Search

The yeast prion [PSI + ] is a self-perpetuating aggregated isoform of the translation termination factor Sup35. [PSI + ] propagation is promoted by moderate levels and antagonized by high levels of the chaperone Hsp104. In agreement with the model postulating that excess Hsp104 acts on [PSI + ] by disaggregating prion polymers, we show that an increase in Sup35 levels, accompanied by an increase in size of prion aggregates, also partially protects [PSI + ] from elimination by excess Hsp104. Despite retention of [PSI + ], excess Hsp104 decreases toxicity of overproduced Sup35 in [PSI + ] strains. A heritable variant of [PSI + ], which has been isolated and is maintained only in the presence of increased levels of Hsp104, is characterized by an abnormally large aggregate size, and exhibits an altered response to overproduction of the Hsp70 chaperone Ssa1. These features resemble the previously described prion generated by a deletion derivative of Sup35, but are not associated with any sequence alteration and are controlled exclusively at the protein level. Our data provide a proof of the existence of conditionally stable prion variants maintained only at altered levels of Hsps, that could in principle be beneficial if the normal cellular function of a prion protein becomes detrimental to the cell in such conditions.

Mutants capable of a high frequency of cytoduction (Hfc+) were obtained in a haploid strain of Saccharomyces cerevisiae, suggesting impaired cytogamy. Nine of the 68 Hfc+ mutants showed the antisuppressor effect with respect to mutations of the SUP35 and SUP45 genes, which code for translation termination factors, or to the [PSI+] factor, which is the prion form of Sup35. Cosegregation of the characters "higher frequency of cytoduction" and "antisuppression" was demonstrated for three Hfc+ mutants. One (HFC12-2) of the Hfc+ mutations exerted a dominant antisuppressor effect with respect to [PSI+] and had no effect on [PSI+] maintenance. On the strength of the results, an interaction was assumed for translation termination components and cytoskeleton proteins, which play a role in karyogamy in yeasts.

According to the nucleated polymerization model, in vivo prion proliferation occurs via dissociation (shearing) of the huge prion polymers into smaller oligomeric 'seeds', initiating new rounds of prion replication. Here, we identify the deletion derivative of yeast prion protein Sup35 (Sup35-Delta22/69) that is specifically defective in aggregate shearing and 'seed' production. This derivative, [PSI+], previously thought to be unable to turn into a prion state, in fact retains the ability to form a prion ([PSI+](Delta22/69)) that can be maintained in selective conditions and transmitted by cytoplasmic infection (cytoduction), but which is mitotically unstable in non-selective conditions. MorePSI+](Delta22/69) retains its mitotic stability defect. The [PSI+](Delta22/69) cells contain more Sup35 protein in the insoluble fraction and form larger Sup35 aggregates compared with the conventional [PSI+] cells. Moderate excess of Hsp104 disaggregase increases transmission of the [PSI+](Delta22/69) prion, while excess Hsp70-Ssa chaperone antagonizes it, opposite to their effects on conventional [PSI+]. Our results shed light on the mechanisms determining the differences between transmissible prions and non-transmissible protein aggregates.

Chromosome stability in suppressor mutants for SUP35 and SUP45 genes coding for translation release factors was studied. We obtained spontaneous and UV-induced sup35 or sup45 mutants in a haploid strain disomic for chromosome III and tested the stability of an extra copy of this chromosome. The majority of the mutants showed increased chromosome instability. This phenotype was correlated with an increased sensitivity to the microtubule-poisoning drug benomyl which affects chromosome segregation at anaphase. Our data suggest that termination-translation factors eRF3 and eRF1 control chromosome transmission at mitotic anaphase in Saccharomyces cerevisiae.