Studies of genetic factors modulating polyglutamine toxicity in the yeast model
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Polyglutamine-expanded fragments, derived from the human huntingtin protein, are aggregation-prone and toxic in yeast cells, bearing endogenous QN-rich proteins in the aggregated (prion) form. Attachment of the proline-rich region targets polyglutamine aggregates to the large perinuclear deposit (aggresome). Aggresome targeting ameliorates polyglutamine cytotoxicity in the presence of the prion form of Rnq1 protein, however, aggresome-forming construct remains toxic in the presence of the prion form of translation termination (release) factor Sup35 (eRF3). Disomy by chromosome II partly ameliorates polyglutamine toxicity in the strains containing Sup35 prion. The chromosome II gene, coding for another release factor, and interaction partner of Sup35, named Sup45 (eRF1), is responsible for amelioration of toxicity. Plasmid-mediated overproduction of Sup45, or expression of the Sup35 derivative that lacks the QN-rich domain and is unable to be incorporated into prion aggregates, also ameliorate polyglutamine toxicity. Protein analysis indicates that polyglutamines alter aggregation patterns of the Sup35 prion and promote aggregation of Sup45, while excess Sup45 counteracts these effects. In the absence of Sup35 prion, disomy by chromosome II is still able to decrease polyglutamine toxicity. However, SUP45 is no longer the gene responsible for such an effect. Taken together with the finding that the presence of both the Rnq1 prion and the Sup35 prion has an additive effect on polyQ toxicity, one gene or few genes on chromosome II are able to ameliorate polyQ toxicity through a SUP45-independent pathway. The identification of such a gene is currently ongoing. Monosomy by chromosome VIII in diploid heterozygous by AQT (Anti-polyQ Toxicity mutants that are disomic by chromosome II) counteracted the effect of AQT. Similarly, deletion of the arg4 gene in chromosome VIII in AQT haploid was able to eliminate the AQT effect. Moreover, analysis of genes involved in the arginine and polyamine synthesis indicated that loss of genes in later stages of arginine biosynthesis causes increase of polyglutamine toxicity. Deletion of genes arg1, arg4, arg8 (arginine pathway) and spe1 (polyamine pathway) all suppressed the Sup35 prion phenotype expression in the nonsense suppression system. Further analysis regarding the mechanisms behind those effects is needed. Our data uncover the mechanisms by which genetic and epigenetic factors may influence polyglutamine toxicity, and demonstrate that one and the same type of polyglutamine deposits could be cytoprotective or cytotoxic, depending on the prion composition of a eukaryotic cell.