseases like type 2 diabetes. In bacteria, amyloids can exert functional roles such as biofilm formation or gene regulation. Up to now, the aggregation mechanism leading to amyloid fibril formation is poorly understood as proteins with different amino acid sequences can fold into similar 3D structures. Understanding the formation of amyloid fibrils constitutes a central challenge for fighting major human health issues such as neurodegenerative diseases and biofilm formation in ports (implantable chambers). Since the dogma linking protein sequence, 3D structure, and function is increasingly disrupted by the growing understanding of the importance of disordered domains in proteins, it is crucial to possess a method capable of building accurate atomic models of amyloids. Aided by the leap forward of cryo-electron microscopy (cryo-EM), which can now routinely achieve sub-nanometric resolutions, it has become the method of choice for studying amyloids. In this chapter, we use the Hfq protein from Escherichia coli as an example to present general protocols in cryo-EM to unveil the structure of bacterial amyloids and improve our knowledge of their aggregation mechanism.

Amyloid and Amyloid-Like Aggregates: Diversity and the Term Crisis

Active accumulation of the data on new amyloids continuing nowadays dissolves boundaries of the term "amyloid". Currently, it is most often used to designate aggregates with cross-β structure. At the same time, amyloids also exhibit a number of other unusual properties, such as: detergent and protease resistance, interaction with specific dyes, and ability to induce transition of some proteins from a soluble form to an aggregated one. The same features have been also demonstrated for the aggregates lacking cross-β structure, which are commonly called "amyloid-like" and combined into one group, although they are very diverse. We have collected and systematized information on the properties of more than two hundred known amyloids and amyloid-like proteins with emphasis on conflicting examples. In particular, a number of proteins in membraneless organelles form aggregates with cross-β structure that are morphologically indistinguishable from the other amyloids, but they can be dissolved in the presence of detergents, which is not typical for amyloids. Such paradoxes signify the need to clarify the existing definition of the term amyloid. On the other hand, the demonstrated structural diversity of the amyloid-like aggregates shows the necessity of their classification.

SynBio and the Boundaries between Functional and Pathogenic RepA-WH1 Bacterial Amyloids

Amyloids are protein polymers that were initially linked to human diseases. Across the whole Tree of Life, many disease-unrelated proteins are now emerging for which amyloids represent distinct functional states. Most bacterial amyloids described are extracellular, contributing to biofilm formation. However, only a few have been found in the bacterial cytosol. This paper reviews from the perspective of synthetic biology (SynBio) our understanding of the subtle line that separates functional from pathogenic and transmissible amyloids (prions).

Amyloids are protein polymers that were initially linked to human diseases. Across the whole Tree of Life, many disease-unrelated proteins are now emerging for which amyloids represent distinct functional states. Most bacterial amyloids described are extracellular, contributing to biofilm formation. However, only a few have been found in the bacterial cytosol. This paper reviews from the perspective of synthetic biology (SynBio) our understanding of the subtle line that separates functional from pathogenic and transmissible amyloids (prions). In particular, it is focused on RepA-WH1, a functional albeit unconventional natural amyloidogenic protein domain that participates in controlling DNA replication of bacterial plasmids. SynBio approaches, including protein engineering and the design of allosteric effectors such as diverse ligands and an optogenetic module, have enabled the generation in RepA-WH1 of an intracellular cytotoxic prion-like agent in bacteria. The synthetic RepA-WH1 prion has the potential to develop into novel antimicrobials.

Intercellular Transmission of a Synthetic Bacterial Cytotoxic Prion-Like Protein in Mammalian Cells

Proteotoxic amyloid seeds can be transmitted between mammalian cells, arguing that the intercellular exchange of prion-like protein aggregates can be a common phenomenon. RepA-WH1 is derived from a bacterial intracellular functional amyloid protein, engineered to become cytotoxic in Escherichia coli. Here, we have studied if such bacterial aggregates can also be transmitted to, and become cytotoxic to, mammalian cells. We demonstrate that RepA-WH1 is capable of entering naive cells, thereby inducing the cytotoxic aggregation of a soluble RepA-WH1 variant expressed in the cytosol, following the same trend that had been described in bacteria. These findings highlight the universality of one of the central principles underlying prion biology: No matter the biological origin of a given prion-like protein, it can be transmitted to a phylogenetically unrelated recipient cell, provided that the latter expresses a soluble protein onto which the incoming protein can readily template its amyloid conformation.

RepA is a bacterial protein that builds intracellular amyloid oligomers acting as inhibitory complexes of plasmid DNA replication. When carrying a mutation enhancing its amyloidogenesis (A31V), the N-terminal domain (WH1) generates cytosolic amyloid particles that are inheritable within a bacterial lineage. Such amyloids trigger in bacteria a lethal cascade reminiscent of mitochondrial impairment in human cells affected by neurodegeneration. To fulfill all the criteria to qualify as a prion-like protein, horizontal (intercellular) transmissibility remains to be demonstrated for RepA-WH1. Since this is experimentally intractable in bacteria, here we transiently expressed in a murine neuroblastoma cell line the soluble, barely cytotoxic RepA-WH1 wild type [RepA-WH1(WT)] and assayed its response to exposure to in vitro-assembled RepA-WH1(A31V) amyloid fibers. In parallel, murine cells releasing RepA-WH1(A31V) aggregates were cocultured with human neuroblastoma cells expressing RepA-WH1(WT). Both the assembled fibers and donor-derived RepA-WH1(A31V) aggregates induced, in the cytosol of recipient cells, the formation of cytotoxic amyloid particles. Mass spectrometry analyses of the proteomes of both types of injured cells pointed to alterations in mitochondria, protein quality triage, signaling, and intracellular traffic. Thus, a synthetic prion-like protein can be propagated to, and become cytotoxic to, cells of organisms placed at such distant branches of the tree of life as bacteria and mammalia, suggesting that mechanisms of protein aggregate spreading and toxicity follow default pathways.

Conformational Priming of RepA-WH1 for Functional Amyloid Conversion Detected by NMR Spectroscopy

How proteins with a stable globular fold acquire the amyloid state is still largely unknown. RepA, a versatile plasmidic DNA binding protein from Pseudomonas savastanoi, is functional as a transcriptional repressor or as an initiator or inhibitor of DNA replication, the latter via assembly of an amyloidogenic oligomer. Its N-terminal domain (WH1) is responsible for discrimination between these functional abilities by undergoing insufficiently understood structural changes. RepA-WH1 is a stable dimer whose conformational dynamics had not been explored. Here, we have studied it through NMR {¹H}-¹⁵N relaxation and H/D exchange kinetics measurements. The N- and the C-terminal α-helices, and the internal amyloidogenic loop, are partially unfolded in solution. S4-indigo, a small inhibitor of RepA-WH1 amyloidogenesis, binds to and tethers the N-terminal α-helix to a β-hairpin that is involved in dimerization, thus providing evidence for a priming role of fraying ends and dimerization switches in the amyloidogenesis of folded proteins.

Infinite Assembly of Folded Proteins in Evolution, Disease, and Engineering

Mutations and changes in a protein's environment are well known for their potential to induce misfolding and aggregation, including amyloid formation. Alternatively, such perturbations can trigger new interactions that lead to the polymerization of folded proteins. In contrast to aggregation, this process does not require misfolding and, to highlight this difference, we refer to it as agglomeration. This term encompasses the amorphous assembly of folded proteins as well as the polymerization in one, two, or three dimensions. We stress the remarkable potential of symmetric homo-oligomers to agglomerate even by single surface point mutations, and we review the double-edged nature of this potential: how aberrant assemblies resulting from agglomeration can lead to disease, but also how agglomeration can serve in cellular adaptation and be exploited for the rational design of novel biomaterials.

Optogenetic Navigation of Routes Leading to Protein Amyloidogenesis in Bacteria

Modulation of liquid-liquid and liquid-hydrogel phase transitions is central to avoid the cytotoxic aggregation of proteins in eukaryotic cells, but knowledge on its relevance in bacteria is limited. Here the power of optogenetics to engineer proteins as light-responsive switches has been used to control the balance between solubility and aggregation for LOV2-WH1, a chimera between the plant blue light-responsive domain LOV2 and the bacterial prion-like protein RepA-WH1. These proteins were first linked by fusing, as a continuous α-helix, the C-terminal photo-transducer Jα helix in LOV2 with the N-terminal domain-closure α1 helix in RepA-WH1, and then improved for light-responsiveness by including mutations in the Jα moiety. In the darkness and in a crowded solution in vitro, LOV2-WH1 nucleates the irreversible assembly of amyloid fibers into a hydrogel. However, under blue light illumination, LOV2-WH1 assembles as soluble oligomers. When expressed in Escherichia coli, LOV2-WH1 forms in the darkness large intracellular amyloid inclusions compatible with bacterial proliferation. Strikingly, under blue light, LOV2-WH1 aggregates decrease in size, while they become detrimental for bacterial growth. LOV2-WH1 optogenetics governs the assembly of mutually exclusive inert amyloid fibers or cytotoxic oligomers, thus enabling the navigation of the conformational landscape of protein amyloidogenesis to generate potential photo-activated anti-bacterial devices (optobiotics).

Modulation of the Aggregation of the Prion-like Protein RepA-WH1 by Chaperones in a Cell-Free Expression System and in Cytomimetic Lipid Vesicles

The accumulation of aggregated forms of proteins as toxic species is associated with fatal diseases such as amyloid proteinopathies. With the purpose of deconstructing the molecular mechanisms of these type of diseases through a Synthetic Biology approach, we are working with a model bacterial prion-like protein, RepA-WH1, expressed in a cell-free system. Our findings show that the Hsp70 chaperone from Escherichia coli, together with its Hsp40 and nucleotide exchange factor cochaperones, modulates the aggregation of the prion-like protein in the cell-free system. Moreover, we observe the same effect by reconstructing the aggregation process inside lipid vesicles. Chaperones reduce the number of aggregates formed, matching previous findings in vivo. We expect that the in vitro approach reported here will help to achieve better understanding and control of amyloid proteinopathies.

Addressing Intracellular Amyloidosis in Bacteria with RepA-WH1, a Prion-Like Protein

Bacteria are the simplest cellular model in which amyloidosis has been addressed. It is well documented that bacterial consortia (biofilms) assemble their extracellular matrix on an amyloid scaffold, yet very few intracellular amyloids are known in bacteria. Here, we describe the methods we have resorted to characterize in Escherichia coli cells the amyloidogenesis, propagation, and dynamics of the RepA-WH1 prionoid. This prion-like protein, a manifold domain from the plasmid replication protein RepA, itself capable of assembling a functional amyloid, causes when expressed in E. coli a synthetic amyloid proteinopathy, the first model for an amyloid disease with a purely bacterial origin. These protocols are useful to study other intracellular amyloids in bacteria.

Outlining Core Pathways of Amyloid Toxicity in Bacteria with the RepA-WH1 Prionoid

The synthetic bacterial prionoid RepA-WH1 causes a vertically transmissible amyloid proteinopathy in Escherichia coli that inhibits growth and eventually kills the cells. Recent in vitro studies show that RepA-WH1 builds pores through model lipid membranes, suggesting a possible mechanism for bacterial cell death. By comparing acutely (A31V) and mildly (ΔN37) cytotoxic mutant variants of the protein, we report here that RepA-WH1(A31V) expression decreases the intracellular osmotic pressure and compromise bacterial viability under either aerobic or anaerobic conditions. Both are effects expected from threatening membrane integrity and are in agreement with findings on the impairment by RepA-WH1(A31V) of the proton motive force (PMF)-dependent transport of ions (Fe³⁺) and ATP synthesis. Systems approaches reveal that, in aerobiosis, the PMF-independent respiratory dehydrogenase NdhII is induced in response to the reduction in intracellular levels of iron. While NdhII is known to generate H₂O₂ as a by-product of the autoxidation of its FAD cofactor, key proteins in the defense against oxidative stress (OxyR, KatE), together with other stress-resistance factors, are sequestered by co-aggregation with the RepA-WH1(A31V) amyloid. Our findings suggest a route for RepA-WH1 toxicity in bacteria: a primary hit of damage to the membrane, compromising bionergetics, triggers a stroke of oxidative stress, which is exacerbated due to the aggregation-dependent inactivation of enzymes and transcription factors that enable the cellular response to such injury. The proteinopathy caused by the prion-like protein RepA-WH1 in bacteria recapitulates some of the core hallmarks of human amyloid diseases.

RepA-WH1 prionoid: Clues from bacteria on factors governing phase transitions in amyloidogenesis

In bacterial plasmids, Rep proteins initiate DNA replication by undergoing a structural transformation coupled to dimer dissociation. Amyloidogenesis of the ‘winged-helix’ N-terminal domain of RepA (WH1) is triggered in vitro upon binding to plasmid-specific DNA sequences, and occurs at the bacterial nucleoid in vivo. Amyloid fibers are made of distorted RepA-WH1 monomers that assemble as single or double intertwined tubular protofilaments. RepA-WH1 causes in E. coli an amyloid proteinopathy, which is transmissible from mother to daughter cells, but not infectious, and enables conformational imprinting in vitro and in vivo; i.e. RepA-WH1 is a ‘prionoid’. Microfluidics allow the assessment of the intracellular dynamics of RepA-WH1: bacterial lineages maintain two types (strains-like) of RepA-WH1 amyloids, either multiple compact cytotoxic particles or a single aggregate with the appearance of a fluidized hydrogel that it is mildly detrimental to growth. The Hsp70 chaperone DnaK governs the phase transition between both types of RepA-WH1 aggregates in vivo, thus modulating the vertical propagation of the prionoid. Engineering chimeras between the Sup35p/[PSI⁺] prion and RepA-WH1 generates [REP-PSI⁺], a synthetic prion exhibiting strong and weak phenotypic variants in yeast. These recent findings on a synthetic, self-contained bacterial prionoid illuminate central issues of protein amyloidogenesis.

Nucleation of Amyloid Oligomers by RepA-WH1-Prionoid-Functionalized Gold Nanorods

Understanding protein amyloidogenesis is an important topic in protein science, fueled by the role of amyloid aggregates, especially oligomers, in the etiology of a number of devastating human degenerative diseases. However, the mechanisms that determine the formation of amyloid oligomers remain elusive due to the high complexity of the amyloidogenesis process. For instance, gold nanoparticles promote or inhibit amyloid fibrillation. We have functionalized gold nanorods with a metal-chelating group to selectively immobilize soluble RepA-WH1, a model synthetic bacterial prionoid, using a hexa-histidine tag (H6). H6-RepA-WH1 undergoes stable amyloid oligomerization in the presence of catalytic concentrations of anisotropic nanoparticles. Then, in a physically separated event, such oligomers promote the growth of amyloid fibers of untagged RepA-WH1. SERS spectral changes of H6-RepA-WH1 on spherical citrate-AuNP substrates provide evidence for structural modifications in the protein, which are compatible with a gradual increase in β-sheet structure, as expected in amyloid oligomerization.

Mechanism of Amyloidogenesis of a Bacterial AAA+ Chaperone

Amyloids are fibrillar protein superstructures that are commonly associated with diseases in humans and with physiological functions in various organisms. The precise mechanisms of amyloid formation remain to be elucidated. Surprisingly, we discovered that a bacterial Escherichia coli chaperone-like ATPase, regulatory ATPase variant A (RavA), and specifically the LARA domain in RavA, forms amyloids under acidic conditions at elevated temperatures. RavA is involved in modulating the proper assembly of membrane respiratory complexes. LARA contains an N-terminal loop region followed by a β-sandwich-like folded core. Several approaches, including nuclear magnetic resonance spectroscopy and molecular dynamics simulations, were used to determine the mechanism by which LARA switches to an amyloid state. These studies revealed that the folded core of LARA is amyloidogenic and is protected by its N-terminal loop. At low pH and high temperatures, the interaction of the N-terminal loop with the folded core is disrupted, leading to amyloid formation.

Functional amyloids as inhibitors of plasmid DNA replication

DNA replication is tightly regulated to constrain the genetic material within strict spatiotemporal boundaries and copy numbers. Bacterial plasmids are autonomously replicating DNA molecules of much clinical, environmental and biotechnological interest. A mechanism used by plasmids to prevent over-replication is 'handcuffing', i.e. inactivating the replication origins in two DNA molecules by holding them together through a bridge built by a plasmid-encoded initiator protein (Rep). Besides being involved in handcuffing, the WH1 domain in the RepA protein assembles as amyloid fibres upon binding to DNA in vitro. The amyloid state in proteins is linked to specific human diseases, but determines selectable and epigenetically transmissible phenotypes in microorganisms. Here we have explored the connection between handcuffing and amyloidogenesis of full-length RepA. Using a monoclonal antibody specific for an amyloidogenic conformation of RepA-WH1, we have found that the handcuffed RepA assemblies, either reconstructed in vitro or in plasmids clustering at the bacterial nucleoid, are amyloidogenic. The replication-inhibitory RepA handcuff assembly is, to our knowledge, the first protein amyloid directly dealing with DNA. Built on an amyloid scaffold, bacterial plasmid handcuffs can bring a novel molecular solution to the universal problem of keeping control on DNA replication initiation.

Electrostatically-guided inhibition of Curli amyloid nucleation by the CsgC-like family of chaperones

Polypeptide aggregation into amyloid is linked with several debilitating human diseases. Despite the inherent risk of aggregation-induced cytotoxicity, bacteria control the export of amyloid-prone subunits and assemble adhesive amyloid fibres during biofilm formation. An Escherichia protein, CsgC potently inhibits amyloid formation of curli amyloid proteins. Here we unlock its mechanism of action, and show that CsgC strongly inhibits primary nucleation via electrostatically-guided molecular encounters, which expands the conformational distribution of disordered curli subunits. This delays the formation of higher order intermediates and maintains amyloidogenic subunits in a secretion-competent form. New structural insight also reveal that CsgC is part of diverse family of bacterial amyloid inhibitors. Curli assembly is therefore not only arrested in the periplasm, but the preservation of conformational flexibility also enables efficient secretion to the cell surface. Understanding how bacteria safely handle amyloidogenic polypeptides contribute towards efforts to control aggregation in disease-causing amyloids and amyloid-based biotechnological applications.

RepA-WH1, the agent of an amyloid proteinopathy in bacteria, builds oligomeric pores through lipid vesicles

RepA-WH1 is a disease-unrelated protein that recapitulates in bacteria key aspects of human amyloid proteinopathies: i) It undergoes ligand-promoted amyloidogenesis in vitro; ii) its aggregates are able to seed/template amyloidosis on soluble protein molecules; iii) its conformation is modulated by Hsp70 chaperones in vivo, generating transmissible amyloid strains; and iv) causes proliferative senescence. Membrane disruption by amyloidogenic oligomers has been found for most proteins causing human neurodegenerative diseases. Here we report that, as for PrP prion and α-synuclein, acidic phospholipids also promote RepA-WH1 amyloidogenesis in vitro. RepA-WH1 molecules bind to liposomes, where the protein assembles oligomeric membrane pores. Fluorescent tracer molecules entrapped in the lumen of the vesicles leak through these pores and RepA-WH1 can then form large aggregates on the surface of the vesicles without inducing their lysis. These findings prove that it is feasible to generate in vitro a synthetic proteinopathy with a minimal set of cytomimetic components and support the view that cell membranes are primary targets in protein amyloidoses.

Unraveling structural polymorphism of amyloid fibers

Amyloid fibers are self-assembling cross β sheet structures whose overall molecular architecture is highly variable. In this issue of Structure, Torreira and colleagues use electron microscopy to resolve ultrastructural polymorphisms of RepA-WH1 amyloids.

Pre-amyloid oligomers of the proteotoxic RepA-WH1 prionoid assemble at the bacterial nucleoid

Upon binding to short specific dsDNA sequences in vitro, the N-terminal WH1 domain of the plasmid DNA replication initiator RepA assembles as amyloid fibres. These are bundles of single or double twisted tubular filaments in which distorted RepA-WH1 monomers are the building blocks. When expressed in Escherichia coli, RepA-WH1 triggers the first synthetic amyloid proteinopathy in bacteria, recapitulating some of the features of mammalian prion diseases: it is vertically transmissible, albeit non-infectious, showing up in at least two phenotypically distinct and interconvertible strains. Here we report B3h7, a monoclonal antibody specific for oligomers of RepA-WH1, but which does not recognize the mature amyloid fibres. Unlike a control polyclonal antibody generated against the soluble protein, B3h7 interferes in vitro with DNA-promoted or amyloid-seeded assembly of RepA-WH1 fibres, thus the targeted oligomers are on-pathway amyloidogenic intermediates. Immuno-electron microscopy with B3h7 on thin sections of E. coli cells expressing RepA-WH1 consistently labels the bacterial nucleoid, but not the large cytoplasmic aggregates of the protein. This observation points to the nucleoid as the place where oligomeric amyloid precursors of RepA-WH1 are generated, and suggests that, once nucleated by DNA, further growth must continue in the cytoplasm due to entropic exclusion.

Engineered bacterial hydrophobic oligopeptide repeats in a synthetic yeast prion, [REP-PSI (+)]

The yeast translation termination factor Sup35p, by aggregating as the [PSI (+)] prion, enables ribosomes to read-through stop codons, thus expanding the diversity of the Saccharomyces cerevisiae proteome. Yeast prions are functional amyloids that replicate by templating their conformation on native protein molecules, then assembling as large aggregates and fibers. Prions propagate epigenetically from mother to daughter cells by fragmentation of such assemblies. In the N-terminal prion-forming domain, Sup35p has glutamine/asparagine-rich oligopeptide repeats (OPRs), which enable propagation through chaperone-elicited shearing. We have engineered chimeras by replacing the polar OPRs in Sup35p by up to five repeats of a hydrophobic amyloidogenic sequence from the synthetic bacterial prionoid RepA-WH1. The resulting hybrid, [REP-PSI (+)], (i) was functional in a stop codon read-through assay in S. cerevisiae; (ii) generates weak phenotypic variants upon both its expression or transformation into [psi (-)] cells; (iii) these variants correlated with high molecular weight aggregates resistant to SDS during electrophoresis; and (iv) according to fluorescence microscopy, the fusion of the prion domains from the engineered chimeras to the reporter protein mCherry generated perivacuolar aggregate foci in yeast cells. All these are signatures of bona fide yeast prions. As assessed through biophysical approaches, the chimeras assembled as oligomers rather than as the fibers characteristic of [PSI (+)]. These results suggest that it is the balance between polar and hydrophobic residues in OPRs what determines prion conformational dynamics. In addition, our findings illustrate the feasibility of enabling new propagation traits in yeast prions by engineering OPRs with heterologous amyloidogenic sequence repeats.