Un modèle prion (études génétique et fonctionnelle de la protéine HET-s chez le champignon filamenteux Podospora anserina)

BORDEAUX2-BU Santé (330632101) / SudocSudocFranceF

Sexual transmission of the [Het-s] prion leads to meiotic drive in Podospora anserina

In the filamentous fungus Podospora anserina, two phenomena are associated with polymorphism at the het-s locus, vegetative incompatibility and ascospore abortion. Two het-s alleles occur naturally, het-s and het-S. The het-s encoded protein is a prion propagating as a self-perpetuating amyloid aggregate. When prion-infected [Het-s] hyphae fuse with [Het-S] hyphae, the resulting heterokaryotic cells necrotize. [Het-s] and [Het-S] strains are sexually compatible. When, however, a female [Het-s] crosses with [Het-S], a significant percentage of het-S spores abort, in a way similar to spore killing in Neurospora and Podospora. We report here that sexual transmission of the [Het-s] prion after nonisogamous mating in the reproductive cycle of Podospora is responsible for the killing of het-S spores. Progeny of crosses between isogenic strains with distinct wild-type or introduced, ectopic het-s/S alleles were cytologically and genetically analyzed. The effect of het-s/S overexpression, ectopic het-s/S expression, absence of het-s expression, loss of [Het-s] prion infection, and the distribution patterns of HET-s/S-GFP proteins were categorized during meiosis and ascospore formation. This study unveiled a het-S spore-killing system that is governed by dosage of and interaction between the [Het-s] prion and the HET-S protein. Due to this property of the [Het-s] prion, the het-s allele acts as a meiotic drive element favoring maintenance of the prion-forming allele in natural populations

Amyloid aggregates of the HET-s prion protein are infectious

The [Het-s] infectious element of the filamentous fungus Podospora anserina is a prion. We have recently reported that recombinant HET-s protein aggregates in vitro into amyloid fibers. In vivo, the protein aggregates specifically in the [Het-s] prion strains. Here, we show that biolistic introduction of aggregated recombinant HET-s protein into fungal cells induces emergence of the [Het-s] prion with a high frequency. Thus, we demonstrate that prion infectivity can be created de novo, in vitro from recombinant protein in this system. Although the amyloid filaments formed from HET-s could transmit [Het-s] efficiently, neither the soluble form of the protein nor amorphous aggregates would do so. In addition, we have found that (i) [Het-s] infectivity correlates with the ability to convert HET-s to amyloids in vitro, (ii) [Het-s] infectivity is resistant to proteinase K digestion, and (iii) HET-s aggregates formed in vivo in [Het-s] strains have the ability to convert the recombinant protein to aggregates. Together, our data designate the HET-s amyloids as the molecular basis of [Het-s] prion propagation

Chaperone-dependent amyloid assembly protects cells from prion toxicity

Protein conformational diseases are associated with the aberrant accumulation of amyloid protein aggregates, but whether amyloid formation is cytotoxic or protective is unclear. To address this issue, we investigated a normally benign amyloid formed by the yeast prion [RNQ+]. Surprisingly, modest overexpression of Rnq1 protein was deadly, but only when preexisting Rnq1 was in the [RNQ+] prion conformation. Molecular chaperones protect against protein aggregation diseases and are generally believed to do so by solubilizing their substrates. The Hsp40 chaperone, Sis1, suppressed Rnq1 proteotoxicity, but instead of blocking Rnq1 protein aggregation, it stimulated conversion of soluble Rnq1 to [RNQ+] amyloid. Furthermore, interference with Sis1-mediated [RNQ+] amyloid formation exacerbated Rnq1 toxicity. These and other data establish that even subtle changes in the folding homeostasis of an amyloidogenic protein can create a severe proteotoxic gain-of-function phenotype and that chaperone-mediated amyloid assembly can be cytoprotective. The possible relevance of these findings to other phenomena, including prion-driven neurodegenerative diseases and heterokaryon incompatibility in fungi, is discussed