20 Yeast Prions and Their Analysis InVivo

20 Yeast Prions and Their Analysis In Vivo Mick F Tuite, Lee J Byrne, Lyne Josse´, Frederique Ness, Nadejda Koloteva-Levine and Brian Cox Department of Biosciences, University of Kent, Canterbury, Kent CT2 7NJ, UK ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

CONTENTS Introduction Yeast prions: a primer Analysis of prion-associated phenotypes Genetic analysis of yeast prions Analysis of prion protein aggregates formed in vivo Eliminating yeast prions Propagon counting Studying prion protein polymerisation in vitro How to recognise a new yeast prion

GdnHCl GFP PrD TSE USA

Guanidine hydrochloride Green fluorescent protein Prion-forming domain Transmissible spongiform encephalopathies Ureidosuccinic acid

~~~~~~ I. INTRODUCTION The existence of protein-only infectious agents (‘prions’) was first established in animals and humans through their association with the fatal neurodegenerative diseases classified as the transmissible spongiform encephalopathies (TSE). The infectious entity in the TSEs is associated with a protease-resistant and conformationally distinct version of the PrP protein (Prusiner et al., 1998). The METHODS IN MICROBIOLOGY, VOLUME 36 0580-9517 DOI:10.1016/S0580-9517(06)36020-5

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Yeast Prions and Their Analysis In Vivo

Abbreviations

infectious form of PrP (called PrPSc) has an identical amino acid sequence to the non-infectious, membrane-associated form (PrPc), but is found largely as high-molecular-weight deposits in the brain. These aggregates have the biophysical characteristics of an amyloid: ordered protein polymers in the form of non-branching fibrils that are rich in b-sheet and, which when stained with Congo Red, exhibit red-green birefringence under polarised light. Prions however differ from most other disease-associated amyloids in that they are transmissible, i.e. the prion form of PrP can be propagated within the host and passed on to other individuals of the same, or closely related, species (Soto et al., 2006, review). While a great deal of attention has been – and continues to be – paid to the disease mechanisms associated with the mammalian TSEs, prions are not unique to mammals. There is now irrefutable evidence that the yeast Saccharomyces cerevisiae has at least three different proteins – Sup35p, Ure2p and Rnq1p – that can generate extrachromosomally inherited phenotypes as a direct consequence of an inherited change in their conformation via a prion-like mechanism (Uptain and Lindquist, 2002; Wickner et al., 2004; Table 1). Het-s, an unrelated prion protein, has also been described in the filamentous fungus Podospora anserina where it controls vegetative incompatibility (Coustou et al., 1997; Maddelein et al., 2002). In contrast to the life-threatening consequences associated with the appearance of, or infection by, PrPSc in humans and animals, yeast cells show no overt ‘disease’ phenotypes if they contain the prion form of any one of these three proteins. While the prion form of two of the proteins – Sup35p and Ure2p – have yet to be found in natural isolates of S. cerevisiae (Jensen et al., 2001; Resende et al., 2003; Nakayashiki et al., 2005), strains carrying the prion form of the Rnq1p protein are frequently found (Resende et al., 2003). This suggests that the presence of the prion form of Sup35p or Ure2p might have a negative impact on the fitness of yeast cells in the wild. However, several studies have shown that yeast cells carrying the prion form of Sup35p are more resistant to various physical and chemical stresses (Eaglestone et al., 1999; True & Lindquist, 2000). Nevertheless, there are characteristic phenotypes associated with the presence of a yeast prion (i.e. [PRION + ]) that do not reflect any underlying change in the sequence of the cell’s genome. Yeast prions can therefore be considered as protein-based epigenetic determinants. In addition to phenotypic differences, one can also define biochemical differences between [PRION + ] cells and their [prion
] (but otherwise isogenic) counterparts, especially with regards the degree of solubility of the underlying prion protein. Yeast prions are transmitted efficiently from cell-to-cell during mitosis and meiosis, indicating that a very effective mechanism must exist that ensures their continued propagation even in rapidly dividing cells (Tuite and Koloteva-Levin, 2004, review). Two different but not mutually exclusive models have been put forward to explain the self-propagation of prions: template-directed refolding 492

Table 1. Native yeast prions and their associated phenotypes Prion

Protein

Prion-associated phenotype(s)

Reference

[URE3]

Ure2p

1. Utilisation of poor N2 sources in ura2 cells 2. Excretion of uracil in wild-type cells in the presence of excess ureidosuccinic acid

Lacroute (1971)

1. Suppression of ade1-14 (red/white colonies) 2. Suppression of ade2-1 in an SUQ5 strain (red/white colonies) 3. Resistance to certain physical and chemical stresses 4. Growth inhibition when Sup35p is over expressed

Chernoff et al. (1995)

No phenotype known High frequency de novo conversion to [PSI + ]

Sondheimer and Lindquist (2000) Derkatch et al. (2001) and Osherovich and Weissman (2001)

Sup35p

[RNQ + ]

Rnq1p

[PIN + ]

Rnq1pa

Cox (1965)

Eaglestone et al. (1999) and True and Lindquist (2000) Chernoff et al. (1993)

a Although the Rnq1p is most commonly associated with the [PIN+] prion, at least eight other proteins, including Ure2p, can also give rise to a prion with this property (Derkatch et al., 2001; Osherovich and Weissman, 2001).

(Prusiner, 1991) and seeded polymerisation (Jarrett and Lansbury, 1993). The more widely accepted seeded polymerisation model is based on the prion protein existing in an altered conformational state which is in a reversible dynamic equilibrium with a soluble form of that protein. The seeding of prion protein polymerisation would be triggered by smaller, perhaps transient, oligomeric form(s) of the protein that in turn arise from the association of conformationally altered prion protein molecules. These forms of the protein have historically been referred to as seeds although we have recently coined the term ‘propagon’ which we define as a self-replicating hereditary particle that is required to maintain the [PRION + ] state (Cox et al., 2003). Thus the propagon, by providing a nucleating activity, drives the polymerisation of both existing and newly synthesised prion protein molecules into the characteristic prion aggregates. 493

Yeast Prions and Their Analysis In Vivo

[PSI + ]

Chernoff et al. (2002)

Stable propagation of the yeast [PRION + ] state also requires propagons to be efficiently distributed during mitosis and meiosis, although whether this is achieved by an active or a passive mechanism has yet to be established. The generation and transmission of propagons in yeast appears to be dependent upon a number of different molecular chaperones with one particular chaperone, the heat-shock-inducible protein Hsp104p, being essential for the propagation of all three native prions (Chernoff et al., 1995; Moriyama et al., 2000; Sondheimer and Lindquist, 2000). In this chapter, we first provide a brief overview of the three yeast (S. cerevisiae) prions in terms of their associated phenotypes and their cell biological and biochemical properties and then go on to review the experimental approaches that can be taken to study the three native prions in vivo. Finally, we consider how one establishes whether or not a new or novel phenotype is associated with the prion-like behaviour of a cellular protein.

~~~~~~ II. YEAST PRIONS: A PRIMER Saccharomyces cerevisiae has at least three proteins that meet the necessary genetic and biochemical criteria to be defined as a prionforming protein: Sup35p which gives rise to the [PSI + ] prion, Ure2p which gives rise to the [URE3] prion and Rnq1p which gives rise to the [RNQ+] or, as it now more routinely referred to, the [PIN + ] prion (Uptain and Lindquist, 2002, a review). None of these proteins share any amino acid sequence identity with the mammalian PrP protein or with each other although they do share some sequence features, as described below. The three prions have impacts on very different biological processes: Sup35p is a translation termination factor, Ure2p regulates nitrogen metabolism via transcriptional modulation, while no specific cellular role has yet been attributed to Rnq1p. Nevertheless, they all share a number of properties in common as originally defined by Wickner (1994) whose pioneering studies on the yeast Ure2p/[URE3] prion provided the first evidence that prions exist in yeast. The key properties are:

 The [PRION + ] state shows a non-Mendelian pattern of inherit-

ance in [PRION + ]  [prion
] genetic crosses consistent with a cytoplasmically located ‘genetic’ determinant.  The [PRION + ] state can only be established and propagated if the nuclear gene that encodes the prion protein is present, i.e. the [PRION + ] state is not maintained if the prion protein gene has been deleted. This can only be verified for Ure2p/[URE3] and Rnq1p/[PIN + ] since the SUP35 gene is essential for viability.  Elimination of the [PRION + ] state by certain non-mutagenic agents results in a viable [prion
] cell that retains the ability to re-establish the [PRION + ] state de novo. 494

 Overproduction of the underlying protein in a [prion
] cell results in a significant elevation in the rate of de novo appearance of the [PRION + ] form of that protein. The proviso here is that the [prion
] must be [PIN + ] (see Section II.C below).

A. The [PSI +] Prion

Yeast Prions and Their Analysis In Vivo

In its [PRION + ] state, the translation termination factor Sup35p (also known as eRF3 – eukaryotic release factor 3) gives rise to [PSI + ] cells, which show a defect in translation termination that can be readily detected by a simple nonsense suppression-based assay (Cox, 1965; Figure 1, see Colour Plate Section). Sup35p physically interacts with at least one other protein, eRF1 (Sup45p), to form the functional release factor needed for polypeptide chain release (Stansfield et al., 1995). The inactivation of Sup35p via prion-mediated aggregation in a [PSI + ] strain would be expected to result in a reduction in levels of the Sup35p:Sup45p functional complex required for translation termination. This in turn would lead to an increase in the frequency with which ribosomes can read through a defined nonsense codon thus giving rise to nonsense suppression.

Figure 1. A simple colony colour assay for the presence of the [PSI + ] prion in Saccharomyces cerevisiae. Either of two different suppressible alleles can be used, the ade2-1 allele and the ade1-14 allele. In both the cases, when the mutation is expressed, i.e. in a [psi
] strain, the cells form red colonies that signal an adenine auxotrophic phenotype. In [PSI + ] cells, suppression of either allele leads to white colonies that are prototrophic and can grow without the provision of exogenous adenine. Note that strains carrying the ade2-1 allele must also carry the weak ochre suppressor tRNASer encoded by the SUQ5 (SUP16) gene (Cox, 1965), whereas the ade1-14 allele can be suppressed directly by [PSI + ] in the absence of a suppressor tRNA. The identity of the amino acid inserted when the UGA codon in the ade1-14 allele is suppressed is unknown but is likely to be tryptophan (encoded by the UGG codon) (See color plate section).

495

In [PSI + ] cells, a significant proportion of the Sup35p in the cell is present in the form of one or more high-molecular-weight aggregates that can be readily sedimented from cell lysates by ultracentrifugation. In prion-free [psi
] cells Sup35p is largely soluble (Patino et al., 1996; Paushkin et al., 1996). The [PSI + ]-associated Sup35p aggregates contain both protease resistant (Paushkin et al., 1996) and SDSresistant forms of Sup35p (Kryndushkin et al., 2003). While it is usually assumed that these aggregates are most likely to be amyloid in nature, this has not been formally demonstrated in vivo, although in vitro polymerisation studies with Sup35p show that it can form amyloid-like fibres in vitro (Glover et al., 1997; King et al., 1997). Two basic [PSI + ] variants have been described which show no differences in the primary amino acid sequence of Sup35p, but which do show phenotypic and biochemical differences. In cells carrying the ‘strong’ [PSI + ] variant, 90% or more of the Sup35p in the cell is usually present in the form of high-molecular-weight aggregates leading to efficient nonsense suppression. In the ‘weak’ [PSI + ] variants, a much greater proportion of the Sup35p is present in the soluble fraction (Uptain et al., 2001) and consequently the nonsense suppression phenotype is weaker (Figure 2). The differences in phenotype most likely arise due to subtle differences in Sup35p conformation, which in turn alter the rate at which new propagons are formed in growing cells. Critical for the de novo formation and propagation of the [PSI + ] prion is the Gln/Asn-rich prion-forming region (PrD) located at the N-terminus of Sup35p, between residues 1 and 97 (Figure 3; Ross et al., 2005, review). This largely unstructured region of the protein consists of two functionally distinct sub-regions (Osherovich et al., 2004): (a) The QN-rich (QNR) region that spans residues 1–40 and is particularly rich in Asn and Gln residues. This region is necessary

Figure 2. Sub-cellular fractionation analysis can be used to distinguish between both [PSI + ] and [psi
] strains and between ‘weak’ and ‘strong’ variants of [PSI + ]. Three different samples, analysed by SDS-PAGE and Western blotting using an anti-Sup35p antibody, are shown: T, total un-fractionated extract; S, soluble fraction after centrifugation at 100 000g and P, the pellet fraction remaining after the ultracentrifugation step. Note that the ‘weak’ [PSI + ] variant has more soluble Sup35p and has a weaker nonsense suppression phenotype compared to the ‘strong’ [PSI + ] variant as judged by both colony colour (where dark tones represent red colony pigmentation) and relative growth on an adenine-deficient medium.

496

for prion aggregate formation and contains a highly amyloidogenic region GNNQQNY between residues 7 and 13 (Diaz-Avalos et al., 2003). (b) The oligopeptide repeat-containing region (OPR), which is required for propagation of the prion form of the protein and contains five imperfect copies of an oligopeptide repeat. The Sup35p-PrD is separated from the functional C-terminal region of the protein molecule by a highly charged M region which may also contribute to the prion-like properties of the Sup35p protein (Figure 3; Liu et al., 2002). Over expression of the Sup35p-PrD or the Sup35p-PrD+M leads to an increase in the rate of de novo induction in a [psi
] cell provided the [PIN + ] prion is present in the cell (Wickner et al., 2001, review).

B. The [URE3+] Prion The main cellular role of the Ure2p prion protein is to regulate nitrogen catabolic gene expression at the level of transcription in response to nitrogen levels. In the presence of excess nitrogen, Ure2p forms a complex with the transcription factor Gln3p and this in turn leads to the sequestration of Gln3p in the cytoplasm and a concomitant reduction in the transcription of genes regulated by Gln3p (Kulkarni et al., 2001). In [URE3] cells, the aggregation of 497

Yeast Prions and Their Analysis In Vivo

Figure 3. Basic organisation of the three native prion proteins of Saccharomyces cerevisiae. For each protein the location of the prion-forming domain (PrD) is indicated. For the Sup35p protein, a more detailed description of the PrD is provided in which the location of the Gln/Asn-rich region (QNR) and the region containing the five copies (and one part copy) of an oligopeptide repeat (OPR) are shown, as defined by Osherovich et al. (2004). The numbers indicate the residue number with the initiator Met being taken as residue 1. The positions of the Met residues that are used to demark the three functionally distinct regions of Sup35p (i.e. PrD, M and C) are also shown.

Ure2p results in the activation of Gln3p and its downstream targets, e.g. the DAL5 gene, which encodes a permease for the uptake of poor nitrogen sources such as allantoate and the structurally related metabolic intermediate ureidosuccinic acid (USA). The Ure2p protein has a C-terminal region (residues 65–354) that shows sequence and structural identity to glutathione-S-transferases and contains the nitrogen regulatory function(s). However, Ure2p does not have glutathione transferase activity, but does appear to have an associated glutathione peroxidase activity even when in its amyloid-like, fibrillar form (Bai et al., 2004). The amino-terminal region of Ure2p (residues 1–65) is required for both its prion-like behaviour in vivo and its seeded polymerisation in vitro, and is particularly rich in Asn residues. The propagation of the [URE3] prion is dependent upon the Hsp104p chaperone although other chaperones, particularly the Ssa1/2p and the Hsp40 Ydj1p have also been implicated in the mechanism which propagates [URE3] (Moriyama et al., 2000).

C. The [RNQ+]/[PIN +] Prion and De Novo Conversion The ability of the Rnq1p protein to form a prion-like determinant was established by Sondheimer and Lindquist (2000) who showed that this protein could take up an insoluble, aggregated form that was propagated by an Hsp104-dependent mechanism. Although the cellular function of soluble Rnq1p remains to be established, when in its prion form, it can facilitate the de novo formation of other prions. Both [PSI + ] and [URE3] prions can also arise de novo, either spontaneously (at a frequency of 10
5) or by over expression of the corresponding protein or its PrD (which elevates the rate of de novo conversion some 100–1000 fold) (Chernoff et al., 1993; Wickner, 1994). Both spontaneous and induced de novo conversion of cells to [PSI + ] require the presence of a second prion originally called [PIN + ] (for [PSI]-inducing prion). [PIN + ] is the prion form of the Rnq1p protein in most [PIN + ] laboratory strains studied although other proteins can form [PIN + ] (Derkatch et al., 2001; Osherovich and Weissman, 2001). How [PIN + ] mediates de novo conversion of a sequence unrelated protein is unknown, but presumably it either sequesters an anti-aggregation factor from cells, which leads to an increase in the rate of spontaneous aggregation of the Sup35p (Osherovich and Weissman, 2001) or the [PIN + ] prion nucleates the polymerisation of soluble forms of Sup35p or Ure2p leading to the formation of the seeds necessary for the propagation of the [PRION + ] state (Derkatch et al., 2001). Recent in vitro studies have provided strong support for the latter model (Derkatch et al., 2004). Rnq1p is a non-essential 405 residue protein rich in Gln and Asn residues (hence RNQ, rich in N and Q) and, like Ure2p and Sup35p, is also able to form amyloid-like fibrils in vitro (Sondheimer and 498

Lindquist, 2000). Although the precise location of the Rnq1p PrD has yet to be mapped, this function resides between residues 130 and 405 in a C-terminal region rich in Asn and Gln residues, i.e. it is a C-terminally located PrD (Sondheimer and Lindquist, 2000). Maintenance of the prion form of Rnq1p requires the Hsp104p chaperone (Sondheimer and Lindquist, 2000), but in addition requires another member of the Hsp40 family, Sis1p (Sondheimer et al., 2001).

~~~~~~ III. ANALYSIS OF PRION-ASSOCIATED The [PSI + ] and [URE3] prions were uncovered in classical genetic screens: a rare [PSI + ] mutant emerged from a genetic screen for nonsense suppressor mutants using an ade2-1-based assay (Cox, 1965), while the [URE3] mutant emerged from a screen for yeast mutants that allowed a ura2 mutant to grow on a minimal medium supplemented with ureidosuccinic and glutamic acids (Lacroute, 1971). In both the cases, it was the unusual genetic behaviour of these mutants that lead their discoverers to conclude that neither mutant could be simply explained by a nuclear gene mutation. It was not until 1994 however that these properties were linked to the existence of prions in yeast (Wickner, 1994). In contrast, the [RNQ/ PIN + ] prion was identified by a rational approach: following the demonstration that the amino-terminal Sup35p-PrD was rich in Gln and Asn residues and a feature important for its prion-like behaviour, sequence-led searches were undertaken for other yeast proteins with similar QNR regions. Over 100 such proteins were identified (Michelitsch and Weissman, 2000; Sondheimer and Lindquist, 2000) but only one of these – to date – has proven to be a prion, i.e. Rnq1p. At least one other protein – New1p – has an QNR region that can functionally replace the equivalent QNR region in the Sup35p-PrD (Osherovich et al., 2004), but whether fulllength New1p forms a prion in the cell remains to be established. Although Rnq1p satisfies all the criteria for to be classified as a prion protein, no cell-level phenotype was originally detected that could readily identify [RNQ + ] strains. With the subsequent discovery that the [RNQ + ] prion can act as a [PSI]-inducing prion (see above), a phenotypic assay has now become available that allows for detection of its presence in yeast strains.

A. The [PSI +] Prion Phenotype [PSI + ] was identified as a mutation that enhanced the ability of the weak suppressor tRNASer encoded by the SUQ5 (also called SUP16) gene to suppress the ade2-1 allele. This allele of ade2 contains a premature UAA codon at codon position 64 (Prokopi, M., 499

Yeast Prions and Their Analysis In Vivo

PHENOTYPES

Koloteva-Levin, N. and Tuite, M. F., unpublished) and when the mutant phenotype is expressed, this leads to red-coloured colonies. There was also some evidence early on that [PSI + ] could weakly suppress nonsense mutations in the absence of the SUQ5 tRNA, e.g. cyc1-72 (Liebman and Sherman, 1979). However, the identification of an ade1 allele (ade1-14) that was relatively efficiently suppressed by [PSI + ] in the absence of a cognate suppressor tRNA yet also had the red/white colour selection, and has lead to it being widely used to assay for [PSI + ]. The ade1-14 allele has a UGA codon in place of UGG codon at position 244 (Nakayashiki et al., 2001; see Figure 1). The availability of the ade1-14 and ade2-1 nonsense alleles therefore provides two simple colony-level assays for the presence of the [PSI + ] prion, namely colony colour and growth on medium lacking adenine. Furthermore, the strength of the phenotype, as defined by varying shades of white and pink and the degree of adenine auxotrophy, allows the ready differentiation between weak and strong [PSI + ] variants: the strong variants are usually white/pale pink and show a strong Ade+ phenotype, while the weak [PSI + ] variants usually give rise to pink/dark pink colonies that only grow relatively weakly on adenine-deficient medium. In our hands, the medium that gives the best distinction in red/white colouration when plating for single colonies is 1/4 YEPD (1% peptone, 0.25% yeast extract, 4% glucose plus 2% agar). Suppression of the auxotrophy associated with the ade1-14 and ade2-1 alleles can be relatively inefficient (especially for weak [PSI + ] variants). Consequently, the addition of a small amount of adenine (usually 1% w/v of normally added levels) or of 2.5 ml YEPD per 100 ml of the standard YNB-based minimal medium lacking adenine, can give greater distinction between the [PSI + ] and [psi
] cells after only 2–3 days growth. Suppression of the ade1-14/ade2-1 markers gives only a qualitative estimate of the efficiency of suppression. For a more quantitative estimate of the relative efficiencies of nonsense suppression, a plasmid-based stop codon read-through system originally described by Firoozan et al. (1991), can be used. This involves the expression of a plasmid-borne PGK-lacZ gene fusion where the two reading frames are separated by one or other of the three stop codons UAA, UAG or UGA. A fourth construct has a sense codon at this position and is used to determine the control (100%) value. Subsequently, a number of variations of this type of bi-cistronic assay have been developed; for example, the plasmid pAC99 which involves two functional cistrons whose products can be independently assayed for in cells, i.e. lacZ encoding b-galactosidase and luc encoding luciferase, separated by a stop codon (Namy et al., 2002). Using extracts prepared from pAC99 transformed cells both the b-galactosidase and luciferase activities are quantified and the ratio of luciferase activity to b-galactosidase activity calculated. These levels are then compared to the equivalent activities in cells expressing a bone fide b-galactosidase–luciferase fusion protein. 500

The relative efficiency of nonsense suppression for a given strain/ stop codon can then be calculated by dividing the luciferase/ b-galactosidase ratio obtained by the same ratio obtained with the in-frame protein fusion control.

The change in nitrogen metabolism that occurs in [URE3] cells can be used to detect the presence of this prion in ura2 cells. The URA2 gene encodes the enzyme aspartate transcarbamylase that catalyses the synthesis of USA. In ura2 cells that are deficient in this enzyme, if the [URE3] prion is present, such cells can utilise sodium ureidosuccinate in the absence of uracil because the Dal5p permease is present. [URE3] can also be detected in cells carrying the wild-type URA2 + gene because [URE3] cells excrete uracil in the presence of excess USA and this can be detected by haloes formed by cross-feeding on a lawn of ura2 cells (Chernoff et al., 2002). An alternative way of detecting [URE3] using a colony colourbased assay has also been established, which avoids the need to select for growth on a USA-containing medium. Schlumpberger et al. (2001) developed a novel ADE2-based reporter for detecting the [URE3] prion in which the wild-type ADE2 gene is placed under the control of the DAL5 promoter (PDAL5). This promoter is regulated by Ure2p and the DAL5 gene product, the Dal5p transporter, is necessary for the uptake of USA. Consequently, when the PDAL5ADE2 reporter is expressed in an ade2 mutant in the presence of ammonium ions, Ure2p binds to Gln3p thereby preventing transcription of the PDAL5ADE2 gene and thus the colony is red. If expressed in a [URE3] ade2 mutant, the release of the Gln3p block results in transcription of the PDAL5ADE2 gene and the colonies are white, adenine prototrophs. Such a colour-based screen for [URE3] has been used to search for compounds that eliminate [URE3] from cells (Bach et al., 2003) and to identify natural variants of the [URE3] prion (Schlumpberger et al., 2001). Because of the intrinsic mitotic instability of many [URE3] isolates it is important to maintain selection for the [URE3] prion in such strains by growth on minimal medium lacking adenine (Kyprianidou, C., Byre, L. J. and Tuite, M. F., unpublished).

C. The [PIN +] Prion Phenotype The prion-based inactivation of Rnq1p function does not lead to a detectable change in phenotype other than allowing for a high rate of de novo formation of the [PSI + ] prion in cells. The constitutive or transient overexpression of either full-length Sup35p or just the Sup35p-PrD in a [psi
] [PIN + ] strain usually leads to a 102–103-fold increase in the frequency of appearance of [PSI + ] cells and in some genetic backgrounds this can be as high as 30% of the cells. To carry 501

Yeast Prions and Their Analysis In Vivo

B. The [URE3] Prion Phenotype

out such an assay, a single copy or multicopy plasmid carrying the SUP35 construct under the transcriptional regulation of the GAL1 promoter is used (Chernoff et al., 2002). This plasmid and a suitable control plasmid (e.g. the expression vector without the SUP35 sequence) are independently introduced into an otherwise isogenic pair of ade1-14 [pin
] or [PIN + ] strains and the cells patched onto an agar plate-containing medium that retains selection for the plasmid and contains galactose to induce expression of the SUP35 gene construct. After 2–3 days growth the cells are replica plated onto a glucose-based adenine-deficient medium to score the Ade+ [PSI + ] cells while switching off the overexpression of the SUP35 construct to avoid any associated toxicity; overexpression of SUP35 is lethal in most [PSI + ] strains (Chernoff et al., 1993). While a relatively straightforward assay, there are a number of complications in assessing the outcome of the assay which need to be considered (Chernoff et al., 2002); for example, nuclear SUP gene mutations can also lead to suppression of the ade1-14 allele.

~~~~~~ IV. GENETIC ANALYSIS OF YEAST PRIONS

A. Meiotic Transmission The unusual pattern of non-Mendelian inheritance for both the [PSI + ] (Cox, 1965) and [URE3] (Lacroute, 1971) traits implicates a genetic determinant located in the cytoplasm. For example, when a [PSI + ] haploid strain is crossed with a [psi
] haploid strain of the opposite mating type, the resulting diploid invariably shows the [PSI + ]-associated nonsense suppression phenotype, i.e. [PSI + ] is genetically dominant. Analysis of the meiotic spores from such a cross usually reveals a clear 4[PSI + ]:0[psi
] segregation pattern for the spores in a single tetrad although some ‘weak’ [PSI + ] variants will often give rise to tetrads containing 3[PSI + ]:1[psi
] spores or even 2[PSI + ]:2[psi
] spores (Derkatch et al., 1996). There are nuclear gene mutations that will also give rise to a [PSI + ]-like phenotype; for example, the ochre tRNATyr suppressor mutants such as SUP4 which are able to suppress ade2-1 in a [psi
] genetic background, but these give rise to two Ade+ (white) to two Ade
(red) spores per tetrad. Inheritance of [URE3] shows similar genetic behaviour to [PSI + ], but while the majority of asci give 4[URE3]:0[ure3] spores, most crosses also produce asci showing 3:1 and 2:2 [URE3]:[ure3] patterns of inheritance indicating that the [URE3] prion shows the high level of meiotic instability as is seen with the weak [PSI + ] variants (see above). The prion form of Rnq1p, in the form of the [PIN + ] prion, is also genetically dominant and when crossed to a [pin
] strain, the majority of asci carry 4[PIN + ]:0[pin
] spores (Derkatch et al., 1996). 502

When carrying out genetic crosses to confirm the presence of a yeast prion ideally the mating partner should be an otherwise isogenic haploid strain in which the mating type has been switched. There is significant variation in the genetic backgrounds of many of the laboratory strains used by yeast researchers and the presence of a number of undefined genetic modifiers can affect the strength of the prion phenotype. A suitable strain can be obtained by taking the prion-free form of the original strain and introducing a plasmid expressing the HO gene. The majority of laboratory strains are defective in the HO gene (i.e. are ho
) and so do not switch their mating type. Expression of the HO-encoded endonuclease triggers the mating type switch and cells can then be readily identified that have lost the plasmid-borne copy of the HO and who had their mating type switched.

One classical method for demonstrating transmission of a genetic determinant through the cytoplasm in fungi is cytoduction. Although not a normal part of the yeast life cycle, nevertheless cytoduction can be used due to the availability of the karyogamy defective kar1 mutant, which blocks the fusion of two haploid nuclei during mating but does not affect plasmogamy, i.e. cell fusion and mixing of the cytoplasm from the two parent strains (Conde and Fink, 1976). In a kar1  KAR1+ cross, after the cells have fused to form a cell containing two haploid nuclei, and thus equivalent to a dikaryon, new haploid daughter cells arise from the dikaryon which contain one or other of the parental nuclei but a mixture of cytoplasm arising from both parents. Figure 4 outlines the strategy that can be used to demonstrate the transfer of the [PSI + ] prion by cytoduction, but the other two yeast prions can also be efficiently transferred by cytoduction.

~~~~~~ V. ANALYSIS OF PRION PROTEIN

AGGREGATES FORMED IN VIVO As with the mammalian prion protein PrP, the aggregated prion forms of Ure2p (Masison and Wickner, 1995) and Sup35p (Paushkin et al., 1996) show an increased resistance to digestion by proteinase K compared to the soluble forms of these proteins, which in turn are readily digested by this proteinase. However, unlike PrPSc digestion with proteinase K, diagnostic and consistent proteinase-resistant protein fragments are not produced for either Sup35p or Ure2p. Proteinase K resistance is therefore usually monitored simply by the relative rate of loss of the full-length prion protein with time of incubation in the presence of the proteinase (Chernoff et al., 2002). 503

Yeast Prions and Their Analysis In Vivo

B. Transmission by Cytoduction

Figure 4. The use of cytoduction to demonstrate the cytoplasmic transmission of yeast prions. The experiment requires a [prion
] recipient strain of the opposite mating type to the strain that is [PRION + ]. The recipient strain carries the kar1 mutation (Conde and Fink, 1976) together with one or more nuclear genetic markers that are different to the markers in the donor strain. In addition, the recipient strain is made a [rho0] petite by growth in the presence of ethidium bromide. Following mating of the donor and recipient strain (Stage 1), the mating mixture will contain a large number of heterokaryons (Stage 2). The mix is then plated onto a selective medium that does not allow either cells carrying the donor nucleus or any rare diploid cells that may form to grow, but does allow haploid cells (cytoductants) carrying the recipient nucleus to grow. The use of a [rho0] petite recipient strain also allows the experimenter to confirm that cytoplasmic transfer has occurred since this would result in cytoductants carrying the recipient nucleus becoming [RHO + ] grande and thus being able to utilise a non-fermentable carbon source such as glycerol. The example shown is for studying the [PSI + ] prion, but can equally well be applied to other prions.

That the Ure2p and Sup35p prion aggregates show increased resistance to proteinase K digestion has been taken as one line of evidence that the yeast prion aggregates are amyloids. Certainly, the recombinant forms of these proteins form proteinase K resistant, amyloid-like structures in vitro (see Section VIII below), but the evidence that they form amyloid-like structures in the cell is less convincing. [URE3] cells engineered to over express Ure2p certainly contain distinctive, filamentous networks of Ure2p in the cytoplasm (Speransky et al., 2001) consistent with an amyloid-like structure, although Ripaud et al. (2003) have suggested, based on proteinase K digestion patterns, that aggregated Ure2p in [URE3] yeast cells is conformationally distinct from the amyloid form of this protein generated in vitro. Another approach is to use amyloid-specific 504

stains on [PRION + ] cells and Kimura et al. (2003) have shown that aggregates made by over expressing either the Rnq1p-PrD or the Sup35p-PrD can be stained in yeast cells by thioflavin-S, an amyloid-binding dye. However, staining [PSI + ] cells with another amyloid-specific dye, 2-(40 -methylaminophenyl) benzothiazole (BTA-1), an uncharged derivative of thioflavin-T (Mathis et al., 2002) also stains a number of aggregates or structures in the [PSI + ] cell but because these structures are also seen in [psi
] cells in which the HSP104 gene deleted, they are unlikely to represent amyloid-like aggregates associated with the prion form of Sup35p or Rnq1p (Byrne, L. J. and Tuite, M. F., unpublished data). In [PSI + ] cells, a certain proportion of the Sup35p protein remains soluble and functional and although the amount is sufficient to ensure cell viability, it is not enough for efficient termination. The relative proportion of soluble:aggregated Sup35p also varies depending on the [PSI + ] variant being studied (Figure 2; Uptain et al., 2001) and this is reflected in the termination phenotype as ‘strong’ variants have a proportionally more severe termination defect than ‘weak’ variants (see Section II.A).

The standard way of establishing whether or not a specific prion protein is present as a high-molecular-weight aggregate is by the use of differential centrifugation of total cell extracts usually prepared from exponentially growing cells. This results in the generation of soluble and pellet fractions and the presence of the respective prion protein in each fraction can be assessed by SDS-PAGE and Western blotting (Figure 2). To prepare total cell extracts for such an analysis, cells are usually disrupted using glass bead lysis and the extract then subjected to centrifugation at 100 000g for between 15 and 30 min at 41C. No preliminary slow speed spin is carried out to remove cell debris as this can result in loss of a significant proportion of the highmolecular-weight forms of the prion protein prior centrifugation. Consequently, it is important to solubilise the pellet fraction after the high-speed spin and prior to electrophoresis by boiling in an SDS-based sample buffer (Ness et al., 2002). The standard sub-cellular fractionation protocol does however need to be optimised for each of the three different yeast prion proteins and/or for different yeast strains under test. For example, although good separation of Sup35p between the soluble and pellet fractions in [PSI + ] and otherwise isogenic [psi
] strains can be achieved by centrifugation for 15 min (Figure 2), good fractionation of Rnq1p in [PIN + ]/[pin
] strains requires a longer spin (30 min) with a higher centrifugal force (Figure 5). Soluble Rnq1p is also a very unstable protein in the presence of a range of protease inhibitors and, following differential centrifugation, the samples need to be analysed immediately by Western blot analysis, and not stored for 505

Yeast Prions and Their Analysis In Vivo

A. Sub-Cellular Fractionation of Prion Aggregates

Figure 5. Sub-cellular fractionation analysis of the Rnq1p protein can be used to distinguish between [PIN + ] and [pin
] strains. Three different samples prepared from the strain 74D-694 were analysed by SDS-PAGE and Western blotting using an anti-Rnq1p antibody: T, total un-fractionated extract; S, soluble fraction after centrifugation at 100 000g and P, the pellet fraction remaining after the ultracentrifugation step.

any length of time, in order to avoid degradation of soluble Rnq1p (Koloteva-Levin, N. and Tuite, M. F., unpublished). Fractionation by differential centrifugation does not result in good fractionation of Ure2p into the expected soluble and pellet fractions in some [URE3]/ [ure3] strain pairs. This raises the question of whether Ure2p actually forms high-molecular-weight aggregates in all [URE3] strains expressing wild-type levels of Ure2p (Fernandez-Bellot et al., 2002). What is also clear is that the nuclear genetic background of the strain can influence the relative distribution of protein between the soluble and pellet fractions in [PRION + ] yeast strains irrespective of the prion type or variant present.

B. Separation of Prion Protein Oligomers Using Agarose Gel Electrophoresis Sedimentation analysis provides a relatively quantitative method for assessing the fraction of the respective prion protein that forms high-molecular-weight aggregates in a [PRION + ] strain compared to a control [prion
] strain. What it does not do is provide information on the nature of the aggregates that are formed, nor on any oligomeric sub-particles that may be present. Such information can be obtained by using a novel electrophoretic method that uses agarose rather than acrylamide as the separation matrix, a method termed semi-denaturing detergent agarose gel electrophoresis (SDD-AGE: Kryndushkin et al., 2003). SDD-AGE was originally developed to study Sup35p aggregates in [PSI + ] strains. In preparing samples for such analysis, Kryndushkin et al. (2003) discovered that treating total yeast cell extracts with SDS and incubating at various temperatures between room temperature and 651C resulted in the disassembly of the highmolecular-weight Sup35p aggregates into more discrete SDS-stable oligomeric forms of Sup35p ranging in size between 10 and 50 506

Yeast Prions and Their Analysis In Vivo

monomers of Sup35p. The relationship between these oligomers and the forms of Sup35p that are important for Sup35p oligomerisation and [PSI + ] propagation in vivo remain to be established. Nevertheless, they show different size distributions in different [PSI + ] variants and also in [PSI + ] cells where the activity of Hsp104p has been inhibited by guanidine hydrochloride (GdnHCl) (Kryndushkin et al., 2003). The SDS-resistant Sup35p-containing oligomers detected in [PSI + ] strains are not discrete entities, but represent a range of sizes between 1.5 and 3.0 MDa (Figure 6). The challenge of finding suitable molecular weight markers for SDDAGE analysis can be met by using chicken pectoralis extract which contains myosin heavy chain (205 kDa), nebulin (740 kDa) and titin (approximately 4 MDa: Kryndushkin et al., 2003). SDD-AGE can also be used to study SDS-stable oligomers of Rnq1p that form in [PIN + ] strains and these oligomers contain between 20 and 100 Rnq1p monomers (Bagriantsev and Liebman, 2004). Differential thermal stability of the Rnq1p oligomers can be used to distinguish between two different variants of the [PIN + ] prion. An alternative approach to fractionating the high-molecularweight aggregates formed by yeast prion proteins is to use sucrose gradient centrifugation (Paushkin et al., 1996). Centrifugation of total cell extracts through a 15–40% sucrose gradient at 170 000g can separate out aggregates from soluble forms of Sup35p although, as with SDD-AGE, the aggregates are relatively dispersed throughout the gradient. The need to remove cell debris prior to loading on the sucrose gradient can however lead to loss of prion aggregates.

Figure 6. The use of semi-denaturing detergent agarose gel electrophoresis (SDDAGE) to study prion aggregates in a [PSI + ] and [psi
] pair of strains. Samples were prepared and fractionated on an agarose gel in the presence of SDS as described by Kryndushkin et al. (2003). Following transfer of the proteins, the membrane was blotted using an anti-Sup35p antibody. The location of the disperse Sup35p-containing polymers in the [PSI + ] strain and the monomeric form of Sup35p present in the [psi
] strain, are indicated. Molecular weight markers used were myosin heavy chain (205 kDa), nebulin (740 kDa) and titin (approximately 4.2 MDa).

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C. GFP Fusion Technology One direct means of assessing whether or not a given strain contains prion-like aggregates of a specific protein is to express in those cells, a fusion protein between the prion-forming domain (PrD) of the protein in question and the widely exploited reporter gene encoding green fluorescent protein (GFP). [A detailed account of the use of GFP as a reporter gene in yeast can be found in Chapter 8, this volume, von der Haar et al.] If cells of a given strain contain the prion in question, the fusion protein is seeded to form discrete fluorescent foci, whereas in the [prion
] cell the fusion protein is usually detected as diffuse fluorescence in the cell’s cytoplasm. The standard strategy is to introduce the gene encoding the prion protein PrD-GFP fusion into the cell on a single copy plasmid with the gene under the control of either the galactose-inducible GAL promoter or the copper-inducible CUP1 promoter (Patino et al., 1996). Using the weaker SUP35 gene promoter results in lower levels of expression of the fusion protein and this can make visualisation of the prion-related foci relatively difficult. The problem with using an efficient promoter to express the fusion protein is that this will also induce the de novo formation of the prion state if the cell is [PIN + ] (see above). In [PSI + ] cells the number and nature of the Sup35pPrD-GFP aggregates (or foci as they are usually referred to as) can show significant variation in both numbers and morphology both within and between different strains (Figure 7, see Colour Plate Section). In some strains a few large discrete foci can be seen while in others, numerous small foci are detected. Different [PSI + ] variants in the same nuclear genetic background also give very different types of Sup35pPrD-GFP foci; ‘weak’ variants have a few large foci while ‘strong’ variants have many small foci (Fernandez-Bellot, E. and Tuite, M. F., unpublished). In cells undergoing de novo conversion as a consequence of over expression of the Sup35p-PrD, one can also detect striking elongated structures (Figure 7, Panel e) which disappear after the [PSI + ] state has stabilised (Zhou et al., 2001). Both Rnq1pPrD-GFP (Sondheimer and Lindquist, 2000) and Ure2pPrD-GFP (Edskes et al., 1999) fusions have also been used. In the case of Rnq1pPrD-GFP, invariably only one of the two types of fluorescence pattern have been reported for [PIN + ] cells: either ‘single dot’ or multidot’ foci and this may diagnose the existence of two different variants of this prion (Bradley and Liebman, 2003). With Ure2pPrD-GFP the situation is slightly more complex. For example, not all [URE3] cells produce distinct foci following the expression of the Ure2pPrD-GFP fusion and problematically, the expression of this fusion in a [URE3] cell results in loss of the [URE3] prion (Edskes et al., 1999). Furthermore, Fernandez-Bellot et al. (2002) showed that while expression of Ure2-GFP leads to the formation of aggregates, this aggregation is not apparently

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associated with [URE3] formation per se. The use of Ure2pPrD-GFP to study the [URE3] prion is therefore of questionable value. There are also a number of other issues that raise concerns about the value of using GFP fusions to study yeast prions in vivo: (a) The variability seen within a single population of cells expressing the fusion where not all cells in the populations show the same behaviour. (b) The way in which cells are prepared for microscopic analysis can modify the appearance of the GFP foci (Chernoff et al., 2002). (c) The change in the number and/or morphology of the foci as cells move from exponential growth to stationary phase (Zhou et al., 2001). (d) A significant proportion of cells showing clear distinct foci are usually dead particularly in the case of [PSI + ] cells (Byrne, L. J. and Tuite, M. F., unpublished).

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Yeast Prions and Their Analysis In Vivo

Figure 7. The use of green fluorescent protein (GFP) fusions to visualise Sup35pbased aggregates in [PSI + ] strains. All cells shown contain a plasmid expressing an identical Sup35pPrD+M-GFP fusion protein whose synthesis was induced using the copper-inducible CUP1 promoter (Patino et al., 1996); (a) a [psi
] strain; (b)–(d) different [PSI + ] variants; (e) a [PIN + ][psi
] strain undergoing de novo conversion to [PSI + ] as a consequence of the over expression of the Sup35pPrD+M-GFP fusion protein (see Zhou et al., 2001) (See color plate section).

The relationship between the GFP foci and the oligomeric forms of the native prion protein necessary for propagation of the prion state is also not established and most likely the observable fluorescent foci are the dead-end products of the aggregation process and play no direct role in prion propagation.

~~~~~~ VI. ELIMINATING YEAST PRIONS Native yeast prions can be readily and rapidly eliminated – in some cases with almost 100% efficiency – from yeast cells either by growing in the presence of various chemical agents or by rational manipulation of the levels of the molecular chaperone Hsp104p (Table 2). None of these agents cause a change in the DNA sequence of the prion gene, i.e. are ‘non-mutagenic’, but rather give rise to a defect in the process by which prions are propagated. Chemical agents that are known to cause gene mutation via DNA sequence damage, e.g. ethyl methane sulphonate (EMS) and ultraviolet light (UV), can also induce prion loss (Cox et al., 1980), but the rate at which this occurs is consistent with the underlying event being a mutation in either the prion gene, that impairs its ability to take up and/or maintain the prion state, or in the HSP104 gene.

A. Elimination by Guanidine Hydrochloride A number of chemical agents have now been described that can eliminate one or more of the native prions (Table 2). The most Table 2. Chemical agents that can eliminate yeast prions Chemical agent

Concentration Mode of action

Guanidine hydrochloride (GdnHCl) Methanol

1–5 mM

Latrunculin A

200–500 mM

KCl

2M

5–10% v/v

6-Aminophenanthridine 0.2 mM

Kastellpaolitines (various)

nd

nd, not determined.

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Inhibits ATPase activity of Hsp104p Unknown Unknown (inhibits actin cytoskeleton) Unknown Unknown (requires 200 mM GdnHCl) Unknown (requires 200 mM GdnHCl)

Reference Tuite et al. (1981) Tuite et al. (1981) BailleulWinslett et al. (2000) Singh et al. (1979) Bach et al. (2003) Bach et al. (2003)

Yeast Prions and Their Analysis In Vivo

effective and widely used means of eliminating [PSI + ], [PIN + ] or [URE3] is by growing cells in the presence of 3–5 mM GdnHCl, a chaotropic protein denaturant (Tuite et al., 1981). Although such treatment also generates a high frequency of mitochondrial [rho
] petite mutants (Juliani et al., 1975), provided the cells are allowed to continually grow for at least 10 generations in the presence of the GdnHCl, then approaching 100% of the cells remaining at the end of the experiment will be [prion
] (Eaglestone et al., 2000; Figure 8, see Colour Plate Section). The reason that GdnHCl has this dramatic effect is not because it is a protein denaturant per se; the normal concentrations used for protein denaturation are normally in the 1–5 M range. Rather, the GdnHCl appears to act as a potent and specific inhibitor of the ATPase activity of the Hsp104p chaperone that is required for propagation of all three prions (Ferreira et al., 2001; Jung et al., 2002; Grimminger et al., 2004). Therefore, in the presence of GdnHCl, Hsp104 chaperone function is impaired and this in turns leads to a failure to produce the new propagons required for the continued propagation of the prion form. Consequently, those propagons that were present at the point in time at which the GdnHCl is added simply dilute out of the population through cell division leading to the emergence of [prion
] cells after only a few generations of growth. Other guanidinium salts, e.g. guanidine dihydrogen sulphate, can also effectively eliminate [PSI + ] from growing cells (Lawrence, C. W., Eaglestone, S. S. and Tuite, M. F., unpublished). In practice, all that is needed to generate [prion
] cells is to patch cells onto an agar plate with a rich medium (e.g. YEPD) containing

Figure 8. The elimination of the [PSI + ] prion from cells grown in the presence of 3 mM guanidine hydrochloride (GdnHCl) over a 30-h period. The experiment was carried out as described in the Protocol 1 and the percent [PSI + ] with time was plotted as shown. The data are combined from three independent experiments. The inset shows the types of colonies that one observes in such an experiment noting in particular that (a) GdnHCl induces a high frequency of mitochondrial petites, and (b) the petite mutation does cause a change in colouration when compared to grande strains with functional mitochondria. Colonies sectored red and white are counted as [PSI + ]. Further details can be found in the text (See color plate section).

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3–5 mM GdnHCl. After 48–72 h growth, the cells should be transferred to a fresh YEPD+GdnHCl plate and to ensure efficient elimination of the prion from all cells, this should then be repeated one or two more times. However, because this treatment does not destroy the propagons that were present in the cells prior to the addition of GdnHCl, there will always be a few [PRION + ] cells remaining in the population (Cox et al., 2003). This phenomenon can be exploited as a means of estimating the number of propagons in a cell (see Section VII.B below). The efficiency of GdnHCl-mediated prion elimination does depend on the medium used with lower concentrations of GdnHCl being required in rich (YEPD) medium than those required to achieve the same effect in defined (YNBbased) medium, being 3 and 5 mM, respectively for efficient [PSI + ] elimination.

B. Elimination by Other Chemical Agents A number of other chemical agents have also been described that cause the loss of [PSI + ] from growing cells (Table 2) but they are generally less efficient than GdnHCl and generally the underlying mechanism is unknown. These include methanol and DMSO (Tuite et al., 1981) and the toxin latrunculin A (Bailleul-Winslett et al., 2000). For latrunculin A, continuing cell growth is not required for the loss of [PSI + ] which suggests that it may act to dissociate key oligomeric forms of Sup35p directly in non-dividing cells. That latrunculin A is known to disrupt the polymeric actin cytoskeleton (Coue et al., 1987) is consistent with this. The kastellpaolitines, a new group of compounds, have recently been reported to eliminate both [PSI + ] and [URE3] and were identified using a cell-based screen for anti-prion compounds (Bach et al., 2003). In this assay, 200 mM GdnHCl was added to the assay medium and although this concentration does not lead to prion elimination per se, it was found to be necessary for detecting the prion-eliminating properties of the kastellpaolitines. Several other prion-eliminating compounds were also identified by Bach et al. (2003) including phenanthridine and 6-aminophenanthridine. Since none of these compounds are effective in the absence of the 200 mM GdnHCl, they most probably work synergistically with the GdnHCl to inhibit Hsp104p, although it is also possible that they are simply membrane-active agents that increase uptake of GdnHCl from the medium leading to a higher than normal intracellular level of GdnHCl than would normally be achieved when cells are grown in 200 mM GdnHCl. Importantly, several of the compounds identified by Bach et al. (2003) also inhibit the formation of PrPSc in cultured animal cells and since mammalian cells do not appear to have an orthologue of Hsp104p, this would suggest that these compounds actually affect prion conversion directly. 512

The molecular chaperone Hsp104p is essential for the propagation of all three yeast prions and, as originally reported by Chernoff et al. (1995), either ablation or overexpression of the HSP104 gene under non-stress conditions, results in the immediate loss of [PSI + ] from growing cells. Overexpression of a mutant form of Hsp104p that is ATPase negative (i.e. carries a double mutation: K218T and K620T) also results in rapid elimination of [PSI + ] even in the presence of the wild-type HSP104 gene, i.e. it has a dominant negative effect (Chernoff et al., 1995). The kinetics of elimination of [PSI + ] by overexpression of wildtype Hsp104p (Figure 9a) are different to what is seen if Hsp104p function is inhibited by GdnHCl (Figure 8) or by overexpression of the ATPase negative (K218T/K620T) allele (Figure 9b) indicating that it may cause prion loss by a different mechanism. The most plausible explanation is that the elevated levels of Hsp104p fully disaggregate prion oligomers including those required for continued propagation of the prion form, although recent biochemical studies provide conflicting evidence for and against this model (Inoue et al., 2004; Shorter and Lindquist, 2004; Krzewska and Melki, 2006). In contrast to the strict dependence of all native yeast prions on Hsp104p for their continued propagation (Sondheimer and Lindquist, 2000; Moriyama et al., 2000) only [PSI + ] is eliminated when Hsp104p is overexpressed (Chernoff et al., 1995; Moriyama et al., 2000), so this does not represent a generic method for yeast prion elimination. However, [PSI + ] elimination by Hsp104p over expression does allow for the easy construction of [PIN + ][psi
] strains. To establish whether or not Hsp104p is required for prion propagation, the most straightforward approach is to use GdnHCl, since the generation of an HSP104 gene knockout can be relatively time consuming. It should be noted however that several artificial prions have been created in yeast that do not apparently require Hsp104p for their continued propagation (e.g. Crist et al., 2003), while some of these ‘non-native’ prions are also susceptible to overexpression of three other chaperone proteins, the Hsp70’s Ssa1p and Ssb1p and the Hsp40 Ydj1p (Kushnirov et al., 2000). [URE3] is also eliminated by overexpression of Ydj1p (Moriyama et al., 2000).

~~~~~~ VII. PROPAGON COUNTING As described in Section VI.A, the addition of GdnHCl to growing yeast [PSI + ] cells results in the failure to generate any new propagons and subsequently, what propagons were present in the cells at the point at which the GdnHCl was added, are diluted out by cell division (Eaglestone et al., 2000). At the end point of this experiment, 513

Yeast Prions and Their Analysis In Vivo

C. Elimination of Yeast Prions by Manipulating the Levels of Chaperone Proteins

Figure 9. The kinetics of elimination of [PSI + ] from cells overexpressing either (A) wild-type Hsp104p or (B) a mutant of Hsp104p which is unable to hydrolyse ATP as a consequence of a double K218T and K620T mutation. Following induction of expression of the respective HSP104 genes by use of a galactose-inducible GAL1 promoter, the percent [PSI + ] cells arising when cells were plated onto 1/4 YEPD were scored over a period of 10–12 generations of growth. The inset shows colonies from this study. Note the difference both in terms of the kinetics of loss and in the types of sectored colonies that arise in the two different experiments.

there will be a number of cells in the population that contain one of the original propagons and the number of these cells gives a readout of the average number of propagons in cells in the population at 514

time ¼ 0. This observation provides a novel means for estimating the number of propagons (n0) in a given strain (Eaglestone et al., 2000).

A. The Kinetic Method

B. The Colony Method The number of propagons can also be estimated by an alternative and technically simpler protocol originally described by Cox et al. (2003). A number of single cells (usually between 10 and 20) from a given [PSI + ] strain are individually micromanipulated onto solid YEPD medium containing 3–5 mM GdnHCl. These cells are allowed to go through at least 10 generations of growth to form a visible microcolony. In the resulting colony there will be a certain number of cells that contain one of the original [PSI + ] propagons. When these cells are returned to a GdnHCl-free medium they become [PSI + ] because there is nothing to prevent the generation of new propagons, which occurs with the numbers doubling every approximately 20 min (Ness et al., 2002). The number of these cells in a given colony therefore gives a direct readout of the number of propagons in the cell at the time the cell is exposed to GdnHCl. The remaining [PSI + ] cells are detected by plating the entire microcolony onto minimal medium selecting for Ade+/[PSI + ] cells. Although some of the Ade+ cells that arise will be due to nuclear tRNA 515

Yeast Prions and Their Analysis In Vivo

In this method cells are allowed to grow continuously for up to 36 h after the addition of GdnHCl by serial dilution into fresh GdnHClcontaining medium (Protocol 1). At various time points throughout the 36 h, cell samples are taken and plated onto YEPD to count the number of [PSI + ] and [psi
] cells in the culture, and the percent [PSI + ] vs. time data are plotted. A significant proportion of the colonies that arise are sectored (Figure 8) and, for the purposes of this analysis, any colony that has a [PSI + ] component is counted as a [PSI + ] cell because at the point at which the cells were plated they must have contained at least one prion seed. Initially, Eaglestone et al. (2000) estimated n0 by a simple binomial method using the kinetics of [PSI + ] loss data, i.e. Bin (n0, 2
g), where g is the number of generations of growth and n0 the number of propagons at time ¼ 0. This gives an estimate for n0 of around 60 per cell (Eaglestone et al., 2000). Subsequently, this model has been replaced by a stochastic model that takes into account a number of key parameters that must be considered, in particular the growth rate of both mother and daughter cells and the relative distribution of propagons between mother and daughter cells (Cole et al., 2004; Cole et al., submitted for publication). Using this model together with the experimentally determined parameters, the estimation for n0 is closer to 600.

Protocol 1. Estimating the number of prion seeds in a [PSI + ] cell using GdnHCl-induced curing.

1. Inoculate a single [PSI + ] colony into 50 ml liquid YEPD medium and grow overnight at 301C in order for the culture to reach mid-to-late exponential phase (OD600
0.5–0.8) by the following morning. 2. Prepare two flasks each containing 50 ml YEPD. To Flask 1 add 3 mM GdnHCl. Flask 2 is the no GdnHCl control. 3. Inoculate both Flasks 1 and 2 with 100 ml of the overnight culture (step 1) and immediately take 3  100 ml aliquots from Flask 1, dilute and plate onto 1/4 YEPD plates so that between 100 and 200 colonies appear on each plate. This is time point t ¼ 0. Continue to incubate both flasks at 301C. 4. At 2 h intervals repeat step 3, taking 3  100 ml aliquots from Flask 1 and making allowances in the dilutions needed, for the increasing density of the culture. Continue taking samples until t ¼ 8 h. 5. Prepare two fresh flasks each containing 50 ml YEPD both containing 3 mM GdnHCl and label Flasks 3 and 4. 6. At t
14 h inoculate Flask 3 with 100 ml from Flask 1 and Flask 4 with the same volume from Flask 2. Allow these cultures to grow at 301C but no sample needs to be taken until the following morning. This staggered inoculation allows the experimenter to get a good night’s sleep. 7. At t
24 h take a fresh set of aliquots from Flasks 1 and 2. The cultures in Flasks 3 and 4 should be diluted (if necessary) into 50 ml fresh YEPD containing 3 mM GdnHCl and 3  100 ml aliquots taken from each flask and process as in step 3. This will be t ¼ 24 for Flask 1 and t ¼ 10 for Flasks 3 and 4. Continue to take two hourly samples for the next 8 h, i.e. till t
32, ensuring the culture remains in exponential phase and diluting into fresh YEPD if necessary. 8. All 1/4 YEPD plates containing cells plated from the different time points in the experiment should be incubated at 301C for a minimum of 4–5 days but no longer than 7 days to allow the full colour of the resulting colonies to develop. Transferring the grown plates to a 41C cold room for 24 h will often enhance colour development allowing best distinction between [PSI + ] and [psi] colonies. 9. From the resulting colony counts: (a) Determine the percent [PSI + ] at each time point counting any colony that has some white sectors as [PSI + ]. Note that the mitochondrial petite mutation can change the colours of colonies (see Figure 8). (b) Calculate the number of viable cell/ml for both [PSI + ]/ [psi] remembering to allow for the different dilutions that were made during the experiment.

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10. These data can then be used to determine n0, the average number of prion seeds at t ¼ 0 using either a simple binomial distribution model (Eaglestone et al., 2000) or the advanced stochastic model (Cole et al., 2004). Solutions and Media Needed YEPD liquid medium: 1% (w/v) yeast extract, 1% (w/v) bactopeptone, 2% (w/v) glucose). 1/4 YEPD solid medium: 0.25% (w/v) yeast extract, 1% (w/v) bactopeptone, 2% (w/v) glucose, 2% (w/v) granulated agar. (NB: This medium gives a better red/white distinction for petites in many strains.) 3 M GdnHCl stock, sterilised by autoclaving. Phosphate buffered saline (PBS): per litre add 8 g NaCl, 0.2 g KCl, 1.44 g Na2HPO4, 0.24 g KH2PO4. Adjust pH 7.4 and sterilise by autoclaving. Use for diluting cell aliquots prior to plating onto YEPD/1/4 YEPD. suppressor mutations, these can be easily identified by replica plating onto fresh YEPD+GdnHCl; only true [PSI + ] turns red. The number of propagons estimated by this method are in good agreement with those obtained using the kinetic method (Cox et al., 2003; Byrne, L. J. et al., unpublished).

~~~~~~ VIII. STUDYING PRION PROTEIN The study of yeast prions is not restricted to the in vivo approaches, which are the focus of this chapter. Much has been learnt about the conformational rearrangements and subsequent protein aggregation for two of the yeast prion proteins (Ure2p and Sup35p) from in vitro studies. In the test tube, both proteins are able to spontaneously undergo conformational rearrangement in the absence of any other proteins or nucleic acids, to generate highly stable fibrils which have the biophysical characteristics of amyloid fibrils (Glover et al., 1997; King et al., 1997; Taylor et al., 1999; Thual et al., 1999). The bulk of these studies have been carried out using the amino-terminal fragments of the protein that have been defined in vivo as being required for prion formation, i.e. the PrDs, although some studies have used full-length proteins (e.g. Krzewska and Melki, 2006). The standard assay for in vitro polymerisation of either full-length or isolated PrDs uses the amyloid-specific dyes Congo Red or Thioflavin-T to monitor the formation of amyloid in vitro. Starting with soluble protein, without any seeding, amyloid fibres usually form within 30–90 h. This time can be reduced to under 15 h by

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Yeast Prions and Their Analysis In Vivo

POLYMERISATION IN VITRO

seeding the assay with either preformed fibres of the protein under investigation (Glover et al., 1997) or by using whole cell extracts prepared from the relevant [PRION + ] strain (Uptain et al., 2001). Importantly, sonication of the preformed fibres prior to their addition to the assay, or gentle agitation of the assay by use of a roller drum can significantly reduce the length of the assay to less than 2 h. At the end point of such assays a significant fraction of the starting protein sample is now found as distinct amyloid-like fibres that can be readily visualised by atomic force microscopy (AFM), transmission electron microscopy (TEM) or scanning transmission electron microscopy (STEM). These high-resolution microscopic techniques can also be used to monitor the aggregation process, for example, by identifying key intermediate oligomeric forms (e.g. Serio et al., 2000) and measuring the rate of amyloid fibre growth (e.g. DePace and Weissman, 2002). To prepare sufficient quantities of the desired PrD for in vitro polymerisation studies requires the use of an E. coli-based highefficiency expression system and engineered versions of the respective PrD with suitable purification tags such as hexa-histidine placed in-frame at either the N- or the C-terminus of the protein. The preferred version of Sup35p used for many in vitro studies contains both the N (PrD) and M domain encompassing residues 1–254, while for Ure2p fragments encompassing either residues 1–65 or residues 1–89 (Taylor et al., 1999; Thual et al., 1999; Baxa et al., 2002; Jiang et al., 2004) are routinely used. The major challenge in preparing suitable protein for in vitro polymerisation studies is the need to have soluble protein, but both the Ure2p and Sup35p-PrDs are highly prone to aggregation when expressed in E. coli, which is perhaps not surprising given their amyloidogenic properties. Consequently, the proteins are usually purified under denaturing conditions using, for example, buffers containing 8 M urea or 4 M GdnHCl. A detailed description of how recombinant Sup35p fragments can be prepared can be found in Chernoff et al. (2002). Encouragingly, some researchers have however been able to produce native, soluble protein from E. coli; for example, Krzewska and Melki (2006) were able to produce soluble full-length Sup35p in E. coli for their in vitro studies. One of the most important questions arising from the use of in vitro polymerisation assays is inevitably ‘What is the relevance of the observed in vitro behaviour to the in vivo behaviour of a yeast prion protein’? The most direct way of answering is to show that the amyloid-like aggregates formed in vitro are able to seed the formation of the prion state of the protein in the living cell (King and Diaz-Avalos, 2004; Tanaka et al., 2004). That this has now been achieved using aggregated forms of the Sup35pN or NM regions provides perhaps the most definitive proof of the protein-only hypothesis of prion replication. In the most straightforward of these assays, Tanaka et al. (2004) generated amyloid-like fibres of Sup35pNM in vitro and then introduced them into [psi
] ade1-14 518

yeast cells using a ‘protein transformation’ protocol. This protocol is essentially the same as that originally developed for plasmid transformation (see Chapter 3, this volume) and involves the generation of sphaeroplasts using lyticase and then, after addition of the protein sample together with a selectable URA3 plasmid, the sphaeroplasts are regenerated and transformed cells selected. The use of a plasmid enables the researcher to first select for Ura+ transformed cells and then, by re-streaking these cells onto a medium that selects for Ura+ Ade+ cells, identify the cells transformed by the protein and have become [PSI + ].

~~~~~~ IX. HOW TO RECOGNISE A NEW YEAST The unusual nature of the phenotypic traits associated with the [PSI + ] and [URE3] prions first came to our attention because of their non-Mendelian mode of inheritance (see Section IV.A). Over the years a large number of other mutants have also emerged from yeast genetic screens, which have an underlying genetic determinant that is inherited in a non-Mendelian fashion. While most of these mutations map to the mitochondrial genome, i.e. are petite mutants, several do not and their basis remains unexplained. For example, Kunz and Ball (1977) reported on a glucosamine-resistant mutant whose determinant was cytoplasmically inherited but was not eliminated by agents (e.g. ethidium bromide) that effectively eliminate mitochondrial DNA from yeast. Now that we recognise that prions exist in yeast and that they can give rise to distinct phenotypes that are not necessarily detrimental to the host cell, there are a number of steps a researcher can take to establish whether or not they have stumbled upon a new prion-based determinant. Based on what we know about the three native prions so far described in S. cerevisiae, in addition to the failure to show Mendelian inheritance patterns for the underlying determinant, four questions need to be answered using relatively straightforward experiments in order to establish the nature of the underlying genetic determinant: (a) Is the genetic determinant lost when cells are cultured in the presence of GdnHCl (see Section VI.A)? (b) Can the ‘mutation’ spontaneously reappear in such cured cells? (c) Is the genetic determinant not maintained in cells carrying a knockout of the non-essential HSP104 gene (see Section VI.C)? (d) Can the determinant be transmitted to other cells in the absence of karyogamy, i.e. cytoduced (see Section IV.B)? While positive answers emerging from these experiments would give one confidence to explore the nature of the genetic determinant, they are not sufficient to unambiguously conclude that a 519

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PRION

prion-based determinant is involved in inheritance of the trait under examination. Critical is identification of the underlying prion protein and the demonstration that (a) the maintenance of the [PRION + ] state depends on the presence of the gene encoding that protein; (b) the protein forms transmissible Hsp104-dependent high– molecular-weight aggregates in [PRION + ] cells but not in [prion
] cells; (c) overexpression of the gene leads to an increase in the de novo appearance of [PRION + ] cells in a [PIN + ] but not in a [pin
] strain; and (d) the protein polymerises in vitro to form self-seeding amyloidlike fibres that, when transformed into a [prion
] cell gives rise at high frequency to the [PRION + ] state (see Section VIII). Should the protein identified satisfy all of these criteria, then one can be confident that one was dealing with a prion-based phenomenon. How does one identify the underlying prion protein? For both [PSI + ] and [URE3], a short list of candidate genes could – and indeed was – quickly drawn up based on what was known about the molecular basis of the associated phenotype. On the other hand, the Rnq1p prion protein was discovered through an attempt to predict what a yeast prion might look like and behave like, i.e. contains a Gln/Asn-rich region at either its N- or C-terminus (Sondheimer and Lindquist, 2000). However, in excess of 100 different yeast proteins contain such regions (Michelitsch and Weissman, 2000) but with the exception of Rnq1p, none of the other candidates has yet emerged as a true prion. Caution must therefore be taken using the rational approach; it is not sufficient to conclude that the protein is a prion just because a Gln/Asn-rich region exists in the protein sequence. That said, there must be a strong possibility that further prions remain to be discovered in S. cerevisiae and perhaps in other fungi?

Acknowledgements The work on yeast prions carried out in the authors’ laboratory is funded by the Biotechnology and Biological Sciences Research Council, the Wellcome Trust, the European Union (APOPIS: LSHMCT-2003-503330) and by the award of a Leverhulme Trust Emeritus Fellowship to BSC.

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Plate 10. A simple colony colour assay for the presence of the [PSI+] prion in Saccharomyces cerevisiae. Either of two different suppressible alleles can be used, the ade2-1 allele and the ade1-14 allele. In both the cases, when the mutation is expressed, i.e. in a [psi ] strain, the cells form red colonies that signal an adenine prototrophic phenotype. In [PSI+] cells, suppression of either allele leads to white colonies that are prototrophic and can grow without the provision of exogenous adenine. Note that strains carrying the ade2-1 allele must also carry the weak ochre suppressor tRNASer encoded by the SUQ5 (SUP16) gene (Cox, 1965), whereas the ade1-14 allele can be suppressed directly by [PSI+] in the absence of a suppressor tRNA. The identity of the amino acid inserted when the UGA codon in the ade1-14 allele is suppressed is unknown but is likely to be tryptophan (encoded by the UGG codon). (See also page 495 of this volume).

Plate 11. The use of green fluorescent protein (GFP) fusions to visualise Sup35p-based aggregates in [PSI+] strains. All cells shown contain a plasmid expressing an identical Sup35pPrD+M-GFP fusion protein whose synthesis was induced using the copper-inducible CUP1 promoter (Patino et al., 1996); (a) a [psi ] strain; (b)–(d) different [PSI+] variants; (e) a [PIN+][psi ] strain undergoing de novo conversion to [PSI+] as a consequence of the over expression of the Sup35pPrD+M-GFP fusion protein (see Zhou et al., 2001). (See also page 509 of this volume).

Plate 12. The elimination of the [PSI+] prion from cells grown in the presence of 3 mM guanidine hydrochloride (GdnHCl) over a 30-h period. The experiment was carried out as described in the Protocol 1 and the percent [PSI+] with time was plotted as shown. The data are combined from three independent experiments. The inset shows the types of colonies that one observes in such an experiment noting in particular that (a) GdnHCl induces a high frequency of mitochondrial petites, and (b) the petite mutation does cause a change in colouration when compared to grande strains with functional mitochondria. Colonies sectored red and white are counted as [PSI+]. Further details can be found in the text. (See also page 511 of this volume).

Plate 13. An SGD ‘‘Chromosomal Features Map’’ of the Saccharomyces genome near the ALD6 gene. This expanded view is obtained by selecting the ORF map on the gene (ALD6 in this case) summary page. The map provides a quick overview of genes or other chromosomal features adjacent to the gene of interest. (See also page 568 of this volume).