Prions as protein-based genetic elements

Mrj is a chaperone of the Hsp40 family that regulates Orb2 oligomerization and long-term memory in Drosophila

Orb2 the Drosophila homolog of cytoplasmic polyadenylation element binding (CPEB) protein forms prion-like oligomers. These oligomers consist of Orb2A and Orb2B isoforms and their formation is dependent on the oligomerization of the Orb2A isoform. Drosophila with a mutation diminishing Orb2A's prion-like oligomerization forms long-term memory but fails to maintain it over time. Since this prion-like oligomerization of Orb2A plays a crucial role in the maintenance of memory, here, we aim to find what regulates this oligomerization. In an immunoprecipitation-based screen, we identify interactors of Orb2A in the Hsp40 and Hsp70 families of proteins. Among these, we find an Hsp40 family protein Mrj as a regulator of the conversion of Orb2A to its prion-like form. Mrj interacts with Hsp70 proteins and acts as a chaperone by interfering with the aggregation of pathogenic Huntingtin. Unlike its mammalian homolog, we find Drosophila Mrj is neither an essential gene nor causes any gross neurodevelopmental defect. We observe a loss of Mrj results in a reduction in Orb2 oligomers. Further, Mrj knockout exhibits a deficit in long-term memory and our observations suggest Mrj is needed in mushroom body neurons for the regulation of long-term memory. Our work implicates a chaperone Mrj in mechanisms of memory regulation through controlling the oligomerization of Orb2A and its association with the translating ribosomes.

Noncanonical usage of stop codons in ciliates expands proteins with structurally flexible Q-rich motifs

Serine(S)/threonine(T)-glutamine(Q) cluster domains (SCDs), polyglutamine (polyQ) tracts and polyglutamine/asparagine (polyQ/N) tracts are Q-rich motifs found in many proteins. SCDs often are intrinsically disordered regions that mediate protein phosphorylation and protein-protein interactions. PolyQ and polyQ/N tracts are structurally flexible sequences that trigger protein aggregation. We report that due to their high percentages of STQ or STQN amino acid content, four SCDs and three prion-causing Q/N-rich motifs of yeast proteins possess autonomous protein expression-enhancing activities. Since these Q-rich motifs can endow proteins with structural and functional plasticity, we suggest that they represent useful toolkits for evolutionary novelty. Comparative Gene Ontology (GO) analyses of the near-complete proteomes of 26 representative model eukaryotes reveal that Q-rich motifs prevail in proteins involved in specialized biological processes, including Saccharomyces cerevisiae RNA-mediated transposition and pseudohyphal growth, Candida albicans filamentous growth, ciliate peptidyl-glutamic acid modification and microtubule-based movement, Tetrahymena thermophila xylan catabolism and meiosis, Dictyostelium discoideum development and sexual cycles, Plasmodium falciparum infection, and the nervous systems of Drosophila melanogaster, Mus musculus and Homo sapiens. We also show that Q-rich-motif proteins are expanded massively in 10 ciliates with reassigned TAAQ and TAGQ codons. Notably, the usage frequency of CAGQ is much lower in ciliates with reassigned TAAQ and TAGQ codons than in organisms with expanded and unstable Q runs (e.g. D. melanogaster and H. sapiens), indicating that the use of noncanonical stop codons in ciliates may have coevolved with codon usage biases to avoid triplet repeat disorders mediated by CAG/GTC replication slippage.

Assembly and catalytic activity of short prion-inspired peptides

Enzymes play a crucial role in biochemical reactions, but their inherent structural instability limits their performance in industrial processes. In contrast, amyloid structures, known for their exceptional stability, are emerging as promising candidates for synthetic catalysis. This article explores the development of metal-decorated nanozymes formed by short peptides, inspired by prion-like domains. We detail the rational design of synthetic short Tyrosine-rich peptide sequences, focusing on their self-assembly into stable amyloid structures and their metallization with biologically relevant divalent metal cations, such as Cu2+, Ni2+, Co2+ and Zn2+. The provided experimental framework offers a step-by-step guide for researchers interested in exploring the catalytic potential of metal-decorated peptides. By bridging the gap between amyloid structures and catalytic function, these hybrid molecules open new avenues for developing novel metalloenzymes with potential applications in diverse chemical reactions.

Exploring the potential of amyloids in biomedical applications: A review

Amyloid is defined as a fibrous quaternary structure formed by assembling protein or peptide monomers into intermolecularly hydrogen linked β-sheets. There is a prevalent issue with protein aggregation and the buildup of amyloid molecules, which results in human neurological illnesses including Alzheimer's and Parkinson's. But it is now evident that many organisms, like bacteria, fungi as well as humans, use the same fibrillar structure to carry out a variety of biological functions, such as structure and protection supporting interface transitions and cell-cell recognition, protein control and storage, epigenetic inheritance, and memory. Recent discoveries of self-assembling amyloidogenic peptides and proteins, based on the amyloid core structure, give rise to interesting biomaterials with potential uses in numerous industries. These functions dramatically diverge from the initial conception of amyloid fibrils as intrinsically diseased entities. Apart from the natural ability of amyloids to spontaneously arrange themselves and their exceptional material characteristics, this aspect has prompted extensive research into engineering artificial amyloids for generating various nanostructures, molecular substances, and combined materials. Here, we discuss significant developments in the artificial design of useful amyloids as well as how amyloid materials serve as examples of how function emerges from protein self-assembly at various length scales.

Concentration and time-dependent amyloidogenic characteristics of intrinsically disordered N-terminal region of Saccharomyces cerevisiae Stm1

Saccharomyces cerevisiae Stm1 protein is a ribosomal association factor, which plays an important role in preserving ribosomes in a nutrition-deprived environment. It is also shown to take part in apoptosis-like cell death. Stm1 N-terminal region (Stm1_N1-113) is shown to recognize purine motif DNA triplex and G-quadruplex. Circular dichroism (CD) spectra of Stm1_N1-113 (enriched in positively-charged Lysine and Arginine; negatively-charged Aspartate; polar-uncharged Threonine, Asparagine, Proline and Serine; hydrophobic Alanine, Valine, and Glycine) collected after 0 and 24 h indicate that the protein assumes beta-sheet conformation at the higher concentrations in contrast to intrinsically disordered conformation seen for its monomeric form found in the crystal structure. Thioflavin-T kinetics experiments indicate that the lag phase is influenced by the salt concentration. Atomic force microscopy (AFM) images collected for a variety of Stm1_N1-113 concentrations (in the range of 1-400 μM) in the presence of 150 mM NaCl at 0, 24, and 48 h indicate a threshold concentration requirement to observe the time-dependent amyloid formation. This is prominent seen at the physiological salt concentration of 150 mM NaCl with the fibrillation observed for 400 μM concentration at 48 h, whereas oligomerization or proto-fibrillation is seen for the other concentrations. Such concentration-dependent fibrillation of Stm1_N1-113 explains that amyloid fibrils formed during the overexpression of Stm1_N1-113 may act as a molecular device to trigger apoptosis-like cell death.

Impact of SARS-CoV-2 ORF6 and its variant polymorphisms on host responses and viral pathogenesis

We and others have previously shown that the SARS-CoV-2 accessory protein ORF6 is a powerful antagonist of the interferon (IFN) signaling pathway by directly interacting with Nup98-Rae1 at the nuclear pore complex (NPC) and disrupting bidirectional nucleo-cytoplasmic trafficking. In this study, we further assessed the role of ORF6 during infection using recombinant SARS-CoV-2 viruses carrying either a deletion or a well characterized M58R loss-of-function mutation in ORF6. We show that ORF6 plays a key role in the antagonism of IFN signaling and in viral pathogenesis by interfering with karyopherin(importin)-mediated nuclear import during SARS-CoV-2 infection both in vitro , and in the Syrian golden hamster model in vivo . In addition, we found that ORF6-Nup98 interaction also contributes to inhibition of cellular mRNA export during SARS-CoV-2 infection. As a result, ORF6 expression significantly remodels the host cell proteome upon infection. Importantly, we also unravel a previously unrecognized function of ORF6 in the modulation of viral protein expression, which is independent of its function at the nuclear pore. Lastly, we characterized the ORF6 D61L mutation that recently emerged in Omicron BA.2 and BA.4 and demonstrated that it is able to disrupt ORF6 protein functions at the NPC and to impair SARS-CoV-2 innate immune evasion strategies. Importantly, the now more abundant Omicron BA.5 lacks this loss-of-function polymorphism in ORF6. Altogether, our findings not only further highlight the key role of ORF6 in the antagonism of the antiviral innate immune response, but also emphasize the importance of studying the role of non-spike mutations to better understand the mechanisms governing differential pathogenicity and immune evasion strategies of SARS-CoV-2 and its evolving variants.

ONE SENTENCE SUMMARY

SARS-CoV-2 ORF6 subverts bidirectional nucleo-cytoplasmic trafficking to inhibit host gene expression and contribute to viral pathogenesis.

Prion Protein Biology Through the Lens of Liquid-Liquid Phase Separation

Conformational conversion of the α-helix-rich cellular prion protein into the misfolded, β-rich, aggregated, scrapie form underlies the molecular basis of prion diseases that represent a class of invariably fatal, untreatable, and transmissible neurodegenerative diseases. However, despite the extensive and rigorous research, there is a significant gap in the understanding of molecular mechanisms that contribute to prion pathogenesis. In this review, we describe the historical perspective of the development of the prion concept and the current state of knowledge of prion biology including structural, molecular, and cellular aspects of the prion protein. We then summarize the putative functional role of the N-terminal intrinsically disordered segment of the prion protein. We next describe the ongoing efforts in elucidating the prion phase behavior and the emerging role of liquid-liquid phase separation that can have potential functional relevance and can offer an alternate non-canonical pathway involving conformational conversion into a disease-associated form. We also attempt to shed light on the evolutionary perspective of the prion protein highlighting the potential role of intrinsic disorder in prion protein biology and summarize a few important questions associated with the phase transitions of the prion protein. Delving deeper into these key aspects can pave the way for a detailed understanding of the critical molecular determinants of the prion phase transition and its relevance to physiology and neurodegenerative diseases.

Bend or break: how biochemically versatile molecules enable metabolic division of labor in clonal microbial communities

In fluctuating nutrient environments, isogenic microbial cells transition into "multicellular" communities composed of phenotypically heterogeneous cells, showing functional specialization. In fungi (such as budding yeast), phenotypic heterogeneity is often described in the context of cells switching between different morphotypes (e.g., yeast to hyphae/pseudohyphae or white/opaque transitions in Candida albicans). However, more fundamental forms of metabolic heterogeneity are seen in clonal Saccharomyces cerevisiae communities growing in nutrient-limited conditions. Cells within such communities exhibit contrasting, specialized metabolic states, and are arranged in distinct, spatially organized groups. In this study, we explain how such an organization can stem from self-organizing biochemical reactions that depend on special metabolites. These metabolites exhibit plasticity in function, wherein the same metabolites are metabolized and utilized for distinct purposes by different cells. This in turn allows cell groups to function as specialized, interdependent cross-feeding systems which support distinct metabolic processes. Exemplifying a system where cells exhibit either gluconeogenic or glycolytic states, we highlight how available metabolites can drive favored biochemical pathways to produce new, limiting resources. These new resources can themselves be consumed or utilized distinctly by cells in different metabolic states. This thereby enables cell groups to sustain contrasting, even apparently impossible metabolic states with stable transcriptional and metabolic signatures for a given environment, and divide labor in order to increase community fitness or survival. We speculate on possible evolutionary implications of such metabolic specialization and division of labor in isogenic microbial communities.

Amyloids as Building Blocks for Macroscopic Functional Materials: Designs, Applications and Challenges

Amyloids are self-assembled protein aggregates that take cross-β fibrillar morphology. Although some amyloid proteins are best known for their association with Alzheimer’s and Parkinson’s disease, many other amyloids are found across diverse organisms, from bacteria to humans, and they play vital functional roles. The rigidity, chemical stability, high aspect ratio, and sequence programmability of amyloid fibrils have made them attractive candidates for functional materials with applications in environmental sciences, material engineering, and translational medicines. This review focuses on recent advances in fabricating various types of macroscopic functional amyloid materials. We discuss different design strategies for the fabrication of amyloid hydrogels, high-strength materials, composite materials, responsive materials, extracellular matrix mimics, conductive materials, and catalytic materials.

Prion-Like Proteins in Phase Separation and Their Link to Disease

Aberrant protein folding underpins many neurodegenerative diseases as well as certain myopathies and cancers. Protein misfolding can be driven by the presence of distinctive prion and prion-like regions within certain proteins. These prion and prion-like regions have also been found to drive liquid-liquid phase separation. Liquid-liquid phase separation is thought to be an important physiological process, but one that is prone to malfunction. Thus, aberrant liquid-to-solid phase transitions may drive protein aggregation and fibrillization, which could give rise to pathological inclusions. Here, we review prions and prion-like proteins, their roles in phase separation and disease, as well as potential therapeutic approaches to counter aberrant phase transitions.

Direct evidence of cellular transformation by prion-like p53 amyloid infection

Tumor suppressor p53 mutations are associated with more than 50% of cancers. Aggregation and amyloid formation of p53 is also implicated in cancer pathogenesis, but direct evidence for aggregated p53 amyloids acting as an oncogene is lacking. Here, we conclusively demonstrate that wild-type p53 amyloid formation imparts oncogenic properties to non-cancerous cells. p53 amyloid aggregates were transferred through cell generations, contributing to enhanced survival, apoptotic resistance with increased proliferation and migration. The tumorigenic potential of p53 amyloid-transformed cells was further confirmed in mouse xenografts, wherein the tumors showed p53 amyloids. p53 disaggregation rescued the cellular transformation and inhibited tumor development in mice. We propose that wild-type p53 amyloid formation contributes to tumorigenesis and can be a potential target for therapeutic intervention. This article has an associated First Person interview with the first author of the paper.

Entropic Bristles Tune the Seeding Efficiency of Prion-Nucleating Fragments

Prions of lower eukaryotes are self-templating protein aggregates with cores formed by parallel in-register beta strands. Short aggregation-prone glutamine (Q)- and asparagine (N)-rich regions embedded in longer disordered domains have been proposed to act as nucleation sites that initiate refolding of soluble prion proteins into highly ordered fibrils, termed amyloid. We demonstrate that a short Q/N-rich peptide corresponding to a proposed nucleation site in the prototype Saccharomyces cerevisiae prion protein Sup35 is sufficient to induce infectious cytosolic prions in mouse neuroblastoma cells ectopically expressing the soluble Sup35 NM prion domain. Embedding this nucleating core in a non-native N-rich sequence that does not form amyloid but acts as an entropic bristle quadruples seeding efficiency. Our data suggest that large disordered sequences flanking an aggregation core in prion proteins act as not only solubilizers of the monomeric protein but also breakers of the formed amyloid fibrils, enhancing infectivity of the prion seeds.

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A short peptide derived from Sup35 (p103–113) forms rigid amyloid fibrils

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p103–113 fibrils can induce infectious Sup35 NM prions in mammalian cells

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Embedding p103–113 in an N-rich sequence increases fibril brittleness

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Increased fibril brittleness enhances prion-inducing capacity

Prion domains as a driving force for the assembly of functional nanomaterials

Amyloids display a highly ordered fibrillar structure. Many of these assemblies appear associated with human disease. However, the controllable, stable, tunable, and robust nature of amyloid fibrils can be exploited to build up remarkable nanomaterials with a wide range of applications in biomedicine and biotechnology. Functional prions constitute a particular class of amyloids. These transmissible proteins exhibit a modular architecture, with a disordered prion domain responsible for the assembly and one or more globular domains that account for the activity. Importantly, the original globular protein can be replaced with any protein of interest, without compromising the fibrillation potential. These genetic fusions form fibrils in which the globular domain remains folded, rendering functional nanostructures. However, in some cases, steric hindrance restricts the activity of these fibrils. This limitation can be solved by dissecting prion domains into shorter sequences that keep their self-assembling properties while allowing better access to the active protein in the fibrillar state. In this review, we will discuss the properties of prion-like functional nanomaterials and the amazing applications of these biocompatible fibrillar arrangements.

Relevance of Electrostatic Charges in Compactness, Aggregation, and Phase Separation of Intrinsically Disordered Proteins

The abundance of intrinsic disorder in the protein realm and its role in a variety of physiological and pathological cellular events have strengthened the interest of the scientific community in understanding the structural and dynamical properties of intrinsically disordered proteins (IDPs) and regions (IDRs). Attempts at rationalizing the general principles underlying both conformational properties and transitions of IDPs/IDRs must consider the abundance of charged residues (Asp, Glu, Lys, and Arg) that typifies these proteins, rendering them assimilable to polyampholytes or polyelectrolytes. Their conformation strongly depends on both the charge density and distribution along the sequence (i.e., charge decoration) as highlighted by recent experimental and theoretical studies that have introduced novel descriptors. Published experimental data are revisited herein in the frame of this formalism, in a new and possibly unitary perspective. The physicochemical properties most directly affected by charge density and distribution are compaction and solubility, which can be described in a relatively simplified way by tools of polymer physics. Dissecting factors controlling such properties could contribute to better understanding complex biological phenomena, such as fibrillation and phase separation. Furthermore, this knowledge is expected to have enormous practical implications for the design, synthesis, and exploitation of bio-derived materials and the control of natural biological processes.

Dynamics of oligomer and amyloid fibril formation by yeast prion Sup35 observed by high-speed atomic force microscopy

The yeast prion protein Sup35, which contains intrinsically disordered regions, forms amyloid fibrils responsible for a prion phenotype [PSI ⁺]. Using high-speed atomic force microscopy (HS-AFM), we directly visualized the prion determinant domain (Sup35NM) and the formation of its oligomers and fibrils at subsecond and submolecular resolutions. Monomers with freely moving tail-like regions initially appeared in the images, and subsequently oligomers with distinct sizes of ∼1.7 and 3 to 4 nm progressively accumulated. Nevertheless, these oligomers did not form fibrils, even after an incubation for 2 h in the presence of monomers. Fibrils appeared after much longer monomer incubation. The fibril elongation occurred smoothly without discrete steps, suggesting gradual conversions of the incorporated monomers into cross-β structures. The individual oligomers were separated from each other and also from the fibrils by respective, identical lengths on the mica surface, probably due to repulsion caused by the freely moving disordered regions. Based on these HS-AFM observations, we propose that the freely moving tails of the monomers are incorporated into the fibril ends, and then the structural conversions to cross-β structures gradually occur.

Structures, functions, and mechanisms of filament forming enzymes: a renaissance of enzyme filamentation

Filament formation by non-cytoskeletal enzymes has been known for decades, yet only relatively recently has its wide-spread role in enzyme regulation and biology come to be appreciated. This comprehensive review summarizes what is known for each enzyme confirmed to form filamentous structures in vitro, and for the many that are known only to form large self-assemblies within cells. For some enzymes, studies describing both the in vitro filamentous structures and cellular self-assembly formation are also known and described. Special attention is paid to the detailed structures of each type of enzyme filament, as well as the roles the structures play in enzyme regulation and in biology. Where it is known or hypothesized, the advantages conferred by enzyme filamentation are reviewed. Finally, the similarities, differences, and comparison to the SgrAI endonuclease system are also highlighted.

A Non-amyloid Prion Particle that Activates a Heritable Gene Expression Program

Spatiotemporal gene regulation is often driven by RNA-binding proteins that harbor long intrinsically disordered regions in addition to folded RNA-binding domains. We report that the disordered region of the evolutionarily ancient developmental regulator Vts1/Smaug drives self-assembly into gel-like condensates. These proteinaceous particles are not composed of amyloid, yet they are infectious, allowing them to act as a protein-based epigenetic element: a prion [SMAUG⁺]. In contrast to many amyloid prions, condensation of Vts1 enhances its function in mRNA decay, and its self-assembly properties are conserved over large evolutionary distances. Yeast cells harboring [SMAUG⁺] downregulate a coherent network of mRNAs and exhibit improved growth under nutrient limitation. Vts1 condensates formed from purified protein can transform naive cells to acquire [SMAUG⁺]. Our data establish that non-amyloid self-assembly of RNA-binding proteins can drive a form of epigenetics beyond the chromosome, instilling adaptive gene expression programs that are heritable over long biological timescales.

Prion soft amyloid core driven self-assembly of globular proteins into bioactive nanofibrils

Amyloids have been exploited to build amazing bioactive materials. In most cases, short synthetic peptides constitute the functional components of such materials. The controlled assembly of globular proteins into active amyloid nanofibrils is still challenging, because the formation of amyloids implies a conformational conversion towards a β-sheet-rich structure, with a concomitant loss of the native fold and the inactivation of the protein. There is, however, a remarkable exception to this rule: yeast prions. They are singular proteins able to switch between a soluble and an amyloid state. In both states, the structure of their globular domains remains essentially intact. The transit between these two conformations is encoded in prion domains (PrDs): long and disordered sequences to which the active globular domains are appended. PrDs are much larger than typical self-assembling peptides. This seriously limits their use for nanotechnological applications. We have recently shown that these domains contain soft amyloid cores (SACs) that suffice to nucleate their self-assembly reaction. Here we genetically fused a model SAC with different globular proteins. We demonstrate that this very short sequence acts as a minimalist PrD, driving the selective and slow assembly of the initially soluble fusion proteins into amyloid fibrils in which the globular proteins retain their native structure and display high activity. Overall, we provide here a novel, modular and straightforward strategy to build active protein-based nanomaterials at a preparative scale.

Intrinsically disordered proteins in the formation of functional amyloids from bacteria to humans

Amyloids are nanoscopic ordered self-assemblies of misfolded proteins that are formed via aggregation of partially unfolded or intrinsically disordered proteins (IDPs) and are commonly linked to devastating human diseases. An enlarging body of recent research has demonstrated that certain amyloids can be beneficial and participate in a wide range of physiological functions from bacteria to humans. These amyloids are termed as functional amyloids. Like disease-associated amyloids, a vast majority of functional amyloids are derived from a range of IDPs or hybrid proteins containing ordered domains and intrinsically disordered regions (IDRs). In this chapter, we describe an account of recent studies on the aggregation behavior of IDPs resulting in the formation of functional amyloids in a diverse range of organisms from bacteria to human. We also discuss the strategies that are used by these organisms to regulate the spatiotemporal amyloid assembly in their physiological functions.

Conceptual Evolution of Cell Signaling

During the last 100 years, cell signaling has evolved into a common mechanism for most physiological processes across systems. Although the majority of cell signaling principles were initially derived from hormonal studies, its exponential growth has been supported by interdisciplinary inputs, e.g., from physics, chemistry, mathematics, statistics, and computational fields. As a result, cell signaling has grown out of scope for any general review. Here, we review how the messages are transferred from the first messenger (the ligand) to the receptor, and then decoded with the help of cascades of second messengers (kinases, phosphatases, GTPases, ions, and small molecules such as cAMP, cGMP, diacylglycerol, etc.). The message is thus relayed from the membrane to the nucleus where gene expression ns, subsequent translations, and protein targeting to the cell membrane and other organelles are triggered. Although there are limited numbers of intracellular messengers, the specificity of the response profiles to the ligands is generated by the involvement of a combination of selected intracellular signaling intermediates. Other crucial parameters in cell signaling are its directionality and distribution of signaling strengths in different pathways that may crosstalk to adjust the amplitude and quality of the final effector output. Finally, we have reflected upon its possible developments during the coming years.

Protein Nanofibrils as Storage Forms of Peptide Drugs and Hormones

Amyloids are highly organized cross β-sheet protein nanofibrils that are associated with both diseases and functions. Thermodynamically amyloids are stable structures as they represent the lowest free energy state that proteins can attain. However, recent studies suggest that amyloid fibrils can be dissociated by a change in environmental parameters such as pH and ionic strength. This reversibility of amyloids can not only be associated with disease, but function as well. In disease-associated amyloids, fibrils can act as reservoirs of cytotoxic oligomers. Recently, in higher organisms such as mammals, hormones were found to be stored in amyloid-like state, where these were reported to act as a reservoir of functional monomers. These hormone amyloids can dissociate to monomers upon release from the secretory granules, and subsequently bind to their respective receptors and perform their functions. In this book chapter, we describe in detail how these protein nanofibrils represent the densest possible peptide packing and are suitable for long-term storage. Thus, mimicking the feature of amyloids to release functional monomers, it is possible to formulate amyloid-based peptide/protein drugs, which can be used for sustained release.

Prion-based nanomaterials and their emerging applications

Protein misfolding and aggregation into highly ordered fibrillar structures have been traditionally associated with pathological processes. Nevertheless, nature has taken advantage of the particular properties of amyloids for functional purposes, like in the protection of organisms against environmental changing conditions. Over the last decades, these fibrillar structures have inspired the design of new nanomaterials with intriguing applications in biomedicine and nanotechnology such as tissue engineering, drug delivery, adhesive materials, biodegradable nanocomposites, nanowires or biosensors. Prion and prion-like proteins, which are considered a subclass of amyloids, are becoming ideal candidates for the design of new and tunable nanomaterials. In this review, we discuss the particular properties of this kind of proteins, and the current advances on the design of new materials based on prion sequences.

More than Just a Phase: Prions at the Crossroads of Epigenetic Inheritance and Evolutionary Change

A central tenet of molecular biology is that heritable information is stored in nucleic acids. However, this paradigm has been overturned by a group of proteins called "prions." Prion proteins, many of which are intrinsically disordered, can adopt multiple conformations, at least one of which has the capacity to self-template. This unusual folding landscape drives a form of extreme epigenetic inheritance that can be stable through both mitotic and meiotic cell divisions. Although the first prion discovered-mammalian PrP-is the causative agent of debilitating neuropathies, many additional prions have now been identified that are not obviously detrimental and can even be adaptive. Intrinsically disordered regions, which endow proteins with the bulk property of "phase-separation," can also be drivers of prion formation. Indeed, many protein domains that promote phase separation have been described as prion-like. In this review, we describe how prions lie at the crossroads of phase separation, epigenetic inheritance, and evolutionary adaptation.

Prion Replication in the Mammalian Cytosol: Functional Regions within a Prion Domain Driving Induction, Propagation, and Inheritance

Prions of lower eukaryotes are transmissible protein particles that propagate by converting homotypic soluble proteins into growing protein assemblies. Prion activity is conferred by so-called prion domains, regions of low complexity that are often enriched in glutamines and asparagines (Q/N).

Prions of lower eukaryotes are transmissible protein particles that propagate by converting homotypic soluble proteins into growing protein assemblies. Prion activity is conferred by so-called prion domains, regions of low complexity that are often enriched in glutamines and asparagines (Q/N). The compositional similarity of fungal prion domains with intrinsically disordered domains found in many mammalian proteins raises the question of whether similar sequence elements can drive prion-like phenomena in mammals. Here, we define sequence features of the prototype Saccharomyces cerevisiae Sup35 prion domain that govern prion activities in mammalian cells by testing the ability of deletion mutants to assemble into self-perpetuating particles. Interestingly, the amino-terminal Q/N-rich tract crucially important for prion induction in yeast was dispensable for the prion life cycle in mammalian cells. Spontaneous and template-assisted prion induction, growth, and maintenance were preferentially driven by the carboxy-terminal region of the prion domain that contains a putative soft amyloid stretch recently proposed to act as a nucleation site for prion assembly. Our data demonstrate that preferred prion nucleation domains can differ between lower and higher eukaryotes, resulting in the formation of prions with strikingly different amyloid cores.

Minimalist Prion-Inspired Polar Self-Assembling Peptides

Nature provides copious examples of self-assembling supramolecular nanofibers. Among them, amyloid structures have found amazing applications as advanced materials in fields such as biomedicine and nanotechnology. Prions are a singular subset of proteins able to switch between a soluble conformation and an amyloid state. The ability to transit between these two conformations is encoded in the so-called prion domains (PrDs), which are long and disordered regions of low complexity, enriched in polar and uncharged amino acids such as Gln, Asn, Tyr, Ser, and Gly. The polar nature of PrDs results in slow amyloid formation, which allows kinetic control of fiber assembly. This approach has been exploited for fabrication of multifunctional materials because in contrast to most amyloids, PrDs lack hydrophobic stretches that can nucleate their aggregation, their assembly depends on the establishment of a large number of weak interactions along the complete domain. The length and low complexity of PrDs make their chemical synthesis for applied purposed hardly affordable. Here, we designed four minimalist polar binary patterned peptides inspired in PrDs, which include the [Q/N/G/S]-Y-[Q/N/G/S] motif frequently observed in these domains: NYNYNYN, QYQYQYQ, SYSYSYS, and GYGYGYG. Despite their small size, they all recapitulate the properties of full-length PrDs, self-assembling into nontoxic amyloids under physiological conditions. Thus, they constitute small building blocks for the construction of tailored prion-inspired nanostructures. We exploited Tyr residues in these peptides to generate highly stable dityrosine cross-linked assemblies for the immobilization of metal nanoparticles in the fibrils surface and to develop an electrocatalytic amyloid scaffold. Moreover, we show that the shorter and more polar NYNNYN, QYQQYQ, and SYSSYS hexapeptides also self-assemble into amyloid-like structures, consistent with the presence of these tandem motifs in human prion-like proteins.

Use of Two Dimensional Semi-denaturing Detergent Agarose Gel Electrophoresis to Confirm Size Heterogeneity of Amyloid or Amyloid-like Fibers

Amyloid or amyloid-like fibers have been associated with many human diseases, and are now being discovered to be important for many signaling pathways. The ability to readily detect the formation of these fibers under various experimental conditions is essential for understanding their potential function. Many methods have been used to detect the fibers, but not without some drawbacks. For example, electron microscopy (EM), or staining with Congo Red or Thioflavin T often requires purification of the fibers. On the other hand, semi-denaturing detergent agarose gel electrophoresis (SDD-AGE) allows detection of the SDS-resistant amyloid-like fibers in the cell extracts without purification. In addition, it allows the comparison of the size difference of the fibers. More importantly, it can be used to identify specific proteins within the fibers by Western blotting. It is less time consuming and more easily accessible to a wider number of labs. SDD-AGE results often show variable degree of heterogeneity. It raises the question whether part of the heterogeneity results from the dissociation of the protein complex during the electrophoresis in the presence of SDS. For this reason, we have employed a second dimension of SDD-AGE to determine if the size heterogeneity seen in SDD-AGE is truly a result of fiber heterogeneity in vivo and not a result of either degradation or dissociation of some of the proteins during electrophoresis. This method allows fast, qualitative confirmation that the amyloid or amyloid-like fibers are not partially dissociating during the SDD-AGE process.

Amyloids Are Novel Cell-Adhesive Matrices

Amyloids are highly ordered peptide/protein aggregates traditionally associated with multiple human diseases including neurodegenerative disorders. However, recent studies suggest that amyloids can also perform several biological functions in organisms varying from bacteria to mammals. In many lower organisms, amyloid fibrils function as adhesives due to their unique surface topography. Recently, amyloid fibrils have been shown to support attachment and spreading of mammalian cells by interacting with the cell membrane and by cell adhesion machinery activation. Moreover, similar to cellular responses on natural extracellular matrices (ECMs), mammalian cells on amyloid surfaces also use integrin machinery for spreading, migration, and differentiation. This has led to the development of biocompatible and implantable amyloid-based hydrogels that could induce lineage-specific differentiation of stem cells. In this chapter, based on adhesion of both lower organisms and mammalian cells on amyloid nanofibrils, we posit that amyloids could have functioned as a primitive extracellular matrix in primordial earth.

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.

Insights into Mechanisms of Transmission and Pathogenesis from Transgenic Mouse Models of Prion Diseases

Prions represent a new paradigm of protein-mediated information transfer. In the case of mammals, prions are the cause of fatal, transmissible neurodegenerative diseases, sometimes referred to as transmissible spongiform encephalopathies (TSEs), which frequently occur as epidemics. An increasing body of evidence indicates that the canonical mechanism of conformational corruption of cellular prion protein (PrP^C) by the pathogenic isoform (PrP^Sc) that is the basis of prion formation in TSEs is common to a spectrum of proteins associated with various additional human neurodegenerative disorders, including the more common Alzheimer's and Parkinson's diseases. The peerless infectious properties of TSE prions, and the unparalleled tools for their study, therefore enable elucidation of mechanisms of template-mediated conformational propagation that are generally applicable to these related disease states. Many unresolved issues remain including the exact molecular nature of the prion, the detailed cellular and molecular mechanisms of prion propagation, and the means by which prion diseases can be both genetic and infectious. In addition, we know little about the mechanism by which neurons degenerate during prion diseases. Tied to this, the physiological role of the normal form of the prion protein remains unclear and it is uncertain whether or not loss of this function contributes to prion pathogenesis. The factors governing the transmission of prions between species remain unclear, in particular the means by which prion strains and PrP primary structure interact to affect interspecies prion transmission. Despite all these unknowns, advances in our understanding of prions have occurred because of their transmissibility to experimental animals, and the development of transgenic (Tg) mouse models has done much to further our understanding about various aspects of prion biology. In this review, we will focus on advances in our understanding of prion biology that occurred in the past 8 years since our last review of this topic.

Structural and functional diversity among amyloid proteins: Agents of disease, building blocks of biology, and implications for molecular engineering

Amyloids have long been associated with protein dysfunction and neurodegenerative diseases, but recent research has demonstrated that some organisms utilize the unique properties of the amyloid fold to create functional structures with important roles in biological processes. Additionally, new engineering approaches have taken advantage of amyloid structures for implementation in a wide variety of materials and devices. In this review, the role of amyloid in human disease is discussed and compared to the functional amyloids, which serve a largely structural purpose. We then consider the use of amyloid constructs in engineering applications, including their utility as building blocks for synthetic biology and molecular engineering. Biotechnol. Bioeng. 2017;114: 7-20. © 2016 Wiley Periodicals, Inc.

Key Points Concerning Amyloid Infectivity and Prion-Like Neuronal Invasion

Amyloid aggregation has been related to an increasing number of human illnesses, from Alzheimer’s and Parkinson’s diseases (AD/PD) to Creutzfeldt-Jakob disease. Commonly, only prions have been considered as infectious agents with a high capacity of propagation. However, recent publications have shown that many amyloid proteins, including amyloid β-peptide, α-synuclein (α-syn) and tau protein, also propagate in a “prion-like” manner. Meanwhile, no link between propagation of pathological proteins and neurotoxicity has been demonstrated. The extremely low infectivity under natural conditions of most non-prion amyloids is far below the capacity to spread exhibited by prions. Nonetheless, it is important to elucidate the key factors that cause non-prion amyloids to become infectious agents. In recent years, important advances in our understanding of the amyloid processes of amyloid-like proteins and unrelated prions (i.e., yeast and fungal prions) have yielded essential information that can shed light on the prion phenomenon in mammals and humans. As shown in this review, recent evidence suggests that there are key factors that could dramatically modulate the prion capacity of proteins in the amyloid conformation. The concentration of nuclei, the presence of oligomers, and the toxicity, resistance and localization of these aggregates could all be key factors affecting their spread. In short, those factors that favor the high concentration of extracellular nuclei or oligomers, characterized by small size, with a low toxicity could dramatically increase prion propensity; whereas low concentrations of highly toxic intracellular amyloids, with a large size, would effectively prevent infectivity.

Amyloids or prions? That is the question

Despite major efforts devoted to understanding the phenomenon of prion transmissibility, it is still poorly understood how this property is encoded in the amino acid sequence. In recent years, experimental data on yeast prion domains allow to start at least partially decrypting the sequence requirements of prion formation. These experiments illustrate the need for intrinsically disordered sequence regions enriched with a particularly high proportion of glutamine and asparagine. Bioinformatic analysis suggests that these regions strike a balance between sufficient amyloid nucleation propensity on the one hand and disorder on the other, which ensures availability of the amyloid prone regions but entropically prevents unwanted nucleation and facilitates brittleness required for propagation.

Specific chaperones and regulatory domains in control of amyloid formation

Many proteins can form amyloid-like fibrils in vitro, but only about 30 amyloids are linked to disease, whereas some proteins form physiological amyloid-like assemblies. This raises questions of how the formation of toxic protein species during amyloidogenesis is prevented or contained in vivo. Intrinsic chaperoning or regulatory factors can control the aggregation in different protein systems, thereby preventing unwanted aggregation and enabling the biological use of amyloidogenic proteins. The molecular actions of these chaperones and regulators provide clues to the prevention of amyloid disease, as well as to the harnessing of amyloidogenic proteins in medicine and biotechnology.

Disaggregases, molecular chaperones that resolubilize protein aggregates

The process of folding is a seminal event in the life of a protein, as it is essential for proper protein function and therefore cell physiology. Inappropriate folding, or misfolding, can not only lead to loss of function, but also to the formation of protein aggregates, an insoluble association of polypeptides that harm cell physiology, either by themselves or in the process of formation. Several biological processes have evolved to prevent and eliminate the existence of non-functional and amyloidogenic aggregates, as they are associated with several human pathologies. Molecular chaperones and heat shock proteins are specialized in controlling the quality of the proteins in the cell, specifically by aiding proper folding, and dissolution and clearance of already formed protein aggregates. The latter is a function of disaggregases, mainly represented by the ClpB/Hsp104 subfamily of molecular chaperones, that are ubiquitous in all organisms but, surprisingly, have no orthologs in the cytosol of metazoan cells. This review aims to describe the characteristics of disaggregases and to discuss the function of yeast Hsp104, a disaggregase that is also involved in prion propagation and inheritance.

The prion-like RNA-processing protein HNRPDL forms inherently toxic amyloid-like inclusion bodies in bacteria

Background

The formation of protein inclusions is connected to the onset of many human diseases. Human RNA binding proteins containing intrinsically disordered regions with an amino acid composition resembling those of yeast prion domains, like TDP-43 or FUS, are being found to aggregate in different neurodegenerative disorders. The structure of the intracellular inclusions formed by these proteins is still unclear and whether these deposits have an amyloid nature or not is a matter of debate. Recently, the aggregation of TDP-43 has been modelled in bacteria, showing that TDP-43 inclusion bodies (IBs) are amorphous but intrinsically neurotoxic. This observation raises the question of whether it is indeed the lack of an ordered structure in these human prion-like protein aggregates the underlying cause of their toxicity in different pathological states.

Results

Here we characterize the IBs formed by the human prion-like RNA-processing protein HNRPDL. HNRPDL is linked to the development of limb-girdle muscular dystrophy 1G and shares domain architecture with TDP-43. We show that HNRPDL IBs display characteristic amyloid hallmarks, since these aggregates bind to amyloid dyes in vitro and inside the cell, they are enriched in intermolecular β-sheet conformation and contain inner amyloid-like fibrillar structure. In addition, despite their ordered structure, HNRPDL IBs are highly neurotoxic.

Conclusions

Our results suggest that at least some of the disorders caused by the aggregation of human prion-like proteins would rely on the formation of classical amyloid assemblies rather than being caused by amorphous aggregates. They also illustrate the power of microbial cell factories to model amyloid aggregation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-015-0284-7) contains supplementary material, which is available to authorized users.

Nonsense codon suppression in fission yeast due to mutations of tRNA(Ser.11) and translation release factor Sup35 (eRF3)

In the fission yeast Schizosaccharomyces pombe, sup9 mutations can suppress the termination of translation at nonsense (stop) codons. We localized sup9 physically to the spctrnaser.11 locus and confirmed that one allele (sup9-UGA) alters the anticodon of a serine tRNA. We also found that another purported allele is not allelic. Instead, strains with that suppressor (renamed sup35-F592S) have a single base pair substitution (T1775C) that introduces an amino acid substitution in the Sup35 protein (Sup35-F592S). Reduced functionality of Sup35 (eRF3), the ubiquitous guanine nucleotide-responsive translation release factor of eukaryotes, increases read-through of stop codons. Tetrad dissection revealed that suppression is tightly linked to (inseparable from) the sup35-F592S mutation and that there are no additional extragenic modifiers. The Mendelian inheritance indicates that the Sup35-F592S protein does not adopt an infectious amyloid state ([PSI (+)] prion) to affect suppression, consistent with recent evidence that fission yeast Sup35 does not form prions. We also report that sup9-UGA and sup35-F592S exhibit different strengths of suppression for opal stop codons of ade6-M26 and ade6-M375. We discuss possible mechanisms for the variation in suppressibility exhibited by the two alleles.

Structural determinants of phenotypic diversity and replication rate of human prions

The infectious pathogen responsible for prion diseases is the misfolded, aggregated form of the prion protein, PrP^Sc. In contrast to recent progress in studies of laboratory rodent-adapted prions, current understanding of the molecular basis of human prion diseases and, especially, their vast phenotypic diversity is very limited. Here, we have purified proteinase resistant PrP^Sc aggregates from two major phenotypes of sporadic Creutzfeldt-Jakob disease (sCJD), determined their conformational stability and replication tempo in vitro, as well as characterized structural organization using recently emerged approaches based on hydrogen/deuterium (H/D) exchange coupled with mass spectrometry. Our data clearly demonstrate that these phenotypically distant prions differ in a major way with regard to their structural organization, both at the level of the polypeptide backbone (as indicated by backbone amide H/D exchange data) as well as the quaternary packing arrangements (as indicated by H/D exchange kinetics for histidine side chains). Furthermore, these data indicate that, in contrast to previous observations on yeast and some murine prion strains, the replication rate of sCJD prions is primarily determined not by conformational stability but by specific structural features that control the growth rate of prion protein aggregates.

Sporadic Creutzfeldt-Jakob disease (sCJD) represents ~90% of all human prion diseases worldwide. This neurodegenerative disease, which is transmissible and invariably fatal, is characterized by variable progression rates and remarkable diversity of clinical and pathological traits. The infectious sCJD prions propagating the pathology mainly in the brain are assemblies of abnormally folded isoform (PrP^Sc) of a host-encoded prion protein (PrP^C). The structure and replication mechanisms of human prions are unknown, and whether the PrP^Sc subtypes identified by proteolytic fragmentation represent distinct strains of sCJD prions has been debated. Here, we isolated sCJD prions from patients with two very distant phenotypes of the disease, compared their structural organization using recently developed biophysical techniques, and investigated their replication in vitro. Our data indicate that these sCJD prions are characterized by different secondary structure organization and quaternary packing arrangements, and that these structural differences are responsible for distinct prion replication rates and unique phenotypic characteristics of the disease. Furthermore, our analysis reveals that, contrary to previous observations for yeast prions, the replication tempo of sCJD prions is determined not so much by their conformational stability but by specific structural features that control the growth speed of prion particles.

Predicting the aggregation propensity of prion sequences

The presence of prions can result in debilitating and neurodegenerative diseases in mammals and protein-based genetic elements in fungi. Prions are defined as a subclass of amyloids in which the self-aggregation process becomes self-perpetuating and infectious. Like all amyloids, prions polymerize into fibres with a common core formed of β-sheet structures oriented perpendicular to the fibril axes which form a structure known as a cross-β structure. The intermolecular β-sheet propensity, a characteristic of the amyloid pattern, as well as other key parameters of amyloid fibril formation can be predicted. Mathematical algorithms have been proposed to predict both amyloid and prion propensities. However, it has been shown that the presence of amyloid-prone regions in a polypeptide sequence could be insufficient for amyloid formation. It has also often been stated that the formation of amyloid fibrils does not imply that these are prions. Despite these limitations, in silico prediction of amyloid and prion propensities should help detect potential new prion sequences in mammals. In addition, the determination of amyloid-prone regions in prion sequences could be very useful in understanding the effect of sporadic mutations and polymorphisms as well as in the search for therapeutic targets.

From molecule to molecule and cell to cell: prion-like mechanisms in amyotrophic lateral sclerosis

Prions, self-proliferating infectious agents consisting of misfolded protein, are most often associated with aggressive neurodegenerative diseases in animals and humans. Akin to the contiguous spread of a living pathogen, the prion paradigm provides a mechanism by which a mutant or wild-type misfolded protein can dominate pathogenesis through self-propagating protein misfolding, and subsequently spread from region to region through the central nervous system. The prion diseases, along with more common neurodegenerative disorders such as Alzheimer's disease, Parkinson's disease and the tauopathies belong to a larger group of protein misfolding disorders termed proteinopathies that feature aberrant misfolding and aggregation of specific proteins. Amyotrophic lateral sclerosis (ALS), a lethal disease characterized by progressive degeneration of motor neurons is currently understood as a classical proteinopathy; the disease is typified by the formation of inclusions consisting of aggregated protein within motor neurons that contribute to neurotoxicity. It is well established that misfolded/aggregated proteins such as SOD1 and TDP-43 contribute to the toxicity of motor neurons and play a prominent role in the pathology of ALS. Recent work has identified propagated protein misfolding properties in both mutant and wild-type SOD1, and to a lesser extent TDP-43, which may provide the molecular basis for the clinically observed contiguous spread of the disease through the neuroaxis. In this review we examine the current state of knowledge regarding the prion-like properties of proteins associated with ALS pathology as well as their possible mechanisms of transmission.

What makes a protein sequence a prion?

Typical amyloid diseases such as Alzheimer's and Parkinson's were thought to exclusively result from de novo aggregation, but recently it was shown that amyloids formed in one cell can cross-seed aggregation in other cells, following a prion-like mechanism. Despite the large experimental effort devoted to understanding the phenomenon of prion transmissibility, it is still poorly understood how this property is encoded in the primary sequence. In many cases, prion structural conversion is driven by the presence of relatively large glutamine/asparagine (Q/N) enriched segments. Several studies suggest that it is the amino acid composition of these regions rather than their specific sequence that accounts for their priogenicity. However, our analysis indicates that it is instead the presence and potency of specific short amyloid-prone sequences that occur within intrinsically disordered Q/N-rich regions that determine their prion behaviour, modulated by the structural and compositional context. This provides a basis for the accurate identification and evaluation of prion candidate sequences in proteomes in the context of a unified framework for amyloid formation and prion propagation.

When amyloids become prions

The conformational diseases, linked to protein aggregation into amyloid conformations, range from non-infectious neurodegenerative disorders, such as Alzheimer disease (AD), to highly infectious ones, such as human transmissible spongiform encephalopathies (TSEs). They are commonly known as prion diseases. However, since all amyloids could be considered prions (from those involved in cell-to-cell transmission to those responsible for real neuronal invasion), it is necessary to find an underlying cause of the different capacity to infect that each of the proteins prone to form amyloids has. As proposed here, both the intrinsic cytotoxicity and the number of nuclei of aggregation per cell could be key factors in this transmission capacity of each amyloid.

Saccharomyces cerevisiae: a sexy yeast with a prion problem

Yeast prions are infectious proteins that spread exclusively by mating. The frequency of prions in the wild therefore largely reflects the rate of spread by mating counterbalanced by prion growth slowing effects in the host. We recently showed that the frequency of outcross mating is about 1% of mitotic doublings with 23–46% of total matings being outcrosses. These findings imply that even the mildest forms of the [PSI+], [URE3] and [PIN+] prions impart > 1% growth/survival detriment on their hosts. Our estimate of outcrossing suggests that Saccharomyces cerevisiae is far more sexual than previously thought and would therefore be more responsive to the adaptive effects of natural selection compared with a strictly asexual yeast. Further, given its large effective population size, a growth/survival detriment of > 1% for yeast prions should strongly select against prion-infected strains in wild populations of Saccharomyces cerevisiae.

Life cycle of cytosolic prions

Prions are self-templating protein aggregates that were originally identified as the causative agent of prion diseases in mammals, but have since been discovered in other kingdoms. Mammalian prions represent a unique class of infectious agents that are composed of misfolded prion protein. Prion proteins usually exist as soluble proteins but can refold and assemble into highly ordered, self-propagating prion polymers. The prion concept is also applicable to a growing number of non-Mendelian elements of inheritance in lower eukaryotes. While prions identified in mammals are clearly pathogens, prions in lower eukaryotes can be either detrimental or beneficial to the host. Prion phenotypes in fungi are transmitted vertically from mother to daughter cells during cell division and horizontally during mating or abortive mating, but extracellular phases have not been reported. Recent findings now demonstrate that in a mammalian cell environment, protein aggregates derived from yeast prion domains exhibit a prion life cycle similar to mammalian prions propagated ex vivo. This life cycle includes a soluble state of the protein, an induction phase by exogenous prion fibrils, stable replication of prion entities, vertical transmission to progeny and natural horizontal transmission to neighboring cells. Our data reveal that mammalian cells contain all co-factors required for cytosolic prion propagation and dissemination. This has important implications for understanding prion-like properties of disease-related protein aggregates. In light of the growing number of identified functional amyloids, cell-to-cell propagation of cytosolic protein conformers might not only be relevant for the spreading of disease-associated proteins, but might also be of more general relevance under non-disease conditions.