Guanidine hydrochloride inhibits the generation of prion "seeds" but not prion protein aggregation in yeast

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

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

Yeast [PSI+] prion aggregates are formed by small Sup35 polymers fragmented by Hsp104

The yeast [PSI+] determinant is related to formation of large prion-like aggregates of the conformationally altered Sup35 protein. Here, we show that these aggregates are composed of small Sup35 prion polymers and associated proteins. In contrast to other protein complexes of yeast lysates, but similarly to amyloid fibers, these polymers are insoluble in SDS at room temperature. The polymers on average are about 30-fold smaller than the aggregates and comprise from 8 to 50 Sup35 monomers. The size of polymers is characteristic of a given [PSI+] variant and differs between the variants. Blocked expression of Hsp104 chaperone causes gradual increase in the size of prion polymers, while inactivation of Hsp104 by guanidine HCl completely stops their fragmentation, which shows indispensability of Hsp104 for this process.

Analysis of the Generation and Segregation of Propagons: Entities That Propagate the [PSI+] Prion in Yeast

The propagation of the prion form of the yeast Sup35p protein, the so-called [PSI+] determinant, involves the generation and partition of a small number of particulate determinants that we propose calling “propagons.” The numbers of propagons in [PSI+] cells can be inferred from the kinetics of elimination of [PSI+] during growth in the presence of a low concentration of guanidine hydrochloride (GdnHCl). Using this and an alternative method of counting the numbers of propagons, we demonstrate considerable clonal variation in the apparent numbers of propagons between different [PSI+] yeast strains, between different cultures of the same [PSI+] yeast strain, and between different cells of the same [PSI+] culture. We provide further evidence that propagon generation is blocked by growth in GdnHCl and that it is largely confined to the S phase of the cell cycle. In addition, we show that at low propagon number there is a bias toward retention of propagons in mother cells and that production of new propagons is very rapid when cells with depleted numbers of propagons are rescued into normal growth medium. The implications of our findings with respect to yeast prion propagation mechanisms are discussed.

[PHI+], a novel Sup35-prion variant propagated with non-Gln/Asn oligopeptide repeats in the absence of the chaperone protein Hsp104

BACKGROUND

The [PSI+] element of the budding yeast is an aggregated form of the translation release factor Sup35 that is propagated and transmitted cytoplasmically in a manner analogous to that of mammalian prions. The N-terminal of Sup35, necessary for [PSI+], contains oligopeptide repeats and multiple Gln/Asn residues.

RESULTS

We replaced the Gln/Asn-rich prion repeats of Sup35 with non-Gln/Asn repeats from heterologous yeast strains. These non-Gln/Asn repeat Sup35s propagated a novel [PSI+] variant, [PHI+], that appeared de novo 103 times more frequent than [PSI+]. [PHI+] was stably inherited in a non-Mendelian fashion, but not eliminated upon the inactivation of Hsp104, unlike known [PSI+] elements. In vitro, non-Gln/Asn repeat domains formed amyloid fibres that were shorter and grew more slowly than did Gln/Asn-rich prion domains, while [PHI+] aggregates were smaller than [PSI+] aggregates in vivo.

CONCLUSIONS

These findings suggest the existence of an alternative, Hsp104-independent pathway to replicate non-Gln/Asn variant Sup35 prion seeds.

Deletion of the Hsp70 chaperone gene SSB causes hypersensitivity to guanidine toxicity and curing of the [PSI+] prion by increasing guanidine uptake in yeast

Yeast Ssb proteins (Ssbp) are ribosome-associated Hsp70 chaperones that function in translation. Elevated levels of Ssbp enhance the ability of over-expressed Hsp104 chaperone to eliminate the yeast [PSI+] prion, while depletion of Ssbp reduces this effect. Millimolar concentrations of guanidine in the growth medium cure yeast cells of prions by inactivating Hsp104. Guanidine is also toxic to yeast, irrespective of the status of Hsp104 and [PSI+]. Strains that lack Ssbp are hypersensitive to guanidine toxicity. Here we show that ssb- cells have normal numbers of [PSI+] "seeds", but can be cured of [PSI+] using one-sixth of the guanidine concentration required to eliminate [PSI+] from SSB cells. Correspondingly, the level of intracellular guanidine was eight-fold higher in ssb- cells than in wild-type cells, which explains all effects of Ssbp depletion on susceptibility to guanidine. The sensitivity of wild-type cells to the effects of guanidine also correlated with guanidine uptake, which was enhanced at low temperature. Guanidine sensitivity of strains mutated in any of 16 ABC membrane transporters, which are implicated in multidrug resistance, was normal. We found that an erg6 mutant that has an altered membrane lipid composition was hypersensitive to guanidine toxicity, but the lipid composition of ssb- cells was identical to that of wild-type cells. Our results suggest that Ssbp depletion does not affect prion seed regeneration, and that elevated guanidine uptake by ssb- cells may be due to increased retention rather than to an alteration in active or passive transport of the compound.

Guanidine reduces stop codon read-through caused by missense mutations in SUP35 or SUP45

Sup35 and Sup45 are essential protein components of the Saccharomyces cerevisiae translation termination factor. Yeast cells harbouring the [PSI(+)] prion form of Sup35 have impaired stop codon recognition (nonsense suppression). It has long been known that the [PSI(+)] prion is not stably transmitted to daughter cells when yeast are grown in the presence of mM concentrations of guanidine hydrochloride (GuHCl). In this paper, Mendelian suppressor mutations whose phenotypes are likewise hidden during growth in the presence of millimolar GuHCl are described. Such GuHCl-remedial Mendelian suppressors were selected under conditions where [PSI(+)] appearance was limiting, and were caused by missense mutations in SUP35 or SUP45. Clearly, anti-suppression caused by growth in the presence of GuHCl is not sufficient to distinguish missense mutations in SUP35 or SUP45, from [PSI(+)]. However, the Mendelian and prion suppressors can be distinguished by subsequent growth in the absence of GuHCl, where only the nonsense suppression caused by the [PSI(+)] prion remains cured. Recent reports indicate that GuHCl blocks the inheritance of [PSI(+)] by directly inhibiting the activity of the protein remodelling factor Hsp104, which is required for the transmission of [PSI(+)] from mother to daughter cells. However, the nonsense suppressor activity caused by the GuHCl-remedial sup35 or sup45 suppressors does not require Hsp104. Thus, GuHCl must anti-suppress the sup35 and sup45 mutations via an in vivo target distinct from Hsp104.